Optimal Selection of the Voltage Modulation Strategy for an Open Winding Multilevel Inverter

An optimal design strategy for Open Winding Asymmetrical Three-Level Inverters is proposed in this paper, to improve the performance in a large set of applications. The Asymmetrical Multilevel Inverter is composed of an open-end winding AC machine connected on one side to a main three-level inverter and, on the other side, to an auxiliary two-level inverter. The main inverter efficiently controls the active power flow operating at a low switching frequency, exploiting a selective harmonic elimination technique, while the auxiliary inverter acts as an active power filter, driven by a conventional high frequency PWM technique. A key point to maximize the efficiency of the entire system is to properly share the task of eliminating the undesired low-frequency voltage harmonics between the two inverters. An optimal strategy is proposed in this paper according to specific figures of merit, which are function of the DC bus voltage and switching frequency of the two inverters.