A Hybrid PWM Strategy for Three-Level Inverter With Unbalanced DC Links

The dc links of the three-level neutral-point-clamped (3L-NPC) converter are needed to work in imbalanced state in some special fields, and a hybrid pulsewidth modulation (PWM) strategy in this case is studied in this paper. The proposed strategy is a combination of the space-vector PWM and carrier-based modulation (CBM) method. The characteristic of the revised space-vector diagram (RSVD) with unbalanced dc links is studied from a new perspective, and based on it, a novel sector distribution method is proposed to identify the utilized switching sequences. Instead of calculating the durations of the adopted space vectors in the asymmetrical and complicated RSVD, the proposed strategy adopts the CBM with well-calculated injected zero-sequence voltage (IZSV) to produce the pulses directly. The IZSV calculating methods to realize the asymmetric control of dc-link voltages for both the continuous and discontinuous PWM are studied in detail. Simulated and experimental results are given to validate the effectiveness.