IMPROVEMENTS IN OR RELATING TO CONVERTER STATIONS

In the field of bipole power transmission schemes there is a need for an improved converter station. [0002] A converter station (10), for a bipole power transmission scheme, comprises a first voltage source converter (12) which has a first terminal (14), for connection to a first transmission conduit, and a second terminal (16) for connection to a return conduit. The first voltage source converter (12) also includes at least one converter limb (18A, 18B, 18C) which extends between the first and second terminals (14, 16). The or each converter limb (18A, 18B, 18C) includes first and second limb portions (20A, 20B, 20C, 22A, 22B, 22C) that are separated by a corresponding first AC terminal (24A, 24B, 24C) which is for connection to a respective phase (A, B, C) of a first AC network. The or each first limb portion (20A, 20B, 20C) includes a first chain-link converter (26) that extends between the associated first AC terminal (24A, 24B, 24C) and the first terminal (14), and the or each second limb portion (22A, 22B, 22C) includes a second chain-link converter (28) which extends between the associated first AC terminal (24A, 24B, 24C) and the second terminal (16). Each chain-link converter (26, 28) includes a plurality of series connected chain-link modules (30), each of which has a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter (26, 28) is controllable to provide a stepped variable voltage source. The converter station (10) additionally includes a second voltage source converter (32) which has a third terminal (34), for connection to a second transmission conduit, and a fourth terminal (36), for connection to the return conduit. The second voltage source converter (32) also includes at least one converter limb (18A, 18B, 18C) which extends between the third and fourth terminals (34, 36). The or each converter limb (18A, 18B, 18C) includes third and fourth limb portions (38A, 38B, 38C, 40A, 40B, 40C) which are separated by a corresponding second AC terminal (42A, 42B, 42C), for connection to a respective phase (A, B, C) of a second AC network. The or each third limb portion (38A, 38B, 38C) includes a third chain-link converter (44) that extends between the associated second AC terminal (42A, 42B, 42C) and the third terminal (34), and the or each fourth limb portion (40A, 40B, 40C) includes a fourth chain-link converter (46) which extends between the associated second AC terminal (42A, 42B, 42C) and the fourth terminal (36). Each chain-link converter (44, 46) includes a plurality of series connected chain-link modules (30), each of which has a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter (44, 46) is controllable to provide a stepped variable voltage source. Also, each of the second and third chain-link converters (28, 44) includes a first number (58) of series-connected chain-link modules (30), and each of the first and fourth chain-link converters (26, 46) includes a second number (52) of series-connected chain-link modules (30), the second number (52) being greater than the first number (58).