Railway electrification system
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Image:CT-Station, Bridgeport.jpg
A railway electrification system is a way of supplying electric power to electric locomotives and multiple units. Electrified railways have advantages over non-electrified railways (usually operated by diesel-powered trains, but in the past also by steam trains). The principal advantage is the superior acceleration of electrically powered trains, but secondary advantages are the lack of exhaust fumes, and the lower maintenance requirements of the traction units. A disadvantage is the capital cost of the electrification equipment, which tends to mitigate against electrification of long distance lines which do not carry a heavy service. Suburban railways with closely-spaced stations and high traffic density are the most likely to be electrified, but main lines carrying heavy and frequent traffic are also electrified in many countries.
Electrification systems can be classified by:
- conductor (third rail or overhead wire)
- voltage
- current (DC or AC)
- AC frequency; single-phase or three-phase (invented by Hungarian Kálmán Kandó)
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[edit] Direct current
Early electric systems used relatively low-voltage DC. Electric motors were fed directly from the traction supply, and were controlled by using a combination of resistors and relays that connected the motors in parallel or series.
The most common voltages are 600 V and 750 V for trams and metros, and 1500 V and 3000 V for railways. The lower voltages are often used with third or fourth rail systems, but voltages above 1000 V are generally limited to overhead wiring for safety reasons.
In the past, rotary converters or mercury arc rectifiers were used to convert utility (mains) AC power to the required DC voltage at the feeder stations. Today, this is usually done by semiconductor rectifiers after transforming the voltage down from the utility supply.
The DC system is quite simple, but it requires thick cables and short distances between feeder stations because of the heavy currents required; additionally, there are significant resistive losses. The feeder stations require constant monitoring, and on many systems, only one train or locomotive could enter one section. The distance between two feeder stations at 3000 V system is about 25 km (15.6 miles).
Auxiliary machinery, such as fans and compressors, are also powered by motors fed directly from the traction supply. Consequently, these motors are often unusually bulky.
1500 V DC 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. In the Czech Republic two local lines remain 1500 V: Tabor - Bechyne and Rybnik - Lipno. In Slovakia, there are two narrow gauge lines in the High-Tatras (one is cog railway),
In the United Kingdom, 1500 V DC was used in 1954 for the Woodhead trans-Pennine route (now closed); the system used regenerative braking, allowing for transfer of energy between climbing and decending trains on the steep approaches to the tunnel. The system was also used for suburban electrification in East London and Manchester, now converted to 25 kV AC. The only UK system now using this voltage is the Tyne and Wear Metro.
3000 V DC is used in Belgium, Italy, Poland, the northern Czech Republic, Slovakia, Slovenia, western Croatia and in the former Soviet Union. It was also formerly used by the Milwaukee Road's extensive electrification across the Continental Divide, and by the Delaware, Lackawanna & Western Railroad (now NJ Transit, converted to 25 kV AC).
Voltages such as 1500 V DC are nominal, and fluctuate up and down from say 1300 V to 1800 V depending on:
- the number of trains drawing current,
- the distance from the substation to the train.
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
[edit] Third Rail
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 require the use of DC, in practice all third-rail systems use 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.
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 vulnerable to disruption caused by ice, snow, and fallen leaves.
DC systems are limited to relatively low voltages, and 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 100 mph (160 km/h) because above that speed reliable contact between the shoe and the rail cannot be maintained. See also Third rail
Some road operating trams (streetcars) also used third rail current collection schemes. In these cases, the third rail was located below street level; the tram picked up the current via a collector accessed through a narrow slot in the road. In the United States, the former trolley system in Washington, D.C. was operated in this manner so as to avoid the unsightly wires and poles associated with electric traction. The evidence of this mode of running can still be seen on the track that runs down the slope on the Northern access to the abandoned Kingsway Tramway Subway (in central London). The slot between the running rails is clearly visible. The slot can easily be confused with the similar looking slot that allows access to a cable hauled tram system (indeed, in at least some cases, the third rail slot was a recycled cable slot).
[edit] Fourth Rail
The London Underground is one of the few networks in the world that uses a four-rail system. The additional rail carries the electrical return that on third-rail and overhead networks is provided by the running rails. On the London Underground a 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 rails are available exclusively for track circuits, of which there are many, though this was not the primary reason for adopting a fourth-rail scheme. Most of the deep level tube lines run in cast iron tunnels (only some of the more recent constructions use concrete tunnel lining). Using a third-rail scheme necessitates that the return current is conducted through one (earthed) running rail. Such current is just as easily able to travel through the cast iron tunnel lining. Unless the joints between the sections are electrically sound, the current will arc across the sections causing considerable damage, or corrode the tunnel segments via electrolysis.
Further there are many cast iron gas and water mains in the vicinity of the tube tunnels, and the return current would travel along these just as easily. Some of these mains date back to the 19th century and the joints between separate sections would certainly not have been designed to be electrically sound, as deep-level electric tube trains were some way off.
The surface sections of the lines are constructed using fourth-rail purely to permit through running of the same trains, there being no other technical reason to do so.
The traction current has no direct earth point, but there are two resistors connected across the traction supply. The centre tap of the resistors is earthed, establishing the reference point between the positive and negative rails. The resistors are large enough to prevent large currents flowing through the earthed infrastructure. The positive resistor is twice as large as the negative resistor, since the positive rail carries twice the voltage of the negative rail.
On lines where London Underground trains run over Network Rail owned lines, sharing the track with third-rail stock, the centre 'negative' rail is directly connected to the return running rail, allowing both types of train to operate.
