25 kV AC railway electrification

Railway electrification systemsusingalternating current(AC) at25kilovolts(kV)are used worldwide, especially forhigh-speed rail.It is usually supplied at the standardutility frequency(typically 50 or 60Hz), which simplifies traction substations. The development of 25kV AC electrification is closely connected with that of successfully using utility frequency.

This electrification is ideal for railways that cover long distances or carry heavy traffic. After some experimentation beforeWorld War IIinHungaryand in theBlack ForestinGermany,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-staterectifiersand related technology. Another reason was the increased clearance required under bridges and in tunnels, which would have required majorcivil engineeringin order to provide the increasedclearanceto live parts. Where existingloading gaugeswere more generous, this was less of an issue.

Railways using older, lower-capacitydirect-currentsystems have introduced or are introducing25 kVAC instead of3 kVDC/1.5 kVDC 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 byKálmán Kandóin Hungary, who used16 kVAC at50 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).^[1]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 theDeutsche Reichsbahnwho electrified part of theHöllentalbahnbetween Freiburg and Neustadt installing a 20kV50Hz ACsystem. This part of Germany was in the French zone of occupation after 1945. As a result of examining the German system in 1951 theSNCFelectrified the line betweenAix-les-BainsandLa Roche-sur-Foronin southern France, initially at the same 20kV but converted to 25kV in 1953. The 25kV system was then adopted as standard in France, but since substantial amounts of mileage south of Paris had already been electrified at 1.5kVDC,SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.^[2]^[3]

The main reason why electrification using utility frequency had not been widely adopted before was the lack of reliability ofMercury arc rectifiersthat could fit on the train. This in turn related to the requirement to useDC series motors,which required the current to be converted from AC to DC and for that arectifieris 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-maintenancethree-phaseAC induction motors. TheN700 Shinkansenuses a three-level converter to convert25 kVsingle-phase AC to1,520 VAC (via transformer) to3 kVDC (via phase-controlled rectifier with thyristor) to a maximum2,300 Vthree-phase AC (via avariable voltage, variable frequencyinverter usingIGBTswithpulse-width modulation) to run the motors. The system works in reverse forregenerative braking.

The choice of25 kVwas 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 that25 kVwas 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 avoidshort circuits,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, whenfour Eurostar trains broke down inside the Channel Tunnel.

Distribution

Electric power for25 kVAC electrification is usually taken directly from the three-phasetransmission system.At the transmission substation, a step-downtransformeris connected across two of the three phases of the high-voltage supply and lowers the voltage to25 kV.This is then fed, sometimes several kilometres away, to a railway feeder station located beside the tracks.Switchgearat 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,neutral sectionsare provided at feeder stations and track sectioning cabins.SVCsare used for load balancing and voltage control.^[4]

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 FrenchTGV.^[5]

Standardisation

Railway electrification using25 kV,50 HzAC has become an international standard. There are two main standards that define the voltages of the system:

EN50163:2004+A1:2007 – "Railway applications. Supply voltages of traction systems"^[6]
IEC60850 – "Railway Applications. Supply voltages of traction systems"^[7]

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	Voltage
Electrification system	Min. non-permanent	Min. permanent	Nominal	Max. permanent	Max. non-permanent
25kV50Hz	17.5kV	19kV	25kV	27.5kV	29kV

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

In countries where60 Hzis the normal grid power frequency,25 kVat60 Hzis used for the railway electrification.

InCanadaon theDeux-Montagnes lineof theMontreal Metropolitan transportation Agency.
InJapanon the Tokaido, Sanyo and KyushuShinkansenlines (using1,435 mmor4 ft8+1⁄2ingauge).
InSouth Koreaon theKorailnetwork.
InTaiwanon theTaiwan High Speed Railline (using1,435 mmor4 ft8+1⁄2ingauge) and onTaiwan Railway Administration's electrified lines (using1,067 mmor3 ft 6 ingauge).
In theUnited Stateson newer electrified portions of theNortheast Corridor(i.e. the New Haven-Boston segment) intercity passenger lines,New Jersey Transitcommuter 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 inTohoku Region,Hokuriku Region,HokkaidoandKyushu,of which Hokuriku and Kyushu are at 60Hz.

