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 between the first and second DC terminals (32,34), each limb including: a phase element having a plurality of switching elements and at least one AC terminal for connection to a respective phase of a multi-phase AC network, the plurality of switching elements configured to be 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 first sub-converter configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to the DC network, the first sub-converter including at least one energy storage device capable of storing and releasing energy to selectively provide a voltage; and a second sub-converter connected in series with the phase element in an electrical block, the first sub-converter connected in parallel with the electrical block, the second sub-converter configured to be controllable to act as a waveform synthesizer to modify a second DC voltage presented to the DC side of the phase element, the second sub-converter including at least one energy storage device capable of storing and releasing energy to selectively provide a voltage, wherein the voltage source converter further includes a controller configured to selectively operate in an energy management mode when there is an imbalance between the AC side voltages (V Ca,V Cb,V Cc) at the AC sides of the phase elements, the controller in the energy management mode configured to operate each limb so as to: control at least one sequence current component of a respective phase current (I a,I b,I c) at the AC side of each phase element to control an exchange of power between the voltage source converter and the AC network; and balance the AC and DC side powers exchanged by each limb with the AC and DC networks (50,58) respectively so that a respective net change in energy stored in the energy storage devices of each limb is controlled to be zero or substantially zero. a phase element having a plurality of switching elements and at least one AC terminal for connection to a respective phase of a multi-phase AC network, the plurality of switching elements configured to be 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 first sub-converter configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to the DC network, the first sub-converter including at least one energy storage device capable of storing and releasing energy to selectively provide a voltage; and a second sub-converter connected in series with the phase element in an electrical block, the first sub-converter connected in parallel with the electrical block, the second sub-converter configured to be controllable to act as a waveform synthesizer to modify a second DC voltage presented to the DC side of the phase element, the second sub-converter including at least one energy storage device capable of storing and releasing energy to selectively provide a voltage, wherein the voltage source converter further includes a controller configured to selectively operate in an energy management mode when there is an imbalance between the AC side voltages (V Ca,V Cb,V Cc) at the AC sides of the phase elements, the controller in the energy management mode configured to operate each limb so as to: control at least one sequence current component of a respective phase current (I a,I b,I c) at the AC side of each phase element to control an exchange of power between the voltage source converter and the AC network; and balance the AC and DC side powers exchanged by each limb with the AC and DC networks (50,58) respectively so that a respective net change in energy stored in the energy storage devices of each limb is controlled to be zero or substantially zero. control at least one sequence current component of a respective phase current (I a,I b,I c) at the AC side of each phase element to control an exchange of power between the voltage source converter and the AC network; and balance the AC and DC side powers exchanged by each limb with the AC and DC networks (50,58) respectively so that a respective net change in energy stored in the energy storage devices of each limb is controlled to be zero or substantially zero.