Firstly, an improved detailed simulation model based on Thevenin equivalent is proposed to analyze the overvoltage and overcurrent of power electronic devices in MMC converter valve, which provides a new idea for converter valve level simulation of multi-level complex system. Then, the effects of different fault moments on the internal and external transient processes of MMC converter valve are explored. The results show that different fault moments will not affect the short-circuit current on the DC side, but have a great impact on the bridge arm current and overcurrent on power electronic devices. When the fault occurs at the voltage zero crossing of the AC side, the power electronic devices inside the converter valve will bear greater overcurrent threat. This provides theoretical basis and data support for fault transient simulation and protection of true bipolar mmc-hvdc system.

Converter Valve; DC Side Fault; MMC-HVDC; Simulation.