Transmission tower - AbsoluteAstronomy.com

A transmission tower is a tall structure

Structure

Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...

, usually a steel lattice tower

Lattice tower

A lattice tower or truss tower is a freestanding framework tower. They can be used as electricity pylons especially for voltages above 100 kilovolts, as a radio tower or as an observation tower....

, used to support an overhead power line

Overhead power line

An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...

. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes. Typical height ranges from 15 to 55 m (49.2 to 180.4 ), although heights in excess of 300 metres (984.3 ft) exist. In addition to steel, other materials may be used, including concrete and wood.

Four major functions of transmission towers are in use: suspension towers, terminal towers, tension towers

Dead-end tower

A dead-end tower uses horizontal strain insulators and occur at the endpoints of conductors. Such endpoints are necessary when interfacing with other modes of power transmission and, due to the inflexibility of the conductors, when significantly altering the direction of the tower line...

, and transposition towers. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution

Visual pollution

Visual pollution is the term given to unattractive and man-made visual elements of a vista, a landscape, or any other thing that a person does not feel comfortable to look at. Visual pollution is an aesthetic issue, referring to the impacts of pollution that impair one's ability to enjoy a vista or...

. Methods to reduce the visual impact include undergrounding

Undergrounding

Undergrounding refers to the replacement of overhead cables providing electrical power or telecommunications, with underground cables. This is typically performed for aesthetic purposes, and increases distribution cost.- Overview and Comparison :...

Naming

"Transmission tower" is the name for the structure used in the industry in the United Kingdom, United States, and other English-speaking countries. The term "pylon" comes from the basic shape of the structure, an obelisk-like structure which tapers toward the top, and is mostly used in the United Kingdom and parts of Europe in everyday colloquial speech. This term is rarely, if ever, used in the United States, as the word "pylon" is commonly used for a multitude of other things, mostly for traffic cone

Traffic cone

Traffic cones, also called traffic pylons, road cones, highway cones, safety cones, construction cones or witches' hats or safety wizards, are usually cone-shaped markers that are placed on roads or footpaths to temporarily redirect traffic in a safe manner...

s. In Canada, the term "hydro tower" comes from the name of local hydroelectric power utility companies.

High voltage AC transmission towers

Three-phase electric power

Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

systems are used for high and extra-high voltage AC

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

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 truss

Truss

In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in...

es (wooden structures are used in Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, and Scandinavia

Scandinavia

Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...

in some cases) and the insulators are either glass or porcelain discs or composite insulators using silicone rubber or EPDM rubber

EPDM rubber

EPDM rubber , a type of synthetic rubber, is an elastomer which is characterized by a wide range of applications. The E refers to ethylene, P to propylene, D to diene and M refers to its classification in ASTM standard D-1418. The M class includes rubbers having a saturated chain of the...

material assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions.

Typically, one or two ground wires are placed on top to intercept lightning and harmlessly divert it to ground.

Towers for high and extra-high voltage are usually designed to carry two or more electric circuits. 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 towers

High-voltage direct current

A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...

(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. On some schemes, the ground conductor is used as electrode line

Electrode line

An electrode line is an electrical connection from a static inverter plant of a high voltage direct current transmission system to the earth grounding electrode...

or ground return. In this case it had to be installed with insulators equipped with surge arrestors on the pylons in order to prevent electrochemical corrision of the pylons. 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, often conductors on both sides of the tower are installed for mechanical reasons. Until the second pole is needed, it is either used as electrode line, 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, as overhead line on a separate right of way or by using the ground conductors.

Electrode line towers are used in some HVDC schemes to carry the power line from the converter station to the grounding electrode. They are similar to structures used for lines with voltages of 10–30kV, but normally carry only one or two conductors.

Railway traction line towers

Towers used for single-phase AC railway traction lines

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...

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.

Towers for different types of currents

It is possible to install circuits for different types of current on the same tower. In principle, it is possible to install AC circuits of different frequency and phase-count or AC and DC circuits on the same pylon.

Usually all circuits of such lines have voltages of 50 kV and more. However, there are some lines of this type for lower voltages, for example when the masts of an overhead wire of a railway carry also circuits of the public three-phase AC grid.

However only two very short sections exist where a pylon carries as well AC as DC circuits: these are the pylons in the terminal of HVDC Volgograd-Donbass on Volga Hydroelectric Power Station and 2 pylons south of Stenkullen, which carry one circuit of HVDC Konti-Skan and the three-phase AC line Stenkullen-Holmbakullen. The overhead section of the electrode line

Electrode line

An electrode line is an electrical connection from a static inverter plant of a high voltage direct current transmission system to the earth grounding electrode...

of Pacific Intertie from Sylmar substation and the electrode line of Square Butte from Adolph substation are installed on AC towers.

