Abstract:

This study aims to examine the significant impact of the frequent starting and braking of the urban rail trains on the voltage of the traction network. A hybrid energy storage system comprising a supercapacitor and battery, which can satisfy the high energy and power requirements of urban rail trains and maintain the voltage stability of the DC traction network to ensure its safe operation, is proposed. A power allocation strategy is designed for a hybrid energy storage system by considering the over-charge, over-discharge, and compensation range of the energy storage elements. Based on the low-pass filtering method, the power allocation strategy combines the supercapacitor’s state-of-charge to allocate and manage the power and energy of the hybrid energy storage system through a low-pass filter. The supercapacitor compensates for the variable high-frequency power, whereas the battery compensates for the corresponding low-frequency power. The simulation results reveal that the voltage of the DC traction network can fluctuate at approximately 1500 V; the battery and supercapacitor can operate within their respective states of charge and will not over-charge or over-discharge. The supercapacitor protects the battery from large shocks and increases the service life of the battery. The proposed hybrid energy storage system and control strategy can not only ensure that the voltage of the DC traction network fluctuates within the required range but also prolong the service life of the energy storage elements; the feedback energy of the urban rail trains can be captured to improve the energy utilization rate.

Key words: hybrid energy storage system, low-pass filtering method, power allocation, energy management, state of charge