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 pulse width 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 paper. 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 (SVB-DPWM), 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 neutral point voltage in single and parallel systems is demonstrated by simulations and experiments.
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