A Novel Charging Strategy to Reduce Voltage Variation for Reconfigurable Energy Storage System of Modern Tram

Due to the voltage limitation, large number of ultracapacitor cells need to be connected in parallel and series to provide a specific voltage level for trams. However, a large scale of series-connected ultracapacitors increase the voltage variation of the DC bus, which affects the size and efficiency of the subsequent power converter stage and undermines the energy utilization of the ultracapacitor. Moreover, ultracapacitors in series-connection inevitably suffer from voltage imbalance, which limits the capacity of energy storage system by the weakest cell. To alleviate the above problems, ultracapacitor parallel-series reconfiguration is employed to reduce the voltage variation in this paper. Particularly, a synchronous equilibrium strategy based on consensus tracking control is designed for voltage balance among series-connected ultracapacitors. First, we mathematically model the ultracapacitor-based energy storage system using the state-space averaging method. Second, the law of reconfiguration and synchronous equilibrium strategy is analyzed and designed. Finally, both simulation and experiment results verify that the proposed charging strategy effectively suppresses the voltage variation of the DC bus and evidently reduces balance time during charging.