Front-End Design Optimization for Downsizing of Power Electronic Transformers in High-Speed Traction Applications Considering Different Modulation Strategies

Power electronic transformers (PETs) are promising for the high-speed traction industry owing advantages such as high power density, high efficiency, and environmental friendliness. The cascade H-bridge (CHB) rectifier at the front end of a PET constitutes a significant proportion of the overall volume and mass of the PET. Modulation strategies for CHBs are divided into three main categories: staircase, carrier phase-shift (CPS), and hybrid modulations. Within these different modulation strategies, there is a tradeoff between the DC-link capacitance and switching loss, leading in turn to a tradeoff between the volume and mass of the DC-link capacitor and heat sink. In this study, the DC-link capacitance was calculated based on a restriction set on the DC-link voltage ripple. For CPS and hybrid modulations, the capacitance was calculated using the amplitude of the second-order harmonic in the DC voltage, whereas for staircase modulation, the worst-case method was proposed to calculate the required DC capacitance. The switching and conduction losses were calculated from the loss function in the HITACHI 6500V IGBT datasheet and the switching characteristics of the different modulation methods. The volume and mass of the heat sink were calculated using the thermal model of the switching device and the switching loss. Among the three considered modulations, hybrid modulation exhibited the highest power density. The accuracy of the calculations was confirmed through experiments and real-scale simulations.