A Carrier-Based Discontinuous PWM for 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 waveform quality. A carrier based discontinuous pulse width modulation (CB-DPWM) with neutral point voltage balancing strategy- that involves the injection of calculated zero-sequence voltage to the original modulation signal - is implemented for the three level T-type converter in this paper. Unlike the space vector based discontinuous pulse width modulation (SVB-DPWM), the CB-DPWM is easier to realize in practical applications. 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 judgement conditions. The efficacy of the proposed method in balancing the neutral point voltage is demonstrated by simulation and experiments for unity, leading and lagging power factor loads.