Capacitor Voltage Balancing of Three-Phase Neutral-Point-Clamped Rectifier Using Modified Reference Vector

This paper presents a control strategy using optimized switching sequences which results in capacitor voltage balancing of a three-phase neutral-point-clamped rectifier. In front end rectifiers, magnitude and lag angle of reference current vector (w.r.t supply voltage vector) varies depending upon amount of reactive power compensation and load current. Accordingly, the effectiveness of small and medium vectors (using SVPWM) for same switching state varies in each sector and its regions. This observation along with detailed analysis of current pattern for both the capacitors and their effect on neutral point voltage forms the basis of the proposed approach, which is to modify the reference vector for changing the number of sampling periods within each sector and its regions. The rectifier exhibits excellent performance in terms of other critical parameters like unity input power factor, low input current THD, minimum possible switching losses, reduced-rippled, and well-regulated dc-bus voltage, particularly, the capacitor voltage balancing. The proposed control algorithm is tested through exhaustive simulations and validated through experimental results obtained from a laboratory prototype of the rectifier.