Transmission tower: Difference between revisions
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==Alternatives to pylons== |
==Alternatives to pylons== |
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Pylons and the cables that they support are generally regarded to be unattractive. An alternative to pylons is underground cables. This is a more expensive solution than cables that are supported by pylons but has aesthetic advantages. There are schemes in various countries to improve the appearance of the environment by removing the pylons and [[undergrounding]] the cables. However, a disadvantage of underground cables is that they have poor heat-dissipation qualities, contrary to cables suspended on towers, which are cooled by the air. The additional [[capacitance]] of the ground also results in less efficient power transmission. |
Pylons and the cables that they support are generally regarded to be unattractive. An alternative to pylons is underground cables. This is a more expensive solution than cables that are supported by pylons but has aesthetic advantages. There are schemes in various countries to improve the appearance of the environment by removing the pylons and [[undergrounding]] the cables. However, a disadvantage of underground cables is that they have poor heat-dissipation qualities, contrary to cables suspended on towers, which are cooled by the air. The additional [[capacitance]] of the ground also results in less efficient power transmission. Perhaps one of the largest disadvantages of cables is that they are far more vulnerable to careless/inadvertent damage by third parties, often in the course of construction work. |
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==See also== |
==See also== |
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Revision as of 08:56, 18 September 2008
An electricity pylon or transmission tower is a tall, usually steel lattice structure used to support overhead electricity conductors for electric power transmission.
High voltage AC transmission towers
Three-phase electric power systems are used for high and extra-high voltage AC transmission lines (50 kV and above). The towers must be designed to carry three (or multiples of three) conductors. The towers are usually steel lattices or trusses (wooden structures are used in Germany in exceptional cases) and the insulators are either glass or porcelain discs or composite Insulators using Silicone Rubber or EPDM rubber material assembled in strings or long rod whose length is dependent on the line voltage and environmental conditions. One or two earth conductors (alternative term: Ground conductors) for lightning protection are often mounted at the top of each tower.
In some countries, towers for high and extra-high voltage are usually designed to carry two or more electric circuits. For double circuit lines in Germany, the "Danube" towers or more rarely, the "fir tree" towers, are usually used. If a line is constructed using towers designed to carry several circuits, it is not necessary to install all the circuits at the time of construction.
Some high voltage circuits are often erected on the same tower as 110 kV lines. Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for railway electrification.
High voltage DC transmission pylons
High voltage direct current (HVDC) transmission lines are either monopolar or bipolar systems. With bipolar systems a conductor arrangement with one conductor on each side of the tower is used. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system. In these cases, conductors are installed on both sides of the tower for mechanical reasons. Until the second pole is needed, it is either grounded, or joined in parallel with the pole in use. In the latter case the line from the converter station to the earthing (grounding) electrode is built as underground cable.
Railway traction line pylons
Towers used for single phase AC railway traction lines are similar in construction to those towers used for 110 kV-three phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve). As a rule, the towers of railway traction lines carry two electric circuits, so they have four conductors. These are usually arranged on one level, whereby each circuit occupies one half of the crossarm. For four traction circuits the arrangement of the conductors is in two-levels and for six electric circuits the arrangement of the conductors is in three levels.
With limited space conditions, it is possible to arrange the conductors of one traction circuit in two levels. Running a traction power line parallel to a high voltage transmission lines for three-phase AC on a separate crossarm of the same tower is possible. If traction lines are led parallel to 380 kV-lines, the insulation must be designed for 220 kV, because in the event of a fault, dangerous overvoltages to the three-phase alternating current line can occur. Traction lines are usually equipped with one earth conductor. In Austria, on some traction circuits, two earth conductors are used.
Assembly
Lattice towers can be assembled horizontally on the ground and erected by push-pull cable, but this method is rarely used because of the large assembly area needed. Lattice towers are more usually erected using a crane or using gin pole method or using derrick or in very inaccessible areas, a helicopter.
Testing of mechanical properties
There are tower testing stations for testing the mechanical properties of towers.
Sign markings
Besides the obligatory high voltage warning sign, electricity towers also frequently possess a sign or circuit identification plate, with the names of the line (either the terminal points of the line or the internal designation of the EVU[vague]) and the tower number. This makes it easier identifying the location of a fault to the power company that owns the tower.
