A voltage source converter comprising first and second DC terminals (32, 34) for connection to a DC network, the voltage source converter including a plurality of limbs connected in series between the first and second DC terminals (32, 34), each limb including: a phase element including a plurality of switching elements switchable to selectively interconnect a DC side voltage at a DC side of the phase element and an AC side voltage at an AC side of the phase element; a DC side sub-converter connected to the DC side of the phase element, the DC side sub-converter configured to be operable as a waveform synthesizer to control the DC side voltage so as to form a DC voltage for presentation to the DC network; and an AC side sub-converter connected to the AC side of the phase element, the AC side sub-converter configured to be operable as a waveform synthesizer to control the AC side voltage so as to form a or a respective AC phase voltage for presentation to a or a respective AC phase of an AC network, wherein the voltage source converter includes a controller programmed to selectively coordinate the operation of the DC and AC side sub-converters (38, 39) as the respective waveform synthesizers to simultaneously: synthesize one or more selected harmonic voltage components to control the or each DC side voltage so as to present a constant or substantially constant DC voltage to the DC network; and cancel the or each selected harmonic voltage component from the or each AC side voltage so as to present a or a respective AC phase voltage free of the or each selected harmonic voltage component to the corresponding AC phase of the AC network, the voltage source converter being characterised in that each limb further includes a winding connected in series with the AC side sub-converter at the AC side of the corresponding phase element, each winding magnetically coupled to a respective one of a plurality of additional windings, and the plurality of additional windings are electrically interconnected in a daisy chain arrangement, preferably a delta arrangement. a phase element including a plurality of switching elements switchable to selectively interconnect a DC side voltage at a DC side of the phase element and an AC side voltage at an AC side of the phase element; a DC side sub-converter connected to the DC side of the phase element, the DC side sub-converter configured to be operable as a waveform synthesizer to control the DC side voltage so as to form a DC voltage for presentation to the DC network; and an AC side sub-converter connected to the AC side of the phase element, the AC side sub-converter configured to be operable as a waveform synthesizer to control the AC side voltage so as to form a or a respective AC phase voltage for presentation to a or a respective AC phase of an AC network, wherein the voltage source converter includes a controller programmed to selectively coordinate the operation of the DC and AC side sub-converters (38, 39) as the respective waveform synthesizers to simultaneously: synthesize one or more selected harmonic voltage components to control the or each DC side voltage so as to present a constant or substantially constant DC voltage to the DC network; and cancel the or each selected harmonic voltage component from the or each AC side voltage so as to present a or a respective AC phase voltage free of the or each selected harmonic voltage component to the corresponding AC phase of the AC network, the voltage source converter being characterised in that each limb further includes a winding connected in series with the AC side sub-converter at the AC side of the corresponding phase element, each winding magnetically coupled to a respective one of a plurality of additional windings, and the plurality of additional windings are electrically interconnected in a daisy chain arrangement, preferably a delta arrangement.