Electric power transmission is the bulk movement of electrical energy from a generating site, such as a

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...

, 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 Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...

. The combined transmission and distribution network is part of

electricity delivery Electricity delivery is the process that starts after generation of electricity in the power station, up to the use by the consumer. The main processes in electricity delivery are, by order: * Transmission * Distribution * Retailing See also * E ...

, known as the electrical grid. Efficient long-distance transmission of electric power requires high

voltages Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to mo ...

. This reduces the losses produced by strong

currents Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

. Transmission lines use either

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

(HVAC) or

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...

(HVDC). The voltage level is changed with

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s. The voltage is stepped up for transmission, then reduced for local distribution. A

wide area synchronous grid A wide area synchronous grid (also called an "interconnection" in North America) is a three-phase electric power grid that has regional scale or greater that operates at a synchronized utility frequency and is electrically tied together during ...

, 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 Western may refer to: Places *Western, Nebraska, a village in the US *Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia *Western Junction, Tasmania, a locality in Australia *Western world, countries that id ...

Eastern Eastern may refer to: Transportation *China Eastern Airlines, a current Chinese airline based in Shanghai *Eastern Air, former name of Zambia Skyways *Eastern Air Lines, a defunct American airline that operated from 1926 to 1991 *Eastern Air Li ...

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...

and

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

. 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 liberalized the regulation of the

electricity market In a broad sense, an electricity market is a system that facilitates the exchange of electricity-related goods and services. During more than a century of evolution of the electric power industry, the economics of the electricity markets had un ...

in ways that led to separate companies handling transmission and distribution.

System

Most North American transmission lines are high-voltage three-phase AC, although

single phase In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads ...

AC is sometimes used in railway electrification systems. DC technology is used for greater efficiency over longer distances (typically hundreds of miles). HVDC technology is also used in submarine power cables (typically longer than 30 miles (50 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 A cascading failure is a failure in a system of interconnected parts in which the failure of one or few parts leads to the failure of other parts, growing progressively as a result of positive feedback. This can occur when a single part fails, in ...

Electricity is transmitted at

high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant sp ...

s to reduce the energy loss that occurs over long distances. Power is usually transmitted through

overhead power line An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-p ...

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 blackout. The US Northeast faced blackouts of 1965,

1977 Events January * January 8 – Three bombs explode in Moscow within 37 minutes, killing seven. The bombings are attributed to an Armenian separatist group. * January 10 – Mount Nyiragongo erupts in eastern Zaire (now the Democrat ...

, 2003, and major blackouts in other US regions in 1996 and 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 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

High-voltage overhead conductors are not covered by insulation. The conductor material is nearly always an

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

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 mm² (#6 American wire gauge) to 750 mm² (1,590,000

circular mil A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch or ). It corresponds to approximately . It is a unit intended for referring to the area of a wire with a circular cross sectio ...

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 conductors) are used for higher capacity. Bundle conductors are used at high voltages to reduce energy loss caused by

corona discharge A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone e ...

. Today, transmission-level voltages are usually 110 kV and above. Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 765 kV are considered

extra high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant spec ...

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 can permit conductors to encroach operating clearances, resulting in a

flashover A flashover is the near-simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated, they undergo thermal decomposition and release flammable gases. Flashover occurs w ...

and loss of supply. Oscillatory motion of the physical line is termed conductor gallop or flutter depending on the frequency and amplitude of oscillation. String of Electrical Pylons in Webster, Texas

String of Electrical Pylons in Webster, Texas

Underground

Electric power can be transmitted by 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. Underground lines are limited by their thermal capacity, which permits less overload or re-rating lines. Long underground AC cables have significant

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...

, which reduces their ability to provide useful power beyond . DC cables are not limited in length by their capacitance.

