Electric power transmission

Electric power transmission

Overview

Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substation
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

s located near demand centers. 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
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

.
Transmission lines, when interconnected with each other, become transmission networks. In the US, these are typically referred to as "power grids" or just "the grid", while in the UK the network is known as the "national grid." North America has three major grids: The Western Interconnection
Western Interconnection
The Western Interconnection is one of the two major alternating current power grids in North America. The other major wide area synchronous grid is the Eastern Interconnection...

; The Eastern Interconnection
Eastern Interconnection
The Eastern Interconnection is one of the two major alternating current power grids in North America. The other major interconnection is the Western Interconnection...

 and the Electric Reliability Council of Texas (or ERCOT) grid.

Historically, transmission and distribution lines were owned by the same company, but since the turn of the 21st century many countries have liberalized
Electricity liberalization
Electricity liberalization refers to the liberalization of electricity markets. As electricity supply is a natural monopoly, this entails complex and costly systems of regulation to enforce a system of competition....

 the electricity market
Electricity market
In economic terms, electricity is a commodity capable of being bought, sold and traded. An electricity market is a system for effecting purchases, through bids to buy; sales, through offers to sell; and short-term trades, generally in the form of financial or obligation swaps. Bids and offers use...

 in ways that have led to the separation of the electricity transmission business from the distribution business.

Transmission lines mostly use high-voltage three-phase
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

 alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC), although single phase
Single-phase electric power
In electrical engineering, single-phase electric power refers to 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 are mostly lighting and heating, with few large electric motors...

 AC is sometimes used in railway electrification system
Railway electrification system
A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world...

s.
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Encyclopedia

Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substation
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

s located near demand centers. 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
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

.
Transmission lines, when interconnected with each other, become transmission networks. In the US, these are typically referred to as "power grids" or just "the grid", while in the UK the network is known as the "national grid." North America has three major grids: The Western Interconnection
Western Interconnection
The Western Interconnection is one of the two major alternating current power grids in North America. The other major wide area synchronous grid is the Eastern Interconnection...

; The Eastern Interconnection
Eastern Interconnection
The Eastern Interconnection is one of the two major alternating current power grids in North America. The other major interconnection is the Western Interconnection...

 and the Electric Reliability Council of Texas (or ERCOT) grid.

Historically, transmission and distribution lines were owned by the same company, but since the turn of the 21st century many countries have liberalized
Electricity liberalization
Electricity liberalization refers to the liberalization of electricity markets. As electricity supply is a natural monopoly, this entails complex and costly systems of regulation to enforce a system of competition....

 the electricity market
Electricity market
In economic terms, electricity is a commodity capable of being bought, sold and traded. An electricity market is a system for effecting purchases, through bids to buy; sales, through offers to sell; and short-term trades, generally in the form of financial or obligation swaps. Bids and offers use...

 in ways that have led to the separation of the electricity transmission business from the distribution business.

Transmission lines mostly use high-voltage three-phase
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

 alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC), although single phase
Single-phase electric power
In electrical engineering, single-phase electric power refers to 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 are mostly lighting and heating, with few large electric motors...

 AC is sometimes used in railway electrification system
Railway electrification system
A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world...

s. High-voltage direct-current
High-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...

 (HVDC) technology is for greater efficiency in very long distances (typically greater than 400 miles, or 600 km); submarine power cable
Submarine power cable
Submarine power cables are major transmission cables for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water...

s (typically longer than 30 miles, or 50 km). HVDC links are increasingly used as isolating features to stabilize against control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems that may bring the rest of the network down.

Electricity is transmitted at high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

s (110 kV or above) to reduce the energy lost in long-distance transmission. Power is usually transmitted through overhead power line
Overhead power line
An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...

s. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.

A key limitation in the distribution of electricity is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated system of control is therefore required to ensure electric generation very closely matches the demand. If supply and demand are not in balance, generation plants and transmission equipment can shut down which, in the worst cases, can lead to a major regional blackout
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...

, such as occurred in the US Northeast blackouts of 1965
Northeast Blackout of 1965
The Northeast blackout of 1965 was a significant disruption in the supply of electricity on November 9, 1965, affecting Ontario, Canada and Connecticut, Massachusetts, New Hampshire, Rhode Island, Vermont, New York, and New Jersey in the United States...

, 1977
New York City blackout of 1977
The New York City blackout of 1977 was an electricity blackout affected most of New York City from July 13, 1977 to July 14, 1977. The only neighborhoods in New York City that were not affected were in southern Queens, and neighborhoods of the Rockaways, which are part of the Long Island Lighting...

, 2003
Northeast Blackout of 2003
The Northeast blackout of 2003 was a widespread power outage that occurred throughout parts of the Northeastern and Midwestern United States and Ontario, Canada on Thursday, August 14, 2003, just before 4:10 p.m....

, and in the west, one in 1996
1996 Western North-America summer blackouts
The 1996 Western North America blackouts were two widespread power outages that occurred across Western Canada, the Western United States, and Northwest Mexico on July 2 and August 10, 1996...

 and 2011. To reduce the risk of such failures, electric transmission networks are interconnected into regional, national or continental wide networks thereby providing multiple redundant
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....

 alternate routes for power to flow should (weather or equipment) failures occur. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available should there be any such failure in another part of the network.

