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Leon Tolbert

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Office: Min Kao 401C
E-mail:
ude.ktu@treblot
Phone: 865-974-8147
Fax: 865-974-5483
Address: Min H. Kao Building, Suite 401C
1520 Middle Drive
Knoxville, TN 37996-2250

Biography

Leon M. Tolbert is the Department Head of the Electrical Engineering and Computer Science Department. He received his Bachelor's in Electrical Engineering with highest honors in 1989 and his M.S. in Electrical Engineering in 1991 from the Georgia Institute of Technology, Atlanta.

He joined the Engineering Division of Lockheed Martin Energy Systems in 1991 and worked on several electrical distribution and power quality projects at the three U.S. Department of Energy plants in Oak Ridge, Tennessee. In 1997, he became a Research Engineer in the Power Electronics and Electric Machinery Research Center (PEEMRC) in the Engineering Science and Technology Division at the Oak Ridge National Laboratory. Some of his projects included thermal and efficiency modeling for hybrid electric vehicles, development of multilevel inverter PWM methods, and testing methods to approximate electric machine efficiency.

In 1999, he received his Ph.D. in Electrical Engineering from the Georgia Institute of Technology. He was appointed as an assistant professor in the Department of Electrical and Computer Engineering at The University of Tennessee in Knoxville in 1999. He does research in the areas of electric power conversion, SiC power devices, multilevel converters, hybrid electric vehicles, and power quality. He is also a participating faculty member of the Graduate Automotive Technology Education (GATE) Center at UT.

Dr. Tolbert is an adjunct participant at the Oak Ridge National Laboratory and conducts joint research at the National Transportation Research Center (NTRC). He is a Registered Professional Engineer in the state of Tennessee, a Senior Member of the IEEE, and a member of the IEEE Industry Applications Society, IEEE Power Electronics Society, and IEEE Power Engineering Society. He is an Associate Editor of the IEEE Power Electronics Letters and the Educational Activities Chairman of the IEEE Power Electronics Society. He is the recipient of a National Science Foundation CAREER Award and the 2001 IEEE Industry Applications Society Outstanding Young Member Award. He has received the following awards at The University of Tennessee: 2003, 2007, 2012, 2013 Engineering Research Fellow Award, Chancellor's Citation for Professional Promise in Research and Creative Achievement in 2003, 2004 Gonzalez Family Faculty Excellence Award in Research, and the Weston Fulton Award for Excellence in Research and Teaching in 2001.

Publications


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Journal Papers
Title
Year
  • 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.

  • 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.

  • 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.

  • 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.

  • Yiwei Ma; Wenchao Cao; Liu Yang; Fei (Fred) 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • Xiaojie Shi; Zhiqiang (Jack) 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.

  • 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).

  • 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.

  • 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.

  • 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 (Fred) 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.

  • 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.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fei Wang; Benjamin J. Blalock
    IEEE Transactions on Industry Applications
    2015

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    This paper presents the paralleling operation of three-phase current-source rectifiers (CSRs) as the front-end power conversion stage of data center power supply systems based on 400-Vdc power delivery architecture, which has been proven to have higher efficiency than traditional ac architectures. A control algorithm of paralleled three-phase CSRs is introduced to achieve balanced outputs and individual rectifier module hot swap, which are required by power supply systems. By using silicon carbide (SiC) power semiconductors, SiC MOSFETs, and Schottky diodes, the power losses of the front-end stage are reduced, and the power supply system efficiency can be further increased. The prototype of a 19-kW front-end rectifier to convert 480 Vac,rms to 400 Vdc, based on three paralleled three-phase CSRs, is developed. Each CSR is an all-SiC converter and designed for high efficiency, and the front-end stage full-load efficiency is greater than 98% from experimental tests. The balanced outputs and individual converter hot swap are realized in the hardware prototype too.

  • 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.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei (Fred) 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.

  • 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.

  • Xiaojie Shi; Bo Liu; Zhiqiang Wang; Yalong Li; Leon M. Tolbert; Fei Wang
    IEEE Transactions on Industrial Electronics
    2015

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    Featuring modularity and high efficiency, a modular multilevel converter (MMC) has become a promising topology in high-voltage direct-current transmission systems. However, its distributed capacitors lead to a more complicated startup process than that of a two-level converter. To fully understand this issue, the charging loops of an MMC rectifier and an MMC inverter during an uncontrolled precharge period are analyzed in this paper, with special focus on the necessity of additional capacitor charging schemes. Moreover, a small-signal model of a capacitor charging loop is first derived according to the internal dynamics of the MMC inverter. Based on this model, a novel startup strategy incorporating an averaging capacitor voltage loop and a feedforward control is proposed, which is capable of an enhanced dynamic response and system stability without sacrificing voltage control precision. The design considerations of the control strategy are also given in detail. Simulation results from a back-to-back MMC system supplying passive loads and experimental results from a scaled-down MMC prototype are provided to support the theoretical analysis and the proposed control scheme.

  • 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.

  • Bailu Xiao; Lijun Hang; Jun Mei; Cameron Riley; Leon M. Tolbert; Burak Ozpineci
    IEEE Transactions on Industry Applications
    2015

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    This paper presents a modular cascaded H-bridge multilevel photovoltaic (PV) inverter for single- or three-phase grid-connected applications. The modular cascaded multilevel topology helps to improve the efficiency and flexibility of PV systems. To realize better utilization of PV modules and maximize the solar energy extraction, a distributed maximum power point tracking control scheme is applied to both single- and three-phase multilevel inverters, which allows independent control of each dc-link voltage. For three-phase grid-connected applications, PV mismatches may introduce unbalanced supplied power, leading to unbalanced grid current. To solve this issue, a control scheme with modulation compensation is also proposed. An experimental three-phase seven-level cascaded H-bridge inverter has been built utilizing nine H-bridge modules (three modules per phase). Each H-bridge module is connected to a 185-W solar panel. Simulation and experimental results are presented to verify the feasibility of the proposed approach.

  • Lakshmi Reddy GopiReddy; Leon M. Tolbert; Burak Ozpineci; João O. P. Pinto
    IEEE Transactions on Industry Applications
    2015

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    Rainflow algorithms are one of the popular counting methods used in fatigue and failure analysis in conjunction with 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 in this paper. A month-long arc furnace load profile is used as a test profile to estimate temperatures in insulated-gate bipolar transistors (IGBTs) 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.

  • 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.

  • 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.

  • 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.

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

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    Overcurrent protection of silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) remains a challenge due to lack of practical knowledge. This paper presents three overcurrent protection methods to improve the reliability and overall cost of SiC MOSFET-based converters. 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. Third, a novel active overcurrent protection scheme through dynamic evaluation of fault current level is proposed. 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 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.

  • Shengnan Li; Leon M. Tolbert; Fei (Fred) 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.

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

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    The number of offboard fast charging stations is increasing as plug-in electric vehicles (PEVs) are more widespread in the world. Additional features on the operation of chargers will result in more benefits for investors, utility companies, and PEV owners. This paper investigates reactive power support operation using offboard PEV charging stations while charging a PEV battery. The topology consists of a three-phase ac-dc boost rectifier that is capable of operating in all four quadrants. The operation modes that are of interest are power-factor-corrected charging operation, and charging and capacitive/inductive reactive power operation. This paper also presents a control system for the PQ command following of a bidirectional offboard charger. The controller only receives the charging power command from a user and the reactive power command (when needed) from a utility, and it adjusts the line current and the battery charging current correspondingly. The vehicle's battery is not affected during the reactive power operation. A simulation study is developed utilizing PSIM, and the control system is experimentally tested using a 12.5-kVA charging station design.

  • 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.

  • 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.

  • 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.

  • Ke Shen; Dan Zhao; Jun Mei; Leon M. Tolbert; Jianze Wang; Mingfei Ban; Yanchao Ji; Xingguo Cai
    IEEE Transactions on Industrial Electronics
    2014

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    A particle swarm optimization (PSO) algorithm-based staircase modulation strategy for modular multilevel converters (MMC) is proposed. To reduce switching losses and device stress, the staircase modulation method has been adopted in high-voltage and high-power energy conversion applications. In particular, the selection of the appropriate iterative initial values of switching angles is a significant step to realize a staircase modulated MMC. The proposed method is able to find the optimum initial values of switching angles, while it has the advantages of global optimization and quadratic convergence, which benefit from the PSO algorithm and Newton method, respectively. The paper presents analytical discussion of the voltage balancing control approach with rotation of switching angles. The main benefit of the efficient switching patterns is that the MMC has lower switching losses and minimum dV/dt stress. Simulation and experimental results are presented to verify the practical feasibility of the proposed scheme for the MMC.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei (Fred) 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • Based on the introduction of a dual-loop current control strategy for a grid-connected inverter, an averaged switching model of a grid-connected inverter with an LCL-filter in discrete domain is built under a stationary frame and a proportional resonant (PR) regulator is adopted in the current loop to track the given fundamental sinusoidal current. The impacts of PR parameters, LCL parameters and digital delay on the root locus are studied, respectively. The parameters of the LCL-filter and the PR current regulator loop are designed to assure system stability and dynamic response during a wide power range by using the pole placements method. Finally, a 10 kW prototype of a grid-connected inverter with an LCL-filter is set up to verify the effectiveness, the practicality and robustness of the proposed PR current design method.

  • 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.

  • 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.

  • 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.

  • Burak Ozpineci; Leon Tolbert
    IEEE Spectrum
    2011

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    Silicon has long been the semiconductor of choice for such power electronics. But soon this ubiquitous substance will have to share the spotlight. Devices made from silicon carbide (SiC)-a faster, tougher, and more efficient alternative to straight silicon-are beginning to take off. Simple SiC diodes have already started to supplant silicon devices in some applica tions. And over the last few years, they've been joined by the first commercially available SiC transistors, enabling anew range of SiC-based power electronics. What's more, SiC wafer manufacturers have steadily reduced the defects in the material while increasing the wafer size, thus driving down the prices of SiC devices. Last year, according to estimates made by wafer maker Cree, the global market for silicon car bide devices topped US $100 million for the first time.

  • 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).

  • 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 po- er 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.

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

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    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.

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

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    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.

  • 2010

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    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.

  • 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.

  • 2009

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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 with the converter at the same time. The modular construction of the MMCCC topology provides autotransformer-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 only this dc transformer (auto) 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 some faults, these redundant modules can be used to replace a faulty module to maintain an uninterrupted operation. Moreover, by obtaining a flexible conversion ratio, the MMCCC converter can transfer power in both directions. Thus, this MMCCC topology could be a solution to establish a power management system among multiple sources and loads having different operating voltages.

  • Burak Ozpineci; Madhu Sudhan Chinthavali; Leon M. Tolbert; Avinash S. Kashyap; H. Alan Mantooth
    IEEE Transactions on Industry Applications
    2009

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    Silicon carbide (SiC) power devices are expected to have an impact on power converter efficiency, weight, volume, and reliability. Currently, only SiC Schottky diodes are commercially available at relatively low current ratings. Oak Ridge National Laboratory has collaborated with Cree and Semikron to build a Si insulated-gate bipolar transistor-SiC Schottky diode hybrid 55-kW inverter by replacing the Si p-n diodes in Semikron's automotive inverter with Cree's made-to-order higher current SiC Schottky diodes. This paper presents the developed models of these diodes for circuit simulators, shows inverter test results, and compares the results with those of a similar all-Si inverter.

  • Engin Ozdemir; Sule Ozdemir; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2009

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    This paper presents a fundamental-frequency-modulated diode-clamped multilevel inverter (DCMLI) scheme for a three-phase stand-alone photovoltaic (PV) system. The system consists of five series-connected PV modules, a six-level 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 using a small-scale laboratory prototype are also presented. The results show the feasibility of the fundamental frequency switching application in three-phase stand-alone PV power systems.