12.5 kV AC at 60 Hz

Some lines in the United States have been electrified at12.5 kV 60 Hzor converted from11 kV 25 Hzto12.5 kV 60 Hz.Use of60 Hzallows direct supply from the 60Hz utility grid yet does not require the larger wire clearance for25 kV 60 Hzor require dual-voltage capability for trains also operating on11 kV 25 Hzlines. Examples are:

Metro-North Railroad'sNew Haven Linefrom Pelham, NY to New Haven, CT (Since 1985; previously 11kV 25Hz).

12 kV at 25 Hz

New Jersey Transit's North Jersey Coast Linefrom Matawan, NJ to Long Branch, NJ (1988–2002; changed to 25kV 60Hz).
Amtrak
SEPTA– Both ex-Reading Railand ex-Pennsylvania Railsides.

6.25 kV AC

Early 50Hz AC railway electrification in the United Kingdom was planned to use sections at6.25 kV ACwhere there was limited clearance under bridges and in tunnels. Rolling stock was dual-voltage with automatic switching between25 kVand6.25 kV.The6.25 kVsections were converted to25 kV ACas 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 atCrewe.A section of25 kVoverhead 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]}

50 kV AC

Occasionally25 kVis doubled to50 kVto obtain greater power and increase the distance between substations. Such lines are usually isolated from other lines to avoid complications from interrunning. Examples are:

TheSishen–Saldanhairon ore railway (50 Hz).
TheDeseret Power Railwaywhich is an isolated coal railway (60 Hz).^[8]
The now closedBlack Mesa and Lake Powell Railroadwhich was also an isolated coal railway (60 Hz).
The now closedTumbler RidgeSubdivision ofBC Rail(60 Hz).^[9]

2 × 25 kV autotransformer system

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

The 2 × 25kVautotransformersystem is asplit-phase electric powersystem which supplies 25kV power to the trains, but transmits power at 50kV to reduce energy losses. It should not be confused with the 50kV 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 50kV, while the voltage between the overhead line (3) and the running rails (4) remains at 25kV. 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 onSan'yō Shinkansenin Japan in 1972 and inParis-Lyon High speed rail linein France in 1981,^[10]and has gone on to be used byNew Zealand Railwaysin 1988,^[11]Indian Railways,^[12]Russian Railways,Italian High Speed Railways, UKHigh Speed 1,most of theWest Coast Main LineandCrossrail,^[13]with some parts of older lines being gradually converted,^{[citation needed]}French lines (LGV lines and some other lines^[14]), most Spanish high-speed rail lines,^[15]Amtrakand some of the Finnish and Hungarian lines.

Diagram of New Haven system forerunner of 2 x 25 kV system.^[16]

The 2 x 25 system traces its roots back in the upgrading of theelectrification of the New York, New Haven and Hartford Railroadwitch was electrified in 1907 andupgraded in 1914.The line was initially electrified with 11 kV 25 Hz system and then upgraded as "2 x 11 kV" system (but it was never named in this way).

Although the railroad considered the 1907 electrification highly successful, two problems required an ultimate redesign of the transmission system. The first was electromagnetic interference in adjacent, parallel telegraph and telephone wires caused by the high currents in the traction power system.

The second was that the system's geographic growth and the evolving state of electrical technology created a need for higher transmission voltages. The railroad could have simply raised the operating voltage of the entire system, however this would have required all the catenary insulators to be upgraded to withstand a higher potential, and replacement of all the locomotive high voltage equipment. And while higher transmission voltages had become common in the seven years since the initial electrification, generators were still limited by economics to a maximum output voltage of around 11 kV.

