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  • Handong Gui; Zheyu Zhang; Ruirui Chen; Ren Ren; Jiahao Niu; Haiguo Li; Zhou Dong; Craig Timms; Fei Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel Costinett; Benjamin B. Choi
    IEEE Transactions on Power Electronics
    2020

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    To better support the superconducting propulsion system in the future aircraft applications, the technologies of high-power high switching frequency power electronics systems at cryogenic temperatures should be investigated. This article presents the development of a 40-kW cryogenically cooled three-level active neutral point clamped inverter with 3 kHz output line frequency and 140 kHz switching frequency. Si mosfets are characterized at cryogenic temperatures, and the results show that they have promising performance such as lower on-resistance and switching loss. The design of the inverter is presented in detail with the special consideration of the cryogenic temperature operation. Moreover, a packaging and integration architecture is designed and fabricated to demonstrate the feasibility and performance of the inverter in the lab. It is able to achieve no leakage with good thermal and air insulation. With the inverter and packaging, the experimental results show that the inverter operates properly at cryogenic temperatures. The loss is measured at different load conditions, and the loss analysis is given, which shows that the cryogenically cooled inverter has 30% less loss than operating at room temperature.

  • Shiqi Ji; Li Zhang; Xingxuan Huang; James Palmer; Fred Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2020

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    Using high voltage (HV) Silicon Carbide (SiC) power semiconductors in the modular multilevel converter (MMC) is promising because of a fewer submodules and lower switching loss compared to conventional Si based solutions. The nearest level pulse width modulation (NL-PWM) is commonly used in the MMC for medium voltage applications. However, with the NL-PWM and existing voltage balancing control, there are many submodules that switch their modes in a control cycle, resulting in a high dv/dt during the deadtime of the power semiconductor, which could be multiple times of the dv/dt of single device. This poses great challenges on the noise immunity and insulation design in the MMC using HV SiC devices, which have very fast switching speed. A novel voltage balancing control, which ensures only two submodules switch their modes in a control cycle, is proposed in this paper, limiting the maximum dv/dt to the dv/dt of single power semiconductor and meanwhile maintaining the voltage balance performance. The proposed voltage balancing control is experimentally validated in a 10 kV SiC MOSFET based MMC with four submodules per arm.

  • Handong Gui; Ruirui Chen; Zheyu Zhang; Jiahao Niu; Ren Ren; Bo Liu; Leon M. Tolbert; Fei Fred Wang; Daniel Costinett; Benjamin J. Blalock; Benjamin B. Choi
    IEEE Transactions on Power Electronics
    2020

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    This article establishes an analytical model for the device drain–source overvoltage related to the two loops in three-level active neutral point clamped (3L-ANPC) converters. Taking into account the nonlinear device output capacitance, two common modulation methods are investigated in detail. The results show that the line switching frequency device usually has higher overvoltage, and the switching speed of the high switching frequency device is not strongly influenced by the multiple loops. By keeping the nonactive clamping switch off, the effect of the nonlinear device output capacitance can be significantly mitigated, which helps reduce the overvoltage. Moreover, the loop inductance can be reduced with vertical loop layout and magnetic cancellation in the printed circuit board and busbar design. A 500-kVA 3L-ANPC converter using silicon carbide mosfets was built and tested. The experimental results validate the overvoltage model of the two modulation methods as well as the busbar design. With the nonactive clamping switch off, the overvoltage of both the high and line switching frequency devices is significantly reduced, which helps achieve higher switching speed.

  • Ren Ren; Handong Gui; Zheyu Zhang; Ruirui Chen; Jiahao Niu; Fei Wang; Leon M. Tolbert; Daniel Costinett; Benjamin J. Blalock; Benjamin B. Choi
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2020

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    In order to evaluate the feasibility of newly developed gallium nitride (GaN) devices in a cryogenically cooled converter, this article characterizes a 650-V enhancement-mode GaN high-electron mobility transistor (GaN HEMT) at cryogenic temperatures. The characterization includes both static and dynamic behaviors. The results show that this GaN HEMT is an excellent device candidate to be applied in cryogenic-cooled applications. For example, transconductance at cryogenic temperature (93 K) is 2.5 times higher than one at room temperature (298 K), and accordingly, peak di/dt during turn-on transients at cryogenic temperature is around 2 times of that at room temperature. Moreover, the ON-resistance of the channel at the cryogenic temperature is only one-fifth of that at room temperature. The corresponding explanations of performance trends at cryogenic temperatures are also given from the view of semiconductor physics. In addition, several device failures were observed during the dynamic characterization of GaN HEMTs at cryogenic temperatures. The ultrafast switching speed-induced high di/dt and dv/dt at cryogenic temperatures amplify the negative effects of parasitics inside the switching loop. Based on failure waveforms, two failure modes were classified, and detailed failure mechanisms caused by ultrafast switching speed are given in this article.

  • Ruirui Chen; Jiahao Niu; Handong Gui; Zheyu Zhang; Fei Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock; Benjamin B. Choi
    IEEE Transactions on Power Electronics
    2020

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    Paralleling three phase three-level inverters is gaining popularity in industrial applications. However, analytical models for the harmonics calculation of a three-level neutral point clamped (NPC) inverter with popular space vector modulation (SVM) are not found in the literature. Moreover, how interleaving angle impacts the dc- and ac-side harmonics and electromagnetic interference (EMI) harmonics in parallel interleaved three-level inverters and how to optimize interleaving angle to reduce these harmonics have not been discussed in the literature. Furthering previous study, this article presents the modeling, analysis, and reduction of harmonics in paralleled and interleaved three-level NPC inverters with SVM. Analytical models for harmonic calculation are developed, and the dc-side harmonics characteristics of an NPC inverter are identified. The impact of interleaving angle on the ac-side voltage and dc-link current harmonics of parallel interleaved three-level NPC inverters is comprehensively studied. The impact of switching frequency and interleaving angle on EMI harmonics is also illustrated. Optimal interleaving angle ranges to reduce these harmonics are derived analytically. The developed models and harmonic reduction analysis are verified experimentally with two paralleled and interleaved three-level NPC inverters.

  • Handong Gui; Ruirui Chen; Jiahao Niu; Zheyu Zhang; Leon M. Tolbert; Fei Fred Wang; Benjamin J. Blalock; Daniel Costinett; Benjamin B. Choi
    IEEE Transactions on Power Electronics
    2020

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    In order to apply power electronics systems to applications such as superconducting systems under cryogenic temperatures, it is necessary to investigate the characteristics of different parts in the power electronics system. This article reviews the influence of cryogenic temperature on power semiconductor devices including Si and wide bandgap switches, integrated circuits, passive components, interconnection and dielectric materials, and some typical cryogenic converter systems. Also, the basic theories and principles are given to explain the trends for different aspects of cryogenically cooled converters. Based on the review, Si active power devices, bulk Complementary metal-oxide-semiconductor (CMOS) based integrated circuits, nanocrystalline and amorphous magnetic cores, NP0 ceramic and film capacitors, thin/metal film and wirewound resistors are the components suitable for cryogenic operation. Pb-rich PbSn solder or In solder, classic printed circuit boards material, most insulation papers and epoxy encapsulant are good interconnection and dielectric parts for cryogenic temperatures.

  • Horacio Daniel Silva-Saravia; Hector Arnaldo Pulgar; Leon M Tolbert; David A Schoenwald; Wenyun Ju
    IEEE Transactions on Sustainable Energy
    2020

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    This paper presents a new control method to enable large-scale solar photovoltaic (PV) plants to damp electromechanical oscillations. The proposed step-down modulation (SDM) control method is based on active power modulation, and it does not require curtailment as in other approaches. After an oscillation event is detected, the PV panel voltage is controlled to transiently deviate the power from its maximum power point (MPP), this power margin is used to modulate active power until the oscillation event is mitigated. Then, the SDM control restores the PV power to its MPP, and it is reset to operate for the next event. The control design, panel voltage strategy, and implementation is tested in a two-area system. A comparison of the SDM control with a curtailment-based PV damping control is also explored in a test case of the 179-bus WECC system with six large-scale PV plants, showing the improved damping capability of the proposed method.

  • Handong Gui; Jingjing Sun; Leon M Tolbert
    IEEE Transactions on Power Electronics
    2020

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    Turn-on loss is the dominant part of the switching loss for SiC MOSFETs in hard switching. It is difficult to reduce turn-on loss with conventional voltage source gate drives (VSGs) because of the limited gate voltage rating and large internal gate resistance of SiC MOSFETs. A charge pump gate drive (CPG) that can reduce the turn-on loss is presented in this paper. By pre-charging the charge-storage capacitor in the gate drive with a charge pump circuit, the gate drive output voltage is pumped up to provide higher gate current during the turn-on transient. As a result, the turn-on time and loss is decreased. Moreover, due to the charge transfer from the charge-storage capacitor to the MOSFET gate capacitance, the pumped output voltage can naturally drop back to a normal value that avoids gate overcharging. The structure of the gate drive is simple, and no additional control is needed. The operation of the proposed CPG is verified with double pulse tests based on SiC MOSFETs. The switching loss of the proposed CPG is reduced by up to 71.7% compared to the conventional VSG at full load condition.

  • Bo Liu; Ren Ren; Fei Fred Wang; Daniel Costinett; Zheyu Zhang
    IEEE Transactions on Power Electronics
    2020

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    The attenuation performance of an electromagnetic interference filter can be significantly degraded by coupling, parasitics, and frequency-dependent nonlinearity, especially in high frequency (HF) range. This article reveals and investigates a mutual capacitive coupling effect in the popular filter structures with T-shaped joint. The mechanism is explained and the impact on filter attenuation is analyzed, which show this coupling is the dominant cause of performance degradation of T-shaped filters and a major cause for other T-shape-related filters. The effect patterns for both common-mode (CM) and differential-mode (DM) filters are analytically derived and further examined in multistage filter structures. Mitigation solutions using PCB slits and grounded shielding are proposed to improve filter transfer gain up to 30 dB in the HF range. A topological strategy is also presented, further enhancing filter attenuation. In addition, the impact of relative positions of the inductors on the coupling capacitance is discussed, and five positions are experimentally studied and compared. Experimental results obtained from three-phase LCL and LCLC filters verify the significance of this coupling and the effectiveness of the mitigation methods.

  • Bo Liu; Ren Ren; Fei Wang; Daniel Costinett; Zheyu Zhang
    IEEE Transactions on Industrial Electronics
    2020

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    This paper studies how an outer fractional winding can impact the equivalent parallel capacitance (EPC) of a differential-mode inductor, which is a critical passive component in a power electronic converter to combat with electromagnetic noises, and proposes a winding scheme that can reduce EPC and increase inductance, achieving both high-frequency filtering performance and high density. To perform these studies, a comprehensive layer capacitance model based on energy equivalence principle is established, which decouples EPC contribution among three elements, i.e., outer fraction layer, layer-to-layer, and layer-to-core, thus enabling the impact evaluation of different winding elements and schemes. Experimental comparison results have validated the accuracy of this EPC model and excellent performance of the proposed winding scheme with EPC reduction by 4×. It reveals that contrary to previous understanding, the inverse winding, in fact, is more effective for EPC reduction than the direct winding in most of the partial layer scenarios, and that by using this scheme with the outer fraction layer, 45% higher inductance and slightly less EPC can be achieved, compared to the single-layer winding design.

  • Pengfei Yao; Xiaohua Jiang; Peng Xue; Shiqi Ji; Fei Wang
    IEEE Transactions on Power Electronics
    2020

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    Transformer flux dc bias is a critical issue, impacting the reliable operation of dual active bridge (DAB) converters especially when ungapped high permeability nanocrystalline core is used. Steady state current dc bias can easily saturate ungapped nanocrystalline transformers, and it is even more dangerous in transient conditions. A dc bias model is proposed to analyze steady state dc bias in different load conditions. The magnetizing current detection is necessary for closed loop control of dc bias, conductors’ position is shown to impact sensor noise, and it is detailed analyzed. To deal with both steady state and transient dc bias, a unified flux balancing control (UFBC) is proposed introducing a predictive bias suppression (PBS) method with closed-loop flux balancing control (CFBC). With the proposed PBS, both primary/secondary current balance and flux balance can be achieved within one switching cycle using UFBC. Power characteristics and interaction between power control and flux balancing control of DAB converters are analyzed, and the CFBC need to work in a low bandwidth due to sensor bandwidth limitation and interaction between power control and flux balancing control. The UFBC is verified on a 300kW cascaded DAB converter prototype.

  • Wenchao Cao; Yiwei Ma; Fei Wang; Leon M. Tolbert; Yaosuo Xue
    IEEE Transactions on Smart Grid
    2020

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    For system planning of three-phase inverter-based islanded ac microgrids, the low frequency instability issue caused by interactions of inverter droop controllers is a major concern. When internal control information of procured commercial inverters is unknown, impedance-based small-signal stability criteria facilitate prediction of resonances in medium and high frequency ranges, but they usually assume the grid fundamental frequency as constant and thus they are incapable of analyzing the low-frequency oscillation of the fundamental frequency in islanded microgrids. Aiming at solving this issue, this paper proposes two stability analysis methods based on terminal characteristics of inverters and passive connection network including the dynamics of the fundamental frequency for analysis of low-frequency stability in islanded multiple-bus microgrids. Based on the Component Connection Method (CCM) to systematically separate inverters from the passive connection network, a general approach is developed to model the microgrid as a multiple-input-multiple-output (MIMO) negative feedback system in the common system d-q reference frame. By applying the generalized Nyquist stability criterion (GNC) to the return-ratio and return-difference matrices of the MIMO system model, the low-frequency stability related to the fundamental frequency can be analyzed using the measured terminal characteristics of inverters. Analysis and simulation of a 37-bus microgrid verify the effectiveness of the proposed stability analysis methods.

  • Dual-active-bridge (DAB) circuit is an excellent candidate for a high-efficiency, high-power density, and bidirectional electric vehicle charger. Unlike resonant circuits employing auxiliary inductors and capacitors, DAB minimizes the usage of passive components. The challenge, however, lies in the difficulty of securing zero-voltage switching (ZVS), particularly at light-to-medium load when using the conventional single-phase-shift (SPS) control. This is of utmost importance not only for the sake of the efficiency, but also for minimizing the switch-bridge crosstalk caused by the hard switching-on, thereby enhancing the system reliability. Although dual-phase-shift (DPS) and triple-phase-shift (TPS) can be the answer, they do introduce side effects such as larger switching-off current. This article systematically integrates SPS, DPS, and TPS to maximize full-power ZVS range for both steady state and transient operations in EV chargers. This article plots ZVS boundaries over the full power range, categorizes all operations into nine modes, and proposes a smooth transition method among all operation modes. Dead band is also incorporated in the ZVS boundary setting. Experimental results on a SiC-based charger validate the effectiveness of this method of widening ZVS range for output voltage of 200-450 Vdc and power of 0-20 kW, achieving smooth transitions among various operation modes, and suppressing the switch crosstalk, thereby securing high charger reliability.

  • Ren Ren; Fanghua Zhang; Bo Liu; Fei Wang; Zichang Chen; Jianping Wu
    IEEE Transactions on Industrial Electronics
    2020

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    Wide bandgap semiconductors with fast switching speed capability are becoming an enabler to achieve the higher power density design. However, the further increase of switching frequency cannot continuously improve the power quality of ac-side waveforms in dc-ac/ac-dc application. The main reason is the distortion of pulsewidth modulation (PWM) voltage at switching point under high switching frequency. In general, there are two contributors causing PWM voltage distortions in bridge-based topologies. One is the deadtime, which occupies higher ratio in the small duty cycle case under high switching frequency and induces the voltage errors. The other one is the turn-off transient in the small load current or around the zero-crossing point of the ac-side current, because the relatively slow rising slope of the drain-source voltage will affect the right duty cycle. To solve this issue, this paper proposes a closed-loop modulation scheme to compensate the duty cycle distortion of PWM voltage based on one-cycle control or charge control. Compared to the traditional feedforward type of compensation, the proposed scheme does not need online calculation and instantaneous inductor current sampling, and it shows potential to be a general scheme for variety of topologies and modulations. Simulations and experiments are carried out on a 400-kHz single-phase full-bridge inverter with 400-Hz fundamental frequency to demonstrate the performance of the proposed approach.

  • Liyan Zhu; Hua Bai; Alan Brown; Matt McAmmond
    IEEE Transactions on Transportation Electrification
    2020

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    Wide-bandgap (WBG) devices are considered to be a better alternative to silicon switches to realize high-efficiency and high-power-density power electronics converters, such as electric vehicle (EV) onboard chargers. The two major challenges of GaN devices are their relatively high cost (~5 times compared to Si) and much smaller footprint than Si, which though is preferred in the high-power-density application is preferred but brings thermal challenges. Much like SiC is paralleled with Si, and GaN could be paralleled with Si to resolve these challenges. In this article, gallium nitride (GaN) high-electron-mobility transistors (HEMTs) are paralleled to various Si MOSFETs. Two different triggering approaches are considered: one adds a time delay between gate signals and the other uses a pulse triggering technique. Both methods ensure that the GaN endures the switching loss, while the Si switches conduct the majority of the current, thereby maximizing the advantages of both types of switches. To follow is a comprehensive study of the critical transient processes, such as the gate cross talking between Si and GaN, current commutation in the dead band, voltage spikes during the turn-off caused by parasitics, the thermal performance, and the cost analysis. Demonstrated success testing this approach at 400 V/80 A provides evidence that this is a possible approach in the onboard EV battery charger applications. The success of testing under 400 V/80 A makes it possible to an onboard EV battery charger.

  • Jie Chen; Yuting Shen; Jiawei Chen; Han Bai; Chunying Gong; Fred Wang
    IEEE Transactions on Transportation Electrification
    2020

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    Multi-pulse AC/DC Rectifiers (MPRs) are widely used in aviation application due to their rugged structure, cost effective, and high reliability features. In this paper, an overview of the recent advances and trends on the MPR technology, mainly the auto-configured transformer based MPRs, and its application in more electric aircrafts are performed. The work covers system topologies, transformer configurations, passive and active harmonic reduction schemes, case study, practical selection and design guideline, and applications. To fairly evaluate the performances of MPRs with different pulse number, necessary simulation studies are carried out under comparable conditions, including power rating, input and output specifications, and transformer configuration etc. Then, an 18-pulse asymmetric DP configured prototype is established based on the simulation evaluation and experimental verification is performed. It is expected that the paper can provide a broad perspective on MPR technology, and, in particular, highlight the latest emerged technology that significantly promotes the performances of MPRs. More importantly, it is desired that the results obtained in this paper can provide an effective selection guideline and design suggestion for researchers and engineers engaged in designing MPRs, especially for aviation application.

  • Handong Gui; Zheyu Zhang; Ruirui Chen; Jiahao Niu; Leon M. Tolbert; Fei Fred Wang; Daniel Costinett; Benjamin J. Blalock; Benjamin B. Choi
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2019

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    To understand the limitation of maximizing the switching speed of SiC low current discrete devices and high current power modules in hard switching applications, double pulse tests are conducted and the testing results are analyzed. For power modules, the switching speed is generally limited by the parasitics rather than the gate drive capability. For discrete SiC devices, the conventional voltage source gate drive (VSG) is not sufficient to maximize the switching speed even if the external gate resistance is minimized. The limitation of existing current source gate drives (CSG) are analyzed, and a CSG dedicated for SiC discrete devices is proposed, which can provide constant current during the switching transient regardless of the high Miller voltage and large internal gate resistance. Compared with the conventional VSG, the proposed CSG achieves 67% faster turnon time and 50% turn-off time, and 68% reduction in switching loss at full load condition.

  • Xiaotong Hu; Tianqi Liu; Yiwei Ma; Yu Su; He Yin; Lin Zhu; Fei Wang; Leon M. Tolbert; Yilu Liu
    IET Generation, Transmission & Distribution
    2019

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    For some distribution networks equipped with smart switches such as Chattanooga Electric Power Board (EPB) system, they can island some areas of the network to mitigate the impact through defensive islanding. However, due to intermittency and uncertainty of renewable-based distributed energy resources (DERs), it is highly likely that the islanded areas would experience insufficient or surplus power. This problem can be relieved by changing the boundaries of islanded areas to incorporate neighbouring load sections (LSs) or disconnect some connected LSs. Considering penetration level and sharply changing rate of renewable energy, it is challenging to define suitable boundaries for islanded areas in real time. Therefore, a two-stage energy management system (EMS) is proposed in this study, which includes day-ahead scheduling stage as well as short-term and real-time control stages. In the first stage, the initial switch combinations of LSs and DERs’ scheduling are obtained through a mixed integer quadratic programming, whereas the second stage is based on rule-based power management algorithm. Finally, a model reduced from real EPB system is used for validating the proposed two-stage EMS. The results successfully verify the effectiveness and performance of the proposed EMS for addressing the energy management of islanded areas under defensive islanding.

  • He Yin; Yiwei Ma; Lin Zhu; Xiaotong Hu; Yu Su; Jim Glass; Fred Wang; Yilu Liu; Leon M. Tolbert
    IET Smart Grid
    2019

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    The design, implementation, and testing of a control system for a flexible microgrid (MG) is presented in this study. The MG controllers can be implemented in a real-world MG with multiple smart switches, photovoltaic panel system, and battery energy storage systems (BESSs). With the benefits from smart switches, the MG has unique characteristics such as dynamic boundary and flexible point of interconnection (POI) concepts. To control such a unique MG and realise the dynamic boundary, an MG central controller and two types of local controllers are implemented. Compared to the MG with fixed boundary, the MG with dynamic boundary can have smaller BESS capacity, better utilisation of renewable energy, and multiple POI options. Also, compared with IEEE Std 2030.7–2017, the topology identification and active and reactive power balance functions are newly designed to realise the dynamic boundary concept. The planned islanding and reconnection functions are modified to realise the flexible POI concept. These functions are introduced including the software architecture, cooperation, and interaction among them. Finally, a hardware-in-the-loop testing platform based on the Opal-RT real-time simulator is set up to verify the performance, realisation of the dynamic boundary, and flexible POI concepts with four comprehensive test scenarios.

  • Zheyu Zhang; Jacob Dyer; Xuanlyu Wu; Fei Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2019

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    Junction temperature is an important design/operation parameter, as well as, a significant indicator of device's health condition for power electronics converters. Compared to its silicon (Si) counterparts, it is more critical for silicon carbide (SiC) devices due to the reliability concern introduced by the immaturity of new material and packaging. This paper proposes a practical implementation using an intelligent gate drive for online junction temperature monitoring of SiC devices based on turn-off delay time as the thermo-sensitive electrical parameter. First, the sensitivity of turn-off delay time on the junction temperature for fast switching SiC devices is analyzed. A gate impedance regulation assist circuit is proposed to enhance the sensitivity by a factor of 60 and approach 736 ps/°C tested in the case study with little penalty on the power conversion performance. Next, an online monitoring unit based on gate assist circuits is developed to monitor the turn-off delay time in real time with the resolution less than 104 ps. As a result, the micro-controller is capable of “reading” junction temperature during the converter operation. Finally, a SiC-based half-bridge inverter is constructed with an intelligent gate drive consisting of the gate impedance regulation circuit and online turn-off delay time monitoring unit. Experimental results demonstrate the feasibility and accuracy of the proposed approach.

  • Bo Liu; Ren Ren; Edward A. Jones; Handong Gui; Zheyu Zhang; Ruirui Chen; Fei Wang; Daniel Costinett
    IEEE Transactions on Power Electronics
    2019

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    This paper identifies extra junction capacitances and switching commutation loops introduced by line-frequency devices (i.e., non-active every other half line cycle) in three-level ac/dc converters and investigates the corresponding effects. Junction capacitances and power loops are well known as the key factors that impact converter switching loss and device stress, thus influence device selection, power stage layout, and thermal design. By examining switching transients of the commonly used T-shaped and I-shaped three-level converters, the cause and mechanism of the extra junction capacitances and power loops are presented. The impacts on switching loss, device voltage stress, and ac-side voltage/current distortion are respectively reported and analyzed. A loss calculation scheme for the three-level converter to include that extra loss is proposed. A power layout scheme to mitigate the device voltage stress is provided. Compensation and modeling of the voltage and current distortion are also proposed. Experimental results conducted on several types of three-level converter prototypes including a gallium nitride based 115 Vac/650 Vdc/1.5-kW/450-kHz Vienna-type rectifier and a SiC MOSFET based 1-kV/10-kW/ 280-kHz three-level active neutral-point-clamped inverter confirm the presented effects and verify the associated analysis and solutions.

  • Jessica D. Boles; Yiwei Ma; Jingxin Wang; Denis Osipov; Leon M. Tolbert; Fred Wang
    IEEE Transactions on Industry Applications
    2019

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    Battery energy storage systems (BESSs) tend to be too costly, restrictive, and require high maintenance for experimental use, but power system tests often need their representation. As a solution, we propose an all-in-one, reconfigurable BESS emulation tool for grid applications that only requires one three-phase voltage-source converter. This emulator provides chemistry-specific battery behavior like previous work, but it also includes the BESS's power electronics interface and control as well as automatic frequency and voltage support functions for the attached power system. Thus, it allows simple, plug-and-play BESS emulation for grid applications. This paper details the construction, verification, and use of the BESS emulator in an existing grid testbed and concludes that it provides an inexpensive, easy-to-use alternative to using real BESSs in power system experiments.

  • Shiqi Ji; Marko Laitinen; Xingxuan Huang; Jingjing Sun; William Giewont; Fei Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2019

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    This paper presents the characterization of the temperature-dependent short-circuit performance of a Gen3 10 kV/20 A silicon carbide (SiC) mosfet. The test platform consisting of a phase-leg configuration and a fast speed 10-kV solid state circuit breaker, with temperature control, is introduced in detail. A novel FPGA-based short-circuit protection circuit having a response time of 1.5 μs is proposed and integrated into the gate driver. The short-circuit protection is validated through the platform. The short-circuit characteristics for both the hard switching fault and fault under load (FUL) types at various dc-link voltages (from 500 V to 6 kV) are tested and discussed. The saturation current increases with dc-link voltage and achieves 360 A at 6 kV. Different from low voltage SiC devices, there is no current spike in FUL type of fault. The temperature-dependent short-circuit performance is also presented from 25 to 125 °C. The difference of short-circuit waveforms at various initial junction temperatures can be neglected. A thermal model of the 10-kV SiC mosfet is built for the junction temperature estimation during the short circuit and for analysis of the initial junction temperature impact on the short-circuit performance.

  • Zheyu Zhang; Leon M. Tolbert; Daniel Costinett; Fei Wang; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2019

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    As wide-bandgap (WBG) devices and applications move from niche to mainstream, a new generation of engineers trained in this area is critical to continue the development of the field. This paper introduces a new hands-on course in characterization of WBG devices, which is an emerging and fundamental topic in WBG-based techniques. First, the lecture-simulation-experiment format based course structure and design considerations, such as safety, are presented. Then, the necessary facilities to support this hands-on course are summarized, including classroom preparation, software tools, and laboratory equipment. Afterward, the detailed course implementation flow is presented to illustrate the approach of close interaction among lecture, simulation, and experiment to maximize students' learning outcomes. Finally, grading for students and course evaluation by students are discussed, highlighting the findings and potential improvements. Detailed course materials are provided via potenntial.eecs.utk.edu/WBGLab for educational use.

  • Yu Ren; Xu Yang; Fan Zhang; Fred Wang; Leon M. Tolbert; Yunqing Pei
    IEEE Transactions on Power Electronics
    2019

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    Semiconductor devices based solid-state circuit breakers (SSCBs) are promising in the dc power distribution system as protective equipment for their ultrashort action time. This letter proposes a topology of SSCB using series connected silicon carbide (SiC) metal oxide semiconductor field effect transistors (mosfets), which only requires a single isolated gate driver. The SSCB has very low cost and high reliability because it only has 13 components including passive components and diodes apart from two SiC mosfets to achieve both balanced voltage distribution during short-circuit interruption duration and reliable positive gate voltage during on-state. The SSCB prototype is built and experimentally verified to interrupt 75 A short-circuit current under the dc-bus voltage of 1200 V within 1.5 μs.

  • Fei Yang; Zhiqiang Wang; Zhenxian Liang; Fei Wang
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2019

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    Silicon carbide (SiC) power modules are promising for high-power applications because of the high breakdown voltage, high operation temperature, low ON-resistance, and fast switching speed. However, the large parasitic inductance in existing package designs results in compromised performance, i.e., long blanking time in the desaturation protection scheme and large overvoltage spikes during the switching transient. Consequently, the benefits of SiC devices are often not fully utilized in practical applications. This paper deals with these two issues and aims at improving the electrical performance of the existing SiC module package. Specifically, a package design with Kelvin drain-to-source connection is first proposed to minimize the blanking time. More than 99% reduction of blanking time is achieved experimentally compared to the conventional package design. Second, a low parasitic inductance package with double-side cooling is proposed to allow the fast switching speed of SiC devices without sacrificing the thermal performance. A power loop inductance of 1.63 nH is realized from Q3D simulation. Verified by the experiment, more than 60% reduction of power loop inductance is achieved in comparison to a previously designed baseline module. At 0-Ω external gate resistance, the turn-off voltage spike is less than 9% of the dc-link voltage under the rated load condition.

  • Kevin P. Schneider; Stuart Laval; Jacob Hansen; Ronald B. Melton; Leslie Ponder; Lance Fox; John Hart; Joshua Hambrick; Mark Buckner; Murali Baggu; Kumaraguru Prabakar; Madhav Manjrekar; Somasundaram Essakiappan; Leon M. Tolbert; Yilu Liu; Jiaojiao Dong; Lin Zhu; Aaron Smallwood; Avnaesh Jayantilal; Chris Irwin; Guohui Yuan
    IEEE Access
    2019

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    Electric distribution systems around the world are seeing an increasing number of utility-owned and non-utility-owned (customer-owned) intelligent devices and systems being deployed. New deployments of utility-owned assets include self-healing systems, microgrids, and distribution automation. Non-utility-owned assets include solar photovoltaic generation, behind-the-meter energy storage systems, and electric vehicles. While these deployments provide potential data and control points, the existing centralized control architectures do not have the flexibility or the scalability to integrate the increasing number or variety of devices. The communication bandwidth, latency, and the scalability of a centralized control architecture limit the ability of these new devices and systems from being engaged as active resources. This paper presents a standards-based architecture for the distributed power system controls, which increases operational flexibility by coordinating centralized and distributed control systems. The system actively engages utility and non-utility assets using a distributed architecture to increase reliability during normal operations and resiliency during extreme events. Results from laboratory testing and preliminary field implementations, as well as the details of an ongoing full-scale implementation at Duke Energy, are presented.

  • Yongsheng Fu; Yu Li; Yang Huang; Hua Bai; Ke Zou; Xi Lu; Chingchi Chen
    IEEE Transactions on Transportation Electrification
    2019

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    Compared with grid to vehicle (G2V) and vehicle to grid (V2G) modes, electric vehicle (EV) battery chargers are more vulnerable to power quality problems at the autonomous mode, i.e., creating its own electric grid during the grid blackout while facing unbalanced and nonlinear loads. Active damping is another common challenge given most charger equip LC filters at grid-end voltage-source inverters (VSIs). In this article, an output-voltage control with harmonic compensation plus virtual impedance term is proposed to achieve high power quality and necessary damping, respectively. The physical meaning of the virtual impedance is further explained, and the voltage loop with PI plus multiresonant terms is analyzed in depth. More importantly, the analytical solutions and procedures to design such controllers considering the time delays are proposed. Finally, simulation results and experimental data on an EV battery charger prototype using SiC devices, which is made of one VSI and one isolated dc/dc converter, validate the effectiveness of the proposed design methodology.

  • Yongsheng Fu; Yu Li; Yang Huang; Xi Lu; Ke Zou; Chingchi Chen; Hua Bai
    IEEE Transactions on Transportation Electrification
    2019

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    As a new application area of electric vehicles, vehicle to home (V2H) recently becomes one of the most attractive research areas. In this paper, a three-phase four-wire inverter using four half-bridge legs is adopted to realize the V2H functionality. Such three-phase inverter acts as the grid-side ac/dc part of the battery charger. In order to deal with the imbalanced load, an independently controlled neutral module has been adopted to form the neutral line and provide the path for the zero-sequence current, together with split dc-bus capacitors. Each phase employs the independent sinusoidal pulse width modulation control with the virtual resistor paralleled with the load, namely, notch-filter-incorporated capacitor voltage feedback control, which damps harmonics around the resonant frequency at various load conditions. In addition, this paper quantifies the load imbalance versus the dc-link voltage oscillation and provides the guidance for the selection of the dc-link capacitance. Finally, a 10-kW three-phase four-wire inverter working at the V2H mode is built and tested, with the three-phase imbalanced load imposed to validate the proposed design and control strategy.

  • Lei Zhu; Dong Jiang; Ronghai Qu; Leon M. Tolbert; Qiao Li
    IEEE Transactions on Transportation Electrification
    2019

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    The integrated starter-generator system (ISGS) is a combination of starter and generator for independent power systems in transportation. It replaces both a conventional starter and generator with a single set of highly integrated devices. A power hardware in the loop simulation that is flexible and can include the hardware under test is used to test the ISGS under representative field conditions. This paper utilizes the special structure of the ISGS and proposes an ISGS emulator (ISGSE) to develop and test the converter. The proposed ISGSE can be used to test a variety of motor drives or rectifiers including dynamic capabilities without necessitating a connection to a large motor load. To emulate the ISGS, the structure and operation principle in different modes are introduced in detail. Also, the issue of stability and accuracy is discussed in this paper. Detailed simulation and experimental comparisons are carried out between the ISGS and the ISGSE, which validates the proposed ISGSE as an effective tool for designing and testing new motor drives.

  • Wei Qian; Juncheng Lu; Hua Bai; Scott Averitt
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2019

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    For any voltage above 600-V dc, it is usually recommended to use 900-1200-V SiC MOSFETs instead of GaN high-electron-mobility transistors (HEMTs), given presently commercial GaN HEMTs have the maximum voltage rating up to 650 V. This paper is an attempt of employing 650-V E-mode GaN HEMTs to build a three-level bidirectional dc/dc converter, with the input as an 800-V battery, the output as the ~400-V dc grid, and half of the switches working at the hard-switching mode. The active-balancing control instead of using clamping diodes in the conventional neutral-point clamping topology is adopted, aiming at higher efficiency. Simulation based on the double-pulse test results shows that such a design strategy with GaN has a better efficiency over the conventional 1200-V SiC MOSFETbased buck/boost converter. Two bottom-cooled GaN HEMTs are in parallel to enhance the power capability and efficiency, which require the special focus on the parasitic parameters. The effects of parasitics, especially the stray inductance in the current commutating loop and the gate drive loop during switching transitions, have been comprehensively analyzed in this paper. Experimental results under 800-V bus voltage are presented to verify the proposed solution.

  • Fei Yang; Zhiqiang Wang; Zheyu Zhang; Steven L Campbell; Fei Wang
    IEEE Transactions on Power Electronics
    2019

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    Middle-point inductance Lmiddle can be introduced in multiple-chip power module package designs. In this paper, the effect of middle-point inductance on switching transients is analyzed first using a frequency-domain analysis. Then a dedicated multiple-chip power module is fabricated with the capability of varying Lmiddle, and extensive switching tests are conducted to evaluate the middle-point inductance's impact. Experiment result shows that the active MOSFET's turn-on loss decreases at higher values of Lmiddle, while its turn-off loss increases. Detailed analysis of this loss variation is presented. In addition to the switching loss variation, it is also observed that different peak voltage stresses are imposed on the active switch and antiparallel diode during the switching transients. Specifically, in the case of lower MOSFET's turn-off, the maximum voltage of the lower MOSFET increases as Lmiddle goes up; however, the peak voltage of the antiparallel diode decreases significantly. The induced voltage spikes during upper MOSFET turn-on process is also evaluated, and an opposite trend is observed experimentally. Analysis of the voltage overshoot variation is discussed. Based on the experimental evaluation and analysis, a multiple-chip power module package design guideline is summarized considering the middle-point inductance's effect.

  • Zheyu Zhang; Ben Guo; Fei Wang
    IEEE Transactions on Power Electronics
    2019

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    Parasitic ringing is commonly observed during the high-speed switching of wide band-gap (WBG) devices. Additional loss contributed by parasitic ringing becomes a concern especially for high switching frequency applications. This paper investigates the effects of parasitic ringing on the switching loss of WBG devices in a phase-leg configuration. An analytical switching loss model considering parasitics in power devices and application circuit is derived. Two switching commutation modes, gate drive dominated mode and power loop dominated mode, are investigated, respectively, and the switching loss induced by damping ringing is identified. It is found that this portion of the loss is at most the energy stored in parasitics, which always exists regardless of the switching speed and parasitic ringing. Therefore, with the given WBG device in the specific application circuit, damping more severe parasitic ringing during faster switching transient would not introduce higher switching loss. Additionally, the extra switching loss induced by resonance among parasitics and crosstalk is investigated. It is observed that severe resonance and its resultant over-voltage during the turn-on transient worsen the crosstalk, causing large shoot-through current and excessive switching loss. The theoretical analysis has been verified by the double pulse test with a 1200-V/50-A SiC-based phase-leg power module.

  • Lei Zhu; Dong Jiang; Ronghai Qu; Leon M. Tolbert; Qiao Li
    IEEE Transactions on Transportation Electrification
    2019

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    The integrated starter-generator system (ISGS) is a combination of starter and generator for independent power systems in transportation. It replaces both a conventional starter and generator with a single set of highly integrated devices. A power hardware in the loop simulation that is flexible and can include the hardware under test is used to test the ISGS under representative field conditions. This paper utilizes the special structure of the ISGS and proposes an ISGS emulator (ISGSE) to develop and test the converter. The proposed ISGSE can be used to test a variety of motor drives or rectifiers including dynamic capabilities without necessitating a connection to a large motor load. To emulate the ISGS, the structure and operation principle in different modes are introduced in detail. Also, the issue of stability and accuracy is discussed in this paper. Detailed simulation and experimental comparisons are carried out between the ISGS and the ISGSE, which validates the proposed ISGSE as an effective tool for designing and testing new motor drives.

  • Wei Qian; Juncheng Lu; Hua Bai; Scott Averitt
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2019

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    For any voltage above 600-V dc, it is usually recommended to use 900-1200-V SiC MOSFETs instead of GaN high-electron-mobility transistors (HEMTs), given presently commercial GaN HEMTs have the maximum voltage rating up to 650 V. This paper is an attempt of employing 650-V E-mode GaN HEMTs to build a three-level bidirectional dc/dc converter, with the input as an 800-V battery, the output as the ~400-V dc grid, and half of the switches working at the hard-switching mode. The active-balancing control instead of using clamping diodes in the conventional neutral-point clamping topology is adopted, aiming at higher efficiency. Simulation based on the double-pulse test results shows that such a design strategy with GaN has a better efficiency over the conventional 1200-V SiC MOSFETbased buck/boost converter. Two bottom-cooled GaN HEMTs are in parallel to enhance the power capability and efficiency, which require the special focus on the parasitic parameters. The effects of parasitics, especially the stray inductance in the current commutating loop and the gate drive loop during switching transitions, have been comprehensively analyzed in this paper. Experimental results under 800-V bus voltage are presented to verify the proposed solution.

  • Fei Yang; Zhiqiang Wang; Zheyu Zhang; Steven L Campbell; Fei Wang
    IEEE Transactions on Power Electronics
    2019

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    Middle-point inductance Lmiddle can be introduced in multiple-chip power module package designs. In this paper, the effect of middle-point inductance on switching transients is analyzed first using a frequency-domain analysis. Then a dedicated multiple-chip power module is fabricated with the capability of varying Lmiddle, and extensive switching tests are conducted to evaluate the middle-point inductance's impact. Experiment result shows that the active MOSFET's turn-on loss decreases at higher values of Lmiddle, while its turn-off loss increases. Detailed analysis of this loss variation is presented. In addition to the switching loss variation, it is also observed that different peak voltage stresses are imposed on the active switch and antiparallel diode during the switching transients. Specifically, in the case of lower MOSFET's turn-off, the maximum voltage of the lower MOSFET increases as Lmiddle goes up; however, the peak voltage of the antiparallel diode decreases significantly. The induced voltage spikes during upper MOSFET turn-on process is also evaluated, and an opposite trend is observed experimentally. Analysis of the voltage overshoot variation is discussed. Based on the experimental evaluation and analysis, a multiple-chip power module package design guideline is summarized considering the middle-point inductance's effect.

  • Zheyu Zhang; Ben Guo; Fei Wang
    IEEE Transactions on Power Electronics
    2019

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    Parasitic ringing is commonly observed during the high-speed switching of wide band-gap (WBG) devices. Additional loss contributed by parasitic ringing becomes a concern especially for high switching frequency applications. This paper investigates the effects of parasitic ringing on the switching loss of WBG devices in a phase-leg configuration. An analytical switching loss model considering parasitics in power devices and application circuit is derived. Two switching commutation modes, gate drive dominated mode and power loop dominated mode, are investigated, respectively, and the switching loss induced by damping ringing is identified. It is found that this portion of the loss is at most the energy stored in parasitics, which always exists regardless of the switching speed and parasitic ringing. Therefore, with the given WBG device in the specific application circuit, damping more severe parasitic ringing during faster switching transient would not introduce higher switching loss. Additionally, the extra switching loss induced by resonance among parasitics and crosstalk is investigated. It is observed that severe resonance and its resultant over-voltage during the turn-on transient worsen the crosstalk, causing large shoot-through current and excessive switching loss. The theoretical analysis has been verified by the double pulse test with a 1200-V/50-A SiC-based phase-leg power module.

  • Jiaojiao Dong; Lin Zhu; Yu Su; Yiwei Ma; Yilu Liu; Fred Wang; Leon M. Tolbert; Jim Glass; Lilian Bruce
    IET Generation, Transmission & Distribution
    2018

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    Owing to the recent power outages caused by extreme events, installing battery energy storage and backup generators is important to improve resiliency for a grid-tied microgrid. In the design stage, the event occurrence time and duration, which are highly uncertain and cannot be effectively predicted, may affect the needed battery and backup generator capacity but are usually assumed to be pre-determined in utility planning tools. This study investigates the optimal battery and backup generator sizing problem considering the stochastic event occurrence time and duration for the grid-tied microgrid under islanded operation. The reliability requirement is quantified by the mean value of the critical customer interruption time in each stochastic islanding time window (ITW), whose length is the duration and the centre is the occurrence time. The stochastic ITW constraint is then transformed to a probability-weighted expression to derive an equivalent Mixed Integer Linear Programming model. Numerical simulations on a realistic grid-tied PV-based microgrid demonstrate that the total cost is reduced by 11.5% considering the stochastic ITW, compared with the deterministic ITW under the same reliability requirement.

  • Bo Liu; Ren Ren; Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang
    IEEE Transactions on Power Electronics
    2018

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    Wide bandgap semiconductors are gradually being adopted in high power-density high efficiency applications, providing faster switching and lower loss, and at the same time imposing new challenges in control and hardware design. In this paper, a gallium nitride-based Vienna-type rectifier with SiC diodes is proposed to serve as the power factor correction stage in a high-density battery charger system targeting for aircraft applications with 800 Hz ac system and 600 V level dc link, where power quality is required according to DO160E standard. To meet the current harmonic requirement, PWM voltage distortion during the turn-off transient, is studied as the main harmonics contributor. The distortion mechanism caused by different junction capacitances of the switching devices is presented. A mitigation scheme considering the nonlinear voltage-dependent characteristics of these capacitances is proposed and then simplified from a pulse-based turn-off compensation method to a general modulation scheme. Simulation and experimental results with a 450 kHz Vienna-type rectifier demonstrate the performance of the proposed approach, showing a THD reduction from 10% to 3% with a relatively low-speed controller.

  • Shuoting Zhang; Bo Liu; Sheng Zheng; Yiwei Ma; Fei Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2018

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    A transmission line emulator has been developed to flexibly represent interconnected ac lines under normal operating conditions in a voltage-source-converter-based power system emulation platform. As the most serious short-circuit fault condition, the three-phase short-circuit fault emulation is essential for power system studies. This paper proposes a model to realize a three-phase short-circuit fault emulation at different locations along a single transmission line or one of several parallel-connected transmission lines. At the same time, a combination method is proposed to eliminate the undesired transients caused by the current reference step changes while switching between the fault state and the normal state. Experiment results verify the developed transmission line three-phase short-circuit fault emulation capability.

  • Yiwei Ma; Jingxin Wang; Fred Wang; Leon M. Tolbert
    Chinese Journal of Electrical Engineering
    2018

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    A Hardware Testbed(HTB) is developed for accurate and flexible emulation and testing of electrical power system and their control, measurement, and protection systems. In the HTB, modular and programmable power electronics converters are used to mimic the static and dynamic characteristics of electrical power components. This paper overviews the development, integration, and application of the HTB, covering emulation principle, hardware and software configuration, and example results of power system research using the HTB. The advantages of the HTB, compared with real-time digital simulation and downscaled hardware-based testing platform are discussed.

  • Bo Liu; Ren Ren; Zheyu Zhang; Ben Guo; Fei Wang; Daniel Costinett
    CPSS Transactions on Power Electronics and Applications
    2018

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    A systematic study on a gallium nitride (GaN) high-electron-mobility transistor (HEMT) based battery charger, consisting of a Vienna-type rectifier plus a dc-dc converter, reveals a common phenomenon. That is, the high switching frequency, and high di/dt and dv/dt noise inside GaN converters may induce a dc drift or low frequency distortion on sensing signals. The distortion mechanisms for different types of sensing errors are identified and practical minimization techniques are developed. Experimental results on the charger system have validated these mechanisms and corresponding approaches, showing an overall reduction of input current total harmonic distortion (THD) by up to 5 percentage points and improved dc-dc output voltage regulation accuracy. The knowledge helps engineers tackle the troublesome issues related to noise.

  • Shiqi Ji; Fei Wang; Leon M. Tolbert; Ting Lu; Zhengming Zhao; Hualong Yu
    IEEE Transactions on Industry Applications
    2018

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    The series connection of insulated gate bipolar transistors (IGBTs) allows operation at voltage levels higher than the rated voltage of one IGBT and has less power semiconductor costs compared to multilevel topologies. However, voltage unbalance during the switching transient is a challenge for series-connected device application. This paper presents an field-programmable gate array (FPGA)-based voltage balancing strategy for multiseries-connected high-voltage (HV)-IGBTs including an FPGA-based active voltage balancing control (AVBC) circuit integrated into the gate driver and the control for multiseries-connected IGBTs. The effectiveness of the control has been experimentally validated in a prototype using four 4.5 kV HV-IGBTs in series connection.

  • Shiqi Ji; Sheng Zheng; Fei Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2018

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    The temperature-dependent characteristics of the third-generation 10-kV/20-A SiC MOSFET including the static characteristics and switching performance are carried out in this paper. The steady-state characteristics, including saturation current, output characteristics, antiparallel diode, and parasitic capacitance, are tested. A double pulse test platform is constructed including a circuit breaker and gate drive with >10-kV insulation and also a hotplate under the device under test for temperature-dependent characterization during switching transients. The switching performance is tested under various load currents and gate resistances at a 7-kV dc-link voltage from 25 to 125 C and compared with previous 10-kV MOSFETs. A simple behavioral model with its parameter extraction method is proposed to predict the temperature-dependent characteristics of the 10-kV SiC MOSFET. The switching speed limitations, including the reverse recovery of SiC MOSFET's body diode, overvoltage caused by stray inductance, crosstalk, heat sink, and electromagnetic interference to the control are discussed based on simulations and experimental results.

  • Chun Gan; Qingguo Sun; Nan Jin; Leon M. Tolbert; Zhibin Ling; Yihua Hu; Jianhua Wu
    IEEE Transactions on Industrial Electronics
    2018

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    This paper proposes a simple and cost-effective current measurement technique for four-phase switched reluctance motor (SRM) control, by splitting the dual bus line of the converter, without pulse injection and voltage penalty. Only two Hall-effect sensors are utilized, where one is installed in the upper bus to measure two phase currents, and the other is placed in the lower bus to measure other two phase currents. In order to realize independent current measurement in the whole turn-on region, switching functions are redesigned so that upper switches of two phases act as the choppers, while lower switches of the other two phases are employed as the choppers. Compared to traditional drives, the developed system requires only two Hall-effect sensors in the dual bus line, without a need for individual phase sensors or additional devices, which reduces the cost and volume for SRM drives. Furthermore, compared to the single-sensor based current measurement scheme, the proposed method has no need to implement pulse injection and will not cause any voltage penalty and current distortion, which also improve the current measurement accuracy and system performance. Simulation and experiments carried out on a 150-W four-phase 8/6 SRM confirm the effectiveness of the proposed technique.

  • Allan Taylor; Juncheng Lu; Liyan Zhu; Kevin Bai; Matt McAmmond; Alan Brown
    IET Power Electronics
    2018

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    As two exemplary candidates of wide-bandgap devices, SiC MOSFETs and GaN HEMTs are regarded as successors of Si devices in medium-to-high-voltage (>1200 V) and low-voltage (<;650 V) domains, respectively, thanks to their excellent switching performance and thermal capability. With the introduction of 650 V SiC MOSFETs and GaN HEMTs, the two technologies are in direct competition in <;650 V domains, such as Level 2 battery chargers for electric vehicles (EVs). This study applies 650 V SiC and GaN to two 240 VAC/7.2 kW EV battery chargers, respectively, aiming to provide a head-to-head comparison of these two devices in terms of overall efficiency, power density, thermal performance, and cost. The charger essentially is an indirect matrix converter with a dual-active-bridge stage handling the power factor correction and power delivery simultaneously. These two chargers utilise the same control strategy, varying the phase-shift and switching frequency to cover the wide input range (80-260 VAC) and wide output range (200 V-450 VDC). Experimental results indicated that at the same efficiency level, the GaN charger is smaller, more efficient and cheaper, while the SiC charger has a better thermal performance.

  • Chun Gan; Nan Jin; Qingguo Sun; Wubin Kong; Yihua Hu; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2018

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    The hybrid electric bus (HEB) presents an emerging solution to exhaust gas emissions in urban transport. This paper proposes a multiport bidirectional switched reluctance motor (SRM) drive for solar-assisted HEB (SHEB) powertrain, which not only improves the motoring performance, but also achieves flexible charging functions. To extend the driving miles and achieve self-charging ability, photovoltaic (PV) panels are installed on the bus to decrease the reliance on fuelsbatteries and charging stations. A bidirectional front-end circuit with a PV-fed circuit is designed to integrate electrical components into one converter. Six driving and five charging modes are achieved. The dc voltage is boosted by the battery in generator control unit (GCU) driving mode and by the charge capacitor in battery driving mode, where the torque capability is improved. Usually, an extra converter is needed to achieve battery charging. In this paper, the battery can be directly charged by the demagnetization current in GCU or PV driving mode, and can be quickly charged by the PV panels and GCUAC grids at SHEB standstill conditions, by utilizing the traction motor windings and integrated converter circuit, without external charging converters. Experiments on a three-phase 128 SRM confirm the effectiveness of the proposed drive and control scheme.

  • A high-frequency injection (HFI) sensorless control for interior permanent magnet synchronous motors with enhanced precision and widened speed range is proposed in this paper. The injection frequency reaches up to 2 kHz under a 50~100 kHz silicon carbide (SiC)-based three-phase inverter. In addition to the high switching frequency, the field-programmable gate array (FPGA) is utilized to achieve high control bandwidth (>200 kHz) when implementing the field-oriented control algorithm. The benefits of high switching frequency and high control bandwidth in senseless controls are explained theoretically, i.e., leaving enough room for the injection frequency by using SiC while tuning down the noise-to-signal ratio by using the FPGA. Experimental results verified that such manners improved the position estimation and lifted the injection frequency effectively, which further allows us to widen the motor speed range under the HFI sensorless control from 0 to 500 r/min with the conventional Si+DSP design to 0~1200 r/min with the proposed SiC+FPGA.

  • Dong Jiang; Puqi Ning; Rixin Lai; Zhihao Fang; Fred Wang
    Chinese Journal of Electrical Engineering
    2018

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    This paper introduces the concept of modular design methodology for hardware design and development of motor drives. The modular design process is first introduced separating the hardware development into three parts: controller, mother board and phase-leg module. The control and circuit function can be decoupled from the phase-leg module development. The hardware update can be simplified with the phase-leg module development and verification. Two design examples are used to demonstrate this method: a DC-fed motor drive with Si IGBTs and an AC-fed motor drive with SiC devices. Design of DC-fed motor drive aims at developing the converter with customized IGBT package for high temperature. Experience with development of the converter with commercial IGBTs simplifies the process. As the AC-fed motor drive is a more complex topology using more advanced devices, the modular design method can simplify and improve the development especially for new packaged devices. Also, the modular design method can help to study the electromagnetic interference (EMI) issue for motor drives, which is presented with an extra design example.

  • Dong Jiang; Zewei Shen; Fei Wang
    IEEE Transactions on Power Electronics
    2018

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    This paper introduces series work of common-mode (CM) voltage reduction for the paralleled inverters. The paralleled inverters' phase-legs are connected through coupling inductors and the combined three-phase currents are provided to the load. Interleaving is an approach to reduce the CM voltage for the paralleled inverters but it cannot eliminate CM voltage. A novel pulse-width-modulation (PWM) method for paralleled inverters which can theoretically achieve zero CM voltage is developed. Considering the basic voltage vectors in each inverter, novel paralleled voltage vectors which have zero CM voltage are proposed to combine the reference voltage vector. The action time's distribution and voltage vectors' sending sequence for each inverter are also introduced. The proposed PWM method can make sure the voltage of the two inverters are balanced in each switching cycle and limits the circulating current through small coupling inductors. Similar to interleaving space vector PWM, the proposed zero CM PWM also has the ability to reduce the output current ripple and electromagnetic interference (EMI). Simulation and experimental results are provided to show the advantage of paralleled inverters in CM voltage reduction and validate the proposed method has good performance to reduce CM current and CM EMI noise.

  • Fei Fred Wang; Bo Liu
    CPSS Transactions on Power Electronics and Applications
    2018

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    The emergence of wide bandgap (WBG) semiconductor devices such as silicon carbide (SiC) and gallium nitride (GaN) devices promises to revolutionize next-generation power electronics converters. Featuring high breakdown electric field, low specific on-resistance, fast switching speed, and high junction temperature capability, these devices are beneficial for the efficiency, power density, reliability, and/or cost of power electronics converters. WBG devices have been employed in some commercial and industrial products with more applications expected in near future. However, extremely fast switching and other superior characteristics of WBG device, and high switching frequency/high voltage/high junction temperature operation, present new design challenges in gate drive and protection, packaging and layout, EMI suppression, and converter control, etc. Addressing these design and application issues is critical to the adoption, commercialization, and success of WBG based power electronics. This special issue intends to report the latest progress in these important areas.

  • A high-frequency injection (HFI) sensorless control for interior permanent magnet synchronous motors with enhanced precision and widened speed range is proposed in this paper. The injection frequency reaches up to 2 kHz under a 50~100 kHz silicon carbide (SiC)-based three-phase inverter. In addition to the high switching frequency, the field-programmable gate array (FPGA) is utilized to achieve high control bandwidth (>200 kHz) when implementing the field-oriented control algorithm. The benefits of high switching frequency and high control bandwidth in senseless controls are explained theoretically, i.e., leaving enough room for the injection frequency by using SiC while tuning down the noise-to-signal ratio by using the FPGA. Experimental results verified that such manners improved the position estimation and lifted the injection frequency effectively, which further allows us to widen the motor speed range under the HFI sensorless control from 0 to 500 r/min with the conventional Si+DSP design to 0~1200 r/min with the proposed SiC+FPGA.

  • Dong Jiang; Puqi Ning; Rixin Lai; Zhihao Fang; Fred Wang
    Chinese Journal of Electrical Engineering
    2018

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    This paper introduces the concept of modular design methodology for hardware design and development of motor drives. The modular design process is first introduced separating the hardware development into three parts: controller, mother board and phase-leg module. The control and circuit function can be decoupled from the phase-leg module development. The hardware update can be simplified with the phase-leg module development and verification. Two design examples are used to demonstrate this method: a DC-fed motor drive with Si IGBTs and an AC-fed motor drive with SiC devices. Design of DC-fed motor drive aims at developing the converter with customized IGBT package for high temperature. Experience with development of the converter with commercial IGBTs simplifies the process. As the AC-fed motor drive is a more complex topology using more advanced devices, the modular design method can simplify and improve the development especially for new packaged devices. Also, the modular design method can help to study the electromagnetic interference (EMI) issue for motor drives, which is presented with an extra design example.

  • Dong Jiang; Zewei Shen; Fei Wang
    IEEE Transactions on Power Electronics
    2018

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    This paper introduces series work of common-mode (CM) voltage reduction for the paralleled inverters. The paralleled inverters' phase-legs are connected through coupling inductors and the combined three-phase currents are provided to the load. Interleaving is an approach to reduce the CM voltage for the paralleled inverters but it cannot eliminate CM voltage. A novel pulse-width-modulation (PWM) method for paralleled inverters which can theoretically achieve zero CM voltage is developed. Considering the basic voltage vectors in each inverter, novel paralleled voltage vectors which have zero CM voltage are proposed to combine the reference voltage vector. The action time's distribution and voltage vectors' sending sequence for each inverter are also introduced. The proposed PWM method can make sure the voltage of the two inverters are balanced in each switching cycle and limits the circulating current through small coupling inductors. Similar to interleaving space vector PWM, the proposed zero CM PWM also has the ability to reduce the output current ripple and electromagnetic interference (EMI). Simulation and experimental results are provided to show the advantage of paralleled inverters in CM voltage reduction and validate the proposed method has good performance to reduce CM current and CM EMI noise.

  • Fei Fred Wang; Bo Liu
    CPSS Transactions on Power Electronics and Applications
    2018

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    The emergence of wide bandgap (WBG) semiconductor devices such as silicon carbide (SiC) and gallium nitride (GaN) devices promises to revolutionize next-generation power electronics converters. Featuring high breakdown electric field, low specific on-resistance, fast switching speed, and high junction temperature capability, these devices are beneficial for the efficiency, power density, reliability, and/or cost of power electronics converters. WBG devices have been employed in some commercial and industrial products with more applications expected in near future. However, extremely fast switching and other superior characteristics of WBG device, and high switching frequency/high voltage/high junction temperature operation, present new design challenges in gate drive and protection, packaging and layout, EMI suppression, and converter control, etc. Addressing these design and application issues is critical to the adoption, commercialization, and success of WBG based power electronics. This special issue intends to report the latest progress in these important areas.

  • Sisi Xiong; Yanjun Yao; Shuangjiang Li; Qing Cao; Tian He; Hairong Qi; Leon Tolbert; Yilu Liu
    IEEE Transactions on Cloud Computing
    2017

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    As one of the most popular cloud services, data storage has attracted great attention in recent research efforts. Key-value (k-v) stores have emerged as a popular option for storing and querying billions of key-value pairs. So far, existing methods have been deterministic. Providing such accuracy, however, comes at the cost of memory and CPU time. In contrast, we present an approximate k-v storage for cloud-based systems that is more compact than existing methods. The tradeoff is that it may, theoretically, return errors. Its design is based on the probabilistic data structure called “bloom filter”, where we extend the classical bloom filter to support key-value operations. We call the resulting design as the kBF (key-value bloom filter). We further develop a distributed version of the kBF (d-kBF) for the unique requirements of cloud computing platforms, where multiple servers cooperate to handle a large volume of queries in a load-balancing manner. Finally, we apply the kBF to a practical problem of implementing a state machine to demonstrate how the kBF can be used as a building block for more complicated software infrastructures.

  • Zheyu Zhang; Jeffery Dix; Fei Fred Wang; Benjamin J. Blalock; Daniel Costinett; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2017

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    This paper presents an intelligent gate drive for silicon carbide (SiC) devices to fully utilize their potential of high switching-speed capability in a phase-leg configuration. Based on the SiC device's intrinsic properties, a gate assist circuit consisting of two auxiliary transistors with two diodes is introduced to actively control gate voltages and gate loop impedances of both devices in a phase-leg configuration during different switching transients. Compared to conventional gate drives, the proposed circuit has the capability of accelerating the switching speed of the phase-leg power devices and suppressing the crosstalk to below device limits. Based on Wolfspeed 1200-V SiC MOSFETs, the test results demonstrate the effectiveness of this intelligent gate drive under varying operating conditions. More importantly, the proposed intelligent gate assist circuitry is embedded into a gate drive integrated circuit, offering a simple, compact, and reliable solution for end-users to maximize benefits of SiC devices in actual power electronics applications.

  • Yutian Cui; Fei Yang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    With the increased cloud computing and digital information storage, the energy requirement of data centers keeps increasing. A high-voltage point of load (HV POL) with an input series output parallel structure is proposed to convert 400 to 1 VDC within a single stage to increase the power conversion efficiency. The symmetrical controlled half-bridge current doubler is selected as the converter topology in the HV POL. A load-dependent soft-switching method has been proposed with an auxiliary circuit that includes inductor, diode, and MOSFETs so that the hard-switching issue of typical symmetrical controlled half-bridge converters is resolved. The operation principles of the proposed soft-switching half-bridge current doubler have been analyzed in detail. Then, the necessity of adjusting the timing with the loading in the proposed method is analyzed based on losses, and a controller is designed to realize the load-dependent operation. A lossless RCD current sensing method is used to sense the output inductor current value in the proposed load-dependent operation. Experimental efficiency of a hardware prototype is provided to show that the proposed method can increase the converter's efficiency in both heavy- and light-load conditions.

  • Zheyu Zhang; Haifeng Lu; Daniel J. Costinett; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    Dead time significantly affects the reliability, power quality, and efficiency of voltage-source converters. For silicon carbide (SiC) devices, considering the high sensitivity of turn-off time to the operating conditions (> 5× difference between light load and full load) and characteristics of inductive loads (> 2× difference between motor load and inductor), as well as large additional energy loss induced by the freewheeling diode conduction during the superfluous dead time (~15% of the switching loss), then the traditional fixed dead time setting becomes inappropriate. This paper introduces an approach to adaptively regulate the dead time considering the current operating condition and load characteristics via synthesizing online monitored turn-off switching parameters in the microcontroller with an embedded preset optimization model. Based on a buck converter built with 1200-V SiC MOSFETs, the experimental results show that the proposed method is able to ensure reliability and reduce power loss by 12% at full load and 18.2% at light load (8% of the full load in this case study).

