Operation and DC Protection of Hybrid DRU-MMC HVDC System for Offshore Wind Integration

Hybrid HVDC transmission with diode rectifier units (DRUs) and modular multilevel converters (MMCs) in parallel has been identified as a promising and economical solution for large-scale wind integration in recent years. This work first studies two typical startup sequences of DRU-MMC HVDC in the literature, then analyzes the transient performance of DRU-MMC during the startup process for both sequences. Based on this analysis, an optimized startup sequence is proposed to reduce the overcurrent during the startup of the DRU-MMC system. Considering that the existing DC fault analysis of DRUMMC HVDC mainly focuses on pole-to-pole (PTG) faults in MMC, this paper specifically studies the fault transients of the DRU-MMC system under PTG fault in MMC, under which, the grounding point of the DRU's passive filter can form a discharge path between the fault point and the DRU, leading to discharge of MMC arms and system tripping. In order to maintain power transmission between offshore and onshore MMC during a PTG fault of the MMC-HVDC, this paper proposes a modified grounding scheme of the DRU's passive filter. The effectiveness of the proposed startup sequence and the modified passive filter configuration is validated by performing electromagnetic simulation (EMT) in MATLAB/Simulink.