A voltage source converter comprises first and second DC terminals for connection to a DC network, and at least one limb connected between the first and second DC terminals. The or each limb includes: a phase element including two parallel-connected sets of series-connected switching elements connected in an H-bridge to define first and second diagonal switching pairs, a respective junction between each set of series-connected switching elements defining an AC terminal for connection to an AC network; and a sub-converter configured to be controllable to act as a voltage waveform synthesizer. The voltage source converter further includes a controller to operate the sub-converter to selectively synthesize a driving commutation voltage to modify a DC side current at a DC side of the H-bridge to minimize any differences in magnitude and direction between the DC side current and an AC side current at an AC side of the H-bridge.