Editing Electric power transmission (section)

== System ==
[[File:Electricity grid simple- North America.svg|thumb|500px|A diagram of an electric power system. The transmission system is in blue.]]

Most North American transmission lines are high-voltage [[Three-phase electric power|three-phase]] AC, although [[single-phase electric power|single phase]] AC is sometimes used in [[railway electrification system]]s. DC technology is used for greater efficiency over longer distances, typically hundreds of miles. [[High-voltage direct current]] (HVDC) technology is also used in [[submarine power cable]]s (typically longer than 30 miles (50&nbsp;km)), and in the interchange of power between grids that are not mutually synchronized. HVDC links stabilize power distribution networks where sudden new loads, or blackouts, in one part of a network might otherwise result in synchronization problems and [[cascading failure]]s.

Electricity is transmitted at [[high voltage]]s to reduce the energy loss due to [[Electrical resistance and conductance|resistance]] that occurs over long distances. Power is usually transmitted through [[overhead power line]]s. [[Underground power transmission]] has a significantly higher installation cost and greater operational limitations, but lowers maintenance costs. Underground transmission is more common in urban areas or environmentally sensitive locations.

Electrical energy must typically be generated at the same rate at which it is consumed. A sophisticated control system is required to ensure that [[power generation]] closely matches demand. If demand exceeds supply, the imbalance can cause generation plant(s) and transmission equipment to automatically disconnect or shut down to prevent damage. In the worst case, this may lead to a cascading series of shutdowns and a major regional [[power outage|blackout]].

The US Northeast faced blackouts in [[Northeast Blackout of 1965|1965]], [[New York City blackout of 1977|1977]], [[Northeast blackout of 2003|2003]], and major blackouts in other US regions in [[1996 Western North America blackouts|1996]] and [[2011 Southwest blackout|2011]]. Electric transmission networks are interconnected into regional, national, and even continent-wide networks to reduce the risk of such a failure by providing multiple [[redundancy (engineering)|redundant]], alternative routes for power to flow should such shutdowns occur. Transmission companies determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure that spare capacity is available in the event of a failure in another part of the network.

=== 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>

=== Underground ===
{{Main|Undergrounding}}

Electric power can be transmitted by [[high-voltage cable|underground power cables]]. Underground cables take up no right-of-way, have lower visibility, and are less affected by weather. However, cables must be insulated. Cable and excavation costs are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair.

In some metropolitan areas, cables are enclosed by metal pipe and insulated with [[dielectric fluid]] (usually an oil) that is either static or circulated via pumps. If an electric fault damages the pipe and leaks dielectric, liquid nitrogen is used to freeze portions of the pipe to enable draining and repair. This extends the repair period and increases costs. The temperature of the pipe and surroundings are monitored throughout the repair period.<ref>{{cite news|url=https://www.nytimes.com/2001/09/16/us/after-attacks-workers-con-edison-crews-improvise-they-rewire-truncated-system.html|title=After the Attacks: The Workers; Con Edison Crews Improvise as They Rewire a Truncated System|first=Neela|last=Banerjee|newspaper=The New York Times |date=September 16, 2001}}</ref><ref>{{cite web|url=http://documents.dps.ny.gov/public/Common/ViewDoc.aspx?DocRefId={5B2369A6-97FC-4613-AD8B-91E23D41AC05} |title=Investigation of the September 2013 Electric Outage of a Portion of Metro-North Railroad's New Haven Line |publisher=documents.dps.ny.gov |date=2014 |access-date=2019-12-29}}</ref><ref>NYSPSC case no. 13-E-0529</ref>

Underground lines are limited by their thermal capacity, which permits less overload or re-rating lines. Long underground AC cables have significant [[capacitance]], which reduces their ability to provide useful power beyond {{convert|50|mi|abbr=off}}. DC cables are not limited in length by their capacitance.