25 kV AC
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25 kV, 50 Hz AC is a type of railway electrification system. It is one of the most common voltages used for railway electrification systems in the world.
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[edit] Overview
This electrification system is ideal for railways that feed long distances or have heavy traffic.
One of the reasons why it was not introduced earlier was the increased clearance distances required where it ran under bridges and in tunnels.
Railways using older and lower capacity direct current systems such as South Africa, Spain and Italy have or are introducing 25 kV AC instead of 3 kV DC for their new high speed lines.
[edit] History
The first railway to use this system was completed in 1951 by SNCF and ran between Aix-Les-Bains and La-Roche-Sur-Furon in France.
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 in turn meant that the current needed to be converted from AC to DC, and for that a rectifier is needed. Until the early 1950's 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 do not have an ideal characteristic for traction purposes. The reason for this is that the control of speed is difficult without varying the frequency, and if you rely on voltage to control speed, then the torque you get at any given speed is not ideal. It is for this reason that DC series motors are the best choice for traction purposes, as they can be controlled by voltage, and have an almost ideal torque vs speed characteristic.
The choice of 25 kV, was not based on some neat and tidy ratio of the supply voltage! It was based on the efficiency of power transmission as a function of voltage and cost. The higher the voltage the lower the current and the better the efficiency, but the higher the cost. It was found that 25kV was an optimum voltage. Above this, the efficiency improves but not significantly, yet the costs increase very significantly (due to higher clearances and larger sized insulators).
[edit] How 25 kV is derived from the Grid Network
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 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.
[edit] 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 60850:2000"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 |
|---|---|---|---|---|---|
| 25000 V, AC, 50 Hz | 17500 V | 19000 V | 25000 V | 27500 V | 29000 V |
This system is now part of the Trans-European Railway Interoperability Standard (1996/48/EC "Interoperability of the Trans-European high-speed rail system" and 2001/16/EC "Interoperability of the Trans-European Conventional rail system").
[edit] See also
[edit] Further Reading
- Nock, O.S. (1966). Britain's New Railway. Ian Allan.
- 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.
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