Pilot Protection for MTDC Grid Based on Phase frequency characteristics of Measured-impedance

The fault transient characteristics in multi-terminal flexible direct current (MTDC) power grid are complex, and the fault current rises rapidly, which put higher demands on the performance of DC line protection. The article is based on a model of MTDC grid for fault transient analysis. Firstly, the phase frequency characteristics of measured-impedance during internal and external faults were derived, revealing the differences in measured-impedance phase at different fault locations. Then, using the norm ratio of the theoretical and actual measuredimpedance phase in multiple frequency bands on both sides of the line, a highly reliable DC line pilot protection scheme is constructed. The simulation results have verified the correctness of the theoretical analysis, indicating that the method can reliably identify different types of internal and external faults and has strong resistance to transition resistance, and is less affected by noise.