  • Zhong Du; Leon M. Tolbert; Burak Ozpineci; John N. Chiasson
    IEEE Transactions on Power Electronics
    2009

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    This paper presents a cascaded H-bridge multilevel inverter that can be implemented using only a single dc power source and capacitors. Standard cascaded multilevel inverters require n dc sources for 2n + 1 levels. Without requiring transformers, the scheme proposed here allows the use of a single dc power source (e.g., a battery or a fuel cell stack) with the remaining n-1 dc sources being capacitors, which is referred to as hybrid cascaded H-bridge multilevel inverter (HCMLI) in this paper. It is shown that the inverter can simultaneously maintain the dc voltage level of the capacitors and choose a fundamental frequency switching pattern to produce a nearly sinusoidal output. HCMLI using only a single dc source for each phase is promising for high-power motor drive applications as it significantly decreases the number of required dc power supplies, provides high-quality output power due to its high number of output levels, and results in high conversion efficiency and low thermal stress as it uses a fundamental frequency switching scheme. This paper mainly discusses control of seven-level HCMLI with fundamental frequency switching control and how its modulation index range can be extended using triplen harmonic compensation.

  • 2009

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    This paper presents the various configurations of a multilevel modular capacitor-clamped converter (MMCCC), and it reveals many useful and new formations of the original MMCCC for transferring power in either an isolated or nonisolated manner. The various features of the original MMCCC circuit are best suited for a multibus system in future plug-in hybrid or fuel-cell-powered vehicles' drive train. The original MMCCC is capable of bidirectional power transfer using multilevel modular structure with capacitor-clamped topology. It has a nonisolated structure, and it offers very high efficiency even at partial loads. This circuit was modified to integrate single or multiple high-frequency transformers by using the intermediate voltage nodes of the converter. On the other hand, a special formation of the MMCCC can exhibit dc outputs offering limited isolation without using any isolation transformer. This modified version can produce a high conversion ratio from a limited number of components and has several useful applications in providing power to multiple low-voltage loads in a hybrid or electric automobile. This paper will investigate the origin of generating ac outputs from the MMCCC and shows how the transformer-free version can be modified to create limited isolation from the circuit. In addition, this paper will compare various modified forms of the MMCCC topology with existing dc-dc converter circuits from compactness and component utilization perspectives.

  • Zhong Du; Burak Ozpineci; Leon M. Tolbert; John N. Chiasson
    IEEE Transactions on Industry Applications
    2009

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    This paper presents a cascaded H-bridge multilevel boost inverter for electric vehicle (EV) and hybrid EV (HEV) applications implemented without the use of inductors. Currently available power inverter systems for HEVs use a dc-dc boost converter to boost the battery voltage for a traditional three-phase inverter. The present HEV traction drive inverters have low power density, are expensive, and have low efficiency because they need a bulky inductor. A cascaded H-bridge multilevel boost inverter design for EV and HEV applications implemented without the use of inductors is proposed in this paper. Traditionally, each H-bridge needs a dc power supply. The proposed design uses a standard three-leg inverter (one leg for each phase) and an H-bridge in series with each inverter leg which uses a capacitor as the dc power source. A fundamental switching scheme is used to do modulation control and to produce a five-level phase voltage. Experiments show that the proposed dc-ac cascaded H-bridge multilevel boost inverter can output a boosted ac voltage without the use of inductors.

  • Miaosen Shen; Fang Zheng Peng; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2008

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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.

  • Zhong Du; Leon M. Tolbert; John N. Chiasson; Burak Ozpineci
    IEEE Transactions on Industrial Electronics
    2008

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    This paper presents a reduced switching-frequency active-harmonic-elimination method (RAHEM) to eliminate any number of specific order harmonics of multilevel converters. First, resultant theory is applied to transcendental equations to eliminate low-order harmonics and to determine switching angles for a fundamental frequency-switching scheme. Next, based on the number of harmonics to be eliminated, Newton climbing method is applied to transcendental equations to eliminate high-order harmonics and to determine switching angles for the fundamental frequency-switching scheme. Third, the magnitudes and phases of the residual lower order harmonics are computed, generated, and subtracted from the original voltage waveform to eliminate these low-order harmonics. Compared to the active-harmonic-elimination method (AHEM), which generates square waves to cancel high-order harmonics, RAHEM has lower switching frequency. The simulation results show that the method can effectively eliminate all the specific harmonics, and a low total harmonic distortion (THD) near sine wave is produced. An experimental 11-level H-bridge multilevel converter with a field-programmable gate-array controller is employed to experimentally validate the method. The experimental results show that RAHEM does effectively eliminate any number of specific harmonics, and the output voltage waveform has low switching frequency and low THD.

  • Kaiyu Wang; John Chiasson; Marc Bodson; Leon M. Tolbert
    IEEE Transactions on Control Systems Technology
    2007

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    Indirect field-oriented control of an induction machine requires knowledge of the rotor time constant to estimate the rotor flux linkages. Here, an online method is presented for estimating the rotor time constant and the stator resistance, both of which vary during operation of the machine due to ohmic heating. The method uses measurements of the stator voltages, stator currents, and their derivatives (first derivative of the voltages and both the first and second derivatives of the currents). The problem is formulated as finding those parameter values that best fit (in a least-squares sense) the model of the induction motor to the measured output data of the motor. This method guarantees that the parameter values are found in a finite number of steps. Experimental results of an online implementation are presented

  • X. Yu; M. R. Starke; L. M. Tolbert; B. Ozpineci
    IET Electric Power Applications
    2007

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    Fuel cells are considered to be one of the most promising sources of distributed energy because of their high efficiency, low environmental impact and scalability. Unfortunately, multiple complications exist in fuel cell operation. Fuel cells cannot accept current in the reverse direction, do not perform well with ripple current, have a low output voltage that varies with age and current, respond sluggishly to step changes in load and are limited in overload capabilities. For these reasons, power converters are often necessary to boost and regulate the voltage as a means to provide a stiff applicable DC power source. Furthermore, the addition of an inverter allows for the conversion of DC power to AC for an utility interface or for the application of an AC motor. To help motivate the use of power conditioning for the fuel cell, a brief introduction of the different types, applications and typical electrical characteristics of fuel cells is presented. This is followed by an examination of the various topologies of DC-DC boost converters and inverters used for power conditioning of fuel cells. Several architectures to aggregate multiple fuel cells for high-voltage/high-power applications are also reviewed.

  • Y. Xu; L. M. Tolbert; J. N. Chiasson; J. B. Campbell; F. Z. Peng
    IET Electric Power Applications
    2007

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    A generalised instantaneous non-active power theory is presented. Comprehensive definitions of instantaneous active and non-active currents, as well as instantaneous, average and apparent powers, are proposed. These definitions have flexible forms that are applicable to different power systems, such as single-phase or multi-phase, periodic or non-periodic and balanced or unbalanced systems. By changing the averaging interval and the reference voltage, various non-active power theories can be derived from this theory. The definitions of instantaneous active and non- active currents provide an algorithm for a STATCOM to calculate the non-active current in the load current. The theory is implemented by the STATCOM, and four cases (three-phase balanced RL load, three-phase unbalanced RL load, diode rectifier load and single-phase load) are tested. The experimental results show that the STATCOM can perform instantaneous non-active power compensation, and both the fundamental non-active component and the harmonics are eliminated from the utility so that nearly unity power factor can be achieved. The STATCOM also has a fast dynamic response for transients.

  • Wenjuan Zhang; Fangxing Li; Leon M. Tolbert
    IEEE Transactions on Power Systems
    2007

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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. 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. Thirdly, the optimization-based models will be categorized as conventional algorithms, intelligent searches, and fuzzy set applications. The conventional algorithms include linear programming, nonlinear programming, mixed-integer nonlinear programming, etc. The intelligent searches include simulated annealing, evolutionary algorithms, and tabu search. 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.

  • Jeremy B. Campbell; Leon M. Tolbert; Curt W. Ayers; Burak Ozpineci; Kirk T. Lowe
    IEEE Transactions on Industry Applications
    2007

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    This paper presents a two-phase cooling method using the R134a refrigerant to dissipate the heat energy (loss) generated by power electronics (PEs), such as those associated with rectifiers, converters, and inverters for a specific application in hybrid-electric vehicles. The cooling method involves submerging PE devices in an R134a bath, which limits the junction temperature of PE devices while conserving weight and volume of the heat sink without sacrificing equipment reliability. First, experimental tests that included an extended soak for more than 850 days were performed on a submerged insulated gate bipolar transistor (IGBT) and gate-controller card to study dielectric characteristics, deterioration effects, and heat-flux capabilities of R134a. Results from these tests illustrate that R134a has high dielectric characteristics and no deterioration of electrical components. Second, experimental tests that included a simultaneous operation with a mock automotive air-conditioner (A/C) system were performed on the same IGBT and gate-controller card. Data extrapolation from these tests determined that a typical automotive A/C system has more than sufficient cooling capacity to cool a typical 30-kW traction inverter. Last, a discussion and simulation of active cooling of the IGBT junction layer with the R134a refrigerant is given. This technique will drastically increase the forward current ratings and reliability of the PE device

  • Surin Khomfoi; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2007

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    A fault diagnostic and reconfiguration method for a cascaded H-bridge multilevel inverter drive (MLID) using artificial-intelligence-based techniques is proposed in this paper. Output phase voltages of the MLID are used as diagnostic signals to detect faults and their locations. It is difficult to diagnose an 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 (NN) classification is applied to the fault diagnosis of an MLID system. Multilayer perceptron networks are used to identify the type and location of occurring faults. The principal component analysis is utilized in the feature extraction process to reduce the NN input size. A lower dimensional input space will also usually reduce the time necessary to train an NN, and the reduced noise can improve the mapping performance. The genetic algorithm is also applied to select the valuable principal components. The proposed network is evaluated with simulation test set and experimental test set. The overall classification performance of the proposed network is more than 95%. A reconfiguration technique is also proposed. The proposed fault diagnostic system requires about six cycles to clear an open-circuit or short-circuit fault. The experimental results show that the proposed system performs satisfactorily to detect the fault type, fault location, and reconfiguration.

  • Mengwei Li Campbell; John Chiasson; Marc Bodson; Leon M. Tolbert
    IEEE Transactions on Automatic Control
    2007

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    A method is proposed 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

  • Faisal H. Khan; Leon M. Tolbert
    IEEE Transactions on Industry Applications
    2007

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    A novel topology of a multilevel modular capacitor-clamped dc-dc converter will be presented in this paper. In contrast to the conventional flying capacitor multilevel dc-dc converter, 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, low on-state voltage drop, and bidirectional power flow management. This paper discusses the construction and operation of the new converter along with a comparison to a conventional converter. Finally, simulation and experimental results are used to validate the concept of this new topology.

  • Surin Khomfoi; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2007

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    In this paper, a fault diagnostic system in a multilevel-inverter using a neural network is developed. 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. Five 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 classification performance of the proposed network between normal and abnormal condition is about 90%, and the classification performance among fault features is about 85%. Thus, by utilizing the proposed neural network fault diagnostic 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

  • Zhong Du; L. M. Tolbert; J. N. Chiasson
    IEEE Transactions on Power Electronics
    2006

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    This paper presents an active harmonic elimination method to eliminate any number of specific higher order harmonics of multilevel converters with equal or unequal dc voltages. First, resultant theory is applied to transcendental equations characterizing the harmonic content to eliminate low order harmonics and to determine switching angles for the fundamental frequency switching scheme and a unipolar switching scheme. Next, the residual higher order harmonics are computed and subtracted from the original voltage waveform to eliminate them. The simulation results show that the method can effectively eliminate the specific harmonics, and a low total harmonic distortion (THD) near sine wave is produced. An experimental 11-level H-bridge multilevel converter with a field programmable gate array controller is employed to implement the method. The experimental results show that the method does effectively eliminate any number of specific harmonics, and the output voltage waveform has low THD.

