View source for 25 kV AC railway electrification

{{Short description|Standard current and voltage settings for most high-speed rail}}
{{Multiple issues|
{{More citations needed|article|date=July 2009}}
{{Unfocused|date=September 2022}}
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[[File:Europe rail electrification en.svg|thumb|300px|[[Railway electrification|Railway electrification system]]s used in Europe:
{{legend|#c0c1c5|Non-electrified}}
{{legend|#003399|750{{nbsp}}V DC}}
{{legend|#5bc6ff|1.5{{nbsp}}kV DC}}
{{legend|#46a43b|3{{nbsp}}kV DC}}
{{legend|#c39467|[[15 kV AC]]}}
{{legend|#ffd617|'''25{{nbsp}}kV AC'''}}
High-speed lines in France, Spain, Italy, the Netherlands, Belgium and Turkey operate under '''25{{nbsp}}kV''', as do high power lines in the former Soviet Union.]]
[[Railway electrification|Railway electrification system]]s using [[alternating current]] (AC) at {{nowrap|25 [[kilovolt]]s (kV)}} are used worldwide, especially for [[high-speed rail]]. It is usually supplied at the standard [[utility frequency]] (typically 50 or 60{{nbsp}}Hz), which simplifies traction substations. The development of 25{{nbsp}}kV AC electrification is closely connected with that of successfully using utility frequency.
[[File:East coast main line. - geograph.org.uk - 103409.jpg|thumbnail|The [[East Coast Main Line]] in the United Kingdom is electrified using 25 kV 50 Hz [[overhead lines]].]]
This electrification is ideal for railways that cover long distances or carry heavy traffic. After some experimentation before [[World War II]] in [[Rail transport in Hungary|Hungary]] and in the [[Black Forest]] in [[Rail transport in Germany|Germany]], it came into widespread use in the 1950s.
One of the reasons it was not introduced earlier was the lack of suitable small and lightweight control and rectification equipment before the development of solid-state [[rectifier]]s and related technology. Another reason was the increased clearance required under bridges and in tunnels, which would have required major [[civil engineering]] in order to provide the increased [[Engineering tolerance|clearance]] to live parts. Where existing [[loading gauge]]s were more  generous, this was less of an issue.
Railways using older, lower-capacity [[direct current|direct-current]] systems have introduced or are introducing {{nowrap|25 kV}} AC instead of {{nowrap|3 kV}} DC/{{nowrap|1.5 kV}} DC for their new high-speed lines.
==History==
The first successful operational and regular use of a utility frequency system dates back to 1931, tests having run since 1922. It was developed by [[Kálmán Kandó]] in Hungary, who used {{nowrap|16 kV}} AC at {{nowrap|50 Hz}}, 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).<ref name="Hollingsworth Cook p. ">{{cite book | last1=Hollingsworth | first1=J. B. | last2=Cook | first2=Arthur F. | title=The great book of trains : featuring 310 locomotives shown in more than 160 full-colour illustrations and 500 photographs | publisher=Salamander Books | publication-place=London | date=1998 | isbn=0-86101-919-9 | oclc=60209873 | pages=254–255}}</ref> 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 1936 by the [[Deutsche Reichsbahn]] who electrified part of the [[Höllentalbahn (Black Forest)|Höllentalbahn]] between Freiburg and Neustadt installing a 20{{nbsp}}[[Volt|kV]]{{nbsp}}50 [[Hertz|Hz]] [[alternating current|AC]] system. This part of Germany was in the French zone of occupation after 1945. As a result of examining the German system in 1951 the [[SNCF]] electrified the line between [[Aix-les-Bains]] and [[La Roche-sur-Foron]] in southern France, initially at the same 20{{nbsp}}kV but converted to 25{{nbsp}}kV in 1953. The 25{{nbsp}}kV system was then adopted as standard in France, but since substantial amounts of mileage south of Paris had already been electrified at 1.5{{nbsp}}kV [[Direct current|DC]], SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.<ref>Haydock, David (1991). ''SNCF''. "Modern Railways" special. London: Ian Allan. {{ISBN|978-0-7110-1980-5}}</ref><ref>Cuynet, Jean (2005). ''La traction électrique en France 1900–2005''. Paris: La Vie du Rail. {{ISBN|2-915034-38-9}}</ref>
The main reason why electrification using utility frequency had not been widely adopted before was the lack of reliability of [[Mercury arc rectifier]]s that could fit on the train. This in turn related to the requirement to use [[Traction motor|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 railway locomotives.
