View source for Overhead power line

{{Short description|Above-ground structure for bulk transfer and distribution of electricity}}
{{About|power lines for general transmission of electrical power|overhead lines used to power road and rail vehicles|Overhead line}}
{{redirect|Power line}}

An '''overhead power line''' is a structure used in [[electric power transmission]] and [[Electric power distribution|distribution]] to transmit electrical energy along large distances. It consists of one or more [[electrical conductor|conductors]] (commonly multiples of three) suspended by [[Transmission tower|towers]] or [[Utility pole|poles]]. Since the surrounding air provides good [[cooling]],   [[electrical insulation|insulation]] along long passages, and allows optical inspection, overhead power lines are generally the lowest-cost method of power transmission for large quantities of electric energy.

==Construction==
[[File:Electricity pylons 330kV in Ukraine.png|thumb|200px|330&nbsp;kV overhead power lines]]

Towers for support of the lines are made of wood (as-grown or laminated), steel or aluminum (either lattice structures or tubular poles), concrete, and occasionally reinforced plastics.  The bare wire conductors on the line are generally made of aluminum<ref>{{Cite web |last=Daware |first=Kiran |date=2017-03-20 |title=Types of Conductors Used in Overhead Power Lines |url=https://peakdemand.com/types-of-conductors-used-in-overhead-power-lines/ |access-date=2025-01-29 |website=Peak Demand Inc |language=en-US}}</ref> (either plain or [[ACSR|reinforced with steel]], or composite materials such as carbon and glass fiber), though some copper wires are used in medium-voltage distribution and low-voltage connections to customer premises. A major goal of overhead power line design is to maintain adequate clearance between energized conductors and the ground so as to prevent dangerous contact with the line, and to provide reliable support for the conductors, resilience to storms, ice loads, earthquakes and other potential damage causes.<ref name="Fink78">[[Donald G. Fink]] and H. Wayne Beaty, ''Standard Handbook for Electrical Engineers, Eleventh Edition'', McGraw-Hill, New York, 1978, {{ISBN|0-07-020974-X}}, Chapter 14 ''Overhead Power Transmission''</ref>
Today,{{When|date=January 2025}} some overhead lines are routinely operated at voltages exceeding 765,000 volts between conductors, with even higher voltages possible in some cases.

== Classification by operating voltage ==
[[File:Łomża linia elektroenergetyczna napowietrzna.jpg|thumb|172px|High- and medium-voltage power lines in [[Łomża]], Poland]]
[[File:Anchor pylon of high-voltage overhead power line 750 kV.jpg|thumb|172px|Extra high-voltage overhead line 750 kV]]
Overhead power transmission lines are classified in the electrical power industry by the range of voltages:
* Low voltage (LV) – less than 1000 Volts, used for connection between a residential or small commercial customer and the utility.
* Medium voltage (MV; distribution) – between 1000 Volts (1 kV) and 69 kV, used for distribution in urban and rural areas.
* High voltage (HV; [[subtransmission]] less than 100&nbsp;kV; subtransmission or transmission at voltages such as 115&nbsp;kV and 138&nbsp;kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.
* Extra high voltage (EHV; transmission) – from 345 kV, up to about 800 kV,<ref name=TGonen>
 {{cite book|  author = Gönen, T. |  title = Electrical Power Transmission System Engineering: Analysis and Design |  publisher = CRC Press  | isbn = 9781482232233
|edition=3rd |  url = https://books.google.com/books?id=6KbNBQAAQBAJ | year = 2014 | pages = }}</ref>{{page needed|date=June 2018}} used for long distance, very high power transmission.
* Ultra high voltage (UHV) – higher than 800 kV. The ''[[Financial Times]]'' reported UHV lines are a "game changer", making a global electricity grid potentially feasible.  [[State Grid Corporation of China|StateGrid]] said that compared to conventional lines, UHV enables the transmission of five times more power, over six times the distance.<ref>{{Cite news
|url= https://www.ft.com/content/2aedab32-6aa1-11e8-8cf3-0c230fa67aec
|title= China's global power play 
|work= [[Financial Times]]
|first= James
|last= Kynge
|date= 8 June 2018
|accessdate=10 June 2018
|url-access=registration}}</ref>

