A Multi-Zero-Sequence Component Injection Algorithm for Five-level Flying Capacitor Rectifier under Unbalanced DC-link Voltages

In this article, the mechanism of current distortions for a five-level flying capacitor rectifier under unbalanced dc-link voltages is analyzed in detail. A multizero-sequence component injection algorithm based on phase-shifted pulsewidth modulation is proposed. In specific, the proposed modulation algorithm divides the fundamental period into clamping and continuous intervals. Considering the inherent topological constraint that the terminal voltage is related to the polarity of the phase current, clamping compensation component is added to three-phase reference signals for clamping one-phase reference signal to zero in order to mitigate current distortions near the zero-crossing points during the clamping interval. Each flying capacitor voltage is regulated by adjusting two switching modulation waves with independent compensation component injection to avoid affecting the average terminal voltage. To improve the quality of dual dc voltages, optimized compensation component is simultaneously injected to three-phase modulation waves for minimizing the ripple component of the average neutral-point current during the continuous interval. In addition, the allowable ranges of both unbalanced voltage degree and unbalanced power degree are theoretically deduced. Finally, simulations and experiments are conducted to verify the effectiveness of the proposed algorithm.