25 kV AC
Encyclopedia
The 25 kV Alternating current railway electrification system is commonly used in railways worldwide, especially for high-speed rail
.
One of the reasons why it was not introduced earlier was the increased clearance distances required where it ran under bridges and in tunnels. Another reason was the lack of suitable control and rectification equipment before the development of solid-state rectifier
s and related technology.
Railways using older, lower-capacity direct current
systems such as France, Russia, South Africa, Spain, Italy, Belgium, Slovakia and The Netherlands have introduced or are introducing AC instead of DC/ DC for their new high-speed lines.
The Channel Tunnel
uses .
in Hungary. He used AC at , asynchronous traction, and an adjustable number of (motor) poles. The first electrified line for testing was Budapest–Dunakeszi–Alag. The first fully electrified line was Budapest–Győr–Hegyeshalom (part of the Budapest–Vienna line). Although Kandó's solution showed a way for the future, railway operators outside of Hungary showed a lack of interest in the design.
The first railway to use this system was completed in 1951 by SNCF
and ran between Aix-les-Bains
and La Roche-sur-Foron
in southern France, initially at 20 kV, but converted to 25 kV in 1953. The 25 kV system was then adopted as standard in France, but since substantial amounts of mileage south of Paris had already been electrified at 1,500 V DC
, the SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.
The main reason why electrification at this voltage had not been used before was the lack of reliability of mercury-arc-type rectifiers
that could fit on the train. This in turn related to the requirement to use DC series motors
, which required the current to be converted from AC to DC and for that a rectifier
is needed. Until the early 1950s, mercury-arc rectifiers were difficult to operate even in ideal conditions and were therefore unsuitable for use in the railway industry.
It was possible to use AC motors (and some railways did, with varying success), but they did not have an ideal characteristic for traction purposes. This was because control of speed is difficult without varying the frequency and reliance on voltage to control speed gives a torque at any given speed that is not ideal. This is why DC series motors were the best choice for traction purposes, as they can be controlled by voltage, and have an almost ideal torque vs speed characteristic.
In the 1990s, high-speed trains began to use lighter, lower-maintenance three-phase
AC induction motors. The N700 Shinkansen
uses a three-level converter to convert single-phase AC to AC (via transformer) to DC (via phase-controlled rectifier with thyristor) to a maximum three-phase AC (via a variable voltage, variable frequency inverter using IGBT
s with pulse-width modulation
) to run the motors. The system works in reverse for regenerative braking
.
The choice of was not based on a neat and tidy ratio of the supply voltage, but rather related to the efficiency of power transmission as a function of voltage and cost. For a given power level, a higher voltage allows for a lower current and usually better efficiency at the greater cost for high-voltage equipment. It was found that was an optimal point, where an even higher voltage would still improve efficiency but not by a significant amount in relation to the higher costs incurred by the need for greater clearance and larger insulators.
supply. This results in an imbalance on the three-phase supply which may affect other customers. This can be overcome by installing static VAR compensator
s or reducing the traction load when the imbalance becomes unacceptable. The system is not insulated from the distribution network, like other systems. Older locomotives and the recuperating electrodynamic brakes
on newer locomotives create electrical noise. It is not necessarily practical to filter this noise from the electricity distribution network, and this has led some countries to prohibit the use of recuperating brakes.
The high voltage leads to a requirement for a slightly higher clearance in tunnels and under overbridges.
To avoid short circuit
s, the high voltage must be protected from moisture. Weather events, such as "the wrong type of snow", have caused failures in the past. An example occurred in December 2009, when four Eurostar trains broke down inside the Channel Tunnel.
At the grid substation, a step-down transformer
is connected across two of the three phases of the high-voltage supply. The transformer lowers the voltage to which is supplied to a railway feeder station located beside the tracks. SVCs
are used for load balancing and voltage control.
Nevertheless, in some cases dedicated single-phase AC power lines were built which run to substations with single phase AC transformers. Such lines were built to supply the French TGV
.
The permissible range of voltages allowed are as stated in the above standards and take into account the number of trains drawing current and their distance from the substation.
This system is now part of the European Union's Trans-European railway interoperability standards (1996/48/EC "Interoperability of the Trans-European high-speed rail system" and 2001/16/EC "Interoperability of the Trans-European Conventional rail system").
