25 kV AC railway electrification

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 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).[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 the Deutsche Reichsbahn who electrified part of the Höllentalbahn between Freiburg and Neustadt installing a 20 kV 50 Hz 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 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 1.5 kV DC, 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 of Mercury arc 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 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 Shinkansen uses a three-level converter to convert 25 kV single-phase AC to 1,520 V AC (via transformer) to 3 kV 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 larger insulators and greater clearance from structures.

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.

Electric power for 25 kV AC electrification is usually taken directly from the three-phase 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 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, neutral sections are provided at feeder stations and track sectioning cabins. SVCs are 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 French TGV.[5]

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:

• EN 50163:2004+A1:2007 – "Railway applications. Supply voltages of traction systems"[6]
• IEC 60850 – "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.

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.

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 × 25 kV autotransformer system is a split-phase electric power system which supplies 25 kV power to the trains, but transmits power at 50 kV 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 transmission line instead of the rail. The overhead line (3) and feeder (5) are on opposite phases so the voltage between them is 50 kV, while the voltage between the overhead line (3) and the running rails (4) remains at 25 kV. Periodic autotransformers (9) divert the return current from the neutral rail, step it up, and send it along the feeder line. This system being initially deployed, in 1981, on France's then new Paris-Lyon High speed rail line,[10] and has gone on to be used by Indian Railways,[11] Russian Railways, Italian High Speed Railways, UK High Speed 1, most of the West Coast Main Line and Crossrail,[12] with some parts of older lines being gradually converted, French lines (LGV lines and some other lines[13]), most Spanish high-speed rail lines,[14] Amtrak and some of the Finnish and Hungarian lines.

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.[17]

• 15 kV AC railway electrification
• List of railway electrification systems
• Rotary phase converter

• Keenor, Garry. Overhead line electrification for railways.
• Boocock, Colin (1991). East Coast Electrification. Ian Allan. ISBN 0-7110-1979-7.
• Gillham, J.C. (1988). The Age of the Electric Train – Electric Trains in Britain since 1883. Ian Allan. ISBN 0-7110-1392-6.
• Glover, John (2003). Eastern Electric. Ian Allan. ISBN 0-7110-2934-2.
• Machefert-Tassin, Yves; Nouvion, Fernand; Woimant, Jean (1980). Histoire de la Traction Electrique, vol.1. La Vie du Rail. ISBN 2-902808-05-4.
• Nock, O.S. (1965). Britain's new railway: Electrification of the London-Midland main lines from Euston to Birmingham, Stoke-on-Trent, Crewe, Liverpool and Manchester. London: Ian Allan. OCLC 59003738.
• Nock, O.S. (1974). Electric Euston to Glasgow. Ian Allan. ISBN 0-7110-0530-3.
• Proceedings of the British Railways Electrification Conference, London 1960 – Railway Electrification at Industrial Frequency. London: British Railways Board. 1960.
• Semmens, Peter (1991). Electrifying the East Coast Route. Patrick Stephens Ltd. ISBN 0-85059-929-6.