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Electricity transmission lines
Structures consisting of cables and supports that are used to transmit and distribute electricity. Transmission lines are a fundamental link in the power system; along with substations, they form electrical networks.
One of the first experimental transmission lines was built in 1882 by the French engineer Marcel Deprez, and carried DC at 1.5-2 kW over 57 km from Miesbach to Munich. In 1991 the first three-phase AC transmission took place, over a distance of more than 170 km between Laufen and Frankfurt in a system designed and built by Russian-born engineer Mikhail Dolivo-Dobrovolsky. The line carried 15kV of electricity at a transmission capacity of 230 kWt and about 75% efficiency.
Electricity transmission lines developed and improved rapidly as a result of the creation of developed electrical networks, which were combined into power systems. Transmission lines can be overhead (cables suspended above the ground or water) or underground/underwater; the latter type uses mainly power cables.
In the case of overhead lines, electricity is transmitted over long distances through cables attached to pylons using isolators. Overhead electricity transmission networks are one of the main links in modern power systems. Voltage depends on the length of the line and its transmission capacity. Overhead transmission lines use uninsulated cables (single-wire, stranded and hollow) made of copper, aluminium, steel-aluminium alloy, and more rarely steel (mainly for rural electrification). The most important characteristics of overhead transmission lines are: l – distance between supports; f – maximum sag; h – lowest (overall) allowable distance between the lowest point of the wire and the ground; l – length of insulator chains; a – distance between adjacent wires (phases) of the line; and H – total height of the support. The construction parameters of overhead transmission lines depend on the nominal voltage of the line, the local terrain and climactic conditions, and also on technical and economic requirements. In non-built-up areas, the lowest point of the line must be at least 5-7 m off the ground; in built-up areas, this rises to 6-8 m.
Various types of supports are used when constructing overhead power lines (see Transmission line supports) Overhead power lines should have good conductivity and mechanical strength, and be resistant to atmospheric and chemical influences. To protect overhead power cables from atmospheric overvoltages in the event of lightning striking the line or close by, ground wires or switches are installed on lines of up to 35 kV.
Underground transmission lines consist of one or more cables, plus stop joints, straight joints, terminations and fasteners; transmission lines that use oil- or gas-filled cables are fed by a supply system and also have an oil (gas) pressure alarm. Underground transmission lines are widely used when laying power grids in urban areas or industrial zones. However, they can cost two to three times more than overhead cables. Cables are set in raceways, boxes or tunnels, and laid in trenches dug at depths of 0.8-1 m. The most economical form of underground transmission line uses up to six cables laid 0.2-0.3 m apart in a single trench. More than 20 cables may be laid in one tunnel.
