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power line[′pau̇·ər ‚līn]
equipment consisting of wires and auxiliary devices; used for transmission or distribution of electric power. It is the component of a power system and, together with electric power substations, it forms power grids.
One of the first experimental power lines (for direct current), with a voltage of 1.5–2.0 kilovolts (kV), was the 57-km Mies-bach-Munich line, built in 1882 by the French scientist M. Deprez. Transmission of three-phase alternating current was first achieved in 1891 over the 170-km Lauffen-Frankfurt lines, designed and built by M. O. Dolivo-Dobrovol’skii. The line operated at a voltage of 15 kV, with a transmitted power of 230 kilovolt-amperes (kVA) and an efficiency of about 75 percent. The first cable lines in Russia (underground lines with a range of 1 km and a voltage of 2 kV) appeared in the late 1870’s. The power supplied to the cable networks was used primarily for lighting private homes. Cable lines with voltages of 6.6, 20, and 35 kV appeared in the early 20th century, because of the electrification of industry and a general increase in power consumption. In 1922 the first 110-kV line was placed in operation (from the Kashira State Regional Electric Power Plant to Moscow). The rapid development and improvement of power lines resulted from the creation of highly developed power grids and their consolidation into power systems. A distinction is made between overhead lines, in which the wires are suspended above land or water, and underground (or underwater) lines, in which power cables are mainly used.
In overhead power lines, power is transmitted over considerable distances through wires attached to supports (poles) by insulators. Overhead lines are a primary component of present-day power systems. The line voltage depends on the length of the line and on the power being transmitted. In overhead lines noninsulated (bare) single-strand, multistrand, or hollow wires are used; they are made of copper, aluminum, or steel-aluminum, and less frequently of steel (mainly for electrification of rural areas).
The main characteristics of overhead power lines are the span l (the distance between adjacent supports), the maximum sag f of the wire in a span, the minimum permissible clearance h between the lowest point of the wire and the ground, the length λ of the insulator chain, the distance a between adjacent wires (phases) of a line, and the total height H of a support. The design parameters of an overhead power line depend on the rated voltage of the line, on the terrain and climatic conditions of the locality, and on technical and economic requirements. (See Table 1 for characteristics of Soviet overhead power lines.)
|Table 1. Main design characteristics of Soviet overhead power lines|
|Line voltage (kV)|
|up to 1||35–110||220–500||750|
|Span l (m)............||40–50||150–200||400–450||400–450|
|Height of support H (m) ....||8–9||13–14||25–30||30–35|
|Distance a (m) .........||0.5||3–4||7–12||15–17|
The permissible distance between the lowest point of the wire and the ground is 5–7 m for uninhabited areas and 6–8 m for inhabited areas.
Overhead power lines use supports of various designs. The wires of an overhead line must have good conductivity and mechanical strength and must be resistant to atmospheric and chemical effects. Lightning-protection cables or lightning arresters are used to protect power lines against atmospheric overvoltages arising during lightning strikes on the line or in the surrounding area; such protective devices are installed in lines with voltages up to 35 kV.
For overhead AC power lines the standard voltages in the USSR are 35, 110, 150, 220, 330,400, 500, and 750 kV. A voltage of 35 kV is widely used for feeder stations of distribution networks (6 and 10 kV); by 1972, the total length of 35-kV power lines was 189,000 km. The distribution networks of most power systems have an operating voltage of 110 kV; the total length of power lines at that voltage is 197,000 km. A voltage of 150 kV is used in distribution networks of the Dneproenergo Power System, in the adjoining areas of the neighboring Kiev, Kharkov, and Odessa power systems, and partially in the Kola Power System. The total length of 150-kV power lines is 6,200 km. Power lines about 100 km long are now being built for voltages of 220–330 kV; the total length of such lines is about 70,000 km. As of 1972, the voltage of 400 kV was used only in the Unified Power System of the South, for connecting with power systems of member countries of the Council for Mutual Economic Assistance (COMECON). Power lines with voltages of 500 kV are built mainly for transmitting power over long distances (more than 100 km). The total length of lines operating at 400–500 kV is about 15,000 km. As of 1972, the only line operating at 750 kV was the experimental line from the Konakovo State Regional Electric Power Plant to Moscow. The first 750-kV industrial power line is under construction for the Unified Power System of the South. The development of 750-kV transmission grids will lead to conversion of the 330-kV grid to a distribution network. An example of a very large power line is the 500-kV line from the Volga Twenty-second Congress of the CPSU Power Plant to Moscow. Its total length is 2,060 km (based on a single circuit). One of the largest power lines abroad is the 500-kV AC line between the power systems of the northwest and southwest Pacific coast of the USA, with a total (single-circuit) length of 1,070 km. A 765-kV line is operating in the USA in the American Electric Power system. In Canada a 735-kV line is operating between the Manicouagan Hydroelectric Power Plant and Quebec and Montreal.
An underground power transmission line consists of one or more cables; plugging cable boxes, joints, and terminals; and fasteners. Lines containing an oil-filled or gas-filled cable are also equipped with systems for filler makeup and for monitoring the oil or gas pressure. Underground power lines are widely used in laying out distribution networks in urban areas or industrial plants. However, their cost is higher than that of overhead networks by a factor of 2–3. Cables are laid underground, in trenches at a depth of 0.8–1.0 m, or in cable channels, blocks, or tunnels. Underground laying of up to six cables in a trench, with cables spaced 0.2–0.3 m apart, is most economical. No fewer than 20 cables may be laid in a tunnel.
In the USSR the standardized rated voltages and wire cross sections are identical for cables and overhead lines (except for those rated at 150 and 750 kV). Distribution cable lines are being provided for voltages of 1, 3, 6, 10, and 20 kV. Feeder cable lines are used at voltages of 35 kV or more. Sometimes 35-kV and 110-kV cable networks are also called distribution lines because of their highly ramified structure. Cable lines are usually used in the construction of power-supply grids for cities and large industrial establishments. Urban power-supply systems in the USSR operate mainly with the following voltage stepdown sequences: 110/35/6/0.4, 110/35/10/0.4, and 110/10/0.4 kV; less frequently, 110/6/0.4 kV.
During the 1960’s DC power transmission by overhead or cable lines has become increasingly significant. There is a DC line operating at ± 400 kV in the USSR. As of 1973, work was under way to construct AC lines for voltages of 1,150–1,200 kV and DC voltages of ±750 kV. Pilot projects are under way for the development of new types of power lines, such as cryogenic and cryosistor lines; lines operating in an elegaz (sulfur hexafluo-ride) atmosphere; semi-interrupted and interrupted lines; high-frequency lines; and lines in which the conducting material is sodium.
REFERENCESPravila ustroistva elektroustanovok, 3rd ed. Moscow-Leningrad, 1964. Elektricheskie sistemy, vols. 2–3. Edited by V. A. Venikov. Moscow, 1971.
Kriukov, K. P., and B. P. Novgorodtsev. Konstruktsii i mekhanicheskii raschet Unii elektroperedachi. Leningrad, 1970.
Elektrifikatsiia SSSR. General editor, P. S. Neporozhnii. Moscow, 1970.
Belorussov, N. I. Elektricheskie kabeli i provoda. Moscow, 1971.