power transmission line

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Electric power transmission is the bulk movement of
electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used loosely, ''electrical energy'' refers to energy that has been converted ''from'' electric potential energy. This energy is supplied by the com ...
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 generation A generation is "all of the people born and living Living or The Living may refer to: ...

, to an
electrical substation A substation is a part of an electrical Electricity generation, generation, electric power transmission, transmission, and electric power distribution, distribution system. Substations transform voltage from high to low, or the reverse, or per ...

. The interconnected lines which facilitate this movement are known as 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 Delivery may refer to: *Delivery (commerce), of goods, e.g.: **Pizza delivery **Milk delivery Film and television *Delivering (film), ''Delivering'' (film), a 1993 short film by Todd ...
. 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 A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for th ...
, known as the
electrical grid An electrical grid is an interconnected network for electricity delivery Electricity delivery is the process that starts after generation of electricity in the power station A power station, also referred to as a power plant and somet ...
. Efficient long-distance transmission of electric power requires high voltages. This reduces the losses produced by heavy current. Transmission lines mostly use high-voltage AC (alternating current), but an important class of transmission line uses
high voltage direct current A high-voltage, direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for the transmission of electrical power, in contrast with the more common alternat ...
. 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 Electrical network, circuits. A varying current in any one coil of the transformer produces a varying magnetic flux ...

s, stepping up the voltage for transmission, then reducing voltage for local distribution and then use by customers. A
wide area synchronous grid A wide area synchronous grid (also called an "interconnection" in North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any ...
, also known as an "interconnection" in North America, directly connects many generators delivering AC power with the same relative ''frequency'' to many consumers. For example, there are four major interconnections in North America (the
Western Interconnection The Western Interconnection is a wide area synchronous grid A wide area synchronous grid (also called an "interconnection" in North America North America is a continent A continent is any of several large landmasses. Genera ...
, the
Eastern Interconnection The Eastern Interconnection is one of the two major alternating-current (AC) electrical grid An electrical grid is an interconnected network for electricity delivery Electricity delivery is the process that starts after generation of e ...
, the
Quebec Interconnection ) , image_shield=Armoiries du Québec.svg , image_flag=Flag of Quebec.svg , coordinates= , AdmittanceDate=July 1, 1867 , AdmittanceOrder=1st, with New Brunswick ("Hope restored") , image_map = New Brunswick in Canada 2.svg , ...
and the
Electric Reliability Council of Texas The Electric Reliability Council of Texas, Inc. (ERCOT) is an American organization that operates Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the ...
(ERCOT) grid). In Europe one large 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 have liberalized the regulation of the
electricity market In economic terms, electricity is a commodity capable of being bought, sold, and traded. An electricity market, also power exchange or PX, is a system enabling purchases, through bids to buy; sales, through offers to sell. Bids and offers use sup ...
in ways that have led to the separation of the electricity transmission business from the distribution business.

# System

Most transmission lines are high-voltage
three-phase Three-phase electric power (abbreviated 3φ) is a common type of alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική ...
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
(AC), although
single phase In electrical engineering, single-phase electric power is the distribution of alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φ ...
AC is sometimes used in
railway electrification system A railway electrification system supplies electric power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used loos ...

s.
High-voltage direct-current A high-voltage, direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for the transmission of electrical power, in contrast with the more common alternat ...
(HVDC) technology is used for greater efficiency over very long distances (typically hundreds of miles). HVDC technology is also used in
submarine power cable A submarine power cable is a Power cable, transmission cable for carrying electric power below the surface of the water.
s (typically longer than 30 miles (50 km)), and in the interchange of power between grids that are not mutually synchronized. HVDC links are used to stabilize large 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 process in a system of interconnected parts in which the failure of one or few parts can trigger the failure of other parts and so on. Such a failure may happen in many types of systems, including power transmission, comp ...
s. Electricity is transmitted at
high voltage High voltage electricity Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (O ...

s (66 kV or above) to reduce the energy loss which occurs in long-distance transmission. Power is usually transmitted through
overhead power line An overhead power line is a structure used in electric power transmission and distributionDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized function ...

s. Underground power transmission has a significantly higher installation cost and greater operational limitations, but reduced maintenance costs. Underground transmission is sometimes used in urban areas or environmentally sensitive locations. A lack of electrical energy storage facilities in transmission systems leads to a key limitation. Electrical energy must be generated at the same rate at which it is consumed. A sophisticated control system is required to ensure that the
power generation Electricity generation is the process of generating electric power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When use ...
very closely matches the demand. If the demand for power 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 shut downs and a major regional blackout. Examples include the US Northeast blackouts of
1965 Events January * January 1 January 1 or 1 January is the first day of the year in the . There are 364 days remaining until the end of the year (365 in s). This day is known as since the day marks the beginning of the year. __TOC__ ...

