25 kV AC railway electrification

Railway electrification systems using alternating current (AC) at 25 kilovolts (kV) are used worldwide, especially for high-speed rail.

One of the reasons it was not introduced earlier was the lack of suitable small and lightweight control and rectification equipment before the development of solid-state rectifiers and related technology.

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.

[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.

Until the early 1950s, mercury-arc rectifiers were difficult to operate even in ideal conditions and were therefore unsuitable for use in railway locomotives.

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.

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.

An example of atmospheric causes occurred in December 2009, when four Eurostar trains broke down inside the Channel Tunnel.

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.

A section of 25 kV overhead line was gradually brought closer to the earthed metalwork of the bridge whilst being subjected to steam from the locomotive's chimney.

Periodic autotransformers (9) divert the return current from the neutral rail, step it up, and send it along the feeder line.

Although the railroad considered the 1907 electrification highly successful, two problems required an ultimate redesign of the transmission system.

The first was electromagnetic interference in adjacent, parallel telegraph and telephone wires caused by the high currents in the traction power system.

The second was that the system's geographic growth and the evolving state of electrical technology created a need for higher transmission voltages.