Asymmetrical fault control strategy for offshore wind power low-frequency transmission system based on segmental dynamic voltage regulation and negative sequence current suppression

Modular multilevel matrix converter (M3C)-based low-frequency AC transmission systems (LFTS) are increasingly applied in offshore wind power transmission. However, asymmetrical faults can lead to several critical issues, including overcurrent in the M3C’s power devices, overvoltage in the healthy phase, and power imbalances across the converter, all of which pose serious threats to system stability and safety. To address these challenges and leverage the high controllability of M3Cs, this paper proposes a fault ride-through control strategy tailored for asymmetrical faults in offshore wind LFTS. The proposed approach dynamically adjusts the low-frequency side voltage of the M3C in segments based on the fault severity, thereby mitigating overvoltage in the non-faulted phases. Additionally, each sub-module restricts short-circuit current surges and suppresses negative-sequence currents to prevent overcurrent damage and minimize power imbalance. The effectiveness of the proposed strategy is validated through simulations conducted in PSCAD/EMTDC, demonstrating that the system maintains stable operation under asymmetrical fault conditions on the low-frequency side.