The study of SPWM control strategy to reduce common-mode interferences in three-phase four-leg inverters

Due to the asymmetry of the structure, the traditional three-phase three-leg inverters produce huge amount of common-mode (CM) voltage and CM current. This paper introduces the theory of reducing CM interferences by using the three-phase four-leg structure and the SPWM control strategy of carrier phase shifting which is applied on the purpose of balance in inverters. The SPWM modulation index in this method is limited to less than 0.666. In order to reduce CM interferences when modulation index is more than 0.666, the forth leg control is changed from exclusive-or to threshold. Under this situation the restraining of CM interferences is effective but not enough. So we put forward an improved method, in which jump edge of SPWM is delayed when zero-state appears. This improved method entirely avoids zero-state and breaks through the modulation index limitation. The effectiveness of this method is clearly shown from the analysis on the simulation of the control strategy in this paper. Thus the three-phase four-leg inverter under this strategy can be widely applied to reduce CM interferences.