Editing Overhead power line (section)

===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.