Switching Angles and DC Link Voltages Optimization for Multilevel Cascade Inverters

The optimization of both dc link voltages and switching angles of fundamental switching strategy for the output waveform of a multilevel cascade inverter is investigated in this study. Previous work in the area of fundamental switching for multilevel inverters is highlighted by the pulse width optimization technique, where the switching angle at each voltage level is optimized, so that the resulting staircase output voltage has the minimum harmonic content. The dc link voltage of each level is however assumed to be the same and constant. As an extension to this work the optimization of both dc link voltages and switching angles of the staircase output voltage is carried out. Harmonic performance of the proposed switching strategy is simulated, results are compared with that of the previous work. It is confirmed theoretically that substantial improvement in the harmonic minimization can be obtained. In practice, the proposed method is specially suitable for the control of multilevel cascade inverters, the latest development of its kind, where each dc voltage can be self-maintained and independently controlled. The description of the multilevel cascade inverters for applications of AC drives as well as Static Var Generation is also given in this paper.