,
1977 Events January * January – The world's first all-in-one home computer (keyboard/screen/tape storage), the Commodore PET, is demonstrated at the Consumer Electronics Show in Chicago. * January 3 – Apple Inc., Apple Computer is incorpor ...
,
2003 2003 was designated the International Year of the Fresh Water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids Total dissolved ...
, and major blackouts in other US regions in
1996 1996 was designated as: * International Year for the Eradication of Poverty Events January * January 3 Events Pre-1600 * 69 – The Roman legions on the Rhine refuse to declare their allegiance to Galba Galba (; born Servius Sulpi ...
and
2011 A series of protests and government overthrows, known as the Arab Spring The Arab Spring ( ar, الربيع العربي) was a series of anti-government protests, uprisings, and armed rebellions that spread across much of the Arab worl ...
. 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 shut downs 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.

High-voltage overhead conductors are not covered by insulation. The conductor material is nearly always an
aluminum Aluminium (aluminum in American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly known as the Uni ...

alloy, made into several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission, but aluminum is lighter, yields only marginally reduced performance and costs much less. Overhead conductors are a commodity supplied by several companies worldwide. Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits. Conductor sizes range from 12 mm2 (#6 American wire gauge) to 750 mm2 (1,590,000
circular mil A circular mil is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentat ...
s area), with varying resistance and
current-carrying capacity Ampacity is a portmanteau A portmanteau (, ) or portmanteau word (from "portmanteau (luggage) A portmanteau is a piece of luggage Baggage or luggage consists of bags, cases, and containers which hold a travel Travel is the moveme ...
. For large conductors (more than a few centimetres in diameter) at power frequency, much of the current flow is concentrated near the surface due to the
skin effect Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble like, for example, an or ...

. The center part of the conductor carries little current, but contributes weight and cost to the conductor. Because of this current limitation, multiple parallel cables (called
bundle conductor An overhead power line is a structure used in electric power transmission and distributionDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized function ...
s) are used when higher capacity is needed. Bundle conductors are also used at high voltages to reduce energy loss caused by
corona discharge A corona discharge is an electrical discharge An electric discharge is the release and transmission of electricity in an applied electric field An electric field (sometimes E-field) is the physical field that surrounds each electric cha ...

. Today, transmission-level voltages are usually considered to be 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
distributionDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized functions, are objects that generalize the classical notion of functions in mathematical analysis. Distr ...
. 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 spe ...

and require different designs compared to equipment used at lower voltages. Since overhead transmission wires depend on air for insulation, the design of these lines requires minimum clearances to be observed to maintain safety. Adverse weather conditions, such as high winds and low temperatures, can lead to power outages. 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 A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temp ...

and loss of supply. Oscillatory motion of the physical line can be termed conductor gallop or flutter depending on the frequency and amplitude of oscillation.

# Underground transmission

Electric power can also be transmitted by underground power cables instead of overhead power lines. Underground cables take up less right-of-way than overhead lines, have lower visibility, and are less affected by bad weather. However, costs of insulated cable and excavation are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair. In some metropolitan areas, underground transmission 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 produces a dielectric leak into the surrounding soil, liquid nitrogen trucks are mobilized to freeze portions of the pipe to enable the draining and repair of the damaged pipe location. This type of underground transmission cable can prolong the repair period and increase repair costs. The temperature of the pipe and soil are usually monitored constantly throughout the repair period. Underground lines are strictly limited by their thermal capacity, which permits less overload or re-rating than overhead lines. Long underground AC cables have significant
capacitance Capacitance is the ratio of the amount of electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' ( ...
, which may reduce their ability to provide useful power to loads beyond . DC cables are not limited in length by their capacitance, however, they do require
HVDC converter station An HVDC converter station (or simply converter station) is a specialised type of electrical substation, substation which forms the terminal equipment for a high-voltage direct current (HVDC) transmission line.Arrillaga, Jos; High Voltage Direct Cur ...
s at both ends of the line to convert from DC to AC before being interconnected with the transmission network.

