An AC Fault Ride Through Method for MMC-HVDC Connecting to Renewable Plants Including Large DC Inductors

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, which hinders energy interactive and results in DC side power delay and voltage oscillation. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage and improve DC side dynamic by regulating the zero-sequence modulation of circulating current suppression. Finally, the feasibility of the proposed method is verified by simulation results.