Patent ID: 8289736
Filing Date: 2012-10-16
Classification: H02J,Y02E

Abstract:
1. An electric power transmission system, comprising: a high voltage direct current line transmission line comprising three conductors; and a converter station arranged at each end of the high voltage direct current line transmission line, the converter station is configured to convert an alternating voltage into a direct voltage for transmitting direct current between said stations in all three conductors, each converter station comprises a voltage source converter comprising at least one phase legs having current valves of turn-off semiconductor devices and rectifying members arranged anti-parallel with the semiconductor devices, the rectifying members being connected in series between a first and a second pole conductor of said conductors and a midpoint being connected to an alternating voltage side of the converter, and a control unit for switching the current valves by controlling the semiconductor devices thereof for converting said alternating voltage into a direct voltage applied to said two pole conductors, wherein the converter station further comprises an extra phase leg of current valves of semiconductor devices of turn-off type and rectifying members connected in anti-parallel therewith connected in series between said two pole conductors on the direct voltage side of said converter, wherein a third of said conductors is connected to a midpoint between current valves of said extra phase leg, and wherein the converter station further comprises an arrangement adapted to control said current valves of said extra phase leg to switch for connecting said third conductor either to the first pole conductor or the second pole conductor for utilizing the third conductor for conducting current between said stations.