  • Liu Yang; Jing Wang; Yiwei Ma; Jingxin Wang; Xiaohu Zhang; Leon M. Tolbert; Fei Fred Wang; Kevin Tomsovic
    IEEE Transactions on Power Electronics
    2017

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    This paper develops a synchronous generator emulator by using a three-phase voltage source converter for transmission level power system testing. Different interface algorithms are compared, and the voltage type ideal transformer model is selected considering accuracy and stability. At the same time, closed-loop voltage control with current feed-forward is proposed to decrease the emulation error. The emulation is then verified through two different ways. First, the output waveforms of the emulator in experiments are compared with the simulation under the same condition. Second, a transfer function perturbation-based error model is obtained and redefined as the relative error for the amplitude and phase between the emulated and the target system over the frequency range of interest. The major cause of the error is investigated through a quantitative analysis of the error with varying parameters.

  • Xiaojie Shi; Zhiqiang Wang; Bo Liu; Yalong Li; Leon M. Tolbert; Fred Wang
    IEEE Transactions on Power Electronics
    2017

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    This paper presents a steady-state model of MMC for the second-order phase voltage ripple prediction under unbalanced conditions, taking the impact of negative-sequence current control into account. From the steady-state model, a circular relationship is found among current and voltage quantities, which can be used to evaluate the magnitudes and initial phase angles of different circulating current components. Moreover, in order to calculate the circulating current in a point-to-point MMC-based HVdc system under unbalanced grid conditions, the derivation of equivalent dc impedance of an MMC is discussed as well. According to the dc impedance model, an MMC inverter can be represented as a series connected R-L-C branch, with its equivalent resistance and capacitance directly related to the circulating current control parameters. Experimental results from a scaled-down three-phase MMC system under an emulated single-line-to-ground fault are provided to support the theoretical analysis and derived model. This new models provides an insight into the impact of different control schemes on the fault characteristics and improves the understanding of the operation of MMC under unbalanced conditions.

  • Zheyu Zhang; Ben Guo; Fei Fred Wang; Edward A. Jones; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    The double pulse test (DPT) is a widely accepted method to evaluate the dynamic behavior of power devices. Considering the high switching-speed capability of wide band-gap devices, the test results are very sensitive to the alignment of voltage and current (V-I) measurements. Also, because of the shoot-through current induced by Cdv/dt (i.e., cross-talk), the switching losses of the nonoperating switch device in a phase-leg must be considered in addition to the operating device. This paper summarizes the key issues of the DPT, including components and layout design, measurement considerations, grounding effects, and data processing. Additionally, a practical method is proposed for phase-leg switching loss evaluation by calculating the difference between the input energy supplied by a dc capacitor and the output energy stored in a load inductor. Based on a phase-leg power module built with 1200-V/50-A SiC MOSFETs, the test results show that this method can accurately evaluate the switching loss of both the upper and lower switches by detecting only one switching current and voltage, and it is immune to V-I timing misalignment errors.

  • Yalong Li; Xiaojie Shi; Bo Liu; Wanjun Lei; Fred Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2017

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    This paper presents the development of a scaled four-terminal high-voltage direct current (HVDC) testbed, including hardware structure, communication architecture, and different control schemes. The developed testbed is capable of emulating typical operation scenarios including system start-up, power variation, line contingency, and converter station failure. Some unique scenarios are also developed and demonstrated, such as online control mode transition and station re-commission. In particular, a dc line current control is proposed, through the regulation of a converter station at one terminal. By controlling a dc line current to zero, the transmission line can be opened by using relatively low-cost HVDC disconnects with low current interrupting capability, instead of the more expensive dc circuit breaker. Utilizing the dc line current control, an automatic line current limiting scheme is developed. When a dc line is overloaded, the line current control will be automatically activated to regulate current within the allowable maximum value.

  • Wenchao Cao; Yiwei Ma; Liu Yang; Fei Wang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2017

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    Small-signal stability is an important concern in three-phase inverter-based ac power systems. The impedance-based approach based on the generalized Nyquist stability criterion (GNC) can analyze the stability related with the medium and high-frequency modes of the systems. However,. the GNC involves the right-half-plane (RHP) pole calculation of return-ratio transfer function matrices, which cannot be avoided for stability analysis of complicated ac power systems. Therefore, it necessitates the detailed internal control information of the inverters, which is not normally available for commercial inverters. To address this issue, this paper introduces the component connection method (CCM) in the frequency domain for stability analysis in the synchronous d-q frame, by proposing a method of deriving the impedance matrix of the connection networks of inverter-based ac power systems. Demonstration on a two-area system and a microgrid shows that: The CCM-enabled approach can avoid the RHP pole calculation of return-ratio matrices and enables the stability analysis by using only the impedances of system components, which could be measured without the need for the internal information. A stability analysis method based on d-q impedances, the CCM, and the determinant-based GNC is also proposed to further simplify the analysis process. Inverter controller parameters can be designed as stability regions in parameter spaces, by repetitively applying the proposed stability analysis method. Simulation and experimental results verify the validity of the proposed stability analysis method and the parameter design approach.

  • Bo Liu; Xiaojie Shi; Yalong Li; Fei Fred Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2017

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    Hybrid ac/dc transmission extends the power transfer capacity of existing long ac lines closer to their thermal limit, by superposing the dc current onto three-phase ac lines through a zigzag transformer. However, this transformer could suffer saturation under unbalanced line impedance conditions. This paper introduces the concept of hybrid line impedance conditioner (HLIC) as a cost-effective approach to compensate for the line unbalance and therefore avoid saturation. The topology and operation principle are presented. The two-level control strategy is described, which enables autonomous adaptive regulation without the need of system-level control. Design and implementation are also analyzed, including dc-link capacitance as one of the key line conditioner components, HLIC installation, and protection under fault conditions. The cost study on this HLIC-based hybrid system is also performed to reveal the benefits of the solution. Simulation results and experimental results based on a down-scaled prototype are provided to verify the feasibility of the proposed approach.

  • Yiwei Ma; Wenchao Cao; Liu Yang; Fei Wang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2017

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    One way to incorporate the increasing amount of wind penetration is to control wind turbines to emulate the behavior of conventional synchronous generators. However, the energy balance is the main issue for the wind turbines to be truly dispatchable by the power system operator such as the generators. This paper presents a comprehensive virtual generator control method for the full converter wind turbine, with a minute-level energy storage in the dc link as the energy buffer. The voltage closed-loop virtual synchronous generator control of the wind turbine allows it to work under both grid-connected and stand-alone condition. Power balance of the wind turbine system is achieved by controlling the rotor speed of the turbine according to the loading condition. With the proposed control, the wind turbine system can enhance the dynamic response, and can be dispatched and regulated by the system operator. The sizing design of the short term energy storage is also discussed in this paper. Experimental results are presented to demonstrate the feasibility and effectiveness of the proposed control method.

  • Weimin Zhang; Fred Wang; Daniel J. Costinett; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    Newly emerged gallium nitride (GaN) devices feature ultrafast switching speed and low on-state resistance that potentially provide significant improvements for power converters. This paper investigates the benefits of GaN devices in an LLC resonant converter and quantitatively evaluates GaN devices' capabilities to improve converter efficiency. First, the relationship of device and converter design parameters to the device loss is established based on an analytical model of LLC resonant converter operating at the resonance. Due to the low effective output capacitance of GaN devices, the GaN-based design demonstrates about 50% device loss reduction compared with the Si-based design. Second, a new perspective on the extra transformer winding loss due to the asymmetrical primary-side and secondary-side current is proposed. The device and design parameters are tied to the winding loss based on the winding loss model in the finite element analysis (FEA) simulation. Compared with the Si-based design, the winding loss is reduced by 18% in the GaN-based design. Finally, in order to verify the GaN device benefits experimentally, 400- to 12-V, 300-W, 1-MHz GaN-based and Si-based LLC resonant converter prototypes are built and tested. One percent efficiency improvement, which is 24.8% loss reduction, is achieved in the GaN-based converter.

  • Shiqi Ji; Zheyu Zhang; Fred Wang
    CES Transactions on Electrical Machines and Systems
    2017

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    Research on high voltage (HV) silicon carbide (SiC) power semiconductor devices has attracted much attention in recent years. This paper overviews the development and status of HV SiC devices. Meanwhile, benefits of HV SiC devices are presented. The technologies and challenges for HV SiC device application in converter design are discussed. The state-of-the-art applications of HV SiC devices are also reviewed.

  • 2017

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    Three-phase inverter-based multibus ac power systems could suffer from the harmonic instability issue. The existing impedance-based stability analysis method using the Nyquist stability criterion once requires the calculation of right-half-plane (RHP) poles of impedance ratios, which would result in a heavy computation burden for complicated systems. In order to analyze the harmonic stability of multibus ac systems consisting of both voltage-controlled and current-controlled inverters without the need for RHP pole calculation, this paper proposes two sequence-impedance-based harmonic stability analysis methods. Based on the summary of all major connection types including mesh, the proposed Method 1 can analyze the harmonic stability of multibus ac systems by adding the components one by one from nodes in the lowest level to areas in the highest system level, and accordingly, applying the stability criteria multiple times in succession. The proposed Method 2 is a generalized extension of the impedance-sum-type criterion to be used for the harmonic stability analysis of any multibus ac systems based on Cauchy's theorem. The inverter controller parameters can be designed in the forms of stability regions in the parameter space, by repetitively applying the proposed harmonic stability analysis methods. Experimental results of inverter-based multibus ac systems validate the effectiveness of the proposed harmonic stability analysis methods and parameter design approach.

  • Yalong Li; Edward A. Jones; Fred Wang
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2017

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    Arm inductor in a modular multilevel converter (MMC) is used to limit the circulating current and dc short circuit fault current. The circulating current in MMC is dominated by second-order harmonic, which can be largely reduced with circulating current suppressing control. By analyzing the mechanism of the circulating current suppressing control, it is found that the circulating current at switching frequency becomes the main harmonic when suppression control is implemented. Unlike the second-order harmonic that circulates only within the three phases, switching frequency harmonic also flows through the dc side and may further cause high-frequency dc voltage harmonic. This paper develops the theoretical relationship between the arm inductance and switching frequency circulating current, which can be used to guide the arm inductance selection. The experimental results with a downscaled MMC prototype verify the existence of the switching frequency circulating current and its relationship with arm inductance.

  • Chongwen Zhao; Daniel Costinett
    IEEE Transactions on Industrial Electronics
    2017

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    Multifrequency wireless power transfer (WPT) is advantageous in facilitating compatibility with different WPT standards. However, implementing a multifrequency transmitter often requires compromises in system size, complexity, power transfer capability, or output regulation. In this paper, a single-inverter-based dual-mode WPT system is proposed. The system employs a multifrequency programmed pulse width modulation scheme. This multifrequency modulated inverter can simultaneously generate and regulate 100-kHz and 6.78-MHz outputs, or multiple frequencies within ranges of 87-300 kHz, which facilitates the development of multistandard WPT technology for consumer electronics. The principle of the proposed modulation is illustrated, where two different frequencies are concurrently modulated using a programmed pulse train of square waveforms for power delivery, while eliminating certain harmonics. Design tradeoffs and constraints are examined through analytical circuit models. Finally, experimental results are provided to verify the method on a gallium-nitride-based WPT prototype.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fred Wang; Zhenxian Liang; Daniel Costinett; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2016

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    This paper presents a comprehensive short-circuit ruggedness evaluation and numerical investigation of up-to-date commercial silicon carbide (SiC) MOSFETs. The short-circuit capability of three types of commercial 1200-V SiC MOSFETs is tested under various conditions, with case temperatures from 25 to 200 °C and dc bus voltages from 400 to 750 V. It is found that the commercial SiC MOSFETs can withstand short-circuit current for only several microseconds with a dc bus voltage of 750 V and case temperature of 200 °C. The experimental short-circuit behaviors are compared, and analyzed through numerical thermal dynamic simulation. Specifically, an electrothermal model is built to estimate the device internal temperature distribution, considering the temperature-dependent thermal properties of SiC material. Based on the temperature information, a leakage current model is derived to calculate the main leakage current components (i.e., thermal, diffusion, and avalanche generation currents). Numerical results show that the short-circuit failure mechanisms of SiC MOSFETs can be thermal generation current induced thermal runaway or high-temperature-related gate oxide damage.

  • Chongwen Zhao; Bradford Trento; Ling Jiang; Edward A. Jones; Bo Liu; Zheyu Zhang; Daniel Costinett; Fei Fred Wang; Leon M. Tolbert; John F. Jansen; Reid Kress; Rick Langley
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2016

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    High power density is a desirable feature of power electronics design, which prompts economic incentives for industrial applications. In this paper, a gallium nitride (GaN)-based 2-kVA single-phase inverter design was developed for the Google Little Box Challenge, which achieves a 102-W/in3 power density. First, the static and dynamic temperature-dependent characteristics of multiple SiC and enhancement-mode GaN FETs are investigated and compared. Based on the device testing results, several topologies of the inverter stage and different power decoupling solutions are compared with respect to the device volume, efficiency, and thermal requirements. Moreover, some design approaches for magnetic devices and the implementation of gate drives for GaN devices are discussed in this paper, which enable a compact and robust system. Finally, a dc notch filter and a hard switching full-bridge converter are combined as the proposed design for the prototype. A 2-kVA prototype is demonstrated, which meets the volume, efficiency, and thermal requirements. The performance of the prototype is verified by the experimental results.

  • Jing Wang; Liu Yang; Yiwei Ma; Jingxin Wang; Leon M. Tolbert; Fei Wang; Kevin Tomsovic
    IEEE Transactions on Power Electronics
    2016

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    A hardware testbed platform emulating multiple-area power system scenario dynamics has been established aiming at multiple time-scale real-time emulations. In order to mimic real power flow situations in the utility system, the load emulators have to behave like real ones in both their static and dynamic characteristics. A constant-impedance, constant-current, and constant-power (ZIP) model has been used for static load types, while a three-phase induction motor model has been built to represent dynamic load types. In this paper, ways of modeling ZIP and induction motor loads and the performance of each load emulator are discussed. Comparisons between simulation and experimental results are shown as well for the validation of the emulator behaviors. A real-time composite power load emulator is then demonstrated with desired characteristics and detailed transients for representing a power system PQ bus dynamics.

  • Ren Ren; Bo Liu; Edward A. Jones; Fei Fred Wang; Zheyu Zhang; Daniel Costinett
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2016

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    Gallium nitride (GaN) heterojunction field-effect transistors are an enabling technology for high-density converter design. This paper proposes a three-level dc-dc converter with dual outputs based on enhancement-mode GaN devices, intended for use as a battery charger in aircraft applications. The charger can output either 28 or 270 V, selected with a jumper, to satisfy the two most common dc bus voltage requirements in airplanes. It operates as an LLC converter in the 28 V mode and as a buck converter in the 270 V mode. In both operation modes, the devices can realize zero voltage switching (ZVS). With the chosen modulation method, the converter can realize automatic voltage balancing of the flying capacitor and the frequency doubling function to act as an interleaved converter. For the LLC mode, the resonant frequency is twice the switching frequency of primary-side switches, and for the buck mode, the frequency of the output inductor current is also twice the switching frequency. This helps to reduce the size of magnetics while maintaining a low switching loss. Also, the converter utilizes a matrix transformer, with resonant parameters designed to reduce conduction loss and avoid ZVS failure. The operating principle of the converter is analyzed and then experimentally verified on a 1.5-kW prototype with 1 MHz resonant frequency.

  • Edward A. Jones; Fei Fred Wang; Daniel Costinett
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2016

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    Gallium nitride (GaN) power devices are an emerging technology that have only recently become available commercially. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This paper reviews the characteristics and commercial status of both vertical and lateral GaN power devices, providing the background necessary to understand the significance of these recent developments. In addition, the challenges encountered in GaN-based converter design are considered, such as the consequences of faster switching on gate driver design and board layout. Other issues include the unique reverse conduction behavior, dynamic Rds,on, breakdown mechanisms, thermal design, device availability, and reliability qualification. This review will help prepare the reader to effectively design GaN-based converters, as these devices become increasingly available on a commercial scale.

  • Yalong Li; Edward A. Jones; Fei Wang
    IEEE Transactions on Power Electronics
    2016

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    Voltage-balancing control in a modular multilevel converter (MMC) impacts not only the voltage difference among submodule capacitors, but also the power device switching patterns. As a result, MMC possesses a nondeterministic switching pattern and its switching frequency is no longer an independent parameter. This paper theoretically investigates how voltage-balancing control influences the switching frequency in the MMC. Equations describing the relationship between the submodule capacitor unbalanced voltage and converter switching frequency are derived. Since unbalanced voltage also impacts the submodule capacitor ripple voltage and voltage/current harmonics, the design interaction between switching frequency and submodule capacitance, as well as the selection of unbalanced voltage are further investigated. Both simulation and experimental verifications are provided.

  • Fei Fred Wang; Zheyu Zhang
    CPSS Transactions on Power Electronics and Applications
    2016

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    This paper overviews the silicon carbide (SiC) technology. The focus is on the benefits of SiC based power electronics for converters and systems, as well as their ability in enabling new applications. The challenges and research trends on the design and application of SiC power electronics are also discussed.

  • Michael Evzelman; M. Muneeb Ur Rehman; Kelly Hathaway; Regan Zane; Daniel Costinett; Dragan Maksimovic
    IEEE Transactions on Power Electronics
    2016

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    Electric-drive vehicles, including hybrid, plug-in hybrid, and electric vehicles, require a high-voltage (HV) battery pack for propulsion and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communication between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for LV low-power dual active bridge (DAB) power converters serving as the bypass converter modules. Furthermore, current sharing is examined and worst-case SOC and current deviations are derived for mismatches in cell capacities, SOCs, and parasitic resistances. Experimental results are presented for a system consisting of 21 series 25 Ah Panasonic lithium-ion NMC battery cells and 21 DAB bypass converters, with combined outputs rated to supply a 650-W auxiliary load.

  • 2016

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    The three-phase current source rectifier (CSR) features a step-down ac-dc voltage conversion function, smaller ac filter size compared with the traditional two-level voltage source rectifier, and inrush current limiting capability. However, large conduction loss of semiconductor devices has limited the wide application of traditional CSRs. In this paper, a new CSR topology, delta-type current source rectifier (DCSR), is proposed to reduce the conduction loss. The proposed rectifier has delta-type connections on its ac input side and its dc-link current can be shared by multiple devices at a given time. This paper introduces the DCSR's operation principle, modulation scheme, and design method. Based on the analysis, the conduction loss can be reduced by up to 20% with the proposed topology. An 8-kW prototype is then built to experimentally verify the performance of the DCSR.

  • Michael Evzelman; M. Muneeb Ur Rehman; Kelly Hathaway; Regan Zane; Daniel Costinett; Dragan Maksimovic
    IEEE Transactions on Power Electronics
    2016

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    Electric-drive vehicles, including hybrid, plug-in hybrid, and electric vehicles, require a high-voltage (HV) battery pack for propulsion and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communication between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for LV low-power dual active bridge (DAB) power converters serving as the bypass converter modules. Furthermore, current sharing is examined and worst-case SOC and current deviations are derived for mismatches in cell capacities, SOCs, and parasitic resistances. Experimental results are presented for a system consisting of 21 series 25 Ah Panasonic lithium-ion NMC battery cells and 21 DAB bypass converters, with combined outputs rated to supply a 650-W auxiliary load.

  • 2016

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    The three-phase current source rectifier (CSR) features a step-down ac-dc voltage conversion function, smaller ac filter size compared with the traditional two-level voltage source rectifier, and inrush current limiting capability. However, large conduction loss of semiconductor devices has limited the wide application of traditional CSRs. In this paper, a new CSR topology, delta-type current source rectifier (DCSR), is proposed to reduce the conduction loss. The proposed rectifier has delta-type connections on its ac input side and its dc-link current can be shared by multiple devices at a given time. This paper introduces the DCSR's operation principle, modulation scheme, and design method. Based on the analysis, the conduction loss can be reduced by up to 20% with the proposed topology. An 8-kW prototype is then built to experimentally verify the performance of the DCSR.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    IEEE Transactions on Power Electronics
    2015

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    Double pulse test (DPT) is a widely accepted method to evaluate the switching characteristics of semiconductor switches, including SiC devices. However, the observed switching performance of SiC devices in a PWM inverter for induction motor drives is almost always worse than the DPT characterization, with slower switching speed, more switching losses, and more serious parasitic ringing. This paper systematically investigates the factors that limit the SiC switching performance from both the motor side and inverter side, including the load characteristics of induction motor and power cable, two more phase legs for the three-phase PWM inverter in comparison with the DPT, and the parasitic capacitive coupling effect between power devices and heat sink. Based on a three-phase PWM inverter with 1200 V SiC MOSFETs, test results show that the induction motor, especially with a relatively long power cable, will significantly impact the switching performance, leading to a switching time increase by a factor of 2, switching loss increase up to 30% in comparison with that yielded from DPT, and serious parasitic ringing with 1.5 μs duration, which is more than 50 times of the corresponding switching time. In addition, the interactions among the three phase legs cannot be ignored unless the decoupling capacitors are mounted close to each phase leg to support the dc bus voltage during switching transients. Also, the coupling capacitance due to the heat sink equivalently increases the junction capacitance of power devices; however, its influence on the switching behavior in the motor drives is small considering the relatively large capacitance of the motor load.

  • Yanjun Yao; Sisi Xiong; Hairong Qi; Yilu Liu; Leon M. Tolbert; Qing Cao
    IEEE Transactions on Smart Grid
    2015

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    With the emerging area of smart grids, one critical challenge faced by administrators of wide-area measurement systems is to analyze and model streaming data with limited resources on their embedded controllers. Usually, streaming data can be modeled as a multiset where each data item has its own frequency. In this paper, we study the problem on how to generate histograms of data items based on their frequency, so we can identify various issues such as power line tripping or line faults under constraints. The primary challenge for achieving this goal using conventional methods is that keeping an individual counter for each unique type of data is too memory-consuming, slow, and costly. In this paper, we describe a novel data structure and its associated algorithms, called the loglog bloom filter, for this purpose. This data structure extends the classical bloom filter with a recent technique called probabilistic counting, so it can effectively generate histograms for streaming data in one pass with sub-linear overhead. Therefore, this method is suitable for data processing in smart grids, where limited computational resources are available on the controllers. We analyze the performance, trade-offs, and capacity of this data structure, and evaluate it with real data traces collected through the frequency disturbance recorders deployed for the FNET/GridEye infrastructure. We demonstrate that this method can identify the frequencies of all unique items with high accuracy and low memory overhead, so that data outliers can be conveniently identified.

  • Xiaojie Shi; Zhiqiang Wang; Bo Liu; Yiqi Liu; Leon M. Tolbert; Fred Wang
    IEEE Transactions on Power Electronics
    2015

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    This paper presents the analysis and control of a multilevel modular converter (MMC)-based HVDC transmission system under three possible single-line-to-ground fault conditions, with special focus on the investigation of their different fault characteristics. Considering positive-, negative-, and zero-sequence components in both arm voltages and currents, the generalized instantaneous power of a phase unit is derived theoretically according to the equivalent circuit model of the MMC under unbalanced conditions. Based on this model, a novel double-line frequency dc-voltage ripple suppression control is proposed. This controller, together with the negative- and zero-sequence current control, could enhance the overall fault-tolerant capability of the HVDC system without additional cost. To further improve the fault-tolerant capability, the operation performance of the HVDC system with and without single-phase switching is discussed and compared in detail. Simulation results from a three-phase MMC-HVDC system generated with MATLAB/Simulink are provided to support the theoretical analysis and proposed control schemes.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei Wang; Zhenxian Liang; Daniel Costinett; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2015

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    This paper presents a board-level integrated silicon carbide (SiC) mosfet power module for high temperature and high power density application. Specifically, a silicon-on-insulator (SOI)-based gate driver capable of operating at 200 °C ambient temperature is designed and fabricated. The sourcing and sinking current capability of the gate driver are tested under various ambient temperatures. Also, a 1200 V/100 A SiC mosfet phase-leg power module is developed utilizing high temperature packaging technologies. The static characteristics, switching performance, and short-circuit behavior of the fabricated power module are fully evaluated at different temperatures. Moreover, a buck converter prototype composed of the SOI gate driver and SiC power module is built for high temperature continuous operation. The converter is operated at different switching frequencies up to 100 kHz, with its junction temperature monitored by a thermosensitive electrical parameter and compared with thermal simulation results. The experimental results from the continuous operation demonstrate the high temperature capability of the power module at a junction temperature greater than 225 °C.

  • Lakshmi Reddy GopiReddy; Leon M. Tolbert; Burak Ozpineci
    IEEE Transactions on Power Electronics
    2015

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    Reliability of power converters and lifetime prediction has been a major topic of research in the last few decades, especially for traction applications. The main failures in high power semiconductors are caused by thermomechanical fatigue. Power cycling and temperature cycling are the two most common thermal acceleration tests used in assessing reliability. The objective of this paper is to study the various power cycling tests found in the literature and to develop generalized steps in planning application specific power cycling tests. A comparison of different tests based on the failures, duration, test circuits, and monitored electrical parameters is presented.

  • Ben Guo; Fei Wang; Rolando Burgos; Eddy Aeloiza
    IEEE Transactions on Power Electronics
    2015

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    The three-phase buck-type rectifier features a step-down ac-dc conversion function, smaller filter size, inrush current limiting capability, and potential for high efficiency, where its switching loss is dependent on the modulation scheme and the specific semiconductors used. In this paper, three different device combinations are compared through experiments on their switching characteristics for the buck rectifier application. It is shown that the switching performance of two series-connected devices becomes worse than a single device due to the superposition of the nonideal semiconductor characteristics. Moreover, the switching loss in the commutation between two switches is usually higher than the one in the commutation between a switch and the freewheeling diode. Taking into consideration both types of commutations, the switching loss of the buck rectifier is then modeled and the analytical equations are derived for four space vector modulation schemes. According to the analysis, each modulation scheme has its own field for high-efficiency application. The most advantageous modulation scheme is identified in this paper for each of the device combinations investigated.

  • Daniel Costinett; Dragan Maksimovic; Regan Zane
    IEEE Transactions on Power Electronics
    2015

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    Nonlinear, voltage-dependent capacitances of power semiconductor devices are capable of having significant impact on the operation of switched-mode power converters. Particularly at high switching frequency, these nonlinearities play a significant role in determining switching times, losses, and converter dynamics during switching transitions. In order to accommodate the well-established design and analysis techniques commonly used for linear circuits, this paper examines the nonlinear voltage-dependence of switching device capacitances and proposes a circuit-oriented analysis technique that allows the parasitic capacitances to be replaced with linear equivalents. The multitude of developed equivalents are verified through full nonlinear simulation in both MATLAB/Simulink and SPICE, as well as through experimental results.

  • Mithat C. Kisacikoglu; Metin Kesler; Leon M. Tolbert
    IEEE Transactions on Smart Grid
    2015

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    This paper presents the design and implementation of a single-phase on-board bidirectional plug-in electric vehicle (PEV) charger that can provide reactive power support to the utility grid in addition to charging the vehicle battery. The topology consists of two-stages: a full-bridge ac-dc boost converter; and a half-bridge bidirectional dc-dc converter. The charger operates in two quadrants in the active-reactive power (PQ) power plane with five different operation modes (i.e., charging-only, charging-capacitive, charging-inductive, capacitive-only, and inductive-only). This paper also presents a unified controller to follow utility PQ commands in a smart grid environment. The cascaded two-stage system controller receives active and reactive power commands from the grid, and results in line current and battery charging current references while also providing a stable dynamic response. The vehicle's battery is not affected during reactive power operation in any of the operation modes. Testing the unified system controller with a 1.44 kVA experimental charger design demonstrates the successful implementation of reactive power support functionality of PEVs for future smart grid applications.

  • Daniel Costinett; Dragan Maksimovic; Regan Zane
    IEEE Transactions on Power Electronics
    2015

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    Nonlinear, voltage-dependent capacitances of power semiconductor devices are capable of having significant impact on the operation of switched-mode power converters. Particularly at high switching frequency, these nonlinearities play a significant role in determining switching times, losses, and converter dynamics during switching transitions. In order to accommodate the well-established design and analysis techniques commonly used for linear circuits, this paper examines the nonlinear voltage-dependence of switching device capacitances and proposes a circuit-oriented analysis technique that allows the parasitic capacitances to be replaced with linear equivalents. The multitude of developed equivalents are verified through full nonlinear simulation in both MATLAB/Simulink and SPICE, as well as through experimental results.

  • Mithat C. Kisacikoglu; Metin Kesler; Leon M. Tolbert
    IEEE Transactions on Smart Grid
    2015

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    This paper presents the design and implementation of a single-phase on-board bidirectional plug-in electric vehicle (PEV) charger that can provide reactive power support to the utility grid in addition to charging the vehicle battery. The topology consists of two-stages: a full-bridge ac-dc boost converter; and a half-bridge bidirectional dc-dc converter. The charger operates in two quadrants in the active-reactive power (PQ) power plane with five different operation modes (i.e., charging-only, charging-capacitive, charging-inductive, capacitive-only, and inductive-only). This paper also presents a unified controller to follow utility PQ commands in a smart grid environment. The cascaded two-stage system controller receives active and reactive power commands from the grid, and results in line current and battery charging current references while also providing a stable dynamic response. The vehicle's battery is not affected during reactive power operation in any of the operation modes. Testing the unified system controller with a 1.44 kVA experimental charger design demonstrates the successful implementation of reactive power support functionality of PEVs for future smart grid applications.

  • Wei Wang; Li He; Penn Markham; Hairong Qi; Yilu Liu; Qing Charles Cao; Leon M. Tolbert
    IEEE Transactions on Smart Grid
    2014

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    A situational awareness system is essential to provide accurate understanding of power system dynamics, such that proper actions can be taken in real time in response to system disturbances and to avoid cascading blackouts. Event analysis has been an important component in any situational awareness system. However, most state-of-the-art techniques can only handle single event analysis. This paper tackles the challenging problem of multiple event detection and recognition. We propose a new conceptual framework, referred to as event unmixing, where we consider real-world events mixtures of more than one constituent root event. This concept is a key enabler for analysis of events to go beyond what are immediately detectable in a system, providing high-resolution data understanding at a finer scale. We interpret the event formation process from a linear mixing perspective and propose an innovative nonnegative sparse event unmixing (NSEU) algorithm for multiple event separation and temporal localization. The proposed framework has been evaluated using both PSS/E simulated cases and real event cases collected from the frequency disturbance recorders (FDRs) of the Frequency Monitoring Network (FNET). The experimental results demonstrate that the framework is reliable to detect and recognize multiple cascading events as well as their time of occurrence with high accuracy.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei Wang; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2014

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    This paper presents an active gate driver (AGD) for IGBT modules to improve their overall performance under normal condition as well as fault condition. Specifically, during normal switching transients, a di/dt feedback controlled current source and current sink is introduced together with a push-pull buffer for dynamic gate current control. Compared to a conventional gate drive strategy, the proposed one has the capability of reducing the switching loss, delay time, and Miller plateau duration during turn-on and turn-off transient without sacrificing current and voltage stress. Under overcurrent condition, it provides a fast protection function for IGBT modules based on the evaluation of fault current level through the di/dt feedback signal. Moreover, the AGD features flexible protection modes, which overcomes the interruption of converter operation in the event of momentary short circuits. A step-down converter is built to evaluate the performance of the proposed driving schemes under various conditions, considering variation of turn-on/off gate resistance, current levels, and short-circuit fault types. Experimental results and detailed analysis are presented to verify the feasibility of the proposed approach.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2014

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    In a phase-leg configuration, the high-switching-speed performance of silicon carbide (SiC) devices is limited by the interaction between the upper and lower devices during the switching transient (crosstalk), leading to additional switching losses and overstress of the power devices. To utilize the full potential of fast SiC devices, this paper proposes two gate assist circuits to actively suppress crosstalk on the basis of the intrinsic properties of SiC power devices. One gate assist circuit employs an auxiliary transistor in series with a capacitor to mitigate crosstalk by gate loop impedance reduction. The other gate assist circuit consists of two auxiliary transistors with a diode to actively control the gate voltage for crosstalk elimination. Based on CREE CMF20120D SiC MOSFETs, the experimental results show that both active gate drivers are effective to suppress crosstalk, enabling turn-on switching losses reduction by up to 17%, and negative spurious gate voltage minimization without the penalty of decreasing the switching speed. Furthermore, both gate assist circuits, even without a negative isolated power supply, are more effective in improving the switching behavior of SiC devices in comparison to the conventional gate driver with a -2 V turn-off gate voltage. Accordingly, the proposed active gate assist circuits are simple, efficient, and cost-effective solutions for crosstalk suppression.

  • Jun Mei; Ke Shen; Bailu Xiao; Leon M. Tolbert; Jianyong Zheng
    IEEE Transactions on Industrial Electronics
    2014

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    This paper presents an improved phase disposition pulsewidth modulation (PWM) (PDPWM) for the modular multilevel converter (MMC) which is based on the selective loop bias mapping (SLBM) method. Its main idea is to change the bias of the PDPWM carrier wave cycling according to the balance situation of the system. This new modulation method can operate at symmetric condition to generate an output voltage with as many as 2N + 1 levels, and by SLBM, the voltages of the upper/lower arm capacitors can be well balanced. Compared to carrier phase-shifted PWM, this method is more easily to be realized and has much stronger dynamic regulation ability. Specially, this method has no issues of sorting, which makes it suitable for MMC with a large number of submodules in one leg. With simulation and experiments, the validity of the proposed method has been shown.

  • Shengnan Li; Leon M. Tolbert; Fei Wang; Fang Zheng Peng
    IEEE Transactions on Power Electronics
    2014

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    This paper proposes a novel packaging method for insulated-gate bipolar transistor (IGBT) modules based on the concepts of P-cell and N-cell. The novel packaging reduces the stray inductance in the current commutation path in a phase-leg module and hence improves the switching behavior. A P-cell- and N-cell-based module and a conventional module are designed. Using finite-element-analysis-based Ansys Q3D Extractor, electromagnetic simulations are conducted to extract the stray inductance from the two modules. Two prototype phase-leg modules based on the two different designs are fabricated. The parasitics are measured using a precision impedance analyzer. Finally, a double pulse tester based-switching characterization is performed to illustrate the effect of stray inductance reduction in the proposed packaging design. The experimental results show the reduction in overshoot voltage with the proposed layout.

  • Zhuxian Xu; Di Zhang; Fei Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2014

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    This paper presents a unified control method for the combined permanent magnet generator (PMG) and active rectifier that can be used in autonomous power systems such as more-electric aircraft requiring high power density and efficiency. With the proposed control, the system can function well without additional boost inductors and rotor position sensors. The design procedure for the control is presented, including current loops, a voltage loop, and a rotor position estimator loop. Simulation and experimental results show that both the dc-link voltage and the reactive power could be controlled effectively. A system efficiency optimization technique is proposed by selecting the permanent magnet flux linkage and determining the operating points at various load and speed conditions. The power density and efficiency of the PMG and active rectifier system are improved with the unified control.

  • Lijun Hang; Bin Li; Ming Zhang; Yong Wang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2014

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    In this paper, the analysis of space vector modulation of a three-phase/wire/level Vienna rectifier is conducted, according to which the implementation of the equivalent carrier-based pulsewidth modulation is deduced theoretically within each separated sector in the diagram of vectors. The voltage balancing ability of dc-link neutral point, which depends on the uneven distribution of short vectors, is analyzed as well. An adaptive and robust controller to balance the output voltage under the unbalanced load limit for different modulation indices is proposed. The proposed controller can work at wide range of unbalanced load condition as well. Furthermore, the maximum unbalanced load is deduced versus the modulation index m when the converter works in unity power factor. An experimental prototype of 2.5 kW was built to verify the effectiveness of the theoretical analysis. Finally, the tested unbalanced limit of outputs for the experimental platform was given under different modulation indices. The output voltages for dual bus are balanced, and the theoretical analysis is verified.

  • Dong Jiang; Fei Wang
    IEEE Transactions on Power Electronics
    2014

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    Current ripple is generated by pulse width modulation (PWM) switching in multiphase voltage source converters (VSCs). This letter introduces a general and fast current ripple prediction method for multiphase VSCs with arbitrary phase numbers. An equivalent converter-load model is derived for the n -phase converter system. By combining the common-mode voltage of both converter terminal and load, the equivalent circuit for each phase can be modeled. The voltage dropping on the ac inductor can be calculated for the 2 n + 2 zones in each switching cycle based on the equivalent circuit for each phase. Then the current ripple can be reconstructed based on the linear di/dt model in each zone. Simulation examples of five- and six-phase converters prove that the current prediction method is accurate. With this real-time prediction method, the current ripple can be controlled in application. An application example of five-phase variable switching frequency PWM is introduced to control the peak current ripple and reduce the switching losses.

  • Metin Kesler; Engin Ozdemir; Mithat C. Kisacikoglu; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2014

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    A three-phase nonlinear load emulator using a power electronic converter is presented in this study. The proposed nonlinear load emulator is intended to be used in an ultrawide-area grid transmission network emulator, also called hardware testbed (HTB). The emulator converter is controlled in rectifier mode to act as the real nonlinear three-phase diode rectifier load. This paper presents an accurate controller for the nonlinear load emulator based on a three-phase diode rectifier system to be used in the HTB. This study also demonstrates simulation and experimental results for verification of the proposed controller.

  • Puqi Ning; Fei Wang; Khai D. T. Ngo
    IEEE Transactions on Power Electronics
    2014

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    An analytical model has been developed for predicting the forced-air cooling system performance, including a detailed optimization process to minimize the total weight. With a design example in a high-density high-temperature SiC converter, the presented design method was verified through numerical simulations and experiments.

  • Dong Jiang; Fei Wang
    IEEE Transactions on Industry Applications
    2014

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    The three-phase pulsewidth-modulation (PWM) converter is one of the most widely used topologies for power conversion. In order to design PWM methods, the influence of PWM methods on the current ripple is needed. This paper studies the current ripple of a three-phase PWM converter with general PWM methods for the design and control of this kind of converter. The current ripple is analyzed with eight different Thevenin equivalent circuits for the eight different voltage vectors. Then, the current-ripple slope and effective time could be achieved for every period. The current ripple could be predicted with both peak and rms values. Analytical predicted results show that discontinuous PWM could generate obviously bigger current ripples than space vector PMW for both peak and rms values with the same conditions. Simulation and experiments are built to verify the analytical results, proving that the theoretical prediction is valid. This analysis provides the basis for the design and control of the PWM method for converters.

  • Zhenxian Liang; Puqi Ning; Fred Wang
    IEEE Transactions on Power Electronics
    2014

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    A thermally integrated packaging structure for an all silicon carbide (SiC) power module was used to realize highly efficient cooling of power semiconductor devices through direct bonding of the power stage and a cold baseplate. The prototype power modules composed of SiC metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes demonstrate significant improvements such as low-power losses and low-thermal resistance. Direct comparisons to their silicon counterparts, which are composed of insulated gate bipolar transistors and PiN diodes, as well as conventional thermal packaging, were experimentally performed. The advantages of this SiC module in efficiency and power density for power electronics systems have also been identified, with clarification of the SiC attributes and packaging advancements.

  • Puqi Ning; Zhenxian Liang; Fred Wang
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2014

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    To further reduce system costs and package volumes of hybrid electric vehicles, it is important to optimize the power module and associated cooling system. This paper reports the thermal performance evaluation and analysis of three commercial power modules and a proposed planar module with different cooling system. Results show that power electronics can be better merged with the mechanical environment. Experiments and simulations were conducted to help further optimization.

  • Zhenxian Liang; Puqi Ning; Fred Wang; Laura Marlino
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2014

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    A multilayer planar interconnection structure was used for the packaging of liquid-cooled automotive power modules. The power semiconductor switch dies are sandwiched between two symmetric substrates, providing planar electrical interconnections and insulation. Two minicoolers are directly bonded to the outside of these substrates, allowing double-sided, integrated cooling. The power switch dies are orientated in a face-up/face-down 3-D interconnection configuration to form a phase leg. The bonding areas between the dies and substrates, and the substrates and coolers are designed to use identical materials and are formed in one heating process. A special packaging process has been developed so that high-efficiency production can be implemented. Incorporating high-efficiency cooling and low-loss electrical interconnections allows dramatic improvements in systems' cost, and electrical conversion efficiency. These features are demonstrated in a planar bond-packaged prototype of a 200 A/1200 V phase-leg power module made of silicon (Si) insulated gate bipolar transistor and PiN diodes.

  • Lijun Hang; Bin Li; Ming Zhang; Yong Wang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2014

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    In this paper, the analysis of space vector modulation of a three-phase/wire/level Vienna rectifier is conducted, according to which the implementation of the equivalent carrier-based pulsewidth modulation is deduced theoretically within each separated sector in the diagram of vectors. The voltage balancing ability of dc-link neutral point, which depends on the uneven distribution of short vectors, is analyzed as well. An adaptive and robust controller to balance the output voltage under the unbalanced load limit for different modulation indices is proposed. The proposed controller can work at wide range of unbalanced load condition as well. Furthermore, the maximum unbalanced load is deduced versus the modulation index m when the converter works in unity power factor. An experimental prototype of 2.5 kW was built to verify the effectiveness of the theoretical analysis. Finally, the tested unbalanced limit of outputs for the experimental platform was given under different modulation indices. The output voltages for dual bus are balanced, and the theoretical analysis is verified.

  • Dong Jiang; Fei Wang
    IEEE Transactions on Power Electronics
    2014

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    Current ripple is generated by pulse width modulation (PWM) switching in multiphase voltage source converters (VSCs). This letter introduces a general and fast current ripple prediction method for multiphase VSCs with arbitrary phase numbers. An equivalent converter-load model is derived for the n -phase converter system. By combining the common-mode voltage of both converter terminal and load, the equivalent circuit for each phase can be modeled. The voltage dropping on the ac inductor can be calculated for the 2 n + 2 zones in each switching cycle based on the equivalent circuit for each phase. Then the current ripple can be reconstructed based on the linear di/dt model in each zone. Simulation examples of five- and six-phase converters prove that the current prediction method is accurate. With this real-time prediction method, the current ripple can be controlled in application. An application example of five-phase variable switching frequency PWM is introduced to control the peak current ripple and reduce the switching losses.

  • Metin Kesler; Engin Ozdemir; Mithat C. Kisacikoglu; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2014

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    A three-phase nonlinear load emulator using a power electronic converter is presented in this study. The proposed nonlinear load emulator is intended to be used in an ultrawide-area grid transmission network emulator, also called hardware testbed (HTB). The emulator converter is controlled in rectifier mode to act as the real nonlinear three-phase diode rectifier load. This paper presents an accurate controller for the nonlinear load emulator based on a three-phase diode rectifier system to be used in the HTB. This study also demonstrates simulation and experimental results for verification of the proposed controller.

  • Puqi Ning; Fei Wang; Khai D. T. Ngo
    IEEE Transactions on Power Electronics
    2014

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    An analytical model has been developed for predicting the forced-air cooling system performance, including a detailed optimization process to minimize the total weight. With a design example in a high-density high-temperature SiC converter, the presented design method was verified through numerical simulations and experiments.

  • Dong Jiang; Fei Wang
    IEEE Transactions on Industry Applications
    2014

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    The three-phase pulsewidth-modulation (PWM) converter is one of the most widely used topologies for power conversion. In order to design PWM methods, the influence of PWM methods on the current ripple is needed. This paper studies the current ripple of a three-phase PWM converter with general PWM methods for the design and control of this kind of converter. The current ripple is analyzed with eight different Thevenin equivalent circuits for the eight different voltage vectors. Then, the current-ripple slope and effective time could be achieved for every period. The current ripple could be predicted with both peak and rms values. Analytical predicted results show that discontinuous PWM could generate obviously bigger current ripples than space vector PMW for both peak and rms values with the same conditions. Simulation and experiments are built to verify the analytical results, proving that the theoretical prediction is valid. This analysis provides the basis for the design and control of the PWM method for converters.

  • Zhenxian Liang; Puqi Ning; Fred Wang
    IEEE Transactions on Power Electronics
    2014

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    A thermally integrated packaging structure for an all silicon carbide (SiC) power module was used to realize highly efficient cooling of power semiconductor devices through direct bonding of the power stage and a cold baseplate. The prototype power modules composed of SiC metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes demonstrate significant improvements such as low-power losses and low-thermal resistance. Direct comparisons to their silicon counterparts, which are composed of insulated gate bipolar transistors and PiN diodes, as well as conventional thermal packaging, were experimentally performed. The advantages of this SiC module in efficiency and power density for power electronics systems have also been identified, with clarification of the SiC attributes and packaging advancements.

  • Puqi Ning; Zhenxian Liang; Fred Wang
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2014

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    To further reduce system costs and package volumes of hybrid electric vehicles, it is important to optimize the power module and associated cooling system. This paper reports the thermal performance evaluation and analysis of three commercial power modules and a proposed planar module with different cooling system. Results show that power electronics can be better merged with the mechanical environment. Experiments and simulations were conducted to help further optimization.

  • Zhenxian Liang; Puqi Ning; Fred Wang; Laura Marlino
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2014

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    A multilayer planar interconnection structure was used for the packaging of liquid-cooled automotive power modules. The power semiconductor switch dies are sandwiched between two symmetric substrates, providing planar electrical interconnections and insulation. Two minicoolers are directly bonded to the outside of these substrates, allowing double-sided, integrated cooling. The power switch dies are orientated in a face-up/face-down 3-D interconnection configuration to form a phase leg. The bonding areas between the dies and substrates, and the substrates and coolers are designed to use identical materials and are formed in one heating process. A special packaging process has been developed so that high-efficiency production can be implemented. Incorporating high-efficiency cooling and low-loss electrical interconnections allows dramatic improvements in systems' cost, and electrical conversion efficiency. These features are demonstrated in a planar bond-packaged prototype of a 200 A/1200 V phase-leg power module made of silicon (Si) insulated gate bipolar transistor and PiN diodes.

  • Fan Xu; Ben Guo; Leon M. Tolbert; Fei Wang; Benjamin J. Blalock
    IEEE Transactions on Industry Applications
    2013

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    The low power losses of silicon carbide (SiC) devices provide new opportunities to implement an ultra high-efficiency front-end rectifier for data center power supplies based on a 400-Vdc power distribution architecture, which requires high conversion efficiency in each power conversion stage. This paper presents a 7.5-kW high-efficiency three-phase buck rectifier with 480-Vac,rms input line-to-line voltage and 400-Vdc output voltage using SiC MOSFETs and Schottky diodes. To estimate power devices' losses, which are the dominant portion of total loss, the method of device evaluation and loss calculation is proposed based on a current source topology. This method simulates the current commutation process and estimates devices' losses during switching transients considering devices with and without switching actions in buck rectifier operation. Moreover, the power losses of buck rectifiers based on different combinations of 1200-V power devices are compared. The investigation and comparison demonstrate the benefits of each combination, and the lowest total loss in the all-SiC rectifier is clearly shown. A 7.5-kW prototype of the all-SiC three-phase buck rectifier using liquid cooling is fabricated and tested, with filter design and switching frequency chosen based on loss minimization. A full-load efficiency value greater than 98.5% is achieved.

  • Fan Xu; Timothy J. Han; Dong Jiang; Leon M. Tolbert; Fei Wang; Jim Nagashima; Sung Joon Kim; Srikanth Kulkarni; Fred Barlow
    IEEE Transactions on Power Electronics
    2013

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    In this paper, a fully integrated silicon carbide (SiC)-based six-pack power module is designed and developed. With 1200-V, 100-A module rating, each switching element is composed of four paralleled SiC junction gate field-effect transistors (JFETs) with two antiparallel SiC Schottky barrier diodes. The stability of the module assembly processes is confirmed with 1000 cycles of -40°C to +200°C thermal shock tests with 1.3°C/s temperature change. The static characteristics of the module are evaluated and the results show 55 mΩ on-state resistance of the phase leg at 200°C junction temperature. For switching performances, the experiments demonstrate that while utilizing a 650-V voltage and 60-A current, the module switching loss decreases as the junction temperature increases up to 150°C. The test setup over a large temperature range is also described. Meanwhile, the shoot-through influenced by the SiC JFET internal capacitance as well as package parasitic inductances are discussed. Additionally, a liquid cooled three-phase inverter with 22.9 cm × 22.4 cm × 7.1 cm volume and 3.53-kg weight, based on this power module, is designed and developed for electric vehicle and hybrid electric vehicle applications. A conversion efficiency of 98.5% is achieved at 10 kHz switching frequency at 5 kW output power. The inverter is evaluated with coolant temperature up to 95°C successfully.

  • Dong Jiang; Fei Wang; Jing Xue
    IEEE Transactions on Industry Applications
    2013

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    This paper analyzes pulsewidth modulation (PWM) methods' impact on the motor drives' common-mode (CM) noise current and CM choke saturation. The modulation in the motor drive terminals serves as the CM noise source. A few improved PWM methods could reduce the CM voltage amplitude in comparison with the conventional space vector PWM and discontinuous PWM. However, some harmonics of the improved PWM methods increase when considering the spectrum. Because the CM loop of the motor drive system is an L-R-C circuit which has its resonant frequency, the CM noise current is highly influenced by the noise near the resonant frequency. This paper studies the CM current with different PWM methods and claims that the design of PWM methods and switching frequency should be together with the CM loop impedance. Reduced CM voltage does not mean reduced CM current. With a CM choke to attenuate the CM noise, the choke size is determined by the CM volt-seconds on the choke. This paper studies the general case and the worst case of the choke size. The conclusions are supported by simulation and experimental results.

  • Ming Zhang; Lijun Hang; Wenxi Yao; Zhengyu Lu; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2013

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    Unbalanced grids introduce performance deterioration for Vienna rectifier topology by producing twice fundamental frequency ripples in dc-link voltage and input active/reactive power. A common current reference generation for the purpose of eliminating the input power ripple, such as dual-frame hybrid vector control, can maintain constant input power and eliminate ripples in dc-link voltage under light voltage unbalanced grids. Under severe unbalanced grids, this type of a control method will fail to work. This paper first analyzes the theoretical operation area of a constant power control method under unbalanced grids, and then a novel control method is proposed. The proposed control method can work under severe unbalanced grids by injecting a small amount of input power ripple and balance the performance of working area and output dc voltage ripples. Finally, the experiment results using the constant power control method are given and validate the performance of the proposed control method.

  • Faete Filho; Helder Zandonadi Maia; Tiago H. A. Mateus; Burak Ozpineci; Leon M. Tolbert; João O. P. Pinto
    IEEE Transactions on Industrial Electronics
    2013

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    A new approach for modulation of an 11-level cascade multilevel inverter using selective harmonic elimination is presented in this paper. The dc sources feeding the multilevel inverter are considered to be varying in time, and the switching angles are adapted to the dc source variation. This method uses genetic algorithms to obtain switching angles offline for different dc source values. Then, artificial neural networks are used to determine the switching angles that correspond to the real-time values of the dc sources for each phase. This implies that each one of the dc sources of this topology can have different values at any time, but the output fundamental voltage will stay constant and the harmonic content will still meet the specifications. The modulating switching angles are updated at each cycle of the output fundamental voltage. This paper gives details on the method in addition to simulation and experimental results.

  • Jun Mei; Bailu Xiao; Ke Shen; Leon M. Tolbert; Jian Yong Zheng
    IEEE Transactions on Power Electronics
    2013

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    This paper proposed an improved phase disposition pulse width modulation (PDPWM) for a modular multilevel inverter which is used for Photovoltaic grid connection. This new modulation method is based on selective virtual loop mapping, to achieve dynamic capacitor voltage balance without the help of an extra compensation signal. The concept of virtual submodule (VSM) is first established, and by changing the loop mapping relationships between the VSMs and the real submodules, the voltages of the upper/lower arm's capacitors can be well balanced. This method does not requiring sorting voltages from highest to lowest, and just identifies the MIN and MAX capacitor voltage's index which makes it suitable for a modular multilevel converter with a large number of submodules in one arm. Compared to carrier phase-shifted PWM (CPSPWM), this method is more easily to be realized in field-programmable gate array and has much stronger dynamic regulation ability, and is conducive to the control of circulating current. Its feasibility and validity have been verified by simulations and experiments.

  • Zhuxian Xu; Dong Jiang; Ming Li; Puqi Ning; Fei Fred Wang; Zhenxian Liang
    IEEE Transactions on Power Electronics
    2013

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    A Si insulated-gate bipolar transistor (IGBT) phase-leg module is developed for operating at 200°C in hybrid electric vehicle applications utilizing the high temperature packaging technologies and appropriate thermal management. The static and switching electrical characteristics of the fabricated power module are tested at various temperatures, showing that the module can operate reliably with increased but acceptable losses at 200°C. The criterion on thermal performance is given to prevent thermal runaway caused by fast increase of the leakage current during a high temperature operation. Afterward, the thermal management system is designed to meet the criterion, the performance of which is evaluated with experiment. Furthermore, two temperature-sensitive electrical parameters, on-state voltage drop and the switching time, are employed for thermal impedance characterization and the junction temperature measurement during converter operation, respectively. Finally, a 10-kW buck converter prototype composed of the module assembly is built and operated at the junction temperature up to 200°C. The experimental results demonstrate the feasibility of operating Si device-based converters continuously at 200°C.

  • Zhuxian Xu; Ming Li; Fei Wang; Zhenxian Liang
    IEEE Transactions on Power Electronics
    2013

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    In order to satisfy the high-density requirement and harsh thermal conditions while reducing cost in future electric and hybrid electric vehicles (HEV), a systematic study of a 1200-V trench-gate field-stop Si insulated gate bipolar transistor (IGBT) operating up to 200°C is performed to determine its feasibility, issues, and application guideline. The device forward conduction characteristics, leakage current, and switching performance are evaluated at various temperatures. Based on the device characterization, the impact of the increased junction temperature on a traction drive converter loss and thermal management is analyzed. It is shown that by extending the device junction temperature to 200°C, the additional 65°C coolant loop can be eliminated without compromising power density and thermal management design. Furthermore, the possible failure mechanisms including latching, short circuit fault, and avalanche capability are tested at elevated temperatures. The criteria considering thermal stability, thermal management, short circuit capability, and avalanche capability are given at 200°C to ensure the safe and reliable operation of Si IGBTs.

  • Lijun Hang; Ming Zhang; Leon M. Tolbert; Zhengyu Lu
    IEEE Transactions on Industrial Electronics
    2013

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    This paper presents a front-end three-phase ac/dc power factor correction rectifier, which is based on the three-level bidirectional-switch Vienna topology. On one hand, the rectifier is designed to operate in continuous-conduction mode (CCM) at full power. However, at reduced load, it operates in discontinuous-conduction mode (DCM). On the other hand, with reduced input inductance, the DCM mode occurs even when the rectifier operates at full power. In this paper, the digitized feedfoward compensation method is proposed for the rectifier to reduce the impact of the switch between DCM and CCM. The theoretical analysis of the proposed method is deduced; furthermore, the control design strategy is given. The experimental results are obtained by using a digitally controlled Vienna rectifier, which validated the proposed compensation method.

  • Ruxi Wang; Dushan Boroyevich; Puqi Ning; Zhiqiang Wang; Fei Wang; Paolo Mattavelli; Khai D. T. Ngo; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2013

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    High-temperature (HT) converters have gained importance in industrial applications where the converters operate in a harsh environment, such as in hybrid electrical vehicles, aviation, and deep-earth petroleum exploration. These environments require the converter to have not only HT semiconductor devices (made of SiC or GaN), but also reliable HT packaging, HT gate drives, and HT control electronics. This paper describes a detailed design process for an HT SiC three-phase PWM rectifier that can operate at ambient temperatures above 100°C. SiC HT planar structure packaging is designed for the main semiconductor devices, and an edge-triggered HT gate drive is also proposed to drive the designed power module. The system is designed to make use of available HT components, including the passive components, silicon-on-insulator chips, and auxiliary components. Finally, a 1.4 kW lab prototype is tested in a harsh environment for verification.

  • Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2013

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    This paper presents a summary of the available single-phase ac-dc topologies used for EV/PHEV, level-1 and -2 on-board charging and for providing reactive power support to the utility grid. It presents the design motives of single-phase on-board chargers in detail and makes a classification of the chargers based on their future vehicle-to-grid usage. The pros and cons of each different ac-dc topology are discussed to shed light on their suitability for reactive power support. This paper also presents and analyzes the differences between charging-only operation and capacitive reactive power operation that results in increased demand from the dc-link capacitor (more charge/discharge cycles and increased second harmonic ripple current). Moreover, battery state of charge is spared from losses during reactive power operation, but converter output power must be limited below its rated power rating to have the same stress on the dc-link capacitor.

  • 2013

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    Compared with the widely used constant switching frequency pulse-width-modulation (PWM) method, variable switching frequency PWM can benefit more because of the extra freedom. Based on the analytical expression of current ripple of three-phase converters, variable switching frequency control methods are proposed to satisfy different ripple requirements. Switching cycle Ts is updated in DSP in every interruption period based on the ripple requirement. Two methods are discussed in this paper. The first method is designed to arrange the current ripple peak value within a certain value and can reduce the equivalent switching frequency and electromagnetic interference (EMI) noise; the second method is designed to keep ripple current RMS value constant and reduce the EMI noise. Simulation and experimental results show that variable switching frequency control could improve the performance of EMI and efficiency without impairing the power quality.

  • Lijun Hang; Ming Zhang; Leon M. Tolbert; Zhengyu Lu
    IEEE Transactions on Industrial Electronics
    2013

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    This paper presents a front-end three-phase ac/dc power factor correction rectifier, which is based on the three-level bidirectional-switch Vienna topology. On one hand, the rectifier is designed to operate in continuous-conduction mode (CCM) at full power. However, at reduced load, it operates in discontinuous-conduction mode (DCM). On the other hand, with reduced input inductance, the DCM mode occurs even when the rectifier operates at full power. In this paper, the digitized feedfoward compensation method is proposed for the rectifier to reduce the impact of the switch between DCM and CCM. The theoretical analysis of the proposed method is deduced; furthermore, the control design strategy is given. The experimental results are obtained by using a digitally controlled Vienna rectifier, which validated the proposed compensation method.

  • Ruxi Wang; Dushan Boroyevich; Puqi Ning; Zhiqiang Wang; Fei Wang; Paolo Mattavelli; Khai D. T. Ngo; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2013

    arrow_drop_down

    High-temperature (HT) converters have gained importance in industrial applications where the converters operate in a harsh environment, such as in hybrid electrical vehicles, aviation, and deep-earth petroleum exploration. These environments require the converter to have not only HT semiconductor devices (made of SiC or GaN), but also reliable HT packaging, HT gate drives, and HT control electronics. This paper describes a detailed design process for an HT SiC three-phase PWM rectifier that can operate at ambient temperatures above 100°C. SiC HT planar structure packaging is designed for the main semiconductor devices, and an edge-triggered HT gate drive is also proposed to drive the designed power module. The system is designed to make use of available HT components, including the passive components, silicon-on-insulator chips, and auxiliary components. Finally, a 1.4 kW lab prototype is tested in a harsh environment for verification.

  • Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2013

    arrow_drop_down

    This paper presents a summary of the available single-phase ac-dc topologies used for EV/PHEV, level-1 and -2 on-board charging and for providing reactive power support to the utility grid. It presents the design motives of single-phase on-board chargers in detail and makes a classification of the chargers based on their future vehicle-to-grid usage. The pros and cons of each different ac-dc topology are discussed to shed light on their suitability for reactive power support. This paper also presents and analyzes the differences between charging-only operation and capacitive reactive power operation that results in increased demand from the dc-link capacitor (more charge/discharge cycles and increased second harmonic ripple current). Moreover, battery state of charge is spared from losses during reactive power operation, but converter output power must be limited below its rated power rating to have the same stress on the dc-link capacitor.

  • 2013

    arrow_drop_down

    Compared with the widely used constant switching frequency pulse-width-modulation (PWM) method, variable switching frequency PWM can benefit more because of the extra freedom. Based on the analytical expression of current ripple of three-phase converters, variable switching frequency control methods are proposed to satisfy different ripple requirements. Switching cycle Ts is updated in DSP in every interruption period based on the ripple requirement. Two methods are discussed in this paper. The first method is designed to arrange the current ripple peak value within a certain value and can reduce the equivalent switching frequency and electromagnetic interference (EMI) noise; the second method is designed to keep ripple current RMS value constant and reduce the EMI noise. Simulation and experimental results show that variable switching frequency control could improve the performance of EMI and efficiency without impairing the power quality.

  • Dong Jiang; Rolando Burgos; Fei Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2012

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    Silicon Carbide (SiC) devices have obvious advantages compared with conventional Si devices, and especially so at high temperatures. This paper aims at developing a method for the characterization of SiC JFET conduction and switching losses at high temperatures as well as the calculation of semiconductor losses in SiC JFET-based converters. To this end, the steady-state performance of SiC JFET and Schottky diodes at different temperatures is studied, and an improved conduction loss evaluation is proposed considering the bidirectional conduction paths of the JFET. Specifically, a SiC JFET bridge test bed is built to measure the switching losses at different temperatures with and without antiparallel diodes, where experimental results show that using SiC Schottky diodes in antiparallel eliminates the reverse recovery of the JFET body diode, improving the switching behavior and reducing the losses of the devices. Further, these test results are used to estimate the losses of a 10-kW ac-dc-ac converter, which shows that the use of Schottky diodes as freewheeling devices helps reduce both conduction and switching losses, presenting an even greater reduction at higher operating temperatures.

  • Mohammad A. Huque; Syed K. Islam; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2012

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    High-temperature power converters (dc-dc, dc-ac, etc.) have enormous potential in extreme environment applications, including automotive, aerospace, geothermal, nuclear, and well logging. For successful realization of such high-temperature power conversion modules, the associated control electronics also need to perform at high temperature. This paper presents a silicon-on-insulator (SOI) based high-temperature gate driver integrated circuit (IC) incorporating an on-chip low-power temperature sensor and demonstrating an improved peak output current drive over our previously reported work. This driver IC has been primarily designed for automotive applications, where the underhood temperature can reach 200 °C. This new gate driver prototype has been designed and implemented in a 0.8 μm, 2-poly, and 3-metal bipolar CMOS-DMOS (Double-Diffused Metal-Oxide Semiconductor) on SOI process and has been successfully tested for up to 200 °C ambient temperature driving a SiC MOSFET and a SiC normally-ON JFET. The salient feature of the proposed universal gate driver is its ability to drive power switches over a wide range of gate turn-ON voltages such as MOSFET (0 to 20 V), normally-OFF JFET (-7 to 3 V), and normally-ON JFET (-20 to 0 V). The measured peak output current capability of the driver is around 5 A and is thus capable of driving several power switches connected in parallel. An ultralow-power on-chip temperature supervisory circuit has also been integrated into the die to safeguard the driver circuit against excessive die temperature (≥220 °C). This approach utilizes increased diode leakage current at higher temperature to monitor the die temperature. The power consumption of the proposed temperature sensor circuit is below 10 μW for operating temperature up to 200 °C.