The research was done using a steam engine beneath a bridge at Crewe. A section of overhead line was gradually brought closer to the earthed metalwork of the bridge whilst being subjected to the steam from the locomotive's chimney. The distance at which a flashover occurred was measured and this was used as a basis from which new clearances between overhead equipment and structures were derived.
system may be used on 25 kV lines to reduce energy losses. It should not be confused with the 50 kV system. The voltage between the overhead line (3) and the feeder line (5) is 50 kV but the voltage between the overhead line (3) and the running rails (4) remains at 25 kV and this is the voltage supplied to the train. This system is used by Indian Railways.
Key
0. Zero volts tapping
1. Supply transformer
2. Power supply
3. Overhead line
4. Running rail
5. Feeder line
6. Pantograph
7. Locomotive transformer
8. Overhead line
9. Autotransformer
10. Running rail
runs in France the voltage was temporarily boosted, to 29.5 kV and 31 kV at different times.
:Category: 25 kV AC locomotives
High-speed rail
High-speed rail is a type of passenger rail transport that operates significantly faster than the normal speed of rail traffic. Specific definitions by the European Union include for upgraded track and or faster for new track, whilst in the United States, the U.S...
.
Overview
This electrification system is ideal for railways that cover long distances and/or carry heavy traffic. After some experimentation before World War II in Hungary and in the Black Forest (Germany), it came into widespread use in the 1950s.One of the reasons why it was not introduced earlier was the increased clearance distances required where it ran under bridges and in tunnels. Another reason was the lack of suitable control and rectification equipment before the development of solid-state rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
s and related technology.
Railways using older, lower-capacity direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
systems such as France, Russia, South Africa, Spain, Italy, Belgium, Slovakia and The Netherlands have introduced or are introducing AC instead of DC/ DC for their new high-speed lines.
The Channel Tunnel
Channel Tunnel
The Channel Tunnel is a undersea rail tunnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is deep...
uses .
History
The first successful operational and regular use of the system dates back to 1931, tests having run since 1922. It was developed by Kálmán KandóKálmán Kandó
Kálmán Kandó de Egerfarmos et Sztregova was a Hungarian engineer, and a pioneer in the development of electric railway traction.-Education:...
in Hungary. He used AC at , asynchronous traction, and an adjustable number of (motor) poles. The first electrified line for testing was Budapest–Dunakeszi–Alag. The first fully electrified line was Budapest–Győr–Hegyeshalom (part of the Budapest–Vienna line). Although Kandó's solution showed a way for the future, railway operators outside of Hungary showed a lack of interest in the design.
The first railway to use this system was completed in 1951 by SNCF
SNCF
The SNCF , is France's national state-owned railway company. SNCF operates the country's national rail services, including the TGV, France's high-speed rail network...
and ran between Aix-les-Bains
Aix-les-Bains
Aix-les-Bains is a commune in the Savoie department in the Rhône-Alpes region in south-eastern France.It is situated on the shore of Lac du Bourget, by rail north of Chambéry.-Geography:...
and La Roche-sur-Foron
La Roche-sur-Foron
La Roche-sur-Foron is a commune in the Haute-Savoie department in the Rhône-Alpes region in south-eastern France.-References:*...
in southern France, initially at 20 kV, but converted to 25 kV in 1953. The 25 kV system was then adopted as standard in France, but since substantial amounts of mileage south of Paris had already been electrified at 1,500 V DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
, the SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.
The main reason why electrification at this voltage had not been used before was the lack of reliability of mercury-arc-type rectifiers
Mercury arc valve
A mercury-arc valve is a type of electrical rectifier used for converting high-voltage or high-current alternating current into direct current . Rectifiers of this type were used to provide power for industrial motors, electric railways, streetcars, and electric locomotives, as well as for...
that could fit on the train. This in turn related to the requirement to use DC series motors
Traction motor
Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion ....
, which required the current to be converted from AC to DC and for that a rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
is needed. Until the early 1950s, mercury-arc rectifiers were difficult to operate even in ideal conditions and were therefore unsuitable for use in the railway industry.
It was possible to use AC motors (and some railways did, with varying success), but they did not have an ideal characteristic for traction purposes. This was because control of speed is difficult without varying the frequency and reliance on voltage to control speed gives a torque at any given speed that is not ideal. This is why DC series motors were the best choice for traction purposes, as they can be controlled by voltage, and have an almost ideal torque vs speed characteristic.
In the 1990s, high-speed trains began to use lighter, lower-maintenance three-phase
Three-phase
In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...
