A controller (40), for controlling a multiphase voltage source converter, comprises a vector control module (42) that is configured to generate multiphase voltage demand waveforms (va, vb, vc) that the voltage source converter subsequently is required to provide. The vector control module (42) generates the multiphase voltage demand waveforms (va, vb, vc) from respective converter voltage output phase waveforms (Va, Vb, and Vc) that are measured at corresponding AC outputs of the voltage source converter. The vector control module (42) includes a transformation element (44) which is configured to transform the respective converter voltage output phase waveforms (Va, Vb, and Vc) into symmetrical sequence rotating reference frame terms. The vector control module (42) also includes a distortion element (46) that is configured to introduce harmonic distortion into the generated multiphase demand waveforms (va, vb, vc). The distortion element (46) includes a time-varying conversion element (48) that is configured to convert the symmetrical sequence terms directly into respective time-varying waveforms (58, 60, 62, 68, 70, 72) utilising only sine and cosine instantaneous reference terms, and a manipulation element (74) which is configured to carry out non-trigonometric manipulation of the time-varying waveforms (58, 60, 62, 68, 70, 72) to introduce the said harmonic distortion into the generated multiphase voltage demand waveforms (va, vb, vc).