NR Electric Co., Ltd., Nanjing 211102, China

Modular multilevel converter (MMC) for flexible DC transmission is introduced as the research object to analyze overvoltage characteristics of the sub-modules. The transient overvoltage and temporary overvoltage problems of the sub-modules in operation are studied. Transient overvoltage of the sub-modules includes turn-off overvoltage of the insulated gate bipolar transistor (IGBT) and reverse recovery overvoltage of the diodes. For the turn-off overvoltage of IGBT in the sub-modules, a two-level turn-off technology is proposed to limit the voltage overshoot; for the reverse recovery overvoltage of diodes in the sub-modules when the bypass switch operates, a bypass method is proposed to suppress the reverse recovery overvoltage of the diodes. The temporary overvoltage problem of the sub-modules caused by system fault is also analyzed, and a sub-module topology with energy-consuming branch is proposed to improve the fault ride-through ability of converters, which can also reduce the number of the sub-modules effectively. For the overvoltage problem of MMC under different fault operation conditions, a control logic strategy of multi-stage overvoltage protection coordination is proposed. And a new redundant power supply scheme is proposed to solve the problem of ‘first start-up failure’ during the MMC starting process, and improve the reliability of system operation. Finally, the experimental and simulation waveforms are given to verify the correctness of the theoretical analysis of transient overvoltage and temporary overvoltage and the validity of the protection scheme.