Aiming at the power imbalance problem caused by power frequency side faults in the offshore wind power flexible low-frequency transmission system based on modular multilevel converter (M3C), this paper proposes a fault ride-through strategy of low-frequency side voltage reduction, which is based on the unbalanced power on both the power frequency and low-frequency sides. First, relying on the mathematical models of the control strategies for the power frequency side and low-frequency side of M3C, the dynamic variation laws of active and reactive power during power frequency side faults are derived, and a low-frequency voltage reduction control strategy is designed. Finally, by linking with the low-voltage ride-through control of the wind farm, the fault ride-through of the entire system is achieved. A simulation system for offshore wind power flexible low-frequency transmission is established based on PSCAD/EMTDC, and the proposed strategy is verified under the fault scenarios of symmetric three-phase short circuit and BC phase short-circuit grounding (asymmetric fault). The simulation results show that the proposed protection method can realize the system's fault ride-through and prevent the overvoltage locking of M3C.
Zuo et al. (Fri,) studied this question.