Editing Electric power transmission

{{Short description|Bulk movement of electrical energy}}
{{other uses|Electric transmission (disambiguation)}}
{{redirect|Power lines|power lines in general|Overhead power line}}
{{Use mdy dates|date=April 2014}}
{{Multiple issues|{{Globalize|date=August 2022}}
{{Update|date=August 2022}}
{{more footnotes|date=August 2022}}}}
[[File:500kV 3-Phase Transmission Lines.png|thumb|Five-hundred kilovolt (500 kV) [[Three-phase electric power]] Transmission Lines at [[Grand Coulee Dam]]. Four circuits are shown. Two additional circuits are obscured by trees on the far right. The entire 6809&nbsp;MW<ref name="GCPP">{{cite web|title=Grand Coulee Powerplant|url=http://www.usbr.gov/projects/Powerplant.jsp?fac_Name=Grand%20Coulee%20Powerplant|publisher=U.S. Bureau of Reclamation|access-date=March 11, 2015|url-status=dead|archive-url=https://web.archive.org/web/20140429162702/http://www.usbr.gov/projects/Powerplant.jsp?fac_Name=Grand%20Coulee%20Powerplant|archive-date=April 29, 2014}}</ref> nameplate generation capacity of the dam is accommodated by these six circuits.]]
'''Electric power transmission''' is the bulk movement of [[electrical energy]] from a [[power generation|generating]] site, such as a [[power plant]], to an [[electrical substation]]. The interconnected lines that facilitate this movement form a ''transmission network''. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as [[electric power distribution]]. The combined transmission and distribution network is part of [[electricity delivery]], known as the [[electrical grid]].

Efficient long-distance transmission of electric power requires high [[voltages]]. This reduces the losses produced by strong [[Electric current|currents]]. Transmission lines use either [[alternating current]] (AC) or [[direct current]] (DC). The voltage level is changed with [[transformer]]s. The voltage is stepped up for transmission, then reduced for local distribution.

A [[wide area synchronous grid]], known as an ''interconnection'' in North America, directly connects generators delivering AC power with the same relative ''frequency'' to many consumers. North America has four major interconnections: [[Western Interconnection|Western]], [[Eastern Interconnection|Eastern]], [[Quebec Interconnection|Quebec]] and [[Texas Interconnection|Texas]]. [[Synchronous grid of Continental Europe|One grid connects most of continental Europe]].

Historically, transmission and distribution lines were often owned by the same company, but starting in the 1990s, many countries [[Electricity liberalization|liberalized]] the regulation of the [[electricity market]] in ways that led to separate companies handling transmission and distribution.<ref name=femp01>{{cite web|url=https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-13906.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-13906.pdf |archive-date=2022-10-09 |url-status=live|title=A Primer on Electric Utilities, Deregulation, and Restructuring of U.S. Electricity Markets|publisher=[[United States Department of Energy]] [[Federal Energy Management Program]] (FEMP)|date=May 2002|access-date=October 30, 2018}}</ref>

== 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.

== History ==
{{Main|History of electric power transmission}}
[[File:New York utility lines in 1890.jpg|thumb|New York City streets in 1890. Besides telegraph lines, multiple electric lines were required for each class of device requiring different voltages.]]

Commercial electric power was initially transmitted at the same voltage used by lighting and mechanical loads. This restricted the distance between generating plant and loads. In 1882, DC voltage could not easily be increased for long-distance transmission. Different classes of loads (for example, lighting, fixed motors, and traction/railway systems) required different voltages, and so used different generators and circuits.<ref name=hughes>{{cite book |url=https://books.google.com/books?id=g07Q9M4agp4C&q=westinghouse+%22universal+system%22&pg=PA122|pages=119–122|author=Thomas P. Hughes|title=Networks of Power: Electrification in Western Society, 1880–1930|publisher=Johns Hopkins University Press|location=Baltimore|isbn=0-8018-4614-5 |year=1993|author-link=Thomas P. Hughes}}</ref><ref name="guarnieri 7-1">{{Cite journal|last=Guarnieri|first=M.|year=2013|title=The Beginning of Electric Energy Transmission: Part One|journal=IEEE Industrial Electronics Magazine|volume=7|issue=1|pages=57–60|doi=10.1109/MIE.2012.2236484|s2cid=45909123}}</ref>

Thus, generators were sited near their loads, a practice that later became known as [[distributed generation]] using large numbers of small generators.<ref name=ncep1>{{cite web|url=https://www.energy.gov/sites/prod/files/oeprod/DocumentsandMedia/primer.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.energy.gov/sites/prod/files/oeprod/DocumentsandMedia/primer.pdf |archive-date=2022-10-09 |url-status=live|title=Electricity Transmission: A primer|publisher=National Council on Electricity Policy|access-date=September 17, 2019}}</ref>

Transmission of [[alternating current]] (AC) became possible after [[Lucien Gaulard]] and [[John Dixon Gibbs]] built what they called the secondary generator, an early [[transformer]] provided with 1:1 turn ratio and open magnetic circuit, in 1881.

The first long distance AC line was {{convert|34|km|abbr=off}} long, built for the 1884 International Exhibition of Electricity in [[Turin, Italy]]. It was powered by a 2&nbsp;kV, 130&nbsp;Hz [[Siemens & Halske]] alternator and featured several Gaulard transformers with primary windings connected in series, which fed incandescent lamps. The system proved the feasibility of AC electric power transmission over long distances.<ref name="guarnieri 7-1"/>

The first commercial AC distribution system entered service in 1885 in via dei Cerchi, [[Rome, Italy]], for public lighting. It was powered by two Siemens & Halske alternators rated 30&nbsp;hp (22&nbsp;kW), 2 kV at 120&nbsp;Hz and used 19&nbsp;km of cables and 200&nbsp;parallel-connected 2&nbsp;kV to 20&nbsp;V step-down transformers provided with a closed magnetic circuit, one for each lamp. A few months later it was followed by the first British AC system, serving [[Grosvenor Gallery]]. It also featured Siemens alternators and 2.4&nbsp;kV to 100&nbsp;V step-down transformers &ndash; one per user &ndash; with shunt-connected primaries.<ref name="guarnieri 7-2">{{Cite journal|last=Guarnieri|first=M.|year=2013|title=The Beginning of Electric Energy Transmission: Part Two|journal=IEEE Industrial Electronics Magazine|volume=7|issue=2|pages=52–59|doi=10.1109/MIE.2013.2256297|s2cid=42790906}}</ref>

Working to improve what he considered an impractical Gaulard-Gibbs design, electrical engineer [[William Stanley, Jr.]] developed the first practical series AC transformer in 1885.<ref name="edisontechcenter.org">{{cite web|url=http://edisontechcenter.org/GreatBarrington.html|title=Great Barrington Experiment|website=edisontechcenter.org}}</ref> Working with the support of [[George Westinghouse]], in 1886 he demonstrated a transformer-based AC lighting system in [[Great Barrington, Massachusetts]]. It was powered by a steam engine-driven 500&nbsp;V Siemens generator. Voltage was stepped down to 100&nbsp;volts using the Stanley transformer to power incandescent lamps at 23&nbsp;businesses over {{convert|4000|ft|m}}.<ref>{{cite web|url=https://ethw.org/William_Stanley|title=William Stanley – Engineering and Technology History Wiki|website=ethw.org|date=August 8, 2017 }}</ref> This practical demonstration of a transformer and alternating current lighting system led Westinghouse to begin installing AC systems later that year.<ref name="edisontechcenter.org"/>

In 1888 the first designs for an [[AC motor]] appeared. These were [[induction motor]]s running on [[polyphase system|polyphase]] current, independently invented by [[Galileo Ferraris]] and [[Nikola Tesla]]. Westinghouse licensed Tesla's design. Practical [[Three-phase electric power|three-phase]] motors were designed by [[Mikhail Dolivo-Dobrovolsky]] and [[Charles Eugene Lancelot Brown]].<ref name="books.google.com">[[Arnold Heertje]], Mark Perlman [https://books.google.com/books?id=qQMOPjUgWHsC&dq=tesla+motors+sparked+induction+motor&pg=PA138 Evolving Technology and Market Structure: Studies in Schumpeterian Economics], p. 138</ref> Widespread use of such motors were delayed many years by development problems and the scarcity of [[Polyphase system|polyphase power systems]] needed to power them.<ref>Carlson, W. Bernard (2013). ''Tesla: Inventor of the Electrical Age''. Princeton University Press. {{ISBN|1-4008-4655-2}}, p. 130</ref><ref>Jonnes, Jill (2004). ''Empires of Light: Edison, Tesla, Westinghouse, and the Race to Electrify the World''. Random House Trade Paperbacks. {{ISBN|978-0-375-75884-3}}, p. 161.</ref>

[[File:Tesla polyphase AC 500hp generator at 1893 exposition.jpg|thumb|right|Westinghouse alternating current [[polyphase system|polyphase]] generators on display at the 1893 [[World's Columbian Exposition|World's Fair in Chicago]], part of their Tesla Poly-phase System. Such polyphase innovations revolutionized transmission.]]

