Electric Power Transmission

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Electric power transmission 1 Electric power transmission Two-circuit, single-voltage power transmission line; "Bundled" 4-ways Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations 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. Transmission lines, when interconnected with each other, become transmission networks. The combined transmission and distribution network is known as the "power grid" in the United States, or just "the grid". In the United Kingdom, the network is known as the "National Grid". A wide area synchronous grid, also known as an "interconnection" in North America, directly connects a large number of generators outputting AC power with the same phase, to a large number of consumers. For example, there are three major interconnections in North America (the Western Interconnection, the Eastern Interconnection and the Electric Reliability Council of Texas (ERCOT) grid), and one large grid for most of continental Europe. Historically, transmission and distribution lines were owned by the same company, but starting in the 1990s, many countries have liberalized the regulation of the electricity market in ways that have led to the separation of the electricity transmission business from the distribution business. System Most transmission lines use high-voltage three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used for greater efficiency in very long distances (typically hundreds of miles (kilometres)), or in submarine power cables (typically longer than 30 miles (50 km)). HVDC links are also used 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 and cascading failures. Diagram of an electric power system; transmission system is in blue Electricity is transmitted at high voltages (120 kV or above) to reduce the energy lost in long-distance transmission. Power is usually transmitted through overhead power lines. 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 electric power is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated control system is required to ensure electric generation very closely matches the demand. If the demand for power exceeds the supply, generation plants and

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Transcript of Electric Power Transmission

Electric power transmission 1

Electric power transmission

Two-circuit, single-voltage power transmissionline; "Bundled" 4-ways

Electric-power transmission is the bulk transfer of electrical energy,from generating power plants to electrical substations located neardemand centers. This is distinct from the local wiring betweenhigh-voltage substations and customers, which is typically referred to aselectric power distribution. Transmission lines, when interconnectedwith each other, become transmission networks. The combinedtransmission and distribution network is known as the "power grid" inthe United States, or just "the grid". In the United Kingdom, the networkis known as the "National Grid".

A wide area synchronous grid, also known as an "interconnection" inNorth America, directly connects a large number of generatorsoutputting AC power with the same phase, to a large number ofconsumers. For example, there are three major interconnections in NorthAmerica (the Western Interconnection, the Eastern Interconnection andthe Electric Reliability Council of Texas (ERCOT) grid), and one largegrid for most of continental Europe.

Historically, transmission and distribution lines were owned by the samecompany, but starting in the 1990s, many countries have liberalized theregulation of the electricity market in ways that have led to theseparation of the electricity transmission business from the distributionbusiness.

System

Most transmission lines use high-voltage three-phase alternating current (AC), although single phase AC issometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used forgreater efficiency in very long distances (typically hundreds of miles (kilometres)), or in submarine power cables(typically longer than 30 miles (50 km)). HVDC links are also used to stabilize against control problems in largepower distribution networks where sudden new loads or blackouts in one part of a network can otherwise result insynchronization problems and cascading failures.

Diagram of an electric power system; transmission system is in blue

Electricity is transmitted at highvoltages (120 kV or above) to reducethe energy lost in long-distancetransmission. Power is usuallytransmitted through overhead powerlines. Underground power transmissionhas a significantly higher cost andgreater operational limitations but issometimes used in urban areas orsensitive locations.

A key limitation in the distribution of electric power is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated control system is required to ensure electric generation very closely matches the demand. If the demand for power exceeds the supply, generation plants and

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transmission equipment can shut down which, in the worst cases, can lead to a major regional blackout, such asoccurred in the US Northeast blackouts of 1965, 1977, 2003, and other regional blackouts in 1996 and 2011. Toreduce the risk of such failures, electric transmission networks are interconnected into regional, national orcontinental wide networks thereby providing multiple redundant alternative routes for power to flow should (weatheror equipment) failures occur. Much analysis is done by transmission companies to determine the maximum reliablecapacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available shouldthere be any such failure in another part of the network.

Four-circuit, two-voltage power transmissionline; "Bundled" 2-ways

Overhead transmission

The electric power transmission grid of the contiguous United States consists of300,000 km of lines operated by 500 companies.

High-voltage overhead conductors are notcovered by insulation. The conductormaterial is nearly always an aluminiumalloy, made into several strands and possiblyreinforced with steel strands. Copper wassometimes used for overhead transmissionbut aluminium is lighter, yields onlymarginally reduced performance and costsmuch less. Overhead conductors are acommodity supplied by several companiesworldwide. Improved conductor materialand shapes are regularly used to allowincreased capacity and modernizetransmission circuits. Conductor sizes rangefrom 12 mm2 (#6 American wire gauge) to

750 mm2 (1,590,000 circular mils area), with varying resistance and current-carrying capacity. Thicker wires wouldlead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close tothe surface of the wire. Because of this current limitation, multiple parallel cables (called bundle conductors) areused when higher capacity is needed. Bundle conductors are also used at high voltages to reduce energy loss causedby corona discharge.

