15 kV AC

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A twisting pylon of a single phase AC 110kV-powerline near Bartholomä in Germany. Lines of these type are used in Germany for the supplying electric railways with single phase AC with 16.7 Hertz. In the substations of the railway, transformers are used for stepping it down to 15 kV

15 kV AC at 16.7 Hz is a railway electrification system used by Germany, Austria, Switzerland, Sweden, and Norway since 1912. The high voltage enables high power 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.

In Sweden it is officially 15 kV AC, 16 2/3 Hz.

This voltage has been superseded by 25 kV AC at 50 Hz or 60 Hz since the 1950s.

Oddly, when Denmark decided to electrify in the 1970s they chose not to use 15 kV AC, which would have linked compatible systems in adjacent Germany and Sweden, but rather 25 kV AC.

The lower frequency of 16.7 Hertz requires either the installation of a special power grid for single phase AC or the usage of substations with rotary and/or static inverters. In Germany (except Mecklenburg-Vorpommern and Saxony-Anhalt), Austria and Switzerland, there are special power grids for single phase AC current with 16.7 Hertz. The voltage of this grid is 110 kV in Germany and Austria and 66 kV, 132 kV in Switzerland.

The middle of these powerlines is grounded by a coil, so each conductor of a 110kV powerline for single phase AC has a voltage of 55 kV against ground. The coil over which the grounding is done is designed so that it compensates ground error currents in case of a defect of the line.

For this grid in Germany, Austria and Switzerland, there are special generators in some power stations, such as the Walchensee Powerstation or the nuclear power plant in Neckarwestheim. There are also power stations exclusively producing single phase AC with a frequency of 16.7 Hertz. Also in use are central plants with rotary machines (and also with static inverters) for the transforming of three phase AC with 50 Hertz of the national power grid into 16.7 Hertz for the railway. Such plants are located in some power stations and on large railway nodes.

Substations in areas where the railway is fed by a single phase AC powerline consist of a switchyard and a transformer for transforming the voltage of the grid (66 kV, 110 kV or 132 kV) into those of the overhead line (15kV).

In Mecklenburg-Western Pomerania, Saxony-Anhalt, Sweden and Norway, there are inverters at the substations. Most use rotary machines, consisting of a three phase AC-machine and a single phase AC-machine for transforming three phase 50 Hz AC of power grid into 16.7 Hz single phase AC for the rail.