Railway electrification system
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Image:Tile Hill train 550.jpg Image:BridgeportCT-Station.jpg
A railway electrification system is a way of supplying electric power to electric locomotives and multiple units.
Such systems can be classified by:
- type of conductor (third rail or overhead wire)
- type of current:
- direct current (DC) or alternating current (AC)
- voltage
- AC frequency
- single-phase or three-phase AC
Contents |
Direct current
Image:NET tram 201-03.jpg Early electric systems used relatively low-voltage DC. Electric motors were fed directly from mains, and were controlled by using a combination of resistors and relays that connected the motors in parallel or series.
The common voltages are 600 V and 750 V for trams and metros, and 1500 V and 3000 V for railways. In the past, Rotary converters or mercury arc rectifiers were used to convert utility (mains) AC power to the required DC voltage. Today, this is usually done by semiconductor rectifiers.
The DC system is quite simple, but requires thick wires and short distances between feeder stations; additionally, there are significant resistive losses.
Auxiliary machinery, such as fans and compressors, are also powered by motors fed directly from mains. Consequently, these motors are often unusually bulky.
The 1500 V DC system is used in The Netherlands, Japan, Ireland, parts of Australia and partially in France. In the United States, 1500 V DC is used in the Chicago area on the Metra (formerly Illinois Central) Electric district and the Chicago, South Shore and South Bend interurban streetcar line.
Image:Tyne&Wear Metrotrain at Kingston Park station.jpg In the United Kingdom, 1500 V DC was used in 1954 for electrifying the trans-Pennine route (now closed) through the Woodhead Tunnel; the system used regenerative braking, allowing for transfer of energy between up and down trains on the steep approach-inclines to the tunnel. The system was also used for suburban electrification in East London, now converted to 25 kV AC. The only UK system now using this voltage is the Tyne and Wear Metro.
The 3000 V DC system is used in Belgium, Italy, Poland, the northern Czech Republic, Slovakia, Slovenia, western Croatia and in the former Soviet Union. 3000 V DC was also formerly used by the Milwaukee Road's extensive electrification across the Continental Divide, as well as by the Delaware, Lackawanna & Western Railroad (now NJ Transit, converted to 25 kV AC).
Note that voltages such as 1500 V DC are nominal voltage which fluctuate up and down from say 1300 V to 1800 V depending on:
- number of trains drawing current,
- distance from substations.
Note also that the common voltages are simple multiples of each other:
- 1200 V DC = 2 x 600 V DC
- 1500 V DC = 2 x 750 V DC
- 3000 V DC = 2 x 1500 V DC
Third Rail
Image:Third Rail.jpg Most electrification systems use overhead wires, but third rail is an option up to about 1200 V. While use of a third-rail does not imply the use of DC, in practice systems using third rail have all used DC because it can carry 41% more power than an AC system operating at the same peak voltage. Third rail is more compact than overhead wires and can be used in smaller diameter tunnels, an important factor for subway systems.
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Third rail systems can be designed to use top contact, side contact, or bottom contact. Top contact is less safe as the live rail is exposed to people treading on the rail unless an insulating hood of some sort is provided. Side and bottom contact third rail can easily have safety shields incorporated, carried by the rail itself. Uncovered top-contact third rails are also vulnerable to disruption caused by ice and snow, and possibly fallen leaves.
Since DC systems are limited to relatively low voltages, this can limit the size and speed of trains and the amount of air-conditioning the trains can provide; this may be a factor favouring overhead wires and high voltage AC even for urban usage. In practice, the top speed of trains on third-rail systems is limited to 100mph (160km/h) because above that speed reliable contact between the "shoe" that collects the power and the rail cannot be maintained. See also Third rail
Fourth Rail
Image:EalingCommon3.jpg The London Underground is one of the few networks in the world that use a fourth rail system. The additional rail carries the electrical return that is provided by the running rails on third rail networks. On the London Underground a conventional top-contact third rail is placed beside the track, energised at +420 V DC, and a top-contact fourth rail is located centrally between the running rails at -210 V DC, which combine to provide a traction voltage of 630 V DC.
