Editing Overhead power line (section)

==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.