A voltage source converter includes at least one limb connected between DC terminals, the or each limb including: a phase element including switching elements to interconnect a DC electrical network and an AC electrical network; an auxiliary sub-converter configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to the DC electrical network; and a tertiary sub-converter connected in parallel with the electrical block and controllable to act as a waveform synthesizer to modify a second DC voltage presented to a DC side of the phase element, the tertiary sub-converter (39) including at least one energy storage device. The voltage source converter includes a controller configured to selectively control the or each tertiary sub-converter to synthesize at least one tertiary voltage component so as to transfer energy to or from that tertiary sub-converter and thereby regulate an energy level of that tertiary sub-converter.