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3-phase high-voltage lines in Washington State

Cable sample: mid 7 conductors steel & the bulk beingaluminium; This conductor is usually referred to as

ACSR (Aluminum conductor, steel reinforced)

Today, transmission-level voltages are usually considered to be110 kV and above. Lower voltages such as 66 kV and 33 kV areusually considered subtransmission voltages but are occasionallyused on long lines with light loads. Voltages less than 33 kV areusually used for distribution. Voltages above 230 kV areconsidered extra high voltage and require different designscompared to equipment used at lower voltages.

Since overhead transmission wires depend on air for insulation,design of these lines requires minimum clearances to be observedto maintain safety. Adverse weather conditions of high wind andlow temperatures can lead to power outages. Wind speeds as lowas 23 knots (43 km/h) can permit conductors to encroach operatingclearances, resulting in a flashover and loss of supply.[1]

Oscillatory motion of the physical line can be termed gallop orflutter depending on the frequency and amplitude of oscillation.

Underground transmission

Electric power can also be transmitted by underground powercables instead of overhead power lines. Underground cables takeup less right-of-way than overhead lines, have lower visibility, andare less affected by bad weather. However, costs of insulated cableand excavation are much higher than overhead construction. Faultsin buried transmission lines take longer to locate and repair.Underground lines are strictly limited by their thermal capacity,which permits less overload or re-rating than overhead lines. Longunderground cables have significant capacitance, which mayreduce their ability to provide useful power to loads.

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History

New York City streets in 1890. Besides telegraph lines,multiple electric lines were required for each class of device

requiring different voltages

In the early days of commercial electric power, transmissionof electric power at the same voltage as used by lighting andmechanical loads restricted the distance between generatingplant and consumers. In 1882, generation was with directcurrent (DC), which could not easily be increased in voltagefor long-distance transmission. Different classes of loads (forexample, lighting, fixed motors, and traction/railwaysystems) required different voltages, and so used differentgenerators and circuits.Wikipedia:Citing sources

Due to this specialization of lines and because transmissionwas inefficient for low-voltage high-current circuits,generators needed to be near their loads. It seemed at thetime, that the industry would develop into what is nowknown as a distributed generation system with large numbersof small generators located near their loads.

In 1886, in Great Barrington, Massachusetts, a 1 kValternating current (AC) distribution system was installed.That same year, AC power at 2 kV, transmitted 30 km, wasinstalled at Cerchi, Italy. At an AIEE meeting on May 16,1888, Nikola Tesla delivered a lecture entitled A New Systemof Alternating Current Motors and Transformers, describing the equipment which allowed efficient generation anduse of polyphase alternating currents. 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 theTesla patents was a key advantage to the Westinghouse Company in offering a complete alternating current powersystem for both lighting and power.

Regarded as one of the most influential electrical innovations, the universal system used transformers to step-upvoltage from generators to high-voltage transmission lines, and then to step-down voltage to local distributioncircuits or industrial customers. By a suitable choice of utility frequency, both lighting and motor loads could beserved. Rotary converters and later mercury-arc valves and other rectifier equipment allowed DC to be providedwhere needed. Generating stations and loads using different frequencies could be interconnected using rotaryconverters. By using common generating plants for every type of load, important economies of scale were achieved,lower overall capital investment was required, load factor on each plant was increased allowing for higher efficiency,a lower cost for the consumer and increased overall use of electric power.

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Nikola Tesla's alternating current polyphase generators on display at the1893 World's Fair in Chicago. Tesla's polyphase innovations

revolutionized transmission

By allowing multiple generating plants to beinterconnected over a wide area, electricityproduction cost was reduced. The most efficientavailable plants could be used to supply the varyingloads during the day. Reliability was improved andcapital investment cost was reduced, since stand-bygenerating capacity could be shared over manymore customers and a wider geographic area.Remote and low-cost sources of energy, such ashydroelectric power or mine-mouth coal, could beexploited to lower energy production cost.

The first transmission of three-phase alternatingcurrent using high voltage took place in 1891during the international electricity exhibition in

Frankfurt. 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-fivetransmission systems each operating at more than 70 kV were in service. The highest voltage then used was150 kV.[2]

The rapid industrialization in the 20th century made electrical transmission lines and grids a critical infrastructureitem in most industrialized nations. Interconnection of local generation plants and small distribution networks wasgreatly spurred by the requirements of World War I, with large electrical generating plants built by governments toprovide power to munitions factories. Later these generating plants were connected to supply civil loads throughlong-distance transmission.[3]

Bulk power transmission

A transmission substation decreases the voltage of incomingelectricity, allowing it to connect from long distance high voltagetransmission, to local lower voltage distribution. It also reroutes

power to other transmission lines that serve local markets. This is thePacifiCorp Hale Substation, Orem, Utah, USA

Engineers design transmission networks to transport theenergy as efficiently as feasible, while at the same timetaking into account economic factors, network safetyand redundancy. These networks use components suchas power lines, cables, circuit breakers, switches andtransformers. The transmission network is usuallyadministered on a regional basis by an entity such as aregional transmission organization or transmissionsystem operator.

