Traction power network

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A traction power network is an electricity grid for the supply of electrified railroads. The installation of separate traction power 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 lines.

[edit] 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. The Mariazeller railway 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.

In Sweden, Norway and some areas of the former GDR three phase AC-current is converted into single phase AC current with a frequency of 16,7 cycles per second at the substations. In these regions there are no traction current lines. Also 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 requried.

In the Republic of South Africa there are extensive AC and DC traction schemes, including 50 kV and 25kV 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.

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 4 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 2 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 380kV 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. For the supply of railways with high traffic and in particular for the supply of high speed railway lines 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.

[edit] Areas with traction power networks

• Germany (except Mecklenburg-Western Pomerania and Saxony-Anhalt), total length 7959km
• Switzerland
• Austria (separate traction power network for the Mariazeller Bahn)
• USA (in New York and Washington DC area for railway lines running with single phase 25 Hz AC)
• South Africa