Voltage source converter

A voltage source converter (30) comprises: first and second DC terminals (32,34) for connection to a DC network (40); a third terminal (36) for connection to ground or a voltage; and a plurality of AC terminals (38), each AC terminal (38) for connection to a respective phase of a multi-phase AC network (42). The voltage source converter (30) includes: a plurality of multi-level switching cells (44), each multi-level switching cell (44) including an AC connecting point and first, second and third further connecting points, the third terminal (36) connected to each third further connecting point of each multi-level switching cell (44), the AC connecting point of each multi-level switching cell (44) being connected to a respective one of the plurality of AC terminals (38), each multi-level switching cell (44) being arranged to permit selectively switching of the corresponding AC terminal (38) into circuit with each of the corresponding further connecting points separately; a switching controller (46) programmed to control the switching of each multi-level switching cell (44) to selectively switch the corresponding AC terminal (38) into circuit with each of the corresponding further connecting points separately; first and second waveform synthesizers (48,50), the first waveform synthesizer (48) connected between the first DC terminal (32) and each first further connecting point of each multi-level switching cell (44), the second waveform synthesizer (50) connected between the second DC terminal (34) and each second further connecting point of each multi-level switching cell (44); and a synthesizer controller (64) programmed to selectively operate each waveform synthesizer (48,50) to synthesize a voltage waveform to control the configuration of the AC voltage at the AC terminal (38) that is switched into circuit with the corresponding further connecting point.