Optimized virtual space vector modulation for three-phase three-level T-type converter

Abstract

The virtual space-vector pulse-width modulation (PWM) control for three level neutral-point-clamped converter (3L-NPC) is ineffective when the modulation ratio is high or the neutral-point (NP) voltage unbalanced due to external factors. In this paper, principles and characteristics of virtual space-vector PWM (VSVPWM) are studied, then an optimized virtual space vector PWM (OVSVPWM) is proposed, which can make up for the shortage of VSVPWM by adjusting the NP current of the virtual medium vector and assigning the proportion of positive and negative small vectors. The well-developed methods to calculate the durations of the virtual vectors for the VSVPWM can be directly utilized in OVSVPWM. In addition, a control strategy of 3L-NPC based on OVSVPWM is proposed, which can adjust the unbalancing degree with a high accuracy for high modulation indexes and load power factors. Simulated and experimental results are given to validate the effectiveness.