A DC fault handling method of the MMC-based DC system

The flexible DC system based on the modular multilevel converter (MMC) has a good development prospect. However, the DC fault handling is still a technical difficulty since the large capacity DC circuit breaker technology and its engineering application are not yet mature. Therefore, the converter-based fault isolation method is widely discussed. One typical idea of the converter-based isolation methods is to create a symmetrical short circuit at the AC system artificially after a DC fault, so the ac-side source could be separated from the DC fault point and the DC fault current can decay to a very small value naturally, which is represented by the double-thyristor switch scheme (DTSS). This paper analyzed the feasibility of the converter-based fault isolation method, including the isolating speed, influence on the ac-system protection and restart of the system. After that, the improved DC fault handling method by introducing damping modules was proposed, which could accelerate the DC fault isolating speed, avoid the maloperation of the ac-system protection and guarantee the fast restart of the system. Finally, the simulation cases based on the PSCAD/EMTDC are carried out to verify the superiority of the proposed scheme.