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Twisting pylon of powerline for single phase AC traction current (110 kV, 16⅔ Hz) near Bartholomä in Germany.

A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of separate traction network generally is only done if the railway in question uses alternating current with a frequency lower than that of the national grid, such as in Germany, Austria and Switzerland.

Alternatively, the three-phase alternating current of the power grid can be converted in substations by rotary transformers or static inverters into the voltage and type of current required by the trains. For railways which run on direct current, this method is always used, as well as for railways which run on single phase AC current of decreased frequency, as in Mecklenburg-Western Pomerania, Saxony-Anhalt, Norway and Sweden. In these areas there are no traction current networks.

Contents

History

Separate power for traction apart from industrial power always has historic roots. There is no reason today to apply different frequencies or current types than for transmission and for industrial usage. However, the advantage with DC current traction was the easier transmission with single copper wires to the feeder points. The advantage with AC current traction is the easier transmission over long distances to the feeder points. Beyond these parameters and securing former investment no evidence exists to stay with different current schemes in networks.

Applications

Dedicated traction current lines are used when railways are supplied with low frequency alternating current. The traction current supply line is connected to substations along the line of the railway and is usually run separately from the overhead catenary wire from which the locomotives are fed.

In countries in which the electric trains run with direct current or with single phase AC current with the frequency of the general power grid, the required conversion of the current is performed in the substations, so again no traction current lines are required.

Traction current supply lines are not usually laid parallel to the railway line, in order to allow a shorter line length and to avoid unnecessary influences to the electrical system near the railway line; this also is applied to the current supply of some rapid-transit railways operating with alternating current in Germany.

It is also possible to lay out the traction current supply on special cross beams right on the overhead wire pylons above the catenary wire. Because the overhead line pylons have a smaller cross section than traction current supply masts the cross beams cannot be too wide, so the standard arrangement of four conductor cables in one level cannot be used. In this case a two-level arrangement is used, or with two electric circuits for double-railed lines the overhead line pylons for both directions are equipped with cross beams for their own traction current system of two conductor cables each.

In densely populated areas there are pylons, which carry circuits for both traction current and for three-phase alternating current for general power. Such lines are found where right of ways are rare. In particular the parallel route of 110 kV and 220 kV three-phase AC is common. The use of 380 kV-power lines on the same pylon requires 220 kV insulators for the traction current line, because in case the 380 kV line fails, voltage spikes can occur along the traction current line, which the 110 kV insulators cannot handle.

As a rule traction current lines use single conductors, however for the supply of railways with high traffic and in particular for the supply of high speed railway lines, two bundle conductors are used.

Around the World

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Austria

The Mariazeller railway in Lower Austria operates on single phase AC at a 25 Hz utility frequency. The railway has its own traction current lines with an operating voltage of 27 kV. These lines are mounted on the pylons of the overhead wire over the catenary wire.

Germany

In Germany single conductors are usually used for traction current lines, but for the ICE train, two bundle conductors are used. The traction current supply lines from the nuclear power station Neckarwestheim to the traction current switching station at Neckarwestheim and from there to the central substation in Stuttgart, Zazenhausen are implemented as a four-bundle conductor circuit.

Scandinavia

In Sweden, Norway and some areas of the former German Democratic Republic three phase AC-current is converted into single phase AC current with a frequency of 16.7 cycles per second at the substations. Unlike in Germany, there are no dedicated power plants for railway electricity. All power comes from general electricity suppliers. In these regions there are no traction current lines, except extra lines along the railways to increase supply for heavily used railways. In Denmark and Finland 50 Hz is used for the main lines (if electrified) and the electricity comes from general suppliers, and much simpler equipment than in Sweden and Norway is needed for conversion.

South Africa

In the Republic of South Africa there are extensive AC and DC traction schemes, including 50 kV and 25 kV AC single phase systems. Electrification in Natal was stimulated by the takeover of the South African Railways' system by the Electricity Supply Commission (now Eskom) based on the Colenso power station.

United Kingdom

In the United Kingdom the British Rail Southern Region 750 V DC electrification system is supplied with power from an extensive 33 kV power distribution network.[citation needed]

Areas with traction power networks

See also


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