Simulation Analysis of Fault Characteristics of Offshore Wind Power DC Delivery System based on Hybrid Series Connection of DRU and MMC

At present, most domestic offshore wind power DC transmission systems are based on MMC (Modular Multilevel Converter) technology. In order to reduce the size of offshore converter platforms and improve transmission efficiency, domestic researchers have proposed a hybrid converter structure combining a DRU (Diode Rectifier Unit) and an MMC. However, current research on this structure mainly focuses on scheme design, and little is known about the electromagnetic transient characteristics of such systems under fault conditions. In this paper, we first construct an electromagnetic transient model of an offshore wind power DC transmission system featuring a series connection of DRU and MMC. Based on this model, we then investigate the system's transient characteristics under typical AC and DC fault conditions. The results show that the hybrid DRU-MMC-based offshore wind power DC transmission system is more likely to pose a threat to equipment insulation during DC-side faults, due to its symmetrical unipolar structure. This study provides valuable insights for the insulation coordination design of such systems.