Fault-tolerant Strategy of MMC with Hybrid Reserved Redundant Submodules

Modular multilevel converters (MMC) are widely used in high-voltage DC transmission and medium-voltage DC distribution networks. In order to improve the reliability of MMC, the arms are usually equipped with redundant submodules (SM), which effectively reduces the risk that the MMC shuts down due to SM failure. Based on the redundant SMs, the fault-tolerant strategy of hybrid cold and hot SMs is improved. Compared with the redundant fault-tolerant strategy with single type of reserved SM, the complexity of the improved algorithm is reduced. On the basic of the improved fault-tolerant strategy, a control strategy for the charging of the cold reserved SM is proposed, which can effectively reduce the impact on the output performance of the MMC during the charging process of cold reserved SM. The validations of the conventional hybrid reserved fault-tolerant strategy and the proposed hybrid reserved fault-tolerant strategy are compared in a 19-level MMC model by the PSCAD/EMTDC platform. According to the simulation results, the proposed fault-torlerant control algorithm can reduce the impact of SMs fault on the performance of MMC.