It was possible to use AC motors (and some railways did, with varying success), but they have had less than ideal characteristics for traction purposes. This is 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 most common choice for traction purposes until the 1990s, 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 Series Shinkansen|N700 Shinkansen]] uses a three-level converter to convert {{nowrap|25 kV}} single-phase AC to {{nowrap|1,520 V}} AC (via transformer) to {{nowrap|3 kV}} DC (via phase-controlled rectifier with thyristor) to a maximum {{nowrap|2,300 V}} three-phase AC (via a [[VVVF|variable voltage, variable frequency]] inverter using [[Insulated-gate bipolar transistor|IGBTs]] with [[pulse-width modulation]]) to run the motors. The system works in reverse for [[Regenerative brake|regenerative braking]].
The choice of {{nowrap|25 kV}} was related to the efficiency of power transmission as a function of voltage and cost, not based on a neat and tidy ratio of the supply voltage. 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 {{nowrap|25 kV}} was an optimal point, where a higher voltage would still improve efficiency but not by a significant amount in relation to the higher costs incurred by the need for larger insulators and greater clearance from structures.
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 of atmospheric causes occurred in December 2009, when [[Eurostar#2009|four Eurostar trains broke down inside the Channel Tunnel]].
==Distribution==
Electric power for {{nowrap|25 kV}} AC electrification is usually taken directly from the three-phase [[Electric power transmission|transmission system]]. At the transmission substation, a step-down [[transformer]] is connected across two of the three phases of the high-voltage supply and lowers the voltage to {{nowrap|25 kV}}. This is then fed, sometimes several kilometres away, to a railway feeder station located beside the tracks. [[Switchgear]] at feeder stations, and at track sectioning cabins located halfway between feeder stations, provides switching to feed the overhead line from adjacent feeder stations if one feeder station loses grid supply.
Since only two phases of the high-voltage supply are used, phase imbalance is corrected by connecting each feeder station to a different combination of phases. To avoid the train pantograph bridging together two feeder stations which may be out-of-phase with each other, [[Overhead line#Neutral section (phase break)|neutral sections]] are provided at feeder stations and track sectioning cabins. [[Static VAR compensator|SVCs]] are used for load balancing and voltage control.<ref>''SVCs for load balancing and trackside voltage control'', ABB Power Technologies. [http://library.abb.com/global/scot/scot221.nsf/veritydisplay/c9f996d4f6c5c601c12570c9004b1754/$File/A02-0196E.pdf] {{webarchive|url=https://web.archive.org/web/20070206165307/http://library.abb.com/GLOBAL/SCOT/scot221.nsf/VerityDisplay/C9F996D4F6C5C601C12570C9004B1754/%24File/A02-0196E.pdf|date=2007-02-06}}</ref>
In some cases dedicated single-phase AC power lines were built to substations with single phase AC transformers. Such lines were built to supply the French [[TGV]].<ref>[http://www.justmystage.com/home/overheadtml/france1.html TGV power] {{webarchive |url=https://web.archive.org/web/20090504213517/http://www.justmystage.com/home/overheadtml/france1.html |date=May 4, 2009 }}</ref>
==Standardisation==
Railway electrification using {{nowrap|25 kV}}, {{nowrap|50 Hz}} AC has become an international standard. There are two main standards that define the voltages of the system:
*EN{{nbsp}}50163:2004+A1:2007 – "Railway applications. Supply voltages of traction systems"<ref>{{cite book|author1=British Standards Institution|title=BS EN 50163:2004+A1:2007 Railway Applications. Supply voltages of traction systems|date=January 2005|doi=10.3403/30103554}}</ref>
*IEC{{nbsp}}60850 – "Railway Applications. Supply voltages of traction systems"<ref>[http://webstore.iec.ch/webstore/webstore.nsf/mysearchajax?Openform&key=60850&sorting=&start=1&onglet=1/ IEC 60850] – "Railway Applications. Supply voltages of traction systems"</ref>
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.