Overhead transmission


High-voltage overhead conductors are not covered by insulation. The conductor material is nearly always an aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 alloy, made into several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission but aluminium is lower in weight for only marginally reduced performance, and much lower in cost. 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
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...

) to 750 mm2 (1,590,000 circular mils area), with varying resistance and current-carrying capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...

, that causes most of the current to flow close to the surface of the wire. Because of this current limitation, multiple parallel cables (called bundle conductors) are used when higher capacity is needed. Bundle conductors are also used at high voltages to reduce corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...

 discharge energy loss.
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 distribution
Electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

Since overhead transmission wires depend on air for insulation, design of these lines requires minimum clearances to be observed to maintain safety. Adverse weather conditions of high wind and low temperatures can lead to power outages: wind speeds as low as 23 knots (45.1 km/h) can permit conductors to encroach operating clearances, resulting in a flashover
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

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

Underground transmission



Electric power can also be transmitted by underground power cables
High voltage cable
A high voltage cable - also called HV cable - is used for electric power transmission at high voltage. High voltage cables of differing types have a variety of applications in instruments, ignition systems, AC and DC power transmission...

 instead of overhead power lines.

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
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 (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 so inefficient that 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
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....

 system with large numbers of small generators located nearby their loads.

In 1886 in Great Barrington, Massachusetts, a 1 kV
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

 alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC) distribution system was installed. That same year, AC power at 2 kV, transmitted 30 km, was installed at Cerchi, Italy. At an AIEE meeting on May 16, 1888, Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

 delivered a lecture entitled A New System of Alternating Current Motors and Transformers, describing the equipment which allowed efficient generation and use of polyphase
Polyphase system
A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...

 alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

s. The transformer, and Tesla's polyphase and single-phase induction motors, were essential for a combined AC distribution system for both lighting and machinery. Ownership of the rights to the Tesla patents was a key advantage to the Westinghouse
Westinghouse Electric (1886)
Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...

 Company in offering a complete alternating current power system for both lighting and power.

Regarded as one of the most influential electrical innovations, the universal system used transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s to step-up voltage from generators to high-voltage transmission lines, and then to step-down voltage to local distribution circuits or industrial customers. By a suitable choice of utility frequency
Utility frequency
The utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...

, both lighting and motor loads could be served. Rotary converter
Rotary converter
A rotary converter is a type of electrical machine which acts as a mechanical rectifier or inverter. It was used to convert AC to DC or DC to AC power before the advent of chemical or solid state power rectification...

s and later mercury-arc valves and other rectifier equipment allowed DC to be provided where needed. Generating stations and loads using different frequencies could be interconnected using rotary converters. By using common generating plants for every type of load, important economies of scale were achieved, lower overall capital investment was required, load factor
Load factor
Load factor may refer to:* Load factor , the ratio of the lift of an aircraft to its weight* Load factor , the ratio of the number of records to the number of addresses within a data structure...

 on each plant was increased allowing for higher efficiency, a lower cost for the consumer and increased overall use of electric power.

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 power or mine-mouth coal, could be exploited to lower energy production cost.

The first transmission of three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition
International Electro-Technical Exhibition - 1891
The 1891 International Electro-Technical Exhibition was held between 16 May and 19 October on the disused site of the three former “Westbahnhöfe” in Frankfurt am Main. The exhibition featured the first long distance transmission of high-power, three-phase electrical current, which was generated...

 in Frankfurt
Frankfurt
Frankfurt am Main , commonly known simply as Frankfurt, is the largest city in the German state of Hesse and the fifth-largest city in Germany, with a 2010 population of 688,249. The urban area had an estimated population of 2,300,000 in 2010...

. A 25 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.

The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the infrastructure in most industrialized nations. Interconnection of local generation plants and small distribution networks was greatly spurred by the requirements of World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

, where large electrical generating plants were built by governments to provide power to munitions factories. Later these 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 feasible, while at the same time taking into account economic factors, network safety and redundancy. These networks use components such as power lines, cables, circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...

s, switches and transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s. The transmission network is usually administered on a regional basis by an entity such as a regional transmission organization
Regional transmission organization
A regional transmission organization in the United States is an organization that is responsible for moving electricity over large interstate areas. Like a transmission system operator , an RTO coordinates, controls and monitors an electricity transmission grid that is larger with much higher...

 or transmission system operator
Transmission system operator
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumersrect 2 243 235 438 Power stationrect 276 317 412 556 Transformer...

.

Transmission efficiency is hugely improved by devices that increase the voltage, and proportionately reduce the current in the conductors, thus keeping the power transmitted nearly equal to the power input. The reduced current flowing through the line reduces the losses in the conductors. According to Joule's Law
Joule's law
Joule's laws are a pair of laws concerning the heat produced by a current and the energy dependence of an ideal gas to that of pressure, volume, and temperature, respectively...

, energy losses are directly proportional to the square of the current. Thus, reducing the current by a factor of 2 will lower the energy lost to conductor resistance by a factor of 4.

This change in voltage is usually achieved in AC circuits using a step-up transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

. DC systems require relatively costly conversion equipment which may be economically justified for particular projects, but are less common currently.

A transmission grid is a network of power stations, transmission circuits, and substations
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

. Energy is usually transmitted within a grid with three-phase
Three-phase
In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...

 AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

. Single phase AC is used only for distribution to end users since it is not usable for large polyphase induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...

s. In the 19th century, two-phase transmission was used but required either three wires with unequal currents or four wires. Higher order phase systems require more than three wires, but deliver marginal benefits.

The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the needed power than to generate it locally. Because nearby loads are often correlated (hot weather in the Southwest portion of the US might cause many people to use air conditioners), electricity often comes from distant sources. Because of the economics of load balancing, wide area transmission grids
Wide area synchronous grid
A wide area synchronous grid, also called an "interconnection" in North America, is a power grid at a regional scale or greater that operates at a synchronized frequency and is electrically tied together during normal system conditions...

 now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow, even if a few links are inoperative.

The unvarying (or slowly varying over many hours) portion of the electric demand is known as the base load
Base load power plant
Baseload is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements...

and is generally served best by large facilities (which are therefore efficient due to economies of scale) with low variable costs for fuel and operations. Such facilities might be nuclear or coal-fired power stations, or hydroelectric, while other renewable energy sources such as concentrated solar thermal
Solar thermal energy
Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat...

 and geothermal power
Geothermal power
Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. Earth's geothermal energy originates from the original formation of the planet and from radioactive decay of minerals...

 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 "base load" but can still add power to the grid. The remaining power demand, if any, is supplied by peaking power plant
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...

s, which are typically smaller, faster-responding, and higher cost sources, such as combined cycle or combustion turbine plants fueled by natural gas.
Long-distance transmission of electricity (thousands of kilometers) is cheap and efficient, with costs of US$0.005–0.02/kWh (compared to annual averaged large producer costs of US$0.01–0.025/kWh, retail rates upwards of US$0.10/kWh, and multiples of retail for instantaneous suppliers at unpredicted highest demand moments). Thus distant suppliers can be cheaper than local sources (e.g., New York City buys a lot of electricity from Canada). Multiple local sources (even if more expensive and infrequently used) can make the transmission grid more fault tolerant to weather and other disasters that can disconnect distant suppliers.

Long distance transmission allows remote renewable energy resources to be used to displace fossil fuel consumption. Hydro and wind sources cannot be moved closer to populous cities, and solar costs are lowest in remote areas where local power needs are minimal. Connection costs alone can determine whether any particular renewable alternative is economically sensible. Costs can be prohibitive for transmission lines, but various proposals for massive infrastructure investment in high capacity, very long distance super grid
Super grid
A super grid is a wide area transmission network that makes it possible to trade high volumes of electricity across great distances. It is sometimes also referred to as a "mega grid".-History:...

 transmission networks could be recovered with modest usage fees.

Grid input


At the generating plants
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 the energy is produced at a relatively low voltage between about 2.3 kV and 30 kV, depending on the size of the unit. The generator terminal voltage is then stepped up by the power station transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 to a higher voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 (115 kV to 765 kV AC, varying by country) for transmission over long distances.

Losses


Transmitting electricity at high voltage reduces the fraction of energy lost to resistance
Joule heating
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...

, which averages around 7%. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the I2R 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 reduced 10-fold to match the lower current the I2R losses are still reduced 10-fold. Long distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. At extremely high voltages, more than 2 MV between conductor and ground, corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

 losses are so large that they can offset the lower resistance loss in the line conductors. Measures to reduce corona losses include conductors having large diameter; often hollow to save weight, or bundles of two or more conductors.

Transmission and distribution losses in the USA were estimated at 6.6% in 1997 and 6.5% in 2007. In general, losses are estimated from the discrepancy between energy produced (as reported by power plants) and energy sold to end customers; the difference between what is produced and what is consumed constitute transmission and distribution losses.

As of 1980, the longest cost-effective distance for DC electricity was determined to be 7000 km (4,349.6 mi). For AC it was 4000 km (2,485.5 mi), though all transmission lines in use today are substantially shorter.

In an alternating current circuit, the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

 and capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

 of the phase conductors can be significant. The currents that flow in these components of the circuit impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

 constitute reactive power, which transmits no energy to the load. Reactive current causes extra losses in the transmission circuit. The ratio of real power (transmitted to the load) to apparent power is the power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...

. As reactive current increases, the reactive power increases and the power factor decreases. For systems with low power factors, losses are higher than for systems with high power factors. Utilities add capacitor banks and other components (such as phase-shifting transformers; static VAR compensator
Static VAr compensator
A static var compensator is an electrical device for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilising the system...

s; physical transposition of the phase conductors; and flexible AC transmission system
Flexible AC transmission system
A flexible alternating current transmission system is a system composed of static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network...

s, FACTS) throughout the system to control reactive power flow for reduction of losses and stabilization of system voltage.

Subtransmission


Subtransmission is part of an electric power transmission system that runs at relatively lower voltages. It is uneconomical to connect all distribution substation
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

s to the high main transmission voltage, because the equipment is larger and more expensive. Typically, only larger substations connect with this high voltage. It is stepped down and sent to smaller substations in towns and neighborhoods. Subtransmission circuits are usually arranged in loops so that a single line failure does not cut off service to a large number of customers for more than a short time. While subtransmission circuits are usually carried on overhead lines
Overhead power line
An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...

, in urban areas buried cable may be used.

