Since　the　current　differential　protection　for　HVDC　transmission　line　is　of　poor　reliability　and　operates　with　long　time　delay,　a　novel　pilot　HVDC　line　protection　scheme　based　on　the　ratio　of　superimposed　current　is　proposed.　The　ratio　of　superimposed　current　at　specific　frequency　measured　at　DC　filter　to　superimposed　current　measured　at　DC　line　is　used　to　identify　the　line　faults.　For　internal　faults,　the　ratios　of　superimposed　current　at　rectifier　side　and　inverter　side　are　both　close　to　1,　while　for　external　faults　one　of　the　two　ratios　is　always　much　greater　than　1.　The　simulative　results　and　the　test　with　field　data　indicate　that　the　proposed　protection　scheme　can　identify　the　internal　and　external　faults　accurately　under　various　fault　conditions.　©　2019,　Electric　Power　Automation　Equipment　Press.　All　right　reserved.