{| class="wikitable"
|-
! rowspan=2 width=20% | Electrification<br />system
! colspan=5 | Voltage
|-
! width=16% | Min.<br />non-permanent
! width=16% | Min.<br />permanent
! width=16% | Nominal
! width=16% | Max.<br />permanent
! width=16% | Max.<br />non-permanent
|-
| {{nowrap|{{val|25}} kV}} 50{{nbsp}}Hz
| {{nowrap|{{val|17.5}} kV}}
| {{nowrap|{{val|19}} kV}}
| {{nowrap|{{val|25}} kV}}
| {{nowrap|{{val|27.5}} kV}}
| {{nowrap|{{val|29}} kV}}
|}
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").
==Variations==
Systems based on this standard but with some variations have been used.
===25 kV AC at 60 Hz===
{{See also|List of railway electrification systems#25 kV AC, 60 Hz|l1=List of railway electrification systems: 25 kV AC, 60 Hz}}
In countries where {{nowrap|60 Hz}} is the normal grid power frequency, {{nowrap|25 kV}} at {{nowrap|60 Hz}} is used for the railway electrification.
*In [[Canada]] on the [[Deux-Montagnes line]] of the [[Agence métropolitaine de transport|Montreal Metropolitan transportation Agency]].
*In [[Japan]] on the Tokaido, Sanyo and Kyushu [[Shinkansen]] lines (using {{Track gauge|1435mm|lk=on|disp=or}} gauge).
*In [[South Korea]] on the [[Korail]] network.
*In [[Taiwan]] on the [[Taiwan High Speed Rail]] line (using {{Track gauge|1435mm|disp=or}} gauge) and on [[Taiwan Railway Administration]]'s electrified lines (using {{Track gauge|1067mm|disp=or}} gauge).
*In the [[United States]] on newer electrified portions of the [[Northeast Corridor]] (i.e. the New Haven-Boston segment) intercity passenger lines, [[New Jersey Transit]] commuter lines, [[Denver]] [[RTD Commuter Rail]], [[Caltrain]], and select isolated short lines.
===20 kV AC at 50 or 60 Hz===
In Japan, this is used on existing railway lines in [[Tohoku Region]], [[Hokuriku Region]], [[Hokkaido]] and [[Kyushu]], of which Hokuriku and Kyushu are at 60{{nbsp}}[[Hz]].
===12.5 kV AC at 60 Hz===
Some lines in the United States have been electrified at {{nowrap|12.5 kV 60 Hz}} or converted from {{nowrap|11 kV 25 Hz}} to {{nowrap|12.5 kV 60 Hz}}.  Use of {{nowrap|60 Hz}} allows direct supply from the 60{{nbsp}}Hz utility grid yet does not require the larger wire clearance for {{nowrap|25 kV 60 Hz}} or require dual-voltage capability for trains also operating on {{nowrap|11 kV 25 Hz}} lines.  Examples are:
*[[Metro-North Railroad]]'s [[New Haven Line]] from Pelham, NY to New Haven, CT (Since 1985; previously 11{{nbsp}}kV 25{{nbsp}}Hz).
=== 12 kV at 25 Hz ===
{{Main|Amtrak's 25 Hz traction power system|SEPTA's 25 Hz traction power system}}
* [[New Jersey Transit|New Jersey Transit's]] [[North Jersey Coast Line]] from Matawan, NJ to Long Branch, NJ (1988–2002; changed to 25{{nbsp}}kV 60{{nbsp}}Hz).