  • Wei Qian; Honnyong Cha; Fang Zheng Peng; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2012

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    This paper presents an alternative to the traditional dc-dc converter interfacing the battery with the inverter dc bus in plug-in hybrid electric vehicle (HEV) traction drives. The boost converter used in commercial HEVs meets with obstacles when it comes to upgrading the power rating and achieving high efficiency while downsizing the converter. A four-level flying-capacitor dc-dc converter is explored that can overcome these drawbacks by dramatically reducing the inductance requirement. A special case of the four-level converter, the 3X dc-dc converter, operates at three discrete output/input voltage ratios, thus further reducing the inductance requirement to a minimal value (almost zero). When further compared to its switched-capacitor dc-dc converter counterparts, the 3X dc-dc converter can be operated at variable output/input voltage ratios without sacrificing efficiency, and it lowers the capacitance requirement by utilizing the parasitic inductance. The operating principle, current ripple analysis, the transient control to limit the inrush current, and power loss analysis are introduced. Experimental results of a 55-kW prototype are provided to demonstrate the principle and analysis of this topology.

  • Parag Kshirsagar; Rolando P. Burgos; Jihoon Jang; Alessandro Lidozzi; Fei Wang; Dushan Boroyevich; Seung-Ki Sul
    IEEE Transactions on Industry Applications
    2012

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    This paper presents a complete design methodology for the sensorless vector control of permanent-magnet synchronous machine (PMSM) motor drives in fan-type applications. The proposed strategy is built over a linear asymptotic state observer used to estimate the PMSM back EMF and a novel tracking controller based on a phase-locked loop system, which, by synchronizing the estimated and actual d-q frames, estimates the rotor speed and position. This paper presents the complete derivation of all associated control loops, namely, state observer; tracking controller; d-q-axis current regulator; speed controller; an antisaturation control loop, which provides inherent operation in the flux-weakening region; and all corresponding antiwindup loops. Detailed design rules are provided for each of these loops, respectively verified through time-domain simulations, frequency-response analysis, and experimental results using a three-phase 7.5-kW PMSM motor drive, validating both the design methodology and the expected performance attained by the proposed control strategy.

  • This paper presents a control method to minimize the total flux in the integrated interphase inductors of paralleled, interleaved three-phase two-level voltage-source converters (VSCs) using discontinuous space vector modulation (DPWM). Specifically, different inductor structures used to limit circulating currents are introduced and compared, and the structure and flux distribution of two types of integrated interphase inductors are analyzed in detail. Based on that, a control method to minimize the total flux in such integrated interphase inductor is proposed for a parallel converter system using interleaved DPWM. The method eliminates the circulating currents during the peak range of the converter output currents; hence the total flux is minimized and only determined by the system load requirements. This control method introduces very limited additional switching actions, which do not significantly affect the converter electrothermal design. Experimental results verify the analysis and the feasibility of the proposed control method.

  • Di Zhang; Fred Wang; Said El-Barbari; Juan A. Sabate; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2012

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    This paper presents an improved asymmetric space vector modulation (ASVM) for two-level voltage source converters (VSCs) when the switching frequency is as low as nine times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly. The penalty of additional switching loss is very limited for high power factor operation. The applications of the improved ASVM in a single VSC or in two interleaved VSCs are shown, respectively. With the optimization of the duration and position of the additional pulses, the ac current THD can be reduced to as low as 50% for single VSC and even lower to less than 25% for interleaved VSCs systems. Such THD reduction has close relationship with space vectors' position, modulation index, and interleaving angle. Improved ASVM can also reduce the amplitude of circulating current in the interleaved VSCs, leading to smaller interphase inductors. Finally, the weights of total inductors needed to meet the same THD requirement are compared to demonstrate the benefits of improved ASVM when different pulsewidth modulation schemes are used. The analysis results are verified by experiments on a demo system.

  • Dong Dong; Timothy Thacker; Igor Cvetkovic; Rolando Burgos; Dushan Boroyevich; Fred Wang; Glenn Skutt
    IEEE Transactions on Smart Grid
    2012

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    Robust system control design and seamless transition between various modes of operation are paramount for multifunctional converters in microgrid systems. This paper proposes a control system for single-phase bidirectional PWM converters for residential power level microgrid systems which is robust and can tolerate transitions between the different modes of operation. This is achieved by means of a common inner ac current-loop. Each of the operating modes has an individually designed outer loop performing the corresponding regulation tasks, most commonly including the ac voltage and the dc voltage regulation. A modified , phase-locked loop (PLL) system is used for system-level operation with both small steady-state error and fast response; and a novel islanding detection algorithm based on PLL stability is proposed to facilitate the transition between grid-connected mode and stand-alone mode. Finally, a frequency-response based design procedure for the proposed control system is presented in detail for all operating modes, and its performance is verified experimentally using a DSP-controlled 6 kW 120 V rms (ac)/ 300 V (dc) laboratory converter prototype.

  • Honggang Sheng; Fei Wang; C. Wesley Tipton IV
    IEEE Transactions on Power Electronics
    2012

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    Fault detection and protection is an important design aspect for any power converter, especially in high-power high-voltage applications, where cost of failure can be high. The three-level dc-dc converter and its varied derivatives are attractive topologies in high-voltage high-power converter applications. The protection method can not only prevent the system failure against unbalanced voltage stresses on the switches, but also provide a remedy for the system as faults occur and save the remaining components. The three-level converter is subject to voltage unbalance in certain abnormal conditions, which can result in switch overvoltage and system failure. The reasons for the unbalanced voltage stresses are fully investigated and categorized. The solutions to each abnormal condition are introduced. In addition to the voltage unbalance, the three-level converters can be protected against multiple faults by the proposed protection method through monitoring the flying capacitor voltage. Phenomena associated with each fault are thoroughly analyzed and summarized. The protection circuit is simple and can be easily implemented, while it can effectively protect the three-level converters and its derivatives, which has been verified by the experiment with a three-level parallel resonant converter.

  • Rolando Burgos; Gang Chen; Fred Wang; Dushan Boroyevich; Willem Gerhardus Odendaal; Jacobus Daniel Van Wyk
    IEEE Transactions on Aerospace and Electronic Systems
    2012

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    This paper presents a reliability-oriented design (ROD) procedure for three-phase power converters in aircraft applications. These require the highest reliability levels for all its components-as high as space applications; hence the need to maximize the reliability of three-phase power converters, which are in increasing demand and use in commercial and military aircrafts as a result of the more-electric aircraft (MEA) initiative. Specifically, the proposed procedure takes reliability up-front in the design process of power converters, carrying out the design in three steps. First, the identification of critical system components; second, the assessment of reliability factors such as risk analysis, failure mode analysis, and fishbone diagrams; and third, the actual design, which is carried out by minimizing system complexity and stress, and by the use of the most reliable components, materials, and structures. To this end, reliability models were developed for all critical components based on the military handbook MIL-HDBK-217F, and field and vendor data. For verification purposes, the paper includes the ROD of a 60 kW three-phase power converter for aircraft applications together with experimental results of the prototype constructed.

  • Wei Qian; Honnyong Cha; Fang Zheng Peng; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2012

    arrow_drop_down

    This paper presents an alternative to the traditional dc-dc converter interfacing the battery with the inverter dc bus in plug-in hybrid electric vehicle (HEV) traction drives. The boost converter used in commercial HEVs meets with obstacles when it comes to upgrading the power rating and achieving high efficiency while downsizing the converter. A four-level flying-capacitor dc-dc converter is explored that can overcome these drawbacks by dramatically reducing the inductance requirement. A special case of the four-level converter, the 3X dc-dc converter, operates at three discrete output/input voltage ratios, thus further reducing the inductance requirement to a minimal value (almost zero). When further compared to its switched-capacitor dc-dc converter counterparts, the 3X dc-dc converter can be operated at variable output/input voltage ratios without sacrificing efficiency, and it lowers the capacitance requirement by utilizing the parasitic inductance. The operating principle, current ripple analysis, the transient control to limit the inrush current, and power loss analysis are introduced. Experimental results of a 55-kW prototype are provided to demonstrate the principle and analysis of this topology.

  • Parag Kshirsagar; Rolando P. Burgos; Jihoon Jang; Alessandro Lidozzi; Fei Wang; Dushan Boroyevich; Seung-Ki Sul
    IEEE Transactions on Industry Applications
    2012

    arrow_drop_down

    This paper presents a complete design methodology for the sensorless vector control of permanent-magnet synchronous machine (PMSM) motor drives in fan-type applications. The proposed strategy is built over a linear asymptotic state observer used to estimate the PMSM back EMF and a novel tracking controller based on a phase-locked loop system, which, by synchronizing the estimated and actual d-q frames, estimates the rotor speed and position. This paper presents the complete derivation of all associated control loops, namely, state observer; tracking controller; d-q-axis current regulator; speed controller; an antisaturation control loop, which provides inherent operation in the flux-weakening region; and all corresponding antiwindup loops. Detailed design rules are provided for each of these loops, respectively verified through time-domain simulations, frequency-response analysis, and experimental results using a three-phase 7.5-kW PMSM motor drive, validating both the design methodology and the expected performance attained by the proposed control strategy.

  • This paper presents a control method to minimize the total flux in the integrated interphase inductors of paralleled, interleaved three-phase two-level voltage-source converters (VSCs) using discontinuous space vector modulation (DPWM). Specifically, different inductor structures used to limit circulating currents are introduced and compared, and the structure and flux distribution of two types of integrated interphase inductors are analyzed in detail. Based on that, a control method to minimize the total flux in such integrated interphase inductor is proposed for a parallel converter system using interleaved DPWM. The method eliminates the circulating currents during the peak range of the converter output currents; hence the total flux is minimized and only determined by the system load requirements. This control method introduces very limited additional switching actions, which do not significantly affect the converter electrothermal design. Experimental results verify the analysis and the feasibility of the proposed control method.

  • Di Zhang; Fred Wang; Said El-Barbari; Juan A. Sabate; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2012

    arrow_drop_down

    This paper presents an improved asymmetric space vector modulation (ASVM) for two-level voltage source converters (VSCs) when the switching frequency is as low as nine times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly. The penalty of additional switching loss is very limited for high power factor operation. The applications of the improved ASVM in a single VSC or in two interleaved VSCs are shown, respectively. With the optimization of the duration and position of the additional pulses, the ac current THD can be reduced to as low as 50% for single VSC and even lower to less than 25% for interleaved VSCs systems. Such THD reduction has close relationship with space vectors' position, modulation index, and interleaving angle. Improved ASVM can also reduce the amplitude of circulating current in the interleaved VSCs, leading to smaller interphase inductors. Finally, the weights of total inductors needed to meet the same THD requirement are compared to demonstrate the benefits of improved ASVM when different pulsewidth modulation schemes are used. The analysis results are verified by experiments on a demo system.

  • Dong Dong; Timothy Thacker; Igor Cvetkovic; Rolando Burgos; Dushan Boroyevich; Fred Wang; Glenn Skutt
    IEEE Transactions on Smart Grid
    2012

    arrow_drop_down

    Robust system control design and seamless transition between various modes of operation are paramount for multifunctional converters in microgrid systems. This paper proposes a control system for single-phase bidirectional PWM converters for residential power level microgrid systems which is robust and can tolerate transitions between the different modes of operation. This is achieved by means of a common inner ac current-loop. Each of the operating modes has an individually designed outer loop performing the corresponding regulation tasks, most commonly including the ac voltage and the dc voltage regulation. A modified , phase-locked loop (PLL) system is used for system-level operation with both small steady-state error and fast response; and a novel islanding detection algorithm based on PLL stability is proposed to facilitate the transition between grid-connected mode and stand-alone mode. Finally, a frequency-response based design procedure for the proposed control system is presented in detail for all operating modes, and its performance is verified experimentally using a DSP-controlled 6 kW 120 V rms (ac)/ 300 V (dc) laboratory converter prototype.

  • Honggang Sheng; Fei Wang; C. Wesley Tipton IV
    IEEE Transactions on Power Electronics
    2012

    arrow_drop_down

    Fault detection and protection is an important design aspect for any power converter, especially in high-power high-voltage applications, where cost of failure can be high. The three-level dc-dc converter and its varied derivatives are attractive topologies in high-voltage high-power converter applications. The protection method can not only prevent the system failure against unbalanced voltage stresses on the switches, but also provide a remedy for the system as faults occur and save the remaining components. The three-level converter is subject to voltage unbalance in certain abnormal conditions, which can result in switch overvoltage and system failure. The reasons for the unbalanced voltage stresses are fully investigated and categorized. The solutions to each abnormal condition are introduced. In addition to the voltage unbalance, the three-level converters can be protected against multiple faults by the proposed protection method through monitoring the flying capacitor voltage. Phenomena associated with each fault are thoroughly analyzed and summarized. The protection circuit is simple and can be easily implemented, while it can effectively protect the three-level converters and its derivatives, which has been verified by the experiment with a three-level parallel resonant converter.

  • Rolando Burgos; Gang Chen; Fred Wang; Dushan Boroyevich; Willem Gerhardus Odendaal; Jacobus Daniel Van Wyk
    IEEE Transactions on Aerospace and Electronic Systems
    2012

    arrow_drop_down

    This paper presents a reliability-oriented design (ROD) procedure for three-phase power converters in aircraft applications. These require the highest reliability levels for all its components-as high as space applications; hence the need to maximize the reliability of three-phase power converters, which are in increasing demand and use in commercial and military aircrafts as a result of the more-electric aircraft (MEA) initiative. Specifically, the proposed procedure takes reliability up-front in the design process of power converters, carrying out the design in three steps. First, the identification of critical system components; second, the assessment of reliability factors such as risk analysis, failure mode analysis, and fishbone diagrams; and third, the actual design, which is carried out by minimizing system complexity and stress, and by the use of the most reliable components, materials, and structures. To this end, reliability models were developed for all critical components based on the military handbook MIL-HDBK-217F, and field and vendor data. For verification purposes, the paper includes the ROD of a 60 kW three-phase power converter for aircraft applications together with experimental results of the prototype constructed.

  • Hairong Qi; Xiaorui Wang; Leon M. Tolbert; Fangxing Li; Fang Z. Peng; Peng Ning; Massoud Amin
    IEEE Transactions on Smart Grid
    2011

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    Energy infrastructure is a critical underpinning of modern society that any compromise or sabotage of its secure and reliable operation has an enormous impact on people's daily lives and the national economy. The massive northeastern power blackout of August 2003 and the most recent Florida blackout have both revealed serious defects in both system-level management and device-level designs of the power grid in handling attacks. At the system level, the control area operators lack the capability to 1) obtain real-time status information of the vastly distributed equipment; 2) respond rapidly enough once events start to unravel; and 3) perform coordinated actions autonomously across the region. At the device level, the traditional hardware lacks the capability to 1) provide reliable frequency and voltage control according to system demands and 2) rapidly reconfigure the system to a secure state through switches and power-electronics based devices. These blackouts were a wake-up call for both the industry and academia to consider new techniques and system architecture design that can help assure the security and reliability of the power grid. In this paper, we present a hardware-in-the-loop reconfigurable system design with embedded intelligence and resilient coordination schemes at both local and system levels that would tackle the vulnerabilities of the grid. The new system design consists of five key components: 1) a location-centric hybrid system architecture that facilitates not only distributed processing but also coordination among geographically close devices; 2) the insertion of intelligence into power electronic devices at the lower level of the power grid to enable a more direct reconfiguration of the physical makeup of the grid; 3) the development of a robust collaboration algorithm among neighboring devices to handle possible faulty, missing, or incomplete information; 4) the design of distributed algorithms to better understand the local state of the power grid; and 5) the adoption of a control-theoretic real-time adaptation strategy to guarantee the availability of large distributed systems. Preliminary evaluation results showing the advantages of each component are provided. A phased implementation plan is also suggested at the end of the discussion.

  • Faete Filho; Leon M. Tolbert; Yue Cao; Burak Ozpineci
    IEEE Transactions on Industry Applications
    2011

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    This work approximates the selective harmonic elimination problem using artificial neural networks (ANNs) to generate the switching angles in an 11-level full-bridge cascade inverter powered by five varying dc input sources. Each of the five full bridges of the cascade inverter was connected to a separate 195-W solar panel. The angles were chosen such that the fundamental was kept constant and the low-order harmonics were minimized or eliminated. A nondeterministic method is used to solve the system for the angles and to obtain the data set for the ANN training. The method also provides a set of acceptable solutions in the space where solutions do not exist by analytical methods. The trained ANN is a suitable tool that brings a small generalization effect on the angles' precision and is able to perform in real time (50-/60-Hz time window).

  • Puqi Ning; Fred Wang; Khai D. T. Ngo
    IEEE Transactions on Power Electronics
    2011

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    To take full advantage of silicon carbide semiconductor devices, high-temperature device packaging needs to be developed. This paper describes potential defects from design and fabrication procedures, and presents a systematic electrical evaluation process to detect such defects. This systematic testing procedure can rapidly detect many defects and reduce the risk in high-temperature packaging testing. A multichip module development procedure that uses this testing procedure is also presented and demonstrated with an example.

  • Ruxi Wang; Fei Wang; Dushan Boroyevich; Rolando Burgos; Rixin Lai; Puqi Ning; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2011

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    It is well known that single-phase pulse width modulation rectifiers have second-order harmonic currents and corresponding ripple voltages on the dc bus. The low-frequency harmonic current is normally filtered using a bulk capacitor in the bus, which results in low power density. However, pursuing high power density in converter design is a very important goal in the aerospace applications. This paper studies methods for reducing the energy storage capacitor for single-phase rectifiers. The minimum ripple energy storage requirement is derived independently of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as the ripple energy storage circuit, which can effectively reduce the energy storage capacitance. The analysis and design are validated by simulation and experimental results.

  • Hui Zhang; Leon M. Tolbert; Burak Ozpineci
    IEEE Transactions on Industry Applications
    2011

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    The application of silicon carbide (SiC) devices as battery interface, motor controller, etc., in a hybrid electric vehicle (HEV) will be beneficial due to their high-temperature capability, high-power density, and high efficiency. Moreover, the light weight and small volume will affect the whole powertrain system in a HEV and, thus, the performance and cost. In this paper, the performance of HEVs is analyzed using the vehicle simulation software Powertrain System Analysis Toolkit (PSAT). Power loss models of a SiC inverter based on the test results of latest SiC devices are incorporated into PSAT powertrain models in order to study the impact of SiC devices on HEVs from a system standpoint and give a direct correlation between the inverter efficiency and weight and the vehicle's fuel economy. Two types of HEVs are considered. One is the 2004 Toyota Prius HEV, and the other is a plug-in HEV (PHEV), whose powertrain architecture is the same as that of the 2004 Toyota Prius HEV. The vehicle-level benefits from the introduction of SiC devices are demonstrated by simulations. Not only the power loss in the motor controller but also those in other components in the vehicle powertrain are reduced. As a result, the system efficiency is improved, and vehicles that incorporate SiC power electronics are predicted to consume less energy and have lower emissions and improved system compactness with a simplified thermal management system. For the PHEV, the benefits are even more distinct; in particular, the size of the battery bank can be reduced for optimum design.

  • Hui Zhang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2011

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    Power electronics is an enabling technology found in most renewable energy generation systems. Because of its superior voltage blocking capabilities and fast switching speeds, silicon carbide (SiC) power electronics are considered for use in power conversion units in wind generation systems in this paper. The potential efficiency gains from the use of SiC devices in a wind generation system are explored by simulations, with the system modeling explained in detail. The performance of the SiC converter is analyzed and compared to its silicon counterpart at different wind speeds, temperatures, and switching frequencies. The quantitative results are based on SiC metal-oxide-semiconductor field-effect transistor (MOSFET) prototypes from Cree and modern Si insulated-gate bipolar transistor (IGBT) products. A conclusion is drawn that the SiC converters can improve the wind system power conversion efficiency and can reduce the system's size and cost due to the low-loss, high-frequency, and high-temperature properties of SiC devices, even for one-for-one replacement for Si devices.

  • 2011

    arrow_drop_down

    This paper presents a control method to limit the common-mode (CM) circulating current between paralleled three-phase two-level voltage-source converters (VSCs) with discontinuous space-vector pulsewidth modulation (DPWM) and interleaved switching cycles. This CM circulating current can be separated into two separate components based on their frequency; the high-frequency component, close to the switching frequency, can be effectively limited by means of passive components; the low-frequency component, close to the fundamental frequency, embodies the jumping CM circulating current observed in parallel VSCs. This is the main reason why it is usually recommended not to implement discontinuous and interleaving PWM together. The origin of this low-frequency circulating current is analyzed in detail, and based on this, a method to eliminate its presence is proposed by impeding the simultaneous use of different zero vectors between the converters. This control method only requires six additional switching actions per line cycle, presenting a minimum impact on the converter thermal design. The analysis and the feasibility of the control method are verified by simulation and experimental results.

  • Timothy Thacker; Dushan Boroyevich; Rolando Burgos; Fei Wang
    IEEE Transactions on Industrial Electronics
    2011

    arrow_drop_down

    A crucial component of grid-connected converters is the phase-locked loop (PLL) control subsystem that tracks the grid voltage's frequency and phase angle. Therefore, accurate fast-responding PLLs for control and protection purposes are required to provide these measurements. This paper proposes a novel feedback mechanism for single-phase PLL phase detectors using the estimated phase angle. Ripple noise appearing in the estimated frequency, most commonly the second harmonic under phase-lock conditions, is reduced or eliminated without the use of low-pass filters, which can cause delays to occur and limits the overall performance of the PLL response to dynamic changes in the system. The proposed method has the capability to eliminate the noise ripple entirely and, under extreme line distortion conditions, can reduce the ripple by at least half. Other modifications implemented through frequency feedback are shown to decrease the settling time of the PLL up to 50%. Mathematical analyses with the simulated and experimental results are provided to confirm the validity of the proposed methods.

  • Dong Jiang; Rixin Lai; Fei Wang; Fang Luo; Shuo Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2011

    arrow_drop_down

    The problem of electromagnetic interference (EMI) plays an important role in the design of power electronic converters, especially for airplane electrical systems. This paper explores techniques to reduce EMI noise in three-phase active front-end rectifier. The Vienna-type rectifier is used as the object. The design approach introduced in this paper is using a high-density EMI filter to satisfy the EMI standard. Design methodology is introduced in the paper by a three-stage LC- LC-L filter structure. In particular, the cause of high noise at high frequencies is studied in experiments, and the coupling effect of the final-stage capacitor and inductors is investigated. In order to reduce the EMI noise in the mid-frequency range, the application of random pulsewidth modulation (PWM) is also presented. The performance of random PWM in a Vienna-type rectifier is verified by theoretical analysis and experimental results. The approaches discussed in this paper significantly reduce the EMI noise in the Vienna-type rectifier, and therefore, the filter size can also be reduced.

  • Dingrong Yi; Chao Wang; Hairong Qi; Linghua Kong; Fengtao Wang; Ali Adibi
    IEEE Transactions on Biomedical Engineering
    2011

    arrow_drop_down

    Multispectral imaging (MSI) is becoming a powerful tool for tissue abnormality detection. Conventional MSI systems, however, are not readily suitable for challenges of routine clinical uses due to the fact that they are expensive, bulky, and time consuming to acquire the data. In this letter we report a novel approach to instrument MSI technology into a handheld, low-cost, standing-alone, real-time operational device that is suitable for home-based health care. It covers techniques used to produce multiple images at discrete signature wavelengths of tissues with a single shot.

  • Dong Dong; Timothy Thacker; Rolando Burgos; Fei Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2011

    arrow_drop_down

    This paper comprehensively investigates and compares different multiloop linear control schemes for single-phase pulsewidth modulation inverters, both in stationary and synchronous (d -q) frames, by focusing on their steady-state error under different loading conditions. Specifically, it is shown how proportional plus resonant (P + R) control and load current feedback (LCF) control can, respectively, improve the steady-state and transient performance of the inverter, leading to the proposal of a PID + R + LCF control scheme. Furthermore, the LCF control and capacitive current feedback control schemes are shown to be subject to stability issues under second and higher order filter loads. Additionally, the equivalence between the stationary frame and d -q frame controllers is discussed depending on the orthogonal term generation method, and a d-q frame voltage control strategy is proposed eliminating the need for the generation of this orthogonal component. This is achieved while retaining all the advantages of operating in the synchronous d-q frame, i.e., zero steady-state error and ease of implementation. All theoretical findings are validated experimentally using a 1.5 kW laboratory prototype.

  • Puqi Ning; Fred Wang; Khai D. T. Ngo
    IEEE Transactions on Power Electronics
    2011

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    To take full advantage of silicon carbide semiconductor devices, high-temperature device packaging needs to be developed. This paper describes potential defects from design and fabrication procedures, and presents a systematic electrical evaluation process to detect such defects. This systematic testing procedure can rapidly detect many defects and reduce the risk in high-temperature packaging testing. A multichip module development procedure that uses this testing procedure is also presented and demonstrated with an example.

  • Ruxi Wang; Fei Wang; Dushan Boroyevich; Rolando Burgos; Rixin Lai; Puqi Ning; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2011

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    It is well known that single-phase pulse width modulation rectifiers have second-order harmonic currents and corresponding ripple voltages on the dc bus. The low-frequency harmonic current is normally filtered using a bulk capacitor in the bus, which results in low power density. However, pursuing high power density in converter design is a very important goal in the aerospace applications. This paper studies methods for reducing the energy storage capacitor for single-phase rectifiers. The minimum ripple energy storage requirement is derived independently of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as the ripple energy storage circuit, which can effectively reduce the energy storage capacitance. The analysis and design are validated by simulation and experimental results.

  • Hui Zhang; Leon M. Tolbert; Burak Ozpineci
    IEEE Transactions on Industry Applications
    2011

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    The application of silicon carbide (SiC) devices as battery interface, motor controller, etc., in a hybrid electric vehicle (HEV) will be beneficial due to their high-temperature capability, high-power density, and high efficiency. Moreover, the light weight and small volume will affect the whole powertrain system in a HEV and, thus, the performance and cost. In this paper, the performance of HEVs is analyzed using the vehicle simulation software Powertrain System Analysis Toolkit (PSAT). Power loss models of a SiC inverter based on the test results of latest SiC devices are incorporated into PSAT powertrain models in order to study the impact of SiC devices on HEVs from a system standpoint and give a direct correlation between the inverter efficiency and weight and the vehicle's fuel economy. Two types of HEVs are considered. One is the 2004 Toyota Prius HEV, and the other is a plug-in HEV (PHEV), whose powertrain architecture is the same as that of the 2004 Toyota Prius HEV. The vehicle-level benefits from the introduction of SiC devices are demonstrated by simulations. Not only the power loss in the motor controller but also those in other components in the vehicle powertrain are reduced. As a result, the system efficiency is improved, and vehicles that incorporate SiC power electronics are predicted to consume less energy and have lower emissions and improved system compactness with a simplified thermal management system. For the PHEV, the benefits are even more distinct; in particular, the size of the battery bank can be reduced for optimum design.

  • Hui Zhang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2011

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    Power electronics is an enabling technology found in most renewable energy generation systems. Because of its superior voltage blocking capabilities and fast switching speeds, silicon carbide (SiC) power electronics are considered for use in power conversion units in wind generation systems in this paper. The potential efficiency gains from the use of SiC devices in a wind generation system are explored by simulations, with the system modeling explained in detail. The performance of the SiC converter is analyzed and compared to its silicon counterpart at different wind speeds, temperatures, and switching frequencies. The quantitative results are based on SiC metal-oxide-semiconductor field-effect transistor (MOSFET) prototypes from Cree and modern Si insulated-gate bipolar transistor (IGBT) products. A conclusion is drawn that the SiC converters can improve the wind system power conversion efficiency and can reduce the system's size and cost due to the low-loss, high-frequency, and high-temperature properties of SiC devices, even for one-for-one replacement for Si devices.

  • 2011

    arrow_drop_down

    This paper presents a control method to limit the common-mode (CM) circulating current between paralleled three-phase two-level voltage-source converters (VSCs) with discontinuous space-vector pulsewidth modulation (DPWM) and interleaved switching cycles. This CM circulating current can be separated into two separate components based on their frequency; the high-frequency component, close to the switching frequency, can be effectively limited by means of passive components; the low-frequency component, close to the fundamental frequency, embodies the jumping CM circulating current observed in parallel VSCs. This is the main reason why it is usually recommended not to implement discontinuous and interleaving PWM together. The origin of this low-frequency circulating current is analyzed in detail, and based on this, a method to eliminate its presence is proposed by impeding the simultaneous use of different zero vectors between the converters. This control method only requires six additional switching actions per line cycle, presenting a minimum impact on the converter thermal design. The analysis and the feasibility of the control method are verified by simulation and experimental results.

  • Timothy Thacker; Dushan Boroyevich; Rolando Burgos; Fei Wang
    IEEE Transactions on Industrial Electronics
    2011

    arrow_drop_down

    A crucial component of grid-connected converters is the phase-locked loop (PLL) control subsystem that tracks the grid voltage's frequency and phase angle. Therefore, accurate fast-responding PLLs for control and protection purposes are required to provide these measurements. This paper proposes a novel feedback mechanism for single-phase PLL phase detectors using the estimated phase angle. Ripple noise appearing in the estimated frequency, most commonly the second harmonic under phase-lock conditions, is reduced or eliminated without the use of low-pass filters, which can cause delays to occur and limits the overall performance of the PLL response to dynamic changes in the system. The proposed method has the capability to eliminate the noise ripple entirely and, under extreme line distortion conditions, can reduce the ripple by at least half. Other modifications implemented through frequency feedback are shown to decrease the settling time of the PLL up to 50%. Mathematical analyses with the simulated and experimental results are provided to confirm the validity of the proposed methods.

  • Dong Jiang; Rixin Lai; Fei Wang; Fang Luo; Shuo Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2011

    arrow_drop_down

    The problem of electromagnetic interference (EMI) plays an important role in the design of power electronic converters, especially for airplane electrical systems. This paper explores techniques to reduce EMI noise in three-phase active front-end rectifier. The Vienna-type rectifier is used as the object. The design approach introduced in this paper is using a high-density EMI filter to satisfy the EMI standard. Design methodology is introduced in the paper by a three-stage LC- LC-L filter structure. In particular, the cause of high noise at high frequencies is studied in experiments, and the coupling effect of the final-stage capacitor and inductors is investigated. In order to reduce the EMI noise in the mid-frequency range, the application of random pulsewidth modulation (PWM) is also presented. The performance of random PWM in a Vienna-type rectifier is verified by theoretical analysis and experimental results. The approaches discussed in this paper significantly reduce the EMI noise in the Vienna-type rectifier, and therefore, the filter size can also be reduced.

  • Dingrong Yi; Chao Wang; Hairong Qi; Linghua Kong; Fengtao Wang; Ali Adibi
    IEEE Transactions on Biomedical Engineering
    2011

    arrow_drop_down

    Multispectral imaging (MSI) is becoming a powerful tool for tissue abnormality detection. Conventional MSI systems, however, are not readily suitable for challenges of routine clinical uses due to the fact that they are expensive, bulky, and time consuming to acquire the data. In this letter we report a novel approach to instrument MSI technology into a handheld, low-cost, standing-alone, real-time operational device that is suitable for home-based health care. It covers techniques used to produce multiple images at discrete signature wavelengths of tissues with a single shot.

  • Dong Dong; Timothy Thacker; Rolando Burgos; Fei Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2011

    arrow_drop_down

    This paper comprehensively investigates and compares different multiloop linear control schemes for single-phase pulsewidth modulation inverters, both in stationary and synchronous (d -q) frames, by focusing on their steady-state error under different loading conditions. Specifically, it is shown how proportional plus resonant (P + R) control and load current feedback (LCF) control can, respectively, improve the steady-state and transient performance of the inverter, leading to the proposal of a PID + R + LCF control scheme. Furthermore, the LCF control and capacitive current feedback control schemes are shown to be subject to stability issues under second and higher order filter loads. Additionally, the equivalence between the stationary frame and d -q frame controllers is discussed depending on the orthogonal term generation method, and a d-q frame voltage control strategy is proposed eliminating the need for the generation of this orthogonal component. This is achieved while retaining all the advantages of operating in the synchronous d-q frame, i.e., zero steady-state error and ease of implementation. All theoretical findings are validated experimentally using a 1.5 kW laboratory prototype.

  • Silicon carbide (SiC)-based field effect transistors (FETs) are gaining popularity as switching elements in power electronic circuits designed for high-temperature environments like hybrid electric vehicle, aircraft, well logging, geothermal power generation etc. Like any other power switches, SiC-based power devices also need gate driver circuits to interface them with the logic units. The placement of the gate driver circuit next to the power switch is optimal for minimising system complexity. Successful operation of the gate driver circuit in a harsh environment, especially with minimal or no heat sink and without liquid cooling, can increase the power-to-volume ratio as well as the power-to-weight ratio for power conversion modules such as a DC-DC converter, inverter etc. A silicon-on-insulator (SOI)-based high-voltage, high-temperature integrated circuit (IC) gate driver for SiC power FETs has been designed and fabricated using a commercially available 0.8--m, 2-poly and 3-metal bipolar-complementary metal oxide semiconductor (CMOS)-double diffused metal oxide semiconductor (DMOS) process. The prototype circuit-s maximum gate drive supply can be 40-V with peak 2.3-A sourcing/sinking current driving capability. Owing to the wide driving range, this gate driver IC can be used to drive a wide variety of SiC FET switches (both normally OFF metal oxide semiconductor field effect transistor (MOSFET) and normally ON junction field effect transistor (JFET)). The switching frequency is 20-kHz and the duty cycle can be varied from 0 to 100-. The circuit has been successfully tested with SiC power MOSFETs and JFETs without any heat sink and cooling mechanism. During these tests, SiC switches were kept at room temperature and ambient temperature of the driver circuit was increased to 200-C. The circuit underwent numerous temperature cycles with negligible performance degradation.

  • Fang Luo; Shuo Wang; Fei Wang; Dushan Boroyevich; Nicolas Gazel; Yong Kang; Andrew Carson Baisden
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    Common-mode (CM) choke saturation is a practical problem in CM filter applications. It is generally believed that the leakage inductance of CM chokes makes the core saturated. This paper analyzes two new mechanisms for CM choke saturation due to CM voltage, and these mechanisms are verified in experiment. CM choke saturation is particularly important for motor drive systems, which have a high CM voltage and comparably higher stray grounding capacitance. A model is established to describe the relationship between the CM voltage and the volume of the CM magnetic components. According to the analysis, line impedance stabilization networks (LISNs) play an important role in the design of CM magnetic components.

  • Puqi Ning; Thomas Guangyin Lei; Fei Wang; Guo-Quan Lu; Khai D. T. Ngo; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper presents the design, development, and testing of a phase-leg power module packaged by a novel planar packaging technique for high-temperature (250°C) operation. The nanosilver paste is chosen as the die-attach material as well as playing the key functions of electrically connecting the devices' pads. The electrical characteristics of the SiC-based power semiconductors, SiC JFETs, and SiC Schottky diodes have been measured and compared before and after packaging. No significant changes (<;5%) are found in the characteristics of all the devices. Prototype module is fabricated and operated up to 400 V, 1.4 kW at junction temperature of 250°C in the continuous power test. Thermomechanical robustness has also been investigated by passive thermal cycling of the module from -55°C to 250°C. Electrical and mechanical performances of the packaged module are characterized and considered to be reliable for at least 200 cycles.

  • Shuo Wang; Yoann Yorrick Maillet; Fei Wang; Rixin Lai; Fang Luo; Dushan Boroyevich
    IEEE Transactions on Industrial Electronics
    2010

    arrow_drop_down

    High-frequency common-mode (CM) electromagnetic-interference (EMI) noise is difficult to suppress in electronics systems. EMI filters are used to suppress CM noise, but their performance is greatly affected by the parasitic effects of the grounding paths. In this paper, the parasitic effects of the grounding paths on an EMI filter's performance are investigated in a motor-drive system. The effects of the mutual inductance between two grounding paths are explored. Guidelines for the grounding of CM EMI filters are derived. Simulations and experiments are finally carried out to verify the theoretical analysis.

  • Rixin Lai; Fei Wang; Rolando Burgos; Dushan Boroyevich; Di Zhang; Puqi Ning
    IEEE Transactions on Industry Applications
    2010

    arrow_drop_down

    The SiC JFET is an attractive semiconductor device due to its superior switching performance and high-temperature operating capability. Its shoot-through protection remains a challenge due to the limited practical knowledge existent on this device and due to its inherent normally on nature. Addressing this limitation, this paper presents a novel shoot-through protection scheme in which a bidirectional switch, compounded by a Si insulated-gate bipolar transistor (IGBT) and a relay,is embedded into the dc-link midpoint in order to detect and clear shoot-through faults, taking advantage of the well-known desaturation protection schemes of IGBTs to protect SiC JFETs. This paper describes in detail the proposed protection mechanism and its circuit design, presenting as well the experimental results that verified the effectiveness of the proposed scheme using, first, Si MOSFETs and second, a 10-kW ac-ac converter system using SiC JFETs.

  • Andrew Carson Baisden; Dushan Boroyevich; Fei Wang
    IEEE Transactions on Industry Applications
    2010

    arrow_drop_down

    Terminal models have been used for various applications. In this paper, a three-terminal model is proposed for electromagnetic-interference (EMI) characterization. The model starts with a power electronic system at a particular operating condition and creates a unique linearized equivalent circuit. Impedances and current/voltage sources define the noise throughout the entire EMI frequency spectrum. All parameters needed to create the model are clearly defined to ensure convergence and maximize accuracy. In addition, the accuracy of the model is confirmed up to 100 MHz for a dc-dc boost converter using both simulation and experimental validation.

  • Faisal H. Khan; Leon M. Tolbert; William E. Webb
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper will present the analytical proof of concept of the multilevel modular capacitor-clamped converter (MMCCC). The quantitative analysis of the charge transfer mechanism among the capacitors of the MMCCC explains the start-up and steady-state voltage balancing. Once these capacitor voltages are found for different time intervals, the start-up and steady-state voltages at various nodes of the MMCCC can be obtained. This analysis provides the necessary proof that explains the stable operation of the converter when a load is connected to the low-voltage side of the circuit. In addition, the analysis also shows how the LV side of the converter is (1/N)th of the HV side excitation when the conversion ratio of the circuit is N. In addition to the analytical and simulation results, experimental results are included to support the analytical proof of concept.

  • Rixin Lai; Yoann Maillet; Fred Wang; Shuo Wang; Rolando Burgos; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This letter presents a novel integration approach for the electromagnetic interference choke. A low-permeability differential-mode (DM) choke is placed within the open window of the common-mode (CM) choke. Both chokes share the same winding structure. With the proposed approach, the footprint of inductors is greatly reduced, and high-DM inductance can be achieved. First, small-signal measurement is carried out to demonstrate the design concept and the symmetry of the proposed structure. Then large-signal experimental results verify the attenuation characteristics, as well as the thermal performance.

  • Shuo Wang; Yoann Yorrick Maillet; Fei Wang; Dushan Boroyevich; Rolando Burgos
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper begins with an analysis of the common-mode (CM) noise in a motor drive system. Based on the developed CM noise model, two cancellation techniques, CM noise voltage cancellation and CM noise current cancellation, are discussed. The constraints and impedance requirements for these two cancellation methods are investigated. An active filter with a feedforward current cancellation technique is proposed, implemented, and tested, and techniques to improve the performance of active filters are explored. It is found that due to the limitations of speed, power loss, and gain bandwidth of active filters, active electromagnetic interference (EMI) filters are not good at suppressing high di/dt or high amplitude noise current. Hybrid filters that include a passive filter and an active filter are proposed to overcome the shortcomings of active filters. Hybrid EMI filters are investigated based on the impedance requirements and frequency responses between the passive and active filters. The experiments show that the proposed active filter can greatly reduce noise by up to 50 dB at low frequencies (LFs), and therefore, the corner frequency of the passive filter can be increased considerably; as a result, the CM inductance of the passive filter is greatly reduced. The power loss of the proposed active EMI filter can be well-controlled in the experiments.

  • Y. Xu; L.M. Tolbert; J.D. Kueck; D.T. Rizy
    IET Power Electronics
    2010

    arrow_drop_down

    A three-phase insulated gate bipolar transistor (IGBT)-based static var compensator (STATCOM) is used for voltage and/or current unbalance compensation. An instantaneous power theory is adopted for real-time calculation and control. Three control schemes - current control, voltage control and integrated control - are proposed to compensate the unbalance of current, voltage or both. The compensation results of the different control schemes in unbalance cases (load current unbalance or voltage unbalance) are compared and analysed. The simulation and experimental results show that the control schemes can compensate the unbalance in load current or in the voltage source. Different compensation objectives can be achieved, that is, balanced and unity power factor source current, balanced and regulated voltage or both, by choosing appropriate control schemes.

  • Yoann Maillet; Rixin Lai; Shuo Wang; Fei Wang; Rolando Burgos; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper presents strategies to reduce both differential-mode (DM) and common-mode (CM) noise using a passive filter in a dc-fed motor drive. The paper concentrates on the type of grounding and the components to optimize filter size and performance. Grounding schemes, material comparison between ferrite and nanocrystalline cores, and a new integrated filter structure are presented. The integrated structure maximizes the core window area and increases the leakage inductance by integrating both CM and DM inductances onto one core. Small-signal and large-signal experiments validate the structure, showing it to have reduced filter size and good filtering performance when compared with standard filters at both low and high frequencies.

  • Fang Luo; Shuo Wang; Fei Wang; Dushan Boroyevich; Nicolas Gazel; Yong Kang; Andrew Carson Baisden
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    Common-mode (CM) choke saturation is a practical problem in CM filter applications. It is generally believed that the leakage inductance of CM chokes makes the core saturated. This paper analyzes two new mechanisms for CM choke saturation due to CM voltage, and these mechanisms are verified in experiment. CM choke saturation is particularly important for motor drive systems, which have a high CM voltage and comparably higher stray grounding capacitance. A model is established to describe the relationship between the CM voltage and the volume of the CM magnetic components. According to the analysis, line impedance stabilization networks (LISNs) play an important role in the design of CM magnetic components.

  • Puqi Ning; Thomas Guangyin Lei; Fei Wang; Guo-Quan Lu; Khai D. T. Ngo; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper presents the design, development, and testing of a phase-leg power module packaged by a novel planar packaging technique for high-temperature (250°C) operation. The nanosilver paste is chosen as the die-attach material as well as playing the key functions of electrically connecting the devices' pads. The electrical characteristics of the SiC-based power semiconductors, SiC JFETs, and SiC Schottky diodes have been measured and compared before and after packaging. No significant changes (<;5%) are found in the characteristics of all the devices. Prototype module is fabricated and operated up to 400 V, 1.4 kW at junction temperature of 250°C in the continuous power test. Thermomechanical robustness has also been investigated by passive thermal cycling of the module from -55°C to 250°C. Electrical and mechanical performances of the packaged module are characterized and considered to be reliable for at least 200 cycles.

  • Shuo Wang; Yoann Yorrick Maillet; Fei Wang; Rixin Lai; Fang Luo; Dushan Boroyevich
    IEEE Transactions on Industrial Electronics
    2010

    arrow_drop_down

    High-frequency common-mode (CM) electromagnetic-interference (EMI) noise is difficult to suppress in electronics systems. EMI filters are used to suppress CM noise, but their performance is greatly affected by the parasitic effects of the grounding paths. In this paper, the parasitic effects of the grounding paths on an EMI filter's performance are investigated in a motor-drive system. The effects of the mutual inductance between two grounding paths are explored. Guidelines for the grounding of CM EMI filters are derived. Simulations and experiments are finally carried out to verify the theoretical analysis.

  • Rixin Lai; Fei Wang; Rolando Burgos; Dushan Boroyevich; Di Zhang; Puqi Ning
    IEEE Transactions on Industry Applications
    2010

    arrow_drop_down

    The SiC JFET is an attractive semiconductor device due to its superior switching performance and high-temperature operating capability. Its shoot-through protection remains a challenge due to the limited practical knowledge existent on this device and due to its inherent normally on nature. Addressing this limitation, this paper presents a novel shoot-through protection scheme in which a bidirectional switch, compounded by a Si insulated-gate bipolar transistor (IGBT) and a relay,is embedded into the dc-link midpoint in order to detect and clear shoot-through faults, taking advantage of the well-known desaturation protection schemes of IGBTs to protect SiC JFETs. This paper describes in detail the proposed protection mechanism and its circuit design, presenting as well the experimental results that verified the effectiveness of the proposed scheme using, first, Si MOSFETs and second, a 10-kW ac-ac converter system using SiC JFETs.

  • Andrew Carson Baisden; Dushan Boroyevich; Fei Wang
    IEEE Transactions on Industry Applications
    2010

    arrow_drop_down

    Terminal models have been used for various applications. In this paper, a three-terminal model is proposed for electromagnetic-interference (EMI) characterization. The model starts with a power electronic system at a particular operating condition and creates a unique linearized equivalent circuit. Impedances and current/voltage sources define the noise throughout the entire EMI frequency spectrum. All parameters needed to create the model are clearly defined to ensure convergence and maximize accuracy. In addition, the accuracy of the model is confirmed up to 100 MHz for a dc-dc boost converter using both simulation and experimental validation.

  • Faisal H. Khan; Leon M. Tolbert; William E. Webb
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper will present the analytical proof of concept of the multilevel modular capacitor-clamped converter (MMCCC). The quantitative analysis of the charge transfer mechanism among the capacitors of the MMCCC explains the start-up and steady-state voltage balancing. Once these capacitor voltages are found for different time intervals, the start-up and steady-state voltages at various nodes of the MMCCC can be obtained. This analysis provides the necessary proof that explains the stable operation of the converter when a load is connected to the low-voltage side of the circuit. In addition, the analysis also shows how the LV side of the converter is (1/N)th of the HV side excitation when the conversion ratio of the circuit is N. In addition to the analytical and simulation results, experimental results are included to support the analytical proof of concept.

  • Rixin Lai; Yoann Maillet; Fred Wang; Shuo Wang; Rolando Burgos; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This letter presents a novel integration approach for the electromagnetic interference choke. A low-permeability differential-mode (DM) choke is placed within the open window of the common-mode (CM) choke. Both chokes share the same winding structure. With the proposed approach, the footprint of inductors is greatly reduced, and high-DM inductance can be achieved. First, small-signal measurement is carried out to demonstrate the design concept and the symmetry of the proposed structure. Then large-signal experimental results verify the attenuation characteristics, as well as the thermal performance.

  • Shuo Wang; Yoann Yorrick Maillet; Fei Wang; Dushan Boroyevich; Rolando Burgos
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper begins with an analysis of the common-mode (CM) noise in a motor drive system. Based on the developed CM noise model, two cancellation techniques, CM noise voltage cancellation and CM noise current cancellation, are discussed. The constraints and impedance requirements for these two cancellation methods are investigated. An active filter with a feedforward current cancellation technique is proposed, implemented, and tested, and techniques to improve the performance of active filters are explored. It is found that due to the limitations of speed, power loss, and gain bandwidth of active filters, active electromagnetic interference (EMI) filters are not good at suppressing high di/dt or high amplitude noise current. Hybrid filters that include a passive filter and an active filter are proposed to overcome the shortcomings of active filters. Hybrid EMI filters are investigated based on the impedance requirements and frequency responses between the passive and active filters. The experiments show that the proposed active filter can greatly reduce noise by up to 50 dB at low frequencies (LFs), and therefore, the corner frequency of the passive filter can be increased considerably; as a result, the CM inductance of the passive filter is greatly reduced. The power loss of the proposed active EMI filter can be well-controlled in the experiments.

  • Y. Xu; L.M. Tolbert; J.D. Kueck; D.T. Rizy
    IET Power Electronics
    2010

    arrow_drop_down

    A three-phase insulated gate bipolar transistor (IGBT)-based static var compensator (STATCOM) is used for voltage and/or current unbalance compensation. An instantaneous power theory is adopted for real-time calculation and control. Three control schemes - current control, voltage control and integrated control - are proposed to compensate the unbalance of current, voltage or both. The compensation results of the different control schemes in unbalance cases (load current unbalance or voltage unbalance) are compared and analysed. The simulation and experimental results show that the control schemes can compensate the unbalance in load current or in the voltage source. Different compensation objectives can be achieved, that is, balanced and unity power factor source current, balanced and regulated voltage or both, by choosing appropriate control schemes.

  • Yoann Maillet; Rixin Lai; Shuo Wang; Fei Wang; Rolando Burgos; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2010

    arrow_drop_down

    This paper presents strategies to reduce both differential-mode (DM) and common-mode (CM) noise using a passive filter in a dc-fed motor drive. The paper concentrates on the type of grounding and the components to optimize filter size and performance. Grounding schemes, material comparison between ferrite and nanocrystalline cores, and a new integrated filter structure are presented. The integrated structure maximizes the core window area and increases the leakage inductance by integrating both CM and DM inductances onto one core. Small-signal and large-signal experiments validate the structure, showing it to have reduced filter size and good filtering performance when compared with standard filters at both low and high frequencies.

  • A 5-kW multilevel modular capacitor-clamped DC-DC converter (MMCCC) with bi-directional power management and real-time fault bypassing capability will be presented in this study. The modular structure of the MMCCC topology was utilised to build this 5-kW converter with necessary redundancy and hot swap feature for industrial and automotive applications including a future plug-in hybrid or fuel-cell powered all electric vehicles. Moreover, the circuit has flexible conversion ratio that leads to establish bi-directional power management for automotive applications mitigating the boost voltage for the fuel-cell or dual battery architecture. In addition, the MMCCC exhibits better component utilisation compared to many capacitor-clamped or classical DC-DC converters based on inductive energy transfer mechanism. Thus, the MMCCC circuit can be made more compact and reliable compared to many other DC-DC converters for high-power applications.

  • A 5-kW multilevel modular capacitor-clamped DC-DC converter (MMCCC) with bi-directional power management and real-time fault bypassing capability will be presented in this study. The modular structure of the MMCCC topology was utilised to build this 5-kW converter with necessary redundancy and hot swap feature for industrial and automotive applications including a future plug-in hybrid or fuel-cell powered all electric vehicles. Moreover, the circuit has flexible conversion ratio that leads to establish bi-directional power management for automotive applications mitigating the boost voltage for the fuel-cell or dual battery architecture. In addition, the MMCCC exhibits better component utilisation compared to many capacitor-clamped or classical DC-DC converters based on inductive energy transfer mechanism. Thus, the MMCCC circuit can be made more compact and reliable compared to many other DC-DC converters for high-power applications.

Conference Papers
Title
Year
  • Jiaojiao Dong; Lin Zhu; Paychuda Kritprajun; Yilu Liu; Leon M. Tolbert; Joshua C. Hambrick; Kevin Schneider; Stuart Laval
    2020 IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT)
    2020

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    In this work, we develop a methodology and tool to quantitatively evaluate the reliability of a self-healing system that considers practical distribution system features such as the distributed energy resources, microgrids, and service restoration strategies. Also, this paper addresses various practical issues when being applied to an actual Duke Energy distribution system, including the design of feasible and practical service restoration strategies that are used to identify the customer interruptions after a fault, and the incorporation of the utility's historical reliability indices that are used to calibrate the failure rate and repair time of distribution system components such as overhead lines and underground cables. This case study demonstrates the effectiveness of the proposed method.

  • Handong Gui; Zheyu Zhang; Ruirui Chen; Ren Ren; Jiahao Niu; Bo Liu; Haiguo Li; Zhou Dong; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    With the development of wide band-gap (WBG) technology, the switching speed of power semiconductor devices increases, which makes circuits more sensitive to parasitics. For three-level active neutral point clamped (3L-ANPC) converters, the over-voltage caused by additional non-active switch loop can be an issue. This paper analyzes the multiple commutation loops in 3L-ANPC converter and summarizes the impact factors of the device over-voltage. The nonlinearity of the output capacitance of the device can significantly influence the over-voltage. A simple control without introducing additional hardware circuit or complex software algorithm is proposed to attenuate the effect of the nonlinear output capacitance. Multi-pulse test is conducted for a 3L-ANPC converter built with silicon carbide (SiC) MOSFETs. With the proposed control, the testing results show that the peak drain-source voltage of both active and non-active switches is reduced by more than 20% compared to the conventional control.

  • Li Zhang; Shiqi Ji; Shida Gu; Xingxuan Huang; James Palmer; William Giewont; Fred Wang; Leon M. Tolbert
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    The performance of the gate drive power supply (GDPS) greatly impacts the safety and reliability of the gate drive for power semiconductor power devices. This paper focuses on the design of isolated gate driver power supply for 10 kV silicon-carbide (SiC) MOSFET for medium-voltage (MV) applications. Different insulation schemes are compared for the high-voltage insulated transformer in GDPS. Impact factors for transformer interwinding capacitance are analyzed, with which, a low inter-winding capacitance design approach is proposed for the high-voltage insulated transformer. Furthermore, an upward-voltage-reference based voltage regulation scheme is proposed for achieving good load regulation without output voltage feedback. Finally, a 20-kV-insulated GDPS is built and tested, and experimental results are presented to verify the effectiveness of the design approach.

  • Li Zhang; Shiqi Ji; Xingxuan Huang; James Palmer; William Giewont; Fred Wang; Leon M. Tolbert
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    The newly emerged 10 kV MOSFETs will be a game changer for the next generation medium-voltage (MV) power converters. While the unique feature of the fast switching speed benefits the development of high efficiency and high power-density MV converters, high dv/dt will be imposed on the power devices and passive components, which will result in accelerated insulation degradation and extreme EMI. This paper investigates the high dv/dt resulting from dead-time insertion in the modular multilevel converter (MMC). For reducing dv/dt, a multiple-step commutation scheme is proposed, in which, the submodules that transfer from bypass-state to insert-state are one-by-one commutated with those transferring from insert-state to bypass-state before the remaining unpaired submodules. Finally, the proposed multiple-step commutation scheme is verified on a single-phase MMC with 10-kV SiC MOSFET.

  • James Palmer; Shiqi Ji; Xingxuan Huang; Li Zhang; William Giewont; Fred Fei Wang; Leon M. Tolbert
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    This paper focuses on the testing approach and validation of a 10 kV SiC MOSFET based Modular Multilevel Converter (MMC) phase-leg meant to interface to a 13.8 kV microgrid. Many of the difficulties associated with testing high-voltage SiC converters are shared regardless of topology, thus the testing setup and challenges associated with it are emphasized while the converter is only briefly summarized. Each component is individually verified, and the entire system is tested up to its rated dc link voltage and power, 25 kV and 35 kVA, respectively.

  • Handong Gui; Ruirui Chen; Ren Ren; Jiahao Niu; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock; Benjamin B. Choi
    2019 20th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2019

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    An analytical model for the device drain-source turn-on overvoltage in three-level active neutral point clamped (3L-ANPC) converters is established in this paper. Considering the two commutation loops in the converter, the relationship between the turn-on overvoltage and the loop inductances is evaluated. The line switching frequency device usually exhibits higher overvoltage, while the high switching frequency device is not strongly influenced by the multiple loops. A 500 kVA 3L-ANPC converter using SiC MOSFETs is tested, and the model is verified with the experimental results.

  • Jiahao Niu; Ruirui Chen; Zheyu Zhang; Handong Gui; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    In paralleled voltage source inverters (VSI), circulating current has both high frequency and low frequency components, and its spectrum highly depends on the modulation scheme. Previous research has mostly focused on the circulating current suppression for paralleled two-level VSIs. Little literature exists on similar analysis for paralleled three-level VSIs using space vector modulation. A detailed circulating current spectrum on full frequency range has not been well developed. This paper presents an improved analytical model for three-level space vector modulation (SVM), considering the impacts of regularly sampled reference and dead time. Then, circulating harmonic currents are determined across the full frequency range for various interleaving angles of two three-level ANPC inverters. The calculated harmonics are also verified by experimental results.

  • Handong Gui; Zheyu Zhang; Ruirui Chen; Jiahao Niu; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    With conventional voltage source gate drives (VSG), the switching speed of SiC MOSFETs is difficult to increase due to large internal gate resistance, high Miller voltage, and limited gate voltage rating. This paper analyzes the requirement of current source gate drive (CSG) for SiC MOSFETs and proposes a CSG that can improve the switching speed and reduce switching loss. With the introduction of bi-directional switches, the influence of the large internal gate resistance of the SiC MOSFET can be mitigated, and sufficient gate current can be guaranteed throughout the switching transient. Therefore, the switching time and loss is reduced. The CSG can be controlled to be a VSG during steady state so the current of the gate drive is discontinuous and the stored energy of the inductor can be returned to the power supply to reduce gate drive loss. Double pulse tests are conducted for a SiC MOSFET with both conventional VSG and the proposed CSG. Testing results show that the switching loss of the proposed CSG is less than one third of the conventional VSG at full load condition.

  • Ruirui Chen; Jiahao Niu; Handong Gui; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    The four-leg topology has been applied to two-level inverters for common-mode (CM) noise elimination. To achieve zero common-mode voltage (CMV), the zero vector typically used in the two-level inverter is not allowed. As a result, the reference cannot be synthesized by nearest three vectors, which introduces a penalty in dc voltage utilization and current THD. This paper applies the fourth-leg to three-level neutral point clamped (NPC) inverter fed motor drives. Unlike the case in the two-level inverter, the reference can be synthesized by the nearest three vectors while zero CMV can be achieved at the same time in a three-level inverter with the fourth-leg. The topology and modulation are presented. The fourth-leg filter structures are investigated, and a fourth-leg filter structure which decouples the fourth-leg from the main circuit power level is proposed for high power applications. The experiment results on a three-level NPC inverter show that with the fourth-leg and presented modulation applied, the CM noise has been significantly reduced, and around 25 dB attenuation can be observed at the first noise peak in the electromagnetic interference (EMI) frequency range.

  • Jiahao Niu; Ruirui Chen; Zheyu Zhang; Handong Gui; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    Paralleling power electronics inverters is an effective way to increase dc-ac system power level. Accurately synchronized switching action and independent closed-loop regulator are necessary to prevent circulating current in paralleled inverters. There are many challenges for the controller design, when the number of paralleled inverters is large, and control period gets short for high switching frequency applications. This paper presents a single controller design based on DSP + FPGA that is suitable for paralleling multiple inverters. A simple synchronization scheme between DSP and FPGA based on universal parallel port (UPP) is proposed to eliminate the synchronization delay among inverters, and independent control of each converter can also be implemented. The controller is built for a system consisting of 4 paralleled three-level, three-phase high frequency ANPC inverters using space vector modulation, and it can be easily adopted to other topologies and modulations. Experimental results have demonstrated the effectiveness of this controller.

  • Ruirui Chen; Jiahao Niu; Handong Gui; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    This paper presents a comprehensive analytical analysis of the ac and dc side harmonics of the three-level active neutral point clamped (ANPC) inverter with space vector modulation (SVM) scheme. An analytical model to calculate the harmonics of a three-level converter with SVM is developed. The ac side output voltage harmonics and dc side current harmonics characteristics are calculated and analyzed. With the developed models, the impact of interleaving on both sides harmonics are studied which considers the modulation index, interleaving angle, and power factor. The analysis provides guideline for interleaving angle optimization to reduce the ac side power filter and dc side dc-link capacitor. The relationship between electromagnetic interference (EMI) filter corner frequency and switching frequency is also analytically derived which provides guideline for switching frequency and EMI filter design optimization. Two paralleled three-level ANPC inverters are constructed and experimental results are presented to verify the analytical analysis.

  • Ruirui Chen; Jiahao Niu; Handong Gui; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    This paper presents the coupled inductor design for interleaved three-level active neutral point clamped (ANPC) inverter considering electromagnetic interference (EMI) noise reduction. Compared to two-level case, the scenarios involved in the three-level space vector modulation (SVM) are more complicated when analyzing the volt-seconds of the coupled inductor for paralleled three-level inverter. At system level, the purpose of converter interleaving is to reduce EMI noise and ripple current in most applications, and coupled inductor design should consider the needs of EMI noise reduction and EMI filter design. These issues are discussed in this paper. The relationship between circulating current and EMI noise is illustrated. EMI filter corner frequency as a function of interleaving angle is analytically derived, and optimal interleaving angle for maximum common-mode (CM) filter and differential-mode (DM) filter corner frequencies is discussed. Coupled inductor design methodology for interleaved three-level inverters with SVM is then presented. Experiments on two interleaved ANPC inverters are conducted. The results verify the coupled inductor design. With the derived optimal interleaving angle, the CM and DM EMI noise are significantly reduced.

  • Dingrui Li; Yiwei Ma; Chengwen Zhang; He Yin; Ishita Ray; Yu Su; Lin Zhu; Fred Wang; Leon M. Tolbert
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    To overcome the limitations of digital simulation (numerical oscillation, limited computing capability of processors, etc.), a converter-based hardware test-bed was developed at CURENT for real-time power grid emulation. However, distribution systems especially microgrids cannot be emulated and tested on the existing hardware test-bed. This paper develops a microgrid test-bed based on the existing hardware test-bed to enable controller testing for microgrids with dynamic boundary. The design and realization of the microgrid hardware test-bed are introduced. The experimental results of the microgrid controller tests are also provided.

  • Handong Gui; Ruirui Chen; Jiahao Niu; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock; Benjamin B. Choi
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    The adoption of SiC devices in high power applications enables higher switching speed, which requires lower circuit parasitic inductance to reduce the voltage overshoot. This paper presents the design of a busbar for a 500 kVA three-level active neutral point clamped (ANPC) converter. The layout of the busbar is discussed in detail based on the analysis of the multiple commutation loops, magnetic canceling effect, and DC-link capacitor placement. The loop inductance of the busbar is verified with simulation, impedance measurements, and converter experiments. The results match with each other, and the inductances of small and large loop are 6.5 nH and 17.5 nH respectively, which is significantly lower than the busbars of NPC type converters in other references.

  • Ruirui Chen; Jiahao Niu; Handong Gui; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    This paper presents harmonic analysis of common-mode reduction (CMR) modulation for three-level voltage source inverters. The analytical model to calculate the harmonics of CMR modulation with arbitrary PWM sequence is developed. The impact of alternative PWM sequences of CMR modulation on harmonics is investigated. New three-state and four-state PWM sequences of CMR are proposed which spread the energy centered in the carrier frequency in the conventional CMR, and thus reduce the voltage peaks in frequency domain. Experiments are conducted on a three-level neutral point clamped inverter. Experiment results verify the developed analytical model and harmonic analysis.