==Structures==
[[File:ЛЭП 10кВ 01.jpg|thumb|172px|Wooden pylon of medium-voltage overhead line 10 kV]]
[[File:Overhead power lines in Dnipro, Ukraine.jpg|thumb|172px|330 and 150&nbsp;kV high-voltage overhead power lines]]

{{See also|Transmission tower}}
Structures for overhead lines take a variety of shapes depending on the type of line. Structures may be as simple as wood [[Utility pole|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 typically used in urban areas.  High-voltage lines are often carried on lattice-type [[Electricity pylon|steel towers]] or pylons. For remote areas, aluminum towers may be placed by [[helicopter]]s.<ref>{{cite web|url=http://www.verticalmag.com/news/article/PoweringUp|archiveurl=https://web.archive.org/web/20151004113042/http://www.verticalmag.com/news/article/PoweringUp|title=Powering Up - Vertical Magazine - The Pulse of the Helicopter Industry|archivedate=4 October 2015|work=verticalmag.com |accessdate=4 October 2015 |url-status=live}}</ref><ref>{{YouTube|GBWHUdPQCH8|[[Sunrise Powerlink]] Helicopter Operations}}</ref> Concrete poles have also been used.<ref name=Fink78/> 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.<ref name=Fink78/> The weight of the conductor must be supported, as well as dynamic loads due to wind and ice accumulation, and effects of vibration. Where conductors are in a straight line, towers need only resist the weight since the tension in the conductors approximately balances with no resultant force on the structure.  Flexible conductors supported at their ends approximate the form of a [[catenary]], and much of the analysis for construction of transmission lines relies on the properties of this form.<ref name=Fink78/>

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 (terminating) 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 power lines to limit the scale of cascading tower failures.<ref name=Fink78/>

Foundations for tower structures may be large and costly, particularly if the ground conditions are poor, such as in wetlands.  Each structure may be stabilized considerably by the use of [[Guy-wire|guy wires]] to counteract some of the forces applied by the conductors.
[[Image:Einebenenleitung.jpg|thumb|Low-profile power lines near an [[airfield]]. Note the red and white transmission towers and [[overhead wire marker]]s (also known as aerial marker balls) on the right.]]
Power lines and supporting structures can be a form of [[visual pollution]]. In some cases the lines are buried to avoid this, but this "[[undergrounding]]" is more expensive and therefore not common.

For a single wood [[utility pole]] structure, a pole is placed in the ground, then three crossarms extend from this, either staggered or all to one side. The insulators are attached to the crossarms. For an "H"-type wood pole structure, two poles are placed in the ground, then a crossbar is placed on top of these, extending to both sides. The insulators are attached at the ends and in the middle. [[Lattice tower]] structures have two common forms. One has a pyramidal base, then a vertical section, where three crossarms extend out, typically staggered. The [[strain insulator]]s are attached to the crossarms. Another has a pyramidal base, which extends to four support points. On top of this a horizontal truss-like structure is placed.

A grounded wire is sometimes strung along the tops of the towers to provide lightning protection. An [[optical ground wire]] is a more advanced version with embedded [[optical fiber]]s for communication. [[Overhead wire marker]]s can be mounted on the ground wire to meet [[International Civil Aviation Organization]] recommendations.<ref>{{cite web|url=http://www.avaids.com/icao.pdf|title=Chapter 6. Visual aids for denoting obstacles|date=2004-11-25|work=Annex 14 Volume I Aerodrome design and operations|publisher=[[International Civil Aviation Organization]]|quote=6.2.8 ... spherical ... diameter of not less than 60 cm. ... 6.2.10 ... should be of one colour.|accessdate=1 June 2011|archive-url=https://web.archive.org/web/20070205130600/http://www.avaids.com/icao.pdf|archive-date=5 February 2007}}</ref>
Some markers include [[Balisor|flashing lamps]] for night-time warning.

