Traction current line

A traction current line is a high voltage transmission line for the supply of railways with traction current for powering locomotives. A network of traction current lines is needed in countries where trains use single phase alternating current of a lower frequency than the public supply, so the electricity cannot easily be taken from the public electricity grid. In these cases, there are separate traction power networks with their own traction current lines. This current is then transformed in electrical substations to supply individual sections of the overhead lines from which the locomotives take their power.

Where traction current lines are used

Traction current lines are used to power the railway systems of countries which use alternating current of a lower frequency than the public supply. This is typically the case in the German-speaking countries of Europe. For example 16.7Hz AC is used in Germany, Austria and Switzerland.

A specific example is the Mariazeller narrow gauge railway in Austria, operating with single phase AC with a frequency of 25 Hz, which 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 lines.

Electrical voltages employed

The voltages used for traction current lines are 110kV in Germany and Austria and 66kV or 132kV in Switzerland.

Traction current lines are operated symmetrically against earth. In the case of 110 kV lines, for example, each conductor has a voltage of 55 kV against earth. The grounding is made in larger substations and in power stations for traction current, using transformers for the cancellation of the earth leakage current. As is the case for all symmetrical powerlines there are also at traction power lines twisting points. A traction powerline for one circuit has usually two conductors. Since most traction current lines possess two electric circuits, four conductors are on the pylons as a rule (in contrast with three-phase alternating current lines, whose number of conductors are an integral multiple of three).

Routing of traction current lines

Traction current lines are not usually laid parallel to the railway line, so as to minimise the line length and to avoid unnecessary influences of electrical system near the railway line. However, there are cases where this practice is not followed (for example, the current supply of some rapid-transit railways operating with alternating current in Germany). In this case, the traction current line is laid on special cross beams of the overhead wire pylons above the overhead line. Because overhead line pylons possess a smaller cross section than traction current masts, these cross beams have to be quite narrow, so the arrangement of four conductor cables in one level, which is standard at traction current lines, cannot be used. Where four conductors are needed, one approach is to employ a two-level arrangement of conductor cables. Alternatively, in cases of double-railed railway lines, the overhead line pylons for both driving directions are equipped with cross beams for the traction current system (two conductor cables).

In densely populated areas, where rights of way are rare, it is common to find pylons which carry electric circuits for traction current as well as those for three-phase alternating current. The latter can be 110kV, 220kV, or, in some cases, 380kV three phase AC lines. In such cases, the traction current lines must use insulators which can cope with the maximum peak-to-peak voltage which can occur between the lines.

Traction current lines are implemented as a rule as single leaders. For the supply of railways with much rail traffic and in particular for the power supply of high speed railway lines such as the German ICE (Inter City Express) trains, conductors of two bundles are used. The traction current lines from the nuclear power station at Neckarwestheim to the traction current switching station at Neckarwestheim, and from the traction current switching station at Neckarwestheim to the central substation in Stuttgart Zazenhausen are implemented as four-bundle conductors.

Traction current lines are always equipped with an earth conductor. In some cases, two earth conductors are used: for example in, Germany, in cases where the traction current line is carried on pylons together with three phase AC current, like the line to the nuclear power station at Neckarwestheim. Similarly, in Austria there are some traction current lines equipped with two earth ropes.

Alternatives to traction current lines

In Sweden, Norway and some areas of the former GDR three phase AC-current from the public grid is converted into single phase AC current with a frequency of 16.7 cycles per second in substations close to the railways. 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 that in these countries no traction current lines are needed.