Pylons carrying three-phase AC circuits of different frequencies may exist in areas, where grids operated with two frequencies come together, e.g. in Japan.

The most important case is given when pylons carry as well three-phase AC circuits for the public grid and single-phase AC for railway use. Such towers exist in Germany, Austria and Switzerland in order to better use right of ways.
In Germany such pylons usually carry two electric circuits for traction current and two for three-phase alternating current. If the three-phase circuits are not arranged in one level, the lowest cross bar carries the traction current circuits. The highest crossbars are, if three-phase circuits are arranged in two or three levels.
In Switzerland, hybrid pylons with three crossbars are used. One half of the pylon carries a three-phase system; the other half carries two traction current circuits. If the traction current line shares the pylon with a three-phase alternating current line operated by more than 220kV, the isolation of the traction current line must be designed for 220kV. Otherwise, over voltages,which overtax an insulation for 110kV—can occur in some failure cases of the 380kV line along the traction current line.

Assembly

Before transmission towers are even erected, prototype towers are tested at tower testing station

Tower testing station

A tower testing station is a special plant for testing various design for towers for transmission lines and similar uses. A tower testing station consists of two steel stands and one or more foundations, on which a sample of the tower can be built...

s. There are a variety of ways they can then be assembled and erected:

They can be assembled horizontally on the ground and erected by push-pull cable. This method is rarely used, however, because of the large assembly area needed.
They can be assembled vertically (in their final upright position). Very tall towers, such as the Yangtze River Crossing
Yangtze River Crossing
The Yangtze River Power Line Crossings refer to overhead power lines that cross the Yangtze River in China. There are at least three power line crossings on the Yangtze River at Jiangyin, Nanjing, and Wuhu. The one at Jiangyin has the tallest electrical pylons in the world.-Jiangyin:One exists...

, were assembled in this way.
A jin-pole
Jin-pole
A jin-pole or gin pole is a rigid pole with a pulley on the end used for the purpose of lifting. The lower portion of the jin-pole is attached to the upper exterior of an existing tower or structure. The jin-pole's free end extends above the existing tower or structure...

crane can be used to assemble lattice towers. This is also used for utility pole
Utility pole
A utility pole is a pole used to support overhead power lines and various other public utilities, such as cable, fibre optic cable, and related equipment such as transformers and street lights. It can be referred to as a telephone pole, power pole, hydro pole, telegraph pole, or telegraph post,...

s.
Helicopters can serve as aerial crane
Aerial crane
Helicopters used to lift heavy loads are called aerial cranes or skycranes. As aerial cranes, helicopters carry loads connected to long cables or slings in order to place heavy equipment when other methods are not available or economically feasible, or when the job must be accomplished in remote or...

s for their assembly in areas with limited accessibility. Towers can also be assembled elsewhere and flown to their place on the transmission right-of-way.

Markers

The International Civil Aviation Organization

International Civil Aviation Organization

The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...

issues recommendations on markers for towers and the power lines suspended between them. Certain jurisdictions will make these recommendations mandatory, for example that certain power lines must have spherical markers placed at intervals, and that obstacle lights be placed on any sufficiently high towers.

Electricity pylons often have an identification number or code placed on the pole in the form of a sign, an identification plate, painted numbers, or anything else the electric company chooses. These tags are usually marked with the names of the line (either the terminal points of the line or the internal designation of the power company) and the tower number. This makes identifying the location of a fault to the power company that owns the tower easier.

Transmission towers, much like other steel lattice towers including broadcasting or cellphone towers, are marked with signs which discourage public access due to the danger of the high voltage. Often this is accomplished with a sign warning of the high voltage; other times the entire access point to the transmission corridor is marked with a sign. Some countries require that lattice steel towers be equipped with a barbed wire

Barbed wire

Barbed wire, also known as barb wire , is a type of fencing wire constructed with sharp edges or points arranged at intervals along the strand. It is used to construct inexpensive fences and is used atop walls surrounding secured property...

barrier approximately 3 metres (9.8 ft) 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 a legal requirement. In the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, all such towers are fitted with barbed wire.

