Pilot Protection for Hybrid DC Transmission Line Based on High-frequency Impedance Curve Matching

Hybrid DC transmission has the advantages of high voltage level and large transmission capacity, which solves the commutation failure problem at the receiving-end of the traditional DC system. However, the traditional DC transmission pilot protection relies on the fault characteristics generated by the filter under the topological symmetry of the sending and receiving end, and is greatly affected by the distributed capacitance. There is no filter at the receiving end in the hybrid DC transmission system, so the traditional DC transmission pilot protection can not be directly applied to the hybrid DC transmission system. In order to solve this problem, a pilot protection is proposed, which does not depend on the filter to generate fault characteristics and is not affected by the distributed capacitance. By analyzing the refraction and reflection of traveling waves, the difference between high-frequency measured impedances of protection with positive and negative line faults is obtained. The fault direction is identified by the waveform matching algorithm and the protection criterion is constructed. In this method, the short window is used to extract the high-frequency impedance characteristics of traveling waves, and the high-frequency characteristics of distributed capacitors are used to form differences, so as to solve the influence of distributed capacitors and improve the operation speed of the protection. Taking an actual project as an example, a two-terminal hybrid DC system model is built for simulation. The simulation results show that the method can quickly, sensitively and reliably identify the faults inside and outside the region, and has a good ability to withstand the transition resistance. © 2021 Automation of Electric Power Systems Press.