  • Miaosen Shen; Jin Wang; A. Joseph; Fang Zheng Peng; L. M. Tolbert; D. J. Adams
    IEEE Transactions on Industry Applications
    2006

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    This paper proposes two constant boost-control methods for the Z-source inverter, which can obtain maximum voltage gain at any given modulation index without producing any low-frequency ripple that is related to the output frequency and minimize the voltage stress at the same time. Thus, the Z-network requirement will be independent of the output frequency and determined only by the switching frequency. The relationship of voltage gain to modulation index is analyzed in detail and verified by simulation and experiments.

  • Mengwei Li; J. Chiasson; M. Bodson; L. M. Tolbert
    IEEE Transactions on Automatic Control
    2006

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    This note considers a differential-algebraic approach to estimating the speed of an induction motor from the measured terminal voltages and currents. In particular, it is shown that the induction motor speed ω satisfies both a second- and a third-order polynomial equation whose coefficients depend on the stator voltages, stator currents, and their derivatives. It is shown that as long as the stator electrical frequency is nonzero, the speed is uniquely determined by these polynomials. The speed so determined is then used to stabilize a dynamic (Luenberger type) observer to obtain a smoother speed estimate. With full knowledge of the machine parameters and filtering of the sensor noise, simulations indicate that this estimator has the potential to provide low speed (including zero speed) control of an induction motor under full rated load.

  • L. M. Tolbert; J. N. Chiasson; Zhong Du; K. J. McKenzie
    IEEE Transactions on Industry Applications
    2005

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    Eliminating harmonics in a multilevel converter in which the separate dc sources vary is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. Assuming that the separate dc sources can be measured, a procedure is given to find all sets of switching angles for which the fundamental is produced while lower order harmonics are eliminated. This is done by first converting the transcendental equations that specify the elimination of the harmonics into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. Experimental results are presented to validate the theory.

  • B. Ozpineci; L. M. Tolbert; J. N. Chiasson
    IEEE Power Electronics Letters
    2005

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    In this letter, a genetic algorithm (GA) optimization technique is applied to determine the switching angles for a cascaded multilevel inverter which eliminates specified higher order harmonics while maintaining the required fundamental voltage. This technique can be applied to multilevel inverters with any number of levels. As an example, in this paper a seven-level inverter is considered, and the optimum switching angles are calculated offline to eliminate the fifth and seventh harmonics. These angles are then used in an experimental setup to validate the results.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Control Systems Technology
    2005

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    A method is presented to compute the switching angles in a multilevel converter so as to produce the required fundamental voltage while at the same time not generate higher order harmonics. Previous work has shown that the transcendental equations characterizing the harmonic content can be converted to polynomial equations which are then solved using the method of resultants from elimination theory. A difficulty with this approach is that when there are several dc sources, the degrees of the polynomials are quite large making the computational burden of their resultant polynomials (as required by elimination theory) quite high. Here, it is shown that the theory of symmetric polynomials can be exploited to reduce the degree of the polynomial equations that must be solved which in turn greatly reduces the computational burden. In contrast to results reported in the literature that use iterative numerical techniques to solve these equations, the approach here produces all possible solutions.

  • Kaiyu Wang; J. Chiasson; M. Bodson; L. M. Tolbert
    IEEE Transactions on Automatic Control
    2005

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    A nonlinear least-squares method is presented for the identification of the induction motor parameters. A major difficulty with the induction motor is that the rotor state variables are not available measurements so that the system identification model cannot be made linear in the parameters without overparametrizing the model. Previous work in the literature has avoided this issue by making simplifying assumptions such as a "slowly varying speed." Here, no such simplifying assumptions are made. The problem is formulated as a nonlinear least-squares identification problem and uses elimination theory (resultants) to compute the parameter vector that minimizes the residual error. The only requirement is that the system must be sufficiently excited. The method is suitable for online operation to continuously update the parameter values. Experimental results are presented.

  • F. Z. Peng; Gui-Jia Su; L. M. Tolbert
    IEEE Transactions on Power Electronics
    2004

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    This paper presents a regenerative passive snubber circuit for pulse-width modulation (PWM) inverters to achieve soft-switching purposes without significant cost and reliability penalties. This passive soft-switching snubber (PSSS) employs a diode/capacitor snubber circuit for each switching device in an inverter to provide low dv/dt and low switching losses to the device. The PSSS further uses a transformer-based energy regenerative circuit to recover the energy captured in the snubber capacitors. All components in the PSSS circuit are passive, thus leading to reliable and low-cost advantages over those soft-switching schemes relying on additional active switches. The snubber has been incorporated into a 150 kVA PWM inverter. Simulation and experimental results are given to demonstrate the validity and features of the snubber circuit.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Power Electronics
    2004

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    The problem of eliminating harmonics in a switching converter is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. In contrast to the well known work of Patel and Hoft and others, here all possible solutions to the problem are found. This is done by first converting the transcendental equations that specify the harmonic elimination problem into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. In particular, it is shown that there are new solutions that have not been previously reported in the literature. The complete solutions for both unipolar and bipolar switching patterns to eliminate the fifth and seventh harmonics are given. Finally, the unipolar case is again considered where the fifth, seventh, 11th, and 13th harmonics are eliminated along with corroborative experimental results.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Power Electronics
    2004

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    A method is presented to compute the switching angles in a multilevel converter so as to produce the required fundamental voltage while at the same time not generate higher order harmonics. Using a staircase fundamental switching scheme, previous work has shown that this is possible only for specific ranges of the modulation index. Here it is shown that, by considering all possible switching schemes, one can extend the lower range of modulation indices for which such switching angles exist. A unified approach is presented to solve the harmonic elimination equations for all of the various switching schemes. In particular, it is shown that all such schemes require solving the same set of equations where each scheme is distinguished by the location of the roots of the harmonic elimination equations. In contrast to iterative numerical techniques, the approach here produces all possible solutions.

  • L. M. Tolbert; W. A. Peterson; T. J. Theiss; M. B. Scudiere
    IEEE Industry Applications Magazine
    2003

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    This article describes a proof-of-concept development for a 7.5 kW gen-set in a family of military gen-sets in the 5 to 60 kW range. Several power electronic converters have been implemented in the design of this proof-of-concept gen-set that is lighter, smaller, and more fuel efficient than conventional fixed-speed gen-sets. The unit also has the flexibility of several selectable voltage and frequency options that greatly reduces the inventory logistics burden for a family of different unit types and sizes. With integrated controls and power electronics, the gen-set is able to run at its most efficient condition to minimize fuel consumption. It would also be possible to control the gen-set such that it ran at other different optimized conditions such as: maximum headroom for supplying load transients; most audibly quiet; and least polluting for certain emissions.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Control Systems Technology
    2003

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    In this work, a method is given to compute the switching angles in a multilevel converter to produce the required fundamental voltage while at the same time cancel out specified higher order harmonics. Specifically, a complete analysis is given for a seven-level converter (three dc sources), where it is shown that for a range of the modulation index mI, the switching angles can be chosen to produce the desired fundamental V1=mI(s4Vdc/π) while making the fifth and seventh harmonics identically zero.

  • Yan Xu; L. M. Tolbert; F. Z. Peng; J. N. Chiasson; Jianqing Chen
    IEEE Power Electronics Letters
    2003

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    This paper presents a new definition of nonactive current from which the definitions of instantaneous active and nonactive power are also derived. The definitions are consistent with the traditional power definitions and valid for single-phase and polyphase systems, as well as periodic and nonperiodic waveforms. The definitions are applied to a shunt compensation system. The paper elaborates on the compensation of three different cases of nonperiodic current: single-phase disturbance, three-phase subharmonics, and three-phase stochastic current. Simulation results give credibility to the applicability of the definition for a diversity of load currents. According to different compensation cases and the goals to be achieved, different averaging time intervals for the compensator are chosen which will determine the compensator's energy storage requirement and the extent of residual distortion in the source current.

  • B. Ozpineci; L. M. Tolbert
    IEEE Power Electronics Letters
    2003

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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. In this study, commercial Si pn and SiC Schottky diodes are tested and characterized, their behavioral static and loss models are derived at different temperatures, and they are compared with respect to each other.

  • L. A. Tolbert; Fang Zheng Peng; T. Cunnyngham; J. N. Chiasson
    IEEE Transactions on Industrial Electronics
    2002

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    This paper presents transformerless multilevel converters as an application for high-power hybrid electric vehicle (HEV) motor drives. Multilevel converters: (1) can generate near-sinusoidal voltages with only fundamental frequency switching; (2) have almost no electromagnetic interference or common-mode voltage; and (3) make an HEV more accessible/safer and open wiring possible for most of an HEV's power system. The cascade inverter is a natural fit for large automotive hybrid electric drives because it uses several levels of DC voltage sources, which would be available from batteries, ultracapacitors, or fuel cells. Simulation and experimental results show how to operate this converter in order to maintain equal charge/discharge rates from the DC sources (batteries, capacitors, or fuel cells) in an HEV.

  • L. M. Tolbert; Fang Zheng Peng; T. G. Habetler
    IEEE Transactions on Power Electronics
    2000

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    When utilized at low amplitude modulation indices, existing multilevel carrier-based PWM strategies have no special provisions for this operating region, and several levels of the inverter go unused. This paper proposes some novel multilevel PWM strategies to take advantage of the multiple levels in both a diode-clamped inverter and a cascaded H-bridges inverter by utilizing all of the levels in the inverter even at low modulation indices. Simulation results show what effects the different strategies have on the active device utilization. A prototype 6-level diode-clamped inverter and an 11-level cascaded H-bridges inverter have been built and controlled with the novel PWM strategies proposed in this paper

  • L. M. Tolbert; Fang Zheng Peng; T. G. Habetler
    IEEE Transactions on Industry Applications
    2000

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    This paper presents the development of a control scheme for a multilevel diode-clamped converter connected in a series-parallel fashion to the electrical system such that it can compensate for deviations in utility voltage (sag, surge, and unbalance) and act as a harmonic and/or reactive current source for a load. New carrier-based multilevel pulsewidth modulation techniques are identified to maximize switch utilization of the two back-to-back diode-clamped inverters that constitute the universal power conditioner. An experimental verification for a six-level power conditioner is given

  • L. M. Tolbert; T. G. Habetler
    IEEE Transactions on Industry Applications
    1999

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    The advent of the transformerless multilevel inverter topology has brought forth various pulsewidth modulation (PWM) schemes as a means to control the switching of the active devices in each of the multiple voltage levels in the inverter. An analysis of how existing multilevel carrier-based PWM affects switch utilization for the different levels of a diode-clamped inverter is conducted. Two novel carrier-based multilevel PWM schemes are presented which help to optimize or balance the switch utilization in multilevel inverters. A 10 kW prototype six-level diode-clamped inverter has been built and controlled with the novel PWM strategies proposed in this paper to act as a voltage-source inverter for a motor drive

  • L. M. Tolbert; Fang Zheng Peng; T. G. Habetler
    IEEE Transactions on Industry Applications
    1999

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    This paper presents transformerless multilevel power converters as an application for high-power and/or high-voltage electric motor drives. Multilevel converters: (1) can generate near-sinusoidal voltages with only fundamental frequency switching; (2) have almost no electromagnetic interference or common-mode voltage; and (3) are suitable for large voltampere-rated motor drives and high voltages. The cascade inverter is a natural fit for large automotive all-electric drives because it uses several levels of DC voltage sources, which would be available from batteries or fuel cells. The back-to-back diode-clamped converter is ideal where a source of AC voltage is available, such as in a hybrid electric vehicle. Simulation and experimental results show the superiority of these two converters over two-level pulsewidth-modulation-based drives

  • J. S. Hsu; J. D. Kueck; M. Olszewski; D. A. Casada; P. J. Otaduy; L. M. Tolbert
    IEEE Transactions on Industry Applications
    1998

