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.　©　The　Institution　of　Engineering　and　Technology　2020.