* [[Amtrak's 25 Hz traction power system|Amtrak]]
*[[SEPTA Regional Rail|SEPTA]] – Both ex-[[Reading Company|Reading Rail]] and ex-[[Pennsylvania Railroad|Pennsylvania Rail]] sides.
===6.25 kV AC===
Early 50{{nbsp}}Hz AC railway electrification in the United Kingdom was planned to use sections at {{nowrap|6.25 kV AC}} where there was limited clearance under bridges and in tunnels. Rolling stock was dual-voltage with automatic switching between {{nowrap|25 kV}} and {{nowrap|6.25 kV}}.  The {{nowrap|6.25 kV}} sections were converted to {{nowrap|25 kV AC}} 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 railway station|Crewe]].  A section of {{nowrap|25 kV}} overhead line was gradually brought closer to the earthed metalwork of the bridge whilst being subjected to 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.{{citation needed|date=July 2011}}
===50 kV AC===
Occasionally {{nowrap|25 kV}} is doubled to {{nowrap|50 kV}} 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 [[Sishen–Saldanha railway line|Sishen–Saldanha]] iron ore railway ({{nowrap|50 Hz}}).
*The [[Deseret Power Railway]] which was an isolated coal railway ({{nowrap|60 Hz}}).<ref name=":0">{{cite web|title=Railroad Coordination Manual Of Instruction, Section 2.1.5 Deseret Power Railway |url=http://shrp2.transportation.org/Documents/RailroadCoordination_MOI.pdf| publisher =Utah Department of Transportation |date=May 2015|access-date=8 November 2016|page=102}}</ref>
*The now closed [[Black Mesa and Lake Powell Railroad]] which was also an isolated coal railway ({{nowrap|60 Hz}}).
*The now closed [[Tumbler Ridge]] Subdivision of [[BC Rail]] ({{nowrap|60 Hz}}).<ref>{{cite web|url=http://www.wcra.org/railwaynews/may2004.htm |title=GF6C #6001 PRESERVED |publisher=West Coast Railway Association, BC |date=May 2004 |access-date=2011-01-09 |url-status=dead |archive-url=https://web.archive.org/web/20090218102742/http://www.wcra.org/railwaynews/may2004.htm |archive-date=February 18, 2009 }}</ref>
===2 × 25 kV autotransformer system===
[[File:Onderdel spoorwegnet.gif|thumb|500px|{{columns-list|colwidth=22em|
1. Supply transformer<br/>
2. Power supply<br/>
3. Overhead line<br/>
4. Running rail<br/>
5. Feeder line<br/>
6. Pantograph<br/>
7. Locomotive transformer<br/>
8. Overhead line<br/>
9. Autotransformer<br/>
10. Running rail
}}]]
The 2 × 25{{nbsp}}kV [[autotransformer]] system is a [[split-phase electric power]] system which supplies 25{{nbsp}}kV power to the trains, but transmits power at 50{{nbsp}}kV to reduce energy losses. It should not be confused with the 50{{nbsp}}kV system. In this system, the current is mainly carried between the overhead line and a feeder transmission line instead of the rail. The overhead line (3) and feeder (5) are on opposite phases so the voltage between them is 50{{nbsp}}kV, while the voltage between the overhead line (3) and the running rails (4) remains at 25{{nbsp}}kV. Periodic autotransformers (9) divert the return current from the neutral rail, step it up, and send it along the feeder line.