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    Unlike testing motor efficiency in a laboratory, certain methods given in IEEE Standard 112 cannot be used for motor efficiency evaluations in the field. For example, it is difficult to load a motor in the field with a dynamometer when the motor is already coupled to driven equipment. The motor efficiency field evaluation faces a different environment from that for which IEEE Standard 112 is chiefly written. A field evaluation method consists of one or several basic methods. This paper separates and compares the basic methods according to their physical natures. Their intrusivenesses and accuracies are also discussed. This paper is useful for field engineers to select or to establish a proper efficiency evaluation method by understanding the theories and error sources of the methods. The basic methods discussed are: nameplate method; slip method; current method; statistical method; equivalent circuit method; segregated loss method; airgap torque method; and shaft torque method

  • L. M. Tolbert; L. J. Degenhardt; J. T. Cleveland
    IEEE Industry Applications Magazine
    1997

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    Lightning continues to be the major cause of outages on overhead power distribution lines. Through laboratory testing and field observations and measurements, the properties of a lightning stroke and its effects on electrical distribution system components are fairly well-understood phenomena. A meticulous compilation of 32 years of historical records has been kept for outage causes, duration, and locations for eight distribution feeders at the Oak Ridge National Laboratory (ORNL). Due to the limited growth of ORNL, the number, length, and location of the 13.8 kV overhead lines have remained the same between 1960 and 1992. Except for noted differences (voltage construction class, length, age, and maximum elevation above a reference level), other factors that could influence the reliability of an overhead line have remained nearly the same. This allowed a meaningful reliability study to be performed on the entire ORNL electrical distribution system. In this article, the main findings of the reliability assessment as it relates to lightning-resistant overhead line construction techniques are out-lined, and a simple and cost-effective method to reduce lightning caused outages is offered. In addition, comparisons are made between the failure rates and causes experienced at ORNL and those in industry surveys. Where large discrepancies exist between survey data and experiences at ORNL, evidence is presented to explain the differences between ORNL's distribution system and those typical of industry

  • T. G. Habetler; F. Profumo; M. Pastorelli; L. M. Tolbert
    IEEE Transactions on Industry Applications
    1992

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    A direct induction machine torque control method based on predictive, deadbeat control of the torque and flux is presented. By estimating the synchronous speed and the voltage behind the transient reactance, the change in torque and flux over the switching period is calculated. The stator voltage required to cause the torque and flux to be equal to their respective reference values is calculated. Space vector PWM is used to define the inverter switching state. An alternative approach to deadbeat control for use in the transient or pulse-dropping mode is also presented. An alternative modulation scheme is presented in which transient performance is improved by specifying the inverter switching states and then calculating the required switched instants to maintain deadbeat control of the flux while reducing the torque error during the entire switching interval. A similar approach is used for a transient in the flux. The implementation of the control scheme using DSP-based hardware is described, with complete experimental results given

Conference Papers
Title
Year
  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • M. Chinthavali; O. C. Onar; S. L. Campbell; L. M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    So far, the charging functionality for vehicles has been integrated either into the traction drive system or to the dc-dc converters in plug-in electric vehicles (PEV). This study features a unique way of combining the wired and wireless charging functionalities with vehicle side boost converter and maintaining the isolation to provide a hybrid plug-in and wireless charging solution to the plug-in electric vehicle users. The proposed integrated charger combined with SiC technology shows the end-to-end and dc-to-dc system efficiencies of 85.9% and 88.9% for wireless charging mode, and 88.8% and 92.4% for the wired charging mode of operation.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • M. Chinthavali; O. C. Onar; S. L. Campbell; L. M. Tolbert
    2015 IEEE Transportation Electrification Conference and Expo (ITEC)
    2015

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    Integrated charger topologies that have been researched so far with dc-dc converters and the charging functionality have no isolation in the system. Isolation is an important feature that is required for user interface systems that have grid connections and therefore is a major limitation that needs to be addressed along with the integrated functionality. The topology proposed in this paper is a unique and a first of its kind topology that integrates a wireless charging system and the boost converter for the traction drive system. The new topology is also compared with an on-board charger system from a commercial electric vehicle (EV). The ac-dc efficiency of the proposed system is 85.1% and the specific power and power density of the onboard components is ~455 W/kg and ~320 W/l.

  • 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.

  • 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.

  • 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.

  • M. Chinthavali; O. C. Onar; S. L. Campbell; L. M. Tolbert
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    Integrated charger topologies that have been researched so far are with the dc-dc converters and the charging functionality usually have no isolation in the system. Isolation is an important feature that is required for user interface systems that have grid connections and therefore is a major limitation that needs to be addressed along with the integrated functionality. This study features a unique way of combining the wired and wireless charging functionalities with vehicle side boost converter integration and maintaining the isolation to provide the best solution to the plug-in electric vehicle (PEV) users. The new performance of the proposed architecture is presented for wired and wireless charging options at different power levels.

  • 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.

  • 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.

  • 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; 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.

  • 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.

  • 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.

  • C. W. Riley; L. M. Tolbert
    2015 IEEE Power & Energy Society General Meeting
    2015

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    In this paper, an I-V tracer which operates autonomously, with no need to be disconnected from its load, will be discussed. The current state of I-V tracers commercially available will be discussed and motivation will be provided for the online autonomous I-V tracer. Design of such an I-V tracer using the single-ended primary inductance converter (SEPIC) will be discussed, and simulation results of the converter operating as an I-V tracer will be presented. Analysis techniques of the I-V curve are also presented.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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; 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).

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.).

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • Hairong Qi; Yilu Liu; Fran Li; Jiajia Luo; Li He; Kevin Tomsovic; Leon Tolbert; Qing Cao
    2011 44th Hawaii International Conference on System Sciences
    2011

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    Energy infrastructure is a critical underpinning of modern society. To ensure its safe and healthy operation, a wide-area situational awareness system is essential to provide high-resolution understanding of the system dynamics such that proper actions can be taken in time in response to power system disturbances and to avoid cascading blackouts. This paper focusses on the high resolution or finer-scale analysis of data obtained through the North American frequency monitoring network (FNET) to reveal hidden information. In the power grid, events seldom occur in an isolated fashion. Cascading events and simultaneous events are more common and realistic. We present a new conceptual framework in event analysis, referred to as event unmixing, where we consider real-world events as a mixture of more than one constituent root event. This concept is a key enabler for the analysis of events to go beyond what are immediately detectable in the system. We interpret the event formation process from spectral mixing perspective and present innovative unsupervised unmixing algorithms to unravel complex events.

  • 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.

  • 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.

  • 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.

  • 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).

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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).

  • M. Chinthavali; L. M. Tolbert; H. Zhang; J. H. Han; F. Barlow; B. Ozpineci
    The 2010 International Power Electronics Conference - ECCE ASIA -
    2010

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    With efforts to reduce the cost, size, and thermal management systems for the power electronics drivetrain in hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), wide band gap semiconductors including silicon carbide (SiC) have been identified as possibly being a partial solution. Research on SiC power electronics has shown their higher efficiency compared to Si power electronics due to significantly lower conduction and switching losses. This paper focuses on the development of a high power module based on SiC JFETs and Schottky diodes. Characterization of a single device, a module developed using the same device, and finally an inverter built using the modules is presented. When tested at moderate load levels compared to the inverter rating, an efficiency of 98.2% was achieved by the initial prototype.

  • 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.

  • 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.

  • Seong Taek Lee; Timothy A. Burress; Leon M. Tolbert
    2009 IEEE International Electric Machines and Drives Conference
    2009

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    This paper introduces a new method for calculating the power factor and output torque by considering the cross saturation between direct-axis (d-axis) and quadrature-axis (q-axis) of an interior permanent magnet synchronous motor (IPMSM). The conventional two-axis IPMSM model is modified to include the cross saturation effect by adding the cross-coupled inductance terms. This paper also contains the new method of calculating the cross-coupled inductance values as well as self-inductance values in d- and q-axes. The analyzed motor is a high-speed brushless field excitation machine that offers high torque per ampere per core length at low speed and weakened flux at high speed, which was developed for the traction motor of a hybrid electric vehicle.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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; 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.

  • 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.

  • Christopher M. Navarro; Shawn D. Hampton; Jong Sung Lee; Nathan L. Tolbert; Terrence M. McLaren; James D. Myers; B. F. Spencer Jr.; Amr S. Elnashai
    2008 IEEE Fourth International Conference on eScience
    2008

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    MAEviz is an open-source project that helps reduce the time from discovery gap that exists between researchers, practitioners, and decision makers by integrating the latest research findings, most accurate data, and new methodologies into a single software product. MAEviz was developed as a platform to perform seismic risk assessment based on the Mid-America Earthquake (MAE) Center research in the consequence-based risk management (CRM) framework. The CRM paradigm provides a philosophical and practical framework for the assessment of the dynamic inter-disciplinary relationship between causes, effects, and effect mitigation for major events and disaster management. MAEviz provides the medium where "what if models and risk reduction strategies can be evaluated by both researchers and decision makers in an effort to avoid a catastrophic impact on society in the event of an earthquake.

  • Shawn D. Hampton; Jong Sung Lee; Nathan L. Tolbert; Terrence M. McLaren; Christopher M. Navarro; James D. Myers; B. F. Spencer Jr.; Amr S. Elnashai
    2008 IEEE Fourth International Conference on eScience
    2008

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    MAEviz is an open-source project that helps reduce the time from discovery gap that exists between researchers, practitioners, and decision makers by integrating the latest research findings, most accurate data, and new methodologies into a single software product. It was developed as a platform to perform seismic risk assessment based on the mid-america earthquake (MAE) center research in the consequence-based risk management (CRM) framework. MAEviz is built upon an open source, extensible software platform developed at NCSA using the eclipse rich client platform (RCP). The example shown in the poster is network-based seismic retrofit (NBSR) analysis. The analysis solves a typical problem faced by decision makers that, given a fixed budget, which combination of bridges and retrofit methods would minimize the societal cost of an earthquake. It clearly shows how new science can be put quickly into the hands of the decision makers, thus bridging the gap between research and practical application. MAEviz is shown to be a powerful tool that can currently be used to assist decision makers in preparing for and mitigating the consequences of seismic hazards. Moreover, the extensible architecture of MAEviz allows it to be easily adapted to integrate newly discovered science and data, both in the area of seismic risk assessment, as well as other future research areas.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • John S. Hsu; Seong-Taek Lee; Leon M. Tolbert
    2008 IEEE Industry Applications Society Annual Meeting
    2008

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    This paper introduces a new high-strength undiffused brushless machine that transfers the stationary excitation magnetomotive force to the rotor without any brushes. For a conventional permanent magnet (PM) machine, the air gap flux density cannot be enhanced effectively but can be weakened. In the new machine, both the stationary excitation coil and the PM in the rotor produce an enhanced air gap flux. The PM in the rotor prevents magnetic flux diffusion between the poles and guides the reluctance flux path. The pole flux density in the air gap can be much higher than what the PM alone can produce. A high-strength machine is thus obtained. The air gap flux density can be weakened through the stationary excitation winding. This type of machine is particularly suitable for electric and hybrid-electric vehicle applications. Patents of this new technology are either granted or pending.

  • 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.

  • 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.

  • Hui Zhang; Leon M. Tolbert; Burak Ozpineci
    2008 IEEE Industry Applications Society Annual Meeting
    2008

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    The application of SiC devices (as battery interface, motor controller, etc.) in a hybrid electric vehicle (HEV) will benefit from their high-temperature capability, high-power density, and high efficiency. Moreover, the light weight and small volume will affect the whole power train system in a HEV, and thus performance and cost. In this work, the performance of HEVs is analyzed using PSAT (powertrain system analysis tool, vehicle simulation software). Power loss models of a SiC inverter are incorporated into PSAT powertrain models in order to study the impact of SiC devices on HEVs. Two types of HEVs are considered. One is the 2004 Toyota Prius HEV, 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 the 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 the vehicles consume less energy and emit less harmful gases. It also makes it possible to improve the system compactness with simplified thermal management system. For the PHEV, the benefits are more distinct. Especially, the size of battery bank can be reduced for optimum design.

