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

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.