History

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. Thus, generators were sited near their loads, a practice that later became known as

distributed generation Distributed generation, also distributed energy, on-site generation (OSG), or district/decentralized energy, is electrical generation and storage performed by a variety of small, grid-connected or distribution system-connected devices referred to ...

using large numbers of small generators. Transmission of

(AC) became possible after

Lucien Gaulard Lucien Gaulard (16 July 1850 – 26 November 1888) invented devices for the transmission of alternating current electrical energy. Biography Gaulard was born in Paris, France in 1850. A power transformer developed by Gaulard of France and John ...

and

John Dixon Gibbs John Dixon Gibbs (1834–1912) was a British engineer and financier who, together with Lucien Gaulard, is often credited as the co-inventor of the AC step-down transformer. The transformer was first demonstrated in 1883 at London's Royal Aquarium ...

built what they called the secondary generator, an early

provided with 1:1 turn ratio and open magnetic circuit, in 1881. The first long distance AC line was long, built for the 1884 International Exhibition of Electricity in

Turin, Italy Turin ( , Piedmontese: ; it, Torino ) is a city and an important business and cultural centre in Northern Italy. It is the capital city of Piedmont and of the Metropolitan City of Turin, and was the first Italian capital from 1861 to 1865. T ...

. It was powered by a 2 kV, 130 Hz

Siemens & Halske Siemens & Halske AG (or Siemens-Halske) was a German electrical engineering company that later became part of Siemens. It was founded on 12 October 1847 as ''Telegraphen-Bauanstalt von Siemens & Halske'' by Werner von Siemens and Johann Ge ...

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. 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 hp (22 kW), 2 kV at 120 Hz and used 19 km of cables and 200 parallel-connected 2 kV to 20 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 The Grosvenor Gallery was an art gallery in London founded in 1877 by Sir Coutts Lindsay and his wife Blanche. Its first directors were J. Comyns Carr and Charles Hallé. The gallery proved crucial to the Aesthetic Movement because it prov ...

. It also featured Siemens alternators and 2.4 kV to 100 V step-down transformers – one per user – with shunt-connected primaries. William-Stanley jr

Working to improve what he considered an impractical Gaulard-Gibbs design, electrical engineer

William Stanley, Jr. William Stanley Jr. (November 28, 1858 – May 14, 1916) was an American physicist born in Brooklyn, New York. During his career, he obtained 129 patents covering a variety of electric devices. In 1913, he also patented an all-steel vacuum bottl ...

developed the first practical series AC transformer in 1885. Working with the support of

George Westinghouse George Westinghouse Jr. (October 6, 1846 – March 12, 1914) was an American entrepreneur and engineer based in Pennsylvania who created the railway air brake and was a pioneer of the electrical industry, receiving his first patent at the age ...

, in 1886 he demonstrated a transformer-based AC lighting system in

Great Barrington, Massachusetts Great Barrington is a town in Berkshire County, Massachusetts, United States. It is part of the Pittsfield, Massachusetts, Metropolitan Statistical Area. The population was 7,172 at the 2020 census. Both a summer resort and home to Ski Butternut, ...

. It was powered by a steam engine-driven 500 V Siemens generator. Voltage was stepped down to 100 Volts using the Stanley transformer to power incandescent lamps at 23 businesses over . This practical demonstration of a transformer and alternating current lighting system led Westinghouse to begin installing AC systems later that year. In 1888 the first designs for an

AC motor An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor ...

appeared. These were induction motors running on polyphase current, independently invented by

Galileo Ferraris Galileo Ferraris (31 October 1847 – 7 February 1897) was an Italian university professor, physicist and electrical engineer, one of the pioneers of AC power system and inventor of the induction motor although he never patented his work. Many ...

and

Nikola Tesla Nikola Tesla ( ; ,"Tesla"
''

three-phase motors were designed by

Mikhail Dolivo-Dobrovolsky Mikhail Osipovich Dolivo-Dobrovolsky (russian: Михаи́л О́сипович Доли́во-Доброво́льский; german: Michail von Dolivo-Dobrowolsky or ''Michail Ossipowitsch Doliwo-Dobrowolski''; – ) was a Russian Empire ...

and Charles Eugene Lancelot Brown.