In the late 1880s and early 1890s smaller electric companies merged into larger corporations such as [[Ganz Works|Ganz]] and [[AEG (German company)|AEG]] in Europe and [[General Electric]] and [[Westinghouse Electric (1886)|Westinghouse Electric]] in the US. These companies developed AC systems, but the technical difference between direct and alternating current systems required a much longer technical merger.<ref name="Thomas Parke Hughes 1930, pages 120-121"/> Alternating current's economies of scale with large generating plants and long-distance transmission slowly added the ability to link all the loads. These included single phase AC systems, poly-phase AC systems, low voltage incandescent lighting, high-voltage arc lighting, and existing DC motors in factories and street cars. In what became a universal system, these technological differences were temporarily bridged via the [[rotary converter]]s and [[motor-generator]]s that allowed the legacy systems to connect to the AC grid.<ref name="Thomas Parke Hughes 1930, pages 120-121">{{cite book|first=Thomas |last=Parke Hughes|title=Networks of Power: Electrification in Western Society, 1880–1930|publisher=JHU Press|year=1993|pages=120–121}}</ref><ref name="Raghu Garud 2009, page 249">{{cite book|first1=Raghu|last1=Garud|first2=Arun|last2=Kumaraswamy|first3= Richard|last3= Langlois|title= Managing in the Modular Age: Architectures, Networks, and Organizations|url=https://archive.org/details/managingmodulara00garu|url-access=limited|publisher= John Wiley & Sons |year=2009| page=[https://archive.org/details/managingmodulara00garu/page/n256 249]|isbn=9781405141949}}</ref> These stopgaps were slowly replaced as older systems were retired or upgraded.

The first transmission of single-phase alternating current using high voltage came in Oregon in 1890 when power was delivered from a hydroelectric plant at [[Willamette Falls]] to the city of [[Portland, Oregon|Portland]] {{convert|14|mi|km}} down river.<ref>{{Cite journal|last=Argersinger|first=R.E.|date=1915|title=Electric Transmission of Power|journal=General Electric Review|volume=XVIII|page=454}}</ref> The first three-phase alternating current using high voltage took place in 1891 during the [[International Electro-Technical Exhibition – 1891|international electricity exhibition]] in [[Frankfurt]]. A 15 kV transmission line, approximately 175&nbsp;km long, connected [[Lauffen, Baden-Württemberg|Lauffen on the Neckar]] and Frankfurt.<ref name="guarnieri 7-2"/><ref>Kiessling F, Nefzger P, Nolasco JF, Kaintzyk U. (2003). ''Overhead power lines''. Springer, Berlin, Heidelberg, New York, p.&nbsp;5</ref>

Transmission voltages increased throughout the 20th&nbsp;century. By 1914, fifty-five transmission systems operating at more than 70 kV were in service. The highest voltage then used was 150&nbsp;kV.<ref>Bureau of Census data reprinted in Hughes, pp.&nbsp;282–283</ref> Interconnecting multiple generating plants over a wide area reduced costs. The most efficient plants could be used to supply varying loads during the day. Reliability was improved and capital costs were reduced, because stand-by generating capacity could be shared over many more customers and a wider area. Remote and low-cost sources of energy, such as [[Hydroelectricity|hydroelectric]] power or mine-mouth coal, could be exploited to further lower costs.<ref name="hughes" /><ref name="guarnieri 7-2"/>

The 20th&nbsp;century's rapid industrialization made electrical transmission lines and grids [[critical infrastructure]]. Interconnection of local generation plants and small distribution networks was spurred by [[World War I]], when large electrical generating plants were built by governments to power munitions factories.<ref>Hughes, pp.&nbsp;293–295</ref>{{clear left}}

== Bulk transmission ==
[[File:Transmissionsubstation.jpg|thumb|A [[Electrical substation|transmission substation]] decreases the voltage of incoming electricity, allowing it to connect from long-distance high-voltage transmission, to local lower voltage distribution. It also reroutes power to other transmission lines that serve local markets. This is the [[PacifiCorp]] Hale Substation, [[Orem, Utah]], US.]]

These networks use components such as power lines, cables, [[circuit breaker]]s, switches and [[transformer]]s. The transmission network is usually administered on a regional basis by an entity such as a [[regional transmission organization]] or [[transmission system operator]].<ref>{{cite web|title=Distribution Substations - Michigan Technological University|url=https://pages.mtu.edu/~avsergue/EET3390/Lectures/CHAPTER6.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://pages.mtu.edu/~avsergue/EET3390/Lectures/CHAPTER6.pdf |archive-date=2022-10-09 |url-status=live|access-date=20 April 2019}}</ref>

Transmission efficiency is improved at higher voltage and lower current. The reduced current reduces heating losses. [[Joule's first law]] states that energy losses are proportional to the square of the current. Thus, reducing the current by a factor of two lowers the energy lost to conductor resistance by a factor of four for any given size of conductor.

The optimum size of a conductor for a given voltage and current can be estimated by Kelvin's law for conductor size, which states that size is optimal when the annual cost of energy wasted in resistance is equal to the annual capital charges of providing the conductor. At times of lower interest rates and low commodity costs, Kelvin's law indicates that thicker wires are optimal. Otherwise, thinner conductors are indicated. Since power lines are designed for long-term use, Kelvin's law is used in conjunction with long-term estimates of the price of copper and aluminum as well as interest rates.

Higher voltage is achieved in AC circuits by using a ''step-up [[transformer]]''. [[High-voltage direct current]] (HVDC) systems require relatively costly conversion equipment that may be economically justified for particular projects such as submarine cables and longer distance high capacity point-to-point transmission. HVDC is necessary for sending energy between unsynchronized grids.

A transmission grid is a network of [[power station]]s, transmission lines, and [[Electrical substation|substations]]. Energy is usually transmitted within a grid with [[Three-phase electric power|three-phase]] [[alternating current|AC]]. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase [[induction motor]]s. In the 19th&nbsp;century, two-phase transmission was used but required either four wires or three wires with unequal currents. Higher order phase systems require more than three wires, but deliver little or no benefit.

[[File:European electricity grid.svg|thumb|The [[wide area synchronous grid|synchronous grids]] of Europe]]

While the price of generating capacity is high, energy demand is variable, making it often cheaper to import needed power than to generate it locally. Because loads often rise and fall together across large areas, power often comes from distant sources. Because of the economic benefits of load sharing, [[wide area synchronous grid|wide area transmission grids]] may span countries and even continents. Interconnections between producers and consumers enables power to flow even if some links are inoperative.

The slowly varying portion of demand is known as the ''[[base load]]'' and is generally served by large facilities with constant operating costs, termed [[firm power]]. Such facilities are nuclear, coal or hydroelectric, while other energy sources such as [[concentrated solar thermal]] and [[geothermal power]] have the potential to provide firm power. Renewable energy sources, such as solar photovoltaics, wind, wave, and tidal, are, due to their intermittency, not considered to be firm. The remaining or ''peak'' power demand, is supplied by [[peaking power plant]]s, which are typically smaller, faster-responding, and higher cost sources, such as [[combined cycle]] or combustion turbine plants typically fueled by natural gas.

Long-distance transmission (hundreds of kilometers) is cheap and efficient, with costs of US$0.005–0.02 per kWh, compared to annual averaged large producer costs of US$0.01–0.025 per kWh, retail rates upwards of US$0.10 per kWh, and multiples of retail for instantaneous suppliers at unpredicted high demand moments.<ref name="limits-of-very-long-distance"/> New York often buys over 1000&nbsp;MW of low-cost hydropower from Canada.<ref>{{cite web|title=NYISO Zone Maps|url=http://www.nyiso.com/public/markets_operations/market_data/maps/index.jsp|publisher=New York Independent System Operator|access-date=10 January 2014|archive-date=December 2, 2018|archive-url=https://web.archive.org/web/20181202015308/http://www.nyiso.com/public/markets_operations/market_data/maps/index.jsp|url-status=dead}}</ref> Local sources (even if more expensive and infrequently used) can protect the power supply from weather and other disasters that can disconnect distant suppliers.

[[File:Electicaltransmissionlines3800ppx.JPG|thumb|A high-power electrical transmission tower, 230&nbsp;kV, double-circuit, also double-bundled]]

Hydro and wind sources cannot be moved closer to big cities, and solar costs are lowest in remote areas where local power needs are nominal. Connection costs can determine whether any particular renewable alternative is economically realistic. Costs can be prohibitive for transmission lines, but high capacity, long distance [[super grid]] transmission network costs could be recovered with modest usage fees.

