A voltage source converter for use in high voltage direct current power transmission

A voltage source converter (VSC) 10 for high voltage direct current (HVDC) power transmission comprises a plurality of converter limbs 12A, 12B, 12C, extending between DC terminals 14, 16, each including first and second limb portions 12A+, 12A-, 12B+, 12B-, 12C+, 12C- separated by an AC terminal 18A, 18B, 18C. Each limb portion includes a chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C-, providing a stepped variable voltage source, and a switching element SW1-6. Each converter limb is selectively operable in a fully-conducting mode, where the primary switching element in each limb portion is conducting, and a partially-conducting mode where the primary switching element in one limb is conducting and the switching element in the other limb is non-conducting. A controller 32 transitions a given converter limb from fully-conducting mode to partially-conducting mode by: transitioning another converter limb from partially conducting mode to fully-conducting mode while the given converter limb remains in fully-conducting mode; and circulating a commutation current (IcomA, IcomB, IcomC) around a commutation loop 40 defined by the converter limb transitioned into fully-conducting mode and the converter limb remaining in fully-conducting mode to facilitate the switching into non-conducting mode of a switching element in the given converter limb