Editing Electric power transmission (section)

=== Overhead ===
{{Main|Overhead power line}}

{{multiple image
 |direction = vertical
 |align = right
 |width = 225
 |image1=Electric power transmission line.JPG
 |image2=Sample cross-section of high tension power (pylon) line.jpg
 |caption1=A four-circuit, two-voltage power transmission line; Bundled 2-ways
 |caption2=A typical [[Aluminium-conductor steel-reinforced cable|ACSR]]. The conductor consists of seven strands of steel surrounded by four layers of aluminium.
}}{{Unreferenced section|date=November 2022}}
High-voltage overhead conductors are not covered by insulation. The conductor material is nearly always an [[aluminium]] alloy, formed of several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission, but aluminum is lighter, reduces yields only marginally and costs much less. Overhead conductors are supplied by several companies. Conductor material and shapes are regularly improved to increase capacity.

Conductor sizes range from 12&nbsp;mm<sup>2</sup> (#6 [[American wire gauge]]) to 1,092&nbsp;mm<sup>2</sup> (2,156,000&nbsp;[[circular mil]]s area), with varying resistance and [[current-carrying capacity]]. For large conductors (more than a few centimetres in diameter), much of the current flow is concentrated near the surface due to the [[skin effect]]. The center of the conductor carries little current but contributes weight and cost. Thus, multiple parallel cables (called [[bundle conductor]]s) are used for higher capacity. Bundle conductors are used at high voltages to reduce energy loss caused by [[corona discharge]].

Today, transmission-level voltages are usually considered to be 110&nbsp;kV and above. Lower voltages, such as 66&nbsp;kV and 33&nbsp;kV, are usually considered [[#Subtransmission|subtransmission]] voltages, but are occasionally used on long lines with light loads. Voltages less than 33&nbsp;kV are usually used for [[electricity distribution|distribution]]. Voltages above 765&nbsp;kV are considered [[high voltage#Power lines|extra high voltage]] and require different designs.

Overhead transmission wires depend on air for insulation, requiring that lines maintain minimum clearances. Adverse weather conditions, such as high winds and low temperatures, interrupt transmission. Wind speeds as low as {{convert|23|kn|km/h}} can permit conductors to encroach operating clearances, resulting in a [[Electric arc|flashover]] and loss of supply.<ref>Hans Dieter Betz, Ulrich Schumann, Pierre Laroche (2009). [https://books.google.com/books?id=U6lCL0CIolYC&dq=Spatial+Distribution+and+Frequency+of+Thunderstorms+and+Lightning+in+Australia+wind+gust&pg=PA187 Lightning: Principles, Instruments and Applications.] Springer, pp.&nbsp;202–203. {{ISBN|978-1-4020-9078-3}}. Retrieved on 13&nbsp;May 2009.</ref> Oscillatory motion of the physical line is termed [[conductor gallop|conductor gallop or flutter]] depending on the frequency and amplitude of oscillation.

<gallery>
File:High Voltage Lines in Washington State.tif|A five-hundred kilovolt (500&nbsp;kV) three-phase transmission tower in Washington State, the line is bundled 3-ways
File:String of Electrical Pylons in Webster, Texas.jpg|Three abreast electrical pylons in Webster, Texas
</gallery>