The solution decided upon by the railroad, after several years of study, was a balancedautotransformersystem.^[16]^[17]^[18]^[19]^[20]^[21]^[22]^[23]^[24]

Boosted voltage

ForTGV world speed recordruns in France the voltage was temporarily boosted, to 29.5kV^[25]and 31kV at different times.^[26]

Multi-system locomotives and trains

Trains that can operate on more than one voltage, such as 3kV and 25kV, are established technologies. Some locomotives in Europe are capable of using four different voltage standards.^[27]

References

^Hollingsworth, J. B.; Cook, Arthur F. (1998).The great book of trains: featuring 310 locomotives shown in more than 160 full-colour illustrations and 500 photographs.London: Salamander Books. pp.254–255.ISBN 0-86101-919-9.OCLC 60209873.
^Haydock, David (1991).SNCF."Modern Railways" special. London: Ian Allan.ISBN 978-0-7110-1980-5
^Cuynet, Jean (2005).La traction électrique en France 1900–2005.Paris: La Vie du Rail.ISBN 2-915034-38-9
^SVCs for load balancing and trackside voltage control,ABB Power Technologies.[1]Archived2007-02-06 at theWayback Machine
^TGV power ArchivedMay 4, 2009, at theWayback Machine
^British Standards Institution (January 2005).BS EN 50163:2004+A1:2007 Railway Applications. Supply voltages of traction systems.doi:10.3403/30103554.
^IEC 60850– "Railway Applications. Supply voltages of traction systems"
^"Railroad Coordination Manual Of Instruction, Section 2.1.5 Deseret Power Railway"(PDF).Utah Department of Transportation. May 2015. p. 102.Retrieved8 November2016.
^"GF6C #6001 PRESERVED".West Coast Railway Association, BC. May 2004. Archived fromthe originalon February 18, 2009.Retrieved2011-01-09.
^Courtois, C. (1993)."Why the 2*25 kV alternative? (autotransformer traction supply)".IEE Colloquium on 50kV Autotransformer Traction Supply Systems - the French Experience:1/1–1/4.
^Tom McGavin (Autumn 1988). "North Island Main Trunk Electrified".New Zealand Railway Observer.45(1).New Zealand Railway and Locomotive Society:49.ISSN 0028-8624.
^"Ministry of Railways (Railway Board)".indianrailways.gov.in.Retrieved2023-07-05.
^"Balfour Beatty gets £16m Crossrail substation contract".www.theconstructionindex.co.uk.Retrieved2023-07-05.
^The remainder of the French lines use 1 × 25 kV booster-transformer system.
^Comparative Study of the Electrification Systems 1×25 kV and 2×25 kV(PDF)(Report). Madrid:Ineco.June 2011.Retrieved2017-03-30.
^^a ^b"Reduction of Inductive Interference from the Power Lines of the New Haven Railroad".Electric Railway Journal.43(18):960–966. May 2, 1914.RetrievedFebruary 15,2011.
^Arthur, W. (1914)."New Haven Improves Method of Electric Operation".Railway Age Gazette.56:988–989.RetrievedFebruary 15,2011.
^Austin, Edwin (1915).Single-phase Electrical Railways.New York: D. Van Nostrand. pp.252–269.RetrievedFebruary 16,2011.
^"Feeding Heavy Single-Phase Load from Three-Phase Units".Electrical World.66(24):1300–1302. 1915.RetrievedFebruary 14,2011..Primarily focused on turbine generator details at Sherman Creek Generating Station.
^"Minimizing Induction from Single-Phase Railway".Electrical World.63(18):984–986. May 2, 1914.RetrievedApril 7,2011.
^"New York, New Haven & Hartford Railroad Electrification,"Westinghouse Electric & Manufacturing Company Special Publication 1698,June 1914. Two drawings are availablehere.
^"Purchased Power for the New Haven".Electric Railway Journal.46(25):1200–1204. December 18, 1915.RetrievedDecember 14,2011.
^Torchio, P. (1916). "Supply of Single-Phase Loads from Central Stations".Transactions of the American Institute of Electrical Engineers.35(2):1293–1313.doi:10.1109/T-AIEE.1916.4765431.ISSN 0096-3860.S2CID 51632492.Discussion of New Haven Substation at West Farms supplied from commercial utility power stations.
^"West Farms Substation of New Haven Railroad".Electrical World.66(24):1365–1367. 1915.RetrievedFebruary 14,2011..Includes a schematic of generator, transformer, and interconnection arrangement.
^"The Test Tracks: an Overview".
^"French Train Hits 357 MPH Breaking World Speed Record".4 April 2007.
^"Traxx locomotive family meets European needs".Railway Gazette International.2008-01-07.Retrieved2019-09-27.Traxx MS (multi-system) for operation on both AC (15 and 25 kV) and DC (1·5 and 3 kV) networks