Capacitor Voltage Ripples Reduction of Full-bridge MMC With Circulating Current Injection Under Single-Line-to-Ground Fault Conditions

Full-bridge submodule (FBSM) voltage ripples can achieve a significant reduction of FBSM-based modular multilevel converter (FB-MMC) with boosted ac voltages. However, the FBSM voltage ripples increase substantially when single-line-to-ground (SLG) faults occur, which may cause damage to the safe operation of FB-MMC. This paper analyzes the FBSM voltage ripples characterization of the FB-MMC under SLG faults conditions. It is found that the fundamental frequency component, which is minor in steady-state operations, becomes dominant in the event of SLG faults leading to a dramatic increase of FBSM voltage ripples. Based on these analyses, a circulating current injection (CCI) method is proposed in order to suppress the main components of the FBSM fundamental frequency fluctuations. Both of the presented analysis and the effectiveness of the proposed CCI method are verified by simulations on a +/- 200 kV FB-MMC system.