===Circuits===
A ''single-circuit transmission line'' carries conductors for only one circuit. For a [[Three-phase electric power|three-phase]] system, this implies that each tower supports three conductors.

A ''double-circuit transmission line''  has two circuits. For three-phase systems, each tower supports and insulates six conductors. Single phase AC-power lines as used for [[Railway electrification system|traction current]] have four conductors for two circuits. Usually both circuits operate at the same voltage.

In HVDC systems typically two conductors are carried per line, but in rare cases only one pole of the system is carried on a set of towers.

In some countries like Germany most power lines with voltages above 100 kV are implemented as double, quadruple or in rare cases even hextuple power line as [[rights of way]] are rare. Sometimes all conductors are installed with the erection of the pylons; often some circuits are installed later. A disadvantage of double circuit transmission lines is that maintenance can be difficult, as either work in close proximity of high voltage or switch-off of two circuits is required.  In case of failure, both systems can be affected.

The largest double-circuit transmission line is the [[Kita-Iwaki Powerline]].

<gallery widths="230" heights="230">
File:Single circuit overhead power line with distribution lines.png|A single-circuit 138 kV line (top) with distribution wires (bottom)
File:Electric Sails.jpg|A double-circuit line
File:Img0289SCE 500kV lines close.JPG|Parallel single-circuit lines
File:Hamilton Beach Pylon (2).JPG|Four circuits on one tower line
File:Wernau Double Pylon1.JPG|six circuits of three different types
File:Stromtrasse bei Mannheim-Seckenheim.jpg|Various powerlines (110/220 kV) in Germany with double and quadruple circuits
</gallery>

==Insulators==
[[File:Power line with ceramic insulators.jpg|thumb|Medium-voltage power lines with ceramic insulators in California]]
[[File:Pylon.detail.arp.750pix.jpg|thumb|Modular suspension insulators are used for high-voltage lines.]]
[[Electrical insulation#High-voltage insulators|Insulators]] must support the conductors and withstand both the normal operating voltage and surges due to switching 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. 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 insulator]]s limited the voltage to no more than 69,000 [[volt]]s. Up to about 33 kV (69 kV in North America) both types are commonly used.<ref name=Fink78/> 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. However, with rising voltage levels, polymer insulators ([[silicone rubber]] based) are seeing increasing usage.<ref>{{cite web |url=http://www.ngk-locke.com/polymer.html |work=NGK-Locke |archive-url=https://archive.today/20120905000404/http://www.ngk-locke.com/polymer.html |archive-date=2012-09-05 |url-status=dead |title=Polymer insulator manufacturer}}</ref> China has already developed polymer insulators having a highest system voltage of 1100 kV and India is currently developing a 1200 kV (highest system voltage) line which will initially be charged with 400 kV to be upgraded to a 1200 kV line.<ref>{{cite web|url= https://www.worldenergynews.com/news/abb-energizes-transformer-record-mln-volts-651995 |title=ABB Energizes Transformer At Record 1.2 Mln Volts|work=World Energy News|accessdate=7 October 2016}}</ref>

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. In cases where these conditions are suboptimal, longer insulators must be used. 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.<ref>{{Cite web |url=http://www.arp-hivoltageinsulators.com/suspension-insulators.html |title=Advanced Rubber Products - Suspension Insulators |access-date=2013-09-17 |archive-date=2022-03-18 |archive-url=https://web.archive.org/web/20220318041111/http://www.arp-hivoltageinsulators.com/suspension-insulators.html |url-status=dead }}</ref>

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 ensures a more even distribution of voltage along the length of the chain of insulator units.

