Overhead powerline
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An overhead powerline is an electric power transmission line suspended by towers or poles. Since most of the insulation is provided by air, overhead powerlines are generally the lowest-cost method of transmission for large quantities of electric power. Towers for support of the lines are made of wood (as-grown or laminated), steel (either lattice structures or tubular poles), concrete, aluminum, and occasionally reinforced plastics. The wire conductors on the line are generally made of aluminum (either plain or reinforced with steel or sometimes composite materials), though some copper wires are used in medium-voltage distribution and low-voltage connections to customer premises.
The invention of the strain insulator was a critical factor in allowing higher voltages to be used. At the end of the 19th century, the limited electrical strength of telegraph-style pin insulators limited the voltage to no more than 40,000 volts. Today overhead lines are routinely operated at RMS voltages exceeding 765,000 volts between conductors, with even higher voltages possible in some cases.
Overhead power transmission lines are classified in the electrical industry by the range of voltages:
- Low voltage - less than 1000 volts, used for connection between a residential or small commercial customer and the utility.
- Medium Voltage - between 1000 volts (1 kV) and to about 33 kV, used for distribution in urban and rural areas
- High Voltage - between 33 kV and about 230 kV, used for transmission of bulk quantities of electric power and connection to very large consumers.
- Extra High Voltage - over 230 kV, up to about 800 kV - long distance, very high power transmission.
- Ultra High Voltage - 800 kV and up
Note that even though a line is classified "low voltage" that contact with energized conductors still presents a risk of electrocution. A main goal of overhead powerline design is to maintain adequate clearance between energized conductors and the ground so as to prevent dangerous contact with the line.
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[edit] Structures
Structures for overhead lines take a variety of shapes depending on the type of line. Structures may be as simple as wood poles directly set in the earth, carrying one or more cross-arm beams to support conductors or "armless" construction with conductors supported on insulators attached to the side of the pole. Tubular steel poles are used in urban areas. High-voltage lines are often carried on lattice-type steel towers or pylons. For remote areas, aluminium towers may be placed by helicopters. Concrete poles have also been used. Poles made of reinforced plastics are also available, but their high cost restricts application.
Each structure must be designed for the loads imposed on it by the conductors. A large transmission line project may have several types of towers, with "tangent" ("suspension" or "line" towers, UK) towers intended for most positions and more heavily constructed towers used for turning the line through an angle, dead-ending a line, or for important river or road crossings. Depending on the design criteria for a particular line, semi-flexible type structures may rely on the weight of the conductors to be balanced on both sides of each tower. More rigid structures may be intended to remain standing even if one or more conductors is broken. Such structures may be installed at intervals in powerlines to limit the scale of cascading tower failures.
Foundations for tower structures may be large and costly, particularly if the ground conditions are poor, such as in wetlands. Each structure may be considerably strengthend by the use of guy wires to resist some of the forces due to the conductors.
In some countries, such as Germany and Austria, it is possible that bridges run over overhead powerlines. Such a crossing exists at Koersch Valley Bridge at Esslingen, Germany. But it is also possible to mount structures for carrying overhead powerlines on bridges. One example is the Danish Storstrøm Bridge.
[edit] Insulators
Insulators must support the conductors and withstand both the normal operating voltage and surges due to swtiching and lightning. Insulators are broadly classified as either pin-type, which support the conductor above the structure, or suspension type, where the conductor hangs below the structure. Up to about 50 kV both types are commonly used. At higher voltages only suspension-type insulators are common for overhead conductors. Insulators are usually made of wet-process porcelain or toughened glass , with increasing use of glass-reinforced polymer insulators.
Suspension insulators are made of multiple units, with the number of unit insulator disks increasing at higher voltages. The number of disks is chosen based on line voltage, lightning withstand requirement, altitude, and environmental factors such as fog, pollution, or salt spray. Longer insulators, with longer creepage distance for leakage current, are required in these cases. Strain insulators must be strong enough mechanically to support the full weight of the span of conductor, as well as loads due to ice accumulation, and wind.
Porcelain insulators may have a semi-conductive glaze finish, so that a small current (a few milliamperes) passes through the insulator. This warms the surface slightly and reduces the effect of fog and dirt accumulation. The semiconducting glaze also insures a more even distribution of voltage along the length of the chain of insulator units.
Insulators for very high voltages, exceeding 200 kV, may have grading rings installed at their terminals. This improves the electric field distribution around the insulator and makes it more resistant to flashover during voltage surges.
[edit] Conductors
Conductor cables of overhead lines are usually stranded aluminum, or aluminum reinforced with steel strands. Some copper is still used especially at lower voltages. Aluminum cables, reinforced with steel (known as ACSR) are primarily used for medium and high voltage transmission lines.
Large conductors are heavy and costly, but have low resistance and so lose little of the powerline's energy. Smaller conductors are less costly, but have a higher resistance and so turn more of the impressed energy into heat. An optimization rule called Kelvin's Law states that the optimum size of conductor for a line is found when the cost of the energy wasted in the conductor is equal to the annual interest paid on that portion of the line construction cost due to the size of the conductors. The optimization problem is made more complex due to additional factors such as varying annual load, varying cost of installation, and by the fact that only definite discrete sizes of cable are commonly made.
