Patent ID: 8014178

Claim:
A converter station for connecting an AC system to a bipolar HVDC transmission line, said station comprising: two converters each comprising a DC-side, each DC-side connected to a first pole of said transmission line, each DC-side also being connected to a first neutral bus 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 comprising two electrode lines connecting members, each electrode line connecting member being connected to at least one of the electrode lines, the DC neutral arrangement further comprising first DC breakers that are closed during bipolar operation of the station so that at least two first current paths are formed from the first neutral bus of the first pole of the transmission line to a second neutral bus of a second pole of the transmission line, wherein said DC neutral arrangement further comprises two connectors, each connector being configured to connect each neutral bus to one of said two electrode line connecting members, and a control device adapted to control a change from bipolar to monopolar operation by controlling said first DC breakers to open said at least two first current paths between said first neutral bus and said second neutral bus and to establish a current path to said electrode line connecting members for diverting the current from said first pole and said second pole thereto, wherein each of said at least two first current paths has at least two of said first DC breakers connected in series for backup of each other, and wherein each of said at least two first current paths is designed to allow a current to flow therethrough for bipolar operation of the station irrespective of the state of said first DC breakers in any other said at least two first current paths.