An AC Fault Ride Through Method for MMC-HVDC System in Offshore Applications Including DC Current-Limiting 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 firstly 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, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. 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, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results.