Non‐unit travelling wave protection method for dc transmission line using waveform correlation calculation

High voltage dc (HVDC) transmission, especially voltage source converter (VSC)‐based HVDC transmission requires high‐speed operation for relay protection. However, traditional derivative‐based non‐unit travelling wave‐based protection (TWP) methods are sensitive to the fault impedance, resulting in low protection sensitivity in case of the high‐impedance fault. First, the fault information containing in the fault travelling wavefront is analysed based on the frequency‐dependent parameters of the dc transmission line and the broadband characteristic of the fault‐point initial travelling wave. It can be concluded that the distortion of the fault travelling wavefront depends on the fault distance and the amplitude of the fault travelling wavefront depends on the fault impedance. Then, a principle of non‐unit TWP for dc transmission line based on waveform correlation calculation is proposed. The proposed method is verified in both thyristor‐based HVDC and VSC HVDC PSCAD test systems. The simulation results verify the high‐speed, high fault‐impedance tolerability and correctness of the proposed protection method.