A Fault-Tolerant Strategy for MMC with Reduced Zero-Sequence Voltage Injection

This paper proposes a novel zero-sequence voltage injection method to maintain stable operation of the modular multilevel converter (MMC) after sub module failures. From the previous researchers' point of view, the zero-sequence voltage is inj ected to output the maximum possible (balanced) line voltage without overmodulation. However, this kind of method would affect the normal operation of the converter but the previous studies did not pai much attention on it. The inj ected fundamental frequency component leads to unequal power factors of the three phases. The presented fault-tolerant strategy is able to reduce the amplitude of the injected zero-sequence voltage (CMV) and its fundamental frequency component (FCMV). Therefore, the above-mentioned negative impacts can be mitigated. In addition, the proposed strategy features a simple and fast calculation of the injected zero-sequence voltage, making itself straightforward to be implemented in real time. The proposed strategy is validated through simulations.