  • Fred Wang; Ruirui Chen; Handong Gui; Jiahao Niu; Leon Tolbert; Daniel Costinett; Benjamin Blalock; Shengyi Liu; John Hull; John Williams; Timothy Messer; Eugene Solodovnik; Darren Paschedag; Vyacheslav Khozikov; Christopher Severns; Benjamin Choi
    2019 AIAA/IEEE Electric Aircraft Technologies Symposium (EATS)
    2019

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    High power inverters will be a key enabler for future aircraft based on hybrid electric or turbo-electric propulsion as envisioned by NASA and Boeing. Cooling a power electronics converter to low temperature, e.g. using cryogenic cooling, can significantly improve the efficiency and power density of a power conversion system. This paper presents the design of a MW cryogenically-cooled power inverter for electric aircraft applications. The power semiconductor and magnetic component characterization, inverter topology and power stage design, modulation and control, EMI noise reduction and filters design, and cooling system design are illustrated. A MW-level inverter prototype has been assembled and tested. The experimental results verify the functionality of the inverter.

  • Le Kong; Shuyao Wang; Nattapat Praisuwanna; Shuoting Zhang; Liang Qiao; Fred Wang; Leon M. Tolbert
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    The modular multilevel converter (MMC) is regarded as a competitive choice for future dc grids at high and medium voltage levels. To study the dc grid stability, a few MMC dc impedance models have been proposed, but their accuracy is limited by the assumption of ideal submodule voltage or neglecting the circulating current control. In this paper, a more accurate MMC dc impedance model is developed considering both the submodule capacitor voltage and circulating current dynamics. Simulation and experiment results indicate that the developed model matches with the actual MMC better than state-of-art models and can predict the dc system stability correctly.

  • Shiqi Ji; Xingxuan Huang; Li Zhang; James Palmer; William Giewont; Fred Wang; Leon M. Tolbert
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    Medium voltage (MV) power converters using high voltage (HV) Silicon Carbide (SiC) power semiconductors result in great benefits in weight, size, efficiency and control bandwidth. However, challenges still exist on the components design considering HV insulation and noise immunity requirements in the MV SiC based power converter. A 5-level MMC based transformer-less grid-connected dc/ac converter is developed for 13.8 kV medium voltage grid using 10 kV SiC MOSFETs. The key components, including gate driver with high dv/dt immunity and fast reliable protection, isolated power supply with low parasitic capacitance, voltage/current sensors with high noise immunity, and passives following related insulation standard are provided. A 25 kV dc-link phase-leg is demonstrated, and the experimental results are presented.

  • Yang Song; Zhifei Zhang; Razieh Kaviani Baghbaderani; Fanqi Wang; Ying Qu; Craig Stuttsy; Hairong Qi
    2019 10th Workshop on Hyperspectral Imaging and Signal Processing: Evolution in Remote Sensing (WHISPERS)
    2019

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    Land cover classification of satellite imagery can provide significant information for many applications, including surface analysis, environmental monitoring, building reconstruction, etc. Land cover classification has been generally performed using unmixing-based or shallow/deep learning approaches, among which the unmixing-based approaches suffer from stability issues due to the complex intrinsic properties of the data, deep learning-based approaches like 2D CNN requires large labeled training set which is often unavailable in satellite images and small ground truth collection leads to spatial discontinuities (as shown in Fig. 1), making 2D CNN approaches unviable. In this paper, we first propose a 1D convolution neural network-based framework applied to each pixel in the spectral domain where we extract descriptive local features for improved classification. Experimental results demonstrate superior classification accuracy through comparison with traditional unmixing-based and neural network methods using just limited number of training samples.

  • Nathan N. Strain; Jingjing Sun; Xingxuan Huang; Daniel J. Costinett; Leon M. Tolbert
    2019 IEEE 7th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2019

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    Data center energy usage is expected to grow in the coming years with the proliferation of cloud services on daily life. The increasing energy demands that data centers will place upon the power grid necessitate the development high efficiency, high power density power supplies. The LLC resonant converter has long been utilized for data center power supplies as the dc-dc transformer to step down the voltage rectified from the ac transmission system to the rack level. As more is demanded of the data center and more information must be processed, space becomes more valuable. The use of wide bandgap materials such as Gallium Nitride (GaN) allows for much faster switching which can lead to higher power density by reducing the size of passive components. However, a higher frequency can lead to much greater switching loss. Zero-voltage switching (ZVS) can be utilized in primary side devices to greatly reduce losses. The achievement of ZVS is dependent on the magnetic design of the LLC transformer. By considering the effects of device capacitance and transformer parasitic capacitance and inductance, ZVS can be achieved to negate the turn-on losses and ensure high efficiency. This paper details the analysis of ZVS for a GaN-based LLC resonant converter with two series-parallel connected transformers.

  • Jingjing Sun; Xingxuan Huang; Nathan N. Strain; Daniel J. Costinett; Leon M. Tolbert
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    This paper details the inductor design and zero-voltage-switching (ZVS) control of a single-phase GaN-based critical-conduction-mode (CRM) totem-pole rectifier with power factor correction (PFC). A full-line-cycle ZVS strategy is derived, and an analytical converter model with ZVS margin is proposed. The boost inductor design is critical for the operation performance of the CRM totem-pole PFC. Based on analytical loss models, the inductor is designed and implemented using a toroidal powder core and litz wire to minimize converter loss and inductor size. Digital on-time control with real-time calculation and zero current detection (ZCD) is used to implement CRM. A 1.5 kW single-phase GaN-based CRM totem-pole PFC prototype is built and tested. With the on-time control, both the inductor current and the output voltage are well regulated. ZVS is realized for the whole line cycle, and the tested efficiency is 98.8% at full load.

  • Xingxuan Huang; James Palmer; Shiqi Ji; Li Zhang; Fred Wang; Leon M. Tolbert; William Giewont
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    This paper focuses on the design and testing of a half bridge submodule based on discrete 10 kV/20 A SiC MOSFETs for a modular multilevel converter (MMC). The rated dc bus voltage of the submodule is 6.5 kV, and the MOSFETs switch with dv/dt up to 100 V/ns. Design considerations and challenges for components in the submodule are presented, especially the gate driver to support the robust continuous operation and the submodule voltage sensor. Systematic testing procedures are developed to fully test the submodule up to 6.5 kV. High voltage insulation and high dv/dt are tackled throughout the design and testing. The designed submodule is validated in the continuous switching test at 6.5 kV with dv/dt up to 100 V/ns.

  • Zhe Yang; Paige Williford; Edward A Jones; Jianliang Chen; Fred Wang; Sandeep Bala; Jing Xu
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    The paper aims to resolve the converter loss discrepancy between calculation and testing results by considering five factors: impacts of parasitic capacitance on switching loss, parameter variation of devices, time-varying power dissipation and junction temperature, thermal modeling using Finite Element Analysis, and some practical issues of the passive components. A 4.5 kW hard-switching inverter prototype using GaN devices was used as an example to demonstrate the improvement of loss model. The results show that after considering the above factors, loss discrepancy reduces from 35.7 W (35%) to 1.4 W (4%) at heavy load and, from 3.9 W (28%) to 2.6 W (less than 16%) at light load.

  • Jingjing Sun; Nathan N. Strain; Daniel J. Costinett; Leon M. Tolbert
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    High-frequency soft-switched gallium-nitride (GaN) based critical conduction mode (CRM) totem-pole power factor correction (PFC) converter is one of the most potential candidates in data center power supplies. However, the high-speed cycle-by-cycle zero current detection (ZCD) brings challenges to zero-voltage-switching (ZVS) control. Current sensing delay (CSD) exists, and the ZCD circuit is sensitive to high di/dt switching noise. In this paper, mechanisms of the ZCD time error are elaborated, and impacts of the current sensing delay on converter switching frequency, inductor current, input current third harmonic distortion (THD), and power loss are analyzed. Qualification time is added within the controller for immunity to the swiching noise, and a CSD embedded converter model is proposed to compensate the ZCD time delay. Also, loss modeling of the CRM totem-pole PFC is conducted to aid in analysis of the proposed theory. A 1.5 kW single-phase CRM totem-pole PFC prototype is tested. Experimental results validiate the analysis, modeling, and the proposed compensation method for current sensing delay.

  • Bo Liu; Ren Ren; Fred Wang; Daniel Costinett; Zheyu Zhang
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    In this paper, a variable frequency soft-switching control for a three-level half bridge (TLHB) buck converter is proposed to achieve wide-range output battery charging function without losing zero voltage switching (ZVS) or high efficiency. The adopted variable frequency triangle-current-modulation (TCM) is based on dc measurement and average-model calculation, thus able to realize ZVS operation fully digitally without current zero-crossing-detection (ZCD) circuits. A top-level average current or output voltage feedback controller further ensures the desired power or output voltage regulation. Experimental results from a GaN based TLHB prototype have shown the reliable TCM control and smooth transition of ZVS operation through the charging procedure.

  • Yiwei Ma; Lin Zhu; Fred Wang; Leon M. Tolbert
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    Virtual synchronous generator control provides a potential solution to connect power electronics interfaced sources to the power system. With its flexible control, the emulated inertia may be adaptively changed to enhance system small-signal stability. However, previously proposed methods may have adverse impact to system transient stability. This paper proposes to improve the methods by using wide-area measurements, namely center of inertia frequency. Experimental results are presented to both demonstrate the limitations of previous control schemes, and the effectiveness of the proposed one. The practical implementation of this method is discussed.

  • Jia Liang; Fanqi Wang; Xiaogang Lin; Hairong Qi; Jayne Wu
    2019 41st Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC)
    2019

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    Cysticercosis is a parasitic infection caused by adult tapeworms, and it constantly plagues the livelihoods of people from subsistence farming communities in developing countries. Diagnosis of Cysticercosis typically requires both central nervous system imaging and serological testing. The most common methods in serological testing are Enzyme-linked Immunosorbent Assay (ELISA) and Enzyme Immuno-electrotransfer Blot (EITB). Both ELISA and EITB methods are excessively time-consuming and labor-intensive. Recent research indicates that a shorter assay time and/or higher sensitivity can be achieved by integrating alternate current electrokinetics (ACEK) with biosensing. However, the raw time-series data is very noisy and the size of the dataset is extremely small, which would bring two potential challenges. On one hand, traditional statistical methods cannot extract features robust enough for high sensitivity as well as high specificity. On the other hand, the small data size limits the usage of automatic feature extractors such as deep neural networks. In this paper, we propose a linear unmixing based approach by exploiting the possibility that the time-series biological signals can be represented as linear combinations of source signals. This paper makes distinctive contributions to the field of bio-signal by introducing the unmixing model from the image processing domain to the time-series domain. Experimental results on the classification of Cysticercosis using 123 samples demonstrate the robustness and superior performance of the linear unmixing method over other conventional classifiers in handling small datasets.

  • Razieh Kaviani Baghbaderani; Fanqi Wang; Craig Stutts; Ying Qu; Hairong Qi
    IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium
    2019

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    Identifying land-cover and specifically the type of the material that constitutes building roofs in urban areas provides important reference information for later procedures including semantic labeling, bridge masking, and 3D reconstruction. In this paper, we present a hybrid unmixing-based classification framework that integrates both class-wise unsupervised unmixing and supervised unmixing that effectively convert the classification problem from the original spectral space to the abundance space, such that the intrinsic characteristics of each material can be better represented. Experimental results demonstrate competitive performance in terms of classification accuracy. In addition, we show that the proposed approach has the capability of handling new region of interest with similar scene content but different illumination geometry and atmospheric composition, which is crucial in classification of satellite images with a limited amount of training data.

  • Jared A. Baxter; Daniel J. Costinett
    2019 20th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2019

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    Modeling plays a vital role in the design of advanced power converters. Commonly, modeling is completed using either dedicated hand analysis, which must be completed individually for each topology, or time-stepping circuit simulations, which are insufficiently rapid for broad analysis considering a wide range of potential designs or operating points. Discrete time state-space modeling of switching converters has shown merits in rapid analysis and generality to arbitrary circuit topologies but is hampered by difficulty incorporating nonlinear elements. In this work, we investigate methods for the incorporation of nonlinear elements into a generalized discrete time state-space modeling framework and showcase the utility of the approach for use in the converter design process.

  • Kamal Sabi; Daniel Costinett
    2019 20th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2019

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    Control based modulation techniques such as Boundary Current Mode (BCM) modulation are used to achieve zero voltage switching (ZVS) and overcome the increased switching losses in power electronics operating at high frequency. A simple control approach to implementing BCM uses dual current programmed mode (DCPM) control, but this approach is highly susceptible to noise and propagation delays at high switching frequency. Propagation delays in the control network cause the inductor current to overshoot its reference by a margin which varies with the instantaneous inductor current slope. This overshoot results in increased losses and introduces low-frequency inductor current distortion, particularly in high switching frequency converters. This work addresses propagation delay challenges by tuning the current sensing circuitry to mitigate impact of sensing delay, resulting in an inherent cancellation of sensing delay without additional control complexity. This approach is simple to implement and offers a flexible current control design for BCM modulation. The operation of this proposed compensation technique is demonstrated experimentally in a GaN-based full bridge inverter.

  • Wenchao Cao; Yiwei Ma; Fred Wang
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    This paper proposes an adaptive control strategy for passivity or phase compensation of inverter output admittance or impedance based on online detection of resonance frequency, so that inverters can be stably integrated into power grids with unknown system information or time-varying structures. The resonance frequency is detected by online fast Fourier transform (FFT) analysis of inverter current within the inverter controller. The passivity of output admittance of current-controlled inverters at the resonance frequency is compensated by emulating a virtual parallel resistor through an additional voltage feed-forward path with a band-pass filter. The phase of output impedance of voltage-controlled inverters at the resonance frequency is compensated by inserting a notch filter in the current feed-forward path. Experimental results validate the effectiveness of the proposed adaptive impedance compensation method in resonance damping and stable integration of inverters into unknown systems.

  • Shuying Zhen; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    A microgrid with dynamic boundary can expand or shrink its boundary depending on available local distributed energy resources (DER). Compared to conventional microgrid with fixed boundary, it can lead to better DER utilization and improved reliability. Previous literature has only considered the operation of a microgrid with a single island that is energized by stable voltage sources. This paper introduces control function designs that can effectively synchronize islands inside a dynamic boundary microgrid, depending on the operation status of each voltage source inside the islands. An overview of the components and rationale within the control function designs is provided, and hardware-in-the-loop simulation results are analyzed to demonstrate the effectiveness of the control functions.

  • Chongwen Zhao; Spencer Cochran; Daniel Costinett; Songnan Yang
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    In order to enable wireless charging of mobile electronics to compete with wired alternatives, wireless receivers need to operate at power levels sufficient to accommodate fast-charging standards. The receiver efficiency, harmonic content and total volume are key design metrics for wireless receivers. In this paper, four candidate topologies are compared, under 20 W fast-changing conditions, with respect to power loss, harmonic distortion, and power density. Each of the metrics are analyzed and verified experimentally. In coordination with the transmitter, a system-level approach to minimizing distortion is presented. Finally, the multilevel switched-capacitor rectifier is demonstrated to be a good candidate for wireless fast charging of mobile devices with high efficiency, small size, and suitable structure for future integration.

  • Ren Ren; Bo Liu; Zhou Dong; Fred Wang
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    For medium to high power application, due to the soft saturation and relatively higher saturation flux density, the powder and amorphous cores are commonly chosen for the DM inductors to achieve high power density in the EMI filter design. One of the issues of these materials is their permeability variation with operating current bias. Even for ac application, the three-phase currents at different switching cycles during one line cycle varies greatly, which leads to a variable DM inductance at different time intervals during one line cycle. Since the three-phase system has the unequal instantaneous currents for three phases, current-bias dependent permeability will cause the unbalanced DM inductance/impedance which converts DM noise to CM noise, also called mixed-mode (MM) noise. In this paper, the characteristics of current-bias dependent permeability are investigated for several commonly adopted DM core materials. In addition, a time domain variable inductance model considering current-bias characteristics is developed and applied to evaluate its impacts on the CM noise and the CM filter design. The phenomenon and proposed model are experimentally verified on a 10 kW three-level ANPC converter.

  • Zhou Dong; Ren Ren; Bo Liu; Fred Wang
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    The accurate leakage inductance modeling of common mode chokes (CMCs) can reduce trial-and-error filter debugging efforts and help avoid the saturation issue caused by the leakage flux. However, the analytical leakage inductance model is difficult to derive due to the complex leakage flux distribution. This paper presents a data-driven approach to model the CMC leakage inductance. A large amount of the leakage inductance data is collected by sweeping the selected input variables for the 3D finite element method (FEM) simulation. Then the data is trained by Artificial Neutral Network (ANN) to regress the nonlinear relationship between the leakage inductance and selected input variables. To verify the proposed method, core ZW4310TC is selected to model the relationship between the leakage inductance and winding parameters. Three CMCs with core ZW4310TC were built with different winding parameters and their leakage inductances were measured to verify the model. Compared with a previous analytical model, the error was reduced from 42.9% to 1.3% at the case where the previous model has the worst accuracy.

  • Wen Zhang; Zheyu Zhang; Fred Wang
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    Coaxial shunt resistors are very useful for measuring the ultra-fast current in wide-bandgap device switching transients. One of their major drawbacks is the relatively large parasitic inductance. The traditional coaxial shunt resistors are reviewed and the relationship between energy rating and parasitic inductance is determined. The parasitic inductance can be greatly reduced with a lower energy rating. A measurement method for characterizing their up to GHz bandwidth is also reported. Lower than expected bandwidth was observed and a fix using measured transfer characteristics is therefore described.

  • Ling Jiang; Daniel Costinett
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    Zero voltage switching (ZVS) has been widely implemented to improve the efficiency and robustness of high switching frequency converters. However, once the converter loses ZVS, the abrupt increase in switching loss decreases efficiency and may damage the converter. In this paper, a voltage slope-based detection method is proposed to detect ZVS status and prevent the converter from continuously hard switching. This detection circuit converts the voltage slope of the switching node to a dc voltage, which is compared with a reference voltage for the ZVS criterion. When hard switching occurs, two actions can be chosen: either shutting down the converter or adjusting the dead time to regain ZVS. The detection is achieved with simple circuitry and little control effort. A 6.78 MHz inverter for WPT application is used to verify this detection method. Experimental results show that ZVS status is detected effectively.

  • Spencer Cochran; Daniel Costinett
    2019 20th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2019

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    Active rectifiers in wireless power transfer systems exhibit many benefits compared to diode rectifiers, including increased efficiency, controllable impedance, and regulation capability. To achieve these benefits, the receivers must synchronize their switching frequency to the transmitter to avoid sub-fundamental beat frequency oscillations. Without additional communication, the receiver must synchronize to locally-sensed signals, such as voltages and currents induced in the power stage by the transmitter. However, the waveforms in the receiver are dependent on both the transmitter and receiver operation, resulting in an internal feedback between sensing and synchronization which prohibits the use of traditional phase-locked-loop design techniques. In this digest, a discrete time state space model is developed and used to derive a small signal model of these interactions for the purpose of designing stable closed-loop synchronization control. A prototype 150 kHz wireless power transfer converter is used to experimentally validate the modeling, showcasing stable synchronization.

  • Ruiyang Qin; Daniel Costinett
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    A multi-layer non-uniform self-resonant coil is proposed in this paper for wireless power transfer applications. Multiple layers of spiral coils are stacked up, and the capacitance between them is controlled by a dielectric material between the copper layers. The coil current is shared among all layers, and the current sharing ratio is determined by the variable-width spiral tracks. Compared with a two-layer self-resonant coil, current sharing across the multi-layer structure reduces total copper loss. The analytical model for the proposed coil is provided and used to optimize the design of the coil. Both finite-element simulation and prototype testing of a three-layer design are used to validate the model. The results confirm the predicted high quality factor series resonant behavior.

  • Kamal Sabi; Daniel Costinett
    2019 IEEE Energy Conversion Congress and Exposition (ECCE)
    2019

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    This paper presents the design and implementation of a boundary current mode (BCM) modulated GaN-based single phase inverter using a combination of bipolar and unipolar switching. Both unipolar and bipolar BCM-switched full bridge inverters are explored in detail in the context of efficiency, output current distortion and leakage current. Although the unipolar switched BCM inverter results in a higher efficiency in comparison to the bipolar switched inverter, it leads to a higher output current distortion at the low frequency zero crossing. On the other hand, the bipolar switched BCM inverter yields a low leakage current and reduced output current distortion, but exhibits lower efficiency. To overcome the low frequency zero crossing current distortion while maintaining a high efficiency, a combination of bipolar and unipolar switching in a BCM inverter is proposed. An experimental prototype has been built to validate the proposed control technique and modulation scheme. The proposed approach achieves a 2% efficiency improvement in comparison to the standard bipolar switched BCM inverter and a THD of 1.15%.

  • Ishita Ray; Leon M. Tolbert
    2019 IEEE Electrical Power and Energy Conference (EPEC)
    2019

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    With increasing deployment of inverter-based sources in microgrids, inverter control methods are constantly being modified and improved. What remains constant, however, is the phase-locked loop (PLL). Regardless of the control technique adopted and the type of microgrid, a PLL is used to synchronize the output PWM signal of the inverter with the main grid or other inverters. But as penetration of inverter-based sources increases and grids become weaker, the impact of PLLs on controller behavior becomes more pronounced. The issues caused by these influences are described in this paper to make a case for finding a solution for inverter synchronization that better fits the needs of inverter-dominated microgrids than PLLs. Simulation results from a study of a low-voltage microgrid supported by parallel inverters are also presented to demonstrate some of these characteristics.

  • Michael Starke; Rong Zeng; Sheng Zheng; Mitchell Smith; Madhu Chinthavali; Zhiqiang Wang; Benjamin Dean; Leon M. Tolbert
    2019 IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT)
    2019

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    Many energy storage systems that use technologies such as batteries are composed of power electronics conditioning systems and battery management systems. These are often produced by multiple manufacturers and require hardware and software integration for full grid functionality. This paper proposes an agent-based framework to support the development of an energy storage system with standardized communications. This framework can be utilized with different power conversion systems with an appropriate hardware interface.

  • Zhe Yang; Harish Suryanarayana; Fred Wang
    2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2019

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    This paper proposes a simple and accurate design method for air gapped inductors. Using inductor models with fringing effect, the proposed method systematically evaluates possible combinations of cores, turn numbers, and air gap lengths within the given constraints, including core geometries and magnetic saturation. Design examples show that the proposed method reduces the number of turns as well as the core size compared to the conventional methods. The method features simplicity in both algorithm and calculation since it only requires a small number of iteration loops. Various models involved in inductor design, including inductance, flux density, loss and thermal, can be readily incorporated into the proposed method.

  • Jiahao Niu; Fred Wang
    2019 IEEE 10th International Symposium on Power Electronics for Distributed Generation Systems (PEDG)
    2019

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    Sorting algorithm is required for operating modular multilevel converter (MMC) with nearest level modulation, to keep the unbalanced submodule capacitor voltage within an acceptable range. Several sorting algorithms have been proposed in previous literatures. However, very few papers discussed about how to select the execution frequency of a sorting algorithm. This paper aims to investigate on the impact of the execution frequency in sorting algorithm on the output voltage harmonics, average switching frequency, submodule capacitor voltage fluctuation of an MMC using nearest level modulation. Multiple MMC models with different number of submodules are built in MATLAB/Simulink. Three different types of sorting algorithms are implemented, and evaluated at various execution frequencies as well as different operating points. Conclusions drawn from the extensive simulations show that to select a sorting algorithm and its execution frequency, all three performance criterions should be considered simultaneous and a minimum execution frequency is always required.

  • Paige Williford; Fred Wang; Sandeep Bala; Jing Xu
    2019 IEEE 7th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2019

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    The short-circuit robustness of 600 V/30 A GaN gate injection transistors (GITs) was evaluated under various operating conditions to determine the worst-case short-circuit scenario. Although a decrease in maximum short-circuit current was observed with higher bus voltage, the short-circuit withstand time decreases dramatically. At room temperature and a bus voltage of 500 V, the short-circuit withstand time is as short as 160 ns as opposed to 220 ns at 400 V. The withstand time also depends on the maximum short-circuit current and can be extended significantly by reducing the drain current during a short-circuit event. The devices were shown to withstand a short-circuit event for considerably longer period with an elevated initial junction temperature as a result of lower drain current. With short-circuit current reduced from 110 A to 70 A, the withstand time was increased from 215 ns at 25°C to over 10 μs at 150°C and Vdc= 400 V. A gate-sensing protection scheme for GaN GIT was evaluated over various operating conditions and shown to successfully protect devices in less than 150 ns after short-circuit condition up to Tj = 150°C and Vdc= 500 V.

  • Wen Zhang; Fred Wang; Zheyu Zhang; and Bernhard Holzinger
    2019 10th International Conference on Power Electronics and ECCE Asia (ICPE 2019 - ECCE Asia)
    2019

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    A fast and reliable overcurrent protection scheme is crucial for the converter reliability. It is also critical for double pulse test stations where newer devices or even engineering samples are tested, and device failures can be costly. A fast overcurrent protection scheme using the direct current measurement in the double pulse test is demonstrated and 7.55 ns fault response delay time is achieved. The total fault clearing time is determined by the fault signal propagation and device switching speed. Around 100 ns and 60 ns fault clearing time is achieved for SiC and GaN devices, respectively. The much faster protection can potentially simplify the gate driver design and reduce the energy rating of the coaxial shunt resistor. Since the overcurrent detection is directly attached to the current measurement, its impact on the measurement bandwidth is also discussed.

  • Paige Williford; Fred Wang; Sandeep Bala; Jing Xu
    2019 IEEE 7th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2019

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    The short-circuit robustness of 600 V/30 A GaN gate injection transistors (GITs) was evaluated under various operating conditions to determine the worst-case short-circuit scenario. Although a decrease in maximum short-circuit current was observed with higher bus voltage, the short-circuit withstand time decreases dramatically. At room temperature and a bus voltage of 500 V, the short-circuit withstand time is as short as 160 ns as opposed to 220 ns at 400 V. The withstand time also depends on the maximum short-circuit current and can be extended significantly by reducing the drain current during a short-circuit event. The devices were shown to withstand a short-circuit event for considerably longer period with an elevated initial junction temperature as a result of lower drain current. With short-circuit current reduced from 110 A to 70 A, the withstand time was increased from 215 ns at 25°C to over 10 μs at 150°C and Vdc= 400 V. A gate-sensing protection scheme for GaN GIT was evaluated over various operating conditions and shown to successfully protect devices in less than 150 ns after short-circuit condition up to Tj = 150°C and Vdc= 500 V.

  • Wen Zhang; Fred Wang; Zheyu Zhang; and Bernhard Holzinger
    2019 10th International Conference on Power Electronics and ECCE Asia (ICPE 2019 - ECCE Asia)
    2019

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    A fast and reliable overcurrent protection scheme is crucial for the converter reliability. It is also critical for double pulse test stations where newer devices or even engineering samples are tested, and device failures can be costly. A fast overcurrent protection scheme using the direct current measurement in the double pulse test is demonstrated and 7.55 ns fault response delay time is achieved. The total fault clearing time is determined by the fault signal propagation and device switching speed. Around 100 ns and 60 ns fault clearing time is achieved for SiC and GaN devices, respectively. The much faster protection can potentially simplify the gate driver design and reduce the energy rating of the coaxial shunt resistor. Since the overcurrent detection is directly attached to the current measurement, its impact on the measurement bandwidth is also discussed.

  • Shuoting Zhang; Shuyao Wang; Nattapat Praisuwanna; Le Kong; Yalong Li; Robert B. Martin; Fred Wang; Leon M. Tolbert
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    Modular multilevel converters (MMCs) have been widely adopted for high voltage direct current (HVDC) applications and have been targeted for use in future dc grids as well. However, most of the MMC research is still limited to digital simulations or relied on hardware demonstrations with a specific setup and limited number of submodules (SMs). In this paper, a MMC test-bed with 10 SMs in each arm is developed that has flexible reconfiguration of the MMC topologies, switching frequencies, and passive element parameters. The MMC test-bed hardware construction, control scheme, and communication architecture are described, and typical MMC scenarios are conducted to verify its multiple functions.

  • Ren Ren; Zheyu Zhang; Bo Liu; Ruirui Chen; Handong Gui; Jiahao Niu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    One of the popular converter topologies applied in high power dc-ac applications is the three-level active neutral point clamped (ANPC). Owing to relatively low switching frequency and slow switching speed of these topologies in high power applications, the commutation loop analysis in these topologies has not been fully conducted, and the over-voltage issue of non-active switches has not been thoroughly analyzed. This paper reveals an over-voltage issue on non-active switches in three level inverters due to multi-commutation loop. The detailed mode analysis during the commutation and related over-voltage issue are given. Finally, Si-based ANPC with 140 kHz switching frequency and SiC-based ANPC converters with 280 kHz switching frequency and high switching speed are tested respectively to compare and verify the over-voltage issue for non-active switches.

  • Jordan Sangid; GaVin Long; Parker Mitchell; Benjamin J. Blalock; Daniel J. Costinett; Leon M. Tolbert
    2018 IEEE 6th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2018

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    This work examines the application of GaN within Class D audio by providing a side-by-side comparison of enhancement-mode GaN devices with currently available silicon MOSFETs with 60 V drain-to-source voltage ratings. GaN in Class D audio will allow for lower heat radiation, smaller circuit footprints, and longer battery life as compared to Si MOSFETs with a negligible trade-off for quality of sound.

  • Ruirui Chen; Zheyu Zhang; Ren Ren; Jiahao Niu; Handong Gui; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Understanding the CM inductor core saturation mechanism and reducing core flux density is critical for CM inductor design optimization. Instead of a time domain method, this paper introduces frequency domain spectrum concept for CM inductor core saturation analysis and design optimization, which will provide designers a better understanding of CM inductor design. First, both core permeability and converter modulation index's opposite influence on DM flux density and CM flux density are identified. Then, CM flux density is further investigated based on the spectrum concept. Three components in the CM inductor which may cause large CM flux density and core saturation are summarized: (1) switching frequency related components, (2) impedance resonance frequency related components, and (3) modulation frequency related components. Each component is investigated for CM flux density reduction and filter design optimization. A connecting AC and DC side midpoint with notch filter structure is proposed to reduce modulation frequency related components. Experiment results are presented to verify the proposed concept and method.

  • Handong Gui; Zheyu Zhang; Ren Ren; Ruirui Chen; Jiahao Niu; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    Although SiC MOSFETs show superior switching performance compared to Si IGBTs, it is unknown whether SiC MOSFETs have the same advantage over Si super junction (SJ) MOSFETs such as CoolMOS. This paper analyzes the switching performance in different switching cell configurations and summarizes the impact factors that influence switching loss. A double pulse test is conducted for a SiC MOSFET and a CoolMOS with the same voltage and current rating. In the FET/diode cell structure, a SiC Schottky diode is used as the upper device to eliminate the reverse recovery, and the testing results show that the SiC MOSFET has 2.4 times higher switching loss than the Si CoolMOS. This can be explained by the smaller transconductance and the higher Miller voltage of the SiC MOSFET. On the other hand, the Si CooMOS has 10 times higher switching loss than the SiC MOSFET in the FET/FET cell structure because of the significant turn-on loss caused by the poor reverse recovery of its body diode.

  • Ruirui Chen; Zheyu Zhang; Ren Ren; Jiahao Niu; Handong Gui; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Unlike conventional passive or active filters, an impedance balancing circuit reduces the common-mode (CM) electromagnetic interference (EMI) noise by establishing an impedance balancing bridge. The EMI noise can be significantly reduced when the impedance bridge is designed to be well balanced. This paper investigates impedance balancing circuits in Dc-fed motor drive systems where both DC input and AC output need to meet EMI standards and thus EMI filters are needed for both sides. An impedance balancing circuit is proposed to reduce both DC and AC side CM noise. Two auxiliary branches are added to the conventional passive filters to establish an impedance bridge and reduce CM noise. The design criteria are presented, and the impact of the proposed impedance balancing circuit on both sides CM noise are investigated. It shows that the proposed impedance balancing circuit can reduce DC side and AC side CM noise based on different mechanisms. The CM noise reduction performance of the proposed method does not depend on the motor and cable models. Experiment results are presented to demonstrate the feasibility and effectiveness of the proposed method.

  • Ruirui Chen; Zhou Dong; Zheyu Zhang; Handong Gui; Jiahao Niu; Ren Ren; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    Superconducting technologies such as motors together with the supporting cryogenic power electronic system are growing in importance in aircraft applications. It is critical to understand the influence of low temperature on filters of the power converter system in these applications. Also, it is worthwhile to investigate whether the converter system can achieve higher efficiency and high power density by utilizing the provided low temperature cooling environment. This paper conducted a comprehensive magnetic core characterization at low temperature to understand the core properties and support filter design at low temperature. The ferrite and nanocrystalline material are characterized from room temperature to cryogenic temperature in a wide range of operating frequencies. The results show that the permeability of ferrite material decreases by a factor of 7~8 and the core loss increases more than 10 times when operating at very low temperature. The permeability of nanocrystalline material decreases to 60% and the core loss increases 1.5~2.5 times when operating at very low temperature. The saturation flux density of both materials has slight increase at low temperature. Based on tested data, a case study of inductor design considering the low temperature cooling environment is presented to illustrate the influence of low temperature on inductor design.

  • Ren Ren; Handong Gui; Zheyu Zhang; Ruirui Chen; Jiahao Niu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    In order to evaluate the feasibility of newly developed GaN devices in a cryogenic-cooled converter, this paper characterizes a 650 V enhancement-mode Gallium-Nitride heterojunction field-effect transistor (GaN HFET) at cryogenic temperatures. The characterization includes two parts: static and dynamic characterization. The results show that this GaN HEMT is an excellent device candidate to be applied in cryogenic-cooled applications. For example, transconductance at cryogenic temperature is 2.5 times of one at room temperature, and accordingly, peak di/dt during turn-on transients at cryogenic temperature is around 2 times of that at room temperature. Moreover, the on-resistance of the channel at cryogenic temperature is only one-fifth of that at room temperature.

  • Zheyu Zhang; Handong Gui; Jiahao Niu; Ruirui Chen; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Due to the low availability, high cost, and limited performance of high voltage power devices in high voltage high power applications, series-connection of low voltage switches is commonly considered. Practically, because of the dynamic voltage unbalance and the resultant reliability issue, switches in series-connection are not popular, especially for fast switching field-effect transistors such as silicon (Si) super junction MOSFETs, silicon carbide (SiC) JFETs, SiC MOSFETs, and gallium nitride (GaN) HEMTs, since their switching performance is highly sensitive to gate control, circuit parasitics, and device parameters. In the end, slight mismatch can introduce severe unbalanced voltage. This paper proposes an active voltage balancing scheme, including 1) tunable gate signal timing unit between series-connected switches with <; 1 ns precision resolution by utilizing a high resolution pulse-width modulator (HRPWM) which has existed in micro-controllers; and 2) online voltage unbalance monitor unit integrated with the gate drive as the feedback. Based on the latest generation 600-V Si CoolMOS, experimental results show that the dynamic voltage can be automatically well balanced in a wide range of operating conditions, and more importantly, the proposed scheme has no penalty for high-speed switching.

  • Handong Gui; Ren Ren; Zheyu Zhang; Ruirui Chen; Jiahao Niu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    To operate a converter at cryogenic temperatures, understanding the characteristics of power semiconductor devices is critical. This paper presents the characterization of state-of-the-art 1.2 kV SiC MOSFETs from leading manufacturers at cryogenic temperatures. The testing setup consisting of a cryogenic chamber, and a liquid nitrogen Dewar is introduced. With a curve tracer and double pulse test, comprehensive characterization of the SiC MOSFETs including both static and switching performance is conducted and evaluated. Test results indicate the on-resistance increases while the breakdown voltage remains relatively constant at cryogenic temperatures. Other characteristics like threshold voltage and switching loss vary significantly at cryogenic temperatures among devices from different manufacturers.

  • Ruirui Chen; Jiahao Niu; Zheyu Zhang; Handong Gui; Ren Ren; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett; Benjamin B. Choi
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    Zero sequence circulating current (ZSCC) exists when paralleled inverters have common dc and ac sides without isolation. Most of the prior work on the ZSCC analysis and suppression depended on paralleled two-level inverters. The scenarios involved in the three-level converters are more complicated. This paper investigates the ZSCC in paralleled three-level active neutral point clamped (ANPC) inverters. The mechanisms causing potential ZSCC jump in three-level paralleled ANPC inverters are analyzed. The ZSCC patterns of different interleaved modulation schemes for three-level converters are illustrated. Then, the active vector dividing concept is extended to three-level converters, and a modulation scheme is proposed to reduce the high frequency ZSCC in three-level converters. Experiments have been conducted on two paralleled three-level inverters. The current jump in ZSCC is observed and mitigated. The ZSCC with proposed modulation scheme is reduced to less than half of the ZSCC with conventional continuous space vector modulation (CSVM) scheme.

  • Yiwei Ma; Xiaotong Hu; He Yin; Lin Zhu; Yu Su; Fred Wang; Leon M. Tolbert; Yilu Liu
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    To better utilize the existing electric power grid distribution network automation of smart switches, a microgrid can expand or shrink its electrical boundary according to available renewable generation. Previous literature only focused on the design of a microgrid with a dynamic boundary, but without considering real-time operation. This paper proposes a microgrid controller that enables operation of microgrid with dynamic boundary and can be integrated into the existing distribution automation system. The architecture of the control system is introduced, and the essential functions such as online topology assessment and synchronization are presented. Simulation results are given to demonstrate the feasibility of the proposed controller.

  • Wen Zhang; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    Switching transient overvoltage is inevitable in hard switching applications, and the faster switching speed of SiC MOSFETs suggests even worse overvoltage. This paper focuses on the turn-on overvoltage. To understand its nature, the switching transient is analyzed, and it shows the turn-on overvoltage is largely independent of load current condition. This phenomenon is verified by characterizing the turn-on overvoltage of a SiC MOFET and a SiC Schottky diode. Finally, a SPICE-based model is also built to understand the switching transient more accurately, and the modeling method can accurately predict the turn-on overvoltage and help select device voltage rating.

  • Jacob Dyer; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Dead-time, device output capacitance, and other non-ideal characteristics cause voltage error for the midpoint PWM voltage of the semiconductor phase-leg employed in a voltage-source inverter (VSI). Voltage-second balancing is a well-known concept to mitigate this distortion and improve converter power quality. This paper proposes a unique voltage-second balancing scheme for a SiC based voltage source inverter using online condition monitoring of turn-off delay time and drain-source voltage rise/fall time. This data is sent to the micro-controller to be used in an algorithm to actively adjust the duty cycle of the input PWM gate signals to match the voltage-second of the non-ideal output voltage with an ideal output voltage-second. The monitoring system also allows for this implementation to eliminate the need for precise current sensing and allows for the implementation to be load independent. Dynamic current sensing is still a developing technology, and each load has a unique effect on the output voltage distortion. Test results for a 1 kW half-bridge inverter implementing this monitoring system and voltage-second balancing scheme show a 70% enhancement on the error against the ideal fundamental current value of the output current and a 2% THD improvement on the output current low frequency harmonics.

  • Shuoting Zhang; Jingxin Wang; Fred Wang; Leon M. Tolbert
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    A power converter based transmission line emulator has been developed for the hardware test-bed (HTB) platform, which emulates power systems by mimicking the system components with universal three-phase voltage source converters. In power grids, transmission line series compensation devices are utilized to enhance the power delivery capability and controllability by regulating the equivalent impedance of a transmission line. This paper proposes a method of mimicking variable capacitors and inductors to combine the series compensation device emulation into the transmission line emulator so that various power system scenarios can be studied. Simulation and experiment results verify the effectiveness of the transmission line emulation with integrated series compensation devices.

  • Jessica D. Boles; Yiwei Ma; Leon M. Tolbert; Fred Wang
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Battery energy storage systems (BESSs) are commonly used for frequency support services in power systems because they have fast response times and can frequently inject and absorb active power. Lithium-ion (Li-ion) BESSs dominate the grid energy storage market now, but Vanadium redox flow (VRB) BESSs are predicted to contend in future markets for large-scale storage systems. Previously, a Li-ion BESS emulator has been developed for a grid emulation system known as the Hardware Testbed (HTB), which consists of converters controlled to emulate different power system components. In this paper, we develop a VRB BESS emulator with a VRB-specific internal battery model and a power electronics interface similar to that of the Li-ion BESS emulator. Then, we compare the effectiveness of the VRB and Li-ion technologies for primary frequency regulation and inertia emulation applications. It is concluded that these two technologies are virtually indistinguishable from the power system's perspective when conducting these services over a short period of time.

  • Bo Liu; Edward Jones; Ren Ren; Zheyu Zhang; Fred Wang; Daniel Costinett
    2018 1st Workshop on Wide Bandgap Power Devices and Applications in Asia (WiPDA Asia)
    2018

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    In this paper, an extra junction capacitance and its associated switching commutation path are identified in three-level ac/dc converters, which were previously overlooked due to the off-state of the related device in half line cycle. The impact of this effect on power loss is analyzed, showing an underestimated switching loss in the traditional loss calculation of three-level converters. Through a proposed loss re-evaluation approach based on energy data of conventional double pulse tester (DPT), the corrected loss matches experimental results obtained from a 450kHz 650 V Gallium Nitride (GaN) based Vienna-type rectifier, showing 17.4% additional switching loss due to this effect. And the dominant extra switching loss is found to be Coss loss instead of overlap loss in WBG converters. Thefore, the effect is severe in high swtiching frequency high-speed wideband gap (WBG) based three-level converters.

  • Bo Liu; Ren Ren; Fred Wang; Daniel J. Costinett; Zheyu Zhang; Yiwei Ma
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    Attenuation performance of an EMI filter can be significantly degraded by coupling, parasitics, and frequency-dependent nonlinearity of magnetic cores. In this paper, the effect due to mutual capacitive coupling in filter structures with T-shape joint is identified and investigated. Its mechanism indicates that this coupling is the dominant cause of performance degradation in T-shape filters. PCB slits and grounded shielding are proposed as two effective mitigation solutions, respectively, and are further combined to improve filter transfer gain up to 40 dB along the high frequency range. Experimental results obtained from a three-phase LCL common-mode (CM) filter verify the significant impact of this coupling and the effectiveness of the proposed mitigation methods.

  • Xingxuan Huang; Shiqi Ji; James Palmer; Li Zhang; Leon M. Tolbert; Fred Wang
    2018 IEEE 6th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2018

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    In a converter based on 10 kV SiC MOSFETs, major sources of parasitic capacitance are the anti-parallel junction barrier schottky (JBS) diode, heat sink, and load inductor. A half bridge phase leg test setup is built to investigate these parasitic capacitors' impact on the switching performance at 6.25 kV. Generally these parasitic capacitors slows down both turn-on and turn-off transient and can cause significant increase in switching energy loss. The impact of the parasitic capacitor in the load inductor is analyzed, which has either very short wire or long wire in series. Switching performance of the phase leg with two different thermal designs are compared to investigate the impact of the parasitic capacitor due to the heat sink. The large parasitic capacitor due to the large drain plate of discrete 10 kV SiC MOSFET for heat dissipation can result in 44.5% increase in switching energy loss at low load current.

  • Yutian Cui; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Data centers consume an ever-increasing amount of electricity because of the rapid growth of cloud computing and digital information storage. A high voltage point of load (HV POL) converter is proposed to convert the 400-VDC distribution voltage to 1-VDC within a single stage to increase the power conversion efficiency. A six-phase input series output parallel (ISOP) connected structure is implemented for the HV POL. The symmetrical controlled half bridge current doubler is selected as the converter topology in the ISOP structure. The full load efficiency is improved by 4% points compared with state of the art products. A voltage compensator has been designed in order to meet the strict dynamic voltage regulation requirement. A laboratory prototype has been built, and experimental results have been provided to verify the proposed HV POL with a single power conversion stage can meet the dynamic voltage regulation requirement for an on-board power supply with higher efficiency compared to the conventional architecture.

  • Shuyao Wang; Shuoting Zhang; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    The frequency decoupling effect of the voltage source converter (VSC)-based high voltage dc (HVDC) transmission makes frequency support unavailable between two ac subsystems interconnected by the HVDC link. Recently, several algorithms have been proposed and demonstrated to improve the ac system stability by implementing inertia emulation (IE) functions in the HVDC VSC stations. However, the capability of the VSC-HVDC to achieve the IE control is not clear. According to the previous research, the IE control strategy has been studied without concerning the impact of the practical control process, which will introduce some time delay into the IE realization. In this paper, a detailed multi-terminal HVDC VSC control module, including the phase-locked loop (PLL) and low pass filter (LPF), is considered in order to analyze the multi-terminal HVDC network impact on the IE performance. The analysis indicates that the frequency performance of the ac network with IE integrated HVDC transmission can be nearly as good as that directly connected with the ac subsystem which involves a high penetration of conventional generation units, except for the short time delay introduced by the VSC control modules. The effectiveness of IE performance will be compromised if the response delay is significant. The simulation results have verified the analysis.

  • Bo Liu; Ren Ren; Zheyu Zhang; Fred Wang; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    As wide bandgap (WBG) semiconductors are gradually adopted for high switching frequency high power-density power converter, new challenges arise from control to hardware design. In this paper, an improved input current sampling method is proposed for three-phase rectifiers to avoid sampling noises when rectifiers are operated at high speed and high switching frequency. Experimental results obtained from a 450-kHz enhancement-mode Gallium Nitride (GaN) high-electron-mobility transistor (HEMT) based three-phase three-level Vienna-type rectifier demonstrate the good performance of the sampling method.

  • Jiaojiao Dong; Lin Zhu; Yilu Liu; D. Tom Rizy; Fei Fred Wang; Leon M. Tolbert; Jim Glass
    2018 Australasian Universities Power Engineering Conference (AUPEC)
    2018

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    Practical distribution system contains many nodes and short branches. As a result, a more efficient and stable distribution state estimation (DSE) algorithm is needed to properly handle the inverse operation of its large-scale ill-conditioned information matrix. This paper proposed a two-stage DSE algorithm to transform the inverse operation into addition and multiplication operations in a recursive manner. It includes pseudo measurement preprocessing in the offline stage where an approximate linear estimator is adopted to relieve most of computation burdens, and real-time correction in the online stage where a nonlinear estimator is used to update the estimation. Numerical results of the method applied to both IEEE standard test systems and an actual distribution system show the accuracy and effectiveness of the proposed algorithm.

  • Shiqi Ji; Marko Laitinen; Xingxuan Huang; Jingjing Sun; Bill Giewont; Leon M. Tolbert; Fred Wang
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    The short circuit performance of a 3rd generation 10 kV/20 A SiC MOSFET with short channel is characterized in this paper. The platform consisting of a phase-leg configuration, which can test both hard switching fault (HSF) and fault under load (FUL) types of fault, is introduced in detail. A Si IGBT based solid state circuit breaker is developed for short circuit test. The short circuit protection having a response time of 1.5 μs is validated by the test platform. The short circuit characteristics for both the HSF and FUL types at 6 kV DC-link are presented and analyzed.

  • Xingxuan Huang; Shiqi Ji; Sheng Zheng; Jingjing Sun; Leon M. Tolbert; Fred Wang; Marko Laitinen; William Giewont
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    The impact of the body diode and the anti-parallel junction barrier Schottky (JBS) diode on the switching performance of the 3rd generation 10 kV SiC MOSFET from Wolfspeed/Cree is investigated in detail at various junction temperatures. The switching performance with and without the anti-parallel JBS diode is tested by specific layout design of the 10 kV MOSFET module. The body diode of the 10 kV SiC MOSFET has excellent reverse recovery performance from 25°C to 125 °C, indicating it is a suitable candidate for freewheeling diode. The application of anti-parallel JBS diode will slow down the turn-off transient and result in lower turn-off dv/dt and higher turn-off energy loss, while its impact on the turn-on transient is not significant.

  • Paige Williford; Edward A. Jones; Zhe Yang; Jianliang Chen; Fred Wang; Sandeep Bala; Jing Xu
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    The dead-time in voltage source converters can have significant impact on power quality and efficiency, and because these losses scale proportionally with switching frequency, it is important to study the impact of these losses in wide bandgap based converters, like SiC and GaN. This paper focuses on the impact of dead-time on the total loss of a 5 kW, single phase, GaN-based inverter, and an experimental model of dead-time losses for various dead-times was developed based on static and dynamic characterization results. The results show that in a GaN-based voltage source inverter (VSI), a 100 ns dead-time setting can attribute up to 30% additional switching loss and is a strong function of load current level and temperature. A comparison of losses between fixed and adaptive dead-time was studied, and an optimal fixed dead-time setting based on the analysis was implemented and verified in experiment.

  • Gabriel Gabian; Jordan Gamble; Benjamin Blalock; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    In this work, a hybrid switched-capacitor/PWM converter is analyzed and designed for battery charging in mobile electronics. Operation of the converter is reviewed to construct a complete analytical loss model based on FET extracted parameters for an integrated circuit implementation. The model is validated with experimental results and compared with other converter topologies in the same application. The loss modeling is used to optimize the physical scaling of the power transistors to minimize total losses.

  • Craig Timms; Liang Qiao; Fred Wang; Zheyu Zhang; Dong Dong
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    This paper presents a new boundary condition for designing phase legs when using the decoupling capacitance method. New SiC MOSFETs have much higher short-circuit currents-over 14X datasheet rating-then comparable Si IGBT devices. The energy draw on the decoupling capacitance due to this can be a large step input that over-voltages the device if not accounted for. Decoupling capacitance requirements have previously been based on switching conditions during normal operation and may not be sufficient for high current devices or modules. Furthermore, fast protection work has focused on lower current discrete devices whereas this issue becomes more prevalent in higher current configurations. Analysis of device over-voltage during short-circuit events is presented along with new sizing guidelines for DC link decoupling capacitance.

  • Craig Timms; Liang Qiao; Fred Wang; Zheyu Zhang; Dong Dong
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    This paper presents the occurrence of potentially destructive oscillation in paralleled power MOSFETs during short-circuit events. Paralleling discrete power devices is desirable in many designs in order to increase power output. Short circuits cause high voltages, saturation current and local temperatures creating unstable environments within devices. Current redistribution can occur between device gates in this environment which can excite oscillation in parallel circuitry if not properly accounted for. Analysis of the phenomenon including experimental results are presented along with mitigation steps.

  • Spencer Cochran; Daniel Costinett
    2018 IEEE 19th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2018

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    Resonant magnetic wireless power transfer (WPT) has potential to improve spatial freedom, allow simultaneous charging of multiple devices, and increase circuit power density. However, many obstacles must first be overcome before the benefits of this technology are fully realized. Due to the high frequency of the WPT carrier, autonomous receiver-side synchronization and control presents significant difficulties. This work addresses the control strategy of a 6.78 MHz GaN-based synchronous WPT rectifier. A zero-crossing current sensing scheme is used to both synchronize the receiver switching frequency and to enable control of its input phase. The rectifier is shown to be capable of regulating its output voltage by changing its input phase, highlighting the value of a circuit that has real-time control over the reactive part of its input impedance. The current sensing, frequency synchronization, and input phase control capabilities of the circuit are demonstrated experimentally and discussed in detail.

  • Kamal Sabi; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    To overcome the increased switching losses in high frequency power electronics, control-based modulation techniques such as boundary current mode (BCM) are commonly used in full bridge inverter and rectifier topologies to guarantee zero voltage switching (ZVS). Traditionally, in order to implement BCM modulation, a combination of current programmed mode (CPM) control and model-based techniques are used. The former is highly susceptible to noise and sensing delay, while the latter is subject to modeling error. In this work, a dual-current programmed mode (DCPM) control circuit for BCM operation is designed and implemented. The proposed control network achieves better noise immunity and low propagation delay at high frequency while regulating peak and valley currents in each period. The operation of this control scheme is demonstrated experimentally using a GaN-based half bridge inverter prototype.

  • Fei Yang; Zhiqiang Jack Wang; Zheyu Zhang; Steven Campbell; Fred Wang; Madhu Chinthavali
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Middle-point inductance Lmiddle can be introduced in power module designs with P-cell/N-cell concept. In this paper, the effect of middle-point inductance on switching transients is analyzed first using frequency domain analysis. Then a dedicated multiple-chip power module is fabricated with the capability of varying Lmiddle. Extensive switching tests are conducted to evaluate the device's switching performance at different values of Lmiddle. Experiment result shows that the active MOSFET's turn-on loss will decrease at higher values of Lmiddle while its turn-off loss will increase. Detailed analysis of this loss variation is presented. In addition to switching loss variation, it is also observed that different voltage stresses are imposed on the active switch and anti-parallel diode. Specifically, in the case of lower MOSFET's turn-off, the maximum voltage of lower MOSFET increases as Lmiddle goes up; however, the peak voltage of anti-parallel diode decreases significantly. The analysis and experiment results will provide design guidelines for multiple-chip power module package design with P-cell/N-cell concept.

  • Yongsheng Fu; Yang Huang; Hua Bai; Xi Lu; Ke Zou; Chingchi Chen
    2018 IEEE 6th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2018

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    With the development of power electronics, three-phase voltage source inverters(VSls) are widely used in industry application. In this paper a SiC device based EV battery charger consisting of dual active bridge(DAB) and three-phase four-wire VSI is built. In order to undertake the unbalanced load, an independently controlled neutral module(ICNM) has been adopted for the VSI control to form the neutral line and provide the path for the zero-sequence current. Split DC-bus capacitors with the fourth leg are equipped to the conventional VSI. Meanwhile, the control performance is improved by paralleling virtual resistors to the load, namely, notch-filter-incorporated capacitor voltage feedback control(NF-CVFC), which not only enhances the quality of the output voltage but also ensures the system stability. Finally, a 10kW prototype is built and tested. Experimental results verified the proposed control strategy and the practical feasibility.

  • Livan Zhu; Hua Kevin Bai; Alan Brown; Matt McAmmond
    2018 IEEE 6th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2018

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    Wide-Bandgap devices are believed to be promising candidates for the next-generation power electronics converter. However, the prohibitive cost and limited variety are still the main constrains before being widely used. One possible solution to mitigate these issues is hybrid switches, a combination of Si MOSFETs and WBG devices. To maximize merits of both GaN HEMT and Si MOSFET, this paper proposes a hybrid switch consisting of two GaN HEMTs and two Si MOSFETs. For such a design, a robust gate-drive loop is critical to secure the safe operation of all switches. As an instructive work, this paper presents a comprehensive analysis of the switching transient process and its impact on the gate-drive loop, practical tuning tips of gate-drive loop design are also given based on the simulation and experimental results. A 400V/ 80A full bridge prototype is developed to validate our design. Experimental result shows that, with enhanced gate-drive loop, we can continuously turn off 400V/80A@100kHz and 400V/40A@300kHz with only one GaN device paralleled to two commercial Si MOSFETs.

  • Jie Li; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    A planar coil structure that exhibits a series self-resonant behavior is developed for wireless power transfer systems which use magnetic resonant coupling. The proposed structure uses two thin, planar, spiral tracks separated by one layer of dielectric. By connecting to alternate ends of each track, the coil shows a series LC impedance, which is often necessary for voltage source inverters. An analytical model for the inductance, capacitance and resistance is used to develop a geometric design method that minimizes resistance given a set of application constraints. Experimental coils made with an FR4 PCB and an Teflon-ceramic PCB verify the proposed structure and modeling.

  • Ling Jiang; Daniel Costinett
    2018 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (Wow)
    2018

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    A single-stage transmitter is reviewed which directly converts a utility ac input to high frequency (6.78 MHz) ac output for wireless power transfer applications. Compared with a two-stage transmitter implementation, this single-stage transmitter obtains high power efficiency with reduced component-count. In this paper, a method is proposed to enable constant current at the output of the single-stage transmitter to accommodate multiple receivers. First, the constant output voltage transmitter is obtained by implementing closed-loop control and a model-based modulation scheme. Then, an impedance matching network is implemented at the output of transmitter to convert the constant voltage to constant current. This feature allows a single transmitter to charge multiple receivers simultaneously. The control methodology is verified using both simulation and a laboratory prototype.

  • Ling Jiang; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    A single-stage transmitter is reviewed, which directly converts a utility ac input to high frequency (6.78 MHz) ac output for wireless power transmission. By integrating two stages (totem-pole PFC rectifier and full bridge inverter) into a single stage, the topology achieves high efficiency and reduced component count. In this paper, a simple auxiliary circuit is added to allow the single-stage transmitter to operate in two modes. At heavy load, the transmitter operates as a totem-pole rectifier with PFC and achieves low distortion of the input current. At light load, the circuit operates as a voltage-doubler rectifier (VDR), extending the ZVS range of the transmitter. As a result, hard switching is avoided and efficiency is improved at light load. This improved single-stage transmitter is verified by a 100 W, GaN-based laboratory prototype.

  • Jie Li; Daniel Costinett
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    The call for higher power and larger spatial freedom has driven the operating frequency of wireless power transfer (WPT) systems into the MHz range. Previous system design methods focus on designing the coils to attain high quality factor while remaining resonant around the working frequency and subsequently designing switching circuit hardware and operating point to minimize the system power loss. This sequential design method over-simplifies the system, neglecting inter-dependencies between system components, which leads to suboptimal designs. This paper presents a system design method that co-optimizes coils, power stages, and circuit operation for maximum overall efficiency, based on accurate models of converters and WPT system. The system model and the importance of co-design are verified by a 6.78 MHz WPT prototype.

  • Chongwen Zhao; Daniel Costinett; Songnan Yang
    2018 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (Wow)
    2018

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    In mobile electronics applications, the high conduction loss on the ac-dc rectifier and the coil is a barrier to the application of wireless charging. In this paper, a wireless charging architecture employing a 7-level switched-capacitor (SC) ac-dc rectifier is investigated, showing a substantial reduction of the conduction losses on the transmitter, the coil and the receiver. The proposed SC rectifier also features a multilevel design with good scalability to accommodate different power ratings, and potentially reduce the harmonic contents of the input voltage. The principle and operation are verified using a 7-level, 20 W, 5-9 Vdc output, 150 kHz prototype. The measured peak dc-to-load system efficiency is above 90%. The high-power density design without bulky magnetic components is suitable for the implementation in low-profile mobile electronics.

  • Saeed Anwar; Daniel Costinett
    2018 IEEE Energy Conversion Congress and Exposition (ECCE)
    2018

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    This paper presents a modeling approach for dual active bridge (DAB) converter in electric vehicle (EV) applications which considers the magnetizing inductance effect of the high-frequency transformer. A Typical DAB transformer has very high magnetizing inductance compared to the leakage inductance. As a result, the magnetizing current can be neglected. However, for integrated and hybrid converters, where the same core is used for both DC excitation and AC excitation, an air gap is used to prevent core saturation. In such applications, the effect of magnetizing inductance needs to be considered for DAB converter modeling. For accurate estimation of the DAB converter, a loss model considering magnetizing inductance is developed in this paper. Finite element analysis (FEA) is performed to model the transformer to evaluate the proximity loss and fringe induced eddy current loss mechanisms. An experimental prototype of the DAB converter is developed to verify the proposed model. Experimental waveforms are presented and compared for different power level and switching frequency.

  • Zheyu Zhang; Handong Gui; Ren Ren; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 AIAA/IEEE Electric Aircraft Technologies Symposium (EATS)
    2018

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    Wide bandgap (WBG) semiconductor devices and cryogenic cooling are key enablers for highly-efficient ultra-dense power electronics converters, which are critical for future more electric aircraft applications. For the development and optimization of a cryogenically-cooled converter, an understanding of power semiconductor characteristics, especially for emerging WBG devices, is critical. This paper focuses on WBG device characterization at cryogenic temperatures. First, the testing setup for cryogenic temperature characterization is introduced. Then several WBG device candidates (e.g., 1200-V SiC MOSFETs and 650-V GaN HEMTs) are characterized from room to cryogenic temperatures. The test results are presented with trends summarized and analyzed, including on-state resistance, breakdown voltage, and switching performance.

  • Fanning Jin; Hua Bai; Dingguo Lu; Bing Cheng
    2018 IEEE 6th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2018

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    As the effort to maximize the potential of SiC devices, the Field Programmable Gate Array (FPGA) is utilized in this paper to achieve high control bandwidth (>200 kHz) when implementing the Field-oriented Control (FOC) algorithm. This can greatly enhance the high-frequency injection (HFI) sensorless control for Permanent Magnet Synchronous Motors (PMSMs) by widening the applicable speed range, for instance, increasing the injection frequency up to 2 kHz from the conventional 500Hz. To offset the cost, it is also validated that one FPGA can control two motors simultaneously.

  • Jared A. Baxter; Daniel A. Merced; Daniel J. Costinett; Leon M. Tolbert; Burak Ozpineci
    2018 IEEE Transportation Electrification Conference and Expo (ITEC)
    2018

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    Automated vehicles require sensors and computer processing that can perceive the surrounding environment and make real time decisions. These additional electrical loads expand the auxiliary load profile, therefore reducing the range of an automated electric vehicle compared to a standard electric vehicle. Furthermore, a fully automated vehicle must be fail-safe from sensor to vehicle control, thus demanding additional electrical loads due to redundancies in hardware throughout the vehicle. This paper presents a review of the sensors needed to make a vehicle automated, the power required for these additional auxiliary loads, and the necessary electrical architectures for increasing levels of robustness.

  • Zheyu Zhang; Handong Gui; Ren Ren; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 AIAA/IEEE Electric Aircraft Technologies Symposium (EATS)
    2018

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    Wide bandgap (WBG) semiconductor devices and cryogenic cooling are key enablers for highly-efficient ultra-dense power electronics converters, which are critical for future more electric aircraft applications. For the development and optimization of a cryogenically-cooled converter, an understanding of power semiconductor characteristics, especially for emerging WBG devices, is critical. This paper focuses on WBG device characterization at cryogenic temperatures. First, the testing setup for cryogenic temperature characterization is introduced. Then several WBG device candidates (e.g., 1200-V SiC MOSFETs and 650-V GaN HEMTs) are characterized from room to cryogenic temperatures. The test results are presented with trends summarized and analyzed, including on-state resistance, breakdown voltage, and switching performance.

  • Fanning Jin; Hua Bai; Dingguo Lu; Bing Cheng
    2018 IEEE 6th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2018

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    As the effort to maximize the potential of SiC devices, the Field Programmable Gate Array (FPGA) is utilized in this paper to achieve high control bandwidth (>200 kHz) when implementing the Field-oriented Control (FOC) algorithm. This can greatly enhance the high-frequency injection (HFI) sensorless control for Permanent Magnet Synchronous Motors (PMSMs) by widening the applicable speed range, for instance, increasing the injection frequency up to 2 kHz from the conventional 500Hz. To offset the cost, it is also validated that one FPGA can control two motors simultaneously.

