Low Modulation Index Operation of a Nine-Level T-Type Converter

This paper presents a study on a recently proposed multilevel converter (MLC) that offers significant advantages over alternative nine-level configurations in terms of component requirements, losses, and efficiency. However, a key challenge en-countered by this converter arises from the difficulty in achieving balanced operation of the flying capacitors (FCs) and the dc-link at low modulation index, due to the absence of redundant states in this operating region. To overcome this challenge, this paper presents a method based on pulse width modulation (PWM) to enables proper operation in this region by seamlessly transitioning to a five-level mode without introducing transients in the FCs' voltage. Importantly, this method is realized without the need for complex control techniques, ensuring simplicity in implementation. The proposed approach is experimentally validated, demonstrating its effectiveness in achieving reliable operation at low modulation index while maintaining the stability of the FCs' voltage. This study contributes to enhancing the operational capabilities of the nine-level converter, enabling its effective utilization in various practical applications.