Harmonic Mitigation in Single Phase Mutually Commutated Converter Systems

An AC/DC converter comprising a cycloconverter and a capacitively snubbered voltage source converter (VSC), coupled by a medium frequency transformer, offers significant advantages. Bidirectional AC/DC power flow as well as voltage transformation and isolation can be achieved. By mutual commutation of the converters, soft switching can be achieved for all semiconductors, thus, link frequency can be increased, allowing smaller transformer and reduced filter components. However, the mutual commutation scheme imposed by the soft switching requirement puts constraints on the PWM pattern causing low frequency harmonics in certain operating points. It is shown that the low frequency harmonics can be eliminated for all points of operation, both in a single bridge configuration and interlaced configuration, common in AC-fed propulsion. A sub-optimal harmonic mitigation method based on a regularly sampled modulator is proposed. By proper oversampling of the reference waveforms substantial reduction of the low frequency harmonic contents can be achieved. The mitigation method is verified by circuit simulation and the feasibility to propulsion is demonstrated.