This system was initially deployed on France's then new [[LGV Sud-Est|Paris-Lyon High speed rail line]] in 1981,<ref>{{Cite journal |last=Courtois |first=C. |date=1993 |title=Why the 2*25 kV alternative? (autotransformer traction supply) |url=https://ieeexplore.ieee.org/document/274637 |journal=IEE Colloquium on 50kV Autotransformer Traction Supply Systems - the French Experience |pages=1/1–1/4}}</ref> and has gone on to be used by [[Rail transport in New Zealand|New Zealand Railways]] in 1988,<ref name="nzro_winter_1988">{{cite journal|journal=New Zealand Railway Observer|title=North Island Main Trunk Electrified|date=Autumn 1988|publisher=[[New Zealand Railway and Locomotive Society]]|issn=0028-8624|volume=45| issue =  1|page=49|author=Tom McGavin}}</ref>[[Central Organization for Railway Electrification#2 x 25 KV System|Indian Railways]],<ref>{{Cite web |title=Ministry of Railways (Railway Board) |url=https://indianrailways.gov.in/railwayboard/view_section.jsp?lang=0&id=0,1,304,366,532,565 |access-date=2023-07-05 |website=indianrailways.gov.in}}</ref> [[Russian Railways]], Italian High Speed Railways, UK [[High Speed 1]], most of the [[West Coast Main Line]] and [[Crossrail]],<ref>{{Cite web |title=Balfour Beatty gets £16m Crossrail substation contract |url=https://www.theconstructionindex.co.uk/news/view/balfour-beatty-gets-16m-crossrail-substation-contract |access-date=2023-07-05 |website=www.theconstructionindex.co.uk |language=en}}</ref> with some parts of older lines being gradually converted,{{citation needed|date=September 2018}} French lines (LGV lines and some other lines<ref>The remainder of the French lines use 1 × 25 kV booster-transformer system.</ref>), most Spanish high-speed rail lines,<ref>{{cite report |title=Comparative Study of the Electrification Systems 1×25 kV and 2×25 kV |location=Madrid |date=June 2011 |publisher=[[ENAIRE#Ineco|Ineco]] |url=http://www.bane.dk/db/filarkiv/14202/INECO%20Comparative%20Study%201x25%20-%202x25.pdf |access-date=2017-03-30}}</ref> [[Amtrak's 60 Hz traction power system|Amtrak]] and some of the Finnish and Hungarian lines.
===Boosted voltage===
For [[TGV world speed record]] runs in France the voltage was temporarily boosted, to 29.5{{nbsp}}kV<ref>{{Cite web
|title=The Test Tracks: an Overview
|url=http://www.railfaneurope.net/tgv/rec-track.html
}}</ref> and 31{{nbsp}}kV at different times.<ref>{{Cite news
|title=French Train Hits 357 MPH Breaking World Speed Record
|date=4 April 2007
|url=https://www.foxnews.com/story/french-train-hits-357-mph-breaking-world-speed-record
}}</ref>
=== 25 kV on broad gauge lines ===
*In [[Argentina]] on the [[Roca Line]] (using {{Track gauge|1676mm|lk=on|disp=or}} gauge).
*In [[Australia]]:
**[[Adelaide]]: part of the suburban network ({{nowrap|50 Hz}}).
*[[Commonwealth of Independent States]]: parts of the network ({{nowrap|50 Hz}}).
*In [[Finland]]: see [[rail transport in Finland]] ({{nowrap|50 Hz}}).
*In [[India]]: see [[rail transport in India]] and [[Central Organisation for Railway Electrification]] ({{nowrap|50 Hz}}).
*In [[Spain]]: the [[Atlantic Axis high-speed rail line|Atlantic Axis]] and the [[Madrid–Galicia_high-speed_rail_line|Madrid-Galicia HSL]] between Ourense and Santiago ({{nowrap|50 Hz}}).
*In [[Portugal]]: see [[list of railway lines in Portugal]] ({{nowrap|50 Hz}}).
=== 25 kV on narrow gauge lines ===
*In [[Australia]]:
**[[Perth]]: entire suburban network, see [[railways in Perth]] ({{nowrap|50 Hz}}).
**[[Queensland]]: see [[rail electrification in Queensland]] ({{nowrap|50 Hz}}).
*In [[Malaysia]]: see [[rail transport in Malaysia]] ({{nowrap|50 Hz}}).