  • Jared A. Baxter; Daniel A. Merced; Daniel J. Costinett; Leon M. Tolbert; Burak Ozpineci
    2018 IEEE Transportation Electrification Conference and Expo (ITEC)
    2018

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    Automated vehicles require sensors and computer processing that can perceive the surrounding environment and make real time decisions. These additional electrical loads expand the auxiliary load profile, therefore reducing the range of an automated electric vehicle compared to a standard electric vehicle. Furthermore, a fully automated vehicle must be fail-safe from sensor to vehicle control, thus demanding additional electrical loads due to redundancies in hardware throughout the vehicle. This paper presents a review of the sensors needed to make a vehicle automated, the power required for these additional auxiliary loads, and the necessary electrical architectures for increasing levels of robustness.

  • Shuoting Zhang; Bo Liu; Sheng Zheng; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2017 IEEE Energy Conversion Congress and Exposition (ECCE)
    2017

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    A hardware test-bed (HTB) has been developed to realize power system emulation by mimicking the system components with universal three-phase voltage source converters (VSCs). The VSC-based transmission line emulator has also been successfully developed to flexibly represent interconnected ac lines under normal operating conditions. As the most serious short-circuit fault condition, the three-phase short-circuit fault emulation is essential for power system studies. This paper proposes a model to realize the three-phase short-circuit fault emulation within the emulated transmission line. At the same time, a combination method is proposed to eliminate the undesired transients caused by the current reference step changes while switching between the fault state and normal state.

  • Shuoting Zhang; Yalong Li; Bo Liu; Xiaojie Shi; Leon M. Tolbert; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Hybrid ac/dc transmission can increase the power transfer capability of long ac transmission lines. High voltage dc (HVDC) converters are needed, and the line-commutated converter (LCC) is used considering the dc fault current controllability. However, zero-sequence current can be generated due to the coupled transmission lines, and it will flow into the HVDC converters as a fundamental frequency current component on dc side (i60). The LCC will convert i60 to dc current components on the ac side, which may cause potential converter transformer saturation. This paper analyzes the influence of coupled transmission lines on i60 and the converter transformer saturation and proposes two possible solutions to avoid converter transformer saturation. The simulation results verify the effectiveness of the proposed methods.

  • Zheyu Zhang; Craig Timms; Jingyi Tang; Ruirui Chen; Jordan Sangid; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Cooling a converter to low temperatures, e.g. using cryogenic cooling, can significantly improve the efficiency and density of a power conversion system. For the development and optimization of a cryogenically-cooled converter, an understanding of power semiconductor characteristics is critical. This paper focuses on the characterization of high-voltage, high-speed switching, power semiconductors at cryogenic temperature. First, the testing setup for cryogenic temperature characterization is introduced. Three testing setups are established for cryogenic switch characterization, including: 1) on-state resistance and forward voltage drop of the body diode, 2) leakage current and breakdown voltage, and 3) switching characteristics. For each testing set up, the corresponding testing configurations, hardware setups, and practical considerations are summarized. Additionally, the test results at cryogenic temperature are illustrated and analyzed for 650-V Si CoolMOS. It is then demonstrated that when the cryogenic temperature test results are compared to that of room temperature, the device performance varies significantly; for example: on-state resistance reduces by 63%, breakdown voltage drops by 31%, switching time decreases and switching energy loss decreases by 26%. Furthermore, the peak dv/dt during transient switching at cryogenic temperature exceeds 100 V/ns which is comparable to the emerging wide bandgap Gallium Nitride devices.

  • Jessica D. Boles; Yiwei Ma; Wenchao Cao; Leon M. Tolbert; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Preparing power systems to better accommodate renewable energy sources has become increasingly more important as penetration levels rise, and energy storage systems are excellent for suppressing the fluctuations of renewable and for providing other ancillary services to the grid. The Hardware Testbed (HTB) is a novel converter-based grid emulator created for studying the needs associated with high renewable penetration, but the system currently lacks battery storage capability. This paper proposes a configurable Lead Acid and Lithium Ion battery storage emulator equipped with a two-stage power electronics interface, which is capable of independent active and reactive power control as well as inertia emulation. Each part of the emulator is described in detail, in terms of both the models used and the control algorithms governing them. The emulator's behavior is simulated, tested, and confirmed to function correctly with the HTB and will be used to study scenarios in which battery storage can be used to support renewables and other dynamic power system needs.

  • Wen Zhang; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon Tolbert; Benjamin Blalock
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    While fast switching brings many benefits, it also presents unwanted ringing during switching transient. In this paper, an increasing magnitude ringing phenomenon is observed during the MOSFET turn-off transient. The unusual phenomenon is replicated in simulation and it is found the MOSFET channel is turned on again after it is turned off. The major cause to this unexpected turn on is found to be common source inductance and a moderate 3 nH one in simulation replicates the severe self-turn-on ringing observed in experiment. This paper reveals the detrimental effect of common source inductance in fast switching. Therefore, Kelvin source connection in circuit and package design is strongly recommended.

  • Jacob Dyer; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    2017 IEEE 5th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2017

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    This paper introduces a dead-time optimization technique for a 2-level voltage source converter (VSC) using turn-off transition monitoring. Dead-time in a VSC impacts power quality, reliability, and efficiency. Silicon carbide (SiC) based VSCs are more sensitive to dead-time from increased reverse conduction losses and turn-off time variability with operating conditions and load characteristics. An online condition monitoring system for SiC devices has been developed using gate drive assist circuits and a micro-controller. It can be leveraged to monitor turn-off time and indicate the optimal dead-time in each switching cycle of any converter operation. It can also be used to specify load current polarity, which is needed for dead-time optimization in an inverter. This is an important distinction from other inverter dead-time elimination/optimization schemes as current around the zero current crossing is hard to accurately detect. A 1kW half-bridge inverter was assembled to test the turn-off time monitoring and dead-time optimization scheme. Results show 91% reduction in reverse conduction power losses in the SiC devices compared to a set dead-time of 500ns switching at 50 kHz.

  • Shiqi Ji; Sheng Zheng; Zheyu Zhang; Fred Wang; Leon M. Tolbert
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Silicon Carbine (SiC) based power semiconductor devices have increased voltage blocking capability, in the meantime, satisfactory switching performance as compared to conventional Silicon (Si) devices. This paper focuses on the latest generation 10 kV / 20 A SiC MOSFETs and investigates their protection schemes and temperature-dependent switching characteristics. A high voltage double pulse test platform is constructed including solid state circuit breaker, gate drive and hot plate under device under test (DPT) for temperature-dependent characterization. A behavioral model is established to analytically investigate switching performance of 10 kV SiC MOSFETs, and the temperature-dependent factors are studied in detail. The experimental results under various load currents and gate resistances from 25 C to 125 C at 7 kV dc-link voltage are presented.

  • Shuoting Zhang; Yalong Li; Fred Wang
    2017 IEEE Energy Conversion Congress and Exposition (ECCE)
    2017

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    Multi-terminal dc (MTDC) grid has been considered as a promising future transmission and distribution system architecture, especially for remote renewable energy integration. However, short-circuit faults can be more detrimental to dc grids than ac grids. The lack of effective and economical dc circuit breakers and potential significant impact of dc fault on the connected ac system have become the main barrier for the dc grid application. This paper analyzes the major differences of dc grid and ac grid under short-circuit fault conditions. After that, the dc fault impact on the connected ac system is evaluated by comparing with an equivalent multi-terminal ac (MTAC) grid. Simulation results indicate that the dc fault impact on the connected ac system stability can be small if fast dc circuit breakers or full-bridge modular multi-level converters (MMCs) are employed. The impact of equivalent multiple ac faults on the connected ac system is small under the defined system scenarios.

  • Edward A. Jones; Paige Williford; Zhe Yang; Jianliang Chen; Fred Wang; Sandeep Bala; Jing Xu; Joonas Puukko
    2017 IEEE 12th International Conference on Power Electronics and Drive Systems (PEDS)
    2017

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    This paper establishes a methodology for maximizing the voltage and current capability of a GaN FET, while maintaining an acceptably low overshoot voltage and junction temperature to prevent damage to the device. Two key contributions of this work are the gate driver design parameters and operating conditions that impact overshoot voltage, and a heatsink design for bottom-side cooling that avoids thermal vias. Additionally, the static and dynamic characterization steps required for this methodology are described, and an example GaN-based full-bridge inverter was designed and tested for experimental verification, using GaN gate injection transistors with capacitive gate driver circuits.

  • Gabriel Gabian; Jordan Gamble; Benjamin Blalock; Daniel Costinett
    2017 IEEE 18th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2017

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    This work presents an analytical model for integrated DC-DC converters at high currents. A loss model is constructed using parameters extracted from simulation or are available in the process manual and are scaled with the size of the device. The loss model is used to compare power converter implementations for varying on-chip size and power loss goals. Buck, 3-Level Buck, and Switched-Capacitor topologies are compared using this analytical model and then implemented in a commercial CMOS process. Validation of the constructed loss model is done through hardware measurements.

  • Spencer Cochran; Daniel Costinett
    2017 IEEE 18th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2017

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    With the growth of magnetic resonance wireless power transfer (WPT), the WPT field sees a push toward higher operating frequencies. New challenges arise as the fundamental frequency increases, namely the relevance of circuit parasitics and the difficulty in preventing harmonic distortion. A 6.78 MHz synchronous rectifier is shown to address the issues of total harmonic distortion (THD) and dynamic loading. This work focuses on further modeling the proposed rectifier, accounting for parasitic conduction losses, THD, input phase control, and characterization of design trade offs. The updated model includes both an exact solution for the complete dynamics of the dead time resonance which, by design, has significant impact on converter harmonics, impedance, and power delivery. The model is compared to the simpler model from previous work and is verified via experimental results.

  • Ling Jiang; Daniel Costinett; Aly Fathy; Songnan Yang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    A new single-stage AC/RF converter is proposed which directly converts a utility AC input to a regulated, high frequency (6.78MHz) AC output for wireless power transmission. The topology integrates a totem pole rectifier operating in discontinuous conduction mode (DCM) and a phase-shift controlled full bridge inverter. Compared to traditional multi-stage approaches, the single-stage approach improves system power efficiency by reducing the number of cascaded conversion stages. In addition, the reduced power semiconductor component count will be a potential benefit for compact size and lower cost. A 100W laboratory prototype has been built to verify theoretical analysis. Experimental measurements show the capability of the converter to provide power factor correction (PFC) with high conversion efficiency and low total harmonic distortion (THD).

  • Gabriel Gabian; Benjamin Blalock; Daniel Costinett
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Battery charging circuits for mobile applications, such as smart phones and tablets, require both small area and low losses. In addition, to reduce the charging time, high current is needed through the converter. In order to reduce conduction losses, low on-resistance of the switches is necessary. However, specific resistance (resistance per unit area) is a strong function of the maximum voltage blocking capability of the transistors. To maintain high efficiency and ensure device reliability, the designed breakdown voltage of the transistors needs to include some margin to account for ringing on the switching node. Bond wires add inductance to the power loop increasing the overshoot voltage. In this work the design, implementation and testing of a 40 W CMOS integrated buck converter with an on chip decoupling capacitor are presented. The design was optimized for a 5V to 4V application with a maximum of 2 W on-chip losses at 10 A with an operating frequency of 1 MHz.

  • Edward A. Jones; Paige Williford; Fred Wang
    2017 IEEE 5th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2017

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    A fast overcurrent protection scheme was developed for GaN gate injection transistors (GITs), harnessing the relationship between the externally measured vgs and id in steady-state operation. This relationship has been characterized in both static and dynamic testing over a wide range of operating conditions, and a circuit has been constructed to implement this control scheme. The circuit uses analog components to integrate the protection feature into a commercially available GIT gate driver. The scheme was experimentally verified in a double pulse test setup for experimental verification, and its total fault response time was recorded at less than 70 ns, with 400 V dc bus and a 30 A threshold. Compared with conventional desaturation protection, which detects faults based on drain voltage rather than gate voltage, the proposed scheme offers benefits in terms of speed, temperature invariance, flexibility in threshold selection, and minimal impact on the GIT's normal switching behavior.

  • Edward A. Jones; Zheyu Zhang; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    The higher switching speed of wide bandgap devices requires new analysis to interpret voltage waveforms during turn-on and turn-off transients. Although the Miller effect remains a dominant feature, the conventional Miller plateau equations do not accurately model the dvds/dt for fast-switching devices such as GaN FETs. This paper derives equations for instantaneous dvds/dt based on static datasheet parameters, considering the Miller effect and the displacement of junction capacitance charges through the saturated channel. These equations will be verified with experimental results for an enhancement-mode GaN FET across a range of operating conditions. Furthermore, the peak dvds/dt is predicted using the derived equations, and shown to be more accurate than other models when compared to GaN experimental results.

  • Saeed Anwar; Daniel J. Costinett
    2017 IEEE Transportation Electrification Conference and Expo (ITEC)
    2017

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    In this paper, a control strategy of an integrated, reconfigurable DC-DC converter for plugin electric vehicles (EVs) is proposed. The integrated converter, capable of operating in both traction and charging modes, can be reconfigured as an interleaved boost or a dual active bridge (DAB) converter. The existing contactors of the battery management system (BMS) are used for reconfiguration between the boost and DAB modes. To maximize overall power conversion efficiency during traction operation, the converter is dynamically reconfigured to operate in the mode with highest efficiency at the present operating point. A switching transition control approach is developed for fast and seamless switching, ensuring zero-voltage turn-on and zero-current turn-off of the BMS contactors. The smooth transitions minimize switching-induced degradation of the BMS contactors, and allow uninterrupted power delivery during mode transitions. The experimental results of the prototype are presented to verify the functionality of the proposed control approach.

  • Chongwen Zhao; Daniel Costinett
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Concurrent dual-frequency ac outputs from a single-inverter configuration potentially benefit many industrial applications, such as induction heating and wireless power transfer. In this paper, a phase-shift dual-frequency selective harmonic elimination (DFSHE) method is proposed to simultaneously generate and regulate two ac outputs at different frequencies from a single full-bridge inverter, which expands the family of DFSHEs. With the phase shift operation, all triplen harmonics of the fundamental are inherently suppressed in the inverter output spectrum, which improves the output THD, and may ease filter design. In addition, an evaluation of the unipolar, bipolar and phase-shift DFSHE techniques is presented, which analyzes design tradeoffs for multi-frequency applications. Finally, experimental results from a 50 W dual-output inverter validate the effectiveness of the proposed method, which agree with theoretical predictions and simulation results.

  • Fei Yang; Zhenxian Liang; Zhiqiang Jack Wang; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    In this paper, a low parasitic inductance SiC power module with double-sided cooling is designed and compared with a baseline double-sided cooled module. With the unique 3D layout utilizing vertical interconnection, the power loop inductance is effectively reduced without sacrificing the thermal performance. Both simulations and experiments are carried out to validate the design. Q3D simulation results show a power loop inductance of 1.63 nH, verified by the experiment, indicating more than 60% reduction of power loop inductance compared with the baseline module. With 0Ω external gate resistance turn-off at 600V, the voltage overshoot is less than 9% of the bus voltage at a load of 44.6A.

  • Fei Yang; Zhenxian Liang; Zhiqiang Jack Wang; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    In this paper, a low parasitic inductance SiC power module with double-sided cooling is designed and compared with a baseline double-sided cooled module. With the unique 3D layout utilizing vertical interconnection, the power loop inductance is effectively reduced without sacrificing the thermal performance. Both simulations and experiments are carried out to validate the design. Q3D simulation results show a power loop inductance of 1.63 nH, verified by the experiment, indicating more than 60% reduction of power loop inductance compared with the baseline module. With 0Ω external gate resistance turn-off at 600V, the voltage overshoot is less than 9% of the bus voltage at a load of 44.6A.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    In this paper, a single stage system which converts 400 V to 1 V within one stage and performs as the high voltage point of load (HV POL) converter for data centers is proposed. A load dependent soft switching method has been proposed for half bridge current doubler with simple auxiliary circuit. The operation principles of the soft switching converter have been analyzed in detail. A lossless RCD current sensing method is used to sense the output current value to reduce the auxiliary circuit loss and turn off loss of secondary side devices as load reduces to achieve higher efficiency. Experimental efficiency has been tested to prove the proposed method can increase the converter's efficiency in both heavy and light load condition. A prototype of the half bridge current doubler circuit has been built to verify the theory.

  • Jacob Dyer; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    2016 IEEE 4th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2016

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    Many intelligent gate drivers being designed for new state-of-the-art WBG devices typically only focus on protection and driving capabilities of the devices. This paper introduces an intelligent gate driver that incorporates online condition monitoring of the WBG devices. For this specific case study, three timing conditions (turn-off delay time, turn-off time, and voltage commutation time) of a silicon carbide (SiC) device are online monitored. This online monitoring system is achieved through gate driver assist circuits and a micro-controller. These conditions are then utilized to develop converter-level benefits for the converter application the SiC devices are placed in. Junction temperature monitoring is realized through turn-off delay time monitoring. Dead-time optimization is achieved with turn-off time monitoring. Dead-time compensation is obtained with turn-off time and voltage commutation time monitoring. The case study converter assembled for testing purposes is a half-bridge inverter using two SiC devices in a phase-leg configuration. All timing conditions are correctly monitored within reasonable difference of the actual condition time. A calibration curve was created to give a direct relationship between turn-off delay time and junction temperature. The half-bridge inverter can operate at 600 Vdc input and successfully obtain a junction temperature measurement through monitored td_off and the calibration curve. Furthermore, the proposed online condition monitoring system is transistor based and suitable for the chip level integration, enabling this practical approach to be cost-effective for end users.

  • Spencer Cochran; Farhan Quaiyum; Aly Fathy; Daniel Costinett; Songnan Yang
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    This work examines the potential of a GaN-based synchronous rectifier as a receiver in 6.78 MHz wireless power transfer (WPT) applications. Compared to a traditional diode-based rectifier, the synchronous rectifier has merits in its additional control freedoms. The active control of switching actions can be used to alter impedance presented to transmitting source, or, when combined with a zero-voltage switching (ZVS) resonant tank, to reduce harmonics generated by the switching actions, and therefore mitigate requirements on the filter network to meet EMI limitations. Analysis of control and hardware design strategies to maximize efficiency, limit total harmonic distortion (THD), and control impedance at the WPT frequency are presented and verified on a prototype experimental platform.

  • Bo Liu; Ren Ren; Edward Jones; Fred Wang; Daniel Costinett; Zheyu Zhang
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Wide bandgap (WBG) semiconductors owing to their low loss and high switching capability, are gradually adopted in high power-density high efficiency applications, and impose new challenges from control to hardware design. In this paper, a Gallium Nitride (GaN) HEMT plus SiC diode based Vienna type rectifier is proposed to serve as the power factor correction stage for a high-density battery charger system. To meet low current harmonic requirement, PWM voltage distortion during turn-off transition, found as the main harmonics contributor, is studied. The distortion mechanism led by different parasitic capacitances of WBG devices is presented. A mitigation scheme is thereafter proposed considering their nonlinear voltage-dependent characteristics and eventually deduced from a pulse-based turn-off compensation to a generic modulation correction. Simulation and experimental results through a 450 kHz enhancement-mode GaN based Vienna type rectifier finally demonstrate the high performance of the proposed approach, showing a THD reduction up to 7% with a relatively low-speed control.

  • Bo Liu; Shuoting Zhang; Sheng Zheng; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Ac transmission line emulator is the bridge to interconnect ac systems to fulfill the grid emulation function, where all the emulator elements such as generators, loads and lines are implemented by universal three-phase voltage source converters. In this paper, three design issues are addressed. First, the impact of ac voltage switching noise on the performance of a transmission line emulator in terms of steady state and dynamic accuracy is described, and an improved sampling algorithm is presented. Then, a new dc offset controller is proposed to mitigate the induced dc current flow by sampled dc offset noise, to guarantee the normal operation of ac line emulator. Furthermore, the stability issues regarding different emulation schemes are analyzed, providing a metric to predict the feasible impedance range that a line emulator can reach and to choose the proper emulation strategy for a specific system. Finally, experimental results obtained from a multi-converter based hardware testbed verify the design schemes.

  • Fengkai Hu; Liu Yang; Jingxin Wang; Yiwei Ma; Kai Sun; Leon M. Tolbert; Fred Wang
    2016 IEEE Power and Energy Society General Meeting (PESGM)
    2016

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    A measurement-based voltage stability assessment and closed-loop control strategy is proposed and demonstrated on the Center for Ultra-Wide-Area Resilient Electric Energy Transmission Networks (CURENT) Hardware Test Bed (HTB) system, a power electronic converter-based research and experiment platform. This new strategy is based on an N+1 buses equivalent proposed recently by Ref. [1] for calculating real-time voltage stability margins on individual tie lines of a load area. Two voltage stability scenarios are designed and implemented on the HTB system that emulates a three-area power system integrating conventional generation, wind generation, and multi-terminal HVDC transmission. The tests validate the effectiveness of real-time monitoring and closed-loop control against voltage instability initiated from one tie line of the load area.

  • Richard Kyle Harris; Benjamin M. McCue; Benjamin D. Roehrs; Charles Roberts; Benjamin J. Blalock; Daniel J. Costinett; Kouros Sariri; George Megyei; Cheng-Po Chen; Avinash Kashyap; Reza Ghandi
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    The properties of silicon carbide (SiC) integrated circuit (IC) processes are discussed and nonlinear-carrier control is proposed as a controller topology that can work within the design challenges presented by SiC. A boost converter with NLC controller is demonstrated using circuit blocks built with SiC IC models.

  • Zheyu Zhang; Fred Wang; Daniel J. Costinett; Leon M. Tolbert; Benjamin J. Blalock; Xuanlyu Wu
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Junction temperature is a critical indicator for health condition monitoring of power devices. Concerning the reliability of emerging silicon carbide (SiC) power semiconductors due to immaturity of new material and packaging, junction temperature measurement becomes more significant and challenging, since SiC devices have low on-state resistance, fast switching speed, and high susceptibility to noise and parasitics in circuit implementations. This paper aims at developing a practical and cost-effective approach for online junction temperature monitoring of SiC devices using turn-off delay time as the thermo-sensitive electrical parameter (TSEP). The sensitivity is analyzed for fast switching SiC devices. A gate impedance regulation assist circuit is designed to improve the sensitivity by a factor of 60 and approach hundreds of ps/°C in the case study with little penalty of the power conversion performance. Also, an online monitoring system based on three gate assist circuits is developed to monitor the turn-off delay time in real time with the resolution within hundreds of ps. In the end, the micro-controller is capable of “reading” junction temperature during the converter operation with less than 0.5 °C measurement error. Two testing platforms for calibration and online junction temperature monitoring are constructed, and experimental results demonstrate the feasibility and accuracy of the proposed approach. Furthermore, the proposed gate assist circuits for sensitivity improvement and high resolution turn-off delay time measurement are transistor based and suitable for chip level integration.

  • Jessica D. Boles; Burak Ozpineci; Leon M. Tolbert; Timothy A. Burress; Curt W. Ayers; Jared A. Baxter
    2016 IEEE Power and Energy Society General Meeting (PESGM)
    2016

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    Transmission line inspection becomes increasingly more important as power system infrastructure ages because proactively identifying line maintenance needs is crucial for minimizing outages. Utilizing robots to conduct such inspections is both safer for humans and less costly in terms of labor, but finding an appropriate on-line robot design presents its own set of challenges. This paper proposes a touch-free transmission line inspection system in which an unmanned aerial vehicle (UAV) conducts all inspection activities and, while doing so, charges from the line via inductive power transmission. Two coil designs are presented and tested for this charging application - one with an air core and one with a line-enclosing core clamp. Finally, the benefits and challenges associated with each design are discussed, along with the general practicality of inductive charging via transmission lines for UAV applications.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    High speed switching of WBG devices causes their switching behavior to be highly susceptible to the parasitics in the circuit, including inductive loads. An inductive load consisting of a motor and power cable significantly worsens the switching speed and losses of SiC MOSFETs in a PWM inverter. This paper focuses on the motor plus power cable based inductive load, and aims at mitigating its negative influence during the switching transient. An auxiliary filter is designed and inserted between the converter and inductive load so that the parasitics of the load will not be “seen” from the converter side during the switching transient. Test results with Cree 1200-V/20-A SiC MOSFETs show that the proposed auxiliary inductor enables the switching performance with a practical inductive load (e.g., motor plus cable based inductive load) to exhibit behavior close to that when the optimally-designed double pulse test load inductor is employed.

  • Ren Ren; Bo Liu; Edward A. Jones; Fred Wang; Zheyu Zhang; Daniel Costinett
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Due to the realization of zero voltage switching (ZVS) under the full load range, LLC resonant converter is widely adopted in the telecom, battery charger and several applications, characterized with high efficiency, high frequency and high power density, to realize DC conversion. Recently, by using Gallium Nitride (GaN) HFETS, switching frequency of LLC converters is further increased. However, ZVS failure cannot be predicted accurately in the high switching frequency condition by only considering traditional constraints generally applied in the low frequency design. The traditional constraints result in a too optimistic estimation of the dead time to obtain ZVS without considering the reverse resonance under the dead time and the design of resonant parameters at high resonant frequency and high load condition. The experiment shows the LLC converter loses ZVS even through the converter satisfies the ZVS constraints proposed by previous paper. In this paper, the failure mode will be investigated in detail and an accurate ZVS boundary is proposed for high frequency LLC converter design. The proposed theory was verified on a 1 MHz, 1500 W LLC prototype.

  • Ren Ren; Bo Liu; Edward A. Jones; Fred Wang; Zheyu Zhang; Daniel Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Gallium Nitride (GaN) HFETS are an enabling technology for high-density converter design. This paper proposes a three-level dc-dc converter with dual outputs based on enhancement-mode GaN devices, intended for use as a battery charger in aircraft applications. The charger can output either 28 V or 270 V, selected with a jumper, which meets the two most common dc bus voltages in airplanes. It operates as an LLC converter in the 28 V mode, and as a buck converter in the 270 V mode. In both operation modes, the devices can realize zero-voltage-switching (ZVS). With the chosen modulation method, the converter can realize the frequency doubling function to act as an interleaved converter. For the LLC mode, the resonant frequency is twice the switching frequency of primary-side switches, and for buck mode, the frequency of the output inductor current is also twice the switching frequency. This helps to reduce the size of magnetics while maintaining low switching loss. Also, the converter utilizes the matrix transformer with resonant parameters designed to avoid ZVS failure. The operation principle of the converter is analyzed and verified on a 1MHz resonant frequency prototype.

  • Xiaojie Shi; Yalong Li; Zhiqiang Wang; Bo Liu; Leon M. Tolbert; Fred Wang
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    This paper presents a steady-state analysis of the modular multilevel converter (MMC) for the second order voltage and current ripple prediction under unbalanced conditions, taking the impact of negative sequence current control into account. Using the circular relationship among current and voltage quantities, the magnitudes and initial phase angles of different circulating current components can be evaluated theoretically. With negative sequence phase current control, the positive, negative and zero sequence circulating currents are generated by more voltage sources and are no longer decoupled. Based on the generic inner relationship among current and voltage quantities, this steady state analysis is applicable to the MMC under both rectifier and inverter operating modes. Experimental results from a scaled down three-phase MMC system are provided to support the theoretical analysis and derived model.

  • Shuoting Zhang; Yiwei Ma; Liu Yang; Fred Wang; Leon M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    A real-time hardware testbed (HTB) has been constructed to emulate the power system by modular regenerative converters. This allows system realistic testing and demonstration with the true measurement, communication, and control. However, the size of the system that can be emulated by the HTB is limited, and certain phenomena are not easy or not needed to be modeled in the HTB. A hybrid emulation platform, which combines real time digital simulator (RTDS) and HTB, is developed in this paper to complement the advantages of RTDS and HTB. A power electronics converter is designed to act as the power interface between the RTDS and the HTB, and an integrated interface with two complementary algorithms is implemented to realize the hybrid emulation stably under different system conditions. At the same time, the closed loop control method under dq0 axis is implemented to realize faster response characteristics, and a time delay correction algorithm is integrated into the Park transformation. Experiment results demonstrate the performance and effectiveness of the hybrid emulation compared with the pure HTB emulation and digital simulation.

  • Ling Jiang; Farshid Tamjid; Chongwen Zhao; Daniel Costinett; Aly Fath; Songnan Yang
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    A two-stage power conversion architecture for the transmitter in wireless power transfer applications is introduced. The system achieves high efficiency at output powers up to 100W, and exhibits constant output current over varying load impedance. A front-end bridgeless totem pole rectifier provides power factor correction (PFC), necessary at the designed power level. This rectifier achieves high efficiency by eliminating the conventional diode full bridge and by achieving soft switching operation. A full bridge inverter, switching at 6.78MHz, generates the AC output. Combined with an output passive filter network, the inverter achieves constant output current with load variation without the need for dynamic feedback control. A prototype system is constructed and tested experimentally to verify operation.

  • Yalong Li; Xiaojie Shi; Fred Wang; Leon M. Tolbert; Jin Liu
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Dc fault protection is a main challenge in voltage source converter (VSC) based multi-terminal high voltage direct current (HVDC) systems. This paper develops a systematic dc fault protection strategy for systems utilizing hybrid dc circuit breakers as the main protection devices. A two-step fault detection method to accommodate the proactive hybrid dc circuit breaker has been simulated and demonstrated with both fast speed and selectivity. The necessities of temporary blocking HVDC converters for both pole-to-pole and pole-to-ground faults have been evaluated, and the corresponding criteria have been established. In order to achieve fast system recovery after the fault clearance, voltage margin control is proposed to simplify the restart sequence for different converters and reduces the dc voltage variation during the process. The overall protection strategy is demonstrated in a 4-terminal HVDC simulation platform, showing a total dc fault recovery time of ~200 ms.

  • Saeed Anwar; Weimin Zhang; Fred Wang; Daniel J. Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    In this paper, an integrated, reconfigurable DC-DC converter for plugin and hybrid Electric Vehicles (EV) is proposed. The converter integrates functionality for both EV powertrain and charging operation into a single unit. During charging, the proposed converter functions as a DAB converter, providing galvanic isolation. For powertrain operation, the converter functions as an interleaved boost converter. During light load powertrain operation, the efficiency of the converter can be further improved by employing the integrated DAB. The proposed integrated converter does not require any extra relays or contactors for charging and powertrain operation. By using such integration, the overall volume and weight of the power electronics circuits, passives and associated cooling system can be improved. In addition, the power flow efficiency from EV battery to the high voltage DC bus for the motor inverter can be improved. The experimental results of the prototype are presented to verify the functionality of the proposed converter.

  • Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang; Ben Guo
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Enhancement-mode GaN HFETs enable efficient high-frequency converter design, but this technology is relatively new and exhibits different characteristics from Si or SiC MOSFETs. GaN performance at elevated temperature is especially unique. Turn-on time increases significantly with temperature, and turn-on losses increase as a result. This phenomenon can be explained based on the relationships between junction temperature and GaN device transconductance, and between transconductance and turn-on time. An analytical relationship between temperature and turn-on loss has been derived for the 650-V GS66508 from GaN Systems, and verified with experimental results. Based on this relationship, a detailed model is developed, and a simplified scaling factor is proposed for estimating turn-on loss in e-mode GaN HFETs, using room-temperature switching characterization and typically published datasheet parameters.

  • Yiwei Ma; Liu Yang; Fred Wang; Leon M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    One way to incorporate the increasing amount of wind penetration is to control wind turbines to emulate the behavior of conventional synchronous generators. This paper presents a comprehensive virtual generator control for the full converter wind turbine considering the power balance. The voltage closed-loop virtual synchronous generator control of the wind turbine allows it to work under both grid-connected and stand-alone condition. Power system control and power dispatch can also be realized through the control. The power balance of the wind turbine system is achieved by controlling the rotor speed of the turbine according to the loading condition. The optional integration of the short term turbine level energy storage is also considered. Experimental results on emulation testbed are presented to demonstrate the feasibility and effectiveness of the proposed control method.

  • Xiaojie Shi; Yalong Li; Leon M. Tolbert; Fred Wang
    2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)
    2016

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    In a multi-terminal HVDC (MTDC) system connecting windfarms, the widely used dc voltage margin and droop control cannot be directly applied to offshore converters due to the lack of existing ac grid. Therefore, the dc voltage regulation only relies on the onshore converters, and its variation will not affect wind power generation. This paper introduces a cascaded droop control scheme for offshore converters and windfarms, which enables autonomous wind power adjustment during onshore station side ac faults, using offshore side ac voltage magnitude as an intermediate variable. Different from the traditional dc voltage droop control (νdc - pdc droop), the embedded dead-band and offshore station transformer leakage impedance will greatly impact the operation point of the MTDC system if the proposed cascade droop control is used, thus requiring special consideration during droop parameters design. Simulation results from a four-terminal HVDC system generated with Matlab/Simulink and experimental results from a scaled down prototype are provided to support the theoretical analysis and proposed control scheme.

  • Shiqi Ji; Fred Wang; Leon Tolbert; Ting Lu; Zhengming Zhao; Hualong Yu
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    The series connection of insulated gate bipolar transistors (IGBTs) allows the operation at voltage levels higher than the rated voltage of one IGBT. However, the technology has not been widely applied due to transient voltage unbalance. Asynchronous gate drive signals, which cause series-connected IGBTs not to turn-on and turn-off at the same time, result in serious unbalanced voltage sharing. This paper presents an active voltage balancing control for multi series connected HV-IGBTs including the active voltage balancing control (AVBC) circuit integrated in the gate driver and the control for multi series connected IGBTs. The effectiveness of the control has been experimentally validated in a 10 kV dc-link voltage converter using four 4.5 kV HV-IGBTs in series connection.

  • Wenchao Cao; Xuan Zhang; Yiwei Ma; Fred Wang
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    To address the instability issue in renewable systems of a radial-line structure with multiple current-controlled interface inverters, this paper proposes a practical stability criterion to easily analyze the system stability and a controller parameter design method to guarantee stable system operation with good oscillation damping performance. The proposed impedance-based sufficient stability criterion does not need the pole calculation of the return ratio matrices, while the phase margin of the system can still be obtained for system dynamic performance evaluation. Based on the phase margin information, design rules of inverter controller parameters are further proposed for system stability. The output admittance model of current-controlled inverters in an arbitrary d-q frame is also derived to facilitate the stability analysis. Simulation and experimental results verify the effectiveness of the proposed stability criterion and controller parameter design method.

  • Zhiyuan Shen; Handong Gui; Leon M. Tolbert
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    According to the analysis of the imbalance in series-connected lithium-ion batteries, the state-of-health (SOH) and the energy utilization is mainly influenced when the cells are fully charged or discharged. Therefore, balancing is not necessarily required all the time. In addition, the energy loss of the balancing system is closely related to the charge transfer among different cells. This paper proposes an active balancing control scheme that utilizes the required charge of each cell as the balancing reference. With the scheme, the charge transfer during the balancing process can be minimized so that the energy loss can be reduced and the balance can be achieved when the cells are fully charged or discharged, which not only improves the battery lifetime but also increases the available energy. The proposed control is applied in a balancing system based on the direct cell-to-cell architecture and bi-directional buck/boost converter. Experimental results have verified the effectiveness of the proposed control.

  • Jing Xue; Fred Wang
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    This paper focuses on the liquid-cooling method of power inductors in electromagnetic interference (EMI) filters for high power motor drive application. A literature study on magnetic cooling methods with encapsulation, potting and liquid-cooled cold plate is carried out. An empirical evaluation method for potting effectiveness is proposed and validated with prototype encapsulation and example potting materials. One simplified experiment-based thermal modeling method for inductors is also developed with the purpose of avoiding time-consuming finite element simulation. Based on the potting evaluation method and simplified thermal modeling, one comprehensive design procedure is summarized.

  • Douglas W. Bouler; Jared Baxter; Daniel Costinett
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    This paper presents a method of optimizing low power boost converters for use in far-field wireless energy harvesting systems. This method uses a database of manufacturer-provided device characteristics of both Silicon and GaN FETs to construct Figures of Merit (FOMs) used for evaluating device technologies for power loss based on boost converter parameters. A loss model is constructed for predicting device power losses and system efficiency over a wide range of operating points. Using the analysis framework, an asynchronous boost converter is constructed and experimentally verified with operation as low as 10 μW with a peak efficiency of 74% at an input power of 300 μW.

  • Chongwen Zhao; Daniel Costinett; Brad Trento; Daniel Friedrichs
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Simultaneous generation of two AC outputs at different frequencies from a single-phase inverter offers practical benefits and control flexibility for many industrial applications. In this paper, a dual-frequency selective harmonic elimination (DFSHE) modulation method is proposed to generate and control two individual frequencies in an H-bridge, while diminishing undesired harmonics. Two AC elements are synthesized independently in the modulation scheme, and thus flexible individual power regulation is achieved via the proposed method. In addition, both unipolar and bipolar DFSHE cases are investigated. Characteristics of the two methods are compared and alternatives are provided for different applications. The generation algorithms of the DFSHE are also studied in this paper, and can be applied to a variety of DC/AC topologies without adding extra switching devices. Finally, the experimental results from a 100W dual-load single-phase inverter verify the effectiveness of proposed method, where 50 kHz and 450 kHz AC outputs are generated and individually regulated.

  • Wenchao Cao; Yiwei Ma; Fred Wang
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Three-phase inverter-based multi-bus ac systems could suffer from the small-signal instability issue due to the dynamic interaction among inverters and passive components in the systems. To address this issue, this paper proposes two harmonic stability analysis methods and an inverter controller parameter design approach for stable system operation. The proposed sequence-impedance-based harmonic stability analysis methods can reduce the computation effort by avoiding the calculation of right-half-plane poles of impedance ratios, as compared with the impedance-based analysis method using Nyquist stability criterion. Therefore, the controller parameters can be designed in the forms of stability regions in the parameter space, by repetitively applying the proposed stability analysis methods. In addition, the proposed stability analysis methods enable the system stability by using only measured component impedances. Experimental results of an inverter-based two-area system validate the effectiveness of the proposed stability analysis methods and parameter design approach.

  • Ling Jiang; Daniel Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    In this paper, a triple active bridge converter is proposed. The topology is capable of achieving ZVS across the full load range with wide input voltage while minimizing heavy load conduction losses to increase overall efficiency. This topology comprises three full bridges coupled by a three-winding transformer. At light load, by adjusting the phase shift between two input bridges, all switching devices can maintain ZVS due to a controlled circulating current. At heavy load, the two input bridges work in parallel to reduce conduction loss. The operation principles of this topology are introduced and the ZVS boundaries are derived. Based on analytical models of power loss, a 200W laboratory prototype has been built to verify theoretical considerations.

  • Chongwen Zhao; Daniel Costinett
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    Multi-frequency wireless power transfer (WPT) is advantageous in facilitating compatibility with different WPT standards. However, implementing a multi-frequency transmitter often requires compromises in system size, complexity, power transfer capability, or output regulation. In this paper, a single-inverter based dual-mode WPT system is proposed. The system employs a multi-frequency programmed pulse width modulation (MFPWM) scheme. This dual-frequency modulated inverter can simultaneously generate and regulate 100 kHz and 6.78 MHz outputs, which facilitates the development of multi-standard WPT technology for consumer electronics. In addition, the principle of the proposed modulation is illustrated, where two different frequencies are concurrently modulated in the programmed pulse train of square waveforms, while eliminating certain harmonics in between. Design tradeoffs and constraints are examined through analytical circuit models. Finally, experimental results are provided to verify the effectiveness of the proposed WPT system.

  • Fei Yang; Zhenxian Liang; Zhiqiang Wang; Fred Wang
    2016 International Symposium on 3D Power Electronics Integration and Manufacturing (3D-PEIM)
    2016

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    In this paper, the parasitic inductance extraction method is studied in detail. It is analyzed that the value of the lumped power loop inductance will be varying at different switching transients. With the aid of Ansys Q3D Extractor, different values of lumped power loop parasitic inductance are obtained at different time intervals during turn-off process for both upper and lower devices. A dedicated 3D Planar Bond All Module with access to both kelvin and terminal drain-to-source voltage is built, and the parasitic inductance of the module is experimentally extracted by comparing those two voltages in double pulse tests. The experiment result shows good agreement with the simulated parasitic inductance value thus validating the extraction and simulation method.

  • Shiqi Ji; Ting Lu; Zhengming Zhao; Hualong Yu; Fred Wang
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    The integration of series connection of insulated gate bipolar transistors (IGBTs) and multi-level can achieve high voltage converters with low total harmonics distortion (THD). However, due to the transient voltage unbalance, the series connection technology has not been widely applied. Asynchronous gate drive signals, which cause series-connected IGBTs not to turn-on and turn-off at the same time, result in serious unbalanced voltage sharing. This paper presents an active voltage balancing control with its electromagnetic compatibility (EMC) design to solve the asynchronous gate signal problem. The effectiveness of the active voltage balancing control has been experimentally validated in a 10kV dc-link voltage three-level bridge using two 4.5kV HV-IGBTs in series-connection.

  • Shiqi Ji; Ting Lu; Zhengming Zhao; Hualong Yu; Fred Wang
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    The integration of series connection of insulated gate bipolar transistors (IGBTs) and multi-level can achieve high voltage converters with low total harmonics distortion (THD). However, due to the transient voltage unbalance, the series connection technology has not been widely applied. Asynchronous gate drive signals, which cause series-connected IGBTs not to turn-on and turn-off at the same time, result in serious unbalanced voltage sharing. This paper presents an active voltage balancing control with its electromagnetic compatibility (EMC) design to solve the asynchronous gate signal problem. The effectiveness of the active voltage balancing control has been experimentally validated in a 10kV dc-link voltage three-level bridge using two 4.5kV HV-IGBTs in series-connection.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    High voltage DC (400 V) power supply architecture is becoming a standard in today's data center power supply. To further convert from 400 V to 1 V, usually several power stages are connected in series. Therefore, even if the efficiency of each power stage is high; the overall system efficiency is limited because of the multiplication of each converter's efficiency. In this paper, a single power stage system which converts 400 V to 1 V directly and performs as the high voltage point of load (HV POL) is proposed. A multi-phase interleaved phase shift pulse width modulation (PWM) DC/DC converter with input series and output parallel (ISOP) connection is selected as the power stage topology. A simplified two phase connected system is discussed in this paper. Common duty cycle control technique is used to control the ISOP connected converters. Input voltage sharing and output current sharing is analyzed with different types of mismatches in the circuit. Finally, the preliminary testing results are given.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    In this paper, the design of a high step down ratio (66:1) phase shift full bridge (PSFB) DC/DC converter used for data center power supplies in terms of primary side MOSFETs selection is covered. A detailed analysis of the converter's operation considering the impact of the output junction capacitance of primary side MOSFETs on the current RMS value has been performed. The study shows that a smaller output junction capacitance will lead to a smaller RMS current value on both primary and secondary side. For the high step down phase shift full bridge converter, transformer winding loss is the dominant loss; the reduction of current through the transformer will lead to a higher efficiency of the whole converter. This phenomenon is observed in experimental waveforms, and its impact on the converter's efficiency is also validated through experiment.

  • Weimin Zhang; Yutian Cui; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    This paper investigates the Gallium Nitride (GaN) devices benefits on the LLC resonant DC-DC converter. First, the relationship between the device parameters and converter current based on an analytical loss model of LLC resonant converter has been established. After that, the loss analysis and comparison between Si-based and GaN-based converter is presented. The GaN-based design demonstrates about 40% loss reduction compared with the Si-based design. An insight on the extra winding loss due to the asymmetrical primary side and secondary side current is presented. The extra winding loss is reduced by 18% with GaN device application. The overall loss breakdown and the experimental result show the 20% overall loss reduction of the GaN-based LLC converter compared with the Si-based LLC converter.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei Fred Wang; Zhenxian Liang; Daniel J. Costinett; Benjamin J. Blalock
    2015 IEEE International Workshop on Integrated Power Packaging (IWIPP)
    2015

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    A board-level integrated silicon carbide (SiC) MOSFET power module is developed in this work for high temperature and high power density applications. Specifically, a silicon-on-insulator (SOI) based gate driver is designed, fabricated and tested at different switching frequencies and temperatures. Also, utilizing high temperature packaging technologies, a 1200 V / 100 A SiC MOSFET phase-leg power module is built. The switching performance of the fabricated power module is fully evaluated at different temperatures up to 225 °C. Moreover, a buck converter prototype incorporating the SOI gate driver and SiC power module is built for high temperature continuous operation. The converter is operated within a wide range from 10 kHz to 100 kHz, with its junction temperature monitored by a thermo-sensitive electrical parameter (TSEP). The experimental results demonstrate that the integrated power module is able to operate at a junction temperature greater of 232 °C.

  • Xiaojie Shi; Bo Liu; Zhiqiang Wang; Yalong Li; Leon M. Tolbert; Fred Wang
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    Due to modularity and high efficiency, modular multilevel converter (MMC) has become a promising topology in high-voltage direct-current (HVDC) transmission systems. However, because of its distributed capacitors, a special capacitor charging process is required in some applications to avoid large inrush arm current. To deal with this issue, the charging loops and associated equivalent circuit of MMC based inverter during uncontrolled pre-charge period are analyzed in this paper, with special focus on the necessity of additional capacitor charging schemes. Moreover, the small signal model of the capacitor charging loop is first derived according to the internal dynamics of the MMC inverter. Based on this model, design considerations of the averaging capacitor voltage control are supplied in detail, which indicates a poor dynamic response of such control due to the resonance among arm inductance and submodule capacitances. To address this problem, a novel feedforward capacitor voltage control is proposed, which can cooperate with the averaging control to obtain enhanced dynamic response and system stability without sacrificing voltage control precision. Simulation and experimental results from a MMC inverter under different load conditions are provided to support the theoretical analysis and proposed control scheme.

  • Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang; Ben Guo; Bo Liu; Ren Ren
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    GaN heterojunction field-effect transistors (HFETs) in the 600-V class are relatively new in commercial power electronics. The GaN Systems GS66508 is the first commercially available 650-V enhancement-mode device. Static and dynamic testing has been performed across the full current, voltage, and temperature range to enable GaN-based converter design using this new device. A curve tracer was used to measure Rds-on across the full operating temperature range, as well as the self-commutated reverse conduction (i.e. diode-like) behavior. Other static parameters such as transconductance and gate current were also measured. A double pulse test setup was constructed and used to measure switching loss and time at the fastest achievable switching speed, and the subsequent over-voltages due to the fast switching were characterized. Based on these results and analysis, an accurate loss model has been developed for the GS66508 to allow for GaN-based converter design and comparison with other commercially available devices in the 600-V class.

  • Zheyu Zhang; Fred Wang; Daniel J. Costinett; Leon M. Tolbert; Benjamin J. Blalock; Haifeng Lu
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    Dead-time in the voltage source converter significantly affects the reliability, power quality and losses. For SiC devices, considering the high sensitivity of turn-off time to the operating conditions (> 5× difference between light load and full load), as well as large extra energy loss induced by reverse conduction during superfluous dead-time (~ 15% of the switching loss), traditional fixed dead-time setting becomes inappropriate. This paper introduces an approach to achieve optimum dead-time for SiC based voltage source converter. First, turn-off behaviors under various operating conditions are investigated, and the relation between optimal dead-times and load currents are established. Second, a practical method for adaptive dead-time regulation is proposed, which consists of a dead-time optimization model and two gate assist circuits to sense the voltage commutation time during turn-off transient. Via synthesizing the monitored switching condition together with the preset dead-time optimization model, the micro-controller is able to online adjust the dead-time. Finally, based on a buck converter with 1200-V SiC MOSFETs, the test results show that by means of the proposed method, the power loss decreases by 12% at full load and 18.2% at light load.

  • Sheng Zheng; Jingxin Wang; Fei Yang; Fred Wang; Leon M. Tolbert; Daniel J. Costinett
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    Continuous variable series reactors (CVSRs), as a cost effective alternative to flexible AC transmission system (FACTS) series compensators, have been proposed to continuously vary the line reactance and control the power flow. The development of the power electronics based dc controller (DCC) is essential and unique to meet the need of CVSR in utility transmission grid applications. In addition to supplying the needed dc current to the CVSR dc winding, the DCC has to deal with the interaction from the ac winding. CVSR, together with DCC, will be installed outdoor in a substation, so the operation environment could be extremely harsh. The detailed design and implementation of the DCC are presented, along with simulations demonstrating the close relationship between the load profile of dc winding and converter output impedance. A 1000 A, 20 kW field prototype has been constructed and tested with a 115 kV, 1500 A CVSR to experimentally verify the performance of the whole CVSR system.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2015 IEEE 3rd Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2015

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    Four factors impact high speed switching of silicon carbide (SiC) devices in voltage source converters, including limited gate driving capability, cross-talk, parasitics associated in switching loop, and parasitics of inductive load. This paper focuses on a solution to mitigate the adverse impact of the aforementioned factors. First, an intelligent gate drive is developed for gate driving capability enhancement and cross-talk suppression. Second, placement and layout design of power devices, gate drive, and power stage board are proposed to minimize parasitics for fast switching and over-voltage mitigation. Third, an auxiliary filter is designed to mitigate the negative impact of inductive load's parasitics during the switching transient. Finally, by utilizing all techniques developed above, a three-phase voltage source inverter with Cree 1200-V/20-A SiC MOSFETs is established. Test results show that the switching behavior of SiC devices in actual three-phase voltage source inverter fed motor drives can mostly repeat the switching performance tested by the optimally-designed double pulse test.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    This paper presents an active gate driver for Silicon Carbide (SiC) devices to fully utilize their potentials of high switching-speed capability in a phase-leg configuration. Based on the SiC device's intrinsic properties, a gate assist circuit consisting of two auxiliary transistors with two diodes is introduced to actively control the gate voltages and gate loop impedances of both devices in a phase-leg during different switching transients. Compared to a conventional gate driver, the proposed circuit has the capability of increasing the switching speed of the phase-leg power devices, suppressing the cross-talk to below device limits. Based on CREE's 2nd generation 1200-V SiC MOSFETs, the test results demonstrate the effectiveness of this active gate driver under various operating conditions. The switching time decreases by up to 28% during turn-on and 50% during turn-off in the prototype circuit, resulting in up to 31% reduction in switching energy loss. In addition, spurious gate voltages induced by cross-talk are limited within the required range.

  • Yalong Li; Xiaojie Shi; Bo Liu; Fred Wang; Leon M. Tolbert; Wanjun Lei
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    This paper presents the implementation of a scaled 4-terminal high-voltage direct current (HVDC) test-bed. The hardware construction, control scheme and communication architecture are described. The typical scenarios such as system start-up, station online recommission, power variation, online mode transition and station failure are emulated in the test-bed. A dc line current control is proposed to allow online disconnecting dc lines by using HVDC disconnectors with low current interrupting capability instead of the expensive dc circuit breaker. This control can be further utilized for dc line current limiting function. When a dc line is overloaded, the line current control will be automatically activated to regulate current below the allowable maximum value.

  • Zheyu Zhang; Zhiqiang Wang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2015 IEEE International Workshop on Integrated Power Packaging (IWIPP)
    2015

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    High reliability of semiconductor power devices is one of the key design objectives for power conversion systems. Fast switching SiC devices are susceptible to cross-talk, and these devices also have limited over-current capability. Both of these issues significantly threaten the reliable operation of SiC-based voltage source converters. This paper proposes two gate assist circuits capable of suppressing cross-talk and preventing shoot-through faults to promote the reliable use of SiC devices within a voltage source converter. Experimental results and detailed analysis are presented to verify the feasibility of the proposed approach.

  • Bo Liu; Sheng Zheng; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    An ultra-wide-area transmission network emulator, also called hardware test-bed (HTB), is being developed to emulate the large-scale interconnected power systems by using regenerative converters. Ac transmission line emulator is a key component in this system to connect two-area grid and to study the ac system's behavior under different scenarios. In this paper, two generic approaches of emulating the ac transmission line are developed based on back-to-back (BTB) voltage source converters (VSC), corresponding to the phasor domain model and discrete time domain model respectively. Two control schemes are presented, both showing less dependency on the communication speed and digital delay, thus enabling high accuracy and the possibility to emulate the dynamics of ac line flow. The impacts of BTB converter losses on the emulation performance are also analyzed, and the corresponding solution is provided. Finally, simulation and experimental results obtained from a scale-down three-phase prototype well verify the modeling and control scheme of the ac line emulation under normal operation and tripping line scenarios.

  • Yu Long; Weimin Zhang; Daniel Costinett; Benjamin B Blalock; Luke L Jenkins
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    A novel resonant gate driver designed for the high-frequency enhancement-mode GaN HEMT power devices is proposed in this work. Simulation results indicate that it reduces gate driving loss more than 50% compared to the conventional non-resonant gate driving topology, and by 20% compared to the existing GaN resonant gate driver. The loss reduction is achieved by partially recovering gate charge to the supply during charging and discharging through a resonant process using an inductance in the gate loop. The resonant condition is managed using the desired turn-on and turn-off driving pulses at the input with specific driving time and pulse width control. These inputs also generate on-chip control signals for safely clamping the GaN power devices during the remaining switching cycle after the resonant transition has concluded. Simulations reveal improved switching waveforms using the proposed gate driver compared to the existing GaN resonant gate driving topologies.

  • Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang; Ben Guo
    2015 IEEE 3rd Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2015

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    Cross conduction is a well-known issue in buck converters and phase-leg topologies, in which fast switching transients cause spurious gate voltages in the synchronous device and a subsequent increase in switching loss. Cross conduction can typically be mitigated with a well-designed gate drive, but this is challenging with WBG devices. Phase legs using SiC and GaN devices can experience heavy cross conduction loss due to their exceptionally fast switching transients. Enhancement-mode GaN heterojunction field-effect transistors (HFETs) in the 600-V class are now commercially available, with switching transients as fast as 200 kV/μs. A double pulse test setup was used to measure the switching loss of one such GaN HFET, with several gate drive circuits and resistances. The results were analyzed and compared to characterize the effects of cross conduction in the active and synchronous devices of a phase-leg topology with enhancementmode GaN HFETs.

  • Yiwei Ma; Liu Yang; Fred Wang; Leon M. Tolbert
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    Short circuit fault emulation is an important capability for power converter based grid emulator. This paper proposes to use a shunt connected voltage source converter to emulate short circuit faults, including single-line-to-ground, double-line-to-ground, line-to-line, and three-phase faults. The operating principle and hardware requirements are discussed first, and control strategies for each type of fault are presented. Simulation and experiments are performed to demonstrate the performances of the fault emulator under various circumstances and validate the effectiveness.

  • Wenchao Cao; Yiwei Ma; Xuan Zhang; Fred Wang
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    This paper proposes a method of sequence impedance measurement of three-phase inverters by using a parallel structure with another inverter as the measurement unit, in order to apply the impedance-based stability analysis of power converter systems. The paralleled inverter not only injects small-signal perturbations but also creates the desired operating conditions for the inverter under test. The measurement setup is simple, since no additional AC source or load banks are needed. First, the sequence impedance model of three-phase inverters is described. Then the measurement setup and injection method are presented. Zero-sequence circulating current reduction and open-loop control with voltage compensation strategies guarantee the measurement accuracy. The agreement between the theoretical analysis and the measurement results in both simulation and experiments verifies the effectiveness of the proposed method.

  • Xuan Zhang; Fred Wang; Wenchao Cao; Yiwei Ma
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    In balanced three-phase systems, source impedance and load admittance matrices in the synchronous rotating (d-q) frame can be used to determine system small-signal stability based on the Generalized Nyquist stability Criterion (GNC). For grid-tied inverters, voltage feed-forward control (VFFC) is widely used due to its fast transient dynamics. Through modeling the d-q frame admittances of three-phase grid-tied inverters with voltage feed-forward control, this paper illustrates instability mechanism and proposes some possible solutions. Simulation and experimental results verify the analysis.

  • Daniel Costinett
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    A concise, analytical method for incorporating the effects of zero voltage switching (ZVS) interval dynamics in small-signal discrete-time models of the dual-active bridge (DAB) converter is detailed. The method allows the influence of the resonant transition between tank inductor and switching device output capacitance to be examined. Importantly, the method does not require the inclusion of an additional state to account for these dynamics, which greatly simplifies the resulting models of converter behavior. The calculations are applicable to any alternate topology where ZVS transition behaviors contribute significantly to converter dynamics, as in many high frequency converters. The method is verified through experimental results on a 1 MHz DAB converter.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    2015 IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT)
    2015

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    This paper presents a MATLAB based dynamic simulation tool for estimating demand response potential from residential loads. First, a review of residential demand response strategies is conducted. Next, the modeling approach used during the development of this tool is described. Markov chain based occupant behavior models constructed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict the dynamic changes in residential power demand on a one-minute time scale. Separate control schemes are used along with these models to simulate different demand response strategies. Finally, simulation results showing the benefits and trade-offs associated with residential demand response programs are presented. Future work will involve using this tool to examine specific utility areas and the development of real-time pricing and incentive program components.

  • Daniel Costinett
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    A concise, analytical method for incorporating the effects of zero voltage switching (ZVS) interval dynamics in small-signal discrete-time models of the dual-active bridge (DAB) converter is detailed. The method allows the influence of the resonant transition between tank inductor and switching device output capacitance to be examined. Importantly, the method does not require the inclusion of an additional state to account for these dynamics, which greatly simplifies the resulting models of converter behavior. The calculations are applicable to any alternate topology where ZVS transition behaviors contribute significantly to converter dynamics, as in many high frequency converters. The method is verified through experimental results on a 1 MHz DAB converter.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    2015 IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT)
    2015

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    This paper presents a MATLAB based dynamic simulation tool for estimating demand response potential from residential loads. First, a review of residential demand response strategies is conducted. Next, the modeling approach used during the development of this tool is described. Markov chain based occupant behavior models constructed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict the dynamic changes in residential power demand on a one-minute time scale. Separate control schemes are used along with these models to simulate different demand response strategies. Finally, simulation results showing the benefits and trade-offs associated with residential demand response programs are presented. Future work will involve using this tool to examine specific utility areas and the development of real-time pricing and incentive program components.

  • Liu Yang; Yiwei Ma; Jingxin Wang; Jing Wang; Xiaohu Zhang; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    A Hardware Test-Bed (HTB) is developed to serve as a platform for power grid emulation. For maximum flexibility, power converters, which can accommodate various control algorithms and behave distinctively based on the applied model and control, is adopted. With the developed emulators, such as generator, load, wind turbine, and PV emulators, diverse research and experiments can be performed by using the HTB. This paper introduces the emulating method, hardware, control and communication structure of the HTB. At the same time, experimental results are compared with simulation to verify the emulation.

  • Weimin Zhang; Ben Guo; Fan Xu; Yutian Cui; Yu Long; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    Wide band gap (WBG) power devices, such as Silicon Carbide (SiC) and Gallium Nitride (GaN) devices, have been innovatively applied in the data center power converters, which are based on the high voltage DC (HVDC) power distribution architecture, to evaluate the potential efficiency improvement. For the front-end AC-DC rectifier, a buck rectifier using SiC devices was implemented. The SiC devices were tested at first to obtain the static and switching characteristics. The number of devices in parallel, the switching frequency and the input/output filters were investigated. A prototype of 7.5 kW, 3 phase 480 VAC input, 400 VDC output front-end rectifier was built and tested. The peak efficiency reaches up to 98.55%, and the full load efficiency is 98.54%. For the intermediate DC-DC bus converter, the impact of the GaN devices on the LLC resonant converter efficiency was evaluated and compared with the Si counterparts. Based on the device loss analysis and the FEA simulation on the transformer winding loss, the GaN devices exhibited the reduced device loss, and also the capabilities to reduce the transformer winding loss. A 300 W, 400 VDC input, 12 VDC output GaN device based DC-DC bus converter was built and tested by 96.3% peak efficiency and 96.1% full load efficiency.

  • Yutian Cui; Fan Xu; Weimin Zhang; Ben Guo; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock; Luke L. Jenkins; Christopher G. Wilson; Jeffrey M. Aggas; Benjamin K. Rhea; Justin D. Moses; Robert N. Dean
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    The energy efficiency of typical data centers is less than 50% because more than half of the power is consumed during power conversion, distribution, cooling, etc. In this paper, a combination of two approaches to improve power supply efficiency is implemented and experimentally verified. One approach uses a high voltage DC architecture, designed to reduce distribution loss and remove unnecessary power conversion stages. The other approach employs wide band gap (WBG) power devices, including silicon carbide (SiC) and gallium nitride (GaN) FETs and diodes, which helps to increase converter efficiency and power density. Scaled down prototypes of all power conversion stages in the data center power supply chain are designed, built, and tested. The advantages of utilizing WBG power devices are illustrated through simulations and then verified by experiment.

  • Yiwei Ma; Liu Yang; Jingxin Wang; Fred Wang; Leon M. Tolbert
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    A real-time reconfigurable hardware test-bed is being constructed to emulate a transmission network of a power system by modular regenerative converters. This test-bed enables flexible research scenarios without the necessities of using the actual power system equipment. This paper presents the emulation of a full-converter wind turbine using a single converter in order to investigate the impact of renewable energy penetration. By integrating physical models and control strategies into the converter controller, the emulator can imitate the behaviors of the wind turbine accurately. Simulation and experiments performed in the test-bed validate the effectiveness of the emulation, and demonstrate the performance of the emulated wind turbine during different power system scenarios.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fred Wang; Zhenxian Liang; Daniel Costinett; Benjamin J. Blalock
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    This paper presents a board-level integrated silicon carbide (SiC) MOSFET power module for high temperature and high power density applications. Specifically, a silicon-on-insulator (SOI) based gate driver capable of operating at 200°C ambient temperature is designed and fabricated. The sourcing and sinking current capability of the gate driver are tested under various ambient temperatures. Also, a 1200 V/100 A SiC MOSFET phase-leg power module is developed utilizing high temperature packaging technologies. The static characteristics, switching performance, and short-circuit behavior of the fabricated power module are fully evaluated at different temperatures. Moreover, a buck converter prototype composed of the SOI gate driver and SiC power module is built for high temperature continuous operation. The converter is operated at different switching frequencies up to 100 kHz, with its junction temperature monitored by a thermo-sensitive electrical parameter (TSEP) and compared with thermal simulation results. The experimental results from the continuous operation demonstrate the high temperature capability of the power module at a junction temperature greater than 225°C.