There is no fixed cutoff between subtransmission and transmission, or subtransmission and distribution
Electric power distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

. The voltage ranges overlap somewhat. 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


At the substations
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

, transformers reduce the voltage to a lower level for distribution
Electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

 to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 132 kV) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (varying by country and customer requirements—see mains power systems).

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 required to be transmitted over very long distances, it is more economical to transmit using direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 instead of alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

. For a long transmission line, the lower losses and reduced construction cost of a DC line can offset the additional cost of converter stations at each end. Also, at high AC voltages, significant (although economically acceptable) amounts of energy are lost due to corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

, the capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

 between phases or, in the case of buried cables, between phases and the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 or water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 in which the cable is buried.

HVDC is also used for long submarine cables
Submarine power cable
Submarine power cables are major transmission cables for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water...

 because over about 30 km length AC can no longer be applied. In that case special high voltage cable
High voltage cable
A high voltage cable - also called HV cable - is used for electric power transmission at high voltage. High voltage cables of differing types have a variety of applications in instruments, ignition systems, AC and DC power transmission...

s for DC are built. Many submarine cable connections – up to 600 km length – are in use nowadays.

HVDC links are sometimes used to stabilize against control problems with the AC electricity flow. The power transmitted by an AC line increases as the phase angle between source end voltage and destination ends increases, but too great a phase angle will allow the generators at either end of the line to fall out of step. Since the power flow in a DC link is controlled independently of the phases of the AC networks at either end of the link, this stability limit does not apply to a DC line, and it can transfer its full thermal rating. A DC link stabilizes the AC grids at either end, since power flow and phase angle can be controlled independently.

In other words, to transmit AC power as AC when needed in either direction between Seattle and Boston
Boston
Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...

 would require the (highly challenging) continuous real-time adjustment of the relative phase
Polyphase system
A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...

 of the two electrical grids. With HVDC instead the interconnection would: (1) Convert AC in Seattle into HVDC. (2) Use HVDC for the three thousand miles of cross country transmission. Then (3) convert the HVDC to locally synchronized AC in Boston, and optionally in other cooperating cities along the transmission route. One prominent example of such a transmission line is the Pacific DC Intertie located in the Western United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

.

Limitations


The amount of power that can be sent over a transmission line is limited. The origins of the limits vary depending on the length of the line. For a short line, the heating of 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 be damaged by overheating. For intermediate-length lines on the order of 100 km (62.1 mi), the limit is set by the voltage drop
Voltage drop
Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...

 in the line. For longer AC lines, system stability sets the limit to the power that can be transferred. Approximately, the power flowing over an AC line is proportional to the sine of the phase angle of the voltage at the receiving and transmitting ends. Since this angle varies depending on system loading and generation, it is undesirable for the angle to approach 90 degrees. Very approximately, the allowable product of line length and maximum load is proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. High-voltage direct current lines are restricted only by thermal and voltage drop limits, since the phase angle is not material to their operation.

Up to now, it has been almost impossible to foresee the temperature distribution along the cable route, so that the maximum applicable current load was usually set as a compromise between understanding of operation conditions and risk minimization. The availability of industrial Distributed Temperature Sensing
Distributed temperature sensing
Distributed temperature sensing systems 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 continuous profile. A high accuracy of temperature...

 (DTS) systems that measure in real time temperatures all along the cable is a first step in monitoring the transmission system capacity. This monitoring solution is based on using passive optical fibers as temperature sensors, either integrated directly inside a high voltage cable or mounted externally on the cable insulation. A solution for overhead lines is also available. In this case the optical fiber is integrated into the core of a phase wire of overhead transmission lines (OPPC). The integrated Dynamic Cable Rating (DCR) or also called Real Time Thermal Rating (RTTR) solution enables not only to continuously monitor the temperature of a high voltage cable circuit in real time, but to safely utilize the existing network capacity to its maximum. Furthermore it provides the ability to the operator to predict the behavior of the transmission system upon major changes made to its initial operating conditions.

Control


To ensure safe and predictable operation the components of the transmission system 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 for the customers.

Load balancing


The transmission system provides for base load and peak load capability
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...

, with safety and fault tolerance margins. The peak load times vary by region largely due to the industry mix. In very hot and very cold climates home air conditioning and heating loads have an effect on 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. This makes the power requirements vary by the season and the 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 the loads with the generation. Thus generation has to be kept matched to the load, to prevent overloading failures of the 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, and it involves synchronization of the generation units
Alternator synchronization
The process of connecting an AC generator to other AC generators is known as synchronization and is crucial for the generation of AC electrical power....

, to prevent large transients and overload conditions.

In distributed power generation
Distributed generation
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....

 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. To load balance the voltage and frequency can be used as a signaling mechanism.

In voltage signaling, the variation of voltage is used 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
In electricity generation, droop speed control is the primary instantaneous system using net frequency deviations to distribute with stability load changes over generating plants....

, 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 turbines, v2g and other distributed storage and generation systems can be connected to the power grid, and interact with it to improve system operation.

Failure protection


Under excess load conditions, the system can be designed to fail gracefully rather than all at once. Brownouts
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...

 occur when the supply power drops below the demand. Blackouts
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...

 occur when the supply fails completely.