AC induction motors. The N700 Shinkansen
N700 Series Shinkansen
The is a Japanese Shinkansen high-speed train with tilting capability developed jointly by JR Central and JR West for use on the Tōkaidō and Sanyō Shinkansen lines, and also operated by JR Kyushu on the Kyushu Shinkansen line....
uses a three-level converter to convert single-phase AC to AC (via transformer) to DC (via phase-controlled rectifier with thyristor) to a maximum three-phase AC (via a variable voltage, variable frequency inverter using IGBT
Insulated gate bipolar transistor
The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device primarily used as an electronic switch and in newer devices is noted for combining high efficiency and fast switching...
s with pulse-width modulation
Pulse-width modulation
Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
) to run the motors. The system works in reverse for regenerative braking
Regenerative brake
A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another form, which can be either used immediately or stored until needed...
.
The choice of was not based on a neat and tidy ratio of the supply voltage, but rather related to the efficiency of power transmission as a function of voltage and cost. For a given power level, a higher voltage allows for a lower current and usually better efficiency at the greater cost for high-voltage equipment. It was found that was an optimal point, where an even higher voltage would still improve efficiency but not by a significant amount in relation to the higher costs incurred by the need for greater clearance and larger insulators.
Disadvantages
A AC system uses only one phase of the normal three-phase electric powerThree-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...
supply. This results in an imbalance on the three-phase supply which may affect other customers. This can be overcome by installing static VAR compensator
Static VAr compensator
A static var compensator is an electrical device for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilising the system...
s or reducing the traction load when the imbalance becomes unacceptable. The system is not insulated from the distribution network, like other systems. Older locomotives and the recuperating electrodynamic brakes
Regenerative brake
A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another form, which can be either used immediately or stored until needed...
on newer locomotives create electrical noise. It is not necessarily practical to filter this noise from the electricity distribution network, and this has led some countries to prohibit the use of recuperating brakes.
The high voltage leads to a requirement for a slightly higher clearance in tunnels and under overbridges.
To avoid short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
s, the high voltage must be protected from moisture. Weather events, such as "the wrong type of snow", have caused failures in the past. An example occurred in December 2009, when four Eurostar trains broke down inside the Channel Tunnel.
Distribution networks
Electric power from a generating station is transmitted to grid substations via overhead pylons at high voltage. In the United Kingdom, this will be , or . Different voltages are used in other countries. This power is transmitted using a three-phase distribution system.At the grid substation, a step-down transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
is connected across two of the three phases of the high-voltage supply. The transformer lowers the voltage to which is supplied to a railway feeder station located beside the tracks. SVCs
Static VAr compensator
A static var compensator is an electrical device for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilising the system...
are used for load balancing and voltage control.
Nevertheless, in some cases dedicated single-phase AC power lines were built which run to substations with single phase AC transformers. Such lines were built to supply the French TGV
TGV
The TGV is France's high-speed rail service, currently operated by SNCF Voyages, the long-distance rail branch of SNCF, the French national rail operator....
.
Standardisation
Railway electrification using , AC has become an international standard. There are two main standards that define the voltages of the system:- BS EN 50163:2004 - "Railway applications. Supply voltages of traction systems"
- IEC 60850 - "Railway Applications. Supply voltages of traction systems"
The permissible range of voltages allowed are as stated in the above standards and take into account the number of trains drawing current and their distance from the substation.
| Electrification system |
Lowest non-permanent voltage |
Lowest permanent voltage |
Nominal voltage |
Highest permanent voltage |
Highest non-permanent voltage |
|---|---|---|---|---|---|
| , AC, 50 Hz | |||||
This system is now part of the European Union's Trans-European railway interoperability standards (1996/48/EC "Interoperability of the Trans-European high-speed rail system" and 2001/16/EC "Interoperability of the Trans-European Conventional rail system").
25 kV AC at 60 Hz
In countries where is the normal grid power frequency, at is used for the railway electrification.- In the United States, newer electrified portions of the Northeast CorridorNortheast CorridorThe Northeast Corridor is a fully electrified railway line owned primarily by Amtrak serving the Northeast megalopolis of the United States from Boston in the north, via New York to Washington, D.C. in the south, with branches serving other cities...
intercity passenger line and New Jersey TransitNew Jersey TransitThe New Jersey Transit Corporation is a statewide public transportation system serving the United States state of New Jersey, and New York, Orange, and Rockland counties in New York State...
commuter lines. - In western Japan, ShinkansenShinkansenThe , also known as THE BULLET TRAIN, is a network of high-speed railway lines in Japan operated by four Japan Railways Group companies. Starting with the Tōkaidō Shinkansen in 1964, the network has expanded to currently consist of of lines with maximum speeds of , of Mini-shinkansen with a...
lines (using gauge) use , contrasting eastern parts which use . - In Canada on the Deux-Montagnes Line of the Montreal Metropolitan transportation AgencyAgence métropolitaine de transportThe Agence métropolitaine de transport is the umbrella organization that plans, integrates, and coordinates public transportation services across Canada's Greater Montreal Region, including the Island of Montreal, Laval , and communities along both the North Shore of the Rivière des Mille-Îles...
