Transient Current Waveform Similarity Based Protection for Flexible DC Distribution System

Unlike the traditional system frequency measurement-based protection in ac distribution systems, the protection in dc systems must deal with complex fault transients and identify the faulty section within milliseconds, as power electronic devices can only endure a fault current for several milliseconds. To address this problem, this paper presents a novel protection principle using transient current waveform similarity for a flexible dc distribution system. Fault current waveform characteristics are presented in detail, and cosine transient waveform similarity is adopted to fully distinguish internal and external faults. Hardware tests and simulation results show that the proposed protection can identify internal faults in less than 0.5 ms based on different fault impedances and measurement noise.