Single Stage Cascaded H-bridge Topology Based Power Electronic Traction Transformer

Power electronic transformer (PET) technology shows great promise for medium to high power conversion systems especially in traction systems. Conventional power electronic traction transformer or solid-state transformer (PETT/SST) for railway traction application has two stage operation: ac-dc that is performed by hard switched active front end (AFE) based on cascaded H-bridges (CHBs) followed by isolated dc-dc converter and a three-phase inverter connecting to the traction motor. This paper presents a novel multilevel architecture that employs a single-stage conversion topology for every module, combining the AC-DC and DC-DC stages into a unified single-stage architecture. The modules are connected in series at the input to form a multilevel topology. The converter is controlled using sine pulse width modulation (SPWM) and phase shift control, which enables the topology to operate at unity power factor (UPF) on the grid side, with bidirectional power flow. The proposed single-stage topology results in a reduction of the switch count in each module, which enhances reliability. Additionally, all switches operate using zero voltage switching (ZVS). The paper outlines the operating principle and analysis of the single-stage module, which forms the basis of the proposed topology. Additionally, the converter is designed for a 4.5 kW rating and the operation of the converter is validated through simulation.