A voltage source converter comprising first and second DC terminals 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, each limb including: at least one terminal for connection to an AC network; and at least one first sub-converter operably connected to the at least one terminal, the first sub-converter configured to be controllable to act as a waveform synthesizer to control a DC voltage for presentation to the DC network, wherein the first sub-converters of each limb are connected in series, and wherein each limb is configured to be operable to exchange a respective DC power (P DC) with the DC network and a respective AC power (P A, P B, P c) with the AC network, wherein the voltage source converter further includes a controller configured to selectively operate the first sub-converters to generate a summed DC voltage including a first AC even harmonic voltage component (v A k, v B k, v C k) and a second AC even harmonic voltage component for presentation to the DC network; characterized in that the controller is further configured to control a circulating current to flow through the first sub-converters in response to a change in the or each AC power (P c) exchanged between at least one of the limbs and the AC network, the circulating current including a direct current component and an even harmonic current component resulting from the second AC even harmonic voltage component, wherein the controller is configured to control the even harmonic current component to interact with the first AC even harmonic voltage component generated by the or each first sub-converter of the at least one limb so as to balance the DC and AC powers (P DC, P c) exchanged by the at least one limb with the DC and AC networks (58,50) respectively. at least one terminal for connection to an AC network; and at least one first sub-converter operably connected to the at least one terminal, the first sub-converter configured to be controllable to act as a waveform synthesizer to control a DC voltage for presentation to the DC network, wherein the first sub-converters of each limb are connected in series, and wherein each limb is configured to be operable to exchange a respective DC power (P DC) with the DC network and a respective AC power (P A, P B, P c) with the AC network, wherein the voltage source converter further includes a controller configured to selectively operate the first sub-converters to generate a summed DC voltage including a first AC even harmonic voltage component (v A k, v B k, v C k) and a second AC even harmonic voltage component for presentation to the DC network; characterized in that the controller is further configured to control a circulating current to flow through the first sub-converters in response to a change in the or each AC power (P c) exchanged between at least one of the limbs and the AC network, the circulating current including a direct current component and an even harmonic current component resulting from the second AC even harmonic voltage component, wherein the controller is configured to control the even harmonic current component to interact with the first AC even harmonic voltage component generated by the or each first sub-converter of the at least one limb so as to balance the DC and AC powers (P DC, P c) exchanged by the at least one limb with the DC and AC networks (58,50) respectively.