Short circuit currents of the SEPTA´s traction 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 and the rails provide the ground return circuit. 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. Since conventional short circuit analyses do not account for the feedback currents through the autotransformers, a novel method for calculation of the fault currents was developed using the Electromagnetic Transients Program (EMTP). Using this method, feeder to trolley, feeder to ground, and trolley to ground fault currents were calculated. This method is readily adaptable to many different feeder, trolley and track trolley configurations