25 kV AC railway electrification

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The 25 kV AC, 50 Hz railway electrification system is commonly used in railways worldwide, especially on high-speed lines.

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 rectifiers and related technology.

Railways using older, lower-capacity direct current systems such as South Africa, Russia, Spain,^[1] Italy, Belgium, Slovakia^[2] and The Netherlands have introduced or are introducing 25 kV AC instead of 3 kV DC/1.5 kV DC for their new high-speed lines.

The Channel Tunnel uses 25 kV 50 Hz.^[3]

History

The first successful use of the 50 Hz system dates back to 1929. It was developed by Kálmán Kandó in Hungary. He 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 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. 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.^[4]

The main reason why electrification at this voltage had not been used before was the 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 motors. The N700 Shinkansen uses a three-level converter to convert 25 kV single-phase AC to 1520 V AC (via transformer) to 3000 V DC (via phase-controlled rectifier with thyristor) to a maximum 2300 V three-phase AC (via a Variable Voltage, Variable Frequency inverter using an Insulated Gate Bipolar Transistor with Pulse Width Modulation) to run the motors. The system works in reverse for regenerative braking.

The choice of 25 kV 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 25 kV 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 25 kV AC system uses only one phase of the normal three-phase power supply. This results in an imbalance on the three-phase supply which may affect other customers. This can be overcome by installing static VAr compensators^[5] or reducing the traction load when the imbalance becomes unacceptable. The system practically is not insulated from the distribution network, like other systems. Older locomotives and the recuperating electrodynamic brakes on the newer locomotives creates noise, and it is almost unable to filter from it the electricity distribution network. Therefore some countries have decided to prohibit the use of recuperating brakes.

Tunnels and overbridges have to be slightly higher to provide adequate electrical clearances^{[citation needed]}.

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 400 kV, 275 kV or 132 kV. 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 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 ^[6].

Nevertheless in some cases dedicated single phase AC powerlines were built, which run to substations with single phase AC transformers. Such lines were built to supply the French TGV.^[7]

Standardisation

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:

• BS EN 50163:2004 - "Railway applications. Supply voltages of traction systems"^[8]
• IEC 60850 - "Railway Applications. Supply voltages of traction systems"^[9]

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").

50 kV AC

Occasionally 25 kV is doubled to obtain greater power, and to increase the distance between substations. Such lines are usually isolated from other lines to avoid complications from interrunning. Three examples are the Black Mesa and Lake Powell Railroad which is an isolated coal railway, the Tumbler Ridge Subdivision of BC Rail^[10] (both 60 Hz) and the Sishen-Saldanha iron ore railway (50 Hz).

60 Hz

In countries where 60 Hz is the normal grid power frequency, 60 Hz is used for the 25 kV railway electrification. In the United States, newer portions of the Northeast Corridor intercity passenger line and New Jersey Transit commuter lines are built to the 25 kV, 60 Hz standard. In western Japan, Shinkansen lines use 60 Hz, contrasting eastern parts which use 50 Hz. 60 Hz is also used in Canada on the Two Mountains line of the Montreal Metropolitan transportation Agency and in South Korea on Korail.

Dual current locomotives and trains

Trains that can operate on more than one current, say 3 kV/25 kV, are established technologies.

See also

• List of current systems for electric rail traction
• Category:25 kV AC locomotives
• 15 kV AC railway electrification

References

Further reading

• 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. 
• Boocock, Colin (1991). East Coast Electrification. Ian Allan. ISBN 0-7110-1979-7. 
• Semmens, Peter (1991). Electrifying the East Coast Route. Patrick Stephens Ltd.. ISBN 0-85059-929-6. 
• Glover, John (2003). Eastern Electric. Ian Allan. ISBN 0-7110-2934-2.