Patent ID: 7830679

Claim:
A high voltage direct current transmission system, comprising: a converter station arranged at one end of a bipolar high voltage direct current transmission line, the converter station configured to connect said transmission line to an AC-system, said converter station comprising two converters each having a DC-side thereof connected to a respective of two poles of said transmission line on high potential and to a neutral bus for a pole of a DC neutral arrangement in common to the converters on zero potential by being grounded, and each converter having an AC-side connected to said AC system, said DC neutral arrangement having a member connecting to electrode lines and the arrangement being provided with first DC breakers enabling breaking of a first current path from the neutral bus of one pole to the neutral bus of the other pole at bipolar operation of the station for changing to monopolar operation thereof, said station also comprising a control device adapted to control said changing from bipolar to monopolar operation by controlling a said first DC breaker to open said first current path between said two buses and establish a current path to said electrode line connecting member for diverting the current from said one pole thereto, an electrode station for earth return of current from one of said poles under monopolar operation of the converter station with the other pole disconnected, said electrode station being connected to said DC neutral arrangement by said electrode lines connecting to said connecting member, wherein said DC neutral arrangement comprises a separate said electrode line connecting member for each of said electrode lines and a connector configured to connect each neutral bus to an optional of said electrode line connecting members, and wherein said electrode lines are dimensioned to be able to at monopolar operation of the converter station transmit substantially full current, to said electrode station through the remaining one or ones of the electrode lines at disconnection of an arbitrary of the electrode lines.