Coordination of MMCs With Hybrid DC Circuit Breakers for HVDC Grid Protection

A high-voltage direct-current (HVDC) grid protection strategy to suppress dc fault currents and prevent overcurrent in the arms of modular multilevel converters (MMCs) is proposed in this paper. The strategy is based on the coordination of half-bridge MMCs and hybrid dc circuit breakers (DCCBs). This is achieved by allowing MMC submodules to be temporarily bypassed prior to the opening of the DCCBs. Once the fault is isolated by the DCCBs, the MMCs will restore to normal operation. The performance of the proposed method is assessed and compared to when MMCs are blocked and when no corrective action is taken. To achieve this, an algorithm for fault detection and discrimination is used and its impact on MMC bypassing is discussed. To assess its effectiveness, the proposed algorithm is demonstrated in PSCAD/EMTDC using a four-terminal HVDC system. Simulation results show that the coordination of MMCs and DCCBs can significantly reduce dc fault current and the absorbed current energy bymore than 70% and 90%, respectively, while keeping MMC arm currents small.