Fault-Tolerant Control of Medium-Voltage Modular Multilevel Converters With Minimum Performance Degradation Under Submodule Failures

As the state-of-the-art voltage-source topology, the modular multilevel converter (MMC) is attractive for various medium- or high-power applications. In MMC, reliability and uninterruptable operation are always seen as primary concerns, which requires the MMC can continue operating even though some of the submodules (SMs) are failed. In this paper, a novel fault-tolerant modulation and control strategy is proposed for MMC in medium voltage applications. Compared with previous works, the proposed strategy provides optimum performances under healthy conditions and minimum performance degradation after SM failures. During healthy conditions, the redundant SMs are fully utilized to decrease SM capacitor voltage for higher reliability and to reduce switching frequency for higher efficiency. While during faulttolerant operation, by appropriate modification of the modulation and controllers, no extra healthy SMs need to be bypassed and the output voltage harmonics of MMC remain unchanged without causing voltage mismatches. Feasibility and effectiveness of the proposed strategy have been verified by simulations in MATLAB/Simulink software and experiments on a downscaled three-phase MMC prototype.