Rolling blackout
Rolling blackout
A rolling blackout, also referred to as load shedding, is an intentionally-engineered electrical power outage where electricity delivery is stopped for non-overlapping periods of time over geographical regions. Rolling blackouts are a last-resort measure used by an electric utility company in order...

s (also called load shedding) are intentionally engineered electrical power outages, used to distribute insufficient power when the demand for electricity exceeds the supply.

Communications


Operators of long transmission lines require reliable communications for control
SCADA
SCADA generally refers to industrial control systems : computer systems that monitor and control industrial, infrastructure, or facility-based processes, as described below:...

 of the power grid and, often, associated generation and distribution facilities. Fault-sensing protective relay
Protective relay
In electrical engineering, a protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected...

s at each end of the line must communicate to monitor the flow of power into and out of the protected line section 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
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....

s and other faults is usually so critical that common carrier
Common carrier
A common carrier in common-law countries is a person or company that transports goods or people for any person or company and that is responsible for any possible loss of the goods during transport...

 telecommunications are insufficiently reliable, and in remote areas a common carrier may not be available. Communication systems associated with a transmission project may use:
  • Microwave
    Microwave
    Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

    s
  • Power line communication
    Power line communication
    Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...

  • Optical fiber
    Optical fiber
    An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

    s

Rarely, and for short distances, a utility will use pilot-wires strung along the transmission line path. Leased circuits from common carriers are not preferred since availability is not under control of the electric power transmission organization.

Transmission lines can also be used to carry data: this is called power-line carrier, or PLC
Power line communication
Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...

. PLC signals can be easily received with a radio for the long wave range.

Optical fibers can be included in the stranded conductors of a transmission line, in the overhead shield wires. These cables are known as optical ground wire (OPGW). Sometimes a standalone cable is used, all-dielectric self-supporting (ADSS) cable, attached to the transmission line cross arms.

Some jurisdictions, such as Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

, prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications common carrier
Common carrier
A common carrier in common-law countries is a person or company that transports goods or people for any person or company and that is responsible for any possible loss of the goods during transport...

. Where the regulatory structure permits, the utility can sell capacity in extra dark fiber
Dark fiber
A dark fiber or unlit fiber is an unused Optical fiber, available for use in fiber-optic communication.The term dark fiber was originally used when referring to the potential network capacity of telecommunication infrastructure, but now also refers to the increasingly common practice of leasing...

s to a common carrier, providing another revenue stream.

Electricity market reform



Some regulators regard electric transmission to be a natural monopoly
Natural monopoly
A monopoly describes a situation where all sales in a market are undertaken by a single firm. A natural monopoly by contrast is a condition on the cost-technology of an industry whereby it is most efficient for production to be concentrated in a single form...

 and there are moves in many countries to separately regulate transmission (see electricity market
Electricity market
In economic terms, electricity is a commodity capable of being bought, sold and traded. An electricity market is a system for effecting purchases, through bids to buy; sales, through offers to sell; and short-term trades, generally in the form of financial or obligation swaps. Bids and offers use...

).

Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

 was the first country to establish a regional transmission organization. In that country transmission operations and market operations are controlled by separate companies. The transmission system operator is Red Eléctrica de España
Red Eléctrica de España
Red Eléctrica de España is a Spanish corporation which operates the nation's power transmission system and electricity grid.The company was created in 1985 by the Spanish Government-owned holding company Instituto Nacional de Industria to provide a unified national power grid.The firm was also...

 (REE) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía – Polo Español, S.A. (OMEL) http://www.omel.es. Spain's transmission system is interconnected with those of France, Portugal, and Morocco.

In the United States and parts of Canada, electrical transmission companies operate independently of generation and distribution companies.

Cost of electric power transmission


The cost of high voltage electricity transmission (as opposed to the costs of electricity distribution
Electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

) is comparatively low, compared to all other costs arising in a consumer's electricity bill. In the UK transmission costs are about 0.2p/kWh compared to a delivered domestic price of around 10 p/kWh.

Research evaluates the level of capital expenditure in the electric power T&D equipment market will be worth $128.9bn in 2011.

Merchant transmission


Merchant transmission is an arrangement where a third party constructs and operates electric transmission lines through the franchise area of an unrelated utility. Advocates of merchant transmission claim that this will create competition to construct the most efficient and lowest cost additions to the transmission grid. Merchant transmission projects typically involve DC lines because it is easier to limit flows to paying customers.

Operating merchant transmission projects in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 include the Cross Sound Cable
Cross Sound Cable
The Cross Sound Cable is a long bipolar high-voltage direct current submarine power cable between New Haven, Connecticut, USA and Shoreham, Long Island New York , USA. The Cross Sound Cable can transmit a maximum power of 330 MW at a voltage of +/- 150 kV DC. The maximum current for Cross Sound...

 from Long Island, New York to New Haven, Connecticut
New Haven, Connecticut
New Haven is the second-largest city in Connecticut and the sixth-largest in New England. According to the 2010 Census, New Haven's population increased by 5.0% between 2000 and 2010, a rate higher than that of the State of Connecticut, and higher than that of the state's five largest cities, and...

, Neptune RTS Transmission Line from Sayreville, N.J., to Newbridge, N.Y, ITC Holdings, Inc. transmission system in the midwest, and Path 15
Path 15
Path 15 is an portion of the north-south power transmission corridor in California, U.S. It forms a part of the Pacific AC Intertie and the California-Oregon Transmission Project....

 in California. Additional projects are in development or have been proposed throughout the United States.