  • Faisal H. Khan; Leon M. Tolbert
    2007 IEEE Industry Applications Annual Meeting
    2007

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    A 5 kW multilevel modular capacitor clamped dc-dc converter (MMCCC) for future hybrid electric vehicle and fuel cell automotive applications will be presented in this paper. The modular structure of the MMCCC topology was utilized to build this 5 kW converter with high reliability and fault bypassing capability. Moreover, the circuit has flexible conversion ratio that leads to establish bi-directional power management for automotive applications. In addition, the MMCCC exhibits better component utilization compared to the well known flying capacitor dc-dc converter. Thus, the MMCCC circuit can be made more compact and reliable compared to many other capacitor clamped dc-dc converters for high power applications.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • Zhong Du; Burak Ozpineci; Leon M. Tolbert
    2007 IEEE Power Electronics Specialists Conference
    2007

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    This paper presents a modulation extension control algorithm for hybrid cascaded H-bridge multilevel converters. The hybrid cascaded H-bridge multilevel motor drive using only a single DC source for each phase is promising for high power motor drive applications since it can greatly decrease the number of required DC power supplies, has high quality output power due to its high number of output levels, and has high conversion efficiency and low thermal stress by using fundamental frequency switching scheme. But one disadvantage of the 7-level fundamental frequency switching scheme is that its modulation index range is too narrow when capacitor's voltage balance is maintained. The proposed modulation extension control algorithm can greatly increase capacitors' charging time and decrease the capacitors' discharging time by injecting triplen harmonics to extend the modulation index range of the hybrid cascaded H-bridge multilevel converters.

  • 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%.

  • 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.

  • 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.

  • 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.

  • 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.

  • Lakshmi R. Gopi Reddy; Leon M. Tolbert; Hui Zhang; Tom F. Cheek Jr.
    2007 IEEE Industry Applications Annual Meeting
    2007

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    A new electronic ballast circuit for High Intensity Discharge (HID) lamps to achieve an "ultra" high efficiency of 95% is proposed In this paper. The first stage of the ballast which Is the power factor correction boost converter has demonstrated an efficiency of >97% using SI MOSFET as the switch. The second stage of the ballast, a DC to high frequency Inverter has been designed to achieve >98% efficiency. Thus the overall efficiency of the ballast Is expected to be >95%. The performance of the "ultra high" efficient electronic ballast when SIC MOSFETS are used Is discussed In the paper. A loss model has been developed to compare the efficiency of the electronic ballast when using Si or SiC devices.

  • Zhong Du; Burak Ozpineci; Leon M. Tolbert; John N. Chiasson
    2007 IEEE Industry Applications Annual Meeting
    2007

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    This paper presents an inductorless cascaded H- bridge multilevel boost inverter for EV and HEV applications. Currently available power inverter systems for HEVs use a DC- DC boost converter to boost the battery voltage for a traditional 3-phase inverter. The present HEV traction drive inverters have low power density, are expensive, and have low efficiency because they need a bulky inductor. An inductorless cascaded H-bridge multilevel boost inverter for EV and HEV applications is proposed in this paper. Traditionally, each H-bridge needs a DC power supply. The proposed inductorless cascaded H-bridge multilevel boost inverter uses a standard 3-leg inverter (one leg for each phase) and an H-bridge in series with each inverter leg which uses a capacitor as the DC power source. Fundamental switching scheme is used to do modulation control and to produce a 5-level phase voltage. Experiments show that the proposed inductorless DC-AC cascaded H-bridge multilevel boost inverter can output a boosted AC voltage.

  • Surin Khomfoi; Leon M. Tolbertt; Burak Ozpineci
    2007 IEEE International Electric Machines & Drives Conference
    2007

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    The ability of cascaded H-bridge multilevel inverter drives (MLID) to operate under faulty condition including AI-based fault diagnosis and reconfiguration system is proposed in this paper. Output phase voltages of a MLID can be used as valuable information to diagnose faults and their locations. It is difficult to diagnose a 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 (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. A lower dimensional input space will also usually reduce the time necessary to train a NN, and the reduced noise may improve the mapping performance. The genetic algorithm (GA) is also applied to select the valuable principal components to train the NN. A reconfiguration technique is also proposed. The proposed system is validated with simulation and experimental results. The proposed 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 proposed 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

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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.

  • 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

  • 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

  • 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.

  • B. Ozpineci; M. S. Chinthavali; L. M. Tolbert; A. Kashyap; H. A. Mantooth
    Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
    2006

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    Silicon carbide (SiC) power devices are expected to have an impact on power converter efficiency, weight, volume, and reliability. Presently, only SiC Schottky diodes are commercially available at relatively low current ratings. Oak Ridge National Laboratory has collaborated with Cree and Semikron to build a Si IGBT-SiC Schottky diode hybrid 55kW inverter by replacing the Si pn diodes in Semikron's automotive inverter with Cree's made-to-order higher current SiC Schottky diodes. This paper presents the developed models of these diodes for circuit simulators, shows inverter test results, and compares the results to those of a similar all-Si inverter.

  • Mengwei Li; J. Chiasson; M. Bodson; L. M. Tolbert
    2006 American Control Conference
    2006

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    Previous work by the authors has described a differential-algebraic approach to speed estimation in induction motors. The method shows that the speed omega can be found by solving for the roots of a polynomial in omega whose coefficients are functions of the stator voltages, stator currents, and their derivatives. Preliminary experimental results are presented in which the speed is estimated using the differential-algebraic method

  • Zhong Du; L. M. Tolbert; J. N. Chiasson; B. Ozpineci
    Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
    2006

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    A method is presented showing that a cascade multilevel inverter can be implemented using only a single DC power source and capacitors. A standard cascade multilevel inverter requires n DC sources for 2n + 1 levels. Without requiring transformers, the scheme proposed here allows the use of a single DC power source (e.g., a battery or a fuel cell stack) with the remaining n-1 DC sources being capacitors. It is shown that one can simultaneously maintain the DC voltage level of the capacitors and choose a fundamental frequency switching pattern to produce a nearly sinusoidal output.

  • Surin Khomfoi; Leon M. Tolbert
    2006 IEEE Workshops on Computers in Power Electronics
    2006

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    A reconfiguration technique for multilevel inverters incorporating a diagnostic system based on 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 (NN) classification is applied to the fault diagnosis of a MLID system. Multilayer perceptron 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. A lower dimensional input space will also usually reduce the time necessary to train a NN, and the reduced noise may improve the mapping performance. The output phase voltage of a MLID can be used to diagnose the faults and their locations. The reconfiguration technique is also proposed. The effects of using the proposed reconfiguration technique at high modulation index are addressed. The proposed system is validated with experimental results. The experimental results show that the proposed system performs satisfactorily to detect the fault type, fault location, and reconfiguration

  • Fangxing Li; Wenjuan Zhang; Leon M. Tolbert; John D. Kueck; D. T. Rizy
    2006 IEEE PES Power Systems Conference and Exposition
    2006

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    The U.S. power industry is under great pressure to provide reactive power or VAr support. Although it is generally known that there are technical benefits for utilities and industrial customers to provide local reactive power support, a thorough quantitative investigation of the economic benefit is greatly needed. This paper seeks to provide a quantitative approach to evaluate the benefits from local reactive power compensation. This paper investigates the benefits including reduced losses, shifting reactive power flow to real power flow, and increased transfer capability. These benefits are illustrated with a simple two-bus model and then presented with a more complicated model using optimal power flow. Tests are conducted on a system with seven buses in two areas. These simulations show that the economic benefits can be significant, if compared with capacity payment to central generators or power factor penalties applied to utilities. This economic value may give utilities a better understanding of the Var benefits to assist their cost-benefit analysis for Var compensation. In addition, since the economic benefits are significant, this paper suggests that the future reactive power market should consider local Var providers

  • 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

  • Faisal H. Khan; Leon M. Tolbert; Fang Z. Peng
    2006 IEEE Workshops on Computers in Power Electronics
    2006

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    This paper will introduce the two basic switching cells, P-cell and N-cell, and their applications in different power electronic circuits. These basic cells have one switching element and one diode. The P-cell is the mirror circuit of the N-cell and vice-versa, and this paper suggests that any power electronic circuit can be analyzed and re-constructed using these basic switching cells. The present paper will describe the construction and operation of the basic switching cells, and it will also show a sequential method to construct several DC-DC converters from these basic switching cells. Moreover, considering the potential of combining these cells in multiple fashions, new kinds of power electronic circuits can be designed

  • 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

  • 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 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

  • Hui Zhang; Leon M. Tolbert; Burak Ozpineci
    2006 IEEE Workshops on Computers in Power Electronics
    2006

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    Most of the present models of silicon carbide (SiC) Schottky diodes are not suitable for evaluating their performance from a system level. The models presented in this paper are specialized for system-level simulations. They are based on basic semiconductor theories and synthesis of some models in the literature. Theoretical and experimental characterization of SiC Schottky power diodes is also involved. The models describe both static and dynamic behaviors of SiC Schottky power diodes. Thermal effects are considered as well for a better evaluation of power losses evaluation and cooling system design. The models were also used to estimate the efficiencies of Si IGBT/SiC Schottky diode hybrid inverter. To validate the simulation, a Si IGBT/SiC Schottky diode hybrid inverter and a Si IGBT inverter were built and tested

  • 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

  • Zhong Du; L. M. Tolbert; J. N. Chiasson
    Twentieth Annual IEEE Applied Power Electronics Conference and Exposition, 2005. APEC 2005.
    2005

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    This paper presents a modulation extension control method for multilevel converters with low switching frequency. The disadvantage of the fundamental frequency switching control method for multilevel converters is its narrow range of modulation indices where solutions exist. To address this problem, a triplen harmonic compensation method is proposed. First, the resultant method and/or Newton climbing method are used to find solutions of the switching angles for the fundamental frequency switching scheme control. Second, a triplen harmonic is injected into the multilevel converters accompanied with the fundamental frequency control signals to reduce the required level number of the DC voltages without changing the fundamental component of the phase voltage. The computational results show that the triplen harmonic method indeed reduces the required DC voltage level number to reduce the hardware cost. A 17-level example was implemented with an 11-level H-bridge multilevel converter and an 8 μs control resolution to demonstrate the triplen harmonic compensation method. The experimental results confirmed the method with this example.

  • 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.

  • Hairong Qi; Wenjuan Zhang; L. M. Tolbert
    Proceedings of the 13th International Conference on, Intelligent Systems Application to Power Systems
    2005

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    Reliable electricity has become an essential underpinning for national security in modern society. The fault-tolerant generic framework proposed herein can prevent potential outages from happening through intelligent agent coordination. Instead of limiting the system to manage existing devices, the developed system is adaptive to the future power grid in years to come. This paper proposes a hardware-in-the-loop multi-agent system with embedded models and functions

  • Wenjuan Zhang; L. M. Tolbert
    IEEE Power Engineering Society General Meeting, 2005
    2005

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    Reactive power planning (RPP) involves optimal allocation and determination of the types and sizes of the installed capacitors. Traditionally, the locations for placing new VAr sources were either simply estimated or directly assumed. Essentially, it is a large-scale nonlinear optimization problem with a large number of variables and uncertain parameters. there are no known ways to solve such nonlinear programming problems (NLP) exactly in a reasonable time. This paper introduces mathematical formulations and the advantages and disadvantages of nine categories of methods, which are divided into two groups, conventional and advanced optimization methods. The first group of methods is often trapped by a local optimal solution; the second one can guarantee the global optimum but needs more computing time. The corresponding development history period is also split into a conventional period and an artificial intelligence period. In the second period, there is a tendency to integrate several approaches to solve the RPP problem.

