Overhead power line
An overhead line (including landline ) is an electric line whose conductors are in contrast to cable isolated from each other only by the intervening air.
The conductors are carried on and on insulators of overhead line masts. For overhead lines at voltages below 50 kV wood, concrete, steel pipe and steel truss masts are used. For voltages above 50 kV lattice steel towers are used at least in Europe generally; for lines of 110 kV voltage level also Stahlrohrmaste are increasingly being used. In order to minimize the risk of electric shock small, overhead lines must comply with a minimum height of four meters above the ground ( for voltages up to 1000 V, for higher voltages the ground clearance of an overhead line must be greater ).
It is in principle allowed in Germany and Austria of passing overhead lines under a viaduct. One can find such a crossing under the Körschtalviadukt near Esslingen am Neckar. But bridges themselves can carry constructions from which overhead lines are fixed, such as the Storstrømsbroen in Denmark.
- 2.1 Overhead lines in alarm systems
- 3.1 catenaries
- 3.2 Reuse lines
- 3.3 Single-wire waveguide
- 3.4 Related Structures
- 8.1 Reference Books
- 8.2 Technical Papers
Overhead lines for power transmission form the overland portion of the power grid for transmission of electrical energy. Previously, they also served to supply current to the low voltage level ( from house to house or on poles, even for street lighting ). They are not to be confused with aerial cables: cables in air an insulated cable is laid on poles. Aerial cable, since they are isolated, fixed without insulators on the mast. In both types of snow can freeze and lead in combination with wind in extreme cases, demolition of the line or breaking of poles. In this case, people should keep a safe distance: at least four meters at 400 kV and dry air of drooping lines, and significantly more in humid weather, and especially when lying on the ground lines. In the latter case, there is a risk of the resulting voltage gradients in conjunction with the so-called step voltage.
However, for voltages above 50 kV today and in the foreseeable future the overhead line usually the most economical form of power line. The cooling by the surrounding air makes it possible to charge overhead lines in the winter when the power consumption is very high high. Due to overhead line monitoring this effect can be optimally utilized.
Overhead lines for electric power transmission are also used for message transmission ( mitverlegte communication cable, fiber optic cable or carrier frequency systems, the use of the conductors themselves).
Conductors of overhead lines are made of copper, Aldrey and composite cables made of steel and aluminum. The latter have because of their lower density at the same weight a larger cross section and thus a higher conductivity than copper cables and are therefore preferably used in high-voltage lines. For voltages above 110 kV AC voltage often to avoid corona phenomena and to increase the natural capacity of the line, so-called bundle conductors used. These consist of several conductor cables connected by spacers and reduce the effective electric field strength edge to values below 17 kV / cm, from which in air ionization begins. For 220 kV lines usually are two bundles, usually used three or four bundle conductors for 380 kV lines.
The maximum continuous temperature of the conductors due to the current load, depending on cable type 70 ° C to 80 ° C and is in the standards DIN 48201 and DIN 48204 specifies. In case of short circuit may briefly the maximum temperature of the conductor at 160 ° C to 170 ° C rise - higher temperatures would result in the claimed to train ropes to a material softening. The economic power density with low thermal heating is 0.7 A / mm ² to 1 A / mm ², ambient temperature, the maximum permissible continuous operating temperature of conductor cables in continuous operation with about 2 A / mm ² to 2.5 A / mm ² at 30 ° C in summer achieved in winter this value is higher. For de-icing of overhead lines in the so-called winter Abtauschaltungen can be used.
A typical conductor cable of a high-voltage line (110 kV ) consists of a siebenadrigen steel core with a total cross -sectional area of 60 mm ², which is encased by a network of 30 aluminum wires with a total area of 257 mm ². For a nominal current of 560 A per line results from six managers a capacity of 215 MVA. With a 380 - kV line with 1300 A per phase, over 900 MVA can be transferred, with the natural power is 600 MW.
