Self-adaptive Droop Control Strategy of DR-MMC Hybrid DC Grid for Offshore Wind Power Transmission

Abstract

The multi-terminal hybrid DC power grid, which is comprised of low-cost, small volume, and high power density diode rectifiers (DRs) and small capacity sending-end offshore modular multilevel converters (MMC), combines the advantages of the two types of converters. The topology research is vital for future deep-sea wind power transmission development, which may realize the black start-up and flexible power transmission of the offshore wind power without any auxiliary equipment. Based on the mathematical model of the AC/DC power flow, the steady-state operation characteristics of the hybrid system with offshore MMC constant voltage/frequency(V/f) and the constant droop coefficient control are initially analyzed. Then, a self-adaptive droop control strategy is proposed to reduce the capacity requirement of the offshore MMC and the fluctuation of the offshore AC voltage under the variable wind power. This proposed control strategy is expected to bring the advantages of MMCs' controllability and DRs' high-efficiency active power transmission ability into full play. Finally, a case study of a five-terminal DR-MMC hybrid DC system for offshore wind power transmission is simulated in the PSCAD/EMTDC. The simulation results verify the effectiveness and superiority of the proposed control strategy.