A Simplified PWM Strategy for a Neutral-Point-Clamped (NPC) Three-Level Converter With Unbalanced DC Links

A simplified pulse width modulation (PWM) strategy for a neutral-point-clamped three-level converter with unbalanced dc links is proposed in this paper to achieve high-quality line-to-line output voltages and to maximize the linear modulation range. The simplified strategy takes the direct output voltage modulation by calculating the special solutions of the voltage-second balance equations without detecting the position of the reference vector in the asymmetrical and complicated space voltage vector diagrams to reduce the calculation time. A novel solution based on the state transition is proposed to extend the maximum linear modulation index to 1.15. Furthermore, the asymmetric control of the split dc link by the proposed PWM is implemented by adjusting the special solutions. The difference between the conventional space vector PWM and the proposed strategy is conducted to illustrate the advantages of the simplified strategy. The effectiveness of the proposed modulation strategy is verified by simulation and experiment results.