  • F. Z. Peng; L. M. Tolbert; F. Khan
    IEEE Workshop Power Electronics Education, 2005.
    2005

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    First Page of the Article

  • J. B. Campbell; L. M. Tolbert; C. W. Ayers; B. Ozpineci
    Twentieth Annual IEEE Applied Power Electronics Conference and Exposition, 2005. APEC 2005.
    2005

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    This paper presents a two-phase cooling method using R134a refrigerant to dissipate the heat energy (loss) generated by power electronics (PE) such as those associated with rectifiers, converters, and inverters for a specific application in hybrid-electric vehicles (HEVs). The cooling method involves submerging PE devices in an R134a bath, which limits the junction temperature of PE devices while conserving weight and volume of the heat sink without sacrificing equipment reliability. First, experimental tests that included an extended soak for more than 300 days were performed on a submerged IGBT and gate-controller card to study dielectric characteristics, deterioration effects, and heat flux capability of R134a. Results from these tests illustrate that R134a has high dielectric characteristics, no deterioration on electrical components, and a heat flux of 114 W/cm 2 for the experimental configuration. Second, experimental tests that included simultaneous operation with a mock automotive air-conditioner (A/C) system were performed on the same IGBT and gate controller card. Data extrapolation from these tests determined that a typical automotive A/C system has more than sufficient cooling capacity to cool a typical 30 kW traction inverter. Last, a discussion and simulation of active cooling of the IGBT junction layer with R134a refrigerant is given. This technique will drastically increase the forward current ratings and reliability of the PE device

  • Zhong Du; L. M. Tolbert; J. N. Chiasson; Hui Li
    Fourtieth IAS Annual Meeting. Conference Record of the 2005 Industry Applications Conference, 2005.
    2005

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    This paper presents an active harmonic elimination modulation control method for the cascaded H-bridges multilevel converter when supplied by unequal DC sources. First, the multilevel converter is decoupled into individual unipolar converters, and the low order harmonics (such as the 5th, 7th, 11th, and 13th) are eliminated by using resultant theory while at the same time the minimum specified harmonic distortion (HD) of a combination of unipolar converters for multilevel converter control is found. Next, the switching angle sets corresponding to the minimum harmonic distortion are used as initial guesses with the Newton climbing method to eliminate the specified higher order harmonics. If the solutions are not available for some modulation indices, the unipolar switching scheme is used to eliminate high order harmonics and the active harmonic elimination method is used to eliminate low order harmonics. This method has lower switching frequency than that of the previously proposed active harmonic elimination method.

  • H. Zhang; L. M. Tolbert; B. Ozpineci; M. S. Chinthavali
    Fourtieth IAS Annual Meeting. Conference Record of the 2005 Industry Applications Conference, 2005.
    2005

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    This paper presents a set of models for a SiC VJFET inverter from device level to system level. The simulations for SiC and Si inverters indicated that the SiC inverter has a much lower junction temperature, much less power loss, significantly enhanced energy efficiency, and a dramatic reduction in heatsink size as compared with the Si inverter. This demonstrated the technical feasibility and benefits of the all-SiC inverter. In addition to the simulations, experimental tests have also been conducted on SiC VJFETs and Schottky diodes for parameter extraction.

  • 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.

  • M. S. Chinthavali; B. Ozpineci; L. M. Tolbert
    Twentieth Annual IEEE Applied Power Electronics Conference and Exposition, 2005. APEC 2005.
    2005

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    Silicon carbide (SiC) unipolar devices have much higher breakdown voltages because of the ten times greater electric field strength of SiC compared with silicon (Si). 4H-SiC unipolar devices have higher switching speeds due to the higher bulk mobility of 4H-SiC compared to other polytypes. Four commercially available SiC Schottky diodes at different voltage and current ratings, an experimental VJFET, and MOSFET samples have been tested to characterize their performance at different temperatures. Their forward characteristics and switching characteristics in a temperature range of -50degC to 175degC are presented. The results of the SiC Schottky diodes are compared with those of a Si pn diode with comparable ratings

  • Kaiyu Wang; J. Chiasson; M. Bodson; L. M. Tolbert
    Proceedings of the 2005, American Control Conference, 2005.
    2005

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    Field-oriented control methodology requires knowledge of the machine parameters and, in particular, the rotor time constant. The interest here is in tracking the value of TR as it changes due to ohmic heating so that an accurate value is available to estimate the rotor flux linkages for a field-oriented controller. The approach presented here uses a nonlinear least-squares approach using measurements of the stator currents and voltages along with the rotor speed. The nature of this technique lends itself directly to an online implementation and therefore can be used to track the rotor time constant. Experimental results are presented to demonstrate the validity of the approach.

  • B. Ozpineci; M. S. Chinthavali; L. M. Tolbert
    2005 IEEE Vehicle Power and Propulsion Conference
    2005

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    Silicon carbide (SiC) power devices are expected to have an impact on power converter efficiency, weight, volume, and reliability. Presently, only SiC Schottky diodes are commercially available at relatively low current ratings. Oak Ridge National Laboratory has collaborated with Cree and Semikron to build a Si IGBT-SiC Schottky diode hybrid 55 kW inverter by replacing the Si pn diodes in Semikron's automotive inverter with Cree's made-to-order higher current SiC Schottky diodes. This paper shows the results obtained from testing this inverter and compares it to a similar all-Si inverter.

  • Z. Du; L. M. Tolbert; J. N. Chiasson
    2005 IEEE 36th Power Electronics Specialists Conference
    2005

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    This paper presents two computed PWM methods for 11-level multilevel converters to eliminate the specified harmonics in the output voltage to decrease total harmonic distortion (THD). The first method uses the fundamental switching scheme to eliminate low order harmonics, and uses the active harmonic elimination method to eliminate higher order harmonics. The second method uses these schemes in the reverse order, the fundamental switching scheme to eliminate higher order harmonics, and the active harmonic elimination method to eliminate low order harmonics. The computational results show that the difference between the THD of the two methods is small, but the second method has lower switching frequency. An experimental 11-level H-bridge multilevel converter was used to implement the algorithm and to validate the two computed PWM methods. The experimental results show that the two methods can effectively eliminate the specific harmonics as expected, while the second method results in a significantly lower switching frequency

  • S. Khomfoi; L. M. Tolbert
    31st Annual Conference of IEEE Industrial Electronics Society, 2005. IECON 2005.
    2005

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    In this paper, a fault diagnosis system in a multilevel-inverter using a neural network is developed. 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. Five multilayer perceptron (MLP) networks are used to identify the type and location of occurring faults. The neural network design process is clearly described. The classification performance of the proposed network between normal and abnormal condition is about 90 %, and the classification performance among fault features is about 85 %. 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.

  • Yan Xu; L. M. Tolbert; J. N. Chiasson; F. Z. Peng
    Fourtieth IAS Annual Meeting. Conference Record of the 2005 Industry Applications Conference, 2005.
    2005

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    This paper presents a theory of instantaneous nonactive power/current. This generalized theory is independent of the number of phases, whether the load is periodic or non-periodic, and whether the system voltages are balanced or unbalanced. By choosing appropriate parameters such as the averaging interval Tc and the reference voltage Vp, the theory has different forms for each specific system application. This theory is consistent with other more traditional concepts. The theory is implemented in a parallel nonactive power compensation system, and several different cases, such as harmonics load, rectifier load, single-phase pulse load, and non-periodic load, are simulated in MATLAB. Unity power factor or pure sinusoidal source current from the utility can be achieved according to different compensation requirements. Furthermore, the dynamic response and its impact on the compensator's energy storage requirement are also presented.

  • Kaiyu Wang; J. Chiasson; M. Bodson; L. M. Tolbert
    IEEE International Conference on Electric Machines and Drives, 2005.
    2005

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    Indirect field oriented control for induction machine requires the knowledge of rotor time constant to estimate the rotor flux linkages. Here an online method for estimating the rotor time constant and stator resistance is presented. The problem is formulated as a nonlinear least-squares problem and a procedure is presented that guarantees the minimum is found in a finite number of steps. Experimental results are presented. Two different approaches to implementing the algorithm online are discussed. Simulations are also presented to show how the algorithm works online

  • Zhong Du; L. M. Tolbert; J. N. Chiasson
    2004 IEEE Workshop on Computers in Power Electronics, 2004. Proceedings.
    2004

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    This paper presents an optimal total harmonic distortion (THD) control algorithm referred to as active harmonic elimination method for cascaded H-bridges multilevel converter control with unequal DC sources. First, the multilevel converter is decoupled into unipolar converters, the low order harmonics, such as the 5th, 7th, 11th and 13th are eliminated by using elimination theory, and the minimum THD combination of unipolar converters for multilevel converter control is found. Next, the magnitudes and phases of the residual higher harmonics are computed and subtracted from the original output voltage waveform to eliminate these higher harmonics. To validate the proposed algorithm, the method is simulated by Matlab first. After the simulation, an experimental 11-level H-bridge multilevel converter with a real-time controller based on Altera FLEX 10 K field programmable gate array (FPGA) is used to implement the algorithm with 8 μs control resolution. The experimental results show that the method can effectively eliminate the specific harmonics as expected, and the output voltage waveforms have low THD.

  • M. Shen; Jin Wang; A. Joseph; F. Z. Peng; L. M. Tolbert; D. J. Adams
    Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting.
    2004

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    This paper proposes two maximum constant boost control methods for the Z-source inverter, which can obtain maximum voltage gain at any given modulation index without producing any low-frequency ripple that is related to the output frequency. Thus the Z-network requirement will be independent of the output frequency and determined only by the switching frequency. The relationship of voltage gain to modulation index is analyzed in detail and verified by simulation and experiment.

  • Z. Du; L. M. Tolbert; J. N. Chiasson
    Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting.
    2004

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    This work presents a programmed PWM method to eliminate specific higher order harmonics of multilevel converters. First, resultant theory is applied to transcendental equations to eliminate low order harmonics and to determine switching angles for the fundamental frequency switching techniques. Next the magnitudes and phases of the residual higher order harmonics are computed, generated, and subtracted from the original voltage waveform to eliminate these higher order harmonics. The simulation results show that the method can effectively eliminate the specific harmonics, and a low total harmonic distortion (THD) near sine wave is produced. An experimental 11-level H-bridge multilevel converter with a first-on, first-off switching strategy, which is used to balance loads between several levels, is employed to validate the method. The experimental results show that the method can effectively eliminate the specific harmonics, and the output voltage waveforms have less THD than that from the fundamental frequency switching techniques.

  • B. Ozpineci; L. M. Tolbert; G. -J. Su; Z. Du
    Applied Power Electronics Conference and Exposition, 2004. APEC '04. Nineteenth Annual IEEE
    2004

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    Static characteristics of fuel cells show more than a 30% difference in the output voltage between no-load to full-load conditions. This inevitable decrease, which is caused by internal losses, reduces the utilization factor of the fuel cells at low loads. Additionally, the converters fed by these fuel cells have to be derated to accommodate higher input voltages at low currents. To increase the utilization of fuel cells and to avoid derating of semiconductors, this paper proposes a level reduction control using a multilevel DC-DC converter. Level reduction is done by inhibiting a certain number of fuel cells when the load current decreases. The inhibited fuel cells can be used in other applications such as charging batteries to further increase their utilization and the efficiency of the system.

  • B. Ozpineci; L. M. Tolbert; Zhong Du
    Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting.
    2004

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    Multilevel converters have been used previously to integrate several fuel cell modules for higher power applications. Some previous publications have also shown improvements in fuel cell utilization by exploiting the static characteristics of fuel cells, which show more than a 30% difference in the output voltage between no-load to full-load conditions. This paper first describes standard fuel cell power electronics interfaces, then reviews a few of the multi input systems and explores additional configurations.

  • Y. Xu; J. N. Chiasson; L. M. Tolbert
    2004 11th International Conference on Harmonics and Quality of Power (IEEE Cat. No.04EX951)
    2004

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    This paper presents a general definition of nonactive current/power and the implementation for a shunt compensation system. This definition is universal for different loads, such as nonperiodic, unbalanced or single phase, and also flexible in terms of the compensation results. Unity power factor, pure sinusoidal source current, or zero nonactive power supply from the utility can be achieved according to different compensation requirements. In addition, the corresponding current rating and energy storage requirements of the compensation system are also presented.

