25 kV AC railway electrification

25 kV alternating current electrification is commonly used in railway electrification systems worldwide, especially for high-speed rail.


A CSR EMU on the Roca Line in Buenos Aires, using 25kV AC.

This electrification is ideal for railways that cover long distances or carry heavy traffic. After some experimentation before World War II in Hungary and in the Black Forest in Germany, it came into widespread use in the 1950s.

One of the reasons why it was not introduced earlier was the lack of suitable small and lightweight control and rectification equipment before the development of solid-state rectifiers and related technology. Another reason was the increased clearance distances required where it ran under bridges and in tunnels, which would have required major civil engineering in order to provide the increased clearance to live parts.

Railways using older, lower-capacity direct current systems have introduced or are introducing 25 kV AC instead of 3 kV DC/1.5 kV DC for their new high-speed lines.


The first successful operational and regular use of the 50 Hz system dates back to 1931, tests having run since 1922. It was developed by Kálmán Kandó in Hungary, who used 16 kV AC at 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). 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 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, SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.[1][2]

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 25 kV single-phase AC to 1,520 V AC (via transformer) to 3,000 V DC (via phase-controlled rectifier with thyristor) to a maximum 2,300 V three-phase AC (via a variable voltage, variable frequency inverter using IGBTs with pulse-width modulation) to run the motors. The system works in reverse for regenerative braking.

The choice of 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 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 greater clearance and larger insulators.


The Indian Traction Power uses a 25 kV autotransformer system to achieve efficiencies greater than the 25 kV system and yet keeping the cost of installation similar to the 25 kV system. Supply,[3] in common terms, the supply for the electric trains run by the Indian Rail uses only two phases of the normal three-phase electric power supply. The usage of the two phases is a special one, feeding a single phase transformer with the two phases instead of the conventional feeding of a phase and a neutral. This increases the load that can be delivered. Feeding the two phases at different feeders will be switched between R-Y, Y-B, R-B, to maintain the overall balance in the system. This results in a slight imbalance on the three-phase supply locally which may affect other customers of the Electricity Board but is better off than using a single phase. To an extent, imbalances can be overcome by installing static VAR compensators[4] 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 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, when four Eurostar trains broke down inside the Channel Tunnel.

Distribution networks

Electric power from a generating station is transmitted to grid substations using a three-phase distribution system.

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 25 kV which is supplied to a railway feeder station located beside the tracks. SVCs are used for load balancing and voltage control.[5]

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.[6]


Railway electrification using 25 kV, 50 Hz AC has become an international standard. There are two main standards that define the voltages of the system:

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.

Nominal Max.
25000 V, AC, 50 Hz 17500 V 19000 V 25000 V 27500 V 29000 V

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").


Systems based on this standard but with some variations have been used.

25 kV AC at 60 Hz

In countries where 60 Hz is the normal grid power frequency, 25 kV at 60 Hz is used for the railway electrification.

12.5 kV AC at 60 Hz

Some lines in the United States have been electrified at 12.5 kV 60 Hz or converted from 11 kV 25 Hz to 12.5 kV 60 Hz. Use of 60 Hz allows direct supply from the 60 Hz utility grid yet does not require the larger wire clearance for 25 kV 60 Hz or require dual-voltage capability for trains also operating on 11 kV 25 Hz lines. Examples are:

6.25 kV AC

Early 50 Hz AC railway electrification in the United Kingdom used sections at 6.25 kV AC where there was limited clearance under bridges and in tunnels. Rolling stock was dual-voltage with automatic switching between 25 kV and 6.25 kV. The 6.25 kV sections were converted to 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. A section of 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.

50 kV AC

Occasionally 25 kV is doubled to 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:

2 x 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
2 × 25 kV overhead line system in France between Paris and Caen

The 2 × 25 kV autotransformer system may be used on 25 kV lines to reduce energy losses. It should not be confused with the 50 kV system. In this system, the current is mainly carried between the overhead line and a feeder instead of the rail. 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, Russian Railways, French lines, 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 kV and 31 kV at different times.

25 kV on narrow gauge lines

Multi-system locomotives and trains

Main article: multi-system (rail)

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.[10]

See also


  1. Haydock, David (1991). SNCF. "Modern Railways" special. London: Ian Allan. ISBN 978-0-7110-1980-5
  2. Cuynet, Jean (2005). La traction électrique en France 1900-2005. Paris: La Vie du Rail. ISBN 2-915034-38-9
  3. http://www.indianrailways.gov.in/railwayboard/uploads/codesmanual/ACTraction-II-P-I/ACTractionIIPartICh1_data.htm
  4. Grunbaum, R. FACTS for dynamic load balancing and voltage support in rail traction, 2007 European Conference on Power Electronics and Applications.
  5. SVCs for load balancing and trackside voltage control, ABB Power Technologies.
  6. TGV power Archived May 4, 2009, at the Wayback Machine.
  7. BS EN 50163:2004 - "Railway applications. Supply voltages of traction systems" (British Standards Institution, 1996). OCLC 228101582
  8. IEC 60850 - "Railway Applications. Supply voltages of traction systems"
  9. "GF6C #6001 PRESERVED". West Coast Railway Association, BC. May 2004. Archived from the original on February 18, 2009. Retrieved 2011-01-09.
  10. "Traxx locomotive family meets European needs". Railway Gazette International. 2008-01-07. Retrieved 2011-01-01. Traxx MS (multi-system) for operation on both AC (15 and 25 kV) and DC (1·5 and 3 kV) networks

Further reading

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