A high-voltage direct-current (HVDC) grid protection strategy to suppress dc fault currents and prevent overcurrent in the arms of modular multi-level converters (MMCs) is proposed in this paper. The strategy is based on the coordination of half-bridge (HB) MMCs and hybrid dc circuit breakers (DCCBs). This is achieved by allowing MMC submodules (SMs) 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 by more than 70 and 90% respectively, while keeping MMC arm currents small.

This letter proves that a grid-following converter can stably connect to a weak grid even short-circuit ratio (SCR) is 1. Root instability causes of this grid-following control are identified including fast control response, insufficient damping, and slow voltage support. A simple and effective tuning method is proposed to stabilize this connection. Three control modes are considered in this tuning method including current control, active power and voltage ( $ PV$ ) control, and active power and reactive power ( $ PQ$ ) control. The switching model of a two-level converter is used for the simulation validation.