Distance Protection for AC Transmission Lines of MMC-HVDC System With Offshore Wind Farms Based on Parameter Identification

The modular multilevel converter for high voltage direct current (MMC-HVDC) technology has become competitive for the long-distance transmission of offshore wind farms. For ac transmission lines between MMC-HVDC system and offshore wind farms, both sides are power electronic equipment, which effect fault characteristics greatly since there is no conventional synchronous generators. Ac faults may cause over-current in MMC arms and block of the converter after several milliseconds, so conventional AC line protection faces great challenges because of short duration of fault current to use. In order to solve this problem, a distance protection based on parameter identification is proposed. This method is not affected by power supply characteristics of power electronic equipment. The proposed protection uses the short-time fault information to realize fault identify. A detailed model of MMC-HVDC system with offshore wind farms is built in SIMULINK, then the effectiveness of the proposed protection is verified. The simulation results show that the proposed method can identify AC faults reliably and quickly.