Modulation Index Design of Full-Bridge MMC for Capacitor Voltage Ripples Reduction Under Single-Line-to-Ground Faults Conditions

The energy storage requirements of full-bridge submodule (FBSM)-based modular multilevel converters (FB-MMCs) can be reduced more than 50% compared to conventional half-bridge SM-based MMCs (HB-MMCs) when negative voltage states of FBSMs are used. However, under single-line-to-ground (SLG) faults conditions, the FBSM voltages ripples increase significantly, which is quite different from HB-MMCs. This article analyzes the characteristics of the FBSM voltage fluctuations under both steady-state and SLG faults conditions, and the impact of modulation index, power factor angle and inductance at the grid side on the FBSM voltage fluctuations is also analyzed. Results indicate that the peak-to-peak value of the FBSM voltage fluctuations has strong correlation with the modulation index. It is also found that the fundamental frequency components become dominant when SLG faults occur, and the ripples are almost twice of the steady-state operations, which may threaten safe operation of FB-MMCs. Based on these analysis, a modulation index design method is proposed for the purpose of balancing the FBSM voltage fluctuations of the FB-MMC under SLG faults conditions. The validity of the proposed modulation index design method is verified by simulation results on a ±200 kV/800 MW FB-MMC.