ac power transmission

AC Power Transmission is the transmission of electric power by alternating current. It is done in most cases by the usage of three phase AC current, but sometimes for railway traction also with single phase AC current.

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. As a rule in electricity grids for the transmission and distribution of electricity three-phase alternating current is used. In Germany the highest three-phase alternating current high-voltage transmission uses voltages of 380kV, abroad partially voltages until 1200kV are used. Depending upon the physical conditions of the transmission network the neutral conductor of the three-phase alternating current high-voltage transmission is with low impedance grounded, grounded inductively or not grounded. For long transmissions with high voltage the neutral point is usually grounded.

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. 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 partially appropriate for the admission of electric circuits for 380kV. However the first three-phase designed for the usage of alternating with 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

The traction current line is a high voltage transmission line for the supply of railways with traction current. It is not to confound with the overhead wire, which supplies directly the electrical traction vehicles with electrical energy, on the contrary serves the traction current line of the supply of substations. In particular for the supply of railways run by alternating current of lower frequency ( 16.7 cycles per second in Germany, Austria and Switzerland) traction current line are used. The used voltages are in Germany and Austria 110 kV, in Switzerland 66 kV or 132 kV. Also the Mariazeller course operating with single phase AC with a frequency of 25 cycles per second has 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 operated symmetrically against earth. At 110 kV traction current lines each conductor has a voltage of 55 kV against earth. The grounding is made in larger substations and in power stations for traction current over induction coils for the deletion 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 three-phase alternating current lines, whose number of conductors are an integral multiple of three). 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 paralell 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. Traction current lines are always equipped - shifted apart from some sections lawn on pylons of overhead wire pylons at - with an earth rope. A use of two ground conductors takes place in Germany only on sections where the traction current line is laid together on pylons for three phase AC current, like the line to the nuclear power station Neckarwestheim. In Austria there are some traction current lines equipped with two earth ropes. 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.