Voltage source converter and control thereof

A voltage source converter comprising:

• a plurality of phase limbs connected between high and low DC terminals;
• wherein each phase limb comprises:
• a high side director switch connecting the high DC terminal to an AC node for that phase limb;
• a low side director switch connecting the AC node to the low DC terminal;
• and
• a chain-link circuit, a first end of the chain-link circuit being directly connected between the high side and low side director switches to be electrically coupled to the AC node;
• first and second additional chain-link circuits, wherein first ends of the first and second additional chain-link circuits are connected to the high side DC terminal and low side DC terminals respectively;
• and
• a converter controller configured to control the director switches of each phase limb in a power cycle and to control the chain-link circuits of each phase limb in a voltage mode to provide a controlled voltage transition at the AC node during the power cycle, wherein said converter controller is operable to control at least some of said chain-link circuits in a current mode to provide a defined current at times during the power cycle;
• wherein each chain-link circuit comprises a plurality of series-connected chain-link modules, each of the chain-link modules comprising at least one energy storage element that can be selectively connected in series with the other modules or bypassed;
• and
• wherein a second end of each of the chain-link circuits is connected to a common node such that, in use, a current can flow from one phase limb to another via the respective chain-link circuits.