A voltage source converter (30) comprises first and second DC terminals (32,34) for connection to a DC network (58). The voltage source converter (30) further includes at least one limb connected between the first and second DC terminals (32,34). The or each limb includes: a phase element (36) including two parallel-connected sets of series-connected switching elements (40) connected in an H-bridge to define first and second diagonal switching pairs, a respective junction between each set of series-connected switching elements (36) defining an AC terminal (40) for connection to an AC network (50); and a sub-converter (38,39) configured to be controllable to act as a voltage waveform synthesiser; wherein the voltage source converter (30) further includes a controller (60) to operate the sub-converter (38,39) to selectively synthesise a driving commutation voltage to modify a DC side current at a DC side of the H-bridge so as to minimise any differences in magnitude and direction between the DC side current and an AC side current at an AC side of the H-bridge and thereby carry out commutation of current from one of the first and second diagonal switching pairs to the other of the first and second diagonal switching pairs.