15 kV AC railway electrification

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A pylon of a single phase AC 110 kV-powerline near Bartholomä in Germany. Lines of these type are used in Germany to supply electric railways with single phase AC at 16.7 hertz. In the substations of the railway, transformers are used to step it down to 15 kV.

The 15 kV AC, 16+²⁄₃ Hz railway electrification system is used in Germany, Austria, Switzerland, Sweden, and Norway. The high voltage enables high power transmission with low losses, while the lower frequency reflects technical limitations at the turn of the 20th century. In particular, the lower frequency reduces flashover problems in the motors, although at the cost of a non-standard line frequency requiring frequency conversion and separate supplies. 16+²⁄₃ Hz is equivalent 1000 rpm. In 1995 the standard^{[clarification needed]} was altered to 16.7 Hz, although 16+²⁄₃ Hz is within the tolerance and is still used in Sweden and Norway.

The disadvantage of 16.7 Hz locomotives as compared to 50 Hz or 60 Hz locomotives is the heavier transformer required (low frequency transformers need to have special construction for enough efficiency).^{[clarification needed]} The Czech Railways encountered this problem when they rebuilt some 25 kV AC, 50 Hz locomotives (series 340), in order to operate on 15 kV AC, 16.7 Hz lines. As a result of using transformers designed for 50 Hz, their tractive effort diminishes to one third of the original rating (around 1000 kW).^{[citation needed]}

Distribution networks

In Germany (except Mecklenburg-Western Pomerania and Saxony-Anhalt), Austria and Switzerland, there are special power grids for single phase AC current at 16.7 Hz; the voltage of these grids is 110 kV in Germany and Austria, 132 kV in Switzerland. This system is called centralized railway energy supply.

In Sweden, Norway, Mecklenburg-Western Pomerania and Saxony-Anhalt, there are no special single phase power grids. The energy is taken directly from the three phase grid (110 kV at 50 Hz), converted to low frequency single phase, and fed into the overhead line. This system is called decentralized railway energy supply.

Generation and conversion

There are two possibilities to supply the centralized system with electricity: either the energy is provided by a special power plant that generates 110 kV (or 132 kV in the Swiss system) alternating current at 16.7 Hz, or the energy is taken from the national power grid (e.g. 110 kV, 50 Hz) and converted into 110 kV (or 132 kV) AC at 16.7 Hz by rotary machines or AC/AC converters. The middle of these power lines is grounded by a coil, so each conductor of the single phase AC power line has a voltage of 55 kV (or 66 kV) with respect to ground potential. This is similar to the split-phase electric power system. The coil over which the grounding is done is designed to compensate ground error currents in case of a defect of the line. At the transformer stations the voltage is transformed from 110 kV (or 132 kV) AC to 15 kV AC, and the energy is fed into the overhead line.

Asynchronous converters

The frequency of 16.7 Hz depends on the necessity to avoid synchronism in parts of the rotary machine, which consists principally of a three phase asynchronous motor and a one phase synchronous generator. Since synchronism sets in at a frequency of 16+²⁄₃ Hz (according to the technical details) in the one phase system, the frequency of the centralized system was set to 16.7 Hz.

Power plants providing 110 kV, 16.7 Hz, are either exclusively producing single phase AC or have special generators, such as the nuclear power plant in Neckarwestheim or the Walchensee power station.

Synchronous converters

The power for the decentralized system is taken directly from the national power grid and directly transformed and converted into 15 kV, 16+²⁄₃ Hz by synchronous-synchronous-converters or static converters. Both systems need additional transformers. The converters consist of a three phase synchronous motor and a one phase synchronous generator. The decentralized system in the north-east of Germany was established by the Deutsche Reichsbahn in the 1980s, because there was no centralized system available in these areas.

The future

25 kV AC systems are now the norm for new railway electrifications, but extensions of existing the 15 kV networks are likely; none of the five countries that use that system have made any indications that they are going to convert to a 25 kV AC system.

References

See also

• List of current systems for electric rail traction
• Category:15 kV AC locomotives
• 25 Hz Power Transmission System
• 25 kV AC railway electrification
• Traction power network

External links

• DB Energie soll Durchleitung von Bahnstrom ermöglichen (German)