Adaptability Analysis of Power Frequency Variation Distance Protection for AC-side of Receiving-end Converter of Offshore Wind Power MMC-HVDC System

Flexible DC transmission is the first choice to solve the problem of large-scale offshore wind power transmission over long distance. However, when the short faults occurs on the AC-side of receiving-end converter, the fault characteristics have fundamentally changed due to the influence of the control strategy, and the adaptability of the power frequency variation distance relay protection of AC line remains to be explored. The different control strategies of power electronic equipment make the equivalent impedance characteristics of power system complex and uncertain, which make the analysis on impedance plane is difficult. Therefore, the fundamental principle of the power frequency variation distance protection on the voltage plane is analyzed. Based on the control strategy of the offshore wind power MMC-HVDC system, the adaptability of the power frequency variation distance protection in the case of different types of faults on the AC side of the receiving-end converter is analyzed on the voltage plane. The results showed that due to the influence of the converter control strategy, when the phase-to-phase short fault occurred on the AC line, the power frequency variation distance protection had a greater risk of refusal to operate. When a phase -to-ground fault occurred, due to the existence of zero-sequence current, the adaptability of the power frequency variation distance protection was better than that of the phase-to-phase short fault. In PSCAD/EMTDC, the model of the wind power MMC-HVDC system was established, and a great number of experiments verified the correctness of the theoretical analysis, which provides a theoretical basis for the study of protection configuration.