Traction power network

From Freepedia

A traction power network is a electricity grid for the supply of electric trains. The installation of separate traction power network is generally only meaningful if the railway in question uses alternating current with a frequency lower than that of the national grid, like in Germany, Austria and Switzerland.

Alternatively, the three-phase alternating current of the power grid can converted in the substations of the railway 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.

Applications

Dedicated traction current lines are used particularly when railways are supplied with low frequency alternating current. The traction current line supplies 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 phaes AC at 25 Hz. 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 lines.

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 in 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 that in these countries no traction current lines exist.

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.

Construction features

Traction current lines are laid usually not parallel to the railway line, in order to allow a short line length and to avoid unnecessary influences of electrical system near the railway line, also this is technically possible and is applied for the current supply of some rapid-transit railways operating with alternating current in Germany. In this case it is also possible that 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 the cross beams should in this case not too wide, so the arrangement of 4 conductor cables in one level, which is standard at traction current lines, cannot be used. In this cases either for traction current lines with two electric circuits at double-railed railway lines the overhead line pylons for both driving directions are equipped with cross beams for traction current system (2 conductor cables) or two-level arrangement of conductor cables are used, whereby each electric circuit uses a mast half.

In densely populated areas there are also pylons, which carry circuits for both traction current and for three-phase alternating current. Such lines are found where rights of way are rare. In particular the parallel course to 110 kV and sometimes to 220-kV-three phase AC line is common. A course parallel to 380-kV-power lines on the same pylon requires the usage of insulators for 220 kV for the traction current line, because in cases of failures of the 380kV line along the traction current line voltage peaks can occur, which isolation for 110 kV cannot stand.

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 for the ICE train also conductors of two bundles are used. The traction current lines from the nuclear power station Neckarwestheim to the traction current switching staion Neckarwestheim and from the traction current switching station Neckarwestheim to the central substation in Stuttgart Zazenhausen are implemented as four-bundle conductors.

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 an Washington DC area for railway lines running with single phase 25 Hz AC)
• South Africa