Aging Effect Analysis of Reactive Power Generation for PV Inverter
This project is to study the impact of reactive power generation on PV inverters in terms of thermal stress. PV inverters are typically designed for active power generation. However, recent IEEE standard 1547 and microgrid operation have started to engage PV inverters in providing ancillary services. The impact of reactive power generation on PV inverter reliability and lifetime remain unrevealed in existing study and literature. This project examines the impact of reactive power generation on grid-tied inverters, such as PV inverters in terms of lifetime and reliability. If successful, this project can 1) provide utilities with supporting data for reactive power engagement design; 2) provide guidance for grid-tied inverter design, particularly grid-tied inverters that are for active power generation priorly; and 3) protect grid-tied inverters from overuse and unnecessary thermal stress.
Junction temperature of inverter IGBT and diode. (a) power factor (PF) = 1, var = 0; (b) PF = 0.9, var = 0.44 p.u.; and (c) PF = 0.9, var = â€“ 0.44 p.u..
This project involves the power loss analysis of PV inverters , electrothermal modeling of PV inverters , and lifetime estimation of PV inverters .
The power loss distribution among inverter semiconductors varies with respect to different output power factor (PF). In IGBT-diode type of PV inverters, providing reactive power will reduce the conduction loss of IGBTs and increase the conduction loss of diodes which increases the diode thermal stress because the equivalent current that flows through the diodes increases as the PF decreases. The theoretical analysis of power loss distribution is researched in this project. The power loss distribution among inverter semiconductors are verified in PLECS simulation.
Semiconductor losses are typically cycling in a fundamental cycle. In addition to the average power loss, the power loss variation during a fundamental cycle also varies with power factor. The average losses determine the mean junction temperature (Tj). The power loss variation in a fundamental cycle determines the junction temperature variation (âˆ†Tj). Both junction temperature and temperature variation greatly influence the lifetime of a semiconductor. This project analytically derives the conduction loss of the semiconductors and shows the results that the junction temperature variation of inverter diodes will increase as the output power factor decreases. The junction temperature variation of semiconductors is evaluated in PLECS simulation.
In addition to loading condition, the inverter filtering inductor will also change the actual inverter power factor and indirectly influence the current distribution of the semiconductors. This project also studies the effect of filtering inductor on the current distribution of semiconductors.
How WBG Can Help
The research findings from this project will provide technical and data support to the Duke Energy Grid Modernization Lab Consortium (GMLC) project. The GMLC project engages PV inverters to provide ancillary services at microgrid contingencies. However, the reliability of the PV inverters to provide ancillary services needs to be assessed. This research has shown that the inverter diodes are more vulnerable to reactive power generation. The load current is more likely to flow through diodes rather than IGBTs when the output power factor is non-unity. PV inverters have started to use SiC Schottky diodes for the antiparallel diodes. This research finding will help with identifying the PV invertersâ€™ capability in providing ancillary services in GMLC project.
- Paychuda Kritprajun
- Yunting Liu
 J. Guo, "Modeling and design of inverters using novel power loss calculation and dc-link current/voltage ripple estimation methods and bus bar analysis", Ph.D. dissertation, Dept. Electr., Comput. Eng., Mcaster Univ., 2017.
 M. Andresen, G. Buticchi, and M. Liserre, "Thermal Stress Analysis and MPPT Optimization of Photovoltaic Systems", IEEE Trans. Ind. Electron., vol. 63, no. 8, pp. 4889-4898, 2016.
 A. Sangwongwanich, Y. Yang, D. Sera, and F. Blaabjerg, "Lifetime Evaluation of Grid-Connected PV Inverters Considering Panel Degradation Rates and Installation Sites", IEEE Trans. Power Electron., vol. 33, no. 2, pp. 1225-1236, 2018.