Capacitor Voltage Balancing Based on Improved Carrier Overlapping PWM Method for a Five-Level T-NNPC Converter

Abstract

T-NNPC inverter is an attractive topology with fewer switches and good withstand voltage for medium voltage applications. In order to effectively achieve five-level output, the conventional method is an MPC strategy that can quickly track current, ensuring good output performance under fundamental frequency conditions. However, the flying capacitor voltage fluctuates, especially under low-frequency conditions. In addition, the CO-PWM is a potential method for this topology due to the broader voltage level distribution in the vertical direction. Nevertheless, the flying capacitor voltage balancing cannot be guaranteed under the high modulation indexes. To improve voltage balance under various modulation indexes and current frequencies, this paper proposes an improved CO-PWM (ICO-PWM) method whose voltage level distribution is modified to better apply in the T-NNPC topology. Compared to the conventional MPC method, the flying capacitor voltage is more stable, and the fluctuation is reduced by more than five times, especially under low-frequency conditions. The effectiveness of the proposed ICO-PWM method is verified by the simulations and experiments.

Keywords

• Multilevel inverter
• flying capacitor voltage balancing
• carrier overlapping PWM
• low current frequency