The　first　AC　500　kV　submarine　cable　circuit　in　China　was　put　into　operation　on　June　2009.　In　order　to　increase　the　transmission　capacity,　it　is　under　consideration　to　convert　this　kraft　paper　insulated　SCLF　cable　circuit　from　AC　to　DC　operation.　From　the　existing　single　circuit　AC　cable　line,　two　circuits　can　be　arranged　under　DC　operation,　one　bipolar　and　one　monopolar.　The　bipolar　circuit　consists　of　two　cables,　a　DC　plus　pole　and　a　DC　minus　pole.　The　monopolar　circuit　consists　of　only　one　cable.　According　to　the　cable　structure　and　circuit　grounding　mode,　return　current　will　flow　through　the　coaxial　metal　layers　of　the　cable　as　well　as　the　seawater　and　earth.　The　long　term　operation　voltage　is　to　be　designed　as　500　kV　based　on　the　insulation　withstand　levels　of　DC　voltage,　polarity　reversal　voltage　and　DC　superimposed　impulse　voltage.　The　permissible　load　current　is　determined　as　1150　A　and　1000　A　using　the　calculation　method　recommended　by　IEC60287　for　different　current　return　modes.　Under　the　designed　DC　operation　scheme,　the　transmission　capacity　will　be　2.5　times　more　than　the　existing　AC.　The　follow-up　testing　and　checking　work　is　underway.