Fault ride-through of flexible low-frequency transmission system for offshore wind power based on voltage reduction method

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.