Since a conductor is a flexible object with uniform weight per unit length, the geometric shape of a conductor strung on towers approximates that of a catenary. The sag of the conductor (vertical distance between the highest and lowest point of the curve) varies depending on the temperature. A minimum overhead clearance must be maintained for safety. Since the temperature of the conductor increases with increasing heat produced by the current through it, it is sometimes possible to increase the power handling capacity by changing the conductors for a type with a lower coefficient of thermal expansion or a higher allowable operating temperature.
Bundle conductors are used for voltages over 200 kV to avoid corona losses. Bundle conductors consist of several conductor cables connected by non-conducting spacers. For 220 kV lines, two-conductor bundles are usually used, for 380 kV lines usually three or even four. In initial testing in Sweden it was found that at higher voltages that the wire conductors would come together due the magnetic fields produced from the single conductors so multiple conductors were used in each phase to reduce this problem.
Overhead power lines are often equipped with a ground conductor, or a shield wire. A ground conductor is a conductor fastened to the top of the towers, which protects the line against lightning strikes. Very high-voltage transmission lines may have two ground conductors. These are either at the outermost ends of the highest cross beam, at two V-shaped mast points, or at a separate cross arm. Older lines may use surge arrestors every few spans in place of a shield wire, this configuration is typically found in the rural areas of the United States.
Medium-voltage distribution lines may have the grounded conductor strung below the phase conductors to provide some measure of protection against tall vehicles or equipment touching the energized line.
While overhead lines are usually bare conductors, rarely overhead insulated cables are used, usually for short distances (less than a kilometer). Insulated cables can be directly fastened to structures without insulating supports. An overhead line with bare conductors insulated by air is typically less costly than a cable with insulated conductors.
[edit] Train power
See main article: Overhead lines
Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains at a distance from the energy supply point. Overhead line is designed on the principle of one or more overhead wires situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regular intervals. The feeder stations are usually fed from a high voltage electrical grid. This electrical grid can be the same as the public grid, but in some cases especially when AC with a frequency, which is not the value of the frequency of the power grid is used, special power grids for traction current are used.
[edit] Further applications
Overhead lines are also occasionally used to supply transmitting antennas, especially for efficient transmission of long, medium and short waves. For this purpose a staggered array line is often used. Along a staggered array line the conductor cables for the supply of the earth net of the transmitting antenna are attached on the exterior of a ring, while the conductor inside the ring, is fastened to insulators leading to the high voltage standing feeder of the antenna.
[edit] Usage of area under overhead powerlines
The area under an overhead line can be used for most purposes, at which the danger does not exist that objects connected with the ground come into the proximity of the conductor cables or at which can be the danger that the conductor cables, insulators or mast constructions are damaged. However under some overhead lines radio reception can be disturbed - in particular at the receipt of signals with frequencies under 10 MHz when using wire antennas. If it is not possible to move the receiving antenna then one should use magnetic antennas (framing or magnetic rod antennas) in this case. During the structural use of the area under overhead lines it is to be remembered that in the winter icing of masts and conductor cables can occur and that falling down pieces of ice can hit parts of buildings and damage them.
[edit] Safety
In the area surrounding overhead lines (and also radio towers, in particular mast antennas) it is dangerous to risk interference; e.g. flying kites or chain balloons, using ladders or operating machinery.
[edit] History
The first overhead line was built on July 14, 1729 by the physicist Stephen Gray, in order to show that one can transfer electricity by that method. It used damp hemp cords, which were fastened to bean bars as a leader. However the first practical use of overhead lines was in the context of the telegraphy. Electric power transmission was accomplished in 1882 with the first high voltage transmission between Munich and Bad Brook. 1891 saw the construction of the first three-phase alternating current overhead line on the occasion of the International Electricity Exhibition in Frankfurt, between Lauffen and Frankfurt.
In 1912 the first 110 kV-overhead powerline entered service followed by the first 220 kV-overhead powerline in 1923. In the 1920s RWE AG built the first overhead line for this voltage and in 1926 built a Rhine crossing with the Pylons of Voerde, two masts 138 meters high.
In Germany in 1957 the first 380 kV-overhead powerline was commissioned (between the transformer station and Rommerskirchen). In the same year the overhead line traversing of the Strait of Messina went into service in Italy, whose pylons served the Elbe crossing 1. This was used as the model for the building of the Elbe crossing 2 in the second half of the 1970s which saw the construction of the highest overhead line pylons of the world. Starting from 1967 in Russia, and also in the USA and Canada, overhead lines for voltage of 765 kV were built. In 1982 overhead powerlines were built in Russia between Elektrostal and the power station at Ekibastusz, this was a three-phase alternating current line at 1200 kV (Powerline Ekibastuz-Kokshetau). In 2003 the building of the highest overhead line commenced in China, the Yangtze River Crossing.
[edit] Similar constructions
- Antenna (Some antennas for lower frequencies are similar to overhead powerlines)
- Overhead line
- Third rail
- radio masts and towers
- electric fence
[edit] References
- Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07-020974-X
- William D. Stevenson, Jr. Elements of Power System Analysis Third Edition,McGraw-Hill, New York (1975) ISBN 0-07-061285-4