Arnold Heertje Arnold Heertje (19 February 1934 – 4 April 2020) was a Dutch economist and professor at the University of Amsterdam, writer and columnist. He became more generally known for his opposition to the Betuweroute. Life and career Heertje was bor ...

, Mark Perlma
Evolving Technology and Market Structure: Studies in Schumpeterian Economics
page 138 Widespread use of such motors were delayed many years by development problems and the scarcity of polyphase power systems needed to power them. In the late 1880s and early 1890s smaller electric companies merged into larger corporations such as

Ganz The Ganz Works or Ganz ( or , ''Ganz companies'', formerly ''Ganz and Partner Iron Mill and Machine Factory'') was a group of companies operating between 1845 and 1949 in Budapest, Hungary. It was named after Ábrahám Ganz, the founder and th ...

and AEG in Europe and

General Electric General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable en ...

and

Westinghouse Electric The Westinghouse Electric Corporation was an American manufacturing company founded in 1886 by George Westinghouse. It was originally named "Westinghouse Electric & Manufacturing Company" and was renamed "Westinghouse Electric Corporation" in ...

in the US. These companies developed AC systems, but the technical difference between direct and alternating current systems required a much longer technical merger. 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 A rotary converter is a type of electrical machine which acts as a mechanical rectifier, inverter or frequency converter. Rotary converters were used to convert alternating current (AC) to direct current (DC), or DC to AC power, before the adv ...

s and motor-generators that allowed the legacy systems to connect to the AC grid. These stopgaps were slowly replaced as older systems were retired or upgraded. Tesla polyphase AC 500hp generator at 1893 exposition

Tesla polyphase AC 500hp generator at 1893 exposition

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 The Willamette Falls is a natural waterfall on the Willamette River between Oregon City, Oregon, Oregon City and West Linn, Oregon, in the United States. It is the largest waterfall in the Northwestern United States by volume, and the seventeen ...

to the city of

Portland Portland most commonly refers to: * Portland, Oregon, the largest city in the state of Oregon, in the Pacific Northwest region of the United States * Portland, Maine, the largest city in the state of Maine, in the New England region of the northeas ...

down river. The first three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in

Frankfurt Frankfurt, officially Frankfurt am Main (; Hessian: , " Frank ford on the Main"), is the most populous city in the German state of Hesse. Its 791,000 inhabitants as of 2022 make it the fifth-most populous city in Germany. Located on it ...

. A 15 kV transmission line, approximately 175 km long, connected Lauffen on the Neckar and Frankfurt. Transission voltages increased throughout the 20th century. By 1914, fifty-five transmission systems operating at more than 70 kV were in service. The highest voltage then used was 150 kV. 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 hydroelectric power or mine-mouth coal, could be exploited to further lower costs. The 20th century's rapid industrialization made electrical transmission lines and grids

critical infrastructure Critical infrastructure (or critical national infrastructure (CNI) in the UK) is a term used by governments to describe assets that are essential for the functioning of a society and economy – the infrastructure. Most commonly associated wi ...

. Interconnection of local generation plants and small distribution networks was spurred by

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

, when large electrical generating plants built by governments to power munitions factories.

Bulk transmission

These networks use components such as power lines, cables,

circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...

s, switches and

s. The transmission network is usually administered on a regional basis by an entity such as a

regional transmission organization A regional transmission organization (RTO) in the United States is an electric power transmission system operator (TSO) that coordinates, controls, and monitors a multi-state electric grid. The transfer of electricity between states is considered i ...

transmission system operator File:Electricity grid simple- North America.svg, 380px, Simplified diagram of AC electricity grid from generation stations to consumers rect 2 243 235 438 Power station rect 276 317 412 556 Transformer rect 412 121 781 400 Electric power transmis ...

. Transmission efficiency is improved at higher voltage and lower current. The reduced current reduces heating losses. Joule's 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

''.

HVDC A high-voltage direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for electric power transmission, in contrast with the more common alternating curre ...