Polymer insulators by nature have hydrophobic characteristics providing for improved wet performance. Also, studies have shown that the specific creepage distance required in polymer insulators is much lower than that required in porcelain or glass. Additionally, the mass of polymer insulators (especially in higher voltages) is approximately 50% to 30% less than that of a comparative porcelain or glass string. Better pollution and wet performance is leading to the increased use of such insulators.

Insulators for very high voltages, exceeding 200 kV, may have [[grading ring]]s installed at their terminals.  This improves the electric field distribution around the insulator and makes it more resistant to flash-over during voltage surges.

==Conductors==
[[File:Sample cross-section of high tension power (pylon) line.jpg|thumb|Sample cross-section of ACSR power line]]

The most common conductor in use for transmission today is [[Aluminium-conductor steel-reinforced cable|aluminum conductor steel reinforced]] (ACSR). Also seeing much use is [[all-aluminum-alloy conductor]] (AAAC). Aluminum is used because it has about half the weight of a comparable resistance copper cable (though larger diameter due to lower [[specific conductivity]]), as well as being cheaper.<ref name=Fink78/> 
Copper was more popular in the past and is still in use, especially at lower voltages and for grounding.

While larger conductors lose less energy due to lower [[electrical resistance]], they are more costly than smaller conductors.  An optimization rule called ''Kelvin's Law'' (named for [[Lord Kelvin]]) 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 by additional factors such as varying annual load, varying cost of installation, and the discrete sizes of cable that are commonly made.<ref name=Fink78/><ref>{{cite web |title=Economic Choice Of Conductor Size - Kelvin's Law |url=https://www.electricaleasy.com/2016/05/economic-choice-of-conductor-size-kelvins-law.html}}</ref>

Since a conductor is a flexible object with uniform weight per unit length, the shape of a conductor strung between two 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 and additional load such as ice cover. A minimum overhead clearance must be maintained for safety. Since the temperature and therefore length of the conductor increase with increasing current through it, it is sometimes possible to increase the power handling capacity (uprate) by changing the conductors for a type with a lower [[coefficient of thermal expansion]] or a higher allowable [[operating temperature]].

[[File:ACSR and ACCC.JPG|thumb|Conventional ACSR (left) and modern carbon core (right) conductors]]

Two such conductors that offer reduced thermal sag are known as composite core conductors (ACCR and [[ACCC conductor]]).  In lieu of steel core strands that are often used to increase overall conductor strength, the ACCC conductor uses a carbon and glass fiber core that offers a coefficient of thermal expansion about 1/10 of that of steel.  While the composite core is nonconductive, it is substantially lighter and stronger than steel, which allows the incorporation of 28% more aluminum (using compact trapezoidal-shaped strands) without any diameter or weight penalty.  The added aluminum content helps reduce line losses by 25 to 40% compared to other conductors of the same diameter and weight, depending upon electric current.  The carbon core conductor's reduced thermal sag allows it to carry up to twice the current ("ampacity") compared to all-aluminum conductor (AAC) or ACSR.