  • B. Ozpineci; L. M. Tolbert; J. N. Chiasson
    2004 IEEE 35th Annual Power Electronics Specialists Conference (IEEE Cat. No.04CH37551)
    2004

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    In this paper, a genetic algorithm (GA) optimization technique is applied to multilevel inverter to determine optimum switching angles for cascaded multilevel inverters for eliminating some higher order harmonics while maintaining the required fundamental voltage. This technique can be applied to multilevel inverters with any number of levels; as an example in this paper, a 7-level inverter is considered, and the optimum switching angles are calculated offline to eliminate the 5th and the 7th harmonics. Then, these angles are used in an experimental setup to validate the results.

  • Kaiyu Wang; J. Chiasson; M. Bodson; L. M. Tolbert
    Power Electronics in Transportation (IEEE Cat. No.04TH8756)
    2004

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    Field-oriented control methodology requires knowledge of the machine parameters, and in particular the rotor time constant. The interest here is in tracking the value of TR as it changes due to ohmic heating so that an accurate value is available to estimate the rotor flux for a field oriented controller. The approach presented here is a nonlinear least-squares method that uses only measurements of the stator currents and voltages along with the rotor speed. The nature of this technique lends itself directly to an online implementation and therefore can be used to track the rotor time constant. Experimental results are presented to demonstrate the validity of the approach.

  • Kaiyu Wang; J. Chiasson; M. Bodson; L. M. Tolbert
    2004 43rd IEEE Conference on Decision and Control (CDC) (IEEE Cat. No.04CH37601)
    2004

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    A nonlinear least-squares method is presented for the identification of the induction motor parameters. A major difficulty with the induction motor is that the rotor state variables are not available measurements so that the system identification model cannot be made linear in the parameters without overparametrizing the model. Previous work in the literature has avoided this issue by making simplifying assumptions such as a "slowly varying speed". Here, no such simplifying assumptions are made. The problem is formulated as a nonlinear system identification problem and uses elimination theory (resultants) to compute the parameter vector that minimizes the residual error. The only assumption is that the system be sufficiently excited. The method is suitable for online operation to continuously update the parameter values. Experimental results are presented.

  • M. Chinthavali; L. M. Tolbert; B. Ozpineci
    IEEE Power Engineering Society General Meeting, 2004.
    2004

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    The development of semiconductor devices is vital for the growth of power electronic systems. Modern technologies like voltage source converter (VSC) based HVDC transmission has been made possible with the advent of power semiconductor devices like GTO thyristors and their high power handling capability. Silicon carbide is the most advanced material among the available wide band gap semiconductors and most SiC devices are currently in the transition from research to manufacturing phase. This paper presents the modeling and design of a loss model for a 4H-SiC GTO thyristor device. The device loss model has been developed based on the device physics and device operation, and simulations have been conducted for various operating conditions. The loss model was integrated in the HVDC transmission system model to study the effects of the Si and SiC devices on the system. The paper focuses on the comparison of Si devices with SiC devices in terms of efficiency and cost savings for a HVDC transmission system.

  • M. S. Chinthavali; L. M. Tolbert; B. Ozpineci
    Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting.
    2004

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    The increase in use of power electronics in transmission and distribution applications is the driving force for development of high power devices. Utility applications like FACTS and HVDC require cost effective and highly efficient converters with high power ratings. SiC power devices have some exceptional physical properties that make them highly reliable at high power, high temperature, and high frequencies. This paper presents the modeling of temperature dependent 4H-SiC GTO thyristor and p-n diode loss models. The conduction and switching losses of the devices for various operating conditions have been simulated and compared for SiC and Si devices. These loss models are integrated with an HVDC transmission system to study the effect of Si and SiC devices on the system in terms of system efficiency and system cost management.

  • M. S. Chinthavali; B. Ozpineci; L. M. Tolbert
    Power Electronics in Transportation (IEEE Cat. No.04TH8756)
    2004

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    Silicon (Si) unipolar devices are limited in breakdown voltages because of the low electric field strength of the material. Silicon carbide (SiC) unipolar devices, on the other hand, have 10 times greater electric field strength and hence they have much higher breakdown voltages compared with Si. They also have low static and dynamic losses compared with Si devices. Four commercially available SiC Schottky diodes at different voltage and current ratings and an experimental SiC VJFET sample have been tested to characterize their performance at different temperatures. Their forward characteristics and switching characteristics in a temperature range of -50 °C to 175 °C are presented. The results for the SiC Schottky diodes are compared with the results for a Si pn diode with comparable ratings. The experimental data were analyzed to obtain the device performance parameters like the on-state resistance and the switching losses.

  • B. Ozpineci; L. M. Tolbert; Zhong Du
    2004 IEEE 35th Annual Power Electronics Specialists Conference (IEEE Cat. No.04CH37551)
    2004

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    Static characteristics of fuel cells show more than a 30% difference in the output voltage between no-load to full-load conditions. This inevitable decrease, which is caused by internal losses, reduces the utilization factor of the fuel cells at low loads. Additionally, the converters fed by these fuel cells have to be derated to accommodate higher input voltages at low currents. To increase the utilization of fuel cells and to avoid derating of semiconductors, this paper proposes a level reduction control using a multilevel inverter. Level reduction is done by inhibiting a certain number of fuel cells when the load current decreases. The inhibited fuel cells can be used in other applications such as charging batteries to further increase their utilization and the efficiency of the system.

  • J. Chiasson; L. M. Tolbert; K. McKenzie; Z. Du
    42nd IEEE International Conference on Decision and Control (IEEE Cat. No.03CH37475)
    2003

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    A method is presented to compute the switching angles in a multilevel converter so as to produce the required fundamental voltage while at the same time not generate higher order harmonics. Previous work has shown that the transcendental equations characterizing the harmonic content can be converted to polynomial equations which are then solved using the method of resultants from elimination theory. A difficulty with this approach is that when there are several DC sources, the degrees of the polynomials are quite large making the computational burden of their resultant polynomials (as required by elimination theory) quite high. In this paper, it is shown that the theory of symmetric polynomials can be exploited to reduce the degree of the polynomial equations that must be solved which in turn greatly reduces the computational burden. In contrast to results reported in the literature that use iterative numerical techniques to solve these equations, the approach here produces all possible solutions.

  • L. M. Tolbert; J. N. Chiasson; F. Z. Peng
    2003 IEEE Power Engineering Society General Meeting (IEEE Cat. No.03CH37491)
    2003

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    A strategy is presented to minimize the voltage total harmonic distortion when controlling a multilevel converter to act as a static var compensator. Use of resultant theory has enabled the complete set of solutions to be found for switching a multilevel inverter's power electronic devices at the fundamental frequency while eliminating the lower order harmonics. A scheme is also presented to regulate the voltage levels of the multilevel inverter so that it operates in an optimum amplitude modulation index regime.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    38th IAS Annual Meeting on Conference Record of the Industry Applications Conference, 2003.
    2003

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    A method is presented to compute the switching angles in a multilevel converter so as to produce the required fundamental voltage while at the same time not generate higher order harmonics. Previous work has shown that the transcendental equations characterizing the harmonic content can be converted to polynomial equations which are then solved using the method of resultants from elimination theory. A difficulty with this approach is that when there are several DC sources, the degrees of the polynomials are quite large making the computational burden of their resultant polynomials (as required by elimination theory) quite high. Here, it is shown that the theory of symmetric polynomials can be exploited to reduce the degree of the polynomial equations that must be solved which in turn greatly reduces the computational burden. In contrast to results reported in the literature that use iterative numerical techniques to solve these equations, the approach here produces all possible solutions.

  • M. Hasanuzzama; S. K. Islam; L. M. Tolbert; M. T. Alam
    International Semiconductor Device Research Symposium, 2003
    2003

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    An analytical model for lateral MOSFET that includes the effects of temperature variation in 4H- and 6H-SiC poly-type is presented in this paper. The model includes the effect of temperature variation on the threshold voltage, the carrier mobility, the body leakage current, and the drain and source contact region resistances. MOSFET device behavior in 4H-SiC is also simulated and compared with 6H-SiC material system.

  • B. Ozpineci; L. M. Tolbert
    Electric Machines and Drives Conference, 2003. IEMDC'03. IEEE International
    2003

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    In this paper, a modular Simulink implementation of an induction machine model is described in a step-by-step approach. With the modular system, each block solves one of the model equations; therefore, unlike black box models, all of the machine parameters are accessible for control and verification purposes. After the implementation, examples are given with the model used in different drive applications, such as open-loop constant V/Hz control and indirect vector control are given. Finally, the use of the model as an Induction generator is demonstrated.

  • B. Ozpineci; Zhong Du; L. M. Tolbert; D. J. Adams; D. Collins
    Industrial Electronics Society, 2003. IECON '03. The 29th Annual Conference of the IEEE
    2003

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    According to SECA program guidelines, solid oxide fuel cells (SOFC) are produced in the form of 3-10 kW modules for residential use. In addition to residential use, these modules can also be used in apartment buildings, hospitals, etc., where a higher power rating would be required. For example, a hospital might require a 250 kW power supply. To provide this power using the SOFC modules, 25 of the 10 kW modules would be required. These modules can be integrated in different configurations to yield the necessary power. This paper shows five different approaches for integrating numerous SOFC modules and will evaluate and compare each one with respect to cost, control complexity, ease of modularity, and fault tolerance.

  • L. M. Tolbert; Y. Xu; J. Chen; F. Z. Peng; J. N. Chiasson
    Power Electronics Specialist Conference, 2003. PESC '03. 2003 IEEE 34th Annual
    2003

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    Based on a new definition of nonactive current/power, this paper presents the application of a parallel active filter for the compensation of nonperiodic currents. Analysis of the compensation characteristics required for a variety of nonperiodic currents such as those associated with arc furnaces is presented. In addition, the corresponding current rating and energy storage requirements of the compensator are also presented.

  • L. M. Tolbert; J. N. Chiasson; K. J. McKenzie; Zhong Du
    Electric Machines and Drives Conference, 2003. IEMDC'03. IEEE International
    2003

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    One promising technology to interface battery packs in electric and hybrid electric vehicles is the multilevel converter. 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.

  • L. M. Tolbert; J. Chiasson; K. McKenzie; Zhong Du
    Applied Power Electronics Conference and Exposition, 2003. APEC '03. Eighteenth Annual IEEE
    2003

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    The problem of eliminating harmonics in a multilevel converter in which the separate DC sources vary is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. Assuming that the separate DC sources can be measured, a procedure is given to find all sets of switching angles for which the fundamental is produced while the 5th and 7th are eliminated. This is done by first converting the transcendental equations that specify the elimination of the harmonics into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. Experimental results are presented to validate the theory.

  • L. M. Tolbert; Y. Xu; J. Chen; F. Z. Peng; J. N. Chiasson
    2003 IEEE Power Engineering Society General Meeting (IEEE Cat. No.03CH37491)
    2003

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    Compensation of irregular currents such as those associated with arc furnaces, transient disturbances, and power electronic converters is presented. The compensation is based on a well-defined nonactive current definition. The basic concepts required in the definition of nonactive current are presented and illustrated by compensation simulations for a variety of different types of nonperiodic currents found in distribution electrical systems, including disturbance, subharmonic, and stochastic currents. Further, based on the compensation objectives for different types of load waveforms, the specifications such as current ratings or capacitance requirements of the active filter are also presented.

  • J. Chiasson; L. M. Tolbert; K. McKenzie; Zhong Du
    Applied Power Electronics Conference and Exposition, 2003. APEC '03. Eighteenth Annual IEEE
    2003

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    The problem of eliminating harmonics in a switching converter is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. In contrast to the well known work of Patel and Hoft (1973, 1974) and others, here all possible solutions to the problem are found. This is done by first converting the transcendental equations that specify the harmonic elimination problem into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. In particular, it is shown that there are new solutions that have not been previously reported in the literature. The complete solutions for both unipolar and bipolar switching patterns to eliminate the 5th and 7th harmonics are given. Finally, the unipolar case is again considered where the 5th 7th, 11th, and 13th harmonics are eliminated along with corroborative experimental results.