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 A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many ...

s, transmission lines, and substations. Energy is usually transmitted within a grid with three-phase AC. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. In the 19th 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. ElectricityUCTE

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 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 The base load (also baseload) is the minimum level of demand on an electrical grid over a span of time, for example, one week. This demand can be met by unvarying power plants, dispatchable generation, or by a collection of smaller intermittent e ...

'' and is generally served by large facilities with constant operating costs, termed

firm power Firm services, also called uninterruptible services, are services, such as electricity (firm power) and natural gas supplies, that are intended to be available at all times during a period covered by an agreement. Also, the service is not subject to ...

. Such facilities are nuclear, coal or hydroelectric, while other energy sources such as

concentrated solar thermal Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated when ...

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 plants, 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. New York often buys over 1000 MW of low-cost hydropower from Canada. Local sources (even if more expensive and infrequently used) can protect the power supply from weather and other disasters that can disconnect distant suppliers. Electicaltransmissionlines3800ppx

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

s, power is produced at a relatively low voltage between about 2.3 kV and 30 kV, depending on the size of the unit. The voltage is then stepped up by the power station

to a higher voltage (115 kV to 765 kV AC) for transmission. In the United States, power transmission is, variously, 230 kV to 500 kV, with less than 230 kV or more than 500 kV as exceptions. The

Western Interconnection The Western Interconnection is a wide area synchronous grid and one of the two major alternating current (AC) power grids in the North American power transmission grid. The other major wide area synchronous grid is the Eastern Interconnection ...

has two primary interchange voltages: 500 kV AC at 60 Hz, and ±500 kV (1,000 kV net) DC from North to South ( Columbia River to

Southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban a ...

) and Northeast to Southwest (Utah to Southern California). The 287.5 kV ( Hoover Dam to

Los Angeles Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the List of municipalities in California, largest city in the U.S. state, state of California and the List of United States cities by population, sec ...

line, via

Victorville Victorville is a city in Victor Valley in San Bernardino County, California. Its population as of the 2020 census was 134,810. History In 1858, Aaron G. Lane came to what is now known as Victorville and founded a waystation called "Lane's Cro ...

) and 345 kV (

Arizona Public Service Arizona Public Service (APS) is the largest electric utility in Arizona, United States. Since 1985, it has been the principal subsidiary of publicly traded S&P 500 member Pinnacle West Capital Corporation, known as AZP Group until 1987. Pinnacl ...

(APS) line) are local standards, both of which were implemented before 500 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 span at 765 kV carrying 1000 MW of power can have losses of 0.5% to 1.1%. A 345 kV line carrying the same load across the same distance has losses of 4.2%. For a given amount of power, a higher voltage reduces the current and thus the resistive losses. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the

I^2 R

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

I^2 R

losses are still reduced ten-fold. Long-distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. At higher voltages, where more than 2,000 kV exists between conductor and ground,

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 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. US transmission and distribution losses were estimated at 6.6% in 1997, 6.5% in 2007 and 5% from 2013 to 2019. 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 . For AC it was , though US transmission lines are substantially shorter. 4.98 MB In any AC line, conductor inductance and

can be significant. Currents that flow solely in reaction to these properties, (which together with the resistance define the 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-shifters; static VAR compensators; and

flexible AC transmission system A flexible alternating current transmission system (FACTS) is a system composed of static equipment used for the alternating current (AC) transmission of electrical energy. It is meant to enhance controllability and increase power transfer capabi ...

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

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 In electrical power transmission, a transposition tower is a transmission tower that changes the relative physical positions of the conductors of a transmission line in a Polyphase system. A transposition tower allows these sections to be connect ...

s at regular intervals along the line using various transposition schemes.

Subtransmission

Subtransmission runs at relatively lower voltages. It is uneconomical to connect all distribution substations 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 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 distribution. Their voltage ranges overlap. Voltages of 69 kV, 115 kV, and 138 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.

Transmission grid exit

Substation transformers reduce the voltage to a lower level for distribution to loads. This distribution is accomplished with a combination of sub-transmission (33 to 132 kV) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage.

