Head end power
From Wikipedia, the free encyclopedia
Head end power (HEP) or Electric train supply (ETS) in the United Kingdom is a rail transport 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
Originally, trains hauled by a steam locomotive would be provided with a supply of steam from the locomotive's boiler for heating the carriages. When diesel locomotives and electric locomotives replaced steam, the steam heating was then supplied by a steam generator. This was oil-fired (in diesel locomotives) or heated by an electric element (in electric locomotives).
At this time, lighting was powered by batteries which were charged by a dynamo underneath each carriage when the train was in motion, and buffet cars would use bottled gas for cooking and water heating.
Later diesels and electric locomotives supplied Electric Train Heating (ETH), which was eventually renamed Electric Train Supply (ETS), to power lighting, heating, ventilation, air conditioning, fans, sockets and kitchen equipment in the train. Each coach has an index relating to the maximum consumption of electricity that that coach could use. The sum of all the indices must not exceed the index of the locomotive.
Although most locomotive-hauled trains take power directly from the locomotive, there have been examples (mainly in continental Europe) where restaurant cars would take power directly from the overhead wires. On modern Diesel multiple unit trains, such as the Virgin Trains Voyager, the engine mounted below each vehicle provides power for that vehicle.
[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. The first advance beyond these came on steam locomotives and passenger cars assigned to commuter service in Boston by the Boston and Maine Railroad. The B&M found that the passenger cars on commuter trains, with low speeds and short periods of sustained running, did not generate enough electricity from their axle generators to keep their lighting batteries charged. The B&M equipped the steam locomotives assigned to commuter service with larger than usual on-board generators and arranged electrical connections from these to all the cars being pulled in order to keep the batteries charged and lights operating. The cars still depended on steam from the locomotive for heating.
Initially when diesel locomotives were introduced, they incorporated special boilers termed steam generators to heat the existing rolling stock. In the late 1950s the Chicago and North Western Railway replaced the steam generators with diesel generator sets on the F7 and E8 locomotives assigned to pull commuter trains. This was a natural evolution as their commuter trains were already receiving low voltage, low amperage power from the locomotives to supplement the electricity from their axle generators in keeping their lighting batteries charged. Sometimes such commuter cars were equipped with propane-engine powered air conditioning. Separate and complex systems of trainlined lighting power, steam heat, and engine-driven air conditioning was ripe for replacement with HEP where a single source provides power for all these functions.
However, while commuter car fleets transitioned to HEP, the intercity trains continued with steam & battery systems. It was only after the coming of Amtrak in 1971, which initially acquired cars and locomotives from the private railroads, that the intercity trains were gradually converted. All cars ordered new by Amtrak were HEP-equipped and the older cars that were retained were eventually converted during overhauls. Amtrak's initial new-built engines were equipped to pull steam-heated trains. 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 50 (UK) or 60 (US) 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 1500 or 1800 rpm through a gearbox. As a noise reduction method, the locomotive's main (traction) generator can also supply HEP, usually at 600 or 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 if powered by the locomotive engine. 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 supplying HEP.[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, in North America, as three-phase AC at 480-V (standard in the US and for Canada's VIA), 575-V (GO Transit, Toronto), or rarely 600-V. Transformers are fitted in each car for reduction to lower voltages.[3]
In the UK, ETS is supplied at 800-V to 1000-V AC/DC two pole (400 or 600-A), 1500-V AC two pole (800-A) or at 415-V 3 phase on the HST
