Fault Characteristic Analysis and a Novel Pilot Protection Scheme for Pole-to-Ground Fault in Flexible DC Distribution Network

Due to the pseudo-bipolar structure of the flexible DC distribution network, the short-circuit current caused by a pole-to-ground fault is weak. The fault characteristics, particularly the steady-state fault characteristics, are not prominent. Therefore, pole-to-ground fault diagnosis and protection have become more challenging. This paper theoretically studies the short-circuit current characteristics of pole-to-ground fault in the flexible DC distribution network, derives the analytical expression of the short-circuit current, and analyzes the directional characteristics. Then, a novel pilot protection scheme based on fault direction comparison is proposed. This scheme compares the positive and negative half-wave integral values of the short-circuit current fault components to judge the fault direction. The fault directions on both sides of the feeder are obtained through the pilot protection channel, thereby distinguishing internal and external faults. The simulation results show that the proposed scheme can reliably identify the faulty feeder and has an excellent ability to perform with high transition resistance. Moreover, the scheme does not require sampling synchronization, which is easy to be applied in engineering.