=== Grid input ===
{{Unreferenced section|date=November 2022}}
At [[power station]]s, power is produced at a relatively low voltage between about 2.3&nbsp;kV and 30&nbsp;kV, depending on the size of the unit. The voltage is then stepped up by the power station [[transformer]] to a higher voltage (115&nbsp;kV to 765&nbsp;kV AC) for transmission.

In the United States, power transmission is, variously, 230&nbsp;kV to 500&nbsp;kV, with less than 230&nbsp;kV or more than 500&nbsp;kV as exceptions.

The [[Western Interconnection]] has two primary interchange voltages: 500&nbsp;kV AC at 60&nbsp;Hz, and ±500&nbsp;kV (1,000&nbsp;kV net) DC from North to South ([[Columbia River]] to [[Southern California]]) and Northeast to Southwest (Utah to Southern California). The 287.5&nbsp;kV ([[Hoover Dam]] to [[Los Angeles]] line, via [[Victorville]]) and 345&nbsp;kV ([[Arizona Public Service]] (APS) line) are local standards, both of which were implemented before 500&nbsp;kV became practical.

=== Losses ===
Transmitting electricity at high voltage reduces the fraction of energy lost to [[Joule heating]], which varies by conductor type, the current, and the transmission distance. For example, a {{convert|100|mile|abbr=on}} span at 765&nbsp;kV carrying 1000&nbsp;MW of power can have losses of 0.5% to 1.1%. A 345&nbsp;kV line carrying the same load across the same distance has losses of 4.2%.<ref>{{cite web|website=American Electric Power|title=Transmission Facts, p.&nbsp;4| url=https://www.aep.com/about/transmission/docs/transmission-facts.pdf |archiveurl=https://web.archive.org/web/20110604181007/https://www.aep.com/about/transmission/docs/transmission-facts.pdf|archivedate=2011-06-04}}</ref> For a given amount of power, a higher voltage reduces the current and thus the [[resistive loss]]es. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the <math>I^2 R</math> losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size (cross-sectional area) is decreased ten-fold to match the lower current, the <math>I^2 R</math> losses are still reduced ten-fold using the higher voltage.

While power loss can also be reduced by increasing the wire's [[Electrical Conductance|conductance]] (by increasing its cross-sectional area), larger conductors are heavier and more expensive. And since conductance is proportional to cross-sectional area, resistive power loss is only reduced proportionally with increasing cross-sectional area, providing a much smaller benefit than the squared reduction provided by multiplying the voltage.

Long-distance transmission is typically done with overhead lines at voltages of 115 to 1,200&nbsp;kV. At higher voltages, where more than 2,000&nbsp;kV exists between conductor and ground, [[corona discharge]] losses are so large that they can offset the lower resistive losses in the line conductors. Measures to reduce corona losses include larger conductor diameter, hollow cores<ref>[http://www.cpuc.ca.gov/environment/info/aspen/deltasub/pea/16_corona_and_induced_currents.pdf California Public Utilities Commission] Corona and induced currents</ref> or conductor bundles.

Factors that affect resistance and thus loss include temperature, spiraling, and the [[skin effect]]. Resistance increases with temperature. Spiraling, which refers to the way stranded conductors spiral about the center, also contributes to increases in conductor resistance. The skin effect causes the effective resistance to increase at higher AC frequencies. Corona and resistive losses can be estimated using a mathematical model.<ref>{{cite web |title=AC Transmission Line Losses |author=Curt Harting |date=October 24, 2010 |publisher=[[Stanford University]] |url=http://large.stanford.edu/courses/2010/ph240/harting1/ |access-date=June 10, 2019}}</ref>

US transmission and distribution losses were estimated at 6.6% in 1997,<ref name="tonto.eia.doe.gov">{{cite web |url=http://tonto.eia.doe.gov/tools/faqs/faq.cfm?id=105&t=3 |archive-url=https://archive.today/20121212061118/http://tonto.eia.doe.gov/tools/faqs/faq.cfm?id=105&t=3 |url-status=dead |archive-date=12 December 2012 |title=Where can I find data on electricity transmission and distribution losses? |date=19 November 2009 |work=Frequently Asked Questions – Electricity |publisher=[[U.S. Energy Information Administration]] |access-date=29 March 2011 }}</ref> 6.5% in 2007<ref name="tonto.eia.doe.gov"/> and 5% from 2013 to 2019.<ref name="eia.gov">{{cite web |url=https://www.eia.gov/tools/faqs/faq.php?id=105&t=3|title=How much electricity is lost in electricity transmission and distribution in the United States? |date=9 January 2019 |work=Frequently Asked Questions – Electricity |publisher=[[U.S. Energy Information Administration]] |access-date=27 February 2019}}</ref> In general, losses are estimated from the discrepancy between power produced (as reported by power plants) and power sold; the difference constitutes transmission and distribution losses, assuming no utility theft occurs.

As of 1980, the longest cost-effective distance for DC transmission was {{convert|7000|km|mi|abbr=off}}. For AC it was {{convert|4000|km|mi|abbr=off}}, though US transmission lines are substantially shorter.<ref name="limits-of-very-long-distance">{{cite web |url=http://www.geni.org/globalenergy/library/technical-articles/transmission/cigre/present-limits-of-very-long-distance-transmission-systems/index.shtml |title=Present Limits of Very Long Distance Transmission Systems | first1 = L. | last1 = Paris | first2 = G. | last2 = Zini | first3 = M. | last3 = Valtorta | first4 = G. | last4 = Manzoni | first5 = A. | last5 = Invernizzi | first6 = N. | last6 = De Franco | first7 = A. | last7 = Vian |year=1984 |work=[[CIGRE]] International Conference on Large High Voltage Electric Systems, 1984 Session, 29&nbsp;August &ndash; 6&nbsp;September |publisher=Global Energy Network Institute |access-date=29 March 2011 |format=PDF}} 4.98&nbsp;MB</ref>

In any AC line, conductor [[inductance]] and [[capacitance]] can be significant. Currents that flow solely in reaction to these properties, (which together with the [[Electrical resistance|resistance]] define the [[electrical impedance|impedance]]) constitute [[reactive power]] flow, which transmits no power to the load. These reactive currents, however, cause extra heating losses. The ratio of real power transmitted to the load to apparent power (the product of a circuit's voltage and current, without reference to phase angle) is the [[power factor]]. As reactive current increases, reactive power increases and power factor decreases.

For transmission systems with low power factor, losses are higher than for systems with high power factor. Utilities add capacitor banks, reactors and other components (such as [[phase-shifter]]s; [[static VAR compensator]]s; and [[flexible AC transmission system]]s, FACTS) throughout the system help to compensate for the reactive power flow, reduce the losses in power transmission and stabilize system voltages. These measures are collectively called 'reactive support'.

=== Transposition ===
{{Unreferenced section|date=November 2022}}
Current flowing through transmission lines induces a [[magnetic field]] that surrounds the lines of each phase and affects the [[inductance]] of the surrounding conductors of other phases. The conductors' mutual inductance is partially dependent on the physical orientation of the lines with respect to each other. Three-phase lines are conventionally strung with phases separated vertically. The mutual inductance seen by a conductor of the phase in the middle of the other two phases is different from the inductance seen on the top/bottom.

Unbalanced inductance among the three conductors is problematic because it may force the middle line to carry a disproportionate amount of the total power transmitted. Similarly, an unbalanced load may occur if one line is consistently closest to the ground and operates at a lower impedance. Because of this phenomenon, conductors must be periodically transposed along the line so that each phase sees equal time in each relative position to balance out the mutual inductance seen by all three phases. To accomplish this, line position is swapped at specially designed [[transposition tower]]s at regular intervals along the line using various [[Transposition (telecommunications)|transposition schemes]].

=== Subtransmission ===
[[File:Cavite, Batangas jf0557 11.jpg|thumb|175px|A 115&nbsp;kV subtransmission line in the [[Philippines]], along with 20&nbsp;kV [[Electric power distribution|distribution]] lines and a [[street light]], all mounted on a wood [[Utility pole|subtransmission pole]]]]
[[File:Wood Pole Structure.JPG|thumb|173px|115 kV H-frame transmission tower]]{{Unreferenced section|date=November 2022}}
Subtransmission runs at relatively lower voltages. It is uneconomical to connect all [[electrical substation|distribution substation]]s to the high main transmission voltage, because that equipment is larger and more expensive. Typically, only larger substations connect with this high voltage. Voltage is stepped down before the current is sent to smaller substations. Subtransmission circuits are usually arranged in loops so that a single line failure does not stop service to many customers for more than a short time.

Loops can be ''normally closed'', where loss of one circuit should result in no interruption, or ''normally open'' where substations can switch to a backup supply. While subtransmission circuits are usually carried on [[Overhead power line|overhead lines]], in urban areas buried cable may be used. The lower-voltage subtransmission lines use less right-of-way and simpler structures; undergrounding is less difficult.

