An Enhanced Fault-Tolerant Scheme for Arm-Multiplexing Modular Multilevel Converters

The arm-multiplexing modular multilevel converter (AM-MMC) is a new compact MMC topology that is especially promising for offshore high-voltage direct current transmission systems. However, the reliable operation of AM-MMCs suffers from submodule (SM) faults. The fault-tolerant scheme is an effective measure to remedy the reliability of AM-MMCs. This paper presents an enhanced fault-tolerant scheme for SM faults. In this scheme, the fault-tolerant structure and control are proposed. Firstly, according to the pivotal constraints of AM-MMCs, the fault-tolerant structure is confirmed, as the redundant SMs distribute in each arm at the same number. Then, the fault-tolerant control composed of SM alternate launching and switching moment reset is proposed. The first method contributes to improving the defects of redundant SMs in normal operation and the second method invokes the redundant SMs in the specified arms to recover the postfault operation to the normal level. Based on the fault-tolerant structure and control, the fault-tolerant capability and performance are significantly enhanced. Moreover, the proposed fault-tolerant scheme consumes less construction cost, tolerates more SM faults and produces higher operation quality. Finally, the effectiveness of the proposed fault-tolerant scheme is validated by simulation results in MATLAB/Simulink.