The power lines and their surroundings must be [[Live-line working|maintained]] by [[Lineman (technician)|linemen]], sometimes assisted by [[helicopter]]s with [[pressure washer]]s or [[circular saw]]s which may work three times faster. However this work often occurs in the dangerous areas of the [[Helicopter height–velocity diagram]],<ref name=vert2015-04>{{cite news |first=Elan |last=Head |url=http://www.verticalmag.com/digital_issue/2015/v14i2/files/82.html |title=High-value cargo |pages=80–90 |work=Vertical Magazine |date=April 2015 |accessdate=11 April 2015 |url-status=dead |archiveurl=https://web.archive.org/web/20150419023320/http://www.verticalmag.com/digital_issue/2015/v14i2/files/82.html |archivedate=19 April 2015 }}</ref><ref name=vert2015-04b>{{cite news |first=Guy R. |last=Maher |url=http://www.verticalmag.com/digital_issue/2015/v14i2/files/94.html |title=A cut above |pages=92–98 |work=Vertical Magazine |date=April 2015 |accessdate=11 April 2015|archive-url=https://web.archive.org/web/20150512160639/http://www.verticalmag.com/news/article/ACutAbove |archive-date=12 May 2015 |url-status=live}}</ref><ref>{{cite magazine |last=Harnesk |first=Tommy |url=http://www.nyteknik.se/nyheter/fordon_motor/flygplan/article3874611.ece |title=Helikoptermonterad motorsåg snabbkapar träden |language=sv |archive-url=https://web.archive.org/web/20150112025125/http://www.nyteknik.se/nyheter/fordon_motor/flygplan/article3874611.ece |archive-date=2015-01-12 |url-status=dead |magazine=[[Ny Teknik]] |date=9 January 2015 |access-date=12 January 2015}}</ref> and the pilot must be qualified for this "[[Helicopter Flight Rescue System|human external cargo]]" method.<ref>{{Cite web |url=https://www.tdworld.com/electric-utility-operations/wapa-helicopters-saving-time-and-money |title=WAPA Helicopters: Saving Time and Money |last=Weger  |first=Travis  |date=2017-11-14 |website=TDWorld |access-date=2017-12-07}}</ref>

===Bundle conductors===
[[File:Pylône électrique détail 2011-2.JPG|thumb|A bundle conductor]]
For transmission of power across long distances, high voltage transmission is employed. Transmission higher than 132&nbsp;kV poses the problem of [[corona discharge]], which causes significant power loss and interference with communication circuits. To reduce this corona effect, it is preferable to use more than one conductor per phase, or bundled conductors.<ref name="Grainger, John J 1994">Grainger, John J. and W. D. Stevenson Jr. Power System Analysis and Design, 2nd edition. McGraw Hill (1994).</ref>

Bundle conductors consist of several parallel cables connected at intervals by spacers, often in a cylindrical configuration. The optimum number of conductors depends on the current rating, but typically higher-voltage lines also have higher current. [[American Electric Power]]<ref>{{Cite news |url=http://tdworld.com/overhead-transmission/six-wire-solution |title=Six Wire Solution |journal=Transmission & Distribution World |first=Bruce |last=Freimark |date=October 1, 2006 |accessdate=March 6, 2007}}</ref> is building 765&nbsp;kV lines using six conductors per phase in a bundle. Spacers must resist the forces due to wind, and magnetic forces during a short-circuit.

[[File:Spacer damper for four-conductor bundles.jpg|thumb|left|upright|Spacer damper for four-conductor bundles]]
[[File:Pylône_électrique_détail_2011-4.JPG|thumb|Bundle conductor attachment]]

Bundled conductors reduce the voltage gradient in the vicinity of the line. This reduces the possibility of corona discharge. At [[extra high voltage]], the electric field [[gradient]] at the surface of a single conductor is high enough to ionize air, which wastes power, generates unwanted audible noise and [[Electromagnetic interference|interferes]] with [[communication system]]s.  The field surrounding a bundle of conductors is similar to the field that would surround a single, very large conductor—this produces lower gradients which mitigates issues associated with high field strength. The transmission efficiency is improved as loss due to corona effect is countered.

Bundled conductors cool themselves more efficiently due to the increased surface area of the conductors, further reducing line losses. When transmitting alternating current, bundle conductors also avoid the reduction in [[ampacity]] of a single large conductor due to the [[skin effect]]. A bundle conductor also has lower [[Reactance (electronics)|reactance]], compared to a single conductor.

While wind resistance is higher, wind-induced oscillation can be damped at bundle spacers. The ice and wind loading of bundled conductors will be greater than a single conductor of the same total cross section, and bundled conductors are more difficult to install than single conductors.

