Hybrid Modulation Strategy with Voltage Balancing Control for Four-Level Neutral-Point Clamped Converters

Capacitor voltage imbalance in four-level (4L) neutral-point clamped (NPC) converters is a direct factor hindering their application. In particular, when they are applied in motor drives, space vector pulse-width modulation (SVPWM) is a more popular scheme, but conventional 4L SVPWM cannot achieve the voltage balancing control of DC-link capacitors, is complex to implement, and requires costly computation. A hybrid modulation method with capacitor voltage-balancing control for 4L NPC converters is proposed. The proposed method is achieved using three-level (3L) SVPWM and two-level (2L) carrier-based pulse-width modulation (CPWM) based on the concept of "4L=3L+2L". Thus, it can be easily implemented on a digital chip because the modulation process is nearly identical to that of 3L SVPWM without the more cumbersome 4L SVPWM algorithm. Meanwhile, any proven optimization scheme of 3L SVPWM can be directly applied to the proposed method to further improve performance. Simulation and experimental results for a 4L active NPC converter demonstrate the effectiveness of the proposed method.