AC Fault Characteristic Analysis and Fault Ride-through of Offshore Wind Farms Based on Hybrid DRU-MMC

With the rapid development of large-scale offshore wind farms, efficient and reliable power transmission systems are urgently needed. Hybrid high-voltage direct current (HVDC) configurations combining a diode rectifier unit (DRU) and a modular multilevel converter (MMC) have emerged as a promising solution, offering advantages in cost-effectiveness and control capability. However, the uncontrollable nature of the DRU poses significant challenges for system stability under offshore AC fault conditions, particularly due to its inability to provide fault current or voltage support. This paper investigates the offshore AC fault characteristics and fault ride-through (FRT) strategy of a hybrid offshore wind power transmission system based on a diode rectifier unit DRU and MMC. First, the dynamic response of the hybrid system under offshore symmetrical three-phase faults is analyzed. It is demonstrated that due to the unidirectional conduction nature of the DRU, its AC current rapidly drops to zero during faults, and the fault current is solely contributed by the wind turbine generators (WTGs) and wind farm MMC (WFMMC). Based on this analysis, a coordinated FRT strategy is proposed, which combines a segmented current limiting control for the wind-turbine (WT) grid-side converters (GSCs) and a constant AC current control for the WFMMC. The strategy ensures effective voltage support during the fault and prevents MMC current saturation during fault recovery, enabling fast and stable system restoration. Electromagnetic transient simulations in PSCAD/EMTDC verify the feasibility of the proposed fault ride-through strategy.