The built mostly for AC overhead lines three or six conductor cables (or bundle conductors ) are stretched. At certain intervals, the position of which changes to Verdrillmasten each other and - at different distances from the earth - the earth. Through this twist of a symmetric capacity is achieved in the three-wire system, this is essential, among other things for the earth fault in compensated networks called.
Marker lamps for high -voltage power lines are near the airport for air traffic control in some regions attached to the conductor cables, such as the system Balisor.
A ground wire is like the conductor cables an electrically conductive cable, but not the power company, but runs above the current-carrying conductors and is usually attached to the mast grounded tips. The earth wire is to protect the current-carrying conductors against lightning strikes. As a rule, overhead lines are equipped with operating voltages above 50 kV with a ground wire.
The ground wire is often an optical waveguide embedded for data transmission. This data transmission capabilities are also provided telecommunications providers provided by the network operators.
For higher demands on the lightning protection power lines are sometimes fitted with two earth wires. They are located either at the outermost ends of the highest cross beam, on a V-shaped mast top or on a separate Erdseiltraverse. In the collinear arrangement of two ground wires are at least always required when a mast top is not present, because here is not sufficient, the scope of a single earth wire.
When insulators are for voltages up to 50 kV hanging or standing isolators are used. The former can withstand higher forces, the latter provide additional security, as in the case of an insulator break the rope ladder falls on the mast. The lines on stationary insulators also represent a risk for large birds that land on the traverse between the insulators or leave and it can easily cause ground faults. To avoid this, some lines are covered in the mast area with bird faucets, or it is installed an additional stand hunting rod at a safe distance above the upper crosshead.
Higher resistance to fracture can be obtained by using two (or more) parallel insulators. For voltages above 50 kV only hanging long rod insulators are used. As the insulator material glass or ceramic is mostly used. For voltages above 200 kV are often chain insulators consisting of two to four long-rod insulators used. For voltages above 100 kV and insulators of high-strength plastic are used (silicone insulators ).
For long distances credits may be caused by wind and other mechanical influences to undesirable mechanical vibrations of the conductor. This can result in mechanical damage to the conductors and insulators. To damp these oscillations near the points of suspension are the overhead line, placed in close proximity to the insulators, Stockbridge vibration damper.
Overhead lines for power transmission are characterized by parameters of the nominal voltage, the natural power and the line impedance. In the table below are for some common voltage levels exemplary standard values summarized:
Many people notice a hum or whirring under power lines in rain, fog, snow or wet weather. These noises are caused by two effects:
- By water droplets, which are on the line cables and generate a low- noise. Droplets are formed on the lines, this by the frequency of the current ( 50 Hz) are excited to vibrate at 100 Hz.
- A higher-frequency noise is caused by micro-discharges ( pre-discharges, corona discharges ) into the air, due to the high electric field strength around the conductor. This higher frequency is dependent on the voltage of the pole and valve geometry.
When the AC voltage of the line has a frequency of 50 Hz, the drop is to each half-wave to a frequency of 100 Hz, which is twice, stimulated. In this case, it changes its shape back during a period of oscillation times of the original spherical form in an elongated form and back again. Thus it gives off like a loudspeaker sound. The more water drops hanging on the line, the louder the hum. The volume is also dependent on the size of water drops - larger drops produce a louder sound. For masts having three-phase system (three or six phase conductors on a pole ), the emitted frequency of the water drops is not affected by time-delayed phase sequence of the individual conductors, it remains at 100 Hz
In countries with 60 Hz power supply, the audible frequency is 120 Hz One tries with suitable coatings or surface structures to promote the discharge of water or to keep the droplet size small.