  • Xiaojie Shi; Zhiqiang Wang; Bo Liu; Yalong Li; Leon M. Tolbert; Fred Wang
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    This paper investigates the prediction of the second order dc voltage ripple in a modular multilevel converter (MMC) based point-to-point high-voltage direct-current (HVDC) system when the rectifier station suffers a single-line-to-ground (SLG) fault. Under this unbalanced condition, the second order dc voltage ripple will transfer to the healthy inverter station and can lead to a potential output voltage distortion. To accurately predict the dc voltage ripple distribution, the equivalent dc side impedances of the MMC inverter station with and without circulating current control are derived separately. It is shown that the MMC inverter station can be regarded as a series connected R-L-C branch in both cases, and the branch values are independent of the adopted current and power control schemes. In addition, long cables with small capacitance and large inductance help to mitigate the voltage ripple in the inverter station. The circulating current control, acting as an active resistance, effectively damps the possible resonance around 120 Hz between the dc cable and the MMC inverter. However, due to the higher equivalent dc impedance, the amplitude of the 2nd order dc voltage ripple in the inverter station is increased. Simulation results from a MMC based HVDC system, and experimental results from a three-phase MMC inverter are provided to support the theoretical analysis.

  • Jing Wang; Liu Yang; Yiwei Ma; Jingxin Wang; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    A hardware test-bed platform emulating multiple-area power system scenario dynamics has been established aiming at multiple time-scale emulations. In order to mimic real power flow situation in the system, the load emulators have to behave like real ones in both its static and dynamic characteristics. A constant-impedance, constant-current, and constant-power (ZIP) model has been used for static load type, while a three-phase induction motor model has been built to represent dynamic load types. In this paper, ways of modeling ZIP and induction motor loads and the performance of each load emulator are discussed. A comparison between simulation and experimental results are shown as well for the validation of the emulator behavior.

  • Yang Xue; Junjie Lu; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    In this paper, a compact planar current sensor is developed to be used in active current balancing applications for parallel-connected Silicon Carbide (SiC) MOSFETs. The designed Rogowski coil allows non-intrusive current measurement with low profile, compact size, and high bandwidth. The sensor circuit design extends both lower and higher cutoff frequency of the sensor, and allows a continuous measurement of current waveforms that contain a DC component. The simulated bandwidth of the proposed current sensor is 2.66 Hz-100 MHz. The measured switching waveforms in the experiment are comparable to a 120 MHz commercial current probe.

  • Yang Xue; Junjie Lu; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Current unbalance in paralleled power electronic devices can affect the performance and reliability of them. In this paper, the factors causing current unbalance in parallel connected silicon carbide (SiC) MOSFETs are analyzed, and the threshold mismatch is identified as the major factor. Then the distribution and temperature dependence of SiC MOSFETs' threshold voltage are studied experimentally. Based on the analysis and study, an active current balancing (ACB) scheme is presented. The scheme directly measures the unbalance current, and eliminates it in closed loop by varying the gate delay to each device. The turn-on and turn-off current unbalance are sensed and independently compensated to yield an optimal performance at both switching transitions. The proposed scheme provides robust compensation of current unbalance in fast-switching wide-band-gap devices while keeping circuit complexity and cost low. The performance of the proposed ACB scheme is verified by both simulation and experimental results.

  • Zhuxian Xu; Weimin Zhang; Fan Xu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper investigates the fast switching characteristics and high temperature performance of the emerging 600 V GaN high-electron-mobility transistor (HEMT) for high efficiency / high temperature applications. First, the inherent switching performance of the GaN HEMT is demonstrated in the double pulse test. The GaN HEMT exhibits superior switching capability, with a di/dt reaching 9.6 A/ns and dv/dt reaching 140 V/ns. Then, the limitations of the fast switching capability by the device packaging and application circuit are analyzed. The interference between the current and gate through common source inductance limits the inherent switching speed. Packaging and circuit layout with small parasitics is critical in achieving fast switching. Finally, the high temperature static and switching characteristics up to 200 °C are also tested and given. The switching performance of the device is temperature insensitive.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel Costinett
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    This paper focuses on understanding the key impacting factors for switching speed of wide bandgap (WBG) devices in a voltage source converter. First, the constraints and challenges of WBG devices during fast switching transients are summarized. Special attention is given to the transient gate-source and drain-source voltages. Second, the impacts of major components in voltage source converter, including gate drivers, parasitics, inductive loads, and cooling systems, on the switching performance of power devices are systematically investigated. The critical parameters for each component are highlighted. Finally, design criteria are suggested to maximize switching speed of WBG devices.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel Costinett
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    Double pulse test (DPT) is a widely accepted method to evaluate the switching characteristics of semiconductor switches, including SiC devices. However, the observed switching performance of SiC devices in a PWM inverter for induction motor drives (IMD) is almost always worse than the DPT characterization, with slower switching speed, more switching losses, and more serious parasitic ringing. This paper systematically investigates the factors that limit the SiC switching performance from both the motor side and inverter side, including the load characteristics of induction motor/power cable, two more phase-legs for the three-phase PWM inverter as compared to the DPT, and the parasitic capacitive coupling effect between power devices and heat sink. Based on the three-phase PWM inverter with 1200 V SiC MOSFETs, the test results show that the induction motor, especially with a relatively long power cable, will significantly impact the switching performance, leading to switching time increase by a factor of 2, switching loss increase up to 30%, and serious parasitic ringing with 1.5 μs duration as compared to that tested by DPT. In addition, the interactions among the three phase-legs cannot be ignored unless the decoupling capacitors are mounted close to each phase-leg to support the dc bus voltage during switching transients. Also, the coupling capacitance induced by the heat sink equivalently increases the junction capacitance of power devices. However, its influence on the switching behavior in the motor drives is small considering the relatively large capacitance of the motor load.

  • Weimin Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel Costinett
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    Gallium Nitride High Electron Mobility Transistor (GaN HEMT) is an emerging wide band gap power device in recent years. Using a cascoded structure, the GaN HEMT can be combined with a low voltage MOSFET to make the combination behave as a normally-off device. This paper investigates the soft-switching behavior of cascode GaN HEMT in the phase-leg structure. The analysis reveals some internal device behaviors during the soft-switching transition, which are not found in the non-cascode device. Due to the internal feedback of the cascode structure, the channel current of the internal GaN HEMT drops to zero quickly, leading to extremely low turn-off loss. However, it has been found that there are switching energy loss dissipated in the internal GaN HEMT during the turn-on transient, although the external waveforms of the cascode GaN HEMT exhibit zero voltage switching. The fundamental reason is that ratio of the sum of MOSFET output capacitance and internal GaN HEMT input capacitance to the internal GaN HEMT output capacitance is quite low. Based on the simulation, by adding additional capacitance on the gate source terminals of internal GaN HEMT, these losses can be mitigated. Experimental tests using a commercially available GaN device are presented which show nearly 400 mW of loss at 1 MHz switching frequency in four different load current conditions.

  • Zheyu Zhang; Ben Guo; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Zhenxian Liang; Puqi Ning
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper investigates the effects of ringing on the switching losses of wide band-gap (WBG) devices in a phase-leg configuration. An analytical switching loss model considering the parasitic inductance, stray resistance, devices' junction capacitances, and reverse recovery characteristics of the freewheeling diode is derived to identify the switching energy dissipation induced by damping ringing. This part of energy is found to be at most the reverse recovery energy and the energy stored in the parasitics, which is a small portion of the total switching energy. But the parasitic ringing causes interference between two devices in a phase-leg (i.e., cross talk). It is observed that during the turn-on transient of one device, the resonance among parasitics results in high overshoot voltage on the complementary device in a phase-leg. It worsens the cross talk, leading to large shoot-through current and excessive switching losses. The analysis results have been verified by double pulse test with a 1200 V SiC MOSFETs based phase-leg power module.

  • Zheyu Zhang; Ben Guo; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Zhenxian Liang; Puqi Ning
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Double pulse tester (DPT) is a widely accepted method to evaluate the switching behavior of power devices. Considering the high switching-speed capability of wide band-gap (WBG) devices, the test results become significantly sensitive to the alignment of voltage and current (V-I) measurement. Also, because of the shoot-through current induced by Cdv/dt, during the switching transient of one device, the switching losses of its complementary device in the phase-leg is non-negligible. This paper summarizes the key issues of DPT, including layout design, measurement considerations, grounding effects and data processing. Among them, the latest probes for switching waveform measurement are compared, the methods of V-I alignment are discussed, and the impact of grounding effects induced by probes on switching waveforms are investigated. Also, for the WBG devices in a phase-leg configuration, a practical method is proposed for switching loss evaluation by calculating the difference between the input energy supplied by a dc capacitor and the output energy stored in a load inductor. Based on a phase-leg power module built with 1200 V SiC MOSFETs, the test results show that regardless of V-I timing alignment, this method can accurately indicate the switching losses of both the upper and lower switches by detecting only one switching current.

  • Ben Guo; Fan Xu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    The overvoltage caused by dc-link inductor current interruption is a serious problem in the current source converters. It becomes an even more challenging issue when the fast-switching SiC MOSFETs are applied as switches in these converters. The protection is required to have nanosecond-level response time to protect the devices. Addressing this challenge, this paper proposes a novel overvoltage protection scheme constituted by a diode bridge and the high-power transient-voltage-suppression (TVS) diodes. It can detect and clamp the overvoltage within less than 50 ns to protect the device from breakdown. When the energy in the inductor is small, it can be dissipated in the TVS diodes. Otherwise, a capacitor in series with a thyristor can be added to absorb the energy. The effectiveness of the protection scheme has been verified by experiments in a 7.5 kW current source rectifier built with SiC MOSFETs.

  • Yu Long; Weimin Zhang; Benjamin Blalock; Leon Tolbert; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    In this paper, an new configurable low-side resonant gate driver circuit based on 5V CMOS process is presented. This gate driver is designed for current Gallium Nitride (GaN) power transistor working up to 10 MHz switching frequency. By updating driving signals and removing off-chip resonant inductors, this gate driver can be working as a conventional non-resonant gate driver. Under resonant gate driving mode, partial of the gate driving power can be recovered to gate driver power supply. Simulation shows up to 30% of gate driving power dissipation reduction can be achieved while driving a single device compared with conventional push-pull gate drivers. We also try to implement a resonant gate driver in a resonant converter. Simulation also shows a similar gate driver power saving is also achieved in a 48-12V LLC resonant DC-DC converter.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Energy efficiency of typical data centers is less than 50% where more than half of the power is consumed during power conversion, distribution, cooling, etc. In this paper, a single power stage architecture that converts 400 V to 1 V directly targeting high system efficiency is proposed. A phase shift full bridge (PSFB) DC/DC converter based input series and output parallel structure (ISOP) is selected due to the high input voltage and large output current operation condition. The latest Gallium Nitride (GaN) FETs are implemented in the prototype circuit because of their low output junction capacitance and zero reverse recovery charge. The high frequency planar transformer is designed correspondingly with consideration of GaN FETs on the primary side. A prototype of the PSFB converter is designed, built, and tested. Preliminary experimental results are provided to verify the design.

  • Jing Xue; Fred Wang; Ben Guo
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    EMI filter design is often influenced by the noise transformations between the common-mode (CM) and differential-mode (DM), which is mainly due to the system unbalance. In this paper, the analysis of the intrinsic and EMI filter unbalance existing in the EMI noise propagation path of the dc-fed three-phase motor drive system is carried out. CM path unbalance in the circuit model is investigated with an example switching state in three-phase space vector pulse-width-modulation (SVPWM). Noise transformation from CM to DM is then verified with prototype experiments. The influence of the DM filter component unbalance on the DM propagation path is analyzed with simulation. After that, the transformation of the DM noise to CM is validated with tests. Influence of the unbalance and noise mode transformation on the EMI filter design is discussed and a comprehensive design procedure for high density EMI filter is proposed.

  • Yalong Li; Xiaojie Shi; Bo Liu; Fred Wang; Wanjun Lei
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    In a modular multilevel converter (MMC), the circulating current control is usually adopted. It can minimize the circulating current in order to reduce the converter power loss, and also provide an active damping which is beneficial for the converter control stability. The circulating current control is normally implemented by adding a compensating component into the modulation signal. Consequently, the maximum modulation index of the fundamental frequency component will be reduced so as to allow room for circulating current control, and the utilization of dc voltage is reduced. In this paper, the impact of circulating current control on the modulation signal in MMC is investigated. The maximum obtainable modulation index of MMC is theoretically derived. It shows that the modulation index reduction is related to the converter submodule capacitance design. If the capacitance is designed for a maximum 10% voltage ripple, the circulating current control could cause as large as a 5% decrease for the maximum modulation index, or 8% for the case with 3rd harmonic component injection. Both simulation and experimental results verify the theoretical analysis.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper develops a three-phase front-end power conversion stage for data center power supplies based on 400 Vdc power delivery architecture, which has been proven to have higher efficiency than traditional AC architectures. The front-end stage is based on three paralleled three-phase current source rectifiers, which have several benefits for this application. A control method is introduced for paralleled three-phase current source rectifiers to achieve balanced outputs and individual rectifier module hot-swap, which are required by power supply systems. By using SiC power semiconductors, the power conversion efficiency of the front-end stage is improved and the whole efficiency of the data center power supply system can be further increased.

  • Brad Trento; Bin Wang; Kai Sun; Leon M. Tolbert
    2014 IEEE PES General Meeting | Conference & Exposition
    2014

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    This paper discusses how a real-time electromechanical oscillation monitoring system (REOMS) can be incorporated into synchrophasor devices. Using a closed loop feedback system consisting of a second order generalized integrator phase-locked loop (SOGI-PLL), this paper shows that real-time frequency, phase, and magnitude of electromechanical oscillations can be tracked using existing synchronized measurement platforms. The approach is demonstrated using simulation and a scaled hardware demonstration platform developed by the Center for Ultra-wide-area Resilient Electric Energy Transmission Networks (CURENT) to test wide-area monitoring and control schemes. Compared to traditional spectral base techniques, the phase lock loop approach offers more flexibility in tracking realtime oscillations, especially when the signal has floating modal properties. Real-time tracking of oscillation becomes extremely important when trying to adopt wide-area control schemes into the power grid, specifically oscillation damping and angular stability.

  • Wenchao Cao; Yiwei Ma; Jingxin Wang; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper proposes a virtual series impedance emulation control strategy for current controlled remote PV or wind farms, in order to enhance the power transfer capability of the transmission line. This approach can be regarded as integrating the functions of the FACTS devices, such as series capacitors, into the grid-side converters of PV or wind farms, by utilizing the reactive power capability of the converters. First, the virtual series impedance emulation control strategy for PV or wind farms in both the operation mode with P and Q control and the operation mode with P and V control is described. Then the capability of virtual impedance emulation is analyzed, considering the converter limitations and plant-level limitations. Comparison with physical series capacitors is carried out to investigate the impact of the proposed method on the grid-side converter design. The effectiveness of the proposed methods is verified by both simulation and experiments.

  • Bo Liu; Xiaojie Shi; Fred Wang; Yalong Li
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    Hybrid ac/dc transmission extends the power transfer capacity of existing long ac lines closer to the thermal limit, by superposing dc current onto ac lines through a zigzag transformer. However, this transformer could suffer saturation under unbalanced line impedance conditions. This paper introduces the concept of hybrid line impedance conditioner (HLIC) as an approach to eliminate the line unbalance, which significantly reduces the design cost of the zigzag transformer and could be integrated into the transformer without requiring high voltage insulation. The topology and operation principle are presented, fully utilizing the capability of full bridge converters to generate a hybrid ac/dc voltage as an active impedance compensator. The selection criterion of the line conditioner key components, especially dc link capacitance is also proposed. Simulation and experimental results are provided.

  • Brad Trento; Leon M. Tolbert; Daniel Costinett
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    This paper describes a grid synchronization technique for single phase systems using only fixed filtering and compensation networks. Compared with other phase locking techniques, the proposed approach has the advantage of requiring no feedback control and a simplistic and predictable design. The paper uses test scenarios and metrics described in IEEE C37-118.1 for testing phasor measurement units (PMUs) to evaluate the performance of the proposed method. The dynamic response of the proposed approach is compared with the second order generalized integrator phase locked loop (SOGI-PLL) and second order generalized integrator frequency locked loop (SOGI-FLL).

  • Lipeng Wan; Zheng Lu; Qing Cao; Feiyi Wang; Sarp Oral; Bradley Settlemyer
    2014 30th Symposium on Mass Storage Systems and Technologies (MSST)
    2014

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    In recent years, non-volatile memory devices such as SSD drives have emerged as a viable storage solution due to their increasing capacity and decreasing cost. Due to the unique capability and capacity requirements in large scale HPC (High Performance Computing) storage environment, a hybrid configuration (SSD and HDD) may represent one of the most available and balanced solutions considering the cost and performance. Under this setting, effective data placement as well as movement with controlled overhead become a pressing challenge. In this paper, we propose an integrated object placement and movement framework and adaptive learning algorithms to address these issues. Specifically, we present a method that shuffle data objects across storage tiers to optimize the data access performance. The method also integrates an adaptive learning algorithm where realtime classification is employed to predict the popularity of data object accesses, so that they can be placed on, or migrate between SSD or HDD drives in the most efficient manner. We discuss preliminary results based on this approach using a simulator we developed to show that the proposed methods can dynamically adapt storage placements and access pattern as workloads evolve to achieve the best system level performance such as throughput.

  • Yalong Li; Edward A. Jones; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    With voltage-balancing control, the voltage difference among sub-module (SM) capacitors in a modular multilevel converter can be reduced. However, this comes at the cost of increased device switching frequency. In this paper, the relationship between the switching frequency and SM capacitor unbalanced voltage is investigated. As the relationship depends on the voltage-balancing control schemes, a popular scheme using a modified sorting method is considered. The switching frequency is found to be inversely proportional to the SM capacitor unbalanced voltage due to the voltage-balancing control and an analytical relationship is derived for the first time. As the SM capacitors voltage-unbalance increases the voltage fluctuation for each SM, the impact of switching frequency on the SM capacitance design is further investigated. The developed analytical relationship can thus be used for the SM capacitance design. Finally, the theoretical analysis is verified by simulation.

  • Yalong Li; Edward A. Jones; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper investigates the dc link voltage ripple in a modular multilevel converter (MMC). It is found that switching-frequency ripple occurs on the dc link voltage in MMC when circulating current suppressing control is implemented. The mechanism of the switching-frequency voltage ripple is investigated and explained. Circulating current suppressing control will manipulate the average values for the three phaseleg voltages to be equal in order to reduce the low frequency circulating current. However, switching-frequency harmonics appear on the phase-leg voltages as a result, introducing the switching-ripple voltage on the dc link. By modeling the dc link voltage, switching-ripple voltage is derived. Experimental results of a three-phase MMC are presented to verify the theoretical analysis.

  • Bo Liu; Xiaojie Shi; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Considering stability, the loadability of long line ac transmission system is typically far below its thermal limitation. To partially address this problem, a hybrid ac/dc approach is investigated in this paper by superposing dc current on ac currents along existing ac lines, and no changes of insulators, towers and conductors are required. First, detailed performance comparison between line commutated converter (LCC) and voltage source converter (VSC) is carried out, which reveals that LCC based bipolar hybrid ac/dc is more suitable in this application due to the controllability of dc fault current on overhead lines. Based on the selected topology, the design methodology of zigzag transformer for dc power injection is proposed, mainly focusing on avoiding dc saturation of magnetic core when ac line impedances are unbalanced. A scaled down prototype is built to demonstrate this design approach and experimental results verify the feasibility of hybrid scheme. Finally, compared to the scheme of converting ac line to pure dc, LCC based hybrid ac/dc can be a much cheaper solution to upgrade HVAC power transfer capacity by 2 times around.

  • Zhenxian Liang; Fred Wang; Leon Tolbert
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    A set of packaging technologies has been developed for promotion of SiC power devices in automotive applications. The technical advances include integrating single side cooling, three-dimensional (3-D) planar electrical interconnection, and integrated double sided direct cooling. The further integration of these features into one packaging process has been demonstrated with highly integrated SiC phase leg power module prototypes. The comprehensive improvements in module's electrical, thermal performance and manufacturability help exploit fully the attributes provided exclusively by the wide bandgap (WBG) power semiconductors. The technical advancements lead to cost-effectiveness, high efficiency, high power density power conversion in electric drive system in modern vehicles.

  • Lakshmi GopiReddy; Leon M. Tolbert; Burak Ozpineci; João O. P. Pinto
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Rainflow algorithms are one of the best counting methods used in fatigue and failure analysis popularly used in semiconductor lifetime estimation models. However, the rainflow algorithm used in power semiconductor reliability does not consider the time dependent mean temperature calculation. The equivalent temperature calculation proposed by Nagode et al is applied to semiconductor lifetime estimation for the first time in this paper. A month long arc furnace load profile is used as a test profile to estimate temperatures in IGBT in a STATCOM for reactive compensation of load. The degradation in the life of the IGBT power device is predicted based on time dependent temperature calculation.

  • Lakshmi Gopi Reddy; Leon M. Tolbert; Burak Ozpineci
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Lifetime estimation of power semiconductors for various applications has gained technical importance. The main failures in high power semiconductors are caused by thermo-mechanical fatigue, mainly in solder and wirebonds, due to different coefficients of thermal expansions of the various packaging materials. Most of the lifetime models do not take all the operating parameters into account. There is a need to develop a generalized lifetime model specific to failure mechanisms that account for all of the operating parameters in an application. This paper presents finite element based stress simulations for varying operating parameters (current, temperature, etc.) for a fixed dimension wire.

  • Bailu Xiao; Leon M. Tolbert
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    A three-phase modular cascaded H-bridge multilevel inverter can be applied to grid-connected photovoltaic (PV) systems. The isolated dc buses make the individual maximum power point tracking (MPPT) control of the connected PV string of each H-bridge module possible. However, with the individual MPPT control, the solar power supplied to each phase of the three-phase system may be different, leading to unbalanced injected grid current. To solve this issue, a control scheme is proposed to improve the PV system efficiency and balance the grid current. The limitation of the proposed control scheme is also discussed. A modular cascaded multilevel PV inverter prototype has been built. Experimental results under different test conditions are presented to validate the proposed ideas.

  • Zheyu Zhang; Fred Wang
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    No power conversion without power semiconductors Power semiconductors is NOTHING without a gate driver! The gate driver will properly drive a power semiconductor and bring the maximum performance. For WBG devices, Driving capability of gate driver IC (rise/fall time, pull-up/pull-down resistance) & CM transient immunity of gate driver isolation are special requirements. The gate driver will protect a power semiconductor and entire converter if something goes wrong. For WBG devices, Cross-talk is easily induced, leading to potential hazard of shoot-through failure and gate terminal reliability issues. A gate assist circuit was introduced for cross-talk suppression. Short circuit capability is limited. The desaturation protection circuit with <; 200 ns response time was described for device reliability enhancement.

  • Edward A. Jones; Fred Wang; Burak Ozpineci
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    Normally-off GaN-on-Si heterojunction field-effect transistors (HFETs) have been developed with up to 650 V blocking capability, fast switching, and low conduction losses in commercial devices. The natively depletion-mode device can be modified to be normally-off using a variety of techniques. For a power electronics engineer accustomed to Si-based converter design, there is inherent benefit to understanding the unique characteristics and challenges that distinguish GaN HFETs from Si MOSFETs. Dynamic Rds-on self-commutated reverse conduction, gate voltage and current requirements, and the effects of very fast switching are explained from an applications perspective. This paper reviews available literature on commercial and near-commercial GaN HFETs, to prepare engineers with Si-based power electronics experience to effectively design GaN-based converters.

  • Ben Guo; Fred Wang; Eddy Aeloiza
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    The conduction loss of semiconductor devices is large in traditional three-phase current source rectifiers (CSR). In this paper, a new CSR topology is proposed to reduce the conduction loss. This rectifier, named Delta-type Current Source Rectifier (DCSR), has delta-type connection on its input side and its dc-link current can be shared by multiple devices at a time. Its principle of operation, modulation scheme and design method are discussed in detail in this paper. Based on the analysis, the conduction loss can be reduced by up to 20% with the proposed topology. A 7.5 kW prototype is then built to experimentally verify the performance of DCSR.

  • Ben Guo; Fred Wang; Eddy Aeloiza
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    To reduce the conduction loss, a novel three-phase current source rectifier, named Delta-type Current Source Rectifier (DCSR), has been proposed in previous research. This rectifier has delta-type connection on its input side, and its dc-link current can be shared by multiple devices at a time to reduce up to 20% conduction loss. A high-efficiency modulation scheme for DCSR has been proposed, where the conduction states involve more switches to share the dc-link current. However, it causes current distortion when the input voltages have intersections. In this paper, the phenomenon is analyzed in detail. The clamped voltage on the diode bridge will fluctuate at the voltage intersections, resulting in false current pulse and distortion. An improved modulation scheme is then proposed for DCSR to reduce the input current distortion without sacrificing much efficiency. Through experiment in a 7.5 kW prototype, its effectiveness is verified and the total harmonic distortion (THD) of the input current is reduced dramatically.

  • Ben Guo; Fred Wang; Eddy Aeloiza; Puqi Ning; Zhenxian Liang
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    To reduce the conduction loss, a novel three-phase current source rectifier, named Delta-type Current Source Rectifier (DCSR), has been proposed in previous paper. This rectifier has delta-type connection on its input side, and its dc-link current can be shared by multiple devices at a time to reduce up to 20% conduction loss. The SiC devices are expected to be the next-generation power devices due to their low conduction and switching losses. In this paper, an all-SiC power module is built to realize a high-density DCSR. The switching performance of the power module is characterized under different operation conditions. Then DCSR is compared with the traditional CSR on both switching speed and switching loss. It is shown that the turn-on speed is accelerated and the switching energy is lower in DCSR. The equivalent parasitic inductance is also lower in DCSR with two paralleled minor commutation loops. The switches can operate at higher switching speed without serious resonance in DCSR.

  • Daniel Costinett; Kelly Hathaway; Muneeb Ur Rehman; Michael Evzelman; Regan Zane; Yoash Levron; Dragan Maksimovic
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Electric-drive vehicles, including hybrid (HEV), plug-in hybrid (PHEV) and electric vehicles (EV), require a high-voltage (HV) battery pack for propulsion, and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communications between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for low-voltage, low-power dual active bridge (DAB) power converters serving as bypass converter modules. Experimental results are presented for a system consisting of two series 3.6 Ah NMC battery cells and two DAB bypass converters, with combined outputs rated to supply a 12 V, 35 W auxiliary load.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    2014 IEEE PES T&D Conference and Exposition
    2014

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    This paper presents a MATLAB based dynamic model for predicting residential power demand. Markov chain based occupant behavior models developed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict residential power demand on a one-second time scale. First, the methods utilized for the modeling of each residential load are presented. Next, an explanation of how these load models are combined with occupant behavior models to predict residential power demand is given. Simulation results showing the overall contribution of each load to the overall residential sector power demand are shown for both winter and summer cases. Finally, future work will involve the use of this high-resolution dynamic residential model to estimate the potential for demand response from residential loads.

  • Francisco J. Azcondo; Regan A. Zane; Dragan Maksimovic; Daniel Costinett
    2014 XI Tecnologias Aplicadas a la Ensenanza de la Electronica (Technologies Applied to Electronics Teaching) (TAEE)
    2014

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    The paper describes the organizational framework and contents of a newly developed course on power electronics for electric drive vehicles. The course is developed and taught synchronously among three universities, with each institution individually managing student registration and assessments, and course administration. The participation of instructors and students from different institutions increases the impact of the course. In addition to the regular classes, followed on campus and remote, the high quality material generated by the instructors is available for the students, including a repository of recorded video lecturers and conferences given by specialist in key topics. Interaction with instructors and among students is promoted using a collaborative on-line tool.

  • Daniel Costinett; Regan Zane; Dragan Maksimovic
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    The small-signal modeling of dual active bridge (DAB) converter dynamics in discrete time is a useful analysis tool due to the ability to incorporate behavior during zero-voltage switching (ZVS) intervals explicitly, and is advantageous for the direct design of digital compensators. While the control-to-output transfer function has been modeled previously, accurate models of the output impedance of the DAB converter which account for the converter behavior during switching intervals have not been proposed. A discrete time model of the DAB output impedance is developed and tested against experimental results for a 1 MHz, 50-to-4 V, 10 W DAB converter. The model is then extended to the analysis of load current step changes asynchronous to the discrete time sampling instances.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    ISGT 2014
    2014

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    This paper presents a statistical method for modeling the behavior of household occupants to estimate residential energy consumption. Using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS), actions carried out by survey respondents are categorized into ten distinct activities. These activities are defined to correspond to the major energy consuming loads commonly found within the residential sector. Next, time varying minute resolution Markov chain based statistical models of different occupant types are developed. Using these behavioral models, individual occupants are simulated to show how an occupant interacts with the major residential energy consuming loads throughout the day. From these simulations, the minimum number of occupants, and consequently the minimum number of multiple occupant households, needing to be simulated to produce a statistically accurate representation of aggregate residential behavior can be determined. Finally, future work will involve the use of these occupant models along side residential load models to produce a high-resolution energy consumption profile and estimate the potential for demand response from residential loads.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    2014 IEEE PES T&D Conference and Exposition
    2014

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    This paper presents a MATLAB based dynamic model for predicting residential power demand. Markov chain based occupant behavior models developed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict residential power demand on a one-second time scale. First, the methods utilized for the modeling of each residential load are presented. Next, an explanation of how these load models are combined with occupant behavior models to predict residential power demand is given. Simulation results showing the overall contribution of each load to the overall residential sector power demand are shown for both winter and summer cases. Finally, future work will involve the use of this high-resolution dynamic residential model to estimate the potential for demand response from residential loads.

  • Francisco J. Azcondo; Regan A. Zane; Dragan Maksimovic; Daniel Costinett
    2014 XI Tecnologias Aplicadas a la Ensenanza de la Electronica (Technologies Applied to Electronics Teaching) (TAEE)
    2014

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    The paper describes the organizational framework and contents of a newly developed course on power electronics for electric drive vehicles. The course is developed and taught synchronously among three universities, with each institution individually managing student registration and assessments, and course administration. The participation of instructors and students from different institutions increases the impact of the course. In addition to the regular classes, followed on campus and remote, the high quality material generated by the instructors is available for the students, including a repository of recorded video lecturers and conferences given by specialist in key topics. Interaction with instructors and among students is promoted using a collaborative on-line tool.

  • Daniel Costinett; Regan Zane; Dragan Maksimovic
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    The small-signal modeling of dual active bridge (DAB) converter dynamics in discrete time is a useful analysis tool due to the ability to incorporate behavior during zero-voltage switching (ZVS) intervals explicitly, and is advantageous for the direct design of digital compensators. While the control-to-output transfer function has been modeled previously, accurate models of the output impedance of the DAB converter which account for the converter behavior during switching intervals have not been proposed. A discrete time model of the DAB output impedance is developed and tested against experimental results for a 1 MHz, 50-to-4 V, 10 W DAB converter. The model is then extended to the analysis of load current step changes asynchronous to the discrete time sampling instances.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    ISGT 2014
    2014

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    This paper presents a statistical method for modeling the behavior of household occupants to estimate residential energy consumption. Using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS), actions carried out by survey respondents are categorized into ten distinct activities. These activities are defined to correspond to the major energy consuming loads commonly found within the residential sector. Next, time varying minute resolution Markov chain based statistical models of different occupant types are developed. Using these behavioral models, individual occupants are simulated to show how an occupant interacts with the major residential energy consuming loads throughout the day. From these simulations, the minimum number of occupants, and consequently the minimum number of multiple occupant households, needing to be simulated to produce a statistically accurate representation of aggregate residential behavior can be determined. Finally, future work will involve the use of these occupant models along side residential load models to produce a high-resolution energy consumption profile and estimate the potential for demand response from residential loads.

  • Liu Yang; Xiaohu Zhang; Yiwei Ma; Jing Wang; Lijun Hang; Keman Lin; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In this project to develop a reconfigurable electrical grid emulator, a Hardware Test-Bed (HTB) is being developed that emulates large scale power system generators and loads by using power electronic converters. Source converters in the HTB system are designed to emulate power generators. A synchronous generator model is implemented in the converter to calculate the voltage references in the dq axis, and a voltage controller is added to achieve zero steady state error. In the HTB, synchronous generator emulators (SGEs) are connected with each other through transformers and transmission line emulators to form a microgrid. To study the parallel behavior and the stability of the SGEs with voltage controller, a small signal statespace model of the multi SGE system is established, and the eigenvalues are then analyzed. Experiments are conducted in the HTB.

  • Liu Yang; Xiaohu Zhang; Yiwei Ma; Jing Wang; Lijun Hang; Keman Lin; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    In this project to develop a reconfigurable electrical grid emulator, a Hardware Test-Bed (HTB) is being developed that emulates large scale power system generators and loads by using power electronic converters. Source converters in the HTB system are designed to emulate generators. A synchronous generator model is implemented in the converter to calculate the voltage references in the dq axis, and a voltage controller is added to achieve zero steady state error. A traditional cascade controller with inner current control and outer voltage control brings additional output impedance to the generator model, and causes voltage tracking error during transients. To minimize the controller output impedance and eliminate controller influence on the generator model, a single voltage loop with current differential feedback is proposed in this paper. Combined with rescaled generator parameters, circulating current elimination, and dead time compensation, simulation and experiments are performed in the HTB. The results verify the effectiveness of the controller and demonstrate the dynamic generator emulator behavior.

  • Weimin Zhang; Yu Long; Yutian Cui; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Stephan Henning; Justin Moses; Robert Dean
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    Transformer loss, comprised of core loss and winding loss, is a critical part in the LLC resonant converter loss. Different winding structures lead to different winding losses and winding capacitances. High winding capacitance will impact the design of the LLC resonant converter. The reason is that high winding capacitance means high winding charge, which must be moved during the dead time to realize the device zero voltage turn-on. As a result, the dead time and magnetizing current will be changed, and the converter loss will be changed as well. This paper first discusses the transformer loss including core loss and winding loss. Then, four different winding structures are analyzed based on a selected core, which show the decrease of AC resistance and the increase of winding capacitance. After that, the winding capacitance model is discussed generally. Finally, the impact of winding capacitance on the design and performance of LLC resonant converter is studied. Two 48 V-12 V, 300 W Si-based and GaN-based LLC resonant converters are designed as platforms to evaluate the impact of winding capacitance. The results indicate that the GaN-based converter is well suited to the transformer with lowest winding loss but highest winding capacitance, since the GaN device's output capacitance is much lower than that of the Si device.

  • Xiaojie Shi; Zhiqiang Wang; Yiwei Ma; Lijun Hang; Leon M. Tolbert; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    An active rectifier with an LCL filter has been proved to be an effective approach to suppress switching harmonic current, guarantee unity power factor, and supply reliable dc voltage in power grids. This paper deals with the modeling and control of an active rectifier with low-volume coupled LCL filter for switching ripple attenuation. Specifically, the equivalent circuit model of three-phase three-column coupled inductors is built taking the variation of coupling coefficient into account. Based on the equivalent circuit, the average model of the active rectifier is derived under abc and dq0 coordinates to design a decoupled controller with excellent steady-state and dynamic performance. In addition, the influences of the coupled LCL filter on current total harmonic distortion (THD) and system stability are investigated under both resistive loads and paralleled inverter loads. Simulation and experimental results from a 30 kVA prototype verify the validity of the proposed model and the effectiveness of the designed controller.

  • Wenchao Cao; Yiwei Ma; Jingxin Wang; Liu Yang; Jing Wang; Fred Wang; Leon M. Tolbert
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper proposes a method of modeling and emulation of a two-stage photovoltaic (PV) inverter system by using a single power converter. The PV emulator is intended to be used in a converter-based power grid emulation system - Hardware Test-bed (HTB), in order to investigate the influence of solar energy sources on the power grid. Both physical components and control strategies of the two-stage PV inverter system are modeled in the converter controller, which enables the emulator to represent the behaviors of the two-stage PV inverter system accurately. The performance of the two-stage PV inverter system emulator in both the MPPT mode and the reserved power control mode under variable solar irradiance circumstances is illustrated by both simulation and experiments in the HTB environment, which verifies the effectiveness of the emulation.

  • Yiwei Ma; Liu Yang; Jing Wang; Xiaojie Shi; Fred Wang; Leon M. Tolbert
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    It is a flexible and effective approach to emulate the behaviors of electric power system components using interconnected parallel power converters. The reduction of unnecessary circulating current is essential for the validity of the test-bed system. The types of the circulating current in the test bed system are discussed in this paper. The causes and the reduction strategies for the switching period circulating current, zero sequence circulating current and lower order harmonics are presented. Simulation and experimental results are given to verify the feasibility.

  • Ben Guo; Fan Xu; Zheyu Zhang; Zhuxian Xu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    An overlap time for two commutating switches is necessary to prevent current interruption in a three-phase buck rectifier, but it may cause input current distortion. In this paper, a modified pulse-based compensation method is proposed to compensate for the overlap time. In addition to the traditional method which places the overlap time based on the voltage polarity, this new method first minimizes the overlap time to reduce its effect and then compensates the pulse width according to the sampled voltage and current. It is verified by experiments that the proposed method has better performance than the traditional method, especially when the line-to-line voltage crosses zero. Another distortion comes from the irregular pulse distribution when two sectors change in a 12-sector space vector PWM. This paper proposes two compensation methods for that scenario as well, compensating the duty cycle and increasing switching frequency near the boundaries of two sectors. It is shown through experiments that both methods can reduce the input current distortion in the buck rectifier.

  • Yang Xue; Junjie Lu; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In high power applications of silicon carbide (SiC) MOSFETs where parallelism is employed, current unbalance can occur and affect the performance and reliability of the power devices. In this paper, factors which cause current unbalance in these devices are analyzed. Among them, the threshold voltage mismatch is identified as a major factor for dynamic current unbalance. The threshold distribution of SiC MOSFETs is investigated, and its effect on current balance is studied in experiments. Based on these analyses, an active current balancing scheme is proposed. It is able to sense the unbalanced current and eliminate it by actively controlling the gate drive signal to each device. The features of fine time resolution and low complexity make this scheme attractive to a wide variety of wide-band-gap device applications. Experimental and simulation results verify the feasibility and effectiveness of the proposed scheme.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fred Wang; Ben J. Blalock
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    Three-phase current source rectifier (CSR) is a promising solution for power supply systems as the buck-type power factor correction converter. By converter paralleling, high power rating and system redundancy can be achieved. However, asymmetrical distribution of load current among converter modules may occur, which can increase power loss or even damage devices. This paper presents the DC-link current control scheme for paralleled current source rectifiers to balance the output currents. Using a master-slave control, the balanced output current distribution and system redundancy are implemented. By correcting zero state duration based on modulation scheme, the circulating current is suppressed without introducing additional power losses, and both positive and negative DC-link currents are balanced.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fred Wang; Ben J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper develops a liquid cooled high efficiency three-phase current source rectifier (CSR) for data center power supplies based on 400 Vdc architecture, using SiC MOSFETs and Schottky diodes. The 98.54% efficiency is achieved at full load. The rectifiers are paralleled to achieve high power ratings and system redundancy. The current balance and hot-swap of paralleled CSRs are realized in simulation using master-slave control. Moreover, an improved modulation scheme through adjustment of the freewheeling state is proposed and verified to effectively suppress the circulating current.

  • Weimin Zhang; Zhuxian Xu; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In recent years, Si power MOSFET is approaching its performance limits, and Gallium Nitride (GaN) HEMT is getting mature. This paper evaluates the 600 V cascode GaN HEMT performance, and compares it with the state-of-the-art Si CoolMOS in LLC resonant converter. First, the static characterization of 600 V cascode GaN HEMT is described in different temperatures. The switching performance is tested by a double pulse tester to provide the turn-off loss reference to the design of LLC resonant converter. Second, a 400 V-12 V/300 W/1 MHz all-GaN-based converter with the 600 V cascode GaN HEMT is compared with a Si-based converter with the 600 V Si CoolMOS. The device output capacitance is a key factor in the design and loss analysis of LLC resonant converter. The design results show that the total GaN device loss of the all-GaN-based converter can be improved by 42% compared with the total Si device loss. Finally, both 400 V-12 V/300 W/1 MHz Si-based and GaN-based LLC resonant converter prototypes are tested and compared with waveforms and efficiency curves.

  • Jing Wang; Yiwei Ma; Liu Yang; Leon M. Tolbert; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A three-phase induction motor emulator using power electronic converters is introduced in this paper. The emulator is intended to be used in an ultra-wide-area grid transmission network emulator represented by regenerative converters structure. The load emulator converter is controlled in rectifier mode to behave like the real induction motor load, whose model is described and programmed in the digital controller. This paper discusses specifically about the induction motor dynamic modeling, numerical method used in the controller, and finally experimental result verification of starting up transient.

  • Zhiqiang Wang; Xiaojie Shi; Yang Xue; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    Overcurrent protection of silicon carbide (SiC) MOSFETs remains a challenge due to lack of practical knowledge. This paper presents two overcurrent protection methods to improve the reliability and overall cost of the SiC MOSFET based converter. First, a solid state circuit breaker (SSCB) composed primarily by a Si IGBT and a commercial gate driver IC is connected in series with the DC bus to detect and clear overcurrent faults. Second, the desaturation technique using a sensing diode to detect the drain-source voltage under overcurrent faults is implemented as well. The design considerations and potential issues of the protection methods are described and analyzed in detail. A phase-leg configuration based step-down converter is built to evaluate the performance of the proposed protection schemes under various conditions, considering variation of fault type, decoupling capacitance, protection circuit parameters, etc. Finally, a comparison is made in terms of fault response time, temperature dependent characteristics, and applications to help designers select a proper protection method.

  • R. L. Greenwell; B. M. McCue; L. M. Tolbert; B. J. Blalock; S. K. Islam
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    High-temperature integrated circuits fill a need in applications where there are obvious benefits to reduced thermal management or where circuitry is placed away from temperature extremes. Examples of these applications include aerospace, automotive, power generation, and well-logging. This work focuses on automotive applications in which the growing demand for hybrid electric vehicles (HEVs), Plug-in-hybrids (PHEVs), and Fuel-cell vehicles (FCVs) has increased the need for high-temperature electronics that can operate at the extreme ambient temperatures that exist under the hood of these vehicles, which can be in excess of 150°C. Silicon carbide (SiC) and other wide-bandgap power switches that can function at these temperature extremes are now entering the market. To take full advantage of their potential, high-temperature capable circuits that can also operate in these environments are required.

  • Xiaojie Shi; Zhiqiang Wang; Leon M. Tolbert; Fred Wang
    2013 IEEE ECCE Asia Downunder
    2013

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    This paper deals with the system structure and operating principle of a modular multilevel converter (MMC) with integrated arm inductors for improved performance. The proposed integrated inductors provide inductances not only for circulating current suppression, but also for switching ripple mitigation. Compared with the conventional MMC structure implemented with two separate inductors connected in both upper and lower arms by two magnetic cores, only one core is required for the arm inductor of each phase. Hence, the overall size, weight, and cost of magnetic components will be much lower than discrete ones. In addition, the relationships between the number of voltage levels, the equivalent differential inductance of the integrated inductor, and the total harmonic distortion (THD) of the phase voltage is analyzed based on the designed integrated inductor. Without differential inductance, the number of voltage levels should be more than 12 with N+1 phase shift PWM (PSPWM) or 8 with 2N+1 PSPWM to bring the THD below 5 %, while this goal can be achieved by 4 sub-modules MMC with only 2 mH differential mode (DM) inductance if N+1 modulation is applied, or 0.5 mH DM inductance if 2N+1 modulation is adopted. Simulation results for a three-phase inverter system are provided to support the theoretical considerations.

  • Xiaojie Shi; Zhiqiang Wang; Leon M. Tolbert; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper deals with DC voltage ripple suppression of the modular multilevel converter (MMC) under single-line-to-ground (SLG) fault condition. First, the instantaneous power of a phase unit is derived theoretically according to the equivalent circuit model of the MMC under unbalanced condition, providing a mathematical explanation of the double-line frequency ripple contained in the dc voltage. Moreover, different characteristics of phase current during three possible SLG faults are analyzed and compared. Based on the derivation and analysis, a quasi-PR controller is proposed to suppress the dc voltage ripple. The proposed controller, combining with the negative and/or zero sequence current controllers, could enhance the overall fault-tolerant capability of the MMC under different types of SLG faults. In addition, no extra cost will be introduced given that only DC voltage is required to be detected. Simulation results from a three-phase MMC based rectifier system generated with the Matlab/Simulink software are provided to support the theoretical considerations.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    Silicon Carbide (SiC) power devices have inherent capability for fast switching. However, in a phase-leg configuration, high dv/dt will worsen the interference between the two devices during a switching transient (i.e., cross talk), leading to slower switching speed, excessive switching losses, and overstress of power devices. Unfortunately, due to intrinsic properties, such as low threshold voltage, low maximum allowable negative gate voltage, and large internal gate resistance, SiC power devices are easily affected by cross talk. This paper proposes a novel gate assist circuit using an auxiliary transistor in series with a capacitor to mitigate cross talk. Based on CMF20120D SiC MOSFETs, the experimental results show that the new gate assist circuit is capable of reducing the turn-on switching loss up to 19.3%, and suppress the negative spurious gate voltage within the maximum allowable negative gate voltage without the penalty of further decreasing the device switching speed. Moreover, in comparison to a conventional gate drive with -2 V turn-off gate voltage, this gate assist circuit without negative isolated power supply is more effective in improving the switching behavior of power devices in a phase-leg. The proposed gate assist circuit is a cost-effective solution for cross talk mitigation.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In a phase-leg configuration, the high switching-speed performance of silicon carbide (SiC) devices is limited by the interaction between the upper and lower devices during the switching transient (cross talk), leading to additional switching losses and overstress of the power devices. To utilize the full potential of fast SiC devices, this paper proposes a gate assist circuit using two auxiliary transistors and a diode to eliminate cross talk. Based on CMF20120D SiC MOSFETs, the experimental results show that this gate assist methodology is effective to suppress cross talk under different operating conditions, enabling turn-on switching losses reduction by up to 19.6%, and negative spurious gate voltage minimization within the maximum allowable negative gate voltage of the power devices without the penalty of reduced switching speed. Moreover, in comparison to the conventional gate driver with -2 V turn-off gate voltage, this gate assist circuit without a negative isolated power supply is more effective in enhancing the switching behavior of power devices in a phase-leg. Accordingly, the proposed gate assist circuit is a cost-effective solution for cross talk suppression.

  • Xiaojie Shi; Zhiqiang Wang; Leon M. Tolbert; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    Modular Multilevel Converter (MMC) has proved to be an effective solution for high power applications, supplying low distorted output voltage and high fault tolerance. This paper presents a detailed performance comparison between phase disposition PWM (PDPWM) and phase shift PWM (PSPWM) schemes under normal condition, over-modulation, as well as carrier non-synchronization condition. Compared to the PSPWM strategy, the PDPWM has smaller line-to-line voltage distortion under normal condition, when the carrier frequencies are adjusted to achieve the same number of switch transitions over one fundamental cycle. In addition, the capacitor voltages are able to keep balanced without additional controllers. Under over-modulation condition, PDPWM can still achieve smaller voltage distortion without capacitor voltage deviation, while obvious voltage differences are observed with PSPWM, which shows an opposite trend toward that of normal condition. Moreover, asynchronous carriers have different impacts on the harmonic cancellation, which needs to be carefully considered in a hardware implementation. Simulation results for a three-phase nine-level inverter system generated with the Matlab/Simulink software are provided to support the theoretical considerations.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Benjamin J. Blalock; Madhu Chinthavali
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper presents a new active overcurrent protection scheme for IGBT modules based on the evaluation of fault current level by measuring the induced voltage across the stray inductance between the Kelvin emitter and power emitter of IGBT modules. Compared with the commonly used desaturation protection, it provides a fast and reliable detection of fault current without any blanking time. Once a short circuit is detected, a current limiting and clamping function is activated to dynamically suppress the transient peak current, thus reducing the considerable energetic and thermal stresses induced upon the power device. Subsequently, a soft turn-off mechanism is employed aiming to reduce surge voltages induced by stray inductance under high current falling rate. Moreover, the proposed method provides flexible protection modes, which overcome the interruption of converter operation in the event of momentary short circuits. The feasibility and effectiveness of the proposed approach have been validated by simulation results with real component models in Saber. A Double Pulse Tester (DPT) based experimental test setup further verifies the proposed protection scheme.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Benjamin J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper addresses the issues of switching behavior of a high power insulated gate bipolar transistor (IGBT) that works in hard switching conditions. First, the voltage and current switching waveforms of IGBT modules are described for an IGBT phase-leg module with an inductive load, and the associated switching losses, reverse recovery current of free-wheeling diodes, voltage overshoot, and EMI noise are analyzed. Based on the analysis, an actively controlled gate drive circuit is proposed, which provides optimization of the fast driving for low switching losses and short switching time, and slow driving for low noise and switching stress. Compared to a conventional gate drive strategy, the proposed active gate driver (AGD) has the capability of reducing the switching losses, delay time, and Miller plateau duration effectively during both turn-on and turn-off transient. Experimental results verify the validity and effectiveness of the proposed gate driving method.

  • Bailu Xiao; Lijun Hang; Cameron Riley; Leon M. Tolbert; Burak Ozpineci
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A three-phase modular cascaded H-bridge multilevel inverter for a grid-connected photovoltaic (PV) system is presented in this paper. To maximize the solar energy extraction of each PV string, an individual maximum power point tracking (MPPT) control scheme is applied, which allows the independent control of each dc-link voltage. PV mismatches may introduce unbalanced power supplied to the three-phase system. To solve this issue, a control scheme with modulation compensation is proposed. The three-phase modular cascaded multilevel inverter prototype has been built. Each H-bridge is connected to a 185 W solar panel. Simulation and experimental results are presented to validate the proposed ideas.

  • Yang Xue; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 IEEE Transportation Electrification Conference and Expo (ITEC)
    2013

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    Buffer circuits are widely used in high-power inverters' gate drives to get enough driving current for power modules or power transistors in parallel. In this paper, designs of buffer circuits to boost the output current for a gate driver IC are investigated. Different buffer topologies are reviewed and their individual advantages and disadvantages analyzed. Based on the analysis, three topologies, specifically the BJT emitter follower, the two NFETs totem pole, and the CMOS buffer, are chosen for further study. Optimizations are performed on these three buffers by taking the driving capability, switching speed, circuit complexity, and cost into account. After that, a test setup is built, and the driving performance of the buffers is characterized and then compared experimentally with a commercial buffer IC with a rated current of 30 A. All three proposed buffers show better performance and lower cost, which verifies the feasibility and effectiveness of the proposed optimization methods. Double pulse test results indicate that the addition of a buffer stage makes the switching performance less sensitive to the load and can achieve significant performance improvement when large or parallel power switches are to be driven.

  • Zhuxian Xu; Fan Xu; Dong Jiang; Wenchao Cao; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper presents a 30 kW Si IGBT based three-phase traction inverter for operating at the junction temperature of 200°C with the reduced cooling and improved efficiency in hybrid electric vehicle (HEV) applications. The high temperature capable Si devices based module is developed for operation with the 105°C high temperature engine coolant, leading to lower cost and higher power density. A variable switching frequency pulse width modulation (VSFPWM) scheme is employed to relieve the negative effect of high temperature on loss. The experimental results demonstrate that the three-phase converter can operate continuously with the 105°C high temperature coolant. The efficiency is increased from 94.98 % to 95.73 % after VSFPWM is applied at full load.

  • Zhuxian Xu; Fan Xu; Puqi Ning; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper presents a 30 kW Si IGBT based three-phase converter for operating at the junction temperature of 200 °C with the high temperature coolant in hybrid electric vehicle applications. First, the Si IGBT phase-leg module is developed for 200 °C operation utilizing high temperature packaging technologies. Then the thermal management system utilizing the integrated pin fin baseplate is adopted to allow improved thermal performance. Afterward, the short circuit current is employed as the temperature sensitive parameter for junction temperature measurement during converter operation. Finally, a 30 kW three-phase converter is implemented. The experimental results demonstrate that the three-phase converter can operate at the junction temperature of 200 °C with the 105 °C high temperature coolant, thus eliminating the need for the additional 65 °C coolant in HEV.

  • Jing Xue; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    High density filter design for electromagnetic interference (EMI) noises is essential for the variable speed motor drive system in modern transportation applications. As often being one of the major causes of bulky and heavy filter designs, the mixed-mode (MM) noise phenomenon in the three-phase motor drive system is discussed in this paper. The generation schemes of the MM noises on both DC and AC sides of the system are analyzed. According to the three-phase voltage vector modulation, the MM noises at different switching conditions are compared and summarized. Based on experimental results from a specified prototype system, the existence of the MM noises and its impact on EMI filter design is verified. Investigations show that the implementation of both CM filter and balancing X-capacitors can help attenuate the MM noises effectively.

  • Siyao Jiang; Weimin Zhang; Bo Liu; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    The desired working condition for the unregulated LLC resonant converter is to have the switching frequency equal to the resonant frequency in order to achieve higher efficiency. However, since the converter works in open-loop condition, the switching frequency usually deviates from the resonant frequency in real converter prototype. In this paper, a novel switching frequency control scheme to track the resonant frequency is proposed. Theoretically, the primary side current of LLC resonant converter is a pure sinusoidal waveform when it works at resonant frequency. The proposed control algorithm is based on tracking the minimum value of the total resonant current harmonic to realize the switching frequency tracking the resonant frequency. A closed-loop digital controller is presented to calculate the total resonant current harmonic to estimate the shape of the waveform. The switching frequency can be well controlled to track the resonant frequency. As a result, the higher efficiency can be achieved.

  • Ming Zhang; Bin Li; Long Huang; Wenxi Yao; Zhengyu Lu; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    Unbalanced grids introduce performance deterioration for the Vienna rectifier topology by producing twice fundamental frequency ripple in the dc-link voltage and input active/reactive power. A traditional control strategy is the input current tracking scheme, which can work under any unbalanced conditions, but it cannot eliminate the input power ripple and the output dc-link voltage ripple. Another class of control strategy for the purpose of eliminating the input power ripple can maintain constant input power and eliminate the dc-link voltage ripple, however, it will fail under severe unbalanced grids. This paper first analyzes the theoretical limits of constant power control method, and based on which a novel control method that injects a small amount of power ripple is proposed to make a compromise between the working area and the output dc voltage ripple. Finally, the experiment results using constant power control method are given and validate the performance of the proposed control method.

  • Bailu Xiao; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A three-phase cascaded voltage source inverter (VSI) for a grid-connected photovoltaic (PV) system is proposed in this paper. The topology consists of three traditional two-level six-switch VSIs interconnected through inductors. Each VSI unit is fed by a string of PV panels. To realize the central control of the proposed system, the equivalent model and average model of the three-phase cascaded VSI are established. To harvest more solar energy, a control scheme with maximum power point tracking (MPPT) control is proposed. Phase-shifted discontinuous PWM (PS-DPWM) is applied to control the switching devices of each VSI unit. Simulation and experimental results are presented to validate the proposed ideas.

  • Bin Li; Ming Zhang; Long Huang; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    The LCCL control strategy, which was brought forward as a solution for the resonance of LCL-filter, provides good steady-state performance but unsatisfactory dynamic performance. In this paper, based on an improved discrete state space model and state observer of the LCCL control, a new optimized pole placement strategy is proposed to achieve the best damping performance of the whole system with LCL-filter, with which the dynamic performance of the system can be greatly improved without additional sensors. Furthermore, the proposed method has almost the same performance in suppressing power grid disturbances and the non-linearity of an inverter, resulting in the same steady-state performance compared with LCCL control strategy. A 10 kW grid-connected inverter with LCL-filter prototype is set up to verify the effectiveness of the proposed control strategy and observer configuration, as well as the overall improvement of system performance.

  • Bin Li; Ming Zhang; Long Huang; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    An averaged switching model of grid-connected inverter using dual-loop current control with LCL-filter in discrete domain is built under stationary frame. A proportional resonant (PR) regulator is adopted in the current-loop to track the given fundamental sinusoidal current without steady state error. In addition, in order to reduce the low frequency harmonics, the resonance of 5th harmonics is adopted. Because the system is of high order and complex, the theory of root locus is proposed to analyze the influence of parameters in PR regulator, the influence of digital delay, and the LCL-filter. Based on the theoretical analysis, the poles can be properly selected to guarantee the stability of the system and the performance of current-loop during wide grid-fed power. Finally, a 10 kW prototype of grid-connected inverter with LCL-filter is set up to verify the effectiveness, practicality, and robustness of the proposed design method.

  • Sensen Liu; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    A natural abc frame based current control loop, which considerably reduces total algorithm complexity, and also helps to reduce the reactive components size and total loss in comparison to the traditional dual current control schemes, is proposed for a Vienna rectifier under unbalanced input conditions. Experimental results are presented to verify the validity of the proposed controller.

  • Alain Sanchez-Ruiz; Gonzalo Abad; Silverio Alvarez; Leon M. Tolbert
    2013 15th European Conference on Power Electronics and Applications (EPE)
    2013

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    This paper proposes a systematic procedure to evaluate, compare and select different modulation techniques for a medium voltage variable speed drive. Several indicators are proposed to be analyzed, mainly based on thermal analysis, voltage limitations, dc bus evaluation and load current quality. The suggested procedure is evaluated on a simulation based analysis in a 6.6 kV adjustable speed drive system with a 5-level H-bridge neutral point clamped (HNPC) inverter topology and diode front end (DFE) as rectifier. The use of selective harmonic elimination (SHE) with different number of angles and space vector modulation (SVM) is assessed depending on the load demands (output frequency, modulation index, etc.).

  • Ke Shen; Bailu Xiao; Jun Mei; Leon M. Tolbert; Jianze Wang; Xingguo Cai; Yanchao Ji
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A modulation reconfiguration based fault-tolerant method is proposed for the modular multilevel converter (MMC) system. A practical method to reconfigure the reference sine wave is proposed from the point of view that the output line-to-line voltages should be guaranteed to be well regulated and not vary. Voltage balancing for sub-module capacitors is achieved while using carrier rotation algorithm based multicarrier pulse width modulation. Experimental results obtained from the laboratory system verified the validity and reliability of the proposed fault-tolerant control algorithm in the MMC application.

  • Jing Xue; Fred Wang; Wenjie Chen
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    High density electromagnetic interference (EMI) filter design, especially on conducted common-mode (CM) noise emission suppression, is essential for variable speed motor drive system in modern transportation applications. As an alternative of the conventional inverter-end (IE) EMI filter, this paper discusses the possibility of implementing an EMI filter on the motor end for the CM noise attenuation. A functional comparison between the inverter and the motor-end CM filter is firstly carried out, concerning the CM noise propagation path in the DC-fed motor drive system. According to their features in the noise propagation, the authors focus on two typical kinds of motor-end CM filters in detail, namely the motor front-end (MFE) filter and the motor chassis-end (MCE) filter. Their performances on improving the output CM impedance and suppressing the CM noise emission are analyzed, based on experimental results from a specified prototype system. Investigations show that the implementation of the MFE filter has similar functions with the IE CM choke, while attaching the MCE filter with the motor can help save the magnetic size and weight from the conventional IE filter. However, special attentions are needed for the safety and parasitic concerns.

  • Yutian Cui; Leon M. Tolbert
    The 1st IEEE Workshop on Wide Bandgap Power Devices and Applications
    2013

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    Energy efficiency of typical data centers is less than 50% where more than half of the power is consumed during power conversion, distribution, cooling, etc. In this paper, a single power stage architecture that converts 400 V directly to 1 V is discussed. Input series and output parallel structure (ISOP) is selected due to the high input voltage and large output current operation condition. Phase shift full bridge (PSFB) DC/DC converter with high step down ratio (66:1) is built with Gallium Nitride (GaN) FETs targeting at high efficiency. This paper mainly focuses on the study of a single 66 V to 1 V converter. Prototypes of the PSFB converter is designed, built, and tested. Preliminary experimental results are provided to verify the design.

  • Wenchao Cao; Fred Wang; Dong Jiang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    In order to reduce the inverter switching loss and system noise of electric vehicle (EV) and hybrid electric vehicle (HEV) motor drives operating in high output torque region, while preventing over-heating and demagnetization of the electric motor, this paper proposes variable switching frequency PWM (VSFPWM) strategies based on on-line prediction of current ripple RMS value for two typical motor drive systems of EV/HEV. First, the instantaneous output current ripple of three-phase inverter with SVPWM is analyzed in the time-domain. Then the current ripple prediction based VSFPWM strategies are proposed for both traction motor drive topologies to meet the current ripple RMS value requirement. Compared with constant switching frequency PWM (CSFPWM) method, the inverter switching loss and noise reduction capabilities of the proposed VSFPWM methods are analyzed. The effectiveness of the proposed methods is verified by both simulation and experiments.

  • Xuning Zhang; Dushan Boroyevich; Paolo Mattavelli; Jing Xue; Fei Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper proposes an EMI filter design and optimization method for both AC and DC side in a DC-fed motor drive system. Based on the noise generation and propagation mechanism, the analysis of common mode (CM) and differential mode (DM) EMI noise equivalent circuits is provided. Based on those equivalent circuits, this paper investigates the system EMI noise between AC and DC side, which shows the interaction between DM and CM noises and the interaction between adding AC and DC side filters. With these considerations, an optimized EMI filter design procedure is proposed to design CM and DM filters for both AC and DC sides. To minimize the impact on the EMI noise of one side caused by adding filter on the other side, certain order must be followed to design AC and DC CM and DM filters. Moreover, the EMI filter weight optimization method is also discussed to get the minimum weight of EMI filter and improve system power density. Simulation and experimental results verify the interaction between AC and DC filters and show that EMI filters can be designed to suppress both AC and DC EMI noise to meet the standard with the proposed EMI filter design method.

  • Yalong Li; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    With circulating current suppressing control, the dominating second-order circulating current in a modular multilevel converter (MMC) can be effectively decreased, and the arm inductance requirement based on the circulating current is thus largely reduced. This paper investigates the extent to which the arm inductance can be reduced. The circulating current at switching frequency is first explored, which is found to be a limitation for arm inductance selection when the circulating current suppressing control is implemented. The theoretical relationship between switching frequency circulating current and arm inductance is further deduced, and the arm inductance selection principle is proposed. Finally, the theoretical analysis is verified by the experiment.

  • Ben Guo; Fred Wang; Rolando Burgos; Eddy Aeloiza
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In the three-phase buck-type rectifier, the current on the dc-link inductor becomes discontinuous under light load condition, at which point the current ripple is larger than the dc current value. Traditional control algorithms and modulation schemes do not work consistently well in discontinuous current mode (DCM), causing input current distortion and output voltage ripple. In this paper, the three-phase buck-type rectifier is modeled and analyzed in DCM. The DCM transfer function is derived and compared with the one for continuous current mode (CCM). It is shown that the pole and gain of the DCM transfer function changes significantly compared to that of CCM. A new modulation scheme for DCM is then proposed, which places the space vectors in such way to keep the dc-link current continuous during the active states. A digital controller is then used to eliminate the sampling error caused by the large current ripple, successfully controlling the rectifier in DCM. Simulation and experimental results are used to verify that the input current distortion and the output voltage ripple are dramatically reduced under the proposed DCM modulation and control strategy.