There is only one unregulated or market interconnector in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

: Basslink
Basslink
Basslink is a high-voltage direct current cable link crossing Bass Strait, connecting the Loy Yang Power Station, Victoria on the Australian mainland to the George Town substation in northern Tasmania...

 between Tasmania and Victoria. Two DC links originally implemented as market interconnectors Directlink
Directlink
Terranora interconnector is a high voltage direct current electricity transmission line between Mullumbimby, New South Wales and Bungalora, New South Wales in Australia. It is one of the two interconnections used to trade electricity between New South Wales and Queensland...

 and Murraylink
Murraylink
Murraylink is an Australian high voltage direct current electricity transmission link between Berri in South Australia and Red Cliffs in Victoria, connecting the two state electricity grids...

 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 other utility businesses.

Health concerns



Some large studies, including a large United States study, have failed to find any link between living near power lines and developing any sickness or diseases such as cancer. One old study from 1997 found that it did not matter how close you were 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 powerlines.

There are established biological effects for acute
Acute toxicity
Acute toxicity describes the adverse effects of a substance that result either from a single exposure or from multiple exposures in a short space of time...

 high level exposure to magnetic fields well above 100 µT
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

 (1000 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 leukaemia, associated with average exposure to residential power-frequency magnetic field above 0.3 (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.

Government policy


Historically, local governments have exercised authority over the grid and have significant disincentives to take action that would benefit states other than their own. Localities with cheap electricity have a disincentive to making interstate commerce in electricity trading easier, since other regions will be able to compete for local energy and drive up rates. Some regulators in Maine for example 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 4 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 energy secretary
United States Secretary of Energy
The United States Secretary of Energy is the head of the United States Department of Energy, a member of the President's Cabinet, and fifteenth in the presidential line of succession. The position was formed on October 1, 1977 with the creation of the Department of Energy when President Jimmy...

 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 2005 energy act
Energy Policy Act of 2005
The Energy Policy Act of 2005 is a bill passed by the United States Congress on July 29, 2005, and signed into law by President George W. Bush on August 8, 2005, at Sandia National Laboratories in Albuquerque, New Mexico...

 giving the Department of Energy the authority to approve transmission if states refuse to act. However, soon after using its power to designate two National Interest Electric Transmission Corridor
National Interest Electric Transmission Corridor
A National Interest Electric Transmission Corridor corridor is a geographic region designated by the United States Department of Energy where electricity transmission limitations are adversely affecting American citizens...

s, 14 senators signed a letter stating the DOE was being too aggressive.

Grids for railways



In some countries where electric train
Electric train
Electric train may refer to:* Diesel-electric locomotive...

s run on low frequency AC (e.g., 16.7 Hz and 25 Hz) power, there are separate single phase traction power network
Traction power network
A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is only done if the railway in question uses alternating current with a frequency lower than that of the national grid, such as...

s operated by the railways. These grids are fed by separate generators in some traction powerstation
Traction powerstation
A traction power station is a power station that produces only traction current, that is, electric current used for railways, trams, trolleybuses or other conveyances. Pure traction power stations are rare and there are many more power stations that generate current for other purposes, such as...

s or by traction current converter plant
Traction current converter plant
A traction substation or traction current converter plant is an electrical substation that converts electric power from the form provided by the electrical power industry for public utility service to an appropriate voltage, current type and frequency to supply railways, trams and/or trolleybuses...

s from the public three phase AC network.

Superconducting cables


High-temperature superconductors 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
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

 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
Consolidated Edison
Consolidated Edison, Inc. is one of the largest investor-owned energy companies in the United States, with approximately $14 billion in annual revenues and $36 billion in assets...

 and American Superconductor
American Superconductor
American Superconductor is an American energy technologies company based in Devens, Massachusetts specializing in the design and manufacture of power systems and superconducting wire...

 have already begun commercial production of such systems. In one hypothetical future system called a SuperGrid
SuperGrid
The SuperGrid refers the existing high voltage transmission grid in Great Britain that was constructed in the 1960s, and to various overseas parallels, including the concept, again in the British context, of increased connectivity to our neighbours...

, 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
Easement
An easement is a certain right to use the real property of another without possessing it.Easements are helpful for providing pathways across two or more pieces of property or allowing an individual to fish in a privately owned pond...

 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, and light rail. Single wire earth return is also used for HVDC over submarine power cables.

Wireless power transmission



Both Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

 and Hidetsugu Yagi
Hidetsugu Yagi
Hidetsugu Yagi was a Japanese electrical engineer. When working at Tohoku University, he wrote several important articles that introduced a new antenna design by his colleague Shintaro Uda to the English-speaking world.The Yagi antenna, patented in 1926, allows directional communication using...

 attempted to devise systems for large scale wireless power transmission, 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
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 or laser transmitters would beam power to a rectenna
Rectenna
A rectenna is a rectifying antenna, a special type of antenna that is used to directly convert microwave energy into DC electricity. Its elements are usually arranged in a multi element phased array with a mesh pattern reflector element to make it directional...

. Major engineering and economic challenges face any solar power satellite project.

