Fault-tolerant control for modular multilevel converter based on zero-sequence voltage injection

As mounting numbers of modular multilevel converters (MMCs) are increasingly applied in the field of high-voltage large-power energy conversion, the loss brought about by MMC failures will be significantly increased. To resolve this problem, a fault-tolerant control strategy for MMC in three-phase three-wire system configuration is proposed. The capacitor voltage redundant sorting method is adopted to overcome the inherent unbalanced power distribution of the phase disposition PWM. The zero-sequence voltage injection method is presented on the basis of elaboration of different fault-tolerant control schemes, to ensure the line voltage will remain constant. Then the modulation signals are reconfigured for the case in excess of the threshold voltage range, which enables the MMC system to possess a reliable fault ride-through capability. Finally, a three-phase 4.5 kVA MMC prototype is built for experimental validation. The experimental results have verified the correctness and feasibility of the scheme proposed.