A Zero-Sequence Component Injection PWM Scheme for Three-Level Neutral Point Clamped Rectifiers with Unbalanced DC-Link Voltages

This article investigates the operating characteristics of three-level neutral-point-clamped rectifiers with unbalanced dc-link voltages. A zero-sequence component injection pulsewidth modulation method is proposed to realize high-quality ac currents and dc voltages under unbalanced condition. First, three-phase reference signals are modified considering the unbalanced voltage factor. Then, according to the existing linear relationship between zero-sequence component and neutral point (NP) current, the first-layer zero-sequence component corresponding to the required NP current is directly calculated for realizing the dynamic control of dual dc voltages. Besides, the second-layer zero-sequence component is simultaneously injected to minimize the ripple components of the NP current. Moreover, the permissible unbalanced power range under different modulation indexes and unbalanced voltage factors is also investigated. Finally, the effectiveness and performance of the proposed method are verified by experimental results.