The DC-link fault is a key issue for the flexible high-voltage direct current system using modular multilevel converter(MMC-HVDC). An auxiliary equipment based processing strategy for MMC-HVDC DC faults is proposed

namely

a novel scheme by bidirectional thyristors in collaboration with a breaker-type zero-loss fault current limiter. The DC-protections based on natural decay of the fault DC current and their shortcomings are analyzed first. It is then proposed that the bidirectional thyristors rather than back-to-back ones are configured and controlled on the AC-side to simplify the control loop. The bidirectional thyristors are controlled to cooperate with the appropriately designed breaker-type zero-loss fault current limiter

which can possess fast current limiting ability while reducing the long short-circuit current rush to AC system during the clearance of the DC-link fault. And the corresponding DC-fault detection and recovery strategy is presented. PSCAD based simulation results finally verify the validity of the proposed protection scheme. This work is supported by National Natural Science Foundation of China(No. 51577072).