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.
Overview
This electrification system is ideal for railways that feed long distances or have heavy traffic. After some experimentation before WWII in Hungary and the Black Forest of 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 etc.
Railways using older and lower capacity direct current systems such as South Africa, Russia, Spain, Italy, Belgium 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.
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 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, the 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 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 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 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 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 25kV 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 only uses one phase of the three phase national power supply, and may have to be broken into smaller sections to use the three phases more evenly.
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 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.
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