Security of control systems


The Federal government of the United States
Federal government of the United States
The federal government of the United States is the national government of the constitutional republic of fifty states that is the United States of America. The federal government comprises three distinct branches of government: a legislative, an executive and a judiciary. These branches and...

 admits that the power grid is susceptible to cyber-warfare
Cyber-warfare
Cyberwarfare refers to politically motivated hacking to conduct sabotage and espionage. It is a form of information warfare sometimes seen as analogous to conventional warfare although this analogy is controversial for both its accuracy and its political motivation.Government security expert...

. The United States Department of Homeland Security
United States Department of Homeland Security
The United States Department of Homeland Security is a cabinet department of the United States federal government, created in response to the September 11 attacks, and with the primary responsibilities of protecting the territory of the United States and protectorates from and responding to...

 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.

Records

  • Highest capacity system: 6.3 GW HVDC Itaipu
    HVDC Itaipu
    The HVDC Itaipu is a High Voltage Direct Current transmission line in Brazil from the Itaipu hydroelectric power plant to the region of São Paulo. The project has two bipolar lines, which run from the generator site at Foz do Iguaçu in Paraná to the "load" site Ibiúna near São Roque, São Paulo...

     (Brazil) (±600 kV DC)
  • Highest transmission voltage (AC): 1.15 MV on Powerline Ekibastuz-Kokshetau
    Powerline Ekibastuz-Kokshetau
    The Ekibastuz–Kokshetau is a power transmission line in Kazakhstan. It runs at 1,150 kV and holds the record for having the highest operating transmission voltage in the world....

     (Kazakhstan)
  • Largest double-circuit transmission, Kita-Iwaki Powerline.
  • Highest pylons
    Electricity pylon
    A transmission tower is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes...

    : Yangtze River Crossing
    Yangtze River Crossing
    The Yangtze River Power Line Crossings refer to overhead power lines that cross the Yangtze River in China. There are at least three power line crossings on the Yangtze River at Jiangyin, Nanjing, and Wuhu. The one at Jiangyin has the tallest electrical pylons in the world.-Jiangyin:One exists...

     (height: 345 m (1,132 ft))
  • Longest power line: Inga-Shaba
    Inga-Shaba
    The Inga–Shaba EHVDC Intertie is a high-voltage direct current overhead electric power transmission line in the Democratic Republic of Congo, linking the Inga hydroelectric complex at the mouth of the Congo River to mineral fields in Shaba...

     (length: 1700 kilometres (1,056 mi))
  • Longest span of power line: 5376 m (17,638 ft) at Ameralik Span
    Ameralik Span
    The Ameralik Span is the longest span of an electrical overhead powerline in the world. It is situated near Nuuk on Greenland and crosses Ameralik fjord with a span width of 5,376 metres at...

  • Longest submarine cables:
    • NorNed
      NorNed
      NorNed is a long HVDC submarine power cable between Feda in Norway and the seaport of Eemshaven in the Netherlands, which interconnects both countries' electricity grids. It is the longest submarine power cable in the world. Budgeted at €550 million, and completed at a cost of €600m, the...

      , North Sea
      North Sea
      In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

       – (length of submarine cable: 580 kilometres (360 mi))
    • Basslink
      Basslink
      Basslink is a high-voltage direct current cable link crossing Bass Strait, connecting the Loy Yang Power Station, Victoria on the Australian mainland to the George Town substation in northern Tasmania...

      , Bass Strait
      Bass Strait
      Bass Strait is a sea strait separating Tasmania from the south of the Australian mainland, specifically the state of Victoria.-Extent:The International Hydrographic Organization defines the limits of the Bass Strait as follows:...

       – (length of submarine cable: 290 kilometres (180 mi), total length: 370.1 kilometres (230 mi))
    • Baltic-Cable, Baltic Sea
      Baltic Sea
      The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

       – (length of submarine cable: 238 kilometres (148 mi), HVDC length: 250 kilometres (155 mi), total length: 262 kilometres (163 mi))
  • Longest underground cables:
    • Murraylink
      Murraylink
      Murraylink is an Australian high voltage direct current electricity transmission link between Berri in South Australia and Red Cliffs in Victoria, connecting the two state electricity grids...

      , Riverland
      Riverland
      The Riverland, is a region of South Australia. It covers the area near the Murray River from where it flows into South Australia downstream to Blanchetown.The major town centres are Renmark, Berri, Loxton, Waikerie and Barmera...

      /Sunraysia
      Sunraysia
      Sunraysia is an area of northwestern Victoria and southwestern New South Wales in Australia, known for its sunshine, grapes and oranges. Its main centre is Mildura....

       – (length of underground cable: 180 kilometres (112 mi))

See also


  • Conductor marking lights
    Conductor marking lights
    Conductor marking lights are a particular type of non-standard aircraft warning lights designed for overhead power lines.-Background:Power distribution on overhead wires, often suspended to widely spaced masts, represents an almost invisible obstacle to low flying aircraft, requiring the...

  • Dynamic demand (electric power)
    Dynamic demand (electric power)
    Dynamic Demand is the name of a semi-passive technology for adjusting load demands on an electrical power grid. The concept is that by monitoring the frequency of the power grid, as well as their own...

  • Demand response
    Demand response
    In electricity grids, demand response is similar to dynamic demand mechanisms to manage customer consumption of electricity in response to supply conditions, for example, having electricity customers reduce their consumption at critical times or in response to market prices...

  • Distributed generation
    Distributed generation
    Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....

