Patent ID: 8867241

Claim:
A method of upgrading a plant for transmitting electric power through High Voltage Direct Current (HVDC), said plant comprising two converter stations interconnected by a pole conductor of a monopolar direct voltage network and each having an alternating voltage side for feeding electric power from one of said alternating voltage sides to the other, each station having a line commutated Current Source Converter having at least one phase leg with one end connected to said pole conductor of the direct voltage network and the other end having an earth connection in the form of a connection to an earth electrode or a return current conductor, the method comprising: interconnecting said stations by a further pole conductor such that said direct voltage network is made bipolar, and providing each station with a Voltage Source Converter having at least one phase leg, one end of which is connected to said further pole conductor and the other end to said earth connection and by that to said Current Source Converter, the method further comprising: providing two switches for both Voltage Source Converters of the plant, and providing means for controlling said four switches so as to enable to change, when the feeding direction of power transmitted between said stations is to be changed, which end of a said phase leg of each converter provided with said switches is connected to one of said pole conductors and which end is connected to said earth connection and by that to the other converter of the station so as to maintain the direction of the current flowing in each said pole conductor upon a said change of the feeding direction of the power transmitted between said stations.