DC line protection with high speed and strong robustness against high fault resistance is essential in high-voltage direct current (HVDC) grids. The conventional voltage-derivative-based protections for DC lines exhibit the problem of low reliability on the occurrence of high-impedance faults. In this paper

fault traveling waves at the fault point and at the line side of the DC reactor are first analyzed using phase-modal transformation. Based on the theoretical analysis of the modal fault-component voltages at the line side of the DC reactor

a novel hybrid protection scheme utilizing the single-ended rate of change of voltage (ROCOV) and waveform similarity coefficient is proposed. It utilizes the hybrid ROCOV criterion and waveform similarity coefficient to identify the faulted line and the amplitudes of the 0- and 1-mode fault-component voltages to identify fault type

and the polarities of the 0- and 1-mode fault-component voltages to identify faulted pole. Simulations under various fault conditions verify that the proposed protection scheme can achieve fast and reliable identification of faulted line

fault type

and faulted pole in 1 ms

and effectively im

prove the reliability against high-impedance faults up to

$1200\ \Omega$

.