A voltage source converter for use in high voltage direct current power transmission and reactive power compensation, the voltage source converter comprising: first and second DC terminals (12, 14) for connection in use to a DC network, three phase elements and three auxiliary converters connected between the first and second DC terminals (12, 14), each phase element including two parallel connected strings, each string including a pair of series-connected primary switching elements, the mid-points between each string of series-connected primary switching elements defining AC terminals for connection in use to a respective phase of an AC network; each auxiliary converter being connected in parallel with a respective one of the phase elements to define a single-phase converter limb and the three single-phase converter limbs being mounted and connected in series to the first and second DC terminals to define a two-terminal DC network for three-phase power transmission; the primary switching elements being configured to be controlled in use to facilitate power conversion between the AC and DC networks (26, 20); each auxiliary converter being configured to be operated in use to act as a waveform synthesizer to modify a first DC voltage presented to the DC network so as to minimise ripple in the DC voltage; each auxiliary converter being configured to be operated in use to synthesize a waveform including at least one rectified, zero-phase sequence triplen harmonic component, characterized in that said at least one rectified, zero-phase sequence triplen harmonic component is configured to cancel, in use, at least one harmonic ripple component in the first DC voltage. first and second DC terminals (12, 14) for connection in use to a DC network, three phase elements and three auxiliary converters connected between the first and second DC terminals (12, 14), each phase element including two parallel connected strings, each string including a pair of series-connected primary switching elements, the mid-points between each string of series-connected primary switching elements defining AC terminals for connection in use to a respective phase of an AC network; each auxiliary converter being connected in parallel with a respective one of the phase elements to define a single-phase converter limb and the three single-phase converter limbs being mounted and connected in series to the first and second DC terminals to define a two-terminal DC network for three-phase power transmission; the primary switching elements being configured to be controlled in use to facilitate power conversion between the AC and DC networks (26, 20); each auxiliary converter being configured to be operated in use to act as a waveform synthesizer to modify a first DC voltage presented to the DC network so as to minimise ripple in the DC voltage; each auxiliary converter being configured to be operated in use to synthesize a waveform including at least one rectified, zero-phase sequence triplen harmonic component, characterized in that said at least one rectified, zero-phase sequence triplen harmonic component is configured to cancel, in use, at least one harmonic ripple component in the first DC voltage.