Auxiliary Power Unit for Railway Vehicles Using Three-Level LLC Converter with Pulse Width Modulation Control

This paper presents the structure of an auxiliary power unit for railway vehicles to reduce the amount of electrical energy conversion and increase efficiency. An auxiliary power unit is a key device for supplying the various control power for railway vehicles. Isolation is required as this provides the necessary power for the passenger car. Therefore, high-efficiency isolated LLC converter has been mainly applied. However, the LLC converter has poor control performance in response to voltage variation. To overcome this problem, the existing system has been implemented by two power conversion stages of the APU : (stages by installing a non-isolated DC / DC converter, such as buck or boost, at the LLC converter input. The heart of this white paper is the analysis of an LLC converter that accepts a wide range of input voltages by applying PWM control. This can achieve volume reduction. The proposed control strategy is verified by PSIM, which is power conversion simulation analysis programs.

I. Introduction

The core function of the Auxiliary Power Unit(APU) is to convert the overhead line voltage to three-phase AC voltage. It is one of the key components of the railway vehicle power system because provides the heating, cooling, lighting and main control power required by the vehicle [1]–[5]. The traditional electrical system of railway vehicles consists of a rectifier, variable-voltage-variable-frequency(VVVF) three-phase inverter, APU and low-frequency transformer which is illustrated in Fig. 1. If the overhead line voltage is DC, the rectifier is not included. The biggest problem with this configuration is that there is a huge transformer for low-frequency isolation.