The advantage of the fourth rail system is that the two running rail are available exclusively for track circuits, of which there are many.
Low-frequency alternating current
Common commutating electric motors can also be fed AC (universal motor), because reversing current in both stator and rotor does not change the direction of torque. However, inductance of the windings makes large motors impractical at standard AC distribution frequencies. Many European countries, including Germany, Austria, Switzerland, Norway, and Sweden have standardised on 15 kV 16-2/3 Hz (one-third the normal mains frequency) single-phase AC (earlier, 6 kV and 7.5 kV systems were in use). In the United States (with its 60 Hz distribution system), 25 Hz (an older, now-obsolete standard mains frequency) is used at 11 kV between Washington, DC and New York City. A 12.5 kV 25 Hz section between New York City and New Haven, Connecticut was converted to 60 Hz in the last third of the 20th century.
Motors are fed through a switching transformer that allows voltage change, so no resistors are required. Auxiliary machinery is driven by low voltage commutating motors, powered from a separate winding of the main transformer, and are reasonably small.
The unusual frequency means that electricity has to be converted from utility power by motor-generators or static inverters at the feeding substations, or generated at altogether separate electric power stations.
Standard frequency alternating current
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The first attempts to use standard-frequency (50 Hz) single-phase AC were made in Hungary in 1930s. However, only in 1950s did this system become widespread.
Today, some locomotives in this system use a transformer and rectifier that provide low-voltage pulsating DC current to motors. Speed is controlled by switching windings in the transformer. More sophisticated locomotives use thyristor or IGBT transistor circuitry to generate chopped or even variable-frequency AC that is then directly consumed by AC traction motors.
This system is quite economical, but it has its drawbacks: the phases of the external power system are loaded unequally, and there is significant electromagnetic interference generated.
The 25 kV 50 Hz single-phase AC system is used in France, Great Britain, Finland, Denmark, Malaysia, the former Soviet Union, the former Yugoslavia (excluding Slovenia and western Croatia, which use 3 kV DC), India, Japan and parts of Australia (namely, all electrification in Queensland and Western Australia), while the USA commonly uses 12.5 and 25 kV at 60 Hz. 25 kV AC is the system of choice for high speed and long distance railways, even if the railways uses a different system for existing trains. This applies to Spain, Italy, South Africa, Taiwan, China, etc.
Multisystem locomotives
Because of the variety of railway electrification systems, which can vary even within a country, trains often have to pass from one system to another. One way this is accomplished is by changing locomotives at the switching stations. These stations have overhead wires that can be switched from one voltage to another, and so the train arrives with one locomotive, and then departs with another. Often however, this is inconvenient and time-consuming.
Another way is to use multisystem locomotives that can operate under several different voltages and current types. In Europe, it is common to use four-system locomotives (DC 1.5 kV, DC 3 kV, AC 15 kV 16-2/3 Hz, AC 25 kV 50 Hz). These locomotives do not have to stop when passing from one electrification system to another, the changeover occurring where the train can coast for a short time.
Eurostar trains through the Channel Tunnel are multisystem: a significant part of the route near London is on southern England's 750 V DC third rail system, the route into Brussels is 3000 V DC overhead, while the rest of the route is 25 kV 50Hz overhead. The need for these trains to use third rail will end upon completion of the Channel Tunnel Rail Link in 2007. Southern England has some overhead/third rail dual-system locomotives and multiple units to allow through running between the 750 V DC third rail system south of London and the 25 kV AC catenary system north and east of London.
In the United States New Jersey Transit uses multisystem ALP-44 and ALP-46 locomotives for its Midtown Direct service into New York, and Amtrak uses multi-system AEM-7, HHP-8 and Acela locomotives on the Northeast Corridor between Washington DC and Boston.
In India, dual-voltage WCAMX series locomotives haul intercity trains out of Mumbai suburban region, which is predominantly under 1500 V DC catenary, unlike the rest of the electrified trackage in India which is under 25 kV 50 Hz catenary.