Voltage source converter

A voltage source converter comprising:

• first, second and third branches, each branch having first and second ends, the first ends of the first, second and third branches being connected to a junction to form a star configuration in which the junction defines a mid-point of the star configuration, the second end of each branch defining a DC terminal, the second ends of the second and third branches being connectable to a first DC voltage, the second ends of the first and second branches being connectable to a second DC voltage, the first branch including at least one first primary switching element, the second branch including at least one second primary switching element, the third branch including a primary inductive element, each of the first and second primary switching elements being switchable to switch the corresponding branch into and out of circuit to transfer power between the first and second DC voltages;
• and
• a converter limb connected in parallel with one of the first and second branches, the converter limb including an auxiliary converter, the auxiliary converter being controllable to selectively provide a voltage source to modify a branch voltage across the branch connected in parallel with the converter limb,
• wherein the auxiliary converter is a chain-link converter, the chain-link converter including a plurality of series-connected modules, each module including at least one secondary switching element and at least one energy storage device, the or each secondary switching element and the or each energy storage device in each module combining to selectively provide a voltage source.