Apparatus for controlling the electric power transmission in a hvdc power transmission system

An apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission line for carrying direct current, DC, said system forming a DC grid, wherein the apparatus comprises a first converter for converting alternating current, AC, to direct current and/or direct current to alternating current and

• a second converter for converting direct current to alternating current and alternating current to direct current,
• each of the first and second converters having an AC side for output and/or input of alternating current and a DC side for output and/or input of direct current, the first converter is connectable via its DC side to the HVDC transmission line and the AC side of the second converter is connected to the AC side of the first converter, wherein the apparatus is further adapted to control the direct current of the HVDC transmission line by introducing a DC voltage in series with the HVDC transmission line, wherein the second converter is adapted to provide, directly or by way of a high frequency transformer connected between the first and the second converter, alternating current to the AC side of the first converter, and/or vice versa, and wherein the second converter is connectable via its DC side to a DC source.