# History

In the early days of commercial electric power, transmission of electric power at the same voltage as used by lighting and mechanical loads restricted the distance between generating plant and consumers. In 1882, generation was with
direct current Direct current (DC) is one-directional flow Flow may refer to: Science and technology * Flow (fluid) or fluid dynamics, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (math ...
(DC), which could not easily be increased in voltage 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. Due to this specialization of lines and because transmission was inefficient for low-voltage high-current circuits, generators needed to be near their loads. It seemed, at the time, that the industry would develop into what is now known as a
distributed generation Distributed generation, also distributed energy, on-site generation (OSG), or district/decentralized energy, is electrical generation A generation is "all of the people born and living Living or The Living may refer to: Common meanings *Life ...
system with large numbers of small generators located near their loads. The transmission of electric power with
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
(AC) became possible after
Lucien Gaulard Lucien Gaulard (16 July 1850 – 26 November 1888) invented devices for the transmission Transmission may refer to: Science and technology * Power transmissionPower transmission is the movement of energy from its place of generation to a locatio ...
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 transformer 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
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Siemens & Halske Siemens & Halske Aktiengesellschaft, 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 Siem ...

alternator and featured several Gaulard "secondary generators" (transformers) with their primary windings connected in series, which fed incandescent lamps. The system proved the feasibility of AC electric power transmission over long distances. The very first AC distribution system to operate was in service in 1885 in via dei Cerchi,
Rome, Italy , established_title = Founded , established_date = 753 BC , founder = King Romulus , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption = The te ...
, 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, which was put into service at the
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 Edward Charles Hallé, Charles Hallé. The gallery proved crucial to the Aesthetic Movem ...

, London. It also featured Siemens alternators and 2.4 kV to 100 V step-down transformers – one per user – with shunt-connected primaries. Working from 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 A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method ...
developed what is considered 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 Entrepreneurship is the creation or extraction of value. With this definition, entrepreneurship is viewed as change, generally entailing risk beyond wh ...

, in 1886 he demonstrated a transformer based alternating current lighting system in
Great Barrington, Massachusetts Great Barrington is a New England town, 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 ...

. Powered by a steam engine driven 500 V Siemens generator, voltage was stepped down to 100 Volts using the new Stanley transformer to power incandescent lamps at 23 businesses along main street with very little power loss over . This practical demonstration of a transformer and alternating current lighting system would lead Westinghouse to begin installing AC based systems later that year. 1888 saw designs for a functional , something these systems had lacked up till then. These were
induction motor An induction motor or asynchronous motor is an AC electric motor in which the electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗm ...
s 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 an inventor of the three-phase induction motor upright=1.15, Cutaway vie ...

and
Nikola Tesla Nikola Tesla ( ; sr-cyr, Никола Тесла, ; 1856 – 7 January 1943) was a Serbian-American inventor An invention is a unique or novel A novel is a relatively long work of narrative fiction, typically written in prose ...

(with Tesla's design being licensed by Westinghouse in the US). This design was further developed into the modern practical
three-phase Three-phase electric power (abbreviated 3φ) is a common type of alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική ...
form by
Mikhail Dolivo-Dobrovolsky Mikhail Osipovich Dolivo-Dobrovolsky (russian: Михаи́л О́сипович Доли́во-Доброво́льский; german: Michail von Dolivo-Dobrowolsky or ''Michail Ossipowitsch Doliwo-Dobrowolski''; pl, Michał Doliwo-Dobrowolski ...
and
Charles Eugene Lancelot Brown 200px Charles Eugene Lancelot Brown (17 June 1863 – 2 May 1924) founded Brown, Boveri & Cie Brown, Boveri & Compagnie (BBC) was a Swiss group of electrical engineering companies. It was founded in Zürich , neighboring_municipalities ...

.
Arnold Heertje Arnold Heertje (19 February 1934 – 4 April 2020) was a Dutch economist An economist is a professional and practitioner in the social science Social science is the branch A branch ( or , ) or tree branch (sometimes referred to ...