No fixed cutoff separates subtransmission and transmission, or subtransmission and [[Electric power distribution|distribution]]. Their voltage ranges overlap. Voltages of 69&nbsp;kV, 115&nbsp;kV, and 138&nbsp;kV are often used for subtransmission in North America. As power systems evolved, voltages formerly used for transmission were used for subtransmission, and subtransmission voltages became distribution voltages. Like transmission, subtransmission moves relatively large amounts of power, and like distribution, subtransmission covers an area instead of just point-to-point.<ref>Donald G. Fink and H. Wayne Beaty. (2007), ''Standard Handbook for Electrical Engineers'' (15th&nbsp;ed.). McGraw-Hill. {{ISBN|978-0-07-144146-9}} section&nbsp;18.5</ref>

=== Transmission grid exit ===
[[electrical substation|Substation]] transformers reduce the voltage to a lower level for [[Electric power distribution|distribution]] to customers. This distribution is accomplished with a combination of sub-transmission (33 to 138&nbsp;kV) and distribution (3.3 to 25&nbsp;kV). Finally, at the point of use, the energy is transformed to end-user voltage (100 to 4160 volts).

== Advantage of high-voltage transmission ==
{{See also|Ideal transformer}}{{Unreferenced section|date=November 2022}}
High-voltage power transmission allows for lesser resistive losses over long distances. This efficiency delivers a larger proportion of the generated power to the loads.

[[File:Power split two resistances.svg|thumb|Electrical grid without a transformer]]
[[File:Transformer power split.svg|thumb|Electrical grid with a transformer]]
In a simplified model, the grid delivers electricity from an [[ideal voltage source]] with voltage <math>V</math>, delivering a power <math>P_V</math>) to a single point of consumption, modelled by a resistance <math>R</math>, when the wires are long enough to have a significant resistance <math>R_C</math>.

If the resistances are [[in series]] with no intervening transformer, the circuit acts as a [[voltage divider]], because the same current <math>I=\frac{V}{R+R_C}</math> runs through the wire resistance and the powered device. As a consequence, the useful power (at the point of consumption) is:
:<math>P_R= V_2\times I = V\frac{R}{R+R_C}\times\frac{V}{R+R_C} = \frac{R}{R+R_C}\times\frac{V^2}{R+R_C} = \frac{R}{R+R_C} P_V</math>
Should an [[ideal transformer]] convert high-voltage, low-current electricity into low-voltage, high-current electricity with a voltage ratio of <math>a</math> (i.e., the voltage is divided by <math>a</math> and the current is multiplied by <math>a</math> in the secondary branch, compared to the primary branch), then the circuit is again equivalent to a voltage divider, but the wires now have apparent resistance of only <math>R_C/a^2</math>. The useful power is then:
:<math>P_R= V_2\times I_2 = \frac{a^2R\times V^2}{(a^2 R+R_C)^2} = \frac{a^2 R}{a^2 R+R_C} P_V = \frac{R}{R+R_C/a^2} P_V</math>

For <math>a>1</math> (i.e. conversion of high voltage to low voltage near the consumption point), a larger fraction of the generator's power is transmitted to the consumption point and a lesser fraction is lost to [[Joule heating]].

== Modeling ==
{{Main|Performance and modelling of AC transmission}}{{Unreferenced section|date=November 2022}}[[File:Transmission Line Black Box.JPG|thumb|upright=1.6|"Black box" model for transmission line]]The terminal characteristics of the transmission line are the voltage and current at the sending (S) and receiving (R) ends. The transmission line can be modeled as a [[black box]] and a 2&nbsp;by&nbsp;2 transmission matrix is used to model its behavior, as follows:

:<math>
\begin{bmatrix}
	V_\mathrm{S}\\
	I_\mathrm{S}\\
\end{bmatrix}
=
\begin{bmatrix}
	A & B\\
	C & D\\
\end{bmatrix}
\begin{bmatrix}
	V_\mathrm{R}\\
	I_\mathrm{R}\\
\end{bmatrix}
</math>

The line is assumed to be a reciprocal, symmetrical network, meaning that the receiving and sending labels can be switched with no consequence. The transmission matrix '''T''' has the properties:
* <math>\det(T) = AD - BC = 1</math>
* <math>A = D</math>

The parameters ''A'', ''B'', ''C'', and ''D'' differ depending on how the desired model handles the line's [[Electrical resistance and conductance|resistance]] (''R''), [[inductance]] (''L''), [[capacitance]] (''C''), and shunt (parallel, leak) [[Electrical conductance|conductance]] ''G''.

The four main models are the short line approximation, the medium line approximation, the long line approximation (with distributed parameters), and the lossless line. In such models, a capital letter such as ''R'' refers to the total quantity summed over the line and a lowercase letter such as ''c'' refers to the per-unit-length quantity.

===Lossless line===
The lossless line approximation is the least accurate; it is typically used on short lines where the inductance is much greater than the resistance. For this approximation, the voltage and current are identical at the sending and receiving ends.
[[File:Losslessline.jpg|thumb|Voltage on sending and receiving ends for lossless line]]
The characteristic impedance is pure real, which means resistive for that impedance, and it is often called surge impedance. When a lossless line is terminated by surge impedance, the voltage does not drop. Though the phase angles of voltage and current are rotated, the magnitudes of voltage and current remain constant along the line. For load&nbsp;>&nbsp;SIL, the voltage drops from sending end and the line ''consumes'' VARs. For load&nbsp;<&nbsp;SIL, the voltage increases from the sending end, and the line ''generates'' VARs.

===Short line===
The short line approximation is normally used for lines shorter than {{cvt|80|km}}. There, only a series impedance ''Z'' is considered, while ''C'' and ''G'' are ignored. The final result is that A&nbsp;=&nbsp;D&nbsp;=&nbsp;1 per unit, B&nbsp;=&nbsp;Z&nbsp;Ohms, and C&nbsp;=&nbsp;0. The associated transition matrix for this approximation is therefore:
:<math>
\begin{bmatrix}
	V_\mathrm{S}\\
	I_\mathrm{S}\\
\end{bmatrix}
=
\begin{bmatrix}
	1 & Z\\
	0 & 1\\
\end{bmatrix}
\begin{bmatrix}
	V_\mathrm{R}\\
	I_\mathrm{R}\\
\end{bmatrix}
</math>

===Medium line===
The medium line approximation is used for lines running between {{cvt|80 and 250|km}}. The series impedance and the shunt (current leak) conductance are considered, placing half of the shunt conductance at each end of the line. This circuit is often referred to as a ''nominal [[Pi (letter)|''π'' (pi)]]'' circuit because of the shape (''π'') that is taken on when leak conductance is placed on both sides of the circuit diagram. The analysis of the medium line produces:

:<math>
\begin{align}
A &= D = 1 + \frac{G Z}{2} \text{ per unit}\\
B &= Z\Omega\\
C &= G \Big( 1 + \frac{G Z}{4}\Big)S
\end{align}
</math>

Counterintuitive behaviors of medium-length transmission lines:

* voltage rise at no load or small current ([[Ferranti effect]])
* receiving-end current can exceed sending-end current

===Long line===
The long line model is used when a higher degree of accuracy is needed or when the line under consideration is more than {{cvt|250|km}} long. Series resistance and shunt conductance are considered to be distributed parameters, such that each differential length of the line has a corresponding differential series impedance and shunt admittance. The following result can be applied at any point along the transmission line, where <math>\gamma</math> is the [[propagation constant]].
:<math>
\begin{align}
A &= D = \cosh(\gamma x) \text{ per unit}\\[3mm]
B &= Z_c \sinh(\gamma x) \Omega\\[2mm]
C &= \frac{1}{Z_c} \sinh(\gamma x) S
\end{align}
</math>

To find the voltage and current at the end of the long line, <math>x</math> should be replaced with <math>l</math> (the line length) in all parameters of the transmission matrix. This model applies the [[Telegrapher's equations]].

== High-voltage direct current ==
{{Main|High-voltage direct current}}

High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead. For a long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the cost of the required converter stations at each end.

HVDC is used for long [[Submarine power cable|submarine cables]] where AC cannot be used because of cable capacitance.<ref>Donald G. Fink, H. Wayne Beatty, ''Standard Handbook for Electrical Engineers 11th Edition'', McGraw Hill, 1978, {{ISBN|0-07-020974-X}}, pages 15–57 and 15–58</ref> In these cases special [[high-voltage cable]]s are used. Submarine HVDC systems are often used to interconnect the electricity grids of islands, for example, between [[Great Britain]] and [[continental Europe]], between Great Britain and Ireland, between [[Tasmania]] and the Australian mainland, between the North and South Islands of New Zealand, between [[New Jersey]] and [[New York City]], and between New Jersey and [[Long Island]]. Submarine connections up to {{convert|600|km}} in length have been deployed.<ref name="guarnieri 7-3">{{Cite journal|last=Guarnieri|first=M.|year=2013|title=The Alternating Evolution of DC Power Transmission|journal=IEEE Industrial Electronics Magazine|volume=7|issue=3|pages=60–63|doi=10.1109/MIE.2013.2272238|s2cid=23610440}}</ref>

HVDC links can be used to control grid problems. The power transmitted by an AC line increases as the [[Electric power#Alternating current|phase angle]] between source end voltage and destination ends increases, but too large a phase angle allows the systems at either end to fall out of step. Since the power flow in a DC link is controlled independently of the phases of the AC networks that it connects, this phase angle limit does not exist, and a DC link is always able to transfer its full rated power. A DC link therefore stabilizes the AC grid at either end, since power flow and phase angle can then be controlled independently.

