Impedance calculations for SEPTA´s rail power distribution system

The Southeastern Pennsylvania Transportation Authority (SEPTA) owns and operates an electrified system of commuter rail lines in Philadelphia and its suburban areas. The traction power supply system is of an autotransformer type and is supplied from three static frequency converters each rated at 15 MVA. The system trolley circuits are supplied at 12 kV, 25 Hz and the feeders are supplied at 24 kV, 25 Hz. The feeder and the trolley circuits are connected through autotransformers at substations spaced along the route at appropriate intervals. The advantage of the autotransformer system is that the traction power is being distributed effectively at 36 kV while it is being utilized at only 12 kV. Recently SEPTA has embarked on an ambitious modernization program under which it plans to replace all autotransformer substations. In the preliminary engineering phase of the program, comprehensive studies of the traction power system were performed. The impedance calculations presented in this paper formed the basis of these studies. A mathematical model was developed for the calculation of trolley to ground, feeder to ground, and feeder to trolley impedances of the distribution system using the Electromagnetic Transients Program. The impedances are expressed in ohms per track, per mile which is suitable for use in load flow and short circuit studies