, Mark Perlma
Evolving Technology and Market Structure: Studies in Schumpeterian Economics
page 138
Practical use of these types of motors would be delayed many years by development problems and the scarcity of poly-phase power systems needed to power them. The late 1880s and early 1890s would see the financial merger of smaller electric companies into a few 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 the ...
and
AEG ''Allgemeine Elektricitäts-Gesellschaft AG'' (AEG; ) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany ...

in Europe and
General Electric General Electric Company (GE) is an American Multinational corporation, multinational Conglomerate (company), conglomerate incorporated in New York State and headquartered in Boston. Until 2021, the company operated through GE Aviation, aviat ...
and
Westinghouse Electric The Westinghouse Electric Corporation was an American manufacturing company founded in 1886 by George Westinghouse George Westinghouse Jr. (October 6, 1846 – March 12, 1914) was an American entrepreneur Entrepreneurship is the creation ...
in the US. These companies continued to develop AC systems but the technical difference between direct and alternating current systems would follow a much longer technical merger. Due to innovation in the US and Europe, alternating current's economy of scale with very large generating plants linked to loads via long-distance transmission was slowly being combined with the ability to link it up with all of the existing systems that needed to be supplied. 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 was becoming a ''universal system'', these technological differences were temporarily being bridged via the development of
rotary converter A rotary converter is a type of electrical machine In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electr ...
s and motor-generators that would allow the large number of legacy systems to be connected to the AC grid. These stopgaps would slowly be replaced as older systems were retired or upgraded. The first transmission of single-phase alternating current using high voltage took place in Oregon in 1890 when power was delivered from a hydroelectric plant at Willamette Falls to the city of Portland downriver. 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 dialects, Hessian: , "Franks, Frank ford (crossing), ford on the Main (river), Main"; french: Francfort-sur-le-Main), is the most populous city in the States of Germany, German state of Hess ...

. A 15 kV transmission line, approximately 175 km long, connected Lauffen on the Neckar and Frankfurt. Voltages used for electric power transmission increased throughout the 20th century. By 1914, fifty-five transmission systems each operating at more than 70 kV were in service. The highest voltage then used was 150 kV. By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as
hydroelectric Hydroelectricity, or hydroelectric power, is electricity produced from hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water Water (chemical formula H2O) is a ...
power or mine-mouth coal, could be exploited to lower energy production cost. The rapid industrialization in the 20th century made electrical transmission lines and grids
critical infrastructure Critical infrastructure (or critical national infrastructure (CNI) in the UK) is a term used by government A government is the system or group of people governing an organized community, generally a state State may refer to: A ...
items in most industrialized nations. The interconnection of local generation plants and small distribution networks was spurred by the requirements of
World War I World War I, often abbreviated as WWI or WW1, also known as the First World War or the Great War, was a global war A world war is "a war engaged in by all or most of the principal nations of the world". The term is usually reserved for ...

, with large electrical generating plants built by governments to provide power to munitions factories. Later these generating plants were connected to supply civil loads through long-distance transmission.

# Bulk power transmission

Engineers design transmission networks to transport the energy as efficiently as possible, while at the same time taking into account the economic factors, network safety and redundancy. 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 An electrical network is an interconnection of electrical component An electronic component is any basic discrete device or physical entity ...

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

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 The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North A ...
or
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 A power station, also referred to as a power plant and sometimes gen ...
. Transmission efficiency is greatly improved by devices that increase the voltage (and thereby proportionately reduce the current), in the line conductors, thus allowing power to be transmitted with acceptable losses. The reduced current flowing through the line reduces the heating losses in the conductors. According to Joule's Law, energy losses are directly proportional to the square of the current. Thus, reducing the current by a factor of two will lower 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 the size is at its optimum when the annual cost of energy wasted in the resistance is equal to the annual capital charges of providing the conductor. At times of lower interest rates, Kelvin's law indicates that thicker wires are optimal; while, when metals are expensive, thinner conductors are indicated: however, power lines are designed for long-term use, so Kelvin's law has to be used in conjunction with long-term estimates of the price of copper and aluminum as well as interest rates for capital. The increase in voltage is achieved in AC circuits by using a ''step-up
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any one coil of the transformer produces a varying magnetic flux ...