The higher frequency, is perceived as crackling noise is derived from pre-discharges which generate ultraviolet radiation and ozone and to be referred to as corona discharge. These discharges can be scanned using corona cameras. The problems associated with corona discharges transmission losses increase with increasing voltage. The cause of this discharge, the high electric field strength at the surface of the electrical conductor, whereby the ambient air is ionized. For this reason, at voltages above 100 kV, by the use of so-called bundle of conductors, the electric field strength at the conductor surface, also referred to as fringe field strength is reduced.
Transition from underground cable to overhead line
Cross beam of an overhead line
An overhead line is a telephone poles run lead, the wires are not insulated in telecommunications. Them to be fixed to the masts, insulators of glass, ceramic or plastic can be used. Shorts between the wires be avoided by a minimum distance between them is maintained. The telephone network in Germany has been converted at a high cost to underground cable (and occasionally aerial cables and ) because overhead lines are often disrupted due to weather: Storms throw masts to which wires can freeze and crack due to its weight, etc. overhead lines for the purpose of communication are now almost completely disappeared in Germany and the western continental Europe. They are found only occasionally for lines of the ground internal telephone network ( BASA ) along nichtelektrifizierter branch lines. However, they are increasingly dismantled with the introduction of GSM -R. The last phone lines with bare wires in the public switched telephone network are likely in the former West Germany in the second half of the 1970s, have been replaced in the new federal states against 1999 by air or underground. However, no date for closing the last telephone transmission line in the public telephone network in Germany can be determined. Aerial cables are often used in rural areas.
Underdeveloped countries, but also rich countries such as the UK, the U.S. and Japan have carried out large portions of their subscriber lines or as overhead lines. In the latter two countries this is mainly for cost reasons, because after the frequently occurring there natural disasters such as earthquakes or storms is a repair of underground cables much more complex and expensive than the bracing new overhead line wires. Due to the antenna effect of overhead lines they can capture the amateur radio and CB radio. While the normal telephone traffic is it actually not affected, but a DSL transmission may be disturbed it when wireless and DSL use the same frequency ranges.
Overhead lines with alarm systems
Until the 1990s, the signal transmission of manually operated fire alarms in public space to the hotlines partly by overhead cables carried. These were mostly single-pole nature and often attached to lampposts or suspensions of street lamps. Also mounts on walls were not rare.
A special form of overhead line are the overhead lines and power rails of electric railways; these must be equipped for the extraction of electrical energy by the pantograph of rail vehicles and therefore are made of solid conductors made of copper alloy.
Also to power transmitting antennas, especially of antennas very powerful transmitter for long, medium and short wave, overhead lines are used occasionally. For this, a fish trap line is often used. In a reuse line form a plurality of parallel conductors with the outer conductors at ground potential, a coaxial line. In the interior of the ring runs attached to insulators, which is under high voltage feed line to the antenna. It is usually also performed as a bundle conductors.
Single-wire waveguides were once used in places with poor reception conditions ( valleys ) for the dissemination of broadcast programs. The along a single transmission line propagating waves could be received by close of duct lying dipole antennas. See Goubau line.
- Antennas ( for long waves are often similar to overhead lines running)
- Self- Radiant transmission tower
- Conductor rail
- Electric fence
Use of the area under a power line
The area under a power line can be used for most purposes for which no risk that objects, which are connected to the ground, can fall into the proximity of the conductors or in which the risk that by the way to use the conductors, insulators or pole structures may be damaged. However, it can under power lines harmful interference to radio reception - especially when receiving signals at frequencies below 10 MHz come with the use of rod antennas. If it is not possible to relocate the receiving antenna, then you should use in this case, magnetic antennas ( frame or ferrite ). In the structural use of the area under overhead lines is to remember that in the winter ice formation can occur at the poles and conductor cables and that falling chunks of ice can cause damage to buildings under certain circumstances.
- In the context of overhead lines (and also of radio towers, especially of radiating transmitting masts ) are prohibited and dangerous to send up kites or captive balloons since. Due to the leash, especially when wet, can cause dangerous currents to flow
- Also, caution should be exercised in dealing with long poles or ladders, especially if they are made of electrically conductive material, especially in low-hanging power lines.
