Voltage source converter

A voltage source converter (30) comprises first and second DC terminals (32,34) for connection to a DC electrical 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 including: a phase element (36) including a plurality of switching elements (40) to interconnect the DC electrical network (58) and an AC electrical network (50); an auxiliary sub-converter (38) configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to the DC electrical network (58); and a tertiary sub-converter (39) connected with the phase element (36) in an electrical block, the auxiliary sub-converter (38) being connected in parallel with the electrical block, the tertiary sub-converter (39) being configured to be controllable to act as a waveform synthesizer to modify a second DC voltage presented to a DC side of the phase element (36), the tertiary sub-converter (39) including at least one energy storage device; wherein the voltage source converter (30) further includes a controller (60) configured to selectively control the or each tertiary sub-converter (39) to synthesize at least one tertiary voltage component so as to transfer energy to or from that tertiary sub-converter (39) and thereby regulate an energy level of that tertiary sub-converter (39).