AC Power Transmission is the transmission of electric power by alternating current. It is done in most cases by the use of three phase AC current; but sometimes single phase AC current is used as traction current for railway traction.

Three-phase alternating current high-voltage transmission

The three-phase alternating current high-voltage transmission is the most important way of the transmission of electricity.

In Germany the highest three-phase alternating current high-voltage transmission uses voltages of 380kV. In other countries voltages up to 1200 kV are used.

History

The first transmission of three-phase alternating current using high voltage took place in the year 1891 on the occasion of the international electricity exhibition in Frankfurt. Between Lauffen at the Neckar and Frankfurt/Main one approx. 175 kilometre long powerline for a voltage of 15 to 25 kv was built in 1891.

Initially transmission lines were supported by porcelain pin-and-sleeve insulators similar to those used for telegraph and telephone lines. However, these reached a practical limit of 40 kV. In 1907 the invention of the strain-type insulators by H. W. Buck of the Niagara Falls Power Corporation and E. M. Hewlett of General Electric allowed practical insulators of any length to be constructed, to allow use of higher voltages.

In 1912 between Lauchhammer and Riesa the first three-phase alternating current with 110kV took place. On April 17th, 1929 there was the inaugauration of the first 220kV-line in Germany running from Brauweiler near cologne, over Kelsterbach near Frankfort, Rheinau near Mannheim, Ludwigsburg/Hoheneck toward Austria. The masts of this line were already were constructed for eventual upgrade to 380kV. However the fist transmission with 380kV took place in Germany on October 5th, 1957 between the substations in Rommerskirchen and Ludwigsburg/Hoheneck. In 1967 the first three-phase alternating current high-voltage transmission with 765kV took place in Canada and in 1982 in the first transmission with 1200kV in former Soviet Union.

Single phase transmission for railway traction

See Railway electrification system. Dedicated traction current lines are used particularly when railways are supplied with low frequency alternating current. A traction current line is a high voltage transmission line for the supply of railways with power for electric locomotives. These lines are separate from the overhead catenary wire which directly supplies the electrical traction vehicles with electrical energy. The traction current line supplies substations along the line of the railway. Voltages for traction current lines are 110 kV in Germany and Austria, and in Switzerland 66 kV or 132 kV. The Mariazeller railway operates on single phaes AC at 25 cycles 25 cycles per second. 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.

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.

There are also pylons, which carry as well electric circuits for traction current and for three-phase alternating current. In particular in the proximity of densely populated areas such lines are be found, since rights of way are rare here. Here 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.

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.

See also

• HVDC