Analysis and mitigation of submodule capacitor overvoltage for MMC-based grid emulator under LVRT test

The modular multilevel converter (MMC)-based grid emulator is recently reported as a promising approach for grid-code compliance testing of next-generation wind turbines. This paper reveals the overvoltage on the submodule (SM) capacitor of the MMC-based grid emulator during the low-voltage ride-through (LVRT) test, and proposes an approach to address such issue. First, a simplified average model is developed to quantify the maximum and minimum SM capacitor voltage during the LVRT test. Then, a SM capacitor energy balancing approach is proposed to mitigate the transient overvoltage, which limits the SM capacitor voltage within 60%~110% of the rated voltage. Simulation results have confirmed the correctness of the theoretical analysis.