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25 kV AC railway electrification

Standard current and voltage settings for most high-speed rail

  • 10 min read
  • 6 chapters

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This electrification is ideal for railways that cover long distances or carry heavy traffic. After some experimentation before World War II in Hungary and in the Black Forest in Germany, it came into widespread use in the 1950s.

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. Another reason was the increased clearance required under bridges and in tunnels, which would have required major civil engineering in order to provide the increased clearance to live parts. Where existing loading gauges were more generous, this was less of an issue.

Railways using older, lower-capacity direct-current systems have introduced or are introducing 25 kV AC instead of 3 kV DC/1.5 kV DC for their new high-speed lines.

History

Many trial sites were developed at the beginning of the 20th century but developing a main frequency electrification was not easy. One of those trials, the Seebach-Wettingen railway electrification trial, not successful at 50 Hz for problems at the motors and interferences with the near telegraph lines, was the beginning of the standardization of the low frequency railway electrification.

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. 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 1936 by the Deutsche Reichsbahn who electrified part of the Höllentalbahn between Freiburg and Neustadt installing a 20 kV 50 Hz AC system. This part of Germany was in the French zone of occupation after 1945. As a result of examining the German system in 1951 the SNCF electrified the line between Aix-les-Bains and La Roche-sur-Foron in southern France, initially at the same 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 1.5 kV DC, SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.

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. 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 railway locomotives.

It was possible to use AC motors (and some railways did, with varying success), but they have had less than ideal characteristics for traction purposes. This is 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 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.

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 kV 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 related to the efficiency of power transmission as a function of voltage and cost, not based on a neat and tidy ratio of the supply voltage. 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 a higher voltage would still improve efficiency but not by a significant amount in relation to the higher costs incurred by the need for larger insulators and greater clearance from structures.

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

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