Novel ZVRT Strategy of All-DC Offshore Wind Power Systems Based on DC Voltage Switching Control

All-DC offshore wind power (OWP) systems have seen rapid development recently, yet they face challenge in meeting zero-voltage ride-through (ZVRT) requirements during onshore AC faults. To address this issue, a novel ZVRT strategy based on DC voltage switching control is proposed. In this scheme, the offshore DC transformer (DCT) seamlessly switches to DC voltage control of the high-voltage DC (HVDC) link upon detecting local HVDC overvoltage, thereby exporting surplus power from the HVDC link to the medium-voltage DC (MVDC) network. When DC wind turbines (WTs) sense overvoltage in the MVDC network, they transition from DC voltage control of the internal low-voltage DC (LVDC) bus to MVDC voltage control, allowing many WTs to jointly regulate the MVDC network voltage. WT's internal DC choppers are employed to dissipate unbalanced power and maintain the LVDC voltage stable. Additionally, a fast fault detection scheme is proposed to enable the DC WTs to rapidly sense onshore AC faults and reduce power generation, thus mitigating overvoltage issues during faults. The proposed approach eliminates the need for onshore energy dissipation devices and achieves 350 ms ZVRT operation without communication, thereby improving system reliability and cost-effectiveness. Simulation results validate the effectiveness of the proposed ZVRT strategy.