Head end power

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.

History

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.

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 steam locomotives assigned to commuter service with larger than normal generators, and arranged electrical connections from these to the cars to keep the batteries charged. The cars still depended on steam from the locomotive for heating.

When diesel locomotives were introduced, they had special boilers (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 commuter service. This was a natural evolution, as their commuter trains were already receiving low voltage, low amperage power from the locomotives to supplement their axle generators in keeping their lighting batteries charged. Sometimes such commuter cars were equipped with propane-engine powered air conditioning. Separate 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.

While commuter fleets transitioned to HEP, intercity trains continued with steam & battery systems. It was only after Amtrak's inauguration in 1971, which initially acquired cars and locomotives from 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 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.

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 (prime mover).

eparate 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.

Locomotive engine

The engine must rotate the HEP generator at a constant speed (rpm) to maintain the required 50 Hz (UK) or 60 Hz (US) AC frequency. 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. For noise reduction, the locomotive's main (traction) generator can supply HEP, usually at 600 or 720 rpm. However, this 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. [ [http://www.trains.com/trn/default.aspx?c=a&id=198 "TRAINS magazine"] ]

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. [ [http://www.nwrail.com/HEP_config.html "Northwest Rail"] ]

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. [ [http://www.nwrail.com/HEP_config.html "Northwest Rail"] ]

In the UK, ETS is supplied at 800-V to 1000-V AC/DC twopole (400 or 600-A), 1500-V AC two pole (800-A) or at 415-V 3 phase on the HST

References

External links

* [http://www.nwrail.com/distributed_products/HEP_index.html Northwest Rail HEP products]
* [http://www.stadcogen.com/railgen_1.html Stadco rail gens]
* [http://webstore.ansi.org/RecordDetail.aspx?sku=APTA+RP-E-015-99&source=wikipedia ANSI HEP recommended practice]
* [http://webstore.ansi.org/RecordDetail.aspx?sku=APTA+RP-E-016-99&source=wikipedia ANSI Recommended Practice for Head End Power Source Characteristics]