Advantage of high-voltage transmission

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. Power split two resistances

In a simplified model, the grid delivers electricity from an ideal voltage source with voltage

V

, delivering a power

P_V

) to a single point of consumption, modelled by a resistance

R

, when the wires are long enough to have a significant resistance

R_C

. If the resistances are in series with no intervening transformer, the circuit acts as a

voltage divider In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the inp ...

, because the same current

I=\frac

runs through the wire resistance and the powered device. As a consequence, the useful power (at the point of consumption) is: :

P_R= V_2\times I = V\frac\times\frac = \frac\times\frac = \frac P_V

Should an

ideal transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's c ...

convert high-voltage, low-current electricity into low-voltage, high-current electricity with a voltage ratio of

a

(i.e., the voltage is divided by

a

and the current is multiplied by

a

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

R_C/a^2

. The useful power is then: :

P_R= V_2\times I_2 = \frac = \frac P_V = \frac P_V

For

a>1

(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

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 by 2 transmission matrix is used to model its behavior, as follows: :

\begin
	V_\mathrm\\
	I_\mathrm\\
\end
=
\begin
	A & B\\
	C & D\\
\end
\begin
	V_\mathrm\\
	I_\mathrm\\
\end

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: *

\det(T) = AD - BC = 1

A = D

The parameters ''A'', ''B'', ''C'', and ''D'' differ depending on how the desired model handles the line's resistance (''R''), inductance (''L''),

(''C''), and shunt (parallel, leak) 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. Losslessline

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 > SIL, the voltage drops from sending end and the line "consumes" VARs. For load < 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 . There, only a series impedance ''Z'' is considered, while ''C'' and ''G'' are ignored. The final result is that A = D = 1 per unit, B = Z Ohms, and C = 0. The associated transition matrix for this approximation is therefore: :

\begin
	V_\mathrm\\
	I_\mathrm\\
\end
=
\begin
	1 & Z\\
	0 & 1\\
\end
\begin
	V_\mathrm\\
	I_\mathrm\\
\end

Medium line

The medium line approximation is used for lines running between . 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 ''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: :

\begin
A &= D = 1 + \frac \text\\
B &= Z\Omega\\
C &= G \Big( 1 + \frac\Big)S
\end

Counterintuitive behaviors of medium-length transmission lines: * voltage rise at no load or small current (

Ferranti effect In electrical engineering, the Ferranti effect is the increase in voltage occurring at the receiving end of a very long (> 200 km) AC electric power transmission line, relative to the voltage at the sending end, when the load is very small, or no ...

) * 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 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

\gamma

is the

propagation constant The propagation constant of a sinusoidal electromagnetic wave is a measure of the change undergone by the amplitude and phase of the wave as it propagates in a given direction. The quantity being measured can be the voltage, the current in a c ...

. :

C &= \frac \sinh(\gamma x) S \end

To find the voltage and current at the end of the long line,

x

should be replaced with

l

(the line length) in all parameters of the transmission matrix. This model applies the

Telegrapher's equations The telegrapher's equations (or just telegraph equations) are a pair of coupled, linear partial differential equations that describe the voltage and current on an electrical transmission line with distance and time. The equations come from Oliver ...

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 cables Submarine cable is any electrical cable that is laid on the seabed, although the term is often extended to encompass cables laid on the bottom of large freshwater bodies of water. Examples include: *Submarine communications cable *Submarine power ...

where AC cannot be used because of cable capacitance. In these cases special

high-voltage cable A high-voltage cable (HV cable) is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation. Cables are considered to be fully insulated. This means that they have a fully rated insulation system t ...

s are used. Submarine HVDC systems are often used to interconnect the electricity grids of islands, for example, between

Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It i ...

and continental Europe, between Great Britain and Ireland, between

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

and the Australian mainland, between the North and South Islands of New Zealand, between

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...

and

New York City New York, often called New York City or NYC, is the most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the most densely populated major city in the Un ...

