Modified Carrier Discontinuous PWM Technique for Mitigating THD and Capacitor Voltage Balancing Problem of T-type NPC Inverters

Neutral point clamped (NPC) multilevel inverters (MLIs) have been widely adopted than other multilevel inverter topologies due to their simple design and superior power density. However, the performance improvement by mitigating several crucial power quality issues is considered as critical research challenge of these inverters, which heavily depend on the applied modulation technique. Harmonic distortion and neutral point voltage imbalance are the most commonly encountered problems of these inverters. In order to mitigate these problems, a modified carrier discontinuous PWM (DPWM) technique is proposed in this paper. The proposed technique is able to mitigate the total harmonic distortion (THD) of the output line voltage as well as improve the voltage balancing performance of the dc-link capacitors. A three-phase T-type NPC inverter system has been utilized for the evaluation purpose. To prove the superiority of the proposed PWM technique, an extensive comparative study has been conducted with the existing PWM techniques. It offers the lowest THD and capacitor voltage difference for all conditions, which can significantly improve the performance of the inverter. The performance of the proposed PWM technique has been tested through simulation in MATLAB-Simulink platform as well as a laboratory prototype.