A Carrier-Based Discontinuous PWM for Single and Parallel Three-Level T-Type Converters With Neutral-Point Potential Balancing

The three-level converters have an inherent problem of the neutral-point (NP) voltage unbalance due to the split dc-link capacitors. This NP voltage unbalance degrades the output voltage/current waveform quality. A carrier-based discontinuous pulsewidth modulation (CB-DPWM) with NP voltage balancing strategy-that involves the injection of calculated zero-sequence voltages to the original modulation signal-is implemented for the three-level T-type converter in this article. The zero-sequence voltages are determined from the space-vector synthesis of the converter output voltages. This method allows the determination of the zero-sequence voltages with few judgment conditions. Unlike the space-vector-based discontinuous pulse width modulation, the CB-DPWM is easier to realize in practical applications. Moreover, the proposed CB-DPWM can also be directly used in parallel systems, which can suppress NP voltage ripples and circulating currents simultaneously by a coordinated strategy. The efficacy of the proposed method in balancing the NP voltage in single and parallel systems is demonstrated by simulations and experiments.