Non-Unit Protection for Flexible DC Lines Based on Mirror Waveform Opening Index of Initial Forward Traveling Wave

Flexible DC grid is of great significance for effective consumption of large-scale renewable energy. However, conventional traveling wave protections for DC transmission lines encounter challenges such as limited tolerance to fault resistance and susceptibility to lightning interference, leading to potential misoperations and reduced the reliability. To tackle these issues, this paper begins by deriving time-domain analytical expressions for the initial forward traveling wave (FTW) under conditions of internal faults, external faults, and lightning interference. These expressions reveal the characteristic differences of the initial FTW under each case. Subsequently, combined with the mathematical method of waveform mirroring and quadratic polynomial fitting, the non-unit protection method based on the mirror waveform opening index of the initial forward traveling wave is proposed. The method takes advantage of the differences in FTW characteristics under different conditions, theoretically unaffected by fault resistance. Extensive simulation results demonstrate that the proposed protection method operates fast and accurately identifies different DC line faults and lightning interference within 1 ms, and it is able to withstand fault resistances up to 1000Ω. Furthermore, the proposed protection method is highly robust to noise conditions and is applicable to various grid topologies.