Parallel-input and series-output configuration of multiple LLC resonant converter for battery back-up system of railway system

Abstract

Recently, in order to cope with power instability of the railway propulsion system, a battery back-up system has been introduced. The cost-effectiveness of high voltage and high capacity batteries has improved thanks to better productivity and higher yield rate of electric vehicle battery packs. However, since the input voltage of a railway traction inverter is very high, at up to 1500V, battery packs cannot be directly adopted for railway traction inverters due to high cost. Therefore, a battery back-up system with a high capacity 400V battery can be employed with a high-efficiency and high-power-density switching power converter. Generally, a conventional boost converter with switching frequency of 20~50kHz has been applied to convert 400V battery voltage to 1500V inverter voltage. However, due to hard-switching, the conventional boost converter cannot achieve both high-efficiency and high-power-density. To maximize high-efficiency and high-power-density, an LLC resonant converter needs to be investigated for the power converter between the battery pack and the traction inverter; this device must have much higher switching frequency, over 200kHz. Furthermore, to increase its power rating up to 50kW, multiple modules of the LLC resonant converter can be introduced and their series or parallel configuration can make a difference. In this paper, the optimal configuration of multiple LLC resonant converters will be investigated with design guidelines to maximize high-efficiency and high-power-density. Finally, a 50kW battery back-up system with optimal configuration of multiple LLC resonant converters will be verified by PSIM simulation results. ? 2020 The Korean Society for Railway. All rights reserved.