Special designs

Sometimes (in particular on steel lattice towers for the highest voltage levels) transmitting plants are installed, and antennas mounted on the top above or below the overhead ground wire. 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 Elbe Crossing 1

Elbe crossing 1

Elbe Crossing 1 is a group of masts providing an overhead crossing of a 220 kV three-phase alternating current electric powerline across the River Elbe. Constructed between 1959 and 1962 as part of the line from Stade to Hamburg north, it consists of four masts:* Each of the two portal masts...

tower, there is a radar facility belonging to the Hamburg

Hamburg

-History:The first historic name for the city was, according to Claudius Ptolemy's reports, Treva.But the city takes its modern name, Hamburg, from the first permanent building on the site, a castle whose construction was ordered by the Emperor Charlemagne in AD 808...

water and navigation office.

For crossing broad valleys, a large distance between the conductors must be maintained to avoid short-circuits caused by conductor cables colliding during storms. To achieve this, sometimes a separate mast or tower is used for each conductor. For crossing wide rivers and straits with flat coastlines, very tall towers must be built due to the necessity of a large height clearance for navigation. Such towers and the conductors they carry must be equipped with flight safety lamps and reflectors.

Two well-known wide river crossings are the Elbe Crossing 1

Elbe crossing 1

and Elbe Crossing 2

Elbe crossing 2

Elbe Crossing 2 is a group of pylons providing overhead lines for four 380 kV three-phase alternating current circuits across the German river Elbe...

. The latter has the tallest overhead line masts in Europe, at 227 metres (744.8 ft) tall. In Spain, the overhead line crossing

Overhead line crossing

An overhead line crossing is the crossing of an obstacle—such as a traffic route, a river, a valley or a strait—by an overhead power line. The style of crossing depends on the local conditions and regulations at the time the power line is constructed. Overhead line crossings can...

pylons in the Spanish bay of Cádiz

Pylons of Cádiz

The Pylons of Cádiz, also known as the Towers of Cádiz, are two tall pylons supporting a double-circuit 132 kV three-phase AC powerline over the bay of Cádiz, Spain starting at Puerto Real Substation at to the substation of the former Cadiz Thermal Power Station situated on the peninsula upon...

have a particularly interesting construction. The main crossing towers are 158 metres (518.4 ft) tall with one crossarm atop a frustum

Frustum

In geometry, a frustum is the portion of a solid that lies between two parallel planes cutting it....

framework construction. The longest overhead line spans are the crossing of the Norwegian Sognefjord (4597 metres (15,082 ft) between two masts) and the Ameralik span in Greenland (5376 metres (17,637.8 ft)). In Germany, the overhead line of the EnBW AG crossing of the Eyachtal has the longest span in the country at 1444 metres (4,737.5 ft).

In order to drop overhead lines into steep, deep valleys, inclined towers are occasionally used. These are utilized at the Hoover Dam

Hoover Dam

Hoover Dam, once known as Boulder Dam, is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the US states of Arizona and Nevada. It was constructed between 1931 and 1936 during the Great Depression and was dedicated on September 30, 1935, by President...

, located in the United States, to descend the cliff walls of the Black Canyon of the Colorado

Black Canyon of the Colorado

The Black Canyon of the Colorado is the canyon on the Colorado River where Hoover Dam was built. The canyon is located on the Colorado River at the state line between Nevada and Arizona. The western wall of the gorge is in the El Dorado Mountains, and the eastern wall is in the Black Mountains of...

. In Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

, a NOK pylon inclined around 20 degrees to the vertical is located near Sargans

Sargans

Sargans is a municipality in the Wahlkreis of Sarganserland in the canton of St. Gallen in Switzerland.Sargans is known for its castle, which dates from before the founding of the Swiss Confederation in 1291...

, St. Gallens. 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.

A new type of pylon will be used in the Netherlands starting in 2010. The pylons were designed as a minimalist structure by Dutch architects Zwarts and Jansma. The use of physical laws for the design made a reduction of the magnetic field possible. Also, the visual impact on the surrounding landscape is reduced.

Tower functions

Tower structures can be classified by the way in which they support the line conductors. Suspension structures support the conductor vertically using suspension insulators. . Strain structures resist net tension in the conductors and the conductors attach to the structure through strain insulators. Dead-end structures support the full weight of the conductor and also all the tension in it, and also use strain insulators.

Where the conductors are straight, a tangent tower is used. Angle towers are used where a line must change direction.

Structures are classified as tangent suspension, angle suspension, tangent strain, angle strain, tangent dead-end and angle dead-end.

Cross arms and conductor arrangement

Generally three conductors are required per AC 3-phase circuit, although single-phase and DC circuits are also carried on towers. Conductors may be arranged in one plane, or by use of several cross-arms may be arranged in a roughly symmetrical, triangulated pattern to balance the impedances of all three phases. If more than one circuit is required to be carried and the width of the line right-of-way does not permit multiple towers to be used, two or three circuits can be carried on the same tower using several levels of cross-arms. Often multiple circuits are the same voltage, but mixed voltages can be found on some structures.