*In [[New Zealand]]: see [[North Island Main Trunk]] and [[Auckland railway electrification]] ({{nowrap|50 Hz}}).
*In [[South Africa]]: see [[rail transport in South Africa]] (25 and {{nowrap|50 kV}} {{nowrap|50 Hz}}).
*In [[Taiwan]]: see [[rail transport in Taiwan]] ({{nowrap|60 Hz}}).
*In [[Thailand]]: see [[SRT Dark Red Line|SRT Dark Red line]] and [[SRT Light Red Line|SRT Light Red line]] ({{nowrap|50 Hz}}).
*In [[Tunisia]]: see [[rail transport in Tunisia]] ({{nowrap|50 Hz}}).
===Other voltages on 50 Hz electrification===
*In [[France]], [[Mont Blanc Tramway]] and [[Chemin de fer du Montenvers]]: {{nowrap|11 kV}}
*In [[Germany]], [[:de:Hambachbahn|Hambachbahn]] and [[:de:Nord-Süd-Bahn (Garzweiler)|Nord-Süd-Bahn]]: {{nowrap|6.6 kV}}
== Multi-system locomotives and trains ==
{{main|Multi-system (rail)}}
Trains that can operate on more than one voltage, such as 3{{nbsp}}kV and 25{{nbsp}}kV, are established technologies. Some locomotives in Europe are capable of using four different voltage standards.<ref>{{cite web|url=https://www.railwaygazette.com/in-depth/traxx-locomotive-family-meets-european-needs/32588.article| title=Traxx locomotive family meets European needs |quote=Traxx MS (multi-system) for operation on both AC (15 and 25 kV) and DC (1·5 and 3 kV) networks| date=2008-01-07|access-date=2019-09-27|publisher=[[Railway Gazette International]] }}</ref>
== See also ==
* [[15 kV AC railway electrification]]
* [[List of railway electrification systems]]
* [[Rotary phase converter#Electric railways|Rotary phase converter]]
== References ==
{{reflist}}
== Further reading ==
* Keenor, Garry. Overhead line electrification for railways.
* {{cite book | last=Boocock | first=Colin | title=East Coast Electrification | publisher=Ian Allan | year=1991| isbn=0-7110-1979-7}}
* {{cite book | last=Gillham | first=J.C. | title=The Age of the Electric Train – Electric Trains in Britain since 1883 | publisher=Ian Allan | year=1988| isbn=0-7110-1392-6}}
* {{cite book | last=Glover | first=John | title=Eastern Electric | publisher=Ian Allan | year=2003 | isbn=0-7110-2934-2}}
* {{cite book | last1=Machefert-Tassin | first1=Yves |last2=Nouvion | first2=Fernand | last3=Woimant | first3=Jean| title=Histoire de la Traction Electrique, vol.1 | publisher=La Vie du Rail | year=1980 | isbn=2-902808-05-4}}
* {{cite book | last=Nock | first=O.S.
| title= Britain's new railway: Electrification of the London-Midland main lines from Euston to Birmingham, Stoke-on-Trent, Crewe, Liverpool and Manchester
| publisher=Ian Allan | location = London
| year=1965|oclc=59003738}}
* {{cite book | last=Nock | first=O.S. | title=Electric Euston to Glasgow | publisher=Ian Allan | year=1974| isbn=0-7110-0530-3}}
* {{cite book | title=Proceedings of the British Railways Electrification Conference, London 1960 – Railway Electrification at Industrial Frequency | publisher=London: British Railways Board | year=1960}}
* {{cite book | last=Semmens | first=Peter | title=Electrifying the East Coast Route | publisher=Patrick Stephens Ltd.| year=1991| isbn=0-85059-929-6}}
{{Railway electrification}}
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[[Category:25 kV AC railway electrification| ]]
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