A waveform-similarity-based protection scheme for the VSC-HVDC transmission lines

•The non-unit protection based on the traveling waveform similarity is proposed for the VSC-HVDC transmission line.•The traveling waveform similarity differences between internal and external faults are proved in theory.•The protection scheme can identify the double-pole-to-ground fault with its positive-pole fault resistance unequal to negative-pole fault resistance accurately.•The scheme provides high performance considering the possible influence factors.•The method improves the protection selectivity, reliability, rapidity, and sensitivity.