SEPTA operates a 25 Hz traction power system in the vicinity of Philadelphia, Pennsylvania that it inherited from the Reading Railroad. This system is separate but similar to the system designed by the Pennsylvania Railroad which is now operated by Amtrak. SEPTA's trains can run over either system as the voltage and frequency presented to the locomotive are essentially identical. However, the ex-Reading system is not electrically connected to the ex-PRR system.
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The Reading Railroad electrified its lines between Reading Terminal in Philadelphia, PA and Norristown, Doylestown, Chestnut Hill, Hatboro, and West Trenton in 1931. This initial electrification utilized catenary electrified at 12 kV, 25 Hz like the competing Pennsylvania Railroad's initial 1915 electrification as well as the 1930s mainline expansion. Unlike the PRR's system, which included multiple power sources and an extensive high voltage transmission network, the Reading's electrification was supplied solely by the rotary converter station at Wayne Junction.
The electrification was extended to include the Fox Chase Line in September 1966. Electrification was again extended from Hatboro to Warminster in the 1970s.[1] Although ownership and operation of the system has changed, the system has remained essentially unchanged with the exception of the replacement of the rotary converter station at Wayne Junction with a solid-state cycloconverter in 1986.
The Reading electrification system was designed for suburban MU operations and did not have to support the main line passenger or through freight operations of the rival Pennsylvania Railroad. While a complex system of high voltage transmission lines was not necessary, the system was large enough to require the use of distribution feeders from the 25 Hz power supply at Wayne Junction. The feeders were set at 24 kV, double the voltage of the traction current in the overhead wires. Most of the rail lines would carry one or two feeders attached to the top of the catenary support masts. These 24 kV substation feeders can be told apart from 12 kV traction feeders by the use of 4-segment insulators as opposed to 3-segment ones.
The 24 kV lines feed a network of 11 substations (12 including the Wayne Junction switchyard) located about every 8–10 miles along the electrified rail lines. Most branches are short enough to only require a single substation near the end of the line, but the Main and West Trenton lines are long enough to require intermediate substations. At each substation the 24 kV feeders are fed through autotransformers to be reduced to the traction voltage of 12 kV which are then passed through circuit breakers before being fed into the overhead catenary wires.
Power is transmitted, effectively, at 36 kV as the 24 kV and 12 kV components are essentially 180° out of phase. This design has several advantages over the PRRs more traditional transmission and distribution scheme. The highest voltage, with respect to ground, within the system is 24 kV. This is significantly less than the 69 kV seen in the PRRs system which required much larger 80 ft towers and larger insulators and trasformers. On a small system such as the Reading's, transmission infastructure of this scale was unnecessary. Secondly, the system uses the 12 kV catenary wire as both transmission and distribution. This 're-use' of the catenary wire allows performing both functions with only two insulated wires. The PRR's system in contrast required three: Two wires for the single phase 138 kV transmission component, and the single wire 12 kV catenary feeder. In both systems, the rail is used as a return path for traction current.
All 25 Hz power on the ex-Reading system is generated at the Wayne Junction static converter plant. This plant was built between 1985 and 1990 and consists of three 15 MVA solid-state, cycloconverter based modules. The system was built by ASEA and is similar technically to the converter later installed at Amtrak's Jerico Park converter.
The inverters replaced two circa 1928 Westinghouse Electric motor generators which were each rated at 21.4 MVA. The capability also formerly existed to supply power from PECO's Somerset Substation which had three motor generators normally used to supply the Pennsylvania Railroad's electrification system. This capability has been removed, as have the Somerset motor generators. The ex-Reading system now operates completely independently from Amtrak's grid.
Wayne Junction receives 230 kV, 60 Hz power from two redundant lines which connect PECO's substations at Waneeta, Tabor, and Pulaski. Two 60 MVA 230/13.2 kV transformers reduce voltage for the static frequency converters. Cross-ties allow removing portions of the supply system from service for maintenance. -
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Each of the former Reading Railroad substations was equipped with transformers and circuit breakers from the American Brown Boveri corporation of Camden, NJ. Most transformers are rated to supply about 2 MVA of power each (a few 4 MVA transformers exist). Each substation also contained a number of oil filled circuit breakers to disconnect both the 12 kV and 24 kV lines under load. The larger substations also have a small equipment and maintenance building to house any control equipment or secondary transformers for generating the 100 Hz signal power.
Due to the robustness and simplicity of the components as of 2010 all the SEPTA substations are equipped with their original equipment now approaching 80 years in service. Due to its position on the abandoned Reading Viaduct the Callowhill has been targeted for replacement to eliminate the need to maintain the overhead wires over the former rail line as feeders to the active tracks.
Sometime in the 1990s the substation at Doylestown suffered a fire rendering it inoperable. This requires the Doylestown Branch east of Lansdale to be powered exclusively from the Lansdale substation. This 'single-end feeding' of a ten mile section of track severely limits the number of electric rail vehicles that can use the branch simultaneously.
| Name | Transformers | Line Served | Coordinates |
|---|---|---|---|
| Ambler | 1 | Main Line | |
| Bethayres | 1 | West Trenton Line | |
| Callowhill (Reading Terminal) | 2 | Main Line | |
| Chestnut Hill | 1 | Chestnut Hill East Line | |
| Doylestown | 1 | Doylestown Branch | |
| Hatboro | 1 | Warminster Line | |
| Jenkintown | 2 | West Trenton and Main Lines | |
| Lansdale | 2 | Main Line | |
| Neshaminy Falls | 2 | West Trenton Line | |
| Norristown | 2 | Norristown Line | |
| Wayne Junction | 3 (step-up) | All | |
| Yardley | 2 | West Trenton Line |