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The '15 kV AC' railway energy supply system at 16.7 Hz respectively 16⅔ Hz is a railway electrification system used by Germany, Austria, Switzerland, Sweden, and Norway since 1912. The high voltage enables high power transmission at 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.
The frequency depends on the system of energy transmission.
In Germany (except Mecklenburg-Western Pomerania and Saxony-Anhalt), Austria and Switzerland, there are special power grids for single phase AC current at a frequency of 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, but the energy is taken directly from the three phase grid with 110 kV at 50 Hz, inverted and fed into the overhead line. This system is called Decentralized Railway Energy Supply.
There are two possibilities to supply the centralized system with energy: The energy is either provided by a special power plant, that generates 110(132) kV, 16.7 Hz, or the energy is taken, similar to the decentralized system, from the national power grid (110 kV, 50 Hz), but inverted into 110(132) kV, 16.7 Hz by rotary machines or static inverters. This Energy is distributed throughout the country. The middle of these power lines is grounded by a coil, so each conductor of an 110(132)kV power line for single phase AC has a voltage of 55 (66) kV against ground. 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(132) kV to 15 kV and the energy is fed into the overhead line. 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 16⅔ 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 Powerstation.
As written above, the energy for the decentralized system is taken directly from the national power grid and also directly transformed and inverted into 15 kV, 16⅔ Hz by synchronous-synchronous-converters or static inverters. 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.
If there is an existing system, it is recommended to use 15 kV, 16.7 (16⅔) Hz. But for new installations it is suggested to use the 25 kV, 50 (60) Hz system. Denmark (since the 1970s) and France have such systems. The technique for this electrification system was first tested in Germany on the Höllentalbahn during the 1930s and 1940s.

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See also

See also

★ Lecture "Energy Supply of Electric Railroads", Dresden University of Technology

★ List of current systems for electric rail traction

★ http://www.udo-leuschner.de/energie-chronik/030808.htm