  • B. Ozpineci; L. M. Tolbert; S. K. Islam; F. Z. Peng
    2002 IEEE 33rd Annual IEEE Power Electronics Specialists Conference. Proceedings (Cat. No.02CH37289)
    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 (ORNL), including SiC device design and system modeling studies, are discussed

  • B. Ozpineci; L. M. Tolbert; S. K. Islam; T. J. Theiss
    Proceedings IEEE 56th Vehicular Technology Conference
    2002

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    Materials and device researchers build switching devices for the circuit researchers to use in their circuits, but they rarely know how and where the devices are going to be used. The circuits people, including power electronics researchers, take the devices as black boxes and use them in their circuits not knowing much about the inside of the devices. The best way to design optimum devices is an interactive design where people designing and building the devices have a close interaction with the people who use them. This study covers the circuit aspects of the SiC power device development. As a contribution to the abovementioned interactive design, in this paper, the device parameters, which need to be improved in order to design better devices, are discussed.

  • L. M. Tolbert; W. A. Peterson; C. P. White; T. J. Theiss; M. B. Scudiere
    Conference Record of the 2002 IEEE Industry Applications Conference. 37th IAS Annual Meeting (Cat. No.02CH37344)
    2002

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    A proof-of-concept military advanced mobile generator set has been developed. The military generator set uses an internal combustion diesel engine to drive a radial-gap permanent magnet alternator at variable speed. The speed of the engine is determined from a user selectable interface that for a given load and ambient thermal conditions controls the engine to run at its most efficient operating point. The variable frequency, variable voltage produced by the permanent magnet alternator is diode-rectified to a high voltage (/spl sim/400 V) DC link, and an inverter is used to produce selectable frequency, controllable AC voltage. As part of the power electronics for this unit, a 7 kW bi-directional DC-DC converter has also been developed. The converter can charge 24 V batteries that are used to start the internal combustion engine and to power auxiliary low voltage DC loads. Additionally, the bi-directional converter can also draw power from the batteries to help maintain the high voltage DC link during severe load transients. Because of stringent weight and volume requirements for this application, the minimum in energy storage elements (high frequency transformers, capacitors, and inductors) was used. This paper presents a description and experimental analysis of this novel DC-DC converter design.

  • B. Ozpineci; L. M. Tolbert; S. K. Islam; M. Hasanuzzaman
    Conference Record of the 2002 IEEE Industry Applications Conference. 37th IAS Annual Meeting (Cat. No.02CH37344)
    2002

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    Materials and device researchers build switching devices for the circuits researchers to use in their circuits, but they rarely know how and where the devices are going to be used. The circuits people, including power electronics researchers, take the devices as black boxes and use them in their circuits not knowing much about the inside of the devices. The best way to design optimum devices is an interactive design where people designing and building the devices have a close interaction with the people who use them. This study covers the circuit aspects of the SiC power device development. As a contribution to the above-mentioned interactive design, in this paper, the device parameters, which need to be improved in order to design better devices, are discussed.

  • F. Z. Peng; L. M. Tolbert; Zhaoming Qian
    2002 IEEE 33rd Annual IEEE Power Electronics Specialists Conference. Proceedings (Cat. No.02CH37289)
    2002

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    Many definitions have been formulated to characterize, detect, and measure active and nonactive current and power for nonsinusoidal and nonperiodic waveforms in electric systems. This paper presents definitions and compensation of nonactive current from the compensation standpoint and provides guidance on how to determine compensation objectives and select detection parameters. The proposed definition is valid to both single and multi-phase power systems. Clear and easy guidance to determine objectives and design parameters of compensation systems is provided

  • F. Z. Peng; Gui-Jia Su; L. M. Tolbert
    2002 IEEE 33rd Annual IEEE Power Electronics Specialists Conference. Proceedings (Cat. No.02CH37289)
    2002

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    This paper presents a regenerative passive snubber circuit for PWM inverters to achieve soft-switching purposes without significant cost and reliability penalties. This passive soft-switching snubber (PSSS) employs a diode/capacitor snubber circuit for each switching device in an inverter to provide low dv/dt and low switching losses to the device. The PSSS further uses a transformer-based energy regenerative circuit to recover the energy captured in the snubber capacitors. All components in the PSSS circuit are passive, thus leading to reliable and low-cost advantages over those soft-switching schemes relying on additional active switches. The snubber has been incorporated into a 150 kVA PWM inverter. Simulation and experimental results are given to demonstrate the validity and features of the snubber circuit.

  • 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.

  • J. Chiasson; L. Tolbert; K. McKenzie; Zhong Du
    2002 IEEE 33rd Annual IEEE Power Electronics Specialists Conference. Proceedings (Cat. No.02CH37289)
    2002

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    A method is given to determine conditions for which the switching angles in a multilevel converter can be chosen to produce the required fundamental voltage while at the same time cancel out higher order harmonics. A complete analysis is given for a 7- level converter where it is shown that for a range of the modulation index mI, the switching angles can be chosen to produce the desired fundamental V1=mI(s4Vdc/π) while making the 5th and 7th harmonics identically zero.

  • 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.

  • M. Bodson; J. Chiasson; L. Tolbert
    Proceedings of the 2001 American Control Conference. (Cat. No.01CH37148)
    2001

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    This paper considers the problem of maximizing the torque of permanent magnet non-salient synchronous motors in the presence of voltage and current constraints. Formulas are given that enable a drive to extract the maximum torque available from a motor in real-time. Both the motoring and braking modes are considered, and it is shown that the well-known field-weakening formula cannot be used with some motors. An example is discussed to illustrate the contributions of the paper

  • J. Chiasson; L. Tolbert
    IEMDC 2001. IEEE International Electric Machines and Drives Conference (Cat. No.01EX485)
    2001

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    A switched reluctance motor is a doubly salient machine and the fact that is has no rotor windings nor permanent magnets, it is a relatively cheap motor to manufacture. However, it is a quite difficult machine to control because the mathematical model is of a nonparametric nonlinear type. It is described in this work how the techniques presented in the existing literature can be used to make the switched reluctance motor an actuator for high-performance control applications

  • L. M. Tolbert; Hairong Qi; F. Z. Peng
    2001 Power Engineering Society Summer Meeting. Conference Proceedings (Cat. No.01CH37262)
    2001

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    A scalable multi-agent paradigm is presented for control of distributed energy resources to achieve higher reliability, higher power quality, and more efficient (optimum) power generation and consumption. A dynamic hybrid multiagent system is proposed in this paper as a means to achieve scalability for control of a large network of power generation, transmission, load, and compensation sources. Example ancillary agents are developed for system stability and harmonic and reactive current compensation.

  • Gui-Jia Su; D. J. Adams; L. M. Tolbert
    2001 IEEE 32nd Annual Power Electronics Specialists Conference (IEEE Cat. No.01CH37230)
    2001

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    A half-bridge inverter is very suitable for single-phase on-line UPS applications because it offers very desirable features. These include fewer active switches, a common neutral connection-not requiring an isolation transformer, and sinusoidal input currents if a power factor correction (PFC) converter is used at the front end. This paper presents a comparative study of PFC converters for single-phase half-bridge UPS inverters. A traditional half-bridge converter and two recently introduced AC-DC/DC-DC boost converters are comparatively investigated for active switch count, voltage stresses on the switches and capability of voltage balance control of the DC bus capacitors. Analytical and experimental results are included in this paper to illustrate performance differences among the converters in terms of input current harmonic distortion and energy efficiency

  • L. M. Tolbert; W. A. Peterson; M. B. Scudiere; C. P. White; T. J. Theiss; J. B. Andriulli; C. W. Ayers; G. Farquharson; G. W. Ott; L. E. Seiber
    Conference Record of the 2001 IEEE Industry Applications Conference. 36th IAS Annual Meeting (Cat. No.01CH37248)
    2001

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    The electronic power conversion system and control for an advanced mobile generator set is described. The military generator set uses an internal combustion diesel engine to drive a radial-gap permanent magnet alternator at variable speed. The speed of the engine is determined from a user selectable interface that for a given load and ambient thermal conditions controls the engine to run at its most efficient operating point. It is also possible to control the engine to run where it is most audibly quiet, at its least-polluting operating point, or at its most reliable, stiffest point such that it is less sensitive to load transients. The variable frequency, variable voltage produced by the permanent magnet alternator is diode-rectified to DC voltage, and an inverter is used to produce selectable frequency, controllable AC voltage. The power conversion system also incorporates a bi-directional DC-DC converter that can charge 24 V batteries that are used to start the IC engine and to power auxiliary loads. The bi-directional converter can also draw power from the batteries to help maintain the DC link during severe load transients. The design of this new generator set offers additional flexibility by being lighter, smaller, and more fuel efficient than a conventional fixed-speed gen-set.

  • B. Ozpineci; L. M. Tolbert; S. K. Islam; M. Hasanuzzaman
    Industrial Electronics Society, 2001. IECON '01. The 27th Annual Conference of the IEEE
    2001

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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 improvements in the performance of power electronics converter systems. These systems with SiC power devices are more compact, lighter, and more efficient, so they are ideal for high-voltage power electronics applications, including hybrid electric vehicle (HEV) traction drives. In this paper, the effect of SiC-based power devices on HEV traction drive losses are investigated. Reductions in heat sink size and device losses with the increase in the efficiency will be analyzed using an averaging model of a three-phase PWM inverter (TPPWMI). For more accurate results, device physics is taken into consideration to find the loss equations for the controllable switches

  • B. Ozpineci; J. O. P. Pinto; L. M. Tolbert
    2001 IEEE International Conference on Systems, Man and Cybernetics. e-Systems and e-Man for Cybernetics in Cyberspace (Cat.No.01CH37236)
    2001

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    As the size and the cost of power semiconductor switches are decreasing, converter topologies with high device count are starting to draw more attention. One such type of converter is the high frequency AC (HFAC) link converters. A popular control method for these converters is pulse density modulation (PDM). The HFAC link voltage of the converter in this paper is a high frequency, three-step, variable pulse-width (PW) square wave voltage waveform. A genetic algorithm approach is used to determine the PW to optimize the output voltage harmonic content

  • F. Z. Peng; L. M. Tolbert
    2000 Power Engineering Society Summer Meeting (Cat. No.00CH37134)
    2000

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    Many definitions have been formulated to characterize, detect and measure active and nonactive current and power for nonsinusoidal and nonperiodic waveforms in electric power systems. This paper presents definitions and compensation of nonactive current from the compensation standpoint

  • L. M. Tolbert; F. Z. Peng
    2000 Power Engineering Society Summer Meeting (Cat. No.00CH37134)
    2000

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    Multilevel inverter structures have been developed to overcome shortcomings in solid-state switching device ratings so that they can be applied to high-voltage electrical systems. The general function of the multilevel inverter is to synthesize a desired AC voltage from several levels of DC voltages. For this reason, multilevel inverters are ideal for connecting either in series or in parallel an AC grid with renewable energy sources such as photovoltaics or fuel cells or with energy storage devices such as capacitors or batteries. In this paper the cascaded H-bridges multilevel inverter is described

  • L. M. Tolbert; T. G. Habetler
    7th IEEE International Power Electronics Congress. Technical Proceedings. CIEP 2000 (Cat. No.00TH8529)
    2000

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    Many definitions have been formulated to characterize nonactive power for nonsinusoidal waveforms in electrical systems, and no single, universally valid power theory has been adopted as a standard for nonactive power. Most of the nonactive power theories formulated thus far have had a particular type of compensation in mind, which has influenced the conventions used in the development of the definitions. Because nonsinusoidal loads are expected to continue to proliferate throughout electrical distribution systems, nonactive power theories will only grow in importance for applications such as nonactive power compensation, harmonic load identification, voltage distortion mitigation, and metering. This paper presents a comprehensive technical survey of the published literature on the topic and briefly outlines the salient points of each of the different theories as well as each one's applicability to active power filtering