Unified Selective Harmonic Elimination for Cascaded H-Bridge Asymmetric Multilevel Inverter

The unequal dc-link voltages in the cascaded H-bridge (CHB) asymmetric multilevel inverters (AMLIs) lead to asymmetry of the equations for selective harmonic elimination (SHE), which increases the difficulty in solving the switching angles. On the other hand, the unequal dc-link voltages provide a large amount of synthesized waveforms which can be used to optimize the performance of inverters. However, as most of the traditional SHE approaches are derived according to a given output waveform, it cannot handle multiple synthesized waveforms. In this paper, the unified SHE equations are derived for all the possible synthesized waveforms under a specific switching angles distribution among the cells. By using the polynomial homotopy continuation (PHC) algorithm, all the possible solutions of the unified SHE equations can be found without the selection of initial values. As the waveforms generated by H-bridge are limited to three levels, some of the solutions are not physically realizable and should be discarded, hence, the criterion is proposed to distinguish these specious solutions. The case of a CHB converter with two cells and six switching angles is studied; in total, there exist 86 groups of candidate solutions and 14 of them are physically realizable. Experiments are also shown to validate the correctness of this unified SHE approach for CHB AMLIs.