, and between New Jersey and Long Island. Submarine connections up to in length have been deployed. HVDC links can be used to control grid problems. The power transmitted by an AC line increases as the 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 Seattle ( ) is a seaport city on the West Coast of the United States. It is the seat of King County, Washington. With a 2020 population of 737,015, it is the largest city in both the state of Washington and the Pacific Northwest regio ...

and

Boston Boston (), officially the City of Boston, is the state capital and most populous city of the Commonwealth of Massachusetts, as well as the cultural and financial center of the New England region of the United States. It is the 24th- mo ...

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 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 The Pacific DC Intertie (also called Path 65) is an electric power transmission line that transmits electricity from the Pacific Northwest to the Los Angeles area using high voltage direct current (HVDC). The line capacity is 3.1 gigawatts, whi ...

located in the Western United States.

Capacity

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 , the limit is set by the

voltage drop Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirab ...

in the line. For longer AC lines, 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 Distributed temperature sensing systems (DTS) are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a cont ...

(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 An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...

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. Furthermore, it allows the operator to predict the behavior of the transmission system to reflect major changes to its initial operating conditions.

Control

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 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 synchronization of the generation units. In 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 Droop speed control is a control mode used for AC electrical power generators, whereby the power output of a generator reduces as the line frequency increases. It is commonly used as the speed control mode of the governor of a prime mover driving a ...

, 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 A wind turbine is a device that converts the kinetic energy of wind into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW added each yea ...

vehicle-to-grid Vehicle-to-grid (V2G), also known as Vehicle-to-home (V2H) or Vehicle-to-load (V2L) describes a system in which plug-in electric vehicles (PEV) sell demand response services to the grid. Demand services are either delivering electricity or by red ...

, virtual power plants, and other locally distributed storage and generation systems can interact with the grid to improve system operation. Internationally, a slow move from a centralized to decentralized power system has 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.

Failure protection

Under excess load conditions, the system can be designed to fail incrementally rather than all at once. Brownouts occur when power supplied drops below the demand. Blackouts occur when the grid fails completely.

Rolling blackout A rolling blackout, also referred to as rota or rotational load shedding, rota disconnection, feeder rotation, or a rotating outage, is an intentionally engineered electrical power shutdown in which electricity delivery is stopped for non-overla ...

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

Communications

Grid operators require reliable communications to manage the grid and associated generation and distribution facilities. Fault-sensing

protective relay In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detecti ...

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 A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circui ...

s and other faults is usually so critical that

common carrier A common carrier in common law countries (corresponding to a public carrier in some civil law systems,Encyclopædia Britannica CD 2000 "Civil-law public carrier" from "carriage of goods" usually called simply a ''carrier'') is a person or compan ...

telecommunications are insufficiently reliable, while in some remote areas no common carrier is available. Communication systems associated with a transmission project may use: *

Microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

s *

Power-line communication Power-line communication (also known as power-line carrier or PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. A wide range of power-line communicat ...

Optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...

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 Power-line communication (also known as power-line carrier or PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. A wide range of power-line communicat ...

(PLC). PLC signals can be easily received with a radio in the long wave range. High Voltage Pylons carrying additional 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 An optical ground wire (also known as an OPGW or, in the IEEE standard, an optical fiber composite overhead ground wire) is a type of cable that is used in overhead power lines. Such cable combines the functions of grounding and communications. A ...

(''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 Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...

, prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications

. Where the regulatory structure permits, the utility can sell capacity in extra

dark fiber A dark fibre or unlit fibre is an unused optical fibre, available for use in fibre-optic communication. Dark fibre may be leased from a network service provider. Dark fibre originally referred to the potential network capacity of telecommunic ...

s to a common carrier.

Market structure

Electricity transmission is generally considered to be a

natural monopoly A natural monopoly is a monopoly in an industry in which high infrastructural costs and other barriers to entry relative to the size of the market give the largest supplier in an industry, often the first supplier in a market, an overwhelming adv ...

