A converter station, for a bipole power transmission scheme, comprising: a first voltage source converter having a first terminal for connection to a first transmission conduit, a second terminal for connection to a return conduit, and at least one converter limb (18A, 18B, 18C) extending between the first and second terminals (14, 16), the or each converter limb (18A, 18B, 18C) including first and second limb portions (20A, 20B, 20C, 22A, 22B, 22C) separated by a corresponding first AC terminal (24A, 24B, 24C) for connection to a respective phase (A, B, C) of a first AC network, the or each first limb portion (20A, 20B, 20C) including a first chain-link converter extending between the associated first AC terminal (24A, 24B, 24C) and the first terminal, the or each second limb portion (22A, 22B, 22C) including a second chain-link converter extending between the associated first AC terminal (24A, 24B, 24C) and the second terminal, and each chain-link converter (26, 28) including a plurality of series connected chain-link modules each having 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; and a second voltage source converter having a third terminal for connection to a second transmission conduit, a fourth terminal for connection to the return conduit, and at least one converter limb (18A, 18B, 18C) extending between the third and fourth terminals (34, 36), the or each converter limb (18A, 18B, 18C) including third and fourth limb portions (38A, 38B, 38C, 40A, 40B, 40C) 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) including a third chain-link converter extending between the associated second AC terminal (42A, 42B, 42C) and the third terminal, the or each fourth limb portion (40A, 40B, 40C) including a fourth chain-link converter extending between the associated second AC terminal (42A, 42B, 42C) and the fourth terminal, and each chain-link converter (44, 46) including a plurality of series connected chain-link modules each having 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, wherein the second terminal and the fourth terminal are electrically connected to one another, characterised in that each of the second and third chain-link converters (28, 44) includes a first number of series-connected chain-link modules, and each of the first and fourth chain-link converters (26, 46) includes a second number of series-connected chain-link modules, the second number being greater than the first number. a first voltage source converter having a first terminal for connection to a first transmission conduit, a second terminal for connection to a return conduit, and at least one converter limb (18A, 18B, 18C) extending between the first and second terminals (14, 16), the or each converter limb (18A, 18B, 18C) including first and second limb portions (20A, 20B, 20C, 22A, 22B, 22C) separated by a corresponding first AC terminal (24A, 24B, 24C) for connection to a respective phase (A, B, C) of a first AC network, the or each first limb portion (20A, 20B, 20C) including a first chain-link converter extending between the associated first AC terminal (24A, 24B, 24C) and the first terminal, the or each second limb portion (22A, 22B, 22C) including a second chain-link converter extending between the associated first AC terminal (24A, 24B, 24C) and the second terminal, and each chain-link converter (26, 28) including a plurality of series connected chain-link modules each having 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; and a second voltage source converter having a third terminal for connection to a second transmission conduit, a fourth terminal for connection to the return conduit, and at least one converter limb (18A, 18B, 18C) extending between the third and fourth terminals (34, 36), the or each converter limb (18A, 18B, 18C) including third and fourth limb portions (38A, 38B, 38C, 40A, 40B, 40C) 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) including a third chain-link converter extending between the associated second AC terminal (42A, 42B, 42C) and the third terminal, the or each fourth limb portion (40A, 40B, 40C) including a fourth chain-link converter extending between the associated second AC terminal (42A, 42B, 42C) and the fourth terminal, and each chain-link converter (44, 46) including a plurality of series connected chain-link modules each having 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, wherein the second terminal and the fourth terminal are electrically connected to one another, characterised in that each of the second and third chain-link converters (28, 44) includes a first number of series-connected chain-link modules, and each of the first and fourth chain-link converters (26, 46) includes a second number of series-connected chain-link modules, the second number being greater than the first number.