A Capacitor Voltage Balancing Hybrid PWM Technique to Improve the Performance of T-Type NPC Inverters

Abstract

Research into solar photovoltaic (PV) fed grid-tied multilevel inverters (MLIs) has increased in recent years due to their distinct features compared to two-level voltage source inverters (VSIs). However, maintaining the power quality of MLIs is a challenging problem that encompasses the reduction of total harmonic distortion (THD), dc-link voltage imbalance issues, power loss reduction and thermal stress mitigation; which are significantly affected by the applied pulse width modulation (PWM) technique. In order to tackle the above mentioned power quality problems, this work presents a modified hybrid PWM technique which combines a modified modulating signal with a modified carrier signal. The comparison between the proposed and the existing PWM techniques is performed under different conditions and using a wide range of modulation index variations. A grid-tied three-phase three-level T-type neutral point clamped (NPC) inverter is adopted for the power quality assessment. The proposed hybrid PWM offers 25.57% output line voltage THD and 11.8 V peak-to-peak dc-link capacitor voltage difference which are lower than existing PWM techniques. Additionally, by reducing the switching and conduction losses of the power switches, the proposed hybrid PWM technique offers lower thermal stress than existing PWM techniques. The comparative analysis between the existing and the proposed hybrid PWM technique is carried out using MATLAB/Simulink and PLECS simulation environments. The proposed hybrid PWM technique is further validated using a reduced scale prototype developed in our research laboratory.

Keywords

• Multilevel inverters
• voltage source inverters
• total harmonic distortion
• dc-link voltage imbalance
• pulse width modulation
• T-type neutral point clamped inverter