25 kV AC railway electrification

The 25 kV AC 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 France, Russia, South Africa, Spain, Italy, Belgium, Slovakia 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.

History

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. 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 outside of Hungary 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, 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, the SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.

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 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 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 lead 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 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 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.

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.

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"
• IEC 60850 - "Railway Applications. Supply voltages of traction systems"

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	Lowest non-permanent voltage	Lowest permanent voltage	Nominal voltage	Highest permanent voltage	Highest non-permanent voltage
25,000 V, AC, 50 Hz	17,500 V	19,000 V	25,000 V	27,500 V	29,000 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").

Variations

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

6,250 V AC

Early 50 Hz AC railway electrification in the United Kingdom used sections at 6,250 V AC where there was limited clearance under bridges and in tunnels. Rolling stock was dual-voltage with automatic switching between 25 kV and 6,250 V. The 6,250 V sections were converted to standard 25 kV sections 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 slowly brought closer to the earthed metalwork of the bridge whilst being subjected to the steam from the locomotive's funnel. 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 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 (both 60 Hz) and the Sishen-Saldanha iron ore railway (50 Hz).

60 Hz

See also: List of current systems for electric rail traction: 25 kV AC, 60 Hz overhead

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 (using 1435 gauge) use 60 Hz, contrasting eastern parts which use 50 Hz. 60 Hz is also used in Canada on the Deux-Montagnes Line of the Montreal Metropolitan transportation Agency, in Pakistan on Pakistan Railways (using 1676 gauge), in South Korea on Korail and in Taiwan on Taiwan High Speed Rail (both using 1435 gauge).

25kV in the narrow gauge lines

In Taiwan: see Rail transport in Taiwan - 60Hz.

In Tunisia - 50Hz.

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.

Multi-system locomotives and trains

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.

See also

• List of current systems for electric rail traction
• Category: 25 kV AC locomotives
• 15 kV AC railway electrification
• Electric power supply of Polish State Railways as example of a railway grid electrified with DC of 3 kV

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.