Comparison of modulation techniques for active split DC-bus three-phase four-leg inverters

Three-phase four-leg inverters are commonly employed to supply unbalanced loads avoiding the use of a transformer. A suitable output filter is necessary to improve the harmonic content of the output voltage and current waveforms. To this aim, different kind of filters, presenting both symmetrical or asymmetrical phase-to-phase and phase-to-neutral behavior, can be used. The paper discusses and analyses the effects of different carrier-based modulation techniques, based on the independent control of the fourth-leg from that of the other phases, with reference to the different filter topologies. The comparison, performed through extensive MATLAB/Simscape simulations and experimental tests, have demonstrated that in case of an asymmetrical filter utilization, such as an Active Split DC-Bus (also known as Split-Link), the conventional continuous and discontinuous Space-Vector modulations produce a significant harmonic content in the output voltages. A detailed problem description and a specific design procedure is presented and validated by experimental tests, allowing a substantial improvement of the inverter performances.