Analysis of a seven‐level asymmetrical hybrid multilevel converter for traction systems

Abstract

A medium-voltage high-power electric traction system, due to its non-linear nature results in serious power quality issues in power system such as low-power factor and high-harmonic distortion. To mitigate the aforementioned problems, this study presents a seven-level asymmetrical hybrid multilevel converter that can be implemented using a single dc source and two capacitors . The proposed converter is a series connection of a five-level cascaded module (CM) converter with an H-bridge cell to meet the demand of medium-voltage high-power traction applications with improved power factor at minimum harmonic distortion. A modulation scheme presented will operate an H-bridge cell and a CM with a fundamental frequency and high-frequency switching, respectively. The unbalancing of dc-link capacitors voltages due to active power transfer requires corrective control action including current control, voltage control and voltage balancing that has been proposed to keep the dc-link capacitor voltage balanced. Working principle of the proposed topology and its mathematical analysis is presented in this work. The effectiveness of the controlled pulse-width modulation strategy and stability of the proposed control method have been validated through simulation and experimental results.