As an example, to adjust the flow of AC power on a hypothetical line between [[Seattle]] and [[Boston]] would require adjustment of the relative phase of the two regional electrical grids. This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the grid. With an HVDC line instead, such an interconnection would:
* Convert AC in Seattle into HVDC;
* Use HVDC for the {{convert|3000|mi|km}} of cross-country transmission; and
* Convert the HVDC to locally synchronized AC in Boston,
(and possibly in other cooperating cities along the transmission route). Such a system could be less prone to failure if parts of it were suddenly shut down. One example of a long DC transmission line is the [[Pacific DC Intertie]] located in the Western United States.

== Capacity ==
<!-- Linked from wind power. -->{{Unreferenced section|date=November 2022}}

The amount of power that can be sent over a transmission line varies with the length of the line. The heating of short line conductors due to line losses sets a thermal limit. If too much current is drawn, conductors may sag too close to the ground, or conductors and equipment may overheat. For intermediate-length lines on the order of {{convert|100|km|mi|abbr=off}}, the limit is set by the [[voltage drop]] in the line. For longer AC lines, [[Power system stability|system stability]] becomes the limiting factor. Approximately, the power flowing over an AC line is proportional to the cosine of the phase angle of the voltage and current at the ends.

This angle varies depending on system loading. It is undesirable for the angle to approach 90 degrees, as the power flowing decreases while resistive losses remain. The product of line length and maximum load is approximately proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. HVDC lines are restricted only by thermal and voltage drop limits, since the phase angle is not material.

Understanding the temperature distribution along the cable route became possible with the introduction of [[distributed temperature sensing]] (DTS) systems that measure temperatures all along the cable. Without them maximum current was typically set as a compromise between understanding of operation conditions and risk minimization. This monitoring solution uses passive [[optical fibers]] as temperature sensors, either inside a high-voltage cable or externally mounted on the cable insulation.

For overhead cables the fiber is integrated into the core of a phase wire. The integrated Dynamic Cable Rating (DCR)/Real Time Thermal Rating (RTTR) solution makes it possible to run the network to its maximum. It allows the operator to predict the behavior of the transmission system to reflect major changes to its initial operating conditions.

=== Reconductoring ===
Some utilities have embraced reconductoring to handle the increase in electricity production. Reconductoring is the replacement-in-place of existing transmission lines with higher-capacity lines. Adding transmission lines is difficult due to cost, permit intervals, and local opposition. Reconductoring has the potential to double the amount of electricity that can travel across a transmission line.<ref name="Pontecorvo">{{Cite web |last=Pontecorvo |first=Emily |date=February 20, 2024 |title=There Is a Stupidly Easy Way To Expand the Grid - Heatmap News |url=https://heatmap.news/climate/clean-energy-grid-reconductoring |access-date=2024-03-06 |website=heatmap.news |language=en}}</ref> 
A 2024 report found the United States behind countries like Belgium and the Netherlands in adoption of this technique to accommodate electrification and renewable energy. <ref name="NYT-AR">{{cite news |url=https://www.nytimes.com/2024/04/09/climate/electric-grid-more-power.html |title=The U.S. Urgently Needs a Bigger Grid. Here's a Fast Solution. |newspaper=[[The New York Times]] |author=Brad Plumer |date=April 14, 2024}}</ref> In April 2022, the [[Biden Administration]] streamlined environmental reviews for such projects, and in May 2022 announced competitive grants for them funded by the 2021 [[Bipartisan Infrastructure Law]] and 2022 [[Inflation Reduction Act]].<ref>{{cite web |url=https://bidenwhitehouse.archives.gov/briefing-room/statements-releases/2024/05/28/fact-sheet-biden-harris-administration-launches-federal-state-initiative-to-bolster-americas-power-grid/ |date=May 28, 2024 |title=FACT SHEET: Biden-⁠Harris Administration Launches Federal-State Initiative to Bolster America's Power Grid |publisher=The [[White House]]}}</ref>

The rate of transmission expansion needs to double to support ongoing electrification and reach emission reduction targets. As of 2022, more than 10,000 power plant and energy storage projects were awaiting permission to connect to the US grid — 95% were zero-carbon resources. New power lines can take 10 years to plan, permit, and build.<ref name="Pontecorvo" />

Traditional power lines use a steel core surrounded by aluminum strands ([[Aluminium-conductor steel-reinforced cable]]). Replacing the steel with a lighter, stronger composite material such as [[carbon fiber]] ([[ACCC conductor]]) allows lines to operate at higher temperatures, with less sag, and doubled transmission capacity. Lowering line sag at high temperatures can prevent wildfires from starting when power lines touch dry vegetation.<ref name="NYT-AR" /> Although advanced lines can cost 2-4x more than steel, total reconductoring costs are less than half of a new line, given savings in time, land acquisition, permitting, and construction.<ref name="Pontecorvo" />

A reconductoring project in southeastern Texas upgraded 240 miles of transmission lines at a cost of $900,000 per mile, versus a 3,600-mile greenfield project that averaged $1.9 million per mile.<ref name="Pontecorvo" />

== Control ==
{{Unreferenced section|date=November 2022}}
To ensure safe and predictable operation, system components are controlled with generators, switches, circuit breakers and loads. The voltage, power, frequency, load factor, and reliability capabilities of the transmission system are designed to provide cost effective performance.

=== Load balancing ===
The transmission system provides for base load and [[peaking power plant|peak load capability]], with margins for safety and fault tolerance. Peak load times vary by region largely due to the industry mix. In hot and cold climates home air conditioning and heating loads affect the overall load. They are typically highest in the late afternoon in the hottest part of the year and in mid-mornings and mid-evenings in the coldest part of the year. Power requirements vary by season and time of day. Distribution system designs always take the base load and the peak load into consideration.

The transmission system usually does not have a large buffering capability to match loads with generation. Thus generation has to be kept matched to the load, to prevent overloading generation equipment.

Multiple sources and loads can be connected to the transmission system and they must be controlled to provide orderly transfer of power. In centralized power generation, only local control of generation is necessary. This involves [[alternator synchronization|synchronization of the generation units]].

In [[distributed generation|distributed power generation]] the generators are geographically distributed and the process to bring them online and offline must be carefully controlled. The load control signals can either be sent on separate lines or on the power lines themselves. Voltage and frequency can be used as signaling mechanisms to balance the loads.

In voltage signaling, voltage is varied to increase generation. The power added by any system increases as the line voltage decreases. This arrangement is stable in principle. Voltage-based regulation is complex to use in mesh networks, since the individual components and setpoints would need to be reconfigured every time a new generator is added to the mesh.

In frequency signaling, the generating units match the frequency of the power transmission system. In [[droop speed control]], if the frequency decreases, the power is increased. (The drop in line frequency is an indication that the increased load is causing the generators to slow down.)

[[Wind turbine]]s, [[vehicle-to-grid]], [[Virtual power plant|virtual power plants]], and other locally distributed storage and generation systems can interact with the grid to improve system operation. Internationally{{Where|date=October 2024}}, a slow move from a centralized to decentralized power systems have taken place. The main draw of locally distributed generation systems is that they reduce transmission losses by leading to consumption of electricity closer to where it was produced.<ref>{{cite web | url = https://www.en-powered.com/blog/the-bumpy-road-to-energy-deregulation | title = The Bumpy Road to Energy Deregulation | publisher = EnPowered | date = 2016-03-28 | access-date = April 6, 2017 | archive-date = April 7, 2017 | archive-url = https://web.archive.org/web/20170407145323/https://www.en-powered.com/blog/the-bumpy-road-to-energy-deregulation | url-status = dead }}</ref>

=== Failure protection ===
Under excess load conditions, the system can be designed to fail incrementally rather than all at once. [[Brownout (electricity)|Brownouts]] occur when power supplied drops below the demand. [[Power outage|Blackouts]] occur when the grid fails completely.

[[Rolling blackout]]s (also called load shedding) are intentionally engineered electrical power outages, used to distribute insufficient power to various loads in turn.

