A Novel SPWM control strategy to reduce common-mode voltage in three-phase four-leg inverters

Because of the unbalanced construction, the conventional three-phase three-leg inverters produce huge amount of common-mode (CM) voltage. This paper proposes a novel sinusoidal pulse width modulation (SPWM) control strategy based on three-phase four-leg structure. This strategy is an improved method on the basis of the SPWM carrier phase-shifting control technology. In the new method, the state of one of the first three legs is exchanged with the latter state of the same leg when zero-state appears. It can ensure the output balance of four legs in inverter under any conditions, and thus reduces the generation of CM voltage. Compared with the previous SPWM control strategies in three-phase four-leg inverter, the state exchange method breaks through the limitation of SPWM modulation index which must be less than 0.666 to avoid the extremely low utilization ratio of direct current (DC) voltage, and reduces the total harmonic distortion factor (THD) of the output voltage. The effectiveness of the control strategy is proved by simulation and experiment results.