, but one that is not inherently linked to generation. Many countries regulate transmission separately from generation. Spain was the first country to establish a

. In that country, transmission operations and electricity markets are separate. The transmission system operator is

Red Eléctrica de España Red Eléctrica de España (; REE) is a partly state-owned and public limited Spanish corporation which operates the national electricity grid in Spain, where it operates the national power transmission system. It also holds assets in Peru, Chile ...

(REE) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía – Polo Español, S.A. (OMEL
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 The Federal Energy Regulatory Commission (FERC) is the United States federal agency that regulates the transmission and wholesale sale of electricity and natural gas in interstate commerce and regulates the transportation of oil by pipeline in ...

'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. 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. Australia has one unregulated or market interconnector - Basslink - between

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

and Victoria (Australia), Victoria. Two DC links originally implemented as market interconnectors, Directlink and Murraylink, were converted to regulated interconnectors. 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. In the United States, the

'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.

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 p per kWh compared to a delivered domestic price of around 10 p per kWh. The level of capital expenditure in the electric power T&D equipment market was estimated to be $128.9 bn in 2011.

Health concerns

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. Other studies, however, reported statistical correlations 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. The New York State Public Service Commission conducted a study 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 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 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 mG magnetic field. 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 Tesla (unit), µT (1 Gauss (unit), G) (1,000 mG). In a residential setting, one study reported "limited evidence of carcinogenicity 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 µT (3 mG) to 0.4 µT (4 mG). These levels exceed average residential power-frequency magnetic fields in homes, which are about 0.07 µT (0.7 mG) in Europe and 0.11 µT (1.1 mG) in North America. The Earth's natural geomagnetic field strength varies over the surface of the planet between 0.035 mT and 0.07 mT (35 µT – 70 µT or 350 mG – 700 mG) while the international standard for continuous exposure is set at 40 mT (400,000 mG or 400 G) for the general public. Tree growth regulators and herbicides may be used in transmission line right of ways, which may have Herbicide#Health and environmental effects, health effects.

Policy by country

United States

The Federal Energy Regulatory Commission (FERC) is the primary regulatory agency of electric power transmission and wholesale electricity sales within the United States. FERC was originally established by Congress in 1920 as the Federal Power Commission and has since undergone multiple name and responsibility modifications. Electric power distribution and the retail sale of power is under state jurisdiction.

Order No. 888

Order No. 888 was adopted by FERC on April 24, 1996. It was "designed to remove impediments to competition in the wholesale bulk power marketplace and to bring more efficient, lower cost power to the Nation’s electricity consumers. The legal and policy cornerstone of these rules is to remedy undue discrimination in access to the monopoly owned transmission wires that control whether and to whom electricity can be transported in interstate commerce." The Order required all public utilities that own, control, or operate facilities used for transmitting electric energy in interstate commerce, to have open access, non-discriminatory transmission tariffs. These tariffs allow any electricity generator to utilize existing power lines to transmit the power that they generate. The Order also permits public utilities to recover the costs associated with providing their power lines as an open access service.

Energy Policy Act of 2005

The Energy Policy Act of 2005 (EPAct) expanded federal authority to regulate power transmission. EPAct gave FERC significant new responsibilities, including enforcement of electric transmission reliability standards and the establishment of rate incentives to encourage investment in electricity transmission. Historically, local governments exercised authority over the grid and maintained significant disincentives to actions that would benefit states other than their own. Localities with cheap electricity have a disincentive to encourage making interstate commerce in electricity trading easier, since other regions would be able to compete for that energy and drive up rates. For example, some regulators in Maine refused to address congestion problems because the congestion protects Maine rates. Further, local constituencies can block or slow permitting by pointing to visual impacts, environmental, and health concerns. In the US, generation is growing four times faster than transmission, but transmission upgrades require the coordination of multiple jurisdictions, complex permitting, and cooperation between a significant portion of the many companies that collectively own the grid. The US national security interest in improving transmission was reflected in the EPAct which gave the Department of Energy the authority to approve transmission if states refused to act.

Specialized transmission

Grids for railways

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

Superconducting cables

High-temperature superconductors (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. 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. 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.

Single-wire earth return

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

Both