Head end power
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Head end power (HEP) is a US rail term for the electrical power distribution system on a passenger train. The power source, usually a locomotive at the front or “head” of a train or a generator car, generates all the electricity for "hotel" power needed by the train.
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[edit] History
[edit] UK
See Electric train supply for UK history
[edit] US
In the days of steam locomotives, cars got their heat from steam supplied by the locomotive. Electricity for train lighting and HVAC came from generators on each car driven either by small engines or by the rotation of the axles. Initially the new diesel locomotives incorporated steam generators to heat the existing rolling stock. In the late 1950s Chicago and North Western Railway replaced the steam generators with diesel generator sets on their F7 and E8 locomotives. As locomotive horsepower increased, it became possible for the locomotive engine to also drive the HEP generator and passenger-train builders and operators began to focus on the diesel locomotive as the source of a train's "hotel" power. However, it was not until 1975 when Amfleet cars and F40PH and P30CH locomotives entered service, that large-scale adoption of HEP started in the US.
[edit] Engine
The HEP generator can be driven by either a separate engine, mounted in the locomotive or generator car, or by the locomotive's own engine
[edit] Separate engines
Engine types vary, but in the US, they are mainly Caterpillar 3412 V12s and Cummins K-Series Inline 6s. Smaller under-car engines for powering short trains are also manufactured, Stadco being one popular brand of under-car generator.
[edit] Locomotive engine
The engine must rotate the HEP generator at a constant speed (rpm) to maintain the required 60 Hz AC frequency output. Therefore, a typical EMD locomotive, in HEP mode, will operate at its full engine speed of 900 rpm, driving the generator at 1800 rpm through a 1:2 gearbox. As a noise reduction method, the locomotive's main (traction) generator can also supply HEP, usually at 720 rpm. However this operating mode is only available when stopped.
The advent of power electronics has allowed the engine to operate over a larger speed range and still supply a constant HEP voltage and frequency by means of inverters.
All power consumed by HEP is at the expense of traction power. The 3200 horsepower (2.4 MW) P32 and the 4000 horsepower (3.0 MW) Genesis-Series P40 reduce to 2900 (2.2 MW) and 3650 horsepower (2.72 MW), respectively, when in HEP mode.[1]
[edit] Electrical loading
HEP power supplies the lighting, HVAC, dining car kitchen and battery charging loads. Individual car electrical loading ranges from 20 kW for a typical car to more than 150 kW for a Dome car with kitchen and dining area, such as Princess Tours Ultra-Dome cars operating in Alaska.[2]
Because of the lengths of trains and the high power requirements, HEP is supplied as three-phase AC at 480 V (common in the US), 575 V (mainly parts of Canada), or (rarely) 600 V. Transformers are fitted in each car for reduction to lower voltages.[3]
[edit] References
[edit] External links
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