== Communications ==
{{Unreferenced section|date=November 2022}}
Grid operators require reliable communications to manage the grid and associated generation and distribution facilities. Fault-sensing [[protective relay]]s at each end of the line must communicate to monitor the flow of power so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored. Protection of the transmission line from [[short circuit]]s and other faults is usually so critical that [[common carrier]] telecommunications are insufficiently reliable, while in some remote areas no common carrier is available. Communication systems associated with a transmission project may use:
* [[Microwave]]s
* [[Power-line communication]]
* [[Optical fiber]]s
Rarely, and for short distances, pilot-wires are strung along the transmission line path. Leased circuits from common carriers are not preferred since availability is not under control of the operator.

Transmission lines can be used to carry data: this is called power-line carrier, or [[power-line communication]] (PLC). PLC signals can be easily received with a radio in the long wave range.
[[File:High Voltage Pylons carrying additional fibre cable in Kenya.jpg|thumb|High-voltage pylons carrying additional optical fibre cable in Kenya]]
Optical fibers can be included in the stranded conductors of a transmission line, in the overhead shield wires. These cables are known as [[optical ground wire]] (''OPGW''). Sometimes a standalone cable is used, all-dielectric self-supporting (''ADSS'') cable, attached to the transmission line cross arms.

Some jurisdictions, such as [[Minnesota]], prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications [[common carrier]]. Where the regulatory structure permits, the utility can sell capacity in extra [[dark fiber]]s to a common carrier.

== Market structure ==
{{Main|Electricity market}}

Electricity transmission is generally considered to be a [[natural monopoly]], but one that is not inherently linked to generation.<ref>{{cite book | title = Handbook on Electricity Markets | first1 = Richard | last1 = Schmalensee | chapter = Strengths and weaknesses of traditional arrangements for electricity supply | page = 16 | date =  2021 | publisher = Edward Elgar Publishing | doi = 10.4337/9781788979955.00008 | isbn = 9781788979955 | s2cid = 244796440 | chapter-url = https://www.elgaronline.com/view/edcoll/9781788979948/9781788979948.00008.xml}}</ref><ref>{{cite web | url = http://www.thehindubusinessline.com/iw/2004/08/15/stories/2004081501201300.htm | title = Power transmission business is a natural monopoly | author = Raghuvir Srinivasan | publisher = The Hindu | work = The Hindu Business Line | date = August 15, 2004 | access-date = January 31, 2008}}</ref><ref>{{cite web | url = http://www.reason.org/commentaries/kiesling_20030818b.shtml | title = Rethink the Natural Monopoly Justification of Electricity Regulation | author = Lynne Kiesling | publisher = Reason Foundation | date = 18 August 2003 | access-date = 31 January 2008 | archive-url = https://web.archive.org/web/20080213034400/http://www.reason.org/commentaries/kiesling_20030818b.shtml | archive-date = February 13, 2008 | url-status = dead }}</ref> Many countries regulate transmission separately from generation.

Spain was the first country to establish a [[regional transmission organization]]. In that country, transmission operations and electricity markets are separate. The transmission system operator is [[Red Eléctrica de España]] (REE) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía – Polo Español, S.A. (OMEL) [https://web.archive.org/web/20040906064835/http://www.omel.es/ OMEL Holding | Omel Holding]. Spain's transmission system is interconnected with those of France, Portugal, and Morocco.

The establishment of RTOs in the United States was spurred by the [[FERC]]'s Order 888, ''Promoting Wholesale Competition Through Open Access Non-discriminatory Transmission Services by Public Utilities; Recovery of Stranded Costs by Public Utilities and Transmitting Utilities'', issued in 1996.<ref>{{cite web|url=https://www.ferc.gov/legal/maj-ord-reg/land-docs/order888.asp|title=FERC: Landmark Orders – Order No. 888|website=www.ferc.gov|access-date=December 7, 2016|archive-url=https://web.archive.org/web/20161219014712/https://www.ferc.gov/legal/maj-ord-reg/land-docs/order888.asp|archive-date=December 19, 2016|url-status=dead}}</ref> In the United States and parts of Canada, electric transmission companies operate independently of generation companies, but in the Southern United States vertical integration is intact. In regions of separation, transmission owners and generation owners continue to interact with each other as market participants with voting rights within their RTO. RTOs in the United States are regulated by the [[Federal Energy Regulatory Commission]].

Merchant transmission projects in the United States include the [[Cross Sound Cable]] from [[Shoreham, New York]] to [[New Haven, Connecticut]], Neptune RTS Transmission Line from [[Sayreville, New Jersey]], to [[New Bridge, New York]], and [[Path 15]] in California. Additional projects are in development or have been proposed throughout the United States, including the [[Lake Erie Connector]], an underwater transmission line proposed by ITC Holdings Corp., connecting Ontario to [[Load serving entity|load serving entities]] in the PJM Interconnection region.<ref>{{cite web |date=8 December 2014 |title=How ITC Holdings plans to connect PJM demand with Ontario's rich renewables |url=http://www.utilitydive.com/news/how-itc-holdings-plans-to-connect-pjm-demand-with-ontarios-rich-renewables/341524/ |website=Utility Dive}}</ref>

Australia has one unregulated or market interconnector – [[Basslink]] – between [[Tasmania]] and [[Victoria (Australia)|Victoria]]. Two DC links originally implemented as market interconnectors, [[Directlink]] and [[Murraylink]], were converted to regulated interconnectors.<ref>{{Cite web |date=2008-07-18 |title=NEMMCO Power System Planning |url=http://www.nemmco.com.au/psplanning/psplanning.html#interconnect |archive-url=https://web.archive.org/web/20080718211829/http://www.nemmco.com.au/psplanning/psplanning.html#interconnect |archive-date=2008-07-18 |access-date=2022-11-14 }}</ref>

A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from the facility so that the beneficiaries pay the toll. Also, it is difficult for a merchant transmission line to compete when the alternative transmission lines are subsidized by utilities with a monopolized and regulated rate base.<ref>{{cite book |author=Fiona Woolf |title=Global Transmission Expansion |date=2003 |publisher=Pennwell Books |isbn=0-87814-862-0 |pages=226, 247}}</ref> In the United States, the [[FERC]]'s Order 1000, issued in 2010, attempted to reduce barriers to third party investment and creation of merchant transmission lines where a public policy need is found.<ref>{{cite web |title=FERC: Industries – Order No. 1000 – Transmission Planning and Cost Allocation |url=https://www.ferc.gov/industries/electric/indus-act/trans-plan.asp |url-status=dead |archive-url=https://web.archive.org/web/20181030205910/https://www.ferc.gov/industries/electric/indus-act/trans-plan.asp |archive-date=October 30, 2018 |access-date=October 30, 2018 |website=www.ferc.gov}}</ref>

== Transmission costs ==
The cost of high voltage transmission is comparatively low, compared to all other costs constituting consumer electricity bills. In the UK, transmission costs are about 0.2&nbsp;p per kWh compared to a delivered domestic price of around 10&nbsp;p per kWh.<ref>[http://www.claverton-energy.com/what-is-the-cost-per-kwh-of-bulk-transmission-national-grid-in-the-uk-note-this-excludes-distribution-costs.html What is the cost per kWh of bulk transmission] / National Grid in the UK (note this excludes distribution costs)</ref>

The level of capital expenditure in the electric power T&D equipment market was estimated to be $128.9&nbsp;bn in 2011.<ref>{{cite web |url=http://www.visiongain.com/Report/626/The-Electric-Power-Transmission-and-Distribution-(T-D)-Equipment-Market-2011-2021 |title=The Electric Power Transmission & Distribution (T&D) Equipment Market 2011–2021 |access-date=June 4, 2011 |archive-url=https://web.archive.org/web/20110618143614/http://www.visiongain.com/Report/626/The-Electric-Power-Transmission-and-Distribution-(T-D)-Equipment-Market-2011-2021 |archive-date=June 18, 2011 |url-status=dead }}</ref>

== Health concerns ==
{{Main|Electromagnetic radiation and health}}

Mainstream scientific evidence suggests that low-power, low-frequency, electromagnetic radiation associated with household currents and high transmission power lines does not constitute a short- or long-term health hazard.