===Ground wires===
[[File:Al OC.jpg|thumb|Aluminum conductor crosslinked polyethylene insulation wire. It is used for 6600V power lines.]]
Overhead power lines are often equipped with a ground conductor (shield wire, static wire, or overhead earth wire). The ground conductor is usually grounded (earthed) at the top of the supporting structure, to minimize the likelihood of direct lightning strikes to the phase conductors.<ref>{{cite book|title=The Art and Science of Lightning Protection| isbn=9780521878111 |url=https://books.google.com/books?id=KO7fVcqispQC&pg=PA205 | last1=Uman | first1=Martin A. | date=24 January 2008 | publisher=Cambridge University Press }}</ref> In circuits with [[earthed neutral]], it also serves as a parallel path with the earth for fault currents. 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 arrester]]s every few spans in place of a shield wire; this configuration is typically found in the more rural areas of the United States. By protecting the line from lightning, the design of apparatus in substations is simplified due to lower stress on insulation. Shield wires on transmission lines may include optical fibers ([[optical ground wire]]s/OPGW), used for communication and control of the power system.

[[File:Fenno-Skan HVDC power line.jpg|thumb|HVDC Fenno-Skan with ground wires used as electrode line]]
At some HVDC converter stations, the ground wire is used also as the electrode line to connect to a distant grounding electrode. This allows the HVDC system to use the earth as one conductor. The ground conductor is mounted on small insulators bridged by lightning arrestors above the phase conductors. The insulation prevents electrochemical corrosion of the pylon.

Medium-voltage distribution lines may also use one or two shield wires, or 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, as well as to provide a neutral line in Wye wired systems.

On some power lines for very high voltages in the former Soviet Union, the ground wire is used for [[Power-line communication|PLC]] systems and mounted on insulators at the pylons.

===Insulated conductors and cable===
Overhead insulated cables are rarely 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.

A more common approach is "covered" line wire. It is treated as bare cable, but often is safer for wildlife, as the insulation on the cables increases the likelihood of a large-wing-span raptor to survive a brush with the lines, and reduces the overall danger of the lines slightly. These types of lines are often seen in the eastern United States and in heavily wooded areas, where tree-line contact is likely. The only pitfall is cost, as insulated wire is often costlier than its bare counterpart. Many utility companies implement covered line wire as jumper material where the wires are often closer to each other on the pole, such as an underground riser/[[pothead]],<!--not [[Cannabis smoking]]--> and on reclosers, cutouts and the like.

=== Dampers ===
[[File:Stockbridge_damper_POV.jpg|thumb|A Stockbridge damper]]
Because power lines can suffer from [[Aeroelasticity#Flutter|aeroelastic flutter]] driven by wind, [[Stockbridge damper]]s are often attached to the lines to reduce the vibrations.

==Compact transmission lines==
{{more citations needed section|date=March 2012}}
A compact overhead transmission line requires a smaller right of way than a standard overhead powerline. Conductors must not get too close to each other. This can be achieved either by short span lengths and insulating crossbars, or by separating the conductors in the span with insulators. The first type is easier to build as it does not require insulators in the span, which may be difficult to install and to maintain.

Examples of compact lines are:
* Lutsk compact overhead powerline {{coord|50.774673|N|25.385215|E|type:landmark|name=Startpoint of Lutsk Compact Overhead Powerline}}
* Hilpertsau-Weisenbach compact overhead line {{coord|48.737898|N|8.355660|E|type:landmark|name=Startpoint of Hilpertsau-Weisenbach Powerline}}

Compact transmission lines may be designed for voltage upgrade of existing lines to increase the power that can be transmitted on an existing right of way.<ref>Beaty, H. Wayne; Fink, Donald G., ''Standard Handbook for Electrical Engineers (15th Edition)''
McGraw-Hill, 2007  978-0-07-144146-9 pages 14-105 through 14-106</ref>

==Low voltage==
[[File:ABC TQ3157 064.JPG|thumb|Aerial bundled cable in [[Old Coulsdon]], [[Surrey]]]]
Low voltage overhead lines may use either bare conductors carried on glass or ceramic insulators or an [[aerial bundled cable]] system. The number of conductors may be anywhere between two (most likely a phase and neutral) up to as many as six (three phase conductors, separate neutral and earth plus street lighting supplied by a common switch); a common case is four (three phase and neutral, where the neutral might also serve as a protective earthing conductor).