''.
HVDC A high-voltage, direct current (HVDC) electric power transmission Electric power transmission is the bulk movement of electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used ...
systems require relatively costly conversion equipment which may be economically justified for particular projects such as submarine cables and longer distance high capacity point-to-point transmission. HVDC is necessary for the import and export of energy between grid systems that are not synchronized with each other. 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 A generation is "all of the people born and living Living or The Living may refer to: ...

s, transmission lines, and . Energy is usually transmitted within a grid with
three-phase Three-phase electric power (abbreviated 3φ) is a common type of alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική ...
AC. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase
induction motor An induction motor or asynchronous motor is an AC electric motor in which the electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗm ...
s. 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. The price of electric power station capacity is high, and electric demand is variable, so it is often cheaper to import some portion of the needed power than to generate it locally. Because loads are often regionally correlated (hot weather in the Southwest portion of the US might cause many people to use air conditioners), electric power often comes from distant sources. Because of the economic benefits of load sharing between regions, wide area transmission grids now span countries and even continents. The web of interconnections between power producers and consumers should enable power to flow, even if some links are inoperative. The unvarying (or slowly varying over many hours) portion of the electric demand is known as the ''
base load The baseload (also base load) on a grid is the minimum level of demand on an electrical grid An electrical grid is an interconnected network for electricity delivery Electricity delivery is the process that starts after generation of electr ...
'' and is generally served by large facilities (which are more efficient due to economies of scale) with fixed costs for fuel and operation. Such facilities are nuclear, coal-fired 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 Solar power is the conversion of renewable energy Renewable energy is energy that is collected from ...
and
geothermal power Geothermal power is electrical power generated from geothermal energy Geothermal energy is the thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical conc ...
have the potential to provide base load power. Renewable energy sources, such as solar photovoltaics, wind, wave, and tidal, are, due to their intermittency, not considered as supplying "base load" but will still add power to the grid. The remaining or 'peak' power demand, is supplied by
peaking power plant Peaking power plants, also known as peaker plants, and occasionally just "peakers", are 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 ...

# Merchant transmission

Merchant transmission is an arrangement where a third party constructs and operates electric transmission lines through the franchise area of an unrelated incumbent utility. Operating 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 entities in the PJM Interconnection region. There is only one unregulated or market interconnector in Australia: Basslink between Tasmania and Victoria (Australia), Victoria. Two DC links originally implemented as market interconnectors, Directlink and Murraylink, have been converted to regulated interconnectors
NEMMCO
A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from the facility so that the beneficiaries will pay the toll. Also, it is difficult for a merchant transmission line to compete when the alternative transmission lines are subsidized by incumbent utility businesses with a monopolized and regulated rate base. In the United States, the FERC's Order 1000, issued in 2010, attempts to reduce barriers to third party investment and creation of merchant transmission lines where a public policy need is found.

# Health concerns

Some large studies, including a large study in the United States, have failed to find any link between living near power lines and developing any sickness or diseases, such as cancer. A 1997 study found that it did not matter how close one was to a power line or a sub-station, there was no increased risk of cancer or illness. The 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, however, have found 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, documented in ''Opinion No. 78-13'' (issued June 19, 1978), to evaluate potential health effects of electric fields. The study's case number is too old to be listed as a case number in the commission's online database, DMM, and so the original study can be difficult to find. The study chose to utilize the electric field strength that was measured at the edge of an existing (but newly built) right-of-way on a 765 kV transmission line from New York to Canada, 1.6 kV/m, as the interim standard maximum electric field at the edge of any new transmission line right-of-way built in New York State after issuance of the order. The opinion also limited the voltage of all 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 study established a magnetic field interim standard of 200 mG at the edge of the right-of-way using the winter-normal conductor rating. This later document can also be difficult to find on the NYSPSC's online database, since it predates the online database system. As a comparison with everyday items, a hair dryer or electric blanket produces a 100 mG – 500 mG magnetic field. An electric razor can produce 2.6 kV/m. Whereas electric fields can be shielded, magnetic fields cannot be shielded, but are usually minimized by optimizing the location of each phase of a circuit in cross-section. When a new transmission line is proposed, within the application to the applicable regulatory body (usually a public utility commission), there is often an analysis of electric and magnetic field levels at the edge of rights-of-way. These analyses are performed by a utility or by an electrical engineering consultant using modelling software. At least one state public utility commission has access to software developed by an engineer or engineers at the Bonneville Power Administration to analyze electric and magnetic fields at edge of rights-of-way for proposed transmission lines. Often, public utility commissions will not comment on any health impacts due to electric and magnetic fields and will refer information seekers to the state's affiliated department of health. There are established biological effects for Acute toxicity, acute ''high'' level exposure to magnetic fields well above 100 Tesla (unit), µT (1 Gauss (unit), G) (1,000 mG). In a residential setting, there is "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 the continuous exposure limit is set at 40 mT (400,000 mG or 400 G) for the general public. Tree Growth Regulator and Herbicide Control Methods 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. It was originally established by Congress in 1920 as the Federal Power Commission and has since undergone multiple name and responsibility modifications. That which is not regulated by FERC, primarily electric power distribution and the retail sale of power, is under the jurisdiction of state authority. Two of the more notable U.S. energy policies impacting electricity transmission are Order No. 888 and the Energy Policy Act of 2005. Order No. 888 adopted by FERC on 24 April 1996, 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." Order No. 888 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 the already existing power lines for the transmission of the power that they generate. Order No. 888 also permits public utilities to recover the costs associated with providing their power lines as an open access service. The Energy Policy Act of 2005 (EPAct) signed into law by congress on 8 August 2005, further expanded the federal authority of regulating power transmission. EPAct gave FERC significant new responsibilities including but not limited to the enforcement of electric transmission reliability standards and the establishment of rate incentives to encourage investment in electric transmission. Historically, local governments have exercised authority over the grid and have significant disincentives to encourage 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 will be able to compete for local energy and drive up rates. For example, some regulators in Maine do not wish to address congestion problems because the congestion serves to keep Maine rates low. Further, vocal local constituencies can block or slow permitting by pointing to visual impact, environmental, and perceived health concerns. In the US, generation is growing four times faster than transmission, but big transmission upgrades require the coordination of multiple states, a multitude of interlocking permits, and cooperation between a significant portion of the 500 companies that own the grid. From a policy perspective, the control of the grid is balkanized, and even former United States Secretary of Energy, energy secretary Bill Richardson refers to it as a ''third world grid''. There have been efforts in the EU and US to confront the problem. The US national security interest in significantly growing transmission capacity drove passage of the Energy Policy Act of 2005, 2005 energy act giving the Department of Energy the authority to approve transmission if states refuse to act. However, soon after the Department of Energy used its power to designate two National Interest Electric Transmission Corridors, 14 senators signed a letter stating the DOE was being too aggressive.