- Under field lines do you refuel not to drive or decant containers with flammable liquid.
- If a torn conductor cable of a power line is down, you should not approach him, or from the proximity with tiny steps removed (step voltage).
The first transmission line in the world was built by the physicist Stephen Gray on July 14, 1729 to show that you can transmit electricity. He used wet as head hemp cords that were attached to string beans. However, there were some practical applications of overhead lines only in the context of telegraphy.
1882 The first overhead line transmission was performed with high voltage between Munich and Mies Bach, with direct current was used with a voltage of 2 kV. The efficiency was 25 %. At the realization of Oskar von Miller were involved and the Frenchman Marcel Deprez.
Made in 1891 the construction of the first three-phase overhead line on the occasion of the International Electricity Exhibition in Frankfurt / Main between Lauffen and Frankfurt am Main. The energy was transported at 10 kV over 176 km, the efficiency was 75 percent.
In 1905 between Moosburg and Munich, the first overhead line with 50 kV operating voltage to operate on.
In 1912, the first 110- kV transmission line ( between Lauchhammer and Riesa ) and in 1922 the first 220 kV transmission line in operation. In the 1920s RWE AG built the first overhead line system for this voltage, which was designed in part already for 380 kV ( North-South line ) and which also includes the 1926 -built Rhine overhead line crossing Voerde with two 138 -meter-high masts. In 1957 in Germany, the first 380 - kV transmission line in operation ( between the substation and Hoheneck Rommerskirchen ).
In the same year went to Italy the overhead line crossing the Strait of Messina in operation, their masts were used as a model for the support poles of the Elbe crossing 1 and until construction of the Elbe crossing 2 in the second half of the 1970s were the tallest overhead line masts in the world.
In 1967 overhead power lines were built for voltages of 765 kV in Russia, the USA and Canada. 1982 was built with 1,150 kV in Russia between Elektrostal and the power plant Ekibastus a three-phase line.
1999, a 500 - kV double-circuit line was built in Japan, which is designed for an operating voltage of 1100 kV, the three-phase line Kita- Iwaki.
2003 took place in China, the construction of the highest overhead line masts far the Yangtze overhead line crossing.
- Longest span: Crossing the Ameralik Fjord, 5366 m
- Longest span of Europe: Crossing the Sognefjord in the place Leikanger, 4597 m
- Longest span Germany: Eyachtalquerung yards, 1,444 m
- Longest line: HVDC Inga - Shaba: 1700 km
- Longest transmission line in Germany: Wolmirstedt - Lubmin
- Highest transmission voltage: three-phase line Ekibastus - Kokshetau, 1150 kV
- Highest transmission voltage DC: HVDC Yunnan - Guangdong ± 800 kV
- High Points of overhead line in Europe: preliminary line 2'720 m above sea level. M.
Overhead line technology:
- Friedrich Kiessling, Peter Nefzger, Ulf Kaintzyk: overhead lines. Heidelberg: Springer, 5th edition 2001, ISBN 978-3-642-62673-9
- Rene Flosdorff, Günther Hilgarth: Electrical energy distribution. Stuttgart: B.G. Teubner 4th edition 1982, ISBN 3-519-36411-5
- Günter Springer: electrical engineering expertise. Wuppertal: Verl Europe teaching aids Nourney, Vollmer 18.Auflage 1989, ISBN 3-8085-3018-9
- Wilfried knee, Klaus Schierack: Electrical System technology - power plants, grids, switchgear, protection devices. Munich, Vienna: Carl Hanser, 1991, ISBN 3-446-15712-3
- Handbook of Telecommunications - Volume 7 Part II - line art - 1973 (PDF)
- Walter Castor: Basics of electrical power supply. HAAG Library, HAAG Electronic Measuring Instruments GmbH, Waldbrunn