  • Xuning Zhang; Dushan Boroyevich; Rolando Burgos; Paolo Mattavelli; Fei Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper presents a comprehensive evaluation of modulation techniques for three-level neutral-point-clamped (NPC) inverters, focusing on the following system aspects: EMI performance, neutral-point voltage ripple, and switching losses. The modulation techniques considered are: nearest three space vectors (NTSV), common-mode reduction (CMR), and common mode elimination (CME). After describing in detail their implementation, an in-depth theoretical analysis on EMI performance is presented based on the space vector sequences employed and the respective space vector dwelling times. Simulation and experimental results in both time and frequency domains are used to verify the techniques and show their effectiveness in terms of EMI noise reduction. An analytical calculation method for the NP voltage ripple is then presented and used to compare the voltage ripple generated by these modulation schemes; experimental results are used to verify the analysis. Lastly, this paper also presents an analytical loss calculation method, which shows that NTSV and CME are comparable in these terms, while CMR can reduce switching losses significantly.

  • Xuning Zhang; Rolando Burgos; Dushan Boroyevich; Paolo Mattavelli; Fei Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper presents a detailed analysis of the impact of “dead time” (DT) on the EMI performance of three-level neutral-point-clamped (3L-NPC) converters using common-mode voltage elimination (CME) modulation, proposing an improved CME modulation method to compensate for this. The implementation method of CME modulation is presented and the benefits and drawbacks are discussed, showing that the benefit of CME modulation is highly related to the DT added to the system, which makes it less practical. The effect of DT on CM voltage generation is then quantified by analyzing the switching states of one phase-leg. This knowledge is used to develop an improved CME modulation method, which has as main feature the proper selection of space vector sequences taking into consideration the current direction of the three phases. This analysis is verified using simulation results and validated using experimental tests conducted with a 2.5 kW 3L-NPC inverter prototype.

  • Puqi Ning; Zhenxian Liang; Fei Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A novel packaging structure for medium power modules featuring power semiconductor switches sandwiched between two symmetric substrates that fulfill electrical conduction and insulation functions is presented. Large bonding areas between dies and substrates allow this packaging technology to offer significant improvements in electrical, thermal performance. Double-sided cooling system was dedicatedly analyzed and designed for different applications.

  • Xuning Zhang; Paolo Mattavelli; Dushan Boroyevich; Fei Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper presents a systematic design method to select the interleaving angle to reduce the EMI noise in a paralleled three-phase voltage-source converters motor drive system. The EMI noise analysis equivalent circuits for a motor drive system are given and double Fourier integral analysis is used to analyze the impact of interleaving on EMI noise sources. With the consideration of noise propagation path impedance, the design method of interleaving angle selection is analyzed in detail. When system switching frequency and system load and source impedance are determined, the optimal interleaving angle can be calculated based on the system impedance resonant frequency and system switching frequency. Verifications are carried out through both the simulation of a 100kW motor drive system and the experiment on a scale-downed 2kW system. The results show that by using calculated optimal interleaving angle based on the method proposed in this paper, the EMI noise can be reduced by 10~12dB in the impedance resonant frequency range which can reduce the EMI filter size significantly.

  • Xuning Zhang; Dushan Boroyevich; Rolando Burgos; Paolo Mattavelli; Fei Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper presents a comprehensive evaluation of modulation techniques for three-level neutral-point-clamped (NPC) inverters, focusing on the following system aspects: EMI performance, neutral-point voltage ripple, and switching losses. The modulation techniques considered are: nearest three space vectors (NTSV), common-mode reduction (CMR), and common mode elimination (CME). After describing in detail their implementation, an in-depth theoretical analysis on EMI performance is presented based on the space vector sequences employed and the respective space vector dwelling times. Simulation and experimental results in both time and frequency domains are used to verify the techniques and show their effectiveness in terms of EMI noise reduction. An analytical calculation method for the NP voltage ripple is then presented and used to compare the voltage ripple generated by these modulation schemes; experimental results are used to verify the analysis. Lastly, this paper also presents an analytical loss calculation method, which shows that NTSV and CME are comparable in these terms, while CMR can reduce switching losses significantly.

  • Xuning Zhang; Rolando Burgos; Dushan Boroyevich; Paolo Mattavelli; Fei Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper presents a detailed analysis of the impact of “dead time” (DT) on the EMI performance of three-level neutral-point-clamped (3L-NPC) converters using common-mode voltage elimination (CME) modulation, proposing an improved CME modulation method to compensate for this. The implementation method of CME modulation is presented and the benefits and drawbacks are discussed, showing that the benefit of CME modulation is highly related to the DT added to the system, which makes it less practical. The effect of DT on CM voltage generation is then quantified by analyzing the switching states of one phase-leg. This knowledge is used to develop an improved CME modulation method, which has as main feature the proper selection of space vector sequences taking into consideration the current direction of the three phases. This analysis is verified using simulation results and validated using experimental tests conducted with a 2.5 kW 3L-NPC inverter prototype.

  • Puqi Ning; Zhenxian Liang; Fei Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A novel packaging structure for medium power modules featuring power semiconductor switches sandwiched between two symmetric substrates that fulfill electrical conduction and insulation functions is presented. Large bonding areas between dies and substrates allow this packaging technology to offer significant improvements in electrical, thermal performance. Double-sided cooling system was dedicatedly analyzed and designed for different applications.

  • Xuning Zhang; Paolo Mattavelli; Dushan Boroyevich; Fei Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper presents a systematic design method to select the interleaving angle to reduce the EMI noise in a paralleled three-phase voltage-source converters motor drive system. The EMI noise analysis equivalent circuits for a motor drive system are given and double Fourier integral analysis is used to analyze the impact of interleaving on EMI noise sources. With the consideration of noise propagation path impedance, the design method of interleaving angle selection is analyzed in detail. When system switching frequency and system load and source impedance are determined, the optimal interleaving angle can be calculated based on the system impedance resonant frequency and system switching frequency. Verifications are carried out through both the simulation of a 100kW motor drive system and the experiment on a scale-downed 2kW system. The results show that by using calculated optimal interleaving angle based on the method proposed in this paper, the EMI noise can be reduced by 10~12dB in the impedance resonant frequency range which can reduce the EMI filter size significantly.

  • Jing Wang; Liu Yang; Yiwei Ma; Xiaojie Shi; Xiaohu Zhang; Lijun Hang; Keman Lin; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    An ultra-wide-area transmission network emulator represented by regenerative converters is developed in this paper. The converters are paralleled to provide and share power similar to electromechanical generators. Others are controlled to emulate loads, such as, induction motors and constant impedance, current, power (ZIP) loads. The structure and control algorithms of these power system component emulators are discussed in detail, and the performance of overall system architecture is presented. As is well-known, the induction motor will induce large power perturbation when it starts. The simulation results clearly show the dynamic response and starting up process of the load.

  • Weimin Zhang; Yu Long; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Stephan Henning; Chris Wilson; Robert Dean
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    Silicon Power MOSFETs, with more than thirty years of development, are widely accepted and applied in power converters. Gallium Nitride (GaN) power devices are commercially available in recent years [1], but the device performance and application have not been fully developed. In this paper, GaN devices are compared with state-of-art Si devices to evaluate the device impact on soft-switching DC-DC converters, like LLC resonant converter. The analytical approach of device selection and comparison are conducted and loss related device parameters are derived. Total device losses are compared between Si and GaN based on these parameters. GaN shows less loss compared with Si, yielding approximately a 20% reduction of total device loss. Two 300 W, 500 kHz, 48 V-12 V GaN-based and Si-based converter prototypes are built and tested. Since the body diode forward voltage drop of GaN device is high, the dead time is adjusted to minimize the body diode conduction period. The peak efficiency of the GaN-based converter is 97.5%, and the full load efficiency is 96.1%, which is around 0.3% higher than the Si-based converter at full load. The test results shows that, although GaN device has lower loss, the improvement of converter efficiency is not much. The reason is that the transformer loss accounts for more than 60% of total loss. Therefore, a transformer which fits the GaN device characteristic need to be further investigated.

  • Zheyu Zhang; Weimin Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    Advanced power semiconductor devices, especially wide band-gap devices, have inherent capability for fast switching. However, due to the limitation of gate driver capability and the interaction between two devices in a phase-leg during switching transient (cross talk), the switching speed is slower than expected in practical use. This paper focuses on identifying the key limiting factors for switching speed. The results provide the basis for improving gate drivers, eliminating interference, and boosting switching speed. Based on the EPC2001 Gallium Nitride transistor, both simulation and experimental results verify that the limiting factors in the gate loop include the pull-up (-down) resistance of gate driver, rise (fall) time and amplitude of gate driver output voltage; among these the rise (fall) time plays the primary role. Another important limiting factor of device switching speed is the spurious gate voltage induced by cross talk between two switches in a phase-leg. This induced gate voltage is not only determined by the switch speed, but also depends on the gate loop impedance, junction capacitance, and operating conditions of the complementary device.

  • Fan Xu; Ben Guo; Leon M. Tolbert; Fred Wang; Ben J. Blalock
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents the characteristics of a 1200 V, 33 A SiC MOSFET and a 1200 V, 60 A SiC schottky barrier diode (SBD). The switching characteristics of the devices are tested by a double pulse test (DPT) based on a current-source structure at voltage levels up to 680 V and current up to 20 A. In addition, based on these devices, a 7.5 kW, three-phase buck rectifier for a 400 Vdc architecture data center power supply is designed. The total loss of this rectifier is calculated full load. The results show that the SiC based buck rectifier can obtain low power loss and smaller weight and volume than a Si based rectifier.

  • Fan Xu; Ben Guo; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    This paper presents a 7.5 kW liquid cooled three-phase buck rectifier which will be used as the front-end rectifier in 400 Vdc architecture data center power supply systems. SiC MOSFETs and SiC Schottky barrier diodes (SBDs) are used in parallel to obtain low power semiconductor losses. Input and output filters are designed and inductor core material is compared to reduce passive component losses. A low-loss modulation scheme and 28 kHz switching frequency are selected to optimize the converter design for efficiency. A prototype of the proposed rectifier is constructed and tested, and greater than 98.5% efficiency is obtained at full load.

  • Zhiqiang Wang; Xiaojie Shi; Yang Xue; Leon M. Tolbert; Benjamin J. Blalock
    2012 IEEE 13th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2012

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    The issues of turn-on performance of a high power insulated gate bipolar transistor (IGBT) that works in hard switching conditions are discussed in detail. First, the turn-on delay time, switching loss, reverse recovery current of the associated free-wheeling diode, and EMI noise are analyzed for an IGBT phase-leg module with an inductive load. Based on the analysis, a novel gate drive circuit combining the slow drive requirements to minimize noise and switching stress, and the fast drive requirements for high-speed switching and low switching energy loss is proposed. Compared to a conventional gate drive circuit, the proposed gate driving strategy is able to effectively reduce the switching loss, delay time, and total switching time during the turn-on transient while the turn-off performance remains unchanged. Simulation and experimental results verify the validity and effectiveness of the proposed gate driving method.

  • Michael Pickelsimer; Leon Tolbert; Burak Ozpineci; John M. Miller
    2012 IEEE International Electric Vehicle Conference
    2012

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    Wireless power transfer has been a popular topic of recent research. Most research has been done to address the limitations of coil-to-coil efficiency. However, little has been done to address the problem associated with the low input power factor with which the systems operate. This paper details the steps taken to analyze a wireless power transfer system from the view of the power grid under a variety of loading conditions with and without power factor correction.

  • B. Diong; L. Dofflemyer; B. Xiao; L. M. Tolbert; F. Filho
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    For relatively low voltage dc sources such as photovoltaic arrays and fuel cell stacks to supply power to typical AC systems, using multilevel inverters is an approach with several advantages. In doing so, while it is possible to achieve optimal THD below 5% by staircase/block modulation of cascaded H-bridge multilevel inverters, this requires the use of several separate dc sources with different non-integer ratio values. This paper proposes having some of these source values be equal to each other to increase sub-system modularity, and replacing some H-bridge cells by a diode-clamped inverter to reduce the required number of separate dc sources, while still achieving near-optimal THD below 5%. In addition, a means of re-charging the diode-clamped inverter's inner capacitors is proposed Analytical, simulation and experimental results are presented for an example hybrid converter with four H-bridge cells in series with a 5-level diode-clamped inverter.

  • Bailu Xiao; Ke Shen; Jun Mei; Faete Filho; Leon M. Tolbert
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    A single-phase cascaded H-bridge multilevel inverter for a grid-connected photovoltaic (PV) system with nonactive power compensation is presented in this paper. To maximize the solar energy extraction of each PV string, an individual maximum power point tracking (MPPT) control scheme is applied, which allows the independent control of each dc-link voltage. A generalized nonactive power theory is applied to generate the nonactive current reference. Within the inverter's capability, the local consumption of nonactive power is provided to realize power factor correction. A single-phase modular cascaded multilevel inverter prototype has been built. Each H-bridge is connected to a 195 W solar panel. Simulation and experimental results are presented to validate the proposed ideas.

  • Yutian Cui; Madhu S. Chinthavali; Fan Xu; Leon M. Tolbert
    2012 IEEE International Symposium on Industrial Electronics
    2012

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    This paper presents recent research on several silicon carbide (SiC) power devices. The devices have been tested for both static and dynamic characteristics, which show the advantages over their Si counterparts. The temperature dependency of these characteristics has also been presented in this paper. Then, simulation work of paralleling operation of SiC power MOSFETs based on a verified device model in Pspice is presented to show the impact of parasitics in the circuit on the switching performance.

  • Zhuxian Xu; Fred Wang; Puqi Ning
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    In this paper, a method is proposed to measure the junction temperatures of IGBT discrete devices and modules using short circuit current. Experimental results show that the short circuit current has good sensitivity, linearity and selectivity, which is suitable to be used as temperature sensitive electrical parameters (TSEP). Test circuit and hardware design are proposed for junction temperature measurement in single phase and three phase converters. By connecting a temperature measurement unit to the converter and giving a short circuit pulse, the IGBT junction temperature can be measured.

  • Long Huang; Bin Li; Zhengyu Lü; Lijun Hang; Leon Tolbert
    2012 IEEE International Symposium on Industrial Electronics
    2012

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    In this paper, the design of PR regulator which is applied on the grid-connected inverter with LCL-filter, is analyzed in details. By modeling the system under stationary frame, the open and close-loop transfer function is given, which is used in analyzing the impact of each parameter on root locus. Using the root locus theory, the proper parameters are selected so that the resonance of LCL-filter is attenuated, the current ripple is suppressed and the sinusoidal input is well tracked, which means the output grid current is of high quality. A 10kW prototype is built and the result of the experiments verified the effectiveness of the proposed PR regulator design.

  • Faete J. T. Filho; Leon M. Tolbert; Burak Ozpineci
    Proceedings of The 7th International Power Electronics and Motion Control Conference
    2012

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    The work developed here proposes a methodology for calculating switching angles for varying DC sources in a multilevel cascaded H-bridges converter. In this approach the required fundamental is achieved, the lower harmonics are minimized, and the system can be implemented in real time with low memory requirements. Genetic algorithm (GA) is the stochastic search method to find the solution for the set of equations where the input voltages are the known variables and the switching angles are the unknown variables. With the dataset generated by GA, an artificial neural network (ANN) is trained to store the solutions without excessive memory storage requirements. This trained ANN then senses the voltage of each cell and produces the switching angles in order to regulate the fundamental at 120 V and eliminate or minimize the low order harmonics while operating in real time.

  • Jing Xue; Fred Wang; Xuning Zhang; Dushan Boroyevich; Paolo Mattavelli
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper focuses on the conducted electromagnetic interference (EMI) noise suppression solution at the ac output terminal of PWM motor drives. Dc-fed voltage source inverter (VSI) is selected as one typical topology. A practical output EMI filter design procedure is proposed. To ensure design accuracy, the procedure includes the system impedance characterization with offline measurement. Meanwhile, different damping methods are studied to avoid oscillation introduced by EMI filters. Prototype experimental results have verified the feasibility of the design procedure for both common-mode (CM) and differential-mode (DM) EMI noise suppression.

  • Bin Li; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    In this paper, the analysis of Space Vector Modulation (SVM) of a three phase/wire/level Vienna rectifier is conducted, according to which the implementation of the equivalent carrier-based PWM is deduced theoretically within each separated sector in the diagram of vectors. Compared with conventional three-level converters, the SVM of Vienna rectifier is different because of its operation principle, which is discussed in detail in this paper. In addition, the ability of the Vienna rectifier to work under unbalanced power grid is demonstrated as well. An adaptive and robust controller to balance the output voltage under the unbalanced load for different modulation indices is further developed. An experimental prototype of 2.5 kW Vienna rectifier was built to verify the validity of the theoretical analysis.

  • Lijun Hang; Leon M. Tolbert; Gang Yan; Jifeng Chen
    Proceedings of The 7th International Power Electronics and Motion Control Conference
    2012

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    This paper presents a front end grid connected power converter which is based on a non-regenerative three-level bidirectional-switch VIENNA rectifier. The whole system is controlled by a digital method. The controller is implemented in carrier based PWM strategy. The control method and the simulation are presented. An experimental prototype is built to verify the theoretical analysis and the simulation. The experimental results are given. The switching frequency of the converter is 100 kHz; the efficiency reaches 96.5% at the rated condition. During the wide input range, the THDi value is controlled to be less than 5%.

  • Ming Zhang; Bin Li; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents one non-regenerative front-end three-phase AC/DC power factor correction (PFC) rectifier, which is based on VIENNA topology. At first, the space vector pulse width modulation method (SVPWM) for this converter is theoretical analyzed and the control method based in d-q frame is given. Then the DC-link voltage utilization ratio is studied based on the SVPWM strategy. The unbalance ability by applying SVPWM method is theoretical analyzed. The theoretical analysis is well verified by the experimental results.

  • Yutian Cui; Madhu Chinthavali; Leon M. Tolbert
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper provides a behavioral model in Pspice for a silicon carbide (SiC) power MOSFET rated at 1200 V / 30 A for a wide temperature range. The Pspice model was built using device parameters extracted through experiment. The static and dynamic behavior of the SiC power MOSFET is simulated and compared to the measured data to show the accuracy of the Pspice model. The temperature dependent behavior was simulated and analyzed. Also, the effect of the parasitics of the circuit on switching behavior was simulated and discussed.

  • Lakshmi Gopi Reddy; Leon M. Tolbert; Burak Ozpineci; Yan Xu; D. Tom Rizy
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    With smart grid integration, there is a need to characterize reliability of a power system by including reliability of power semiconductors in grid related applications. In this paper, the reliability of IGBTs in a STATCOM application is presented for two different applications, power factor correction and harmonic elimination. The STATCOM model is developed in EMTP, and analytical equations for average conduction losses in an IGBT and a diode are derived and compared with experimental data. A commonly used reliability model is used to predict reliability of IGBT.

  • Lakshmi GopiReddy; Leon M. Tolbert; Burak Ozpineci
    IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society
    2012

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    Rainflow algorithms are one of the best counting methods used in fatigue and failure analysis [17]. There have been many approaches to the rainflow algorithm, some proposing modifications. Graphical Rainflow Method (GRM) was proposed recently with a claim of faster execution times [10]. However, the steps of the graphical method of rainflow algorithm, when implemented, do not generate the same output as the four-point or ASTM standard algorithm. A modified graphical method is presented and discussed in this paper to overcome the shortcomings of graphical rainflow algorithm. A fast rainflow algorithm based on four-point algorithm but considering point comparison than range comparison is also presented. A comparison between the performances of the common rainflow algorithms [6-10], including the proposed methods, in terms of execution time, memory used, and efficiency, complexity, and load sequences is presented. Finally, the rainflow algorithm is applied to temperature data of an IGBT in assessing the lifetime of a STATCOM operating for power factor correction of the load. From 5-minute data load profiles available, the lifetime is estimated to be at 3.4 years.

  • Dong Jiang; Fei Wang
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    Compared with the widely used constant switching frequency PWM method, variable switching frequency PWM can benefit more because of the extra freedom. Based on the analytical expression of current ripple of three-phase converter discussed in “Study of Analytical Current Ripple of Three-Phase PWM Converter” in APEC 2012, variable switching frequency PWM (VSFPWM) methods are proposed to satisfy different ripple requirements. Two methods are discussed in this paper. The first method (VSFPWM1) is designed to arrange the current ripple peak value within a certain value, and can reduce the equivalent switching frequency and EMI noise; the second method (VSFPWM2) is designed to keep ripple current RMS value constant and reduce the EMI noise. Simulation and experimental results show that variable switching frequency control could improve the performance.

  • Zhuxian Xu; Dong Jiang; Ming Li; Puqi Ning; Fred Wang; Zhenxian Liang
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    A Si IGBT phase-leg module is designed and fabricated for operating at 200 °C in hybrid electrical vehicle (HEV) applications. First, a phase-leg package design is given including die selection, material selection, and layout design. Then the static and switching characterization of the fabricated module is conducted at various temperatures. The losses for a kW phase-leg in three-phase motor drive are calculated based on the characterization. Thermal performance of the proposed package and cooling is then evaluated with both (finite element analysis) FEA simulation and experiments. The simulation and experimental results agree well, which show that the proposed packaging design and cooling approach can maintain the Si IGBT junction temperature below 200 °C with 105 °C coolant. Finally, a buck converter composed of the phase-leg module is operated successfully with the device junction temperature heated up to 200 °C, which demonstrates the high temperature operation ability of the designed package module.

  • Jing Xue; Fred Wang
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    Single and three-phase common-mode (CM) inductors play a vital role in conductive CM noise suppression for motor drive system. In this paper, the CM conductive noise propagation path in the dc-fed motor drive system is analyzed. One measurement-based characterization method is discussed for determining the noise propagation path. Meanwhile, modeling methods for the impedance of both single and three-phase CM inductors are reviewed and verified experimentally. With the CM inductor impedance model and CM propagation path characterization, the suppression of both DC and AC CM noise is accurately predicted to be within a 5 dBμA design margin from 10 kHz to 30 MHz. Based on the modeling and verification, this paper also proposes a practical method for high density CM inductors design using MATLAB Optimization Toolbox®.

  • Dong Jiang; Fei Wang
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    The effects of the current ripple of three-phase PWM converters are very important for the design and control of this kind of converter, which is the most popular topology for the energy conversion system. Based on the previous study of single-phase converter's current ripple, analytical current ripple of three-phase converter is studied in this paper with the Thevenin equivalent circuit of 8 different voltage vectors. With the analytical expression, current ripple peak value and RMS value could be predicted and fit the simulation results and experimental results well. This analysis provides the basis for variable switching frequency PWM method discussed in the paper of “Variable Switching Frequency PWM for Three-phase Converter for Loss and EMI Improvement” in APEC 2012.

  • Ben Guo; Fred Wang; Rolando Burgos
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    The three-phase buck-type rectifier has advantages as front-end converter for high efficiency power supplies in telecommunication and data centers. In this paper, the different commutation types of a three-phase buck rectifier with a freewheeling diode are analyzed through experiments using different semiconductor devices. Further, the switching loss of the converter is modeled and calculated for four space vector modulation schemes. It is shown that when the switches include minority carrier devices, such as Si PiN diode, IGBT and Reverse Blocking IGBT (RB-IGBT), more switching loss will occur in the commutation between two switches than between a switch and the freewheeling diode. This difference can be reduced if majority carrier devices, such as SiC Schottky diodes, are used in series with the switches. The modulator can be arranged to eliminate the specific transition which has the most switching loss. According to the analysis, each modulation scheme has its own field for high efficiency application. The advantageous modulation scheme is given for different device combinations in this paper.

  • Zhuxian Xu; Fred Wang
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper investigates the short-circuit capability and the failure mechanisms of Si trench gate field-stop IGBT under high temperature operation conditions through experiments. First, the test circuits are proposed for IGBT short circuit capability evaluation in different types of short circuit conditions. A hardware setup is built accordingly, and used to evaluate experimentally the IGBT short circuit failures caused by thermal destruction, thermal runaway and latch-up at both 25°C and 200°C. The critical short circuit time is given for high temperature operation under different short circuit conditions. The experimental results show that although the critical short circuit time is reduced at 200°C operation, it is still adequate for the protection circuit to shut down the devices safely. The Si trench gate field-stop IGBT exhibits good short circuit ruggedness at 200°C operation.

  • Ming Zhang; Bin Li; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents one front-end three-phase AC/DC power factor correction (PFC) rectifier, which is based on three-level bidirectional-switch VIENNA topology. On one hand, the rectifier, which is designed to operate in continuous-conduction mode (CCM) at full power, operates at reduced load, discontinuous-conduction mode (DCM) occurs in a zone which is close to the crossover of the line voltage. On the other hand, with the reduced input inductance, the DCM mode occurs even when the rectifiers operate at full power. In this paper, the digitized feedfoward compensation method is proposed for the rectifier to reduce the impact of the switch between DCM and CCM. The theoretical analysis of the proposed method is deduced, furthermore, the control design strategy is given. The experimental results are obtained by using a digitally controlled VIENNA rectifier, which validated the proposed compensation method.

  • Xuning Zhang; Dushan Boroyevich; Paolo Mattavelli; Fred Wang
    Proceedings of The 7th International Power Electronics and Motion Control Conference
    2012

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    This work proposes a frequency-domain EMI prediction model for common mode (CM) and differential mode (DM) noises for the purpose of designing EMI filters before the system construction. The parameters in the model can be extracted from system detail switching model or measured from the real system. The Double Fourier Integral Transformation (DFIT) method is used to calculate the noise sources in the model. When the system topology modulation method and modulation index is fixed, the system EMI noise can be predicted from the calculation of the equivalent model. Verifications are carried out through simulation and experiment system by comparing the calculated EMI spectrums and simulated and measured EMI spectrums. Based on the proposed model, this paper also proves that the resonances in EMI noise propagation path will have a significant impact on the EMI filter design for DC fed motor drive system with long cables.

  • Yan Xu; Huijuan Li; Leon M. Tolbert
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    A hybrid microgrid control architecture is proposed, which combines centralized control and distributed control to achieve a fully-functional microgrid control strategy. A complete model of the microgrid has been built. Coordination and power sharing strategies have been developed for the inverters and microgrid controller, and the microgrid can be operated in on-grid mode and islanding mode, and is able to transition between the two modes smoothly with minimum load disruption and shedding during transitions. A multi-input proportional-integral control is proposed to eliminate the transients caused by the inverter control mode transition. In islanding mode, secondary frequency and voltage control strategies are proposed besides droop control methods. Assisted with the microgrid controller and the communication between the microgrid controller and the individual source and load, the microgrid can achieve optimized power flow and energy utilization, and the system efficiency can also be improved.

  • Sara Ahmed; R. Burgos; Chris Roy; D. Boroyevich; Paolo Mattavelli; F. Wang
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents complete Verification, Validation, and Uncertainty Quantification (VV&UQ) procedures that are applied to a two-level boost rectifier. The goal of this VV&UQ study is the improvement of the modeling procedure for power electronics systems, and the full assessment of the boost rectifier model (as an example) predictive capabilities. Modeling and simulation of large systems is always the preferred solution to avoid repetitive hardware, and to minimize the cost. However, developing the accurate models and making sure it matches the hardware is always a challenge. This paper provides a set of procedures that if followed one can claim the model is validated.

  • Ming Zhang; Bin Li; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents one front-end three-phase AC/DC power factor correction (PFC) rectifier, which is based on three-level bidirectional-switch VIENNA topology. On one hand, the rectifier, which is designed to operate in continuous-conduction mode (CCM) at full power, operates at reduced load, discontinuous-conduction mode (DCM) occurs in a zone which is close to the crossover of the line voltage. On the other hand, with the reduced input inductance, the DCM mode occurs even when the rectifiers operate at full power. In this paper, the digitized feedfoward compensation method is proposed for the rectifier to reduce the impact of the switch between DCM and CCM. The theoretical analysis of the proposed method is deduced, furthermore, the control design strategy is given. The experimental results are obtained by using a digitally controlled VIENNA rectifier, which validated the proposed compensation method.

  • Xuning Zhang; Dushan Boroyevich; Paolo Mattavelli; Fred Wang
    Proceedings of The 7th International Power Electronics and Motion Control Conference
    2012

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    This work proposes a frequency-domain EMI prediction model for common mode (CM) and differential mode (DM) noises for the purpose of designing EMI filters before the system construction. The parameters in the model can be extracted from system detail switching model or measured from the real system. The Double Fourier Integral Transformation (DFIT) method is used to calculate the noise sources in the model. When the system topology modulation method and modulation index is fixed, the system EMI noise can be predicted from the calculation of the equivalent model. Verifications are carried out through simulation and experiment system by comparing the calculated EMI spectrums and simulated and measured EMI spectrums. Based on the proposed model, this paper also proves that the resonances in EMI noise propagation path will have a significant impact on the EMI filter design for DC fed motor drive system with long cables.

  • Yan Xu; Huijuan Li; Leon M. Tolbert
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    A hybrid microgrid control architecture is proposed, which combines centralized control and distributed control to achieve a fully-functional microgrid control strategy. A complete model of the microgrid has been built. Coordination and power sharing strategies have been developed for the inverters and microgrid controller, and the microgrid can be operated in on-grid mode and islanding mode, and is able to transition between the two modes smoothly with minimum load disruption and shedding during transitions. A multi-input proportional-integral control is proposed to eliminate the transients caused by the inverter control mode transition. In islanding mode, secondary frequency and voltage control strategies are proposed besides droop control methods. Assisted with the microgrid controller and the communication between the microgrid controller and the individual source and load, the microgrid can achieve optimized power flow and energy utilization, and the system efficiency can also be improved.

  • Sara Ahmed; R. Burgos; Chris Roy; D. Boroyevich; Paolo Mattavelli; F. Wang
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents complete Verification, Validation, and Uncertainty Quantification (VV&UQ) procedures that are applied to a two-level boost rectifier. The goal of this VV&UQ study is the improvement of the modeling procedure for power electronics systems, and the full assessment of the boost rectifier model (as an example) predictive capabilities. Modeling and simulation of large systems is always the preferred solution to avoid repetitive hardware, and to minimize the cost. However, developing the accurate models and making sure it matches the hardware is always a challenge. This paper provides a set of procedures that if followed one can claim the model is validated.

  • R. L. Greenwell; B. M. McCue; L. Zuo; M. A. Huque; L. M. Tolbert; B. J. Blalock; S. K. Islam
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

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    The growing demand for hybrid electric vehicles (HEVs) has increased the need for high-temperature electronics that can operate at the extreme temperatures that exist under the hood. This paper presents a high-voltage, high-temperature SOI-based gate driver for SiC FET switches. The gate driver is designed and implemented on a 0.8-micron BCD on SOI process. This gate driver chip is intended to drive SiC power FETs for DC-DC converters and traction drives in HEVs. To this end, the gate driver IC has been successfully tested up to 200°C. Successful operation of the circuit at this temperature with minimal or no heat sink, and without liquid cooling, will help to achieve higher power-to-volume as well as power-to-weight ratios for the power electronics modules in HEVs.

  • Fan Xu; Dong Jiang; Jing Wang; Fred Wang; Leon M. Tolbert; Timothy J. Han; Sung Joon Kim
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This paper presents a SiC JFET-based, 200°C, 50 kW three-phase inverter module and evaluates its electrical performance. With 1200 V, 100 A rating of the module, each switching element is composed of four paralleled SiC JFETs with two anti-parallel SiC Shottky Barrier Diodes (SBDs). The substrate layout inside the module is designed to reduce package parasitics. Then, experimental static characteristics of the module are obtained over a wide range of temperature, and low on-state resistance is shown up to 200°C. The dynamic performance of this module is evaluated by double pulse test up to 150°C, under 650 V dc bus voltage and 60 A drain current, with different turn-on and turn-off gate resistances. The current unbalance phenomenon and phase-leg shoot-through problem are analyzed too. The results by simulation and experiments show that the causes of shoot-through are JFET inside parameters, package parasitics, and high temperature. The switching losses of this module at different temperatures are shown at the end.

  • Fan Xu; Dong Jiang; Jing Wang; Fred Wang; Leon M. Tolbert; Timothy Junghee Han; Jim Nagashima; Sung Joon Kim
    8th International Conference on Power Electronics - ECCE Asia
    2011

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    Research on silicon carbide (SiC) power electronics has shown their advantages in high temperature and high efficiency applications. This paper presents a SiC JFET based, 200°C, 50 kW three-phase inverter module and evaluates its electrical performance. With 1200 V, 100 A rating of the module, each switching element is composed of four paralleled SiC JFETs (1200 V/25 A each) and two anti parallel SiC Shottky Barrier Diodes (SBDs). The substrate layout inside the module is designed to reduce package parasitics. Then, experimental static characteristics of the module are obtained over a wide range of temperature, and low on-state resistance is shown up to 200°C. A gate driver, with different turn-on, turn-off gate resistances and RCD network, is designed to optimize the switching performances. The module is verified to have low power loss, fast switching characteristics at 650 V dc bus voltage, 60 A drain current, in both simulation and experiments. Finally, switching time and losses, obtained from simulation and experiment, are compared.

  • Dong Jiang; Jing Xue; Fred Wang; Min H. Kao
    2011 IEEE Electric Ship Technologies Symposium
    2011

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    This paper proposes a new Modular Multilevel Cascade Converter (MMCC) topology for future high density medium-voltage motor drive. The proposed topology utilizes the active ripple energy storage method for rectifier capacitance design. Simulation has been done in this paper and proves that the proposed topology works well, and the active energy storage method can significantly reduce the DC-link capacitor. Power density improvement is quantified for an example system to show the benefits of the proposed topology.

  • Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert; Fred Wang
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

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    Vehicle to grid (V2G) power transfer has been under research for more than a decade because of the large energy reserve of an electric vehicle battery and the potential of thousands of these connected to the grid. In this study a complete analysis of the front end inverter of a non-isolated bidirectional EV/PHEV charger capable of V2G reactive power compensation is presented.

  • Dong Jiang; Fan Xu; Fei Wang; Leon M. Tolbert; Timothy J. Han; Sung Joon Kim
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This paper studies the performance of a newly designed 1200V/60A three-phase SiC power module based on parallel SiC JFETs and diodes. The conduction and the switching performance are tested from room temperature to 150°C. The switching speed of the module increases when temperature rises. In the switching performance test, the gate driver speed could bring false peak in turn-off waveform. The experimental results show that the false peak is cause by Differential-mode (DM) noises but not Common-mode (CM) noises. Finally the losses and efficiency of this power module are evaluated.

  • Shengnan Li; Leon M. Tolbert; Fred Wang; Fang Zheng Peng
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

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    This paper proposes a novel packaging method for power electronics modules based on the concepts of P-cell and N-cell. It can reduce the stray inductance in the current commutation path in a phase-leg module and hence improve the switching behavior. Two IGBT phase-leg modules, specifically a P-cell and N-cell based module and a conventional module are designed. Using Ansoft Q3D Extractor, electromagnetic simulation is carried out to extract the stray inductance from the two modules. Switching behavior with different package parasitics is studied based on Saber simulation. Two prototype phase-leg modules based on two different designs are fabricated. The parasitics are measured using a precision impedance analyzer. The measurement results agree with the simulation very well.

  • Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert
    8th International Conference on Power Electronics - ECCE Asia
    2011

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    More battery powered electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) will be introduced to the market in 2011 and beyond. PHEVs/EVs potentially have the capability to fulfill the energy storage needs of the electric grid by supplying ancillary services such as reactive power compensation, voltage regulation, and peak shaving since they carry an on-board battery charger. However, to allow bidirectional power transfer, the PHEV battery charger should be designed to manage such reactive power capability. This study shows how bidirectional four quadrant operation affects the design stage of a conventional unidirectional charger and the operation of the battery pack. Mainly, the subjects that are discussed are the following: required topology updates, dc link capacitor (voltage and current), ac inductor (current), rectifier (power loss), and battery pack (voltage and current).

  • Fan Xu; Leon M. Tolbert; Yan Xu
    Proceedings of the 2011 14th European Conference on Power Electronics and Applications
    2011

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    Due to the widespread use of non-sinusoidal, non-periodic loads and the existence of distorted voltages, many definitions of non-active power for non-sinusoidal and non-periodic waveforms have been formulated. This paper investigates the major similarities and discrepancies of three non-active power theories which are widely used in control algorithms for shunt compensation systems. The three approaches are FBD, PQ and generalized non-active power theories. The evaluation and comparison of these three theories as the non-active power compensation strategies for single-phase system, three-phase system with unbalanced load, sub-harmonic load, and distorted system voltage will be included in this paper. The conclusions are based on both the simulation and the experimental results of different compensation objectives.

  • Bailu Xiao; Faete Filho; Leon M. Tolbert
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This paper presents a single-phase cascaded H-bridge multilevel inverter for a grid-connected photovoltaic (PV) system with nonactive power compensation. A generalized nonactive power theory is applied to generate the nonactive current reference. Within the inverter's capability, nonactive power required by the local load is provided to improve the grid power quality. To minimize harmonics and achieve zero error tracking, a hybrid controller composed of a proportional controller and a repetitive controller is applied to current control. A single-phase 11-level cascaded multilevel inverter is considered in both simulation and experimental tests. Each H-bridge is connected to a 195 W solar panel. Simulation and experimental results are presented to validate the proposed ideas.

  • Dong Jiang; Fei Wang; Jing Xue
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This paper analyzes the common-mode (CM) current and CM filter size for the variable-speed motor drives with different PWM methods. The principle of CM current and CM choke saturation are studied. Some previous proposed CM deduction PWM methods such as active zero states PWM (AZSPWM) and near state PMW (NSPWM) can reduce the CM voltage peak value, However, they may not obviously reduce the CM current and CM choke volt-seconds, due to increased 3rd order switching harmonics and CM loop parameters. PWM methods should work properly with the CM loop impedance. Also, new CM volt-seconds relationship with PWM methods is proposed with consideration of CM modulation signals. Simulation and experimental results are presented to verify the analysis.

  • Faete Filho; Leon Tolbert; Burak Ozpineci
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This work presents an approach to determine the input voltage value of each cell in a cascade H-bridge multilevel inverter using a sensor at the output of the inverter to eliminate all the dc voltage sensors measuring the individual source voltages. The input voltages can be equal or unequal. The MOSFET device datasheet, the ambient temperature, and the modulation strategy are utilized to estimate the switch voltage drop to compensate for the measurement. The output voltage is then processed by a DSP unit that uses the signals that command the switches to estimate the voltage at each cell. Simulation and experimental results are shown for a seven-level cascade multilevel inverter operating under a RLC load.

  • Puqi Ning; Fred Wang; Dong Jiang; Di Zhang; Rixin Lai; Dushan Boroyevich; Khai Ngo; Rolando Burgos; Kamiar Karimi; Vikram Immanuel; Eugene Solodovnik
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This paper presents the development and experimental performance of a 10 kW, all SiC, 250°C junction temperature high-power-density three-phase ac-dc-ac converter. The electromagnetic interference filter, thermal system, high temperature package, and gate drive design are discussed in detail. Finally, tests confirming the feasibility and validating the theoretical basis of the prototype converter system are described.

  • Zhuxian Xu; Di Zhang; Fred Wang; Dushan Boroyevich
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

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    This paper presents a unified control method for the combined permanent magnet generator (PMG) and three-phase boost rectifier that can be used in autonomous power systems such as more-electric aircraft requiring high power density. With the unified control, the paper shows the system can function well without additional boost inductors. The design procedure for the control is presented, including current loops, voltage loop, reactive power loop and rotor position estimator loop. Simulation and experimental results show that both the DC link voltage and reactive power could be controlled effectively. In addition, the paper proposes a method to optimize the overall system efficiency by appropriate reactive power distribution. The power density and efficiency of the PMG and rectifier system is improved with the unified control.

  • Lakshmi GopiReddy; Leon M. Tolbert; Burak Ozpineci; Yan Xu; Tom Rizy
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    Most of the failures in IGBTs are caused by thermal fatigue. Hence, the thermal analysis of IGBTs for each particular application is an important step in determining their lifetime. In this paper, the thermal analysis of a STATCOM is presented for two different applications, power factor correction and harmonic elimination. The STATCOM model is developed in EMTP for the above mentioned functions. The analytical equations for average conduction losses in an IGBT and a diode are derived. The electrothermal model is used to estimate the temperature of the IGBT. A comparative analysis of the thermal stresses on the IGBT with various parameters such as power factor, harmonic frequency, and harmonic amplitude is presented as a basis for future reliability testing of IGBTs in FACTS applications.

  • Zhuxian Xu; Ming Li; Fred Wang; Zhenxian Liang
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    In order to satisfy the high density requirement and harsh thermal conditions in future hybrid vehicles, a systematic study of Si IGBT operating at 200°C is performed to determine its feasibility, issues and application guideline. First, the device forward conduction characteristics, leakage current, and switching performance at various temperatures are evaluated through both analytical and lab evaluation. Based on the device characterization, the loss and thermal study is then performed, which provides the guideline for packaging and cooling design. Finally, the possible failure mechanisms at high temperatures including latching and short circuit fault have been tested to ensure the safe and reliable operation of Si IGBTs.

  • Fang Luo; Xuning Zhang; Dushan Boroyevich; Paolo Mattevelli; Jing Xue; Fred Wang; Nicolas Gazel
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

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    This paper discusses EMI filter design methodology for DC-fed three-phase motor drive system. An analysis of noise source and propagation path impedances of motor drive systems is conducted, a new filter design model for both AC side and DC side filter is proposed based on the impedance mismatching between the EMI filter impedances and noise source/load impedances. Using this model, the in circuit attenuation of EMI filter can be accurately predicted. The interaction between AC and DC filters is also studied in this analysis. By applying some constraint, EMI filters can be designed. This new design procedure is suitable for both DC and AC side EMI filters. Simulation and experimental results give a firm support to the proposed method.

  • Dong Jiang; Fred Wang
    2011 IEEE Electric Ship Technologies Symposium
    2011

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    SiC devices have obvious advantages comparing with conventional Si devices especially in high temperature range. This paper aims at developing a method of characterization SiC JFET conduction and switching performance in high temperature and calculating the loss of SiC JFET converters. Experimental results show that with SiC Schottky diode as anti-paralleling diode the reverse recovery in switching is improved and switching loss is less. Also, the turn-off time will decrease when the temperature rises, showing a better performance in high temperature. With the test results, the loss estimation method is developed. Then losses of two typical three-phase AC-DC-AC converters are calculated. Experimental results show that with Schottky diode as anti-paralleling diode, both conduction losses and switching losses can be reduced especially at high temperature.

  • Puqi Ning; Khai Ngo; Fred Wang
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This study confirms that high stresses between the metallization layer and ceramic lead to significant failures in DBC substrate. The driving forces behind several different failure modes are discussed. Further understanding of these failure mechanisms enables the modules to be engineered for longtime operation and helps to enhance the reliability of system-level operation.

  • Madhu Chinthavali; Puqi Ning; Yutian Cui; Leon M. Tolbert
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

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    This paper presents an analysis of single discrete silicon carbide (SiC) JFET and BJT devices and their parallel operation. The static and dynamic characteristics of the devices were obtained over a wide range of temperature to study the scaling of device parameters. The static parameters like on-resistance, threshold voltage, current gains, transconductance, and leakage currents were extracted to show how these parameters would scale as the devices are paralleled. A detailed analysis of the dynamic current sharing between the paralleled devices during the switching transients and energy losses at different voltages and currents is also presented. The effect of the gate driver on the device transient behavior of the paralleled devices was studied, and it was shown that faster switching speeds of the devices could cause mismatches in current shared during transients.

  • Di Zhang; Puqi Ning; Dushan Boroyevich; Fred Wang; Rolando Burgos; Kamiar Karimi; Vikram Immanuel; Eugene Solodovnik
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    With interleaving, DPWM and SiC semiconductors, the design of a 15 kW high power density three-phase ac-dc rectifier is presented. The development of main passive components, main active components, and the system are reported in detail. With the presented technologies, the specific power density can be pushed to more than 2kW/lb.

  • Di Zhang; Fred Wang; Rolando Burgos; Dushan Boroyevich
    2011 IEEE Energy Conversion Congress and Exposition
    2011

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    This paper presents the use of inter-phase interleaving for three-phase voltage source converter (VSC). With the proposed inter-phase interleaving, the output three-phase voltages can be balanced even though low non-triple carrier ratios, such as 5 or 7, are used for the VSC. First, the impacts of sideband harmonic components on the fundamental component are analyzed in detail. The validity of analysis is demonstrated by the explanation of beat phenomenon related to non-integer carrier ratio. Based on the analysis, the unbalance issue of output voltages due to non-triple carrier ratio is explained. After that, the inter-phase interleaving control method is proposed to correct such unbalance. With this control method, the output voltages are balanced even with non-triple carrier ratios. The corresponding penalty of potential higher current THD is also discussed in detail. The analysis and control method can be applied to both two-level and multi-level converters. Experimental results verify the analysis and the feasibility of the proposed control method.

  • Puqi Ning; Fred Wang; Khai D. T. Ngo
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

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    Taking full advantage of SiC devices, a team from Oak Ridge National Laboratory, the University of Tennessee and Virginia Polytechnic Institute and State University have designed, developed, and tested a phase-leg power module based on a high temperature wirebond package. Details of the layout, gate drive, and cooling system designs are described. Continuous power tests confirmed that our design process produced a high density power module that operated successfully at high junction temperatures.

  • I. Cvetkovic; D. Boroyevich; D. Dong; P. Mattavelli; R. Burgos; M. Jaksic; G. Francis; Z. Shen; S. Ahmed; F. Wang
    8th International Conference on Power Electronics - ECCE Asia
    2011

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    Hybrid electronic power distribution systems have majority of loads interfaced to energy sources through power electronics converters. Furthermore, all alternative, sustainable, and distributed energy sources can only be connected to electric grid through power electronics equipment. However, one of the main challenges in designing and developing of these hybrid ac/dc systems has been modeling and analysis of dynamic interactions between converters at the synchronous and higher frequencies. In order to address these problems, this paper employs terminal-behavioral modeling of power converters as possible methodology for analysis, system-level design, stability, and subsystem inter actions study. Some simulation and experimental results are both shown for model verification and validation purposes.

  • Puqi Ning; Khai Ngo; Fred Wang
    2011 IEEE Energy Conversion Congress and Exposition
    2011

    arrow_drop_down

    This study confirms that high stresses between the metallization layer and ceramic lead to significant failures in DBC substrate. The driving forces behind several different failure modes are discussed. Further understanding of these failure mechanisms enables the modules to be engineered for longtime operation and helps to enhance the reliability of system-level operation.

  • Madhu Chinthavali; Puqi Ning; Yutian Cui; Leon M. Tolbert
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

    arrow_drop_down

    This paper presents an analysis of single discrete silicon carbide (SiC) JFET and BJT devices and their parallel operation. The static and dynamic characteristics of the devices were obtained over a wide range of temperature to study the scaling of device parameters. The static parameters like on-resistance, threshold voltage, current gains, transconductance, and leakage currents were extracted to show how these parameters would scale as the devices are paralleled. A detailed analysis of the dynamic current sharing between the paralleled devices during the switching transients and energy losses at different voltages and currents is also presented. The effect of the gate driver on the device transient behavior of the paralleled devices was studied, and it was shown that faster switching speeds of the devices could cause mismatches in current shared during transients.

  • Di Zhang; Puqi Ning; Dushan Boroyevich; Fred Wang; Rolando Burgos; Kamiar Karimi; Vikram Immanuel; Eugene Solodovnik
    2011 IEEE Energy Conversion Congress and Exposition
    2011

    arrow_drop_down

    With interleaving, DPWM and SiC semiconductors, the design of a 15 kW high power density three-phase ac-dc rectifier is presented. The development of main passive components, main active components, and the system are reported in detail. With the presented technologies, the specific power density can be pushed to more than 2kW/lb.

  • Di Zhang; Fred Wang; Rolando Burgos; Dushan Boroyevich
    2011 IEEE Energy Conversion Congress and Exposition
    2011

    arrow_drop_down

    This paper presents the use of inter-phase interleaving for three-phase voltage source converter (VSC). With the proposed inter-phase interleaving, the output three-phase voltages can be balanced even though low non-triple carrier ratios, such as 5 or 7, are used for the VSC. First, the impacts of sideband harmonic components on the fundamental component are analyzed in detail. The validity of analysis is demonstrated by the explanation of beat phenomenon related to non-integer carrier ratio. Based on the analysis, the unbalance issue of output voltages due to non-triple carrier ratio is explained. After that, the inter-phase interleaving control method is proposed to correct such unbalance. With this control method, the output voltages are balanced even with non-triple carrier ratios. The corresponding penalty of potential higher current THD is also discussed in detail. The analysis and control method can be applied to both two-level and multi-level converters. Experimental results verify the analysis and the feasibility of the proposed control method.

  • Puqi Ning; Fred Wang; Khai D. T. Ngo
    2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2011

    arrow_drop_down

    Taking full advantage of SiC devices, a team from Oak Ridge National Laboratory, the University of Tennessee and Virginia Polytechnic Institute and State University have designed, developed, and tested a phase-leg power module based on a high temperature wirebond package. Details of the layout, gate drive, and cooling system designs are described. Continuous power tests confirmed that our design process produced a high density power module that operated successfully at high junction temperatures.

  • I. Cvetkovic; D. Boroyevich; D. Dong; P. Mattavelli; R. Burgos; M. Jaksic; G. Francis; Z. Shen; S. Ahmed; F. Wang
    8th International Conference on Power Electronics - ECCE Asia
    2011

    arrow_drop_down

    Hybrid electronic power distribution systems have majority of loads interfaced to energy sources through power electronics converters. Furthermore, all alternative, sustainable, and distributed energy sources can only be connected to electric grid through power electronics equipment. However, one of the main challenges in designing and developing of these hybrid ac/dc systems has been modeling and analysis of dynamic interactions between converters at the synchronous and higher frequencies. In order to address these problems, this paper employs terminal-behavioral modeling of power converters as possible methodology for analysis, system-level design, stability, and subsystem inter actions study. Some simulation and experimental results are both shown for model verification and validation purposes.

  • Faete Filho; Leon M. Tolbert; Yue Cao; Burak Ozpineci
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    This work approximates the selective harmonic elimination problem using Artificial Neural Networks (ANN) to generate the switching angles in an 11-level full bridge cascade inverter powered by five varying DC input sources. Five 195 W solar panels were used as the DC source for each full bridge. The angles were chosen such that the fundamental was kept constant and the low order harmonics were minimized or eliminated. A non-deterministic method is used to solve the system for the angles and to obtain the data set for the ANN training. The method also provides a set of acceptable solutions in the space where solutions do not exist by analytical methods. The trained ANN shows to be a suitable tool that brings a small generalization effect on the angles' precision.

  • Shengnan Li; Leon M. Tolbert; Fred Wang; Fang Zheng Peng
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    This paper introduces the concepts of two basic switching cells, P-cell and N-cell, along with their implications in power electronic circuits. The basic switching cells exist in almost every power electronic circuit. To take advantage of these structures, this paper proposes a novel packaging method for power electronics modules. The proposed packaging method uses the basic switching cells as the unit in a module, instead of traditional anti-parallel connection of active switch and diode. This rearrangement can reduce the stray inductance in the current commutation pass; therefore, the performance and reliability of the power device module and the power electronic system can be improved. A conventional phase leg module and a proposed module are modeled. Electromagnetic simulation is carried out to extract the stray inductance from the two modules. Switching behavior under different package parasitics is studied based on Saber simulation.

  • Faete Filho; Yue Cao; Leon M. Tolbert
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    This paper presents a single-phase 11-level (5 H-bridges) cascade multilevel DC-AC grid-tied inverter. Each inverter bridge is connected to a 200 W solar panel. OPAL-RT lab was used as the hardware in the loop (HIL) real-time control system platform where a Maximum Power Point Tracking (MPPT) algorithm was implemented based on the inverter output power to assure optimal operation of the inverter when connected to the power grid as well as a Phase Locked Loop (PLL) for phase and frequency match. A novel SPWM scheme is proposed in this paper to be used with the solar panels that can account for voltage profile fluctuations among the panels during the day. Simulation and experimental results are shown for voltage and current during synchronization mode and power transferring mode to validate the methodology for grid connection of renewable resources.

  • Jing Xue; Fred Wang; Dushan Boroyevich; Zhiyu Shen
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    In this paper, the design and comparison of single and three-phase transformers in Dual Active Bridge (DAB) Converter are carried out. DAB converter topology and operating schemes are introduced. And core-type transformers design optimization methods are proposed for both single and three-phase DAB converters (DAB1 and DAB3). Based on DAB topology assumptions, single and three-phase transformers are compared in volume with various design constraints. And analysis shows that comparison result varies at different cases of flux and thermal limits. Scaled-down DAB1 and DAB3 systems have been built and temperature rises of the transformers have been tested for verification.

  • Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    Electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) are becoming a part of the electric grid day by day. Chargers for these vehicles have the ability to make this interaction better for the consumer and for the grid. Vehicle to grid (V2G) power transfer has been under research for more than a decade because of the large energy reserve of an electric vehicle battery and the potential of thousands of these connected to the grid. Rather than discharging the vehicle batteries, reactive power compensation in particular is beneficial for both consumers and for the utility. However, certain adverse effects or requirements of reactive power transfer should be defined before a design stage. To understand the dynamics of this operation, this study investigates the effect of reactive power transfer on the charger system components, especially on the dc-link capacitor and the battery.

  • Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    Plug-in hybrid electric vehicles (PHEVs) potentially have the capability to fulfill the energy storage needs of the electric grid by supplying ancillary services such as reactive power compensation, voltage regulation, and peak shaving. However, in order to allow bidirectional power transfer, the PHEV battery charger should be designed to manage such capability. While many different battery chargers have been available since the inception of the first electric vehicles (EVs), on-board, conductive chargers with bidirectional power transfer capability have recently drawn attention due to their inherent advantages in charging accessibility, ease of use, and efficiency. In this paper, a reactive power compensation case study using just the inverter dc-link capacitor is evaluated when a PHEV battery is under charging operation. Finally, the impact of providing these services on the batteries is also explained.

  • Olumide Aluko; Travis M. Smith; Leon M. Tolbert
    IEEE PES General Meeting
    2010

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    Wind energy is the fastest growing source of renewable energy in the power industry and it will continue to grow worldwide as many countries are developing plans for its future development. For power system operators, this increasing contribution of wind energy to the grid poses new challenges that need to be addressed in order to ensure the reliability and security of the electric power grid. One of the main concerns by system operators is the ability of wind turbines to ride through faults without disconnecting from the grid according to FERC-661. This paper analyzes a three phase fault event on a wind plant modeled in EMTP and investigates the behavior of the doubly fed induction generator (DFIG) during grid fault conditions.

  • Dong Jiang; Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    PM motor's speed sensorless control does not work well in the low speed. To avoid this issue, open-loop control is usually used to start the motor, and then it is switched to sensorless close-loop control. The transition between the two control modes can cause a transient during the starting process. This transient can be undesirable especially for motor drives with front-end rectifier and small energy storage components, such as small dc capacitors in the case of voltage source inverter (VSI) drive. This paper studies the principle of dc bus transient in sensorless control start-up process and proposes a method to reduce the oscillation: before closing the sensorless loop, the reference current is adjusted to continuously track the real motor rotor position to the estimated rotor position, and the speed regulator is pre-calculated to generate the q-axis reference current before closing the speed loop. An experiment is conducted using a Vienna-type rectifier and a VSI based motor drive with back-EMF observer based sensorless control on a PM motor. Experimental results show that with the proposed method, the DC bus voltage transient is obviously reduced.

  • Ruxi Wang; Fred Wang; Dushan Boroyevich; Puqi Ning
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    It is well known that there exist second-order harmonic current and corresponding ripple voltage on dc bus for single phase PWM rectifiers. The low frequency harmonic current is normally filtered using a bulk capacitor in the bus which results in low power density. This paper proposed an active ripple energy storage method that can effectively reduce the energy storage capacitance. The feed-forward control method and design considerations are provided. Simulation and 15 kW experimental results are provided for verification purposes.

  • Hui Zhang; Leon M. Tolbert; Jung Hee Han; Madhu S. Chinthavali; Fred Barlow
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    Power electronics play an important role in electricity utilization from generation to end customers. Thus, high-efficiency power electronics help to save energy and conserve energy resources. Research on silicon carbide (SiC) power electronics has shown their better efficiency compared to Si power electronics due to the significant reduction in both conduction and switching losses. Combined with their high-temperature capability, SiC power electronics are more reliable and compact. This paper focuses on the development of such a high efficiency, high temperature inverter based on SiC JFET and diode modules. It involves the work on high temperature packaging (>200°C), inverter design and prototype development, device characterization, and inverter testing. A SiC inverter prototype with a power rating of 18 kW is developed and demonstrated. When tested at moderate load levels compared to the inverter rating, an efficiency of 98.2% is achieved by the initial prototype without optimization, which is higher than most Si inverters.

  • Fang Luo; Andrew Carson Baisden; Dushan Boroyevich; Khai Ngo; Fred Wang; Paolo Mattavelli; Luisa Coppola; Nicolas Gazel; Yong Kang
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    The transmission line busbar filter is a good choice for a high-density EMI containment solution. This paper presents a study of the application of a transmission-line busbar filter in a motor drive system. To examine the potential improvement for busbar filters, low-frequency attenuation and basic modeling of busbar filters in a motor drive are studied. Based on the knowledge gained from the modeling, an improved busbar filter is proposed and fabricated. Experimental results show a significant improvement on busbar filter attenuation.

  • S. Ahmed; D. Boroyevich; F. Wang; R. Burgos
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    This paper presents an enhanced average model of a voltage source inverter (VSI) that can accurately predict some of the low frequency phenomena only seen by the switching models. These phenomena include the dead time, voltage drop across switching devices (switch & diode), different modulation and minimum pulse width. Simulation and experimental results with a 2 kW prototype is used for validation purposes. The paper shows the simulation results of both complete switching and proposed average model depicting a great match between the two. The enhanced average model showed very good matching with the switching model, about 2% difference in time domain and also harmonic spectrum analysis.

  • Fang Luo; Shuo Wang; Fred Wang; Dushan Boroyevich; Nicolas Gazel; Yong Kang
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    Common mode (CM) choke saturation is a practical problem in common-mode filter applications. It's generally believed that the leakage inductance of common-mode chokes makes the core saturated. This paper analyzes two new mechanisms for common-mode choke saturation due to common-mode voltage, and these mechanisms are verified in experiment. Common-mode choke saturation is particularly important for motor drive systems, which have a high common-mode voltage and comparably higher stray grounding capacitance. A model is established to describe the relationship between the common-mode voltage and the volume of the common-mode magnetic components. According to the analysis, LISNs (line impedance stabilization networks) play an important role in the design of CM magnetic components.

  • Ruxi Wang; Puqi Ning; Dushan Boroyevich; Milisav Danilovic; Fred Wang; Rajashekara Kaushik
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    High temperature (HT) converters have become more and more important in industrial applications where the converters will operate in a harsh environment. These environments require the converter to have not only high-temperature semiconductor devices (SiC, GaN) but also high-temperature control electronics. This paper describes a design process for a three-phase PWM rectifier. The SiC JFET planar structure is used for the main semiconductor devices. Other high-temperature components, including the passive components, silicon-on-insulator chips and the auxiliary components are studied and summarized. Finally, a 1.4 kW lab prototype is fabricated and tested for verification.

  • Seong Taek Lee; Leon M. Tolbert
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    This paper shows how to maximize the effect of the slanted air-gap structure of an interior permanent magnet synchronous motor with brushless field excitation (BFE) for application in a hybrid electric vehicle. The BFE structure offers high torque density at low speed and weakened flux at high speed. The unique slanted air-gap is intended to increase the output torque of the machine as well as to maximize the ratio of the back-emf of a machine that is controllable by BFE. This irregularly shaped air-gap makes a flux barrier along the d-axis flux path and decreases the d-axis inductance; as a result, the reluctance torque of the machine is much higher than a uniform air-gap machine, and so is the output torque. Also, the machine achieves a higher ratio of the magnitude of controllable back-emf. The determination of the slanted shape was performed by using magnetic equivalent circuit analysis and finite element analysis (FEA).

  • Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    This paper proposes a new average model and a control approach for the three-phase three-level non-regenerate rectifier (Vienna-type rectifier) with unbalanced dc load. State space analysis is first carried out to achieve the relationship between the voltage unbalance, load conditions and the control duty cycle. With the implementation of an optimum zero-sequence component, a simplified average model for the dc output stage with unbalanced load is obtained. Based on the developed model, a new control approach and the criteria of control parameter selection are presented. The simulation and experiment results validate the proposed control scheme.

  • Di Zhang; Fred Wang; Said El-Barbari; Juan Sabate; Dushan Boroyevich
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    This paper presents an improved asymmetric space vector modulation (ASVM) for two level voltage source converters (VSCs) when the switching frequency is only 9 times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly with very limited penalty. The applications of improved ASVM in a single VSC or two interleaved VSCs systems are shown separately. With optimization, the ac current THD can be reduced to as low as 50% for single VSC and even lower to 20% for interleaved VSCs systems. Such THD reduction has close relationship with modulation index and interleaving angle. In addition, improved ASVM can also reduce the amplitude of circulating current which mainly determined the size of inter-phase inductors. Finally, the weights of total inductors needed to meet the same THD requirement are compared to demonstrate the benefits of improved ASVM when different PWM schemes are used. The analysis results are verified by experiments on a demo system.

  • Surin Khomfoi; Nattapat Praisuwanna; Leon M. Tolbert
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    A hybrid cascaded multilevel inverter application for renewable energy resources including a reconfiguration technique is developed. The objective of this research is to propose an alternative topology of hybrid cascaded multilevel inverter applied to a low voltage dc microgrid in telecommunication buildings. The modified PWM technique is also developed to reduce switching losses. Also, the proposed topology can reduce the number of required power switches compared to a traditional cascaded multilevel inverter. A possible reconfiguration technique after faulty condition is also discussed. PSIM (PowerSim) and Simulink/MATLAB are used to simulate the circuit operation and control signal. A 3-kW prototype is developed. The switching losses of the proposed multilevel inverter are also investigated. By using the modified PWM technique and reconfiguration method, the proposed hybrid inverter can improve system efficiency and reliability. The proposed inverter efficiency is 97% under tested condition. The results show that proposed hybrid inverter topology is a promising method for a low voltage dc microgrid interfacing with renewable energy resources.