==Train power==
{{main|Overhead line}}

Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains. Overhead line is designed on the principle of one or more overhead wires situated over rail tracks. Feeder stations at regular intervals along the overhead line supply power from the high-voltage grid. For some cases low-frequency AC is used, and distributed by a special [[traction current]] network.

==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.

==Use of area under overhead power lines==
Use of the area below an overhead line is limited because objects must not come too close to the energized conductors. Overhead lines and structures may shed ice, creating a hazard. Radio reception can be impaired under a power line, due both to shielding of a receiver antenna by the overhead conductors, and by partial discharge at insulators and sharp points of the conductors which creates radio noise.

In the area surrounding the overhead lines it is dangerous to risk interference; e.g. flying kites or  balloons, using ladders or operating machinery.

Overhead distribution and transmission lines near [[airfield]]s are often marked on maps, and the lines themselves marked with conspicuous plastic reflectors, to warn pilots of the presence of conductors.

Construction of overhead power lines, especially in wilderness areas, may have significant environmental effects. Environmental studies for such projects may consider the effect of bush clearing, changed migration routes for migratory animals, possible access by predators and humans along transmission corridors, disturbances of fish habitat at stream crossings, and other effects.

==Aviation accidents ==
[[File:High voltage transmission line aviation obstruction marker.jpg|thumb|An aviation obstruction marker on a high-voltage overhead transmission line reminds pilots of the presence of an overhead line. Some markers are lit at night or have strobe lights.]]
General aviation, hang gliding, paragliding, skydiving, balloon, and kite flying must avoid accidental contact with power lines. Nearly every kite product warns users to stay away from power lines. Deaths occur when aircraft crash into power lines. Some power lines are marked with obstruction markers, especially near air strips or over waterways that may support floatplane operations. The placement of power lines sometimes use up sites that would otherwise be used by hang gliders.<ref>[https://retasite.wordpress.com/2014/09/10/aircraft-accidents-due-to-overhead-power-lines-updated/ Aircraft Accidents Due to Overhead Power Lines]</ref><ref>{{Cite web |url=http://www.thefreelibrary.com/Pacific+Gas+and+Electric+Company+Reminds+Customers+About+Flying+Kites...-a084267973 |title=Pacific Gas and Electric Company Reminds Customers About Flying Kites Safely. |access-date=2014-10-20 |archive-date=2014-10-20 |archive-url=https://archive.today/20141020164701/http://www.thefreelibrary.com/Pacific+Gas+and+Electric+Company+Reminds+Customers+About+Flying+Kites...-a084267973 |url-status=dead }}</ref>

==History==
The first transmission of electrical impulses over an extended distance was demonstrated on July 14, 1729 by the physicist [[Stephen Gray (scientist)|Stephen Gray]].{{Citation needed|date=May 2014}} The demonstration used damp hemp cords suspended by silk threads (the low resistance of metallic conductors not being appreciated at the time).

However the first practical use of overhead lines was in the context of [[electrical telegraph|telegraphy]]. By 1837 experimental commercial telegraph systems ran as far as  20&nbsp;km (13 miles).   Electric power transmission was accomplished in 1882 with the first high-voltage transmission between [[Miesbach–Munich Power Transmission|Munich and Miesbach]] (60&nbsp;km). 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 power line entered service followed by the first 220 kV-overhead power line 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.

