A novel control strategy for AC fault ride through in a MTDC system with offshore wind farms

This paper proposes a novel control strategy-Coordinated DC Voltage Control (CDVC)-to enhance the AC fault ride through capability of Multi-terminal HVDC system. It is able to keep DC voltage constant and distribute power fast and smoothly, independent on line impedance parameters. Combined with this original control strategy, a series of AC fault ride through methods, including voltage reduction, frequency increase and DC chopper methods are applied to avoid overvoltage of HVDC lines. Compared with the conventional voltage droop control, the proposed CDVC strategy can keep DC voltage at rated value on different conditions and make full use of active power by adjusting power distribution ratio during fault periods. The control effect is validated in two scenarios by a five-terminal HVDC system based on PSCAD/EMTDC.