Amtrak's 25 Hz Traction Power System - Substations

Substations

The PRR's original 1915 electrification made use of four substations, at Arsenal Bridge, West Philadelphia, Bryn Mawr, and Paoli. The Arsenal Bridge substation stepped-up 13.2 kV, 25 Hz power supplied from PECO's Schuylkill power station on Christian Street to 44 kV for distribution. The remaining three substations reduced the 44 kV distribution voltage to 11 kV trolley voltage. The substations were operated from adjacent signal towers. They used typical period concrete buildings to house the transformers and switchgear while the line terminals were on the roof. From 1918 onwards outdoor stations were used and when the main line electrification began in 1928 the stations became large open air structures using lattice steel frameworks to mount the 132 kV terminations and switchgear. By 1935 new stations were connected to remote supervision systems allowing power directors to open and close switches and breakers from central offices without having to go through the tower operators.

Today about 55 substations are part of Amtrak's network. Substations are spaced on average 8 miles apart and feed 12 kV catenary circuits in both directions along the line. Thus the catenary is segmented (via section breaks, also called 'sectionalizations' by the PRR) at each substation, and each substation feeds both sides of a catenary's section break. A train traveling between two substations draws power through both transformers.

A typical substation includes two to four 138/12 kV transformers, 138 kV air switches that permit isolation of individual transformers, shutdown one of the two 138 kV feeders, or cross-connection from one feeder to another. The output of the transformers is routed to track catenary via 12 kV circuit breakers and air disconnect switches. Cross-connect switches allow one transformer to feed all catenary lines.

The PRR substation architecture was based on a long distance, high speed railway. The substation spacing ensures that any train is never more than 4 or 5 miles from the nearest substation, which minimized voltage drop. One disadvantage to the substation design as originally built by the PRR concerns its lack of 138 kV circuit breakers. Essentially all segmentation of the 138 kV system must be done by hand. This makes rapid isolation of a fault on the 138 kV line very slow. Faults in one part of the line also affect the entire distribution system since it is impossible for the 138 kV transmission to protect or reconfigure itself during a fault condition. High voltage faults generally are cleared by opening converter output breakers, which causes a concurrent loss of the converter. The system does not degrade gracefully under high voltage faults. Rather than isolating, for example, the south 138 kV feeder between Washington and Perryville, the system would require opening converter output breakers at Jericho Park and Safe Harbor. This results in loss of much more of the network than is required to simply isolate the fault.