A Novel Fault Ride-Through Strategy Based on Capacitor Energy Storage Inside MMC

Once modular multilevel converter (MMC) dc-side short-circuit fault occurs, the conventional fault ride-through (FRT) strategy indeed eliminates dc fault current quickly with the utilization of full-bridge submodules. Thereupon, ac-side active power declines sharply. However, instantaneous high power outage will cause ac-grid power and frequency oscillation and voltage instability, especially in MMC-based high-voltage direct-current transmission system. In order to solve this problem, a novel FRT strategy based on capacitor energy storage (CES) inside MMC (FRT-CES) is proposed, which can accomplish the clearance of dc fault current, ac-side grid support, and stable operation of MMC simultaneously. Through theoretical analysis and mathematical derivation, the prerequisite of CES utilization and universal design of improving ac-side transient characteristics are proposed and quantified. Based on the quantified results, the potential capability of MMC inner CES can be excavated to attenuate the transient change of ac-side active power after dc fault. Also, the proposed universal design is applicable to other MMCs with different capacity ratings. Finally, the threat of dc fault and its negative impact propagating into ac grid are both suppressed. Simulation and experimental results are provided to demonstrate the validity of the proposed FRT-CES strategy.