Single-ended MMC-MTDC line protection based on dual-frequency amplitude ratio of traveling wave.

• The proposed DC line protection scheme based on the amplitude ratio of dual-frequency traveling wave can identify the internal and external faults reliably. • The sensibility under the internal fault with fault resistance of 1500Ω is still high. • The required time window is only 1.2ms after the traveling wave propagates to the protection device, which satisfies the fast protection requirement of MMC-MTDC grids. • The method of calculating h and the criterion for selecting the sampling frequency can be applied to any system. Fast and reliable DC line protection is an essential challenge for the modular multilevel converter (MMC) based multi-terminal direct current (MTDC) grid. This paper proposed a novel single-ended protection scheme for DC lines in MMC-MTDC grids. Firstly, the values of the initial voltage traveling wave (IVTW) generated by internal and external faults are calculated by using the equivalent model of the MTDC grid, respectively. Then a criterion for identifying internal and external faults is constructed based on the amplitude ratio of the low-frequency component of the IVTW to its high-frequency component. The amplitude of the IVTW in both frequency bands is concisely expressed by the modulus maximums of stationary wavelet transform (SWT). Additionally, the criteria for discriminating the fault direction and the faulted pole are also proposed, which are both based on the integral values of the fault component currents on positive- and negative-pole lines. Lastly, simulation results verify the effectiveness of the proposed protection scheme. [ABSTRACT FROM AUTHOR]