An Improved Droop Control Strategy for VSC-Based MVDC Traction Power Supply System

Due to the development of advanced power electronic techniques, voltage-source-converter-based medium voltage dc (MVDC) traction power supply system (VSC-based MVDC TPSS) could be an alternative supply solution for high-speed railways (HSRs) in the future. The distributed configuration of the MVDC system, along with dynamic load and catenary resistance, makes the control of such system a real challenge. In this paper, an improved droop control method is proposed to obtain a better performance of system operation for the specific supply environment without any communication links. The proposed control scheme can eliminate the voltage deviation due to droop action and improve catenary network voltage control dynamics by using voltage-shifting and load current feedforward control approaches simultaneously. Additionally, the detailed model of the proposed control scheme is derived and the stability in the case of moving load is critically tested. Finally, a simulation model based on MATLAB/Simulink and an experimental prototype have been established to validate the effectiveness of the proposed control method.

I. Introduction

Recently, great achievements have been made in the field of dc systems, such as the practical engineering applications of voltage-source-converter-based high-voltage dc transmission (VSC-HVDC), VSC-based multi-terminal high-voltage dc (VSC-MTDC) systems, more electric aircraft (MEA), and dc microgrid, which calls for a rethink about dc TPSS for high-speed railways (HSRs) [1]– [5]. The idea of establishing VSC-based medium voltage dc (MVDC) TPSS for electric traction was proposed in [4] and Fig. 1 shows the typical structure of the system. Compared to the 25-kV ac railway electrification system, the power quality problems, such as unbalance, reactive power, and harmonic distortion, will be greatly improved because of the excellent performance of VSC. Meanwhile, null reactance of the catenary network without neutral sections will substantially increase the power supply distance of a traction substation (TSS). In addition, the full single dc catenary network can ensure the continuous high-speed motion of trains and provide a physically distributed bus to which renewable energy sources (RESs) and energy storage systems (ESSs) can be easily accessed [4], [5]. Fig. 1.

Structure of VSC-based MVDC TPSS.