, - In Pakistan on Pakistan RailwaysPakistan RailwaysThis article is about the rail company in Pakistan. For technical details and operations see: Transport in Pakistan.Pakistan Railways is a national state-owned rail transport service of Pakistan, head-quartered in Lahore. It is administered by the federal government under the Ministry of Railways....
(using gauge), - In South Korea on KorailKorailKorea Railroad Corporation , promoted as Korail , is the national railroad operator in South Korea.Korail operates passenger and freight trains throughout South Korea...
and in Taiwan on Taiwan High Speed RailTaiwan High Speed RailTaiwan High Speed Rail is a high-speed rail line that runs approximately along the west coast of the Republic of China from the national capital of Taipei to the southern city of Kaohsiung...
(both using gauge).
6.25 kV AC
Early 50 Hz AC railway electrification in the United Kingdom used sections at where there was limited clearance under bridges and in tunnels. Rolling stock was dual-voltage with automatic switching between and . The sections were converted to standard as a result of research work that demonstrated that the distance between live and earthed equipment could be reduced from that originally thought to be necessary.The research was done using a steam engine beneath a bridge at Crewe. A section of overhead line was gradually brought closer to the earthed metalwork of the bridge whilst being subjected to the steam from the locomotive's chimney. The distance at which a flashover occurred was measured and this was used as a basis from which new clearances between overhead equipment and structures were derived.
50 kV AC
Occasionally is doubled to to obtain greater power and increase the distance between substations. Such lines are usually isolated from other lines to avoid complications from interrunning. Examples are:- The Black Mesa and Lake Powell Railroad which is an isolated coal railway .
- The Tumbler Ridge Subdivision of BC Rail .
- The Sishen-Saldanha iron ore railway .
2 x 25 kV autotransformer system
The 2 x 25 kV autotransformerAutotransformer
An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where...
system may be used on 25 kV lines to reduce energy losses. It should not be confused with the 50 kV system. The voltage between the overhead line (3) and the feeder line (5) is 50 kV but the voltage between the overhead line (3) and the running rails (4) remains at 25 kV and this is the voltage supplied to the train. This system is used by Indian Railways.
Key
0. Zero volts tapping
1. Supply transformer
2. Power supply
3. Overhead line
4. Running rail
5. Feeder line
6. Pantograph
7. Locomotive transformer
8. Overhead line
9. Autotransformer
10. Running rail
Boosted voltage
For TGV world speed recordTGV world speed record
The TGV holds a series of world speed records achieved by SNCF, the French national railway, and its industrial partners...
runs in France the voltage was temporarily boosted, to 29.5 kV and 31 kV at different times.
25kV in the narrow gauge lines
- In TaiwanTaiwanTaiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
: see Rail transport in TaiwanRail transport in TaiwanTaiwan has an extensive network of railways . Though no longer as dominant as it once was, Taiwan's high population density continues to make rail transport an extremely important form of transportation, especially along the densely populated western corridor...
. - In TunisiaTunisiaTunisia , officially the Tunisian RepublicThe long name of Tunisia in other languages used in the country is: , is the northernmost country in Africa. It is a Maghreb country and is bordered by Algeria to the west, Libya to the southeast, and the Mediterranean Sea to the north and east. Its area...
. - In Queensland, Australia: see Rail electrification in QueenslandRail electrification in QueenslandElectrification of the Brisbane suburban network with a 25 kV AC overhead power supply commenced in 1976. The suburban network was integrated with the construction of a cross-river rail Link between Roma Street and South Brisbane stations in 1978...
. - In New ZealandNew ZealandNew Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
: see North Island Main Trunk Railway - In Perth, Australia. Entire suburban network, see: Transperth TrainsTransperth TrainsTransperth Trains is a division of the Public Transport Authority of Western Australia. It is responsible for operating Perth’s urban passenger rail system, as part of the Transperth network.- Lines :...
Multi-system locomotives and trains
Trains that can operate on more than one voltage, say 3 kV/25 kV, are established technologies. Some locomotives in Europe are capable of using four different voltage standards.See also
:Category: 25 kV AC locomotives
- 15 kV AC railway electrification
- Electric power supply of Polish State Railways as example of a railway grid electrified with DC of 3 kV