A Composite Voltage-Balancing Method for Four-Level NPC Inverters

Four-level neutral-point-clamped (NPC) inverter suffers from the neutral-point (NP) voltage-balancing problem, which seriously hinders its application in the industry. Carrier-overlapped pulsewidth modulation (COPWM) is a newly proposed carrier-based modulation method which can solve this problem. However, the total switching transitions under this modulation method are doubled, and hence the switching losses are increased. In order to alleviate this problem, a composite modulation and voltage-balancing method is proposed in this article. For a low modulation index range under 0.5, phase-disposition pulsewidth modulation (PDPWM) is utilized and the relationship between the NP currents and the reference phase voltage is derived. A zero-sequence voltage injection-based NP voltage balancing method is proposed. For a high modulation index range more than 0.5, a decoupled voltage-balancing method based on COPWM is used. By combining the two modulations and voltage-balancing methods, the NP voltages can be balanced under the full modulation index range with reduced switching losses. Simulation and experimental results are presented to confirm the validity of this method.