Fault-Tolerant Control Strategy for Sub-Module Faults of Modular Multilevel Converters

Because of the sub-modules fault, redundant sub-modules may be exhausted in the long-term operation of modular multilevel converter (MMC). In order to ensure that the MMC does not stop running under the extreme conditions of sub-module failure, this paper proposes a novel neutral point offset fault-tolerant control strategy for MMC. This method can maintain the symmetrical operation between the system lines only by injecting the fundamental frequency voltage modulation component, and it is simple to realize. Moreover, it does not need to raise the operating voltage of the faulty phase sub-module, which reduces the voltage stress of the sub-module switching device. Finally, the simulation and experiment are carried out to verify the effectiveness of the proposed fault-tolerant control strategy.