A study on stationary supercapacitor sets for voltage droops compensation of streetcar feeder lines

Electrical storage devices can be very useful in electrified mass transit and in particular in trolleybus lines and tramways. In fact, storage devices are capable of supporting the line voltages of old lines, avoiding the opening of minimum voltage protective relays, especially when the new generation vehicles of greater power circulate. Among electrical storage devices, supercapacitors seem to be the most suitable for the application in traction systems, due to their high power densities and short charge/discharge times. The paper shows that the boosting of the line voltage can be more easily performed when supercapacitors are placed wayside. Their correct sizing requires the definition of suitable criteria, which take into account the topology of the line, the type of electrical supply and the movement of the vehicles. For this reason, the sizing criterion suggested in the paper refers to the optimization theory. The design procedure is capable of individuating the minimum set of the supercapacitors that guarantee at the same time the substation currents minimization and the line voltage stabilization. The sizing procedure has been applied to the sample case of a branch of the tramway in the city of Naples. Numerical results show the capabilities of the methodology in the selection of both the location and the capacitance of supercapacitors and that the set designed in this way is well tailored for the requirements of the line.