Head-end power

thumb|[[MBTA Commuter Rail car with U.S. standard head-end power electrical connection cables]]

In rail transport, head-end power (HEP), also known as electric train supply (ETS), or even known by the name Head-on Generation (HOG) in India, is the electrical power distribution system on a passenger train. The power source, usually a locomotive (or a generator car) at the front or 'head' of a train, provides the electricity used for heating, lighting, electrical and other 'hotel' needs. The maritime equivalent is hotel electric power. A successful attempt by the London, Brighton and South Coast Railway in October 1881 to light the passenger cars on the London to Brighton route heralded the beginning of using electricity to light trains in the world.

History

Oil lamps were introduced in 1842 to light trains. Economics drove the Lancashire and Yorkshire Railway to replace oil with coal gas lighting in 1870, but a gas cylinder explosion on the train led them to abandon the experiment.

In 1887, steam-driven generators in the baggage cars of the Florida Special and the Chicago Limited trains in the US supplied electric lighting to all the cars of the train by wiring them, to introduce the other form of head-end power.

The oil-gas lighting provided a higher intensity of light compared to electric lighting and was more popularly used until September 1913, when an accident on the Midland Railway at Aisgill caused a large number of passenger deaths. This accident prompted railways to adopt electricity for lighting the trains.

Head-end power car

thumb|A [[China Railway KD<sub>25K</sub> generator car at Beijing railway station.]]

A head-end power car (also called a generator car) is a rail car that supplies head-end power ("HEP"). Since most modern locomotives supply HEP, they are now mostly used by heritage railways that use older locomotives, or by railroad museums that take their equipment on excursions. Some head-end power cars started out as other forms of rolling stock that have been rebuilt with diesel generators and fuel tanks to supply HEP to the passenger equipment.

thumb|Combined luggage and electric power carriage

thumb|A disused [[British Rail Mark 3 generator car which was converted from a sleeping car to provide electric power for the Nightstar international sleeper train project (eventually cancelled)]]

thumb|Indian Railways power car

Although diesel-powered cars are more common, electric ones also exist and are used to provide power to trains when hauled by locomotives without HEP, or when not attached to a locomotive.

Engine

The HEP generator can be driven by either a separate engine mounted in the locomotive or generator car, or by the locomotive's prime mover.

Separate engines

Genset-supplied HEP is usually through an auxiliary diesel unit that is independent from the main propulsion (prime mover) engine. Such engine/generator sets are generally installed in a compartment in the rear of the locomotive. The prime mover and the HEP genset share fuel supplies.

Smaller under-car engine/generator sets for providing electricity on short trains are also manufactured.

Locomotive prime mover

In many applications, the locomotive's prime mover provides both propulsion and head-end power. If the HEP generator is driven by the engine then it must run at a constant speed (RPM) to maintain the required 50 Hz or 60 Hz AC line frequency. An engineer will not have to keep the throttle in a higher run position, as the onboard electronics control the speed of the engine to maintain the set frequency.

More recently, locomotives have adopted the use of a static inverter, powered from the traction generator, which allows the prime mover to have a larger RPM range.

When derived from the prime mover, HEP is generated at the expense of traction power. For example, the General Electric P32 and P40 locomotives are derated to , respectively, when supplying HEP. The Fairbanks-Morse P-12-42 was one of the first HEP equipped locomotives to have its prime mover configured to run at a constant speed, with traction generator output regulated solely by varying excitation voltage.

One of the first tests of HEP powered by an EMD locomotive's prime mover was in 1969, on Milwaukee Road EMD E9 #33C, which was converted to have a constant speed rear engine.

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.

Voltage

thumb|Connection cables between two [[China Railway 25T coaches]]

North America

Because of train lengths and the high power requirements in North America, HEP is supplied as three-phase AC at 480 V (standard in the US), 575 V, or 600 V. Transformers are fitted in each car for reduction to lower voltages. such as on the Amtrak Cascades train or Iarnród Éireann's CAF Mark 4 Driving Van Trailer (with twin MAN 2846 LE 202 (320 kW) / Letag (330 kVA) engine / generator sets, assembled by GESAN). KiwiRail (New Zealand) use AG class luggage-generator vans for their Tranz Scenic passenger services; Tranz Metro on the Wairarapa line use SWG class passenger carriages with part of the interior adapted to house a generator. The Ringling Bros. and Barnum & Bailey Circus train used at least one custom-built power car that supplied HEP to its passenger coaches to avoid reliance upon host railway locomotives hauling the train.

In UK and Sweden the high-speed trains IC125 and X2000 have 50 Hz 3-phase power bus.