  • Double-circuit transmission line
  • Electricity distribution
    Electricity distribution
    File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

  • Electricity market
    Electricity market
    In economic terms, electricity is a commodity capable of being bought, sold and traded. An electricity market is a system for effecting purchases, through bids to buy; sales, through offers to sell; and short-term trades, generally in the form of financial or obligation swaps. Bids and offers use...

  • Electricity pylon
    Electricity pylon
    A transmission tower is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes...

  • Electromagnetic Transients Program
    Emtp
    EMTP is an acronym for ElectroMagnetic Transients Program. It is usually part of a battery of software tools targeting a slice of the spectrum of design and operation problems presented by Electric Power Systems to the Electrical Engineer, that of the so called "electromagnetic transients" and...

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

     (FACTS)
  • Geomagnetically induced current
    Geomagnetically induced current
    Geomagnetically induced currents , affecting the normal operation of long electrical conductor systems, are a manifestation at ground level of space weather. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations, which manifest also in the...

    , (GIC
    GIC
    The acronym GIC can refer to :* Guaranteed Investment Certificate, a financial instrument* Guaranteed Investment Contract, an insurance contract* Gender Identity Clinic, for the treatment of disorders related to gender variance...

    )
  • Green power grid
  • Grid-tied electrical system
    Grid-tied electrical system
    A grid-tied electrical system, also called tied to grid or grid tie system, is a semi-autonomous electrical generation or grid energy storage system which links to the mains to feed excess capacity back to the local mains electrical grid...

  • High-voltage direct current
    High-voltage direct current
    A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...

     (HVDC)
  • Infrastructure
    Infrastructure
    Infrastructure is basic physical and organizational structures needed for the operation of a society or enterprise, or the services and facilities necessary for an economy to function...

  • List of high voltage underground and submarine cables
  • Load profile
    Load profile
    In electrical engineering, a load profile is a graph of the variation in the electrical load versus time. A load profile will vary according to customer type , temperature and holiday seasons....


  • Mains electricity
    Mains electricity
    Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

  • Mains power around the world (list of voltage by country)
  • Miesbach-Munich Power Transmission
    Miesbach-Munich Power Transmission
    Miesbach–Munich Power Transmission was the firsttransmission of electrical energy over a large distance.It took place in 1882 between a steam engine situated near Miesbach and the glass palace of Munich, where world's first international electricity exhibition took place. The voltage used was 2000...

  • Off-the-grid
    Off-the-grid
    The term off-the-grid or off-grid refers to living in a self-sufficient manner without reliance on one or more public utilities....

    , living without public utility
  • Overhead power line
    Overhead power line
    An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...

  • Path 27 and Path 46
    Path 46
    Path 46, also called West of Colorado River, Arizona-California West-of-the-River Path , is a set of many alternating current high-voltage transmission lines that are located in southeast California and Nevada up to the Colorado River...

  • Power line communication
    Power line communication
    Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...

    s (PLC)
  • Power System Harmonics
    Power system harmonics
    Power system harmonics are integer multiples of the fundamental power system frequency. Power system harmonics are created by non-linear devices connected to the power system...

  • Power outage
    Power outage
    A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...

  • Radio frequency power transmission
    Radio frequency power transmission
    Radio frequency power transmission is the transmission of the output power of a transmitter to an antenna. When the antenna is not situated close to the transmitter, special transmission lines are required....

  • Relative cost of electricity generated by different sources
  • Submarine power cable
    Submarine power cable
    Submarine power cables are major transmission cables for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water...

  • Traction current
  • Traction power network
    Traction power network
    A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is only done if the railway in question uses alternating current with a frequency lower than that of the national grid, such as...

  • Three-phase electric power
    Three-phase electric power
    Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

  • V2G
  • Wheeling (electric power transmission)
    Wheeling (electric power transmission)
    In electric power transmission, wheeling is a term used to describe either of the following:* the act of transporting electric power over transmission lines, or;...

  • Wireless energy transfer
    Wireless energy transfer
    Wireless energy transfer or wireless power is the transmission of electrical energy from a power source to an electrical load without artificial interconnecting conductors. Wireless transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible...

  • 25Hz Power Transmission System
    25Hz Power Transmission System
    Amtrak's 25 Hz Traction Power System is a traction power grid operated by Amtrak along the southern portion of its Northeast Corridor : the 363 route miles between Washington, D.C. and New York City and the 104 route miles between Philadelphia and Harrisburg, Pennsylvania. The Pennsylvania...


Further reading

  • Grigsby, L. L., et al. The Electric Power Engineering Handbook. USA: CRC Press. (2001). ISBN 0-8493-8578-4
  • Thomas P. Hughes
    Thomas P. Hughes
    Thomas Parke Hughes is an American Historian of Technology. He is an emeritus professor of history at the University of Pennsylvania and is a visiting professor at MIT and Stanford.He received his Ph.D. from the University of Virginia in 1953....

    , Networks of Power: Electrification in Western Society 1880–1930, The Johns Hopkins University Press,Baltimore 1983 ISBN 0-8018-2873-2, an excellent overview of development during the first 50 years of commercial electric power
  • Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system". (Transmission of power; polyphase system; Tesla patents)
  • Pansini, Anthony J, E.E., P.E. undergrounding electric lines. USA Hayden Book Co, 1978. ISBN 0-8104-0827-9

External links