Tubular steel

Poles made of tubular steel generally are assembled at the factory and placed on the right-of-way afterward. Because of its durability and ease of manufacturing and installation, many utilities in recent years prefer the use of monopolar steel or concrete towers over lattice steel for new power lines and tower replacements.

In Germany

Energy in Germany

Energy in Germany describes energy and electricity production, consumption and import in Germany. Energy policy of Germany will describe the politics of Germany related to energy more in detail. Electricity sector in Germany is the main article of electricity in Germany...

steel tube pylons are also established predominantly for medium voltage lines, in addition, for high voltage transmission lines or two electric circuits for operating voltages by up to 110 kV. Steel tube pylons are also frequently used for 380 kV lines in France

Energy in France

Energy in France describes energy and electricity production, consumption and import in France. Energy policy of France will describe more in detail the politics of France related to energy. Electricity sector in France is the main article of electricity in France.-Overview:-Companies:Électricité...

, and for 500 kV lines in the USA.

Lattice steel

A lattice steel tower is a steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

framework construction. Lattice steel towers are used for power lines

Overhead power line

of all voltages, and are the most common type for high-voltage transmission lines.

A lattice tower is usually assembled at the location where it is to be erected. This makes very tall towers possible (up to 100 metres—in special cases even higher, as in the Elbe crossing 1

Elbe crossing 1

and Elbe crossing 2

Elbe crossing 2

Elbe Crossing 2 is a group of pylons providing overhead lines for four 380 kV three-phase alternating current circuits across the German river Elbe...

). Assembly of lattice steel towers can be done using a crane

Crane (machine)

A crane is a type of machine, generally equipped with a hoist, wire ropes or chains, and sheaves, that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of...

. Lattice steel towers are generally made of angle-profiled steel beams (L- or T-beam

T-beam

A T-beam, used in construction, is a load-bearing structure of reinforced concrete, wood or metal, with a t-shaped cross section. The top of the t-shaped cross section serves as a flange or compression member in resisting compressive stresses...

s). For very tall towers, truss

Truss

es are often used.

Wood

Wood is a material which is limited in use in high-voltage transmission. Because of the limited height of available trees the maximum height of wooden pylons is limited (approximately 30 metres). Wood is rarely used for lattice framework; they are instead used to build multi-pole structures, such as H-frame and K-frame structures. The voltages they carry are also limited, such as in Germany

Germany

, where wood structures only carry voltages up to approximately 30 kV.
In coutries such as Canada

Canada

or United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

wooden towers carry voltages up to 345 kV; these can be less costly than steel structures and take advantage of the surge voltage insulating properties of wood.

Concrete

Concrete pylons are used in Germany

Germany

normally only for lines with operating voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

s below 30kV. In exceptional cases concrete pylons are used also for 110kV-lines, as well as for the public grid or for the railway traction current grid. In Switzerland

Switzerland

, concrete pylons with heights of up to 59.5 metres (world's tallest pylon of prefabricated concrete at Littau

Littau

Littau is a former municipality, and now part of the city of Lucerne, in the district of Lucerne in the canton of Lucerne in Switzerland. On 1 January 2010 the municipality of Littau merged into the municipality of Lucerne.-History:...

) are used for 380kV-overhead lines. Concrete poles are also used in Canada.

Concrete pylons, which are not prefabricated, are also used for constructions taller than 60 metres. One example is a 66 metres tall pylon of a 380 kV powerline near Reuter West Power Plant in Berlin. Such pylons look like industrial chimneys. In China some pylons for lines crossing rivers were built of concrete. The tallest of these pylons belong to the Yangtze Powerline crossing at Nanjing with a height of 257 metres.

Aluminum

Where towers must be placed in inaccessible terrain by helicopters, the extra material cost of aluminum towers will be offset by lower installation cost. Design of aluminum lattice towers is similar to that for steel, but must take into account aluminum's lower Young's modulus

Young's modulus

Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...

Notable pylons

See Lattice tower##Steel lattice towers for a list that includes notable transmission towers.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Naming

High voltage AC transmission towers

High voltage DC transmission towers

Railway traction line towers

Towers for different types of currents

Assembly

Markers

Special designs

Tower functions

Cross arms and conductor arrangement

Tubular steel

Lattice steel

Wood

Concrete

Aluminum

Notable pylons

See also

External links