Line voltage regulation of urban transit systems using supercapacitors

In DC electric railway system, when the railcar starts, the electric power is absorbed from the line, and then the line voltage is instantaneously collapsed to provide a large accelerating torque. On the other hand, when the railcar brakes, the regenerative power is created and it increases the line voltage at the pantograph of the railcar. If it is possible to install an energy storage system near the train, the regenerated power can be saved to this system. This paper describes the control of energy storage system which consists of a supercapacitor bank and a bidirectional DC-DC converter. Supercapacitor can be used for storing the regenerative energy and for peak power requirement to increase the efficiency and life cycle of the system. This paper presents a method to identify the parameters of supercapacitor equivalent circuit by using the experimental results. In order to verify the validity of this method, parameters of a Maxwell BCAP 3000F are extracted through the experiments and then the equivalent circuit model using these parameters is implemented for PSiM simulation. And the control strategy of a bidirectional DC-DC converter is described to meet the performance for the initial charging mode and power control mode. Finally the experimental data in laboratory and at Kyoungsan test site for urban light rail road system are provided in detail. From the simulation and experimental results, the validity of this energy storage system using capacitor is verified.