A few lines of the Paris Metro also operate on a '4 rail' power scheme, but for a very different reason. It is not strictly a 4 rail scheme as they run on rubber tyres running on a pair of narrow roadways made of steel, and in some places, concrete. Since the tyres do not conduct the return current well, two conductor rails are provided outside of the running 'roadways', so at least electrically, it fits as a 4 rail scheme. The trains are designed to operate from either polarity of supply, because some lines use reversing loops at one end, causing the train to be reversed during every complete journey. Rubber tyres also run against the side contact conductor rails to guide the train on its track. Conventional rails are provided inside the 'roadways' to facilitate the operation of maintenance equipment, and movement of conventional rail stock. The rubber tyres were intended to provide a smoother ride and less vibration to surrounding buildings. They succeed in doing this, but at the expense of considerable running noise inside the trains and very short tyre life. Due to frictional losses, the energy consumption is significantly higher than that of similar trains equipped with steel wheels, running on steel rails.
[edit] 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 16⅔ 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.
In such a system, the traction motors can be fed through a transformer with multiple taps. Changing the taps allows the motor voltage to be changed without requiring power-wasting resistors. 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.
[edit] Standard frequency alternating current
The first attempts to use standard-frequency 50 Hz single-phase AC were made in Hungary in 1930s, by Kálmán Kandó on line between Budapest-Nyugati and Alag, using 16 kV. 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 winding taps on 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, not to mention acoustic noise.
The 25 kV 50 Hz single-phase AC system is used in France, Great Britain, Finland, Denmark, Hungary, Malaysia, the former Soviet Union, the former Yugoslavia (excluding Slovenia and western Croatia, which use 3 kV DC), India, Japan, New Zealand, and the Australian states of 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 railway uses a different system for existing trains. This applies to Spain, Italy, South Africa, Taiwan, China, etc.
To prevent the risk of out of phase supplies mixing, sections of line fed from different feeder station must be kept strictly isolated. This is achieved by Neutral Sections (also known as Phase Breaks), usually provided at and midway between feeder stations, although typically only half are in use at any time, the others being provided to allow a feeder station to be shut down and power provided from adjacent feeder stations. Neutral Sections usually consist of an earthed section of wire which is separated from the live wires on either side by insulating material, typically ceramic beads, designed so the pantograph will smoothly run from one section to the other. The earthed section prevents an arc being drawn from one live section to the other, as the voltage difference may be higher than the normal system voltage if the live sections are on different phases, and the protective circuit breakers may not be able to safety interrupt the considerable current that would flow. To prevent the risk of an arc being drawn across from one section of wire to earth, when passing through the neutral section the train must be coasting and the circuit breakers must be open. In many cases this is done manually by the driver. To help them, a warning board is provided both just before the neutral section and also an advanced warning some distance before. A further board is then provided after the neutral section to tell the driver they can reclose the circuit breaker, although the driver must not do this until the rear pantograph has passed this board. In the UK, a system known as Automatic Power Control (APC) is in use which automatically opens and closes the circuit breaker, this being achieved by using sets of permanent magnets alongside the track communicating with a detector on the train. The only action needed by the driver is to shut off power and coast, and therefore warning boards are still provided at and on the approach to neutral sections.
[edit] 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. The switching stations have very sophisticated components, and they are very expensive.
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 (1.5 kV DC, 3 kV DC, 15 kV 16⅔ Hz AC, 25 kV 50 Hz AC). These locomotives do not have to stop when passing from one electrification system to another, the changeover occurring where the train coasts for a short time.
In the Czech Republic and Slovakia, the railways have both 3000 V DC and 25 kV AC systems, but there are no switching stations - the two sytems meet at breaks on overhead wires. Except for two breaks (Kutná Hora and somewhere in South-Moravia), they are all away from stations.
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 750 V DC third rail south of London and the 25 kV AC overhead north and east of London.
Early AC electrification systems in the UK used 6.25kV in urban areas, to prevent the need to raise many bridges to meet with the clearance standards of the time. This system was used on suburban routes in Glasgow and East London, the latter having been upgaded from 1500V DC. Further away from the cities, though, the voltage switched to 25kV. A system known as "Automatic Power Control" was developed to allow trains to automatically switch between the voltages whilst moving. All the driver had to do was shut off power and coast until clear of the neutral section; the system automatically opened the circuit breaker, detected a change in voltage and switched over the transformer to the correct input voltage setting, then closed the circuit breaker. This system proved unreliable, with the voltage changeover often failing to work. Since by this time enough knowledge of high voltage electrification was known, the rules were able to be changed to allow 25kV to be used in areas where 6.25kV had previously been used, and all new electrification was then at 25kV, with the 6.25kV systems graually disappearing, in particular spurred on by the desire to eliminate dual voltage equipment from new trains, the last line to retain 6.25kV being the semi-isolated London, Tilbury and Southend line, converted in 1983.
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 both cases, through trains run on both newer, 25 kV 60 Hz built or refurbished by their respective agencies since the 1980s, and older, 12 kV 25 Hz inherited from the now-defunct Pennsylvania Railroad. The latter dates to the 1930s, when the Pennsylvania upgraded its electrified network from 650 V DC third rail.
In India, dual-voltage WCAMX series locomotives haul intercity trains out of Mumbai suburban region, which is predominantly 1500 V DC, unlike the rest of the electrified trackage in India, which is 25 kV 50 Hz.
[edit] See also
- High-speed rail
- List of current systems for electric rail traction
- Maglev train
- Overhead lines
- Railroad electrification in the United States
- Railway electrification in Great Britain
- Third rail
- Tram
[edit] External links
de:Bahnstrom fr:Système d'électrification ferroviaire nl:Elektrificatie ja:鉄道の電化 zh:電氣化鐵路