Some studies failed to find any link between living near power lines and developing any sickness or diseases, such as cancer. A 1997 study reported no increased risk of cancer or illness from living near a transmission line.<ref>[http://www.abc.net.au/rn/talks/8.30/helthrpt/stories/s175.htm Power Lines and Cancer] {{Webarchive|url=https://web.archive.org/web/20110417202936/http://www.abc.net.au/rn/talks/8.30/helthrpt/stories/s175.htm |date=April 17, 2011 }}, The Health Report / ABC Science - Broadcast on 7&nbsp;June 1997 (Australian Broadcasting Corporation)</ref> Other studies, however, reported [[statistical correlation]]s between various diseases and living or working near power lines. No adverse health effects have been substantiated for people not living close to power lines.<ref>{{Cite web |date=2007-12-24 |title=WHO {{!}} Electromagnetic fields and public health |url=http://www.who.int/mediacentre/factsheets/fs322/en/ |access-date=2022-11-14 |archive-url=https://web.archive.org/web/20071224020021/http://www.who.int/mediacentre/factsheets/fs322/en/ |archive-date=December 24, 2007 }}</ref>

The [[New York State Public Service Commission]] conducted a study<ref>''Opinion No. 78-13'' (issued June 19, 1978)</ref> to evaluate potential health effects of electric fields. The study measured the electric field strength at the edge of an existing right-of-way on a 765 kV transmission line. The field strength was 1.6&nbsp;kV/m, and became the interim maximum strength standard for new transmission lines in New York State. The opinion also limited the voltage of new transmission lines built in New York to 345&nbsp;kV. On September 11, 1990, after a similar study of magnetic field strengths, the NYSPSC issued their ''Interim Policy Statement on Magnetic Fields''. This policy established a magnetic field standard of 200 mG at the edge of the right-of-way using the winter-normal conductor rating. As a comparison with everyday items, a hair dryer or electric blanket produces a 100 mG – 500&nbsp;mG magnetic field.<ref>{{cite web|url=http://documents.dps.ny.gov/public/Common/ViewDoc.aspx?DocRefId=%7BED95C2A2-2DEA-4FFC-A8DA-CD9C39F5D361%7D|title=EMF Report for the CHPE|pages=1–4|publisher=TRC|date=March 2010|access-date=November 9, 2018}}</ref><ref>{{cite web|url=https://www.transpower.co.nz/sites/default/files/publications/resources/EMF-fact-sheet-3-2009.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.transpower.co.nz/sites/default/files/publications/resources/EMF-fact-sheet-3-2009.pdf |archive-date=2022-10-09 |url-status=live|title=Electric and Magnetic Field Strengths|publisher=Transpower New Zealand Ltd|page=2|access-date=November 9, 2018}}</ref>

Applications for a new transmission line typically include an analysis of electric and magnetic field levels at the edge of rights-of-way. [[Public utilities commission|Public utility commissions]] typically do not comment on health impacts.

Biological effects have been established for [[Acute toxicity|acute]] high level exposure to magnetic fields above 100&nbsp;[[Tesla (unit)|μT]] (1&nbsp;[[Gauss (unit)|G]]) (1,000&nbsp;mG). In a residential setting, one study reported "limited evidence of [[carcinogen]]icity in humans and less than sufficient evidence for carcinogenicity in experimental animals", in particular, childhood leukemia, associated with average exposure to residential power-frequency magnetic field above 0.3&nbsp;μT (3&nbsp;mG) to 0.4&nbsp;μT (4&nbsp;mG). These levels exceed average residential power-frequency magnetic fields in homes, which are about 0.07&nbsp;μT (0.7&nbsp;mG) in Europe and 0.11&nbsp;μT (1.1&nbsp;mG) in North America.<ref name="WHOFactsheet322">{{cite web |url=https://www.who.int/mediacentre/factsheets/fs322/en/index.html|archive-url=https://web.archive.org/web/20070701204347/http://www.who.int/mediacentre/factsheets/fs322/en/index.html|url-status=dead|archive-date=July 1, 2007|title= Electromagnetic fields and public health|access-date=23 January 2008 |date=June 2007|work= Fact sheet No.&nbsp;322|publisher=[[World Health Organization]]}}</ref><ref name="NIEHS">{{cite web|url=http://www.niehs.nih.gov/health/docs/emf-02.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.niehs.nih.gov/health/docs/emf-02.pdf |archive-date=2022-10-09 |url-status=live |title=Electric and Magnetic Fields Associated with the Use of Power |access-date=29 January 2008 |date=June 2002 |publisher=[[National Institute of Environmental Health Sciences]] }}</ref>

The Earth's natural geomagnetic field strength varies over the surface of the planet between 0.035&nbsp;mT and 0.07&nbsp;mT (35&nbsp;μT – 70&nbsp;μT or 350 mG – 700 mG) while the international standard for continuous exposure is set at 40&nbsp;mT (400,000 mG or 400&nbsp;G) for the general public.<ref name="WHOFactsheet322"/>

Tree growth regulators and herbicides may be used in transmission line right of ways,<ref>{{cite web|title=Transmission Vegetation Management NERC Standard FAC-003-2 Technical Reference Page 14/50|url=http://www.nerc.com/docs/standards/sar/FAC-003-2_White_Paper_2009Sept9.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.nerc.com/docs/standards/sar/FAC-003-2_White_Paper_2009Sept9.pdf |archive-date=2022-10-09 |url-status=live|website=nerc.com}}</ref> which may have [[Herbicide#Health and environmental effects|health effects]].

== Specialized transmission ==

=== Grids for railways ===
{{Main|Traction power network}}

In some countries where [[electric locomotive]]s or [[electric multiple unit]]s run on low frequency AC power, separate single phase [[traction power network]]s are operated by the railways. Prime examples are countries such as Austria, Germany and Switzerland that utilize AC technology based on 16&nbsp;<sup>2</sup>''/''<sub>3</sub>&nbsp;Hz. Norway and Sweden also use this frequency but use conversion from the 50&nbsp;Hz public supply; Sweden has a 16&nbsp;<sup>2</sup>''/''<sub>3</sub>&nbsp;Hz traction grid but only for part of the system.

=== Superconducting cables ===
[[High-temperature superconductor]]s (HTS) promise to revolutionize power distribution by providing lossless transmission. The development of superconductors with transition temperatures higher than the boiling point of [[liquid nitrogen]] has made the concept of superconducting power lines commercially feasible, at least for high-load applications.<ref>{{cite journal |doi=10.1109/77.920339 |author=Jacob Oestergaard |journal=IEEE Transactions on Applied Superconductivity |title=Energy losses of superconducting power transmission cables in the grid |year=2001 |volume=11 |issue=1 |page=2375|bibcode=2001ITAS...11.2375O |s2cid=55086502 |display-authors=etal|url=http://orbit.dtu.dk/files/4280307/%C3%B8stergaard.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://orbit.dtu.dk/files/4280307/%C3%B8stergaard.pdf |archive-date=2022-10-09 |url-status=live }}</ref> It has been estimated that waste would be halved using this method, since the necessary refrigeration equipment would consume about half the power saved by the elimination of resistive losses. Companies such as [[Consolidated Edison]] and [[American Superconductor]] began commercial production of such systems in 2007.<ref>{{cite web |last=((New Scientist and Reuters)) |date=22 May 2007 |title=Superconducting power line to shore up New York grid |url=https://www.newscientist.com/article/dn11907-superconducting-power-line-to-shore-up-new-york-grid/ |website=New Scientist}}</ref>

Superconducting cables are particularly suited to high load density areas such as the business district of large cities, where purchase of an [[easement]] for cables is costly.<ref>{{cite web |url=http://www.futureenergies.com/modules.php?name=News&file=article&sid=237 |title=Superconducting cables will be used to supply electricity to consumers |access-date=June 12, 2014 |archive-url=https://web.archive.org/web/20140714161200/http://www.futureenergies.com/modules.php?name=News&file=article&sid=237 |archive-date=July 14, 2014 |url-status=dead }}</ref>

{| class="wikitable sortable"
|+HTS transmission lines<ref>{{cite web |url=https://spectrum.ieee.org/biomedical/imaging/superconductivitys-first-century/3 |title=Superconductivity's First Century |access-date=August 9, 2012 |archive-url=https://web.archive.org/web/20120812011121/https://spectrum.ieee.org/biomedical/imaging/superconductivitys-first-century/3 |archive-date=August 12, 2012 |url-status=dead }}</ref>
|-
! Location !! Length (km) !! Voltage (kV) !! Capacity (GW) !! Date
|-
|Carrollton, Georgia || || || || 2000
|-
|align=left|Albany, New York<ref>{{cite web|url=http://www.superpower-inc.com/content/hts-transmission-cable|title=HTS Transmission Cable|website=www.superpower-inc.com}}</ref>|| 0.35 || 34.5 || 0.048 ||2006
|-
|[[Holbrook Superconductor Project|Holbrook, Long Island]]<ref>{{cite web|url=http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/hightempsuperconductors/|archive-url=https://web.archive.org/web/20120403014711/http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/hightempsuperconductors/|url-status=dead|archive-date=April 3, 2012|title=IBM100 - High-Temperature Superconductors|date=August 10, 2017|website=www-03.ibm.com}}</ref>|| 0.6 || 138 || 0.574 || 2008
|-
|align=left|[[Tres Amigas SuperStation|Tres Amigas]]|| || || 5 || Proposed 2013
|-
|align=left|Manhattan: Project Hydra|| || || || Proposed 2014
|-
|align=left|Essen, Germany<ref>{{cite web|url=https://www.powermag.com/high-temperature-superconductor-technology-stepped-up/|title=High-Temperature Superconductor Technology Stepped Up|first=03/01/2012 &#124; Sonal|last=Patel|date=March 1, 2012|website=POWER Magazine}}</ref><ref>{{cite web|url=https://phys.org/news/2014-05-longest-superconducting-cable-worldwide.html|title=Operation of longest superconducting cable worldwide started|website=phys.org}}</ref>|| 1 || 10 || 0.04 || 2014
|}

=== Single-wire earth return ===
{{Main|Single-wire earth return}}{{Unreferenced section|date=November 2022}}
Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line for supplying single-phase electrical power to remote areas at low cost. It is principally used for [[rural electrification]], but also finds use for larger isolated loads such as water pumps. Single-wire earth return is also used for HVDC over submarine power cables.