<!-- Milestones of years and voltages table here -->
In 1953, the first 345 kV line was put into service by [[American Electric Power]] in the [[United States]]. In Germany in 1957 the first 380 kV overhead power line 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 of Messina|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. Earlier, in 1952, the first 380 kV line was put into service in [[Sweden]], in 1000&nbsp;km (625 miles) between the more populated areas in the south and the largest hydroelectric power stations in the north.
Starting from 1967 in Russia, and also in the USA and Canada, overhead lines for voltage of 765 kV were built. In 1985 [[Powerline Ekibastuz-Kokshetau|overhead power line]] was built in Soviet Union between [[Kokshetau]] and the power station at [[Ekibastuz]], this was a three-phase alternating current line at 1150 kV. In 1999, in Japan the first powerline designed for 1000 kV with 2 circuits were built, the [[Kita-Iwaki Powerline]]. In 2002 the building of the highest overhead line commenced in China, the [[Yangtze River Crossing]], its two {{convert|346.5|m|ft|0|abbr=on}} high [[suspension tower]]s beginning service in 2004.<ref>{{Cite web|url=https://structurae.net/en/structures/jiangyin-yangtze-high-voltage-crossing-pylons|title = Jiangyin Yangtze High-Voltage Crossing Pylons (Jiangyin, 2004)}}</ref>

In the 21st century, replacing steel with carbon fiber cores ([[advanced reconductoring]]) became a way for utilities to increase transmission capacity without increasing the amount of land used.

==Mathematical analysis==
An overhead power line is one example of a [[transmission line]]. At power system frequencies, many useful simplifications can be made for lines of typical lengths. For analysis of power systems, the distributed resistance, series inductance, shunt leakage resistance and shunt capacitance can be replaced with suitable lumped values or simplified networks.

===Short and medium line model===
A short length of a power line (less than 80&nbsp;km) can be approximated with a resistance in series with an inductance and ignoring the shunt admittances. This value is not the total impedance of the line, but rather the series impedance per unit length of line. For a longer length of line (80–250&nbsp;km), a shunt capacitance is added to the model. In this case it is common to distribute half of the total capacitance to each side of the line. As a result, the power line can be represented as a [[two-port network]], such as with ABCD parameters.<ref name=Glover21>J. Glover, M. Sarma, and T. Overbye, ''Power System Analysis and Design, Fifth Edition'', Cengage Learning, Connecticut, 2012, {{ISBN|978-1-111-42577-7}}, Chapter 5 ''Transmission Lines: Steady-State Operation''</ref>

The circuit can be characterized as
:<math>Z = z l = (R + j \omega L)l </math>
where
*''Z'' is the total series line [[Electrical impedance|impedance]]
*''z'' is the series impedance per unit length
*''l'' is the line length
*<math>\omega \ </math> is the [[sinusoidal]] [[angular frequency]]
The medium line has an additional shunt [[admittance]]
:<math>Y = y l = j \omega C l </math>
where
*''Y'' is the total shunt line admittance
*''y'' is the shunt admittance per unit length

<gallery widths="230px" heights="100px" perrow="2">
File:Short Line Approximation.png|Short length of power line
File:Med Line Approximation.png|Medium length of power line
</gallery>

== See also ==
* [[Aerial cable]]
* [[Conductor gallop]]
* [[Conductor marking lights]]
* [[CU project controversy]]
* [[Overhead cable]]
* [[Overhead line]]
* [[Raptor conservation]]
* [[Third rail]]
* [[Operation Outward]]
* [[Powerline river crossings in the United Kingdom]]
* [[Wireless powerline sensor|Wireless monitoring of overhead power lines]]

==References==
{{Reflist|30em}}

==Further reading==
{{refbegin}}
* William D. Stevenson, Jr. ''Elements of Power System Analysis Third Edition'', McGraw-Hill, New York (1975) {{ISBN|0-07-061285-4}}

== External links ==
* {{Commons-inline|Overhead power lines}}

{{refend}}
{{Electricity delivery}}

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[[Category:Overhead power lines| ]]
[[Category:Electric power distribution]]