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Recently, high-voltage motor direct drive systems have been put to practical use, and various multilevel PWM strategies have also been proposed. This paper describes a multilevel PWM strategy (our group calls “Carrier Phase Selection (CPS) Method”) that has the lowest line voltage harmonic distortion in order to prevent the degradation of high-voltage motor winding insulations. This method takes the adverse effect of a deadtime into consideration, specifically selects the shift direction and determines phase difference of carrier waves in accordance with the CPS-definition (which is not influenced of a deadtime) when the voltage command makes transition to different level regions in each phase. Therefore, a favorable output waveform without instantaneous voltage serge is achieved even if the line voltage level changes. Moreover, the switching transitions across all switching devices are well-balanced, so the utilization of inverter unit cells is equalized. It is important factor when designing the entire system. Fig. 1 shows simulated waveforms of CPS method. It turns out that the utilization of single-phase inverter outputs is equalized and well-balanced switching. Additionally, CPS method gives rise to the lowest harmonic distortion for the line voltage waveform theoretically; in fact, output level always changes stepwise without causing level-skip.

A Multilevel PWM Strategy suitable for High-Voltage Motor Direct Drive Systems in Consideration of the Adverse Effect of a Deadtime