=== Wireless power transmission ===
{{Main|Wireless power transfer}}{{Unreferenced section|date=November 2022}}
Both [[Nikola Tesla]] and [[Hidetsugu Yagi]] attempted to devise systems for large scale wireless power transmission in the late 1800s and early 1900s, without commercial success.

In November 2009, LaserMotive won the NASA 2009 Power Beaming Challenge by powering a cable climber 1&nbsp;km vertically using a ground-based laser transmitter. The system produced up to 1&nbsp;kW of power at the receiver end. In August 2010, NASA contracted with private companies to pursue the design of laser power beaming systems to power low earth orbit satellites and to launch rockets using laser power beams.

Wireless power transmission has been studied for transmission of power from [[solar power satellite]]s to the earth. A high power array of [[microwave]] or laser transmitters would beam power to a [[rectenna]]. Major engineering and economic challenges face any solar power satellite project.

== Security ==
{{Globalize|date=March 2013}}

The [[federal government of the United States]] stated that the American power grid was susceptible to [[cyber-warfare]].<ref>{{cite news|url=http://news.bbc.co.uk/2/hi/technology/7990997.stm|title=Spies 'infiltrate US power grid'|date=April 9, 2009|website=BBC News |first=Maggie |last=Shiels}}</ref><ref>{{cite news|url=http://www.cnn.com/2009/TECH/04/08/grid.threat/index.html?iref=newssearch#cnnSTCVideo |title=Hackers reportedly have embedded code in power grid |website=CNN |date=April 9, 2009}}</ref> The [[United States Department of Homeland Security]] works with industry to identify vulnerabilities and to help industry enhance the security of control system networks.<ref>{{cite news|url=https://in.reuters.com/article/cyberattack-usa-idINN0853911920090408|archive-url=https://web.archive.org/web/20160113235120/http://in.reuters.com/article/cyberattack-usa-idINN0853911920090408|url-status=dead|archive-date=January 13, 2016|title=UPDATE 2-US concerned power grid vulnerable to cyber-attack|newspaper=Reuters|date=April 8, 2009
|first1=Steve |last1=Holland |first2=Randall |last2=Mikkelsen}}</ref>

In June 2019, Russia conceded that it was "possible" its [[Electricity sector in Russia|electrical grid]] is under cyber-attack by the [[United States]].<ref>{{cite news |title=US and Russia clash over power grid 'hack attacks |url=https://www.bbc.com/news/technology-48675203 |work=BBC News |date=18 June 2019}}</ref> ''The New York Times'' reported that American hackers from the [[United States Cyber Command]] planted malware potentially capable of disrupting the Russian electrical grid.<ref>{{cite news |title=How Not To Prevent a Cyberwar With Russia |url=https://www.wired.com/story/russia-cyberwar-escalation-power-grid/ |magazine=[[Wired (magazine)|Wired]] |date=18 June 2019 |first=Andy |last=Greenberg}}</ref>

== Records ==
* Highest capacity system: [[Ultra-high-voltage_electricity_transmission_in_China#UHV_circuits_completed_or_under_construction|12 GW Zhundong–Wannan]] (准东-皖南)±1100&nbsp;kV HVDC.<ref>{{cite web|url=https://www.e-fermat.org/files/communication/Li-COMM-ASIAEM2015-2017-Vol21-May-Jun.-017.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.e-fermat.org/files/communication/Li-COMM-ASIAEM2015-2017-Vol21-May-Jun.-017.pdf |archive-date=2022-10-09 |url-status=live|title=Development of UHV Transmission and Insulation Technology in China}}</ref><ref>{{cite web|url=http://www.xj.xinhuanet.com/2019-09/27/c_1125048315.htm|archive-url=https://web.archive.org/web/20190930234443/http://www.xj.xinhuanet.com/2019-09/27/c_1125048315.htm|url-status=dead|archive-date=September 30, 2019|title=准东-皖南±1100千伏特高压直流输电工程竣工投运|website=xj.xinhuanet.com}}</ref>
* Highest transmission voltage (AC):
** planned: 1.20&nbsp;MV (Ultra-High Voltage) on Wardha-Aurangabad line (India), planned to initially operate at 400 kV.<ref>{{cite journal |url=http://tdworld.com/overhead_transmission/powergrid-research-development-201301/ |title=India Steps It Up |journal=Transmission & Distribution World | date=January 2013}}</ref>
** worldwide: 1.15&nbsp;MV (Ultra-High Voltage) on [[Powerline Ekibastuz-Kokshetau|Ekibastuz-Kokshetau line]] ([[Kazakhstan]])
* Largest double-circuit transmission, [[Kita-Iwaki Powerline]] (Japan).
* Highest [[Transmission tower|towers]]: [[Yangtze River Crossing]] (China) (height: {{convert|345|m|ft|0|abbr=on|disp=or}})
* Longest power line: [[Inga-Shaba]] ([[Democratic Republic of Congo]]) (length: {{convert|1700|km|mi|0|disp=or}})
* Longest span of power line: {{convert|5376|m|ft|0|abbr=on}} at [[Ameralik Span]] ([[Greenland]], Denmark)
* Longest submarine cables:
** [[North Sea Link]], (Norway/United Kingdom) – (length of submarine cable: {{convert|720|km|mi|0|disp=or}})
** [[NorNed]], [[North Sea]] (Norway/Netherlands) – (length of submarine cable: {{convert|580|km|mi|0|disp=or}})
** [[Basslink]], [[Bass Strait]], (Australia) – (length of submarine cable: {{convert|290|km|mi|0|disp=or}}, total length: {{convert|370.1|km|mi|0|disp=or}})
** [[Baltic Cable]], [[Baltic Sea]] (Germany/Sweden) – (length of submarine cable: {{convert|238|km|mi|0|disp=or}}, [[High-voltage direct current|HVDC]] length: {{convert|250|km|mi|0|disp=or}}, total length: {{convert|262|km|mi|0|disp=or}})
* Longest underground cables:
** [[Murraylink]], [[Riverland]]/[[Sunraysia]] (Australia) – (length of underground cable: {{convert|170|km|mi|0|disp=or}})

== See also ==
{{Portal|Energy}}
{{div col|colwidth=30em}}
* [[Dynamic demand (electric power)]]
* [[Demand response]]
* [[List of energy storage power plants]]
* [[Traction power network]]
* [[Backfeeding]]
* [[Conductor marking lights]]
* [[Double-circuit transmission line]]
* [[Emtp|Electromagnetic Transients Program]] (EMTP)
* [[Flexible AC transmission system]] (FACTS)
* [[Geomagnetically induced current]], (GIC)
* [[Graphene-clad wire]]
* [[Grid-tied electrical system]]
* [[List of high-voltage underground and submarine cables]]
* [[Load profile]]
* [[National Grid (disambiguation)|National Grid]] (disambiguation)
* [[Power-line communication]]s (PLC)
* [[Power system simulation]]
* [[Radio frequency power transmission]]
* [[Wheeling (electric power transmission)]]
{{div col end}}

==References==
{{reflist}}

==Further reading==
* Grigsby, L. L., et al. ''The Electric Power Engineering Handbook''. US: CRC Press. (2001). {{ISBN|0-8493-8578-4}}
* [[Thomas P. Hughes|Hughes, Thomas P.]], ''Networks of Power: Electrification in Western Society 1880–1930'', The Johns Hopkins University Press, Baltimore 1983 {{ISBN|0-8018-2873-2}}, an excellent overview of development during the first 50 years of commercial electric power
* {{cite book | author=Reilly, Helen | title= Connecting the Country – New Zealand's National Grid 1886–2007| location=Wellington| publisher= Steele Roberts| year=2008| pages = 376 pages | isbn=978-1-877448-40-9}}
* Pansini, Anthony J, E.E., P.E. ''undergrounding electric lines''. US: Hayden Book Co, 1978. {{ISBN|0-8104-0827-9}}
* Westinghouse Electric Corporation, "''Electric power transmission patents; Tesla polyphase system''". (Transmission of power; polyphase system; [[Tesla patents]])

{{Commons category|Electric power transmission}}
{{Wiktionary|grid electricity}}

{{Electricity generation}}
{{Authority control}}

{{DEFAULTSORT:Electric Power Transmission}}
[[Category:Electric power transmission| ]]
[[Category:Electrical engineering]]
[[Category:Monopoly (economics)]]
[[Category:Electrical safety]]