  • Dushan Boroyevich; Zheng Chen; Fang Luo; Khai Ngo; Puqi Ning; Ruxi Wang; Di Zhang; Fred Wang; Rolando Burgos; Rixin Lai; Shuo Wang
    2010 6th International Conference on Integrated Power Electronics Systems
    2010

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    Over the past ten years, there has been increased incorporation of electronic power processing into alternative, sustainable, and distributed energy sources, as well as energy storage systems, cars, airplanes, ships, homes, data centers, and the power grid. The goals have been to reduce the size, weight, and maintenance and operational costs of these power systems, while increasing overall energy efficiency, safety, and reliability. This paper summarizes some of the authors' research efforts in the last four years on the improvements in power density and physical integration of power converters, mostly for vehicular applications. Several approaches to integration of active components into high-temperature modules are presented together with examples of the evaluation and modeling of 1.2 kV SiC JFET and MOSFET. Possible improvements in the power density through hybrid passive and active integration of an EMI filter and of an energy storage capacitor in single-phase PWM rectifier are also shown. Examples of converter integration for a 10 kW motor drive with active front-end using SiC devices operating at 250°C, and for paralleling three-phase boost rectifiers with interleaved PWM are presented.

  • Dong Jiang; Rixin Lai; Fred Wang; Fang Luo; Shuo Wang; Dushan Boroyevich
    The 2010 International Power Electronics Conference - ECCE ASIA -
    2010

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    The problem of electromagnetic interference (EMI) plays an important role in the design of power electronic converters, especially for airplane electrical systems. This paper explores techniques to reduce EMI noise in three-phase Vienna-type rectifiers. The design approach is using a high-density EMI filter to satisfy the EMI standard. In particular, the cause of high noise at high frequencies is studied in experiments, and the coupling effect of the final stage capacitor and inductors is investigated. The use of random PWM for EMI noise reduction is also presented. The performance of random PWM in a Vienna-type rectifier is verified by theoretical analysis and experimental results. The approaches discussed in this paper significantly reduce the EMI noise in the Vienna-type rectifier, and therefore the filter size can also be reduced.

  • Fang Luo; Andrew Carson Baisden; Dushan Boroyevich; Khai Ngo; Fred Wang; Paolo Mattavelli; Luisa Coppola; Nicolas Gazel; Yong Kang
    2010 IEEE Energy Conversion Congress and Exposition
    2010

    arrow_drop_down

    The transmission line busbar filter is a good choice for a high-density EMI containment solution. This paper presents a study of the application of a transmission-line busbar filter in a motor drive system. To examine the potential improvement for busbar filters, low-frequency attenuation and basic modeling of busbar filters in a motor drive are studied. Based on the knowledge gained from the modeling, an improved busbar filter is proposed and fabricated. Experimental results show a significant improvement on busbar filter attenuation.

  • S. Ahmed; D. Boroyevich; F. Wang; R. Burgos
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

    arrow_drop_down

    This paper presents an enhanced average model of a voltage source inverter (VSI) that can accurately predict some of the low frequency phenomena only seen by the switching models. These phenomena include the dead time, voltage drop across switching devices (switch & diode), different modulation and minimum pulse width. Simulation and experimental results with a 2 kW prototype is used for validation purposes. The paper shows the simulation results of both complete switching and proposed average model depicting a great match between the two. The enhanced average model showed very good matching with the switching model, about 2% difference in time domain and also harmonic spectrum analysis.

  • Fang Luo; Shuo Wang; Fred Wang; Dushan Boroyevich; Nicolas Gazel; Yong Kang
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

    arrow_drop_down

    Common mode (CM) choke saturation is a practical problem in common-mode filter applications. It's generally believed that the leakage inductance of common-mode chokes makes the core saturated. This paper analyzes two new mechanisms for common-mode choke saturation due to common-mode voltage, and these mechanisms are verified in experiment. Common-mode choke saturation is particularly important for motor drive systems, which have a high common-mode voltage and comparably higher stray grounding capacitance. A model is established to describe the relationship between the common-mode voltage and the volume of the common-mode magnetic components. According to the analysis, LISNs (line impedance stabilization networks) play an important role in the design of CM magnetic components.

  • Ruxi Wang; Puqi Ning; Dushan Boroyevich; Milisav Danilovic; Fred Wang; Rajashekara Kaushik
    2010 IEEE Energy Conversion Congress and Exposition
    2010

    arrow_drop_down

    High temperature (HT) converters have become more and more important in industrial applications where the converters will operate in a harsh environment. These environments require the converter to have not only high-temperature semiconductor devices (SiC, GaN) but also high-temperature control electronics. This paper describes a design process for a three-phase PWM rectifier. The SiC JFET planar structure is used for the main semiconductor devices. Other high-temperature components, including the passive components, silicon-on-insulator chips and the auxiliary components are studied and summarized. Finally, a 1.4 kW lab prototype is fabricated and tested for verification.

  • Seong Taek Lee; Leon M. Tolbert
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    This paper shows how to maximize the effect of the slanted air-gap structure of an interior permanent magnet synchronous motor with brushless field excitation (BFE) for application in a hybrid electric vehicle. The BFE structure offers high torque density at low speed and weakened flux at high speed. The unique slanted air-gap is intended to increase the output torque of the machine as well as to maximize the ratio of the back-emf of a machine that is controllable by BFE. This irregularly shaped air-gap makes a flux barrier along the d-axis flux path and decreases the d-axis inductance; as a result, the reluctance torque of the machine is much higher than a uniform air-gap machine, and so is the output torque. Also, the machine achieves a higher ratio of the magnitude of controllable back-emf. The determination of the slanted shape was performed by using magnetic equivalent circuit analysis and finite element analysis (FEA).

  • Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    This paper proposes a new average model and a control approach for the three-phase three-level non-regenerate rectifier (Vienna-type rectifier) with unbalanced dc load. State space analysis is first carried out to achieve the relationship between the voltage unbalance, load conditions and the control duty cycle. With the implementation of an optimum zero-sequence component, a simplified average model for the dc output stage with unbalanced load is obtained. Based on the developed model, a new control approach and the criteria of control parameter selection are presented. The simulation and experiment results validate the proposed control scheme.

  • Di Zhang; Fred Wang; Said El-Barbari; Juan Sabate; Dushan Boroyevich
    2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2010

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    This paper presents an improved asymmetric space vector modulation (ASVM) for two level voltage source converters (VSCs) when the switching frequency is only 9 times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly with very limited penalty. The applications of improved ASVM in a single VSC or two interleaved VSCs systems are shown separately. With optimization, the ac current THD can be reduced to as low as 50% for single VSC and even lower to 20% for interleaved VSCs systems. Such THD reduction has close relationship with modulation index and interleaving angle. In addition, improved ASVM can also reduce the amplitude of circulating current which mainly determined the size of inter-phase inductors. Finally, the weights of total inductors needed to meet the same THD requirement are compared to demonstrate the benefits of improved ASVM when different PWM schemes are used. The analysis results are verified by experiments on a demo system.

  • Surin Khomfoi; Nattapat Praisuwanna; Leon M. Tolbert
    2010 IEEE Energy Conversion Congress and Exposition
    2010

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    A hybrid cascaded multilevel inverter application for renewable energy resources including a reconfiguration technique is developed. The objective of this research is to propose an alternative topology of hybrid cascaded multilevel inverter applied to a low voltage dc microgrid in telecommunication buildings. The modified PWM technique is also developed to reduce switching losses. Also, the proposed topology can reduce the number of required power switches compared to a traditional cascaded multilevel inverter. A possible reconfiguration technique after faulty condition is also discussed. PSIM (PowerSim) and Simulink/MATLAB are used to simulate the circuit operation and control signal. A 3-kW prototype is developed. The switching losses of the proposed multilevel inverter are also investigated. By using the modified PWM technique and reconfiguration method, the proposed hybrid inverter can improve system efficiency and reliability. The proposed inverter efficiency is 97% under tested condition. The results show that proposed hybrid inverter topology is a promising method for a low voltage dc microgrid interfacing with renewable energy resources.

  • Dushan Boroyevich; Zheng Chen; Fang Luo; Khai Ngo; Puqi Ning; Ruxi Wang; Di Zhang; Fred Wang; Rolando Burgos; Rixin Lai; Shuo Wang
    2010 6th International Conference on Integrated Power Electronics Systems
    2010

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    Over the past ten years, there has been increased incorporation of electronic power processing into alternative, sustainable, and distributed energy sources, as well as energy storage systems, cars, airplanes, ships, homes, data centers, and the power grid. The goals have been to reduce the size, weight, and maintenance and operational costs of these power systems, while increasing overall energy efficiency, safety, and reliability. This paper summarizes some of the authors' research efforts in the last four years on the improvements in power density and physical integration of power converters, mostly for vehicular applications. Several approaches to integration of active components into high-temperature modules are presented together with examples of the evaluation and modeling of 1.2 kV SiC JFET and MOSFET. Possible improvements in the power density through hybrid passive and active integration of an EMI filter and of an energy storage capacitor in single-phase PWM rectifier are also shown. Examples of converter integration for a 10 kW motor drive with active front-end using SiC devices operating at 250°C, and for paralleling three-phase boost rectifiers with interleaved PWM are presented.

  • Dong Jiang; Rixin Lai; Fred Wang; Fang Luo; Shuo Wang; Dushan Boroyevich
    The 2010 International Power Electronics Conference - ECCE ASIA -
    2010

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    The problem of electromagnetic interference (EMI) plays an important role in the design of power electronic converters, especially for airplane electrical systems. This paper explores techniques to reduce EMI noise in three-phase Vienna-type rectifiers. The design approach is using a high-density EMI filter to satisfy the EMI standard. In particular, the cause of high noise at high frequencies is studied in experiments, and the coupling effect of the final stage capacitor and inductors is investigated. The use of random PWM for EMI noise reduction is also presented. The performance of random PWM in a Vienna-type rectifier is verified by theoretical analysis and experimental results. The approaches discussed in this paper significantly reduce the EMI noise in the Vienna-type rectifier, and therefore the filter size can also be reduced.

  • Wei Qian; Fang Z. Peng; Miaosen Shen; Leon M. Tolbert
    2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
    2009

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    This paper presents a bidirectional 3X dc-dc multiplier/divider that can interface the battery with the inverter dc bus for hybrid electric vehicle (HEV) traction drives. Compared with traditional multi-level dc-dc converter, this converter can have three output/input voltage ratios with smooth transition. The control method to limit the transient inrush current, as well as a novel regenerative clamping circuit for multi-level dc-dc converters is provided in this paper. By utilizing the parasitic inductance or the minimum inductance from an air core inductor, the size and weight of the converter can be significantly reduced. Its magnetic-less feature and high efficiency provide the potential of the high temperature operation with the future wide bandgap devices. Experimental results are given to verify the operating principle and the design concepts of this topology.

  • Leon M. Tolbert; Fang Zheng Peng; Faisal H. Khan; Shengnan Li
    2009 IEEE 6th International Power Electronics and Motion Control Conference
    2009

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    This paper will introduce two basic switching cells, P-cell and N-cell, along with their implications and applications in power electronic circuits. The concept of switching cells in power electronic circuits started in the late 1970's. The basic cells presented in this paper have one switching element (transistor) and one diode. The P-cell is the mirror circuit of the N-cell and vice-versa, and this paper suggests that (1) most power electronic circuits can be analyzed and re-constructed using these basic switching cells, (2) single, dual, and 6-pack switching modules should be configured and laid-out according to the basic switching cells and not necessarily the conventional way used by industry, and (3) many benefits such as minimal parasitic inductance and dead-time elimination or minimization may come about. The present paper will describe the construction and operation of these basic switching cells, and it will also show a sequential method to reconstruct several classical dc-dc converters, a voltage source inverter (VSI), and a current source inverter (CSI) using these basic switching cells. In addition, the use of basic switching cells introduces some new topologies of dc-dc converters that originate from the buck, boost, and Cuk converter for negative input voltages. This paper will also illustrate the experimental results of the new and existing topologies constructed from basic switching cells.

  • Seong Taek Lee; Leon M. Tolbert
    2009 IEEE Energy Conversion Congress and Exposition
    2009

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    This paper introduces a new analytical method for performing the output torque calculations of an interior permanent magnet synchronous motor (IPMSM) including both permanent magnet and reluctance torque components. This method works well when using a 2-dimensional magnetic equivalent circuit of a machine by omitting the step of calculating the inductance values which are required for the calculation of the reluctance torque. The analysis results show that this method is very useful in the first design step before simulating a model using finite elements analysis (FEA). Also, this method can be applied to any type of synchronous machine.

  • Seong Taek Lee; Leon M. Tolbert
    2009 IEEE Energy Conversion Congress and Exposition
    2009

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    This paper introduces a new air-gap structure, the slanted air-gap, for increasing the speed limit of an interior permanent magnet synchronous motor (IPMSM) for application in a hybrid electric vehicle. This unique slanted air-gap is intended to maximize the ratio of the back-emf of a machine that is controllable by brushless field excitation (BFE). The BFE structure offers high torque per ampere per core length at low speed and weakened flux at high speed. Although the irregularly shaped air-gap reduces the air-gap flux, it also makes a flux barrier along the d-axis flux path and decreases the d-axis inductance. Therefore, the reluctance torque of the machine increases to compensate for the decreased permanent magnet torque; as a result, the machine achieves a higher ratio of the magnitude of controllable back-emf without losing the high torque capability resulting from the BFE.

  • Shengnan Li; Burak Ozpineci; Leon M. Tolbert
    2009 IEEE Energy Conversion Congress and Exposition
    2009

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    In hybrid electric vehicles (HEV), a battery-powered three-phase inverter is used to drive the traction motor. Due to the switching behavior of this inverter, significant harmonic currents are present on the DC side of the inverter. Traditionally, a bulky capacitor is used to filter these harmonics. In this paper, an active filtering method is evaluated to substitute for the DC bus capacitor. The active power filter (APF), composed of power electronic switches and an inductor, works as a current-source inverter. The operation principle of the proposed method is described and implemented in Matlab/Simulink. The method has been proposed before but the practical feasibility of this method has not been evaluated. In this paper, several crucial design parameters in association with the filtering effect, such as voltage band and the values of the inductor and the smoothing capacitor are identified, and the dependence of system performance on these parameters is illustrated. Finally, the underlying problems for practical implementation are discussed.

  • Hui Zhang; Leon M. Tolbert
    2009 4th IEEE Conference on Industrial Electronics and Applications
    2009

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    The state-of-the-art SiC JFETs are characterized. Three-phase full-bridge inverter power loss models based on experimental data are established and used to estimate inverter efficiency. The impact of load power, temperature, and switching frequency on inverter efficiency is analyzed and demonstrated. The efficiency of the SiC JFET inverters based on present device quality is above 98% with full load current, and more efficient than most conventional Si inverters, especially at high temperature and high frequency.

  • Madhu Chinthavali; Hui Zhang; Leon M. Tolbert; Burak Ozpineci
    2009 Brazilian Power Electronics Conference
    2009

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    This paper presents a study of silicon carbide (SiC) technology which includes device characterization and modeling, inverter simulation, and test results for several prototype inverters. The static and dynamic characteristics of discrete devices and half bridge modules are presented. Test results of a 55 kW hybrid inverter with SiC Schottky diodes and an 18 kW all-SiC inverter using SiC JFETs and Schottky diodes are demonstrated.

  • Wei Qian; Fang Z. Peng; Leon M. Tolbert
    2009 IEEE Vehicle Power and Propulsion Conference
    2009

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    The design of a 55 kW 3X dc-dc converter is presented for hybrid electric vehicle (HEV) traction drives. It can interface the battery with the inverter dc bus with three output/input voltage ratios and have smooth transition in voltage ratio changes. By making use of the parasitic inductance, the size and weight of the converter are significantly reduced. Its magnetic-less feature and high efficiency provide the great potential for the very high temperature operation. The circuit parameter design and the circuit modeling are provided. Experimental results are given to verify the analysis and design concepts.

  • Fang Z. Peng; Yun Wei Li; Leon M. Tolbert
    2009 IEEE Power & Energy Society General Meeting
    2009

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    This paper discusses control and protection of power electronics interfaced distributed generation (DG) systems in a customer-driven microgrid (CDM). Particularly, the following topics will be addressed: microgrid system configurations and features, DG interfacing converter topologies and control, power flow control in grid-connected operation, islanding detection, autonomous islanding operation with load shedding and load demand sharing among DG units, and system/DG protection. Most of the above mentioned control and protection issues should be embedded into the DG interfacing converter control scheme. Some case study results are also shown in this paper to further illustrate the above mentioned issues.

  • John N. Chiasson; Zhong Du; Burak Özpineci; Leon M. Tolbert
    Proceedings of the 48h IEEE Conference on Decision and Control (CDC) held jointly with 2009 28th Chinese Control Conference
    2009

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    A cascade multilevel inverter consisting of a standard 3-leg inverter supplied by a DC source and three full H-bridges each supplied by a capacitor is considered for use as a motor drive. The capacitor H-bridges can only supply reactive voltage to the motor while the standard three leg inverter can supply both reactive and active voltage. A switching control algorithm is presented that shows this inverter topology can be used as an AC drive achieving considerable performance advantages (e.g., higher motor speed) compared to using a standard 3-leg inverter while at the same time regulating the capacitor voltages. The converter controller is a fundamental frequency switching controller based on programmed PWM to achieve higher efficiency (less power losses in the switches) compared to high-frequency PWM approaches. As is well known, the programmed PWM switching times are computed assuming the drive is in sinusoidal steady-state, that is, the derived switching angles achieve the fundamental while rejecting specified harmonics if the voltage waveforms are in sinusoidal steady-state. Here it shown that the switching commands to the converter can be implemented in a smooth fashion for voltage waveform commands whose frequency and amplitudes are continuously varying.

  • Madhu Chinthavali; Hui Zhang; Leon M. Tolbert; Burak Ozpineci
    2009 Brazilian Power Electronics Conference
    2009

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    This paper presents a study of silicon carbide (SiC) technology which includes device characterization and modeling, inverter simulation, and test results for several prototype inverters. The static and dynamic characteristics of discrete devices and half bridge modules are presented. Test results of a 55 kW hybrid inverter with SiC Schottky diodes and an 18 kW all-SiC inverter using SiC JFETs and Schottky diodes are demonstrated.

  • Wei Qian; Fang Z. Peng; Leon M. Tolbert
    2009 IEEE Vehicle Power and Propulsion Conference
    2009

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    The design of a 55 kW 3X dc-dc converter is presented for hybrid electric vehicle (HEV) traction drives. It can interface the battery with the inverter dc bus with three output/input voltage ratios and have smooth transition in voltage ratio changes. By making use of the parasitic inductance, the size and weight of the converter are significantly reduced. Its magnetic-less feature and high efficiency provide the great potential for the very high temperature operation. The circuit parameter design and the circuit modeling are provided. Experimental results are given to verify the analysis and design concepts.

  • Fang Z. Peng; Yun Wei Li; Leon M. Tolbert
    2009 IEEE Power & Energy Society General Meeting
    2009

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    This paper discusses control and protection of power electronics interfaced distributed generation (DG) systems in a customer-driven microgrid (CDM). Particularly, the following topics will be addressed: microgrid system configurations and features, DG interfacing converter topologies and control, power flow control in grid-connected operation, islanding detection, autonomous islanding operation with load shedding and load demand sharing among DG units, and system/DG protection. Most of the above mentioned control and protection issues should be embedded into the DG interfacing converter control scheme. Some case study results are also shown in this paper to further illustrate the above mentioned issues.

  • John N. Chiasson; Zhong Du; Burak Özpineci; Leon M. Tolbert
    Proceedings of the 48h IEEE Conference on Decision and Control (CDC) held jointly with 2009 28th Chinese Control Conference
    2009

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    A cascade multilevel inverter consisting of a standard 3-leg inverter supplied by a DC source and three full H-bridges each supplied by a capacitor is considered for use as a motor drive. The capacitor H-bridges can only supply reactive voltage to the motor while the standard three leg inverter can supply both reactive and active voltage. A switching control algorithm is presented that shows this inverter topology can be used as an AC drive achieving considerable performance advantages (e.g., higher motor speed) compared to using a standard 3-leg inverter while at the same time regulating the capacitor voltages. The converter controller is a fundamental frequency switching controller based on programmed PWM to achieve higher efficiency (less power losses in the switches) compared to high-frequency PWM approaches. As is well known, the programmed PWM switching times are computed assuming the drive is in sinusoidal steady-state, that is, the derived switching angles achieve the fundamental while rejecting specified harmonics if the voltage waveforms are in sinusoidal steady-state. Here it shown that the switching commands to the converter can be implemented in a smooth fashion for voltage waveform commands whose frequency and amplitudes are continuously varying.

  • M. A Huque; S. K. Islam; B. J. Blalock; C. Su; R. Vijayaraghavan; L M. Tolbert
    2008 IEEE International Symposium on Industrial Electronics
    2008

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    In recent years increasing demand for hybrid electric vehicle has generated the need for reliable and low-cost high-temperature electronics which can operate at the extreme temperatures that exists under the hood. A high-voltage and high-temperature gate-driver integrated circuit for SiC FET switches is designed and implemented in a 0.8-micron Silicon-on-Insulator high-voltage process. First prototype chip has been successfully tested up to 200degC ambient temperature without any heat sink or cooling mechanism. This gate-driver chip is intended to drive SiC power FETs of the DC-DC converters in a hybrid electric vehicle. The converter modules along with the gate-driver chip will be placed very close to the engine where the temperature can reach up to 175degC. Successful operation of the chip at this temperature with or without minimal heat sink and without liquid cooling will help achieve greater power-to-volume as well as power-to-weight ratios for the power electronics module. A second prototype has also been designed with more robust features.

  • Wenjuan Zhang; Fangxing Li; Leon Tolbert
    2008 IEEE/PES Transmission and Distribution Conference and Exposition
    2008

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    Summary form only given. The key of reactive power planning (RPP), or Var planning, is the optimal allocation of reactive power sources considering location and size. Traditionally, the locations for placing new Var sources were either simply estimated or directly assumed. Recent research works have presented some rigorous optimization-based methods in RPP. This paper will first review various objectives of RPP. The objectives may consider many cost functions such as variable Var cost, fixed Var cost, real power losses, and fuel cost. Also considered may be the deviation of a given voltage schedule, voltage stability margin, or even a combination of different objectives as a multi-objective model. Secondly, different constraints in RPP are discussed. These different constraints are the key of various optimization models, identified as Optimal Power Flow (OPF) model, Security Constrained OPF (SCOPF) model, and SCOPF with Voltage-Stability consideration (SCOPF-VS). Thirdly, the optimization-based models will be categorized as conventional algorithms, intelligent searches, and fuzzy set applications. The conventional algorithms include Linear Programming (LP), Nonlinear Programming (NLP), Mixed Integer Non- Linear Programming (MINLP), etc. The intelligent searches include Simulated Annealing (SA), Evolutionary Algorithms (EA), and Tabu Search (TS). The fuzzy set applications in RPP address the uncertainties in objectives and constraints. Finally, this paper will conclude the discussion with a summary matrix for different objectives, models, and algorithms.

  • Wenjuan Zhang; Fangxing Li; Leon M. Tolbert
    2008 IEEE/PES Transmission and Distribution Conference and Exposition
    2008

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    The key of reactive power planning (RPP), or Var planning, is the optimal allocation of reactive power sources considering location and size. First, the relationships of Var compensation, total transfer capability (TTC), and fuel cost are introduced in this paper. Second, the enumeration approach for RPP is briefly described. Although time-consuming, it provides a global view of the relationship between the system cost and local Var compensation, which is useful for benchmarking purposes. Third, the voltage stability constrained optimal power flow (VSCOPF) model with two sets of variables (TSV) approach is used to combine a large number of OPFs in the enumeration approach to achieve an efficient model. The two sets of variables correspond to the normal operating point and the collapse point, respectively. The computational complexity of TSV is tremendously reduced. Different from the previous work using Var cost minimization as the objective, this work proposes to use the total system cost (fuel cost and Var cost) minimization as the objective. This leads to significantly different results. The observed results have important implication to RPP, especially under the deregulated environment. That is, it verifies that RPP should consider the impact to system dispatch considering generation cost. The results from the TSV approach are also benchmarked with the enumeration approach. Finally, conclusions are presented.

  • Wenjuan Zhang; Fangxing Li; Leon M. Tolbert
    2008 Third International Conference on Electric Utility Deregulation and Restructuring and Power Technologies
    2008

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    The natural gas price surged in 2004. As a result, the marginal cost of some generators burning gas also rose sharply. This paper is in response to the sharp increase in gas price and the corresponding generator marginal cost. This paper will first investigate the benefits of Var compensation including reduced losses (B1), shifting reactive power flow to real power flow (B2), and increased transfer capability (B3). Then, an OPF-based quantitative approach is used to assess the three benefits. Finally, the scheme of Var economic benefits sensitivity analysis to generator marginal cost is proposed. Tests are conducted on a system with seven buses in two areas. The simulation results show that a positive relationship exists between the generator marginal cost in the load center and the Var economic benefits, and a negative relationship exists between the generator marginal cost in the generation center and the Var economic benefits.

  • Michael Starke; Fangxing Li; Leon M. Tolbert; Burak Ozpineci
    2008 IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century
    2008

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    Many studies comparing AC and DC systems have focused on efficiency, stability, and controllability, but have not compared the maximum transfer capability. In this paper, the maximum transfer capability of an AC system and two DC systems, one with two lines and another with three, is determined through the continuation power flow method and compared. The results reveal that significant gains can be achieved by moving to a DC system with three lines.

  • F. J. T. Filho; T. H. A. Mateus; H. Z. Maia; B. Ozpineci; J. O. P. Pinto; L. M. Tolbert
    2008 IEEE Power Electronics Specialists Conference
    2008

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    A new approach for selective harmonic elimination in a 7-level cascaded multilevel inverter with separate DC sources will be presented. As opposed to previous research in this area, the DC sources feeding the multilevel inverter are considered to be varying in time. This method uses genetic algorithms to obtain switching angles offline for different DC source values and uses neural networks to determine the switching angles that correspond to the real-time values of the DC sources. This implies that each one of the DC sources of this topology can have different values at any time but the output fundamental voltage will stay constant and the harmonic will still meet the specifications. The paper gives details on the approach used, together with simulation and experimental results.

  • Haiwen Liu; Leon M. Tolbert; Burak Ozpineci; Zhong Du
    2008 51st Midwest Symposium on Circuits and Systems
    2008

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    A hybrid multilevel inverter model based on PSIM and MATLAB/SIMULINK is presented in this paper. It consists of a standard 3-leg inverter (one leg for each phase) and H-bridge in series with each inverter leg. The inverter can be used in hybrid electric vehicles (HEV) and electric vehicles (EV). The co-simulation model is employed in order to take full advantage of different power electronics simulation software. Specifically, the main circuit model is developed using PSIM, and the control model is developed using MATLAB/SIMULINK. An experimental 5-level hybrid inverter is tested, which is controlled by multilevel carrier-based PWM signals. The simulation yields a good estimation for the test results of the inverter.

  • Hui Zhang; Leon M. Tolbert
    2008 34th Annual Conference of IEEE Industrial Electronics
    2008

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    The potential impact of SiC devices on a wind generation system is explored by simulations in this work. The system modeling is explained in detail. Most recent SiC MOSFET prototypes are obtained, tested, and used to form a bi-directional converter in the simulation. The performance of the SiC converter is analyzed and compared to its Si counterpart at different temperatures and frequencies. A conclusion is drawn that the SiC converters can improve the wind system efficiency, conserve energy, and reduce system size and cost due to the low-loss, high-frequency, and high-temperature properties of SiC devices even for one-for-one replacement for Si devices.

  • Michael Starke; Leon M. Tolbert; Burak Ozpineci
    2008 IEEE/PES Transmission and Distribution Conference and Exposition
    2008

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    Environmentally friendly technologies such as photovoltaics and fuel cells are DC sources. In the current power infrastructure, this necessitates converting the power supplied by these devices into AC for transmission and distribution which adds losses and complexity. The amount of DC loads in our buildings is ever-increasing with computers, monitors, and other electronics entering our workplaces and homes. This forces another conversion of the AC power to DC, adding further losses and complexity. This paper proposes the use of a DC distribution system. In this study, an equivalent AC and DC distribution system are compared in terms of efficiency.

  • Haiwen Liu; Leon M. Tolbert; Surin Khomfoi; Burak Ozpineci; Zhong Du
    2008 IEEE Power Electronics Specialists Conference
    2008

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    A hybrid cascaded multilevel inverter with PWM method is presented in this paper. It consists of a standard 3-leg inverter (one leg for each phase) and H-bridge in series with each inverter leg. It can use only a single DC power source to supply a standard 3-leg inverter along with three full H-bridges supplied by capacitors. Multilevel carrier- based PWM method is used to produce a five-level phase voltage. The inverter can be used in hybrid electric vehicles (HEV) and electric vehicles (EV). A simulation model based on PSIM and MATLAB/SIMULINK is developed. An experimental 5 kW prototype inverter is built and tested. The results experimentally validate the proposed PWM hybrid cascaded multilevel inverter.

  • Haiwen Liu; Leon M. Tolbert; Burak Ozpineci; Zhong Du
    2008 34th Annual Conference of IEEE Industrial Electronics
    2008

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    This paper presents a hybrid cascaded multilevel inverter for electric vehicles (EV) / hybrid electric vehicles (HEV) and utility interface applications. The inverter consists of a standard 3-leg inverter (one leg for each phase) and H-bridge in series with each inverter leg. It can use only a single DC power source to supply a standard 3-leg inverter along with three full H-bridges supplied by capacitors or batteries. Both fundamental frequency and high switching frequency PWM methods are used for the hybrid multilevel inverter. An experimental 5 kW prototype inverter is built and tested. The above two switching control methods are validated and compared experimentally.

  • 2008

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    This paper presents a new technique to obtain isolated dc voltage outputs from a capacitor clamped DC-DC converter. The multilevel modular capacitor clamped converter (MMCCC) has several key features that make it possible to generate AC outputs (10 kHz) from a DC-DC converter circuit. Using those high frequency AC outputs, the MMCCC circuit can incorporate single or multiple high frequency transformers to generate isolated AC outputs. These isolated outputs can be rectified and filtered to obtain unidirectional or bi-directional DC outputs. Using another MMCCC converter stage or an active full bridge block, the AC port can be made bi-directional to transfer power in both directions. By adopting the MMCCC topology to achieve isolated outputs, it is possible to simultaneously integrate multiple DC sources in an isolated and non-isolated manner. This paper will investigate the origin of the AC outputs in the MMCCC circuit, and present an analytical approach to estimating the isolated DC output voltage. Finally, experimental results will be presented for further verification of the concept.

  • E. Ozdemir; S. Ozdemir; L. M. Tolbert; B. Ozpineci
    2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
    2008

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    This paper presents a fundamental frequency modulated multilevel inverter scheme for use with a three-phase stand-alone photovoltaic (PV) system. The system consists of four series connected PV arrays, a five-level diode-clamped multilevel inverter (DCMLI) generating fundamental modulation staircase three-phase output voltages, and a three-phase induction motor as the load. In order to validate the proposed concept, simulation studies and experimental measurements, done using a small-scale laboratory prototype, are also presented. The results show the feasibility of the fundamental switching application in three- phase stand-alone PV power systems.

  • Faisal H. Khan; Leon M. Tolbert
    2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
    2008

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    A multilevel dc-dc converter with programmable conversion ratio (CR) is presented in this paper. This converter is a modified version of the MMCCC converter. A universal version of the MMCCC is developed in this paper, and the CR can be easily changed within a wide range. The MMCCC converter is based on capacitor-clamped topology, and the conversion ratio of the circuit depends on the number of active modules. However, like any other capacitor-clamped circuit, the MMCCC circuit requires a large number of transistors and capacitors to attain a high conversion ratio (CR). In this paper, a new circuit module will be introduced that can be connected in a cascade pattern to form the new converter. By using the new modular cell, it is possible to attain very high conversion ratio using a limited number of components, and thus more compactness compared to the predecessor MMCCC circuit can be achieved.

  • 2008

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    This paper presents a new technique to obtain isolated dc voltage outputs from a capacitor clamped DC-DC converter. The multilevel modular capacitor clamped converter (MMCCC) has several key features that make it possible to generate AC outputs (10 kHz) from a DC-DC converter circuit. Using those high frequency AC outputs, the MMCCC circuit can incorporate single or multiple high frequency transformers to generate isolated AC outputs. These isolated outputs can be rectified and filtered to obtain unidirectional or bi-directional DC outputs. Using another MMCCC converter stage or an active full bridge block, the AC port can be made bi-directional to transfer power in both directions. By adopting the MMCCC topology to achieve isolated outputs, it is possible to simultaneously integrate multiple DC sources in an isolated and non-isolated manner. This paper will investigate the origin of the AC outputs in the MMCCC circuit, and present an analytical approach to estimating the isolated DC output voltage. Finally, experimental results will be presented for further verification of the concept.

  • E. Ozdemir; S. Ozdemir; L. M. Tolbert; B. Ozpineci
    2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
    2008

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    This paper presents a fundamental frequency modulated multilevel inverter scheme for use with a three-phase stand-alone photovoltaic (PV) system. The system consists of four series connected PV arrays, a five-level diode-clamped multilevel inverter (DCMLI) generating fundamental modulation staircase three-phase output voltages, and a three-phase induction motor as the load. In order to validate the proposed concept, simulation studies and experimental measurements, done using a small-scale laboratory prototype, are also presented. The results show the feasibility of the fundamental switching application in three- phase stand-alone PV power systems.

  • Faisal H. Khan; Leon M. Tolbert
    2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
    2008

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    A multilevel dc-dc converter with programmable conversion ratio (CR) is presented in this paper. This converter is a modified version of the MMCCC converter. A universal version of the MMCCC is developed in this paper, and the CR can be easily changed within a wide range. The MMCCC converter is based on capacitor-clamped topology, and the conversion ratio of the circuit depends on the number of active modules. However, like any other capacitor-clamped circuit, the MMCCC circuit requires a large number of transistors and capacitors to attain a high conversion ratio (CR). In this paper, a new circuit module will be introduced that can be connected in a cascade pattern to form the new converter. By using the new modular cell, it is possible to attain very high conversion ratio using a limited number of components, and thus more compactness compared to the predecessor MMCCC circuit can be achieved.

  • M. A Huque; R. Vijayaraghavan; M. Zhang; B. J. Blalock; L M. Tolbert; S. K. Islam
    2007 IEEE Power Electronics Specialists Conference
    2007

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    A high-voltage and high-temperature gate-driver chip for SiC FET switches is designed and fabricated using 0.8- micron, 2-poly and 3-metal BCD on SOI process. It can generate output voltage swing from -5 V to 30 V and can operate up to 175degC ambient temperature. This gate-driver chip is intended to drive SiC power FETs in DC-DC converters in a hybrid electric vehicle. The converter modules along with the gate-driver chip will be placed very close to the engine where the temperature can reach up to 175degC. Successful operation of the chip at this temperature without heat sink and liquid cooling will help to achieve greater power-to-volume as well as power-to-weight ratios for the power electronics module. Initial test results presented in this paper also validate the simulation.

  • Wenjuan Zhang; Fangxing Li; Leon M. Tolbert
    2007 International Power Engineering Conference (IPEC 2007)
    2007

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    This paper will first discuss the quantitative economic benefits from reactive power (Var) compensation. The benefits can be categorized as reduced losses (B1), exchanging reactive power flow to real power flow (B2), and increased transfer capability (B3). Then, the benefits in the three categories are applied to Var planning considering different locations and amounts of Var compensation. The study on Var planning presented in this paper is a straightforward, two-step approach: 1) it utilizes an Optimal Power Flow (OPF) model to update the Total Transfer Capability (TTC) in order to give a more accurate evaluation of Var benefits; 2) it then performs three OPFs for each Var location and amount combination to evaluate the Var benefits in three categories. Although this approach may be time- consuming, it does give a full spectrum and insightful information about the benefits under different categories if a Var compensator is installed at a specific location in various amounts. Hence, the sensitivity of economic benefits under different categories will be easily obtained. This approach may be used for future researches for benchmarking. It is also interesting to observe that the test results from a seven-bus system in this paper shows that it is not always economically efficient if Var compensation increases continuously.

  • 2007

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    A new capacitor clamped modular dc-dc converter with bi-directional power handling capability will be presented in this paper. This inductor-free design is modular, and it is possible to integrate multiple loads and sources simultaneously in the converter. Moreover, this 5 kW dc-dc converter can produce multiple ac outputs to feed power to ac loads or transformers to get further control over the conversion ratio of the circuit. This high efficiency modular converter has flexible conversion ratio, and it could be successfully used in a multi-bus power system by virtue of its inherent power management properties.

  • Faisal H. Khan; Leon M. Tolbert
    APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
    2007

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    A multilevel modular capacitor clamped dc-dc converter (MMCCC) will be presented in this paper with some of its advantageous features. By virtue of the modular nature of the converter, it is possible to integrate multiple loads and sources to the converter at the same time. The modular construction of the MMCCC topology provides transformer like taps in the circuit, and depending on the conversion ratio of the converter, it becomes possible to connect several dc sources and loads at these taps. The modularity of the new converter is not limited to this transformer like operation, but also provides redundancy and fault bypass capability in the circuit. Using the modularity feature, some redundant modules can be operated in bypass state, and during any fault, these redundant modules can be used to replace a faulty module to maintain an uninterrupted operation. Thus, this MMCCC topology could be a solution to establish a power management system among multiple sources and loads having different operating voltages.

  • Surin Khomfoi; Leon M. Tolbert
    APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
    2007

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    A genetic-algorithm-based selective principal component neural network method for fault diagnosis system in a multilevel inverter is proposed in this paper. Multilayer perceptron (MLP) networks are used to identify the type and location of occurring faults from inverter output voltage measurement. Principal component analysis (PCA) is utilized to reduce the neural network input size. A lower dimensional input space will also usually reduce the time necessary to train a neural network, and the reduced noise may improve the mapping performance. The genetic algorithm is also applied to select the valuable principal components. The neural network design process including principal component analysis and the use of genetic algorithm is clearly described. The comparison among MLP neural network (NN), principal component neural network (PC-NN), and genetic algorithm based selective principal component neural network (PC-GA-NN) are performed. Proposed networks are evaluated with a simulation test set and an experimental test set. The PC-NN has improved overall classification performance from NN by about 5% points, whereas PC-GA-NN has better overall classification performance from NN by about 7.5% points. The overall classification performance of the proposed networks is more than 90%.

  • Sule Ozdemir; Engin Ozdemir; Leon M. Tolbert; Surin Khomfoi
    2007 7th International Conference on Power Electronics and Drive Systems
    2007

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    In this study, elimination of harmonics in a five- level diode-clamped multilevel inverter (DCMLI) has been implemented by using fundamental modulation switching. The proposed method eliminates harmonics by generating negative harmonics with switching angles calculated for selective harmonic elimination method. In order to confirm the proposed method, first Matlab/Simulink and PSIM simulation results are given. Then the proposed method is also validated by experiments with Opal-RT controller and a 10 kW three- phase, five-level DCMLI prototype.

  • Miaosen Shen; Fang Z. Peng; Leon M. Tolbert
    2007 IEEE Power Electronics Specialists Conference
    2007

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    A multilevel dc/dc power conversion system with multiple dc sources is proposed in this paper. With this conversion system, the output voltage can be changed almost continuously without any magnetic components. With this magnetic-less system, very high temperature operation is possible. Power loss and efficiency analysis is provided in the paper. Comparison results show that the system does not require more semiconductors or capacitance than the traditional boost converter. Experimental results are provided to confirm the analysis and control concept.

  • John N. Chiasson; Burak Ozpineci; Leon M. Tolbert
    APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
    2007

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    The interest here is in using a single DC power source to construct a 3-phase 5-level cascade multilevel inverter to be used as a drive for a PM traction motor. The 5-level inverter consists of a standard 3-leg inverter (one leg for each phase) and an H-bridge in series with each inverter leg, which use a capacitor as a DC source. It is shown that one can simultaneously maintain the regulation of the capacitor voltage while achieving an output voltage waveform which is 25% higher than that obtained using a standard 3-leg inverter by itself.

  • John Chiasson; Burak Ozpineci; Zhong Du; Leon M. Tolbert
    2007 IEEE International Electric Machines & Drives Conference
    2007

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    A cascade multilevel inverter is a power electronic device built to synthesize a desired AC voltage from several levels of DC voltages. Such inverters have been the subject of research in the last several years, where the DC levels were considered to be identical in that all of them were either batteries, solar cells, etc. Similar to previous results in the literature, the work here shows how a cascade multilevel inverter can be used to obtain a voltage boost at higher speeds for a three-phase PM drive using only a single DC voltage source. The input of a standard three-leg inverter is connected to the DC source and the output of each leg is fed through an H-bridge (which is supplied by a capacitor) to form a cascade multilevel inverter. A fundamental switching scheme is used, which achieves the fundamental in the output voltage while eliminating the fifth harmonic. A new contribution in this paper is the development of explicit conditions in terms of the power factor and modulation index for which the capacitor voltage of the H-bridges can be regulated while simultaneously maintaining the aforementioned output voltage. This is then used for a PM motor drive showing the machine can attain higher speeds due to the higher output voltage of the multilevel inverter compared to using just a three-leg inverter.

  • Yan Xu; John D. Kueck; Leon M. Tolbert; D. Tom Rizy
    2007 IEEE Power Electronics Specialists Conference
    2007

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    A three-phase insulated gate bipolar transistor (IGBT)-based parallel active filter is used for current and/or voltage unbalance compensation. An instantaneous power theory is adopted for real-time calculation and control. Three control schemes, current control, voltage control, and integrated control are proposed to compensate the unbalance of current, voltage, or both. The compensation results of the different control schemes in unbalance cases (load unbalance or voltage source unbalance) are compared and analyzed. The simulation and experimental results show that the control schemes can compensate the unbalance in load current or in the voltage source. Different compensation objectives can be achieved, i.e., balanced and unity power factor source current, balanced and regulated voltage, or both, by choosing appropriate control schemes.

  • Yan Xu; Leon M. Tolbert; D. Tom Rizy; John D. Kueck
    2007 IEEE Power Engineering Society General Meeting
    2007

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    The nonactive-power-related ancillary services provided by distributed energy (DE) resources are categorized by voltage regulation, reactive power compensation, power factor correction, voltage and/or current unbalance compensation, and harmonics compensation. An instantaneous nonactive power theory is adopted to control the DE system to provide these ancillary services. Three control schemes, including nonactive current compensation, power factor correction, and voltage regulation, are developed which can perform one or more of the ancillary services. The control schemes are implemented in a DE system in simulation and experiments. The simulation and the experimental results show that DE is feasible for providing nonactive-power-related ancillary services.

  • Surin Khomfoi; Leon M. Tolbert
    2007 7th International Conference on Power Electronics and Drive Systems
    2007

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    A fault detection and reconfiguration technique for a cascaded H-bridge 11-level inverter drives during faulty condition is proposed in this paper. The ability of cascaded H-bridge multilevel inverter drives (MLID) to operate under faulty condition is also discussed. Output phase voltages of a MLID can be used as a diagnostic signal to detect faults and their locations. Al-based techniques are used to perform the fault classification. A neural network (NN) classification is applied to the fault diagnosis of a MLID system. Multilayer perceptron (MLP) networks are used to identify the type and location of occurring faults. The principal component analysis (PCA) is utilized in the feature extraction process to reduce the NN input size. The genetic algorithm (GA) is also applied to select the valuable principal components to train the NN. A reconfiguration technique is also developed. The developed system is validated with simulation and experimental results. The developed fault diagnostic system requires about 6 cycles (-100 ms at 60 Hz) to clear an open circuit and about 9 cycles (~150 ms at 60 Hz) to clear a short circuit fault. The experiment and simulation results are in good agreement with each other, and the results show that the developed system performs satisfactorily to detect the fault type, fault location, and reconfiguration.

  • Sule Ozdemir; Engin Ozdemir; Leon M. Tolbert; Surin Khomfoi
    2007 7th International Conference on Power Electronics and Drive Systems
    2007

    arrow_drop_down

    In this study, elimination of harmonics in a five- level diode-clamped multilevel inverter (DCMLI) has been implemented by using fundamental modulation switching. The proposed method eliminates harmonics by generating negative harmonics with switching angles calculated for selective harmonic elimination method. In order to confirm the proposed method, first Matlab/Simulink and PSIM simulation results are given. Then the proposed method is also validated by experiments with Opal-RT controller and a 10 kW three- phase, five-level DCMLI prototype.

  • Miaosen Shen; Fang Z. Peng; Leon M. Tolbert
    2007 IEEE Power Electronics Specialists Conference
    2007

    arrow_drop_down

    A multilevel dc/dc power conversion system with multiple dc sources is proposed in this paper. With this conversion system, the output voltage can be changed almost continuously without any magnetic components. With this magnetic-less system, very high temperature operation is possible. Power loss and efficiency analysis is provided in the paper. Comparison results show that the system does not require more semiconductors or capacitance than the traditional boost converter. Experimental results are provided to confirm the analysis and control concept.

  • John N. Chiasson; Burak Ozpineci; Leon M. Tolbert
    APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
    2007

    arrow_drop_down

    The interest here is in using a single DC power source to construct a 3-phase 5-level cascade multilevel inverter to be used as a drive for a PM traction motor. The 5-level inverter consists of a standard 3-leg inverter (one leg for each phase) and an H-bridge in series with each inverter leg, which use a capacitor as a DC source. It is shown that one can simultaneously maintain the regulation of the capacitor voltage while achieving an output voltage waveform which is 25% higher than that obtained using a standard 3-leg inverter by itself.

  • John Chiasson; Burak Ozpineci; Zhong Du; Leon M. Tolbert
    2007 IEEE International Electric Machines & Drives Conference
    2007

    arrow_drop_down

    A cascade multilevel inverter is a power electronic device built to synthesize a desired AC voltage from several levels of DC voltages. Such inverters have been the subject of research in the last several years, where the DC levels were considered to be identical in that all of them were either batteries, solar cells, etc. Similar to previous results in the literature, the work here shows how a cascade multilevel inverter can be used to obtain a voltage boost at higher speeds for a three-phase PM drive using only a single DC voltage source. The input of a standard three-leg inverter is connected to the DC source and the output of each leg is fed through an H-bridge (which is supplied by a capacitor) to form a cascade multilevel inverter. A fundamental switching scheme is used, which achieves the fundamental in the output voltage while eliminating the fifth harmonic. A new contribution in this paper is the development of explicit conditions in terms of the power factor and modulation index for which the capacitor voltage of the H-bridges can be regulated while simultaneously maintaining the aforementioned output voltage. This is then used for a PM motor drive showing the machine can attain higher speeds due to the higher output voltage of the multilevel inverter compared to using just a three-leg inverter.

  • Yan Xu; John D. Kueck; Leon M. Tolbert; D. Tom Rizy
    2007 IEEE Power Electronics Specialists Conference
    2007

    arrow_drop_down

    A three-phase insulated gate bipolar transistor (IGBT)-based parallel active filter is used for current and/or voltage unbalance compensation. An instantaneous power theory is adopted for real-time calculation and control. Three control schemes, current control, voltage control, and integrated control are proposed to compensate the unbalance of current, voltage, or both. The compensation results of the different control schemes in unbalance cases (load unbalance or voltage source unbalance) are compared and analyzed. The simulation and experimental results show that the control schemes can compensate the unbalance in load current or in the voltage source. Different compensation objectives can be achieved, i.e., balanced and unity power factor source current, balanced and regulated voltage, or both, by choosing appropriate control schemes.

  • Yan Xu; Leon M. Tolbert; D. Tom Rizy; John D. Kueck
    2007 IEEE Power Engineering Society General Meeting
    2007

    arrow_drop_down

    The nonactive-power-related ancillary services provided by distributed energy (DE) resources are categorized by voltage regulation, reactive power compensation, power factor correction, voltage and/or current unbalance compensation, and harmonics compensation. An instantaneous nonactive power theory is adopted to control the DE system to provide these ancillary services. Three control schemes, including nonactive current compensation, power factor correction, and voltage regulation, are developed which can perform one or more of the ancillary services. The control schemes are implemented in a DE system in simulation and experiments. The simulation and the experimental results show that DE is feasible for providing nonactive-power-related ancillary services.

  • Surin Khomfoi; Leon M. Tolbert
    2007 7th International Conference on Power Electronics and Drive Systems
    2007

    arrow_drop_down

    A fault detection and reconfiguration technique for a cascaded H-bridge 11-level inverter drives during faulty condition is proposed in this paper. The ability of cascaded H-bridge multilevel inverter drives (MLID) to operate under faulty condition is also discussed. Output phase voltages of a MLID can be used as a diagnostic signal to detect faults and their locations. Al-based techniques are used to perform the fault classification. A neural network (NN) classification is applied to the fault diagnosis of a MLID system. Multilayer perceptron (MLP) networks are used to identify the type and location of occurring faults. The principal component analysis (PCA) is utilized in the feature extraction process to reduce the NN input size. The genetic algorithm (GA) is also applied to select the valuable principal components to train the NN. A reconfiguration technique is also developed. The developed system is validated with simulation and experimental results. The developed fault diagnostic system requires about 6 cycles (-100 ms at 60 Hz) to clear an open circuit and about 9 cycles (~150 ms at 60 Hz) to clear a short circuit fault. The experiment and simulation results are in good agreement with each other, and the results show that the developed system performs satisfactorily to detect the fault type, fault location, and reconfiguration.

  • Yan Xu; Leon M. Tolbert; John N. Chiasson; Jeremy B. Campbell; Fang Z. Peng
    Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting
    2006

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    This paper presents a generalized nonactive power theory, in which the instantaneous currents (active and nonactive) and instantaneous powers (active and nonactive) are defined. This theory is implemented in a parallel nonactive power compensation system. The theory is valid if the system is three-phase or single-phase, sinusoidal or non-sinusoidal, periodic or non-periodic, balanced or unbalanced. Four cases, three-phase balanced RL load, three-phase unbalanced RL load, diode rectifier load, and single-phase RL load are tested in the experiments. Subharmonic load compensation and non-periodic load compensation are simulated in Matlab. The simulation and experimental results not only verify the validity of the theory, but also show that this theory can perform instantaneous nonactive power compensation with fast dynamic response

  • M. Li; J.N. Chiasson; M. Bodson; L.M. Tolbert
    2006 CES/IEEE 5th International Power Electronics and Motion Control Conference
    2006

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    A differential-algebraic method is used to estimate the rotor time constant TR of an induction motor without measurements of the rotor speed/position. The method consists of solving for the roots of a polynomial equation in TR whose coefficients depend only on the stator currents, stator voltages, and their derivatives. Experimental results are presented

  • M. Li; J. N. Chiasson; L. M. Tolbert
    2006 CES/IEEE 5th International Power Electronics and Motion Control Conference
    2006

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    The widespread use of non-linear loads and power electronics converters has increased the generation of non-sinusoidal and non-periodic currents and voltages in power systems. Reactive power compensation or control is an important part of a power system to minimize power transmission losses. Given a modulation index, the switch times can be chosen to achieve the fundamental while eliminating specific harmonics. However, the resulting total harmonic distortion (THD) depends on the modulation index. This work considers the control of the DC capacitor voltage in such a way that one can operate at the modulation index which results in the minimum THD. This paper presents the development of specific control algorithms for a cascaded multilevel inverter to be used for static VAr compensation

  • Zhong Du; Leon M. Tolbert; John N. Chiasson; Burak Ozpineci; Hui Li; Alex Q. Huang
    2006 37th IEEE Power Electronics Specialists Conference
    2006

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    This paper presents a hybrid cascaded H-bridge multilevel motor drive control scheme for electric/hybrid electric vehicles where each phase of a three-phase cascaded multilevel converter can be implemented using only a single DC source and capacitors for the other DC sources. Traditionally, each phase of a three-phase cascaded multilevel converter requires n DC sources for 2n + 1 output voltage levels. In this paper, a scheme is proposed that allows the use of a single DC source as the first DC source with the remaining n − 1 DC sources being capacitors. It is shown that a simple 7-level equal step output voltage switching control can simultaneously maintain the balance of DC voltage levels of the capacitors, eliminate specified low order non-triplen harmonics, and produce a nearly sinusoidal three-phase output voltage. This scheme therefore provides the capability to produce higher voltages at higher speeds (where they are needed) with a low switching frequency method for motor drive application, which has inherent low switching losses and high conversion efficiency. This control scheme especially fits fuel cell electric vehicle motor drive applications and hybrid electric vehicle motor drive applications.

  • Hui Li; Zhong Du; Kaiyu Wang; Leon M. Tolbert; Danwei Liu
    Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting
    2006

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    Different circuit configurations have been researched to combine clean energy sources and energy storage elements. This paper proposes a hybrid energy system to integrate the variable-speed wind turbine, fuel cell, and battery using a cascaded H-bridge converter. One of the advantages of this topology is that it still can obtain the regulated output voltage if one or more energy sources are diminished. In addition, the topology can be easily extended to connect more sources without increasing the circuit and control complexity; therefore, it is beneficial for distributed energy generation. Different operation modes are analyzed in detail. The control schemes were developed to extract maximum wind power and charge/discharge the battery with fast dynamics. The simulation and experimental results are provided to confirm the theoretical analysis

  • Hui Zhang; Leon M. Tolbert; Burak Ozpineci; Madhu S. Chinthavali
    Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting
    2006

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    The purpose of this work is to provide validated models to estimate the performance of a SiC-based converter as a utility interface in battery systems. System design and modeling are described in detail. Simulations are done for both a SiC JFET converter and its Si counterpart based on the quality of tested devices. The simulation results indicate that in both charging and discharging modes, the SiC converter has a better performance compared to the Si one. (1) With the same heatsink size and ambient temperature, great advantages in efficiency and junction temperatures were found in the SiC-based converter. (2) With the same thermal limit, large savings in system weight and volume combined with a high efficiency were found in the SiC-based converter

  • Faisal H. Khan; Leon M. Tolbert
    Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting
    2006

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    A novel topology of multilevel modular capacitor clamped dc-dc converter (MMCCC) is presented in this paper. In contrast to the conventional flying capacitor multilevel dc-dc converter (FCMDC), this new topology is completely modular and requires a simpler gate drive circuit. Moreover, the new topology has many advantageous features such as high frequency operation capability, low input/output current ripple, lower on-state voltage drop, and bi-directional power flow management. This paper discusses the construction and operation of the new converter along with a comparison with a conventional converter. Finally, the simulation and experimental results validate the concept of this new topology

  • Surin Khomfoi; Leon M. Tolbert
    2006 37th IEEE Power Electronics Specialists Conference
    2006

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    A fault diagnosis system In a multilevel-Inverter using a compact neural network is proposed in this paper. It is difficult to diagnose a multilevel-inverter drive (MLID) system using a mathematical model because MLID systems consist of many switching devices and their system complexity has a nonlinear factor. Therefore, a neural network classification is applied to the fault diagnosis of a MLID system. Multilayer perceptron (MLP) networks are used to identify the type and location of occurring faults from inverter output voltage measurement. The neural network design process is clearly described. The principal component analysis (PCA) is utilized to reduce the neural network input size. A lower dimensional input space will also usually reduce the time necessary to train a neural network, and the reduced noise may improve the mapping performance. The comparison between MLP neural network (NN) and PC neural network (PC-NN) are performed. Both proposed networks are evaluated with simulation test set and experimental test set. The PC-NN has improved overall classification performance from NN by about 5% points. The overall classification performance of the proposed networks is more than 90%. Thus, by utilizing the proposed neural network fault diagnosis system, a better understanding about fault behaviors, diagnostics, and detections of a multilevel inverter drive system can be accomplished. The results of this analysis are identified in percentage tabular form of faults and switch locations.

  • Kaiyu Wang; M. Bodson; J. Chiasson; L.M. Tolbert
    Proceedings of the 44th IEEE Conference on Decision and Control
    2005

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    A Hammerstein model is a system model in which the inputs go through a static nonlinearity followed by a linear time-invariant system. Often the static nonlinearity is modeled as a polynomial nonlinearity in the inputs or as a piecewise constant nonlinearity. Such models are nonlinear in the unknown parameters and therefore present a challenging identification problem. In this work, the authors show that elimination theory can be used to solve exactly for parameter values that minimize a least-square criterion. Thus, the approach guarantees the minimum can be found in a finite number of steps, unlike iterative methods that are currently used.

  • Mengwei Li; J. Chiasson; M. Bodson; L. Tolbert
    Proceedings of the 44th IEEE Conference on Decision and Control
    2005

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    This paper describes a new approach to estimating the speed of an induction motor from the measured terminal voltages and currents without the use of a speed/position sensor. The new observer uses a purely algebraic speed estimator to stabilize a dynamic speed estimator and it is shown that it has the potential to provide low speed (including zero speed) control of an induction motor under full rated load.

  • Kaiyu Wang; M. Bodson; J. Chiasson; L.M. Tolbert
    Proceedings of the 44th IEEE Conference on Decision and Control
    2005

    arrow_drop_down

    A Hammerstein model is a system model in which the inputs go through a static nonlinearity followed by a linear time-invariant system. Often the static nonlinearity is modeled as a polynomial nonlinearity in the inputs or as a piecewise constant nonlinearity. Such models are nonlinear in the unknown parameters and therefore present a challenging identification problem. In this work, the authors show that elimination theory can be used to solve exactly for parameter values that minimize a least-square criterion. Thus, the approach guarantees the minimum can be found in a finite number of steps, unlike iterative methods that are currently used.

  • Mengwei Li; J. Chiasson; M. Bodson; L. Tolbert
    Proceedings of the 44th IEEE Conference on Decision and Control
    2005

    arrow_drop_down

    This paper describes a new approach to estimating the speed of an induction motor from the measured terminal voltages and currents without the use of a speed/position sensor. The new observer uses a purely algebraic speed estimator to stabilize a dynamic speed estimator and it is shown that it has the potential to provide low speed (including zero speed) control of an induction motor under full rated load.

  • B. Ozpinecil; L.M. Tolbert; S.K. Islam
    Power Electronics in Transportation, 2002
    2002

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    The emergence of silicon carbide- (SiC-) based power semiconductor switches, with their superior features compared with silicon- (Si-) based switches, has resulted in substantial improvement in the performance of power electronics converter systems. These systems with SiC power devices have the qualities of being more compact, lighter, and more efficient; thus, they are ideal for high-voltage power electronics applications such as a hybrid electric vehicle (HEV) traction drive. More research is required to show the impact of SIC devices in power conversion systems. In this study, findings of SIC research at Oak Ridge National Laboratory (OWL), TN, USA, including SIC device design and system modeling studies, are discussed.

  • L.M. Tolbert; J. Chiasson; K. McKenzie; Zhong Du
    Power Electronics in Transportation, 2002
    2002

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    One promising technology to interface battery packs in electric and hybrid electric vehicles are multilevel converters. In the work presented here, it is shown how the switching times (angles) in a multilevel inverter can be chosen to achieve a required fundamental voltage and not generate specific higher order harmonics. The method gives a complete solution to the problem in that all possible solutions are found.

  • B. Ozpinecil; L.M. Tolbert; S.K. Islam
    Power Electronics in Transportation, 2002
    2002

    arrow_drop_down

    The emergence of silicon carbide- (SiC-) based power semiconductor switches, with their superior features compared with silicon- (Si-) based switches, has resulted in substantial improvement in the performance of power electronics converter systems. These systems with SiC power devices have the qualities of being more compact, lighter, and more efficient; thus, they are ideal for high-voltage power electronics applications such as a hybrid electric vehicle (HEV) traction drive. More research is required to show the impact of SIC devices in power conversion systems. In this study, findings of SIC research at Oak Ridge National Laboratory (OWL), TN, USA, including SIC device design and system modeling studies, are discussed.

  • L.M. Tolbert; J. Chiasson; K. McKenzie; Zhong Du
    Power Electronics in Transportation, 2002
    2002

    arrow_drop_down

    One promising technology to interface battery packs in electric and hybrid electric vehicles are multilevel converters. In the work presented here, it is shown how the switching times (angles) in a multilevel inverter can be chosen to achieve a required fundamental voltage and not generate specific higher order harmonics. The method gives a complete solution to the problem in that all possible solutions are found.

  • A. Rodriguez; H. Nour; F. Wang; S.J. Dale
    1985 EIC 17th Electrical/Electronics Insulation Conference
    1985

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    Contamination on power system insulation can cause flashovers and subsequent outages when moisture deposits on the surface of the insulators to form electrolytes. Fog and light drizzle have been observed to be the most conducive form of moisture leading to flashovers; whereas, rain will wash off most of the soluble contaminants on the insulation's surface. The initial wetting is therefore a critical stage in the flashover process of contaminated insulators. This paper describes flashover tests made in a fog chamber on dc energized suspension insulators. The test insulators were preheated and the effect of the insulator's relative surface temperature on the flashover voltage was noted. The test results indicate that if the insulator's body heat can keep the surface sufficiently dry through this critical wetting stage, there can be a significant increase in the flashover voltage: as high as four times that of an insulator with a surface temperature equal to the air temperature and under the same contamination and fog conditions. This finding could be applied to abate contamination problems on insulation at severe pollution sites. To generate the required heating, an impedance element can be embedded in an insulator with a castable dielectric body. Calculations indicate that the energy necessary to obtain this internal body heating effect will be comparable to the energy losses due to leakage current and partial arcing caused by the contaminant layer.

  • A. Rodriguez; H. Nour; F. Wang; S.J. Dale
    1985 EIC 17th Electrical/Electronics Insulation Conference
    1985

    arrow_drop_down

    Contamination on power system insulation can cause flashovers and subsequent outages when moisture deposits on the surface of the insulators to form electrolytes. Fog and light drizzle have been observed to be the most conducive form of moisture leading to flashovers; whereas, rain will wash off most of the soluble contaminants on the insulation's surface. The initial wetting is therefore a critical stage in the flashover process of contaminated insulators. This paper describes flashover tests made in a fog chamber on dc energized suspension insulators. The test insulators were preheated and the effect of the insulator's relative surface temperature on the flashover voltage was noted. The test results indicate that if the insulator's body heat can keep the surface sufficiently dry through this critical wetting stage, there can be a significant increase in the flashover voltage: as high as four times that of an insulator with a surface temperature equal to the air temperature and under the same contamination and fog conditions. This finding could be applied to abate contamination problems on insulation at severe pollution sites. To generate the required heating, an impedance element can be embedded in an insulator with a castable dielectric body. Calculations indicate that the energy necessary to obtain this internal body heating effect will be comparable to the energy losses due to leakage current and partial arcing caused by the contaminant layer.