Control Strategies for Start-Up and AC Fault Ride-Through in DRU–MMC Hybrid HVDC Offshore Transmission System

As offshore wind power continues to expand into deeper waters, hybrid HVDC systems combining diode rectifier units (DRUs) and modular multilevel converters (MMCs) have gained attention for their compactness, reduced cost, and high reliability. However, practical deployment is hindered by the unidirectional conduction of DRUs during startup and instability under offshore AC faults. This paper proposes a coordinated startup strategy based on local auxiliary power and an enhanced fault ride-through control scheme tailored to the DRU-MMC topology. A 2-GW electromagnetic transient model is developed in PSCAD/EMTDC for validation. Simulation results show that the proposed methods enable smooth offshore converter energization, reliable wind turbine synchronization, effective fault-induced overcurrent suppression, and rapid post-fault recovery. The results verify the feasibility and robustness of the proposed control strategies for large-scale offshore wind power transmission.