# Special transmission

## Grids for railways

In some countries where electric locomotives or electric multiple units run on low frequency AC power, there are separate single phase traction power networks operated by the railways. Prime examples are countries in Europe (including Austria, Germany and Switzerland) which utilize the older 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 of electrical power. 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 the waste would be halved using this method, since the necessary refrigeration equipment would consume about half the power saved by the elimination of the majority of resistive losses. Some companies such as Consolidated Edison and American Superconductor have already begun commercial production of such systems. In one hypothetical future system called a SuperGrid, the cost of cooling would be eliminated by coupling the transmission line with a liquid hydrogen pipeline. 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 would be very 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 for an electrical grid 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
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and Hidetsugu Yagi attempted to devise systems for large scale wireless power transmission in the late 1800s and early 1900s, with no commercial success. In November 2009, LaserMotive won the NASA 2009 Power Beaming Challenge by powering a cable climber 1 km vertically using a ground-based laser transmitter. The system produced up to 1 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 satellites 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 of control systems

The Federal government of the United States admits that the power grid is susceptible to cyber-warfare. The United States Department of Homeland Security works with industry to identify vulnerabilities and to help industry enhance the security of control system networks, the federal government is also working to ensure that security is built in as the U.S. develops the next generation of 'smart grid' networks. In June 2019, Russia has conceded that it is "possible" its Electricity sector in Russia, electrical grid is under cyber-attack by the United States. ''The New York Times'' reported that American hackers from the United States Cyber Command planted malware potentially capable of disrupting the Russian electrical grid.

# Records

* Highest capacity system: 12 GW Zhundong–Wannan（准东-皖南）±1100 kV HVDC. * Highest transmission voltage (AC): **planned: 1.20 MV (Ultra High Voltage) on Wardha-Aurangabad line (India) – under construction. Initially will operate at 400 kV. **worldwide: 1.15 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: ) * Longest power line: Inga-Shaba (Democratic Republic of Congo) (length: ) * Longest span of power line: at Ameralik Span (Greenland, Denmark) * Longest submarine cables: **North Sea Link, (Norway/United Kingdom) – (length of submarine cable: ) **NorNed, North Sea (Norway/Netherlands) – (length of submarine cable: ) **Basslink, Bass Strait, (Australia) – (length of submarine cable: , total length: ) **Baltic Cable, Baltic Sea (Germany/Sweden) – (length of submarine cable: ,
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length: , total length: ) * Longest underground cables: **Murraylink, Riverland/Sunraysia (Australia) – (length of underground cable: )