In some countries, electricity towers of lattice steel have to be equipped with a barbed wire barrier approximately 3 metres above ground in order to deter unauthorized climbing. Such barriers can often be found on towers close to roads or other areas with easy public access, even where there is not such a requirement.
Special designs
Antennas for low power FM radio, television, and mobile phone services are sometimes erected on pylons, especially on the steel masts carrying high voltage cables.
To build branches, quite impressive constructions must occasionally be used. This also applies occasionally to twisting masts that divert three-level conductor cables.
Sometimes (in particular on steel framework pylons for the highest voltage levels) transmitting plants are installed. Usually these installations are for mobile phone services or the operating radio of the power supply firm, but occasionally also for other radio services, like directional radio. Thus transmitting antennas for low-power FM radio and television transmitters were already installed on pylons. On the carrying pylon of the Elbe Crossing 1 there is a radar facility belonging to the Hamburg water and navigation office.
For crossing broad valleys, a large distance between the conductor cables must be maintained to avoid short-circuits caused by conductor cables colliding during storms. Sometimes a separate pylon is used for each conductor. For crossing wide rivers and straits with flat coastlines very high pylons must be built, because a large height clearance is needed for navigation. Such masts must be equipped with flight safety lamps.
Two well-known crossings of wide rivers are the Elbe Crossing 1 and Elbe Crossing 2. The latter has the highest overhead line masts in Europe (height: 227 meters). The pylons of the overhead line crossing of the bay of Cádiz, Spain have a particularly interesting construction. They consist of 158-meter-high carrying pylons with one cross beam atop a frustum framework construction. The largest spans of overhead lines are the crossing of the Norwegian Sognefjord (span between two masts of 4,597 meters) and the Ameralik span in Greenland (span width: 5,376 meters). In Germany the overhead line of the EnBW AG crossing of the Eyachtal has the largest span in the country, a width of 1,444 meters.
In order to drop overhead lines into steep, deep valleys, inclined pylons are occasionally used. An example of this type of pylon is located at the Hoover dam in the USA. In Switzerland a NOK pylon inclined around 20 degrees to the vertical is located near Sargans. Highly sloping masts are used on two 380 kV pylons in Switzerland, the top 32 meters of one of them being bent by 18 degrees to the vertical.
Power station chimneys are sometimes equipped with crossbars for fixing conductors of the outgoing lines. Because of possible problems with corrosion by the flue gases, such constructions are very rare.
Types of pylons
Specific functions
- anchor pylons (or strainer pylons) or terminal towers utilize tension insulators and occur at the endpoints of conductors.
- pine pylon — an electricity pylon for two circuits of three-phase AC current, a which the conductors are arranged in three levels. In pine pylons the lowest crossbar has a wider span than that in the middle and this one a larger span than that on the top.
- Transposing pylons are anchor or terminal pylons at which the conductors are "transposed" so that they exchange sides of the pylon.
- long-distance anchor pylon
- branch pylon
- anchor portal
- termination pylon
Materials used
- wood pylon
- concrete pylon
- steel tube pylon
- lattice steel pylon
- concrete filled steel tube pylon
- stobie pole
Conductor arrangements
Specific locations
- roof stand
- crossing pylon
- bridge mounted structures, as on Storstrøm Bridge
Specific purposes
Pylons in art and culture
For the movie Among Giants a pylon, the meanwhile dismantled Pink Pylon, was coloured in pink. In Ruhrpark, a big mall in Bochum, Germany, there is a pylon decorated with balls.
The North Korean official emblem has a pylon and a dam on it.
Pylons of special interest
Alternatives to pylons
Pylons and the cables that they support are generally regarded to be unattractive. An alternative to pylons is underground cables. This is a more expensive solution than cables that are supported by pylons but has aesthetic advantages. There are schemes in various countries to improve the appearance of the environment by removing the pylons and undergrounding the cables. However, a disadvantage of underground cables is that they have poor heat-dissipation qualities, contrary to cables suspended on towers, which are cooled by the air. The additional capacitance of the ground also results in less efficient power transmission. Perhaps one of the largest disadvantages of cables is that they are far more vulnerable to careless/inadvertent damage by third parties, often in the course of construction work.
See also
External links
- Flash Bristow's pylon photo gallery and pylon FAQ
- Pictures of Pylons and technical information
- Remarkable Pylons
- Pylon Photographs and Art
- Power Station Konakovskaya GRES, at which chimneys serve as electricity pylons
- Hungarian Pylons
 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (February 2008) |