Transmission efficiency is greatly improved by devicesthat increase the voltage, (and thereby proportionatelyreduce the current) in the line conductors, thus allowingpower to be transmitted with acceptable losses. Thereduced current flowing through the line reduces theheating losses in the conductors. According to Joule'sLaw, energy losses are directly proportional to thesquare of the current. Thus, reducing the current by afactor of 2 will lower the energy lost to conductor resistance by a factor of 4 for any given size of conductor.

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The optimum size of a conductor for a given voltage and current can be estimated by Kelvin's law for conductor sizewhich states that the size is at its optimum when the annual cost of energy wasted in the resistance is equal to theannual capital charges of providing the conductor. At times of lower interest rates Kelvin's law indicates that thickerwires are optimal, while when metals are expensive thinner conductors are indicated: however power lines aredesigned for long-term usage so Kelvin's law has to be used in conjunction with long-term estimates of the price ofcopper and aluminium as well as interest rates for capital.The increase in voltage is achieved in AC circuits by using a step-up transformer. HVDC systems require relativelycostly conversion equipment which may be economically justified for particular projects such as submarine cablesand longer distance high capacity point to point transmission. HVDC is necessary for the import and export ofenergy between grid systems that are not synchronised with each other.A transmission grid is a network of power stations, transmission lines, and substations. Energy is usually transmittedwithin a grid with three-phase AC. Single-phase AC is used only for distribution to end users since it is not usablefor large polyphase induction motors. In the 19th century, two-phase transmission was used but required either fourwires or three wires with unequal currents. Higher order phase systems require more than three wires, but deliverlittle or no benefit.

The synchronous grids of the European Union.

The price of electric power station capacity is high, andelectric demand is variable, so it is often cheaper toimport some portion of the needed power than togenerate it locally. Because loads are often regionallycorrelated (hot weather in the Southwest portion of theUS might cause many people to use air conditioners),electric power often comes from distant sources.Because of the economic benefits of load sharingbetween regions, wide area transmission grids nowspan countries and even continents. The web ofinterconnections between power producers andconsumers should enable power to flow, even if somelinks are inoperative.

The unvarying (or slowly varying over many hours)portion of the electric demand is known as the baseload and is generally served by large facilities (whichare more efficient due to economies of scale) with fixedcosts for fuel and operation. Such facilities are nuclear,

coal-fired or hydroelectric, while other energy sources such as concentrated solar thermal and geothermal powerhave 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 plants, which are typically smaller,faster-responding, and higher cost sources, such as combined cycle or combustion turbine plants fueled by naturalgas.

Long-distance transmission of electricity (thousands of kilometers) is cheap and efficient, with costs ofUS$0.005–0.02/kWh (compared to annual averaged large producer costs of US$0.01–0.025/kWh, retail ratesupwards of US$0.10/kWh, and multiples of retail for instantaneous suppliers at unpredicted highest demandmoments). Thus distant suppliers can be cheaper than local sources (e.g., New York often buys over 1000 MW ofelectricity from Canada). Multiple local sources (even if more expensive and infrequently used) can make thetransmission grid more fault tolerant to weather and other disasters that can disconnect distant suppliers.

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A high-power electrical transmission tower

Long-distance transmission allows remoterenewable energy resources to be used todisplace fossil fuel consumption. Hydro andwind sources cannot be moved closer topopulous cities, and solar costs are lowest inremote areas where local power needs areminimal. Connection costs alone can determinewhether any particular renewable alternative iseconomically sensible. Costs can be prohibitivefor transmission lines, but various proposals formassive infrastructure investment in highcapacity, very long distance super gridtransmission networks could be recovered withmodest usage fees.

Grid inputAt the power stations the power is produced at a relatively low voltage between about 2.3 kV and 30 kV, dependingon the size of the unit. The generator terminal voltage is then stepped up by the power station transformer to a highervoltage (115 kV to 765 kV AC, varying by the transmission system and by country) for transmission over longdistances.

LossesTransmitting electricity at high voltage reduces the fraction of energy lost to resistance, which varies depending onthe specific conductors, the current flowing, and the length of the transmission line. For example, a 100 mile 765 kVline carrying 1000 MW of energy can have losses of 1.1% to 0.5%. A 345 kV line carrying the same load across thesame distance has losses of 4.2%.[4] For a given amount of power, a higher voltage reduces the current and thus theresistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the current by acorresponding factor of 10 and therefore the I2R losses by a factor of 100, provided the same sized conductors areused in both cases. Even if the conductor size (cross-sectional area) is reduced 10-fold to match the lower current theI2R losses are still reduced 10-fold. Long-distance transmission is typically done with overhead lines at voltages of115 to 1,200 kV. At extremely high voltages, more than 2,000 kV exists between conductor and ground, coronadischarge losses are so large that they can offset the lower resistive losses in the line conductors. Measures to reducecorona losses include conductors having larger diameters; often hollow to save weight,[5] or bundles of two or moreconductors.Transmission and distribution losses in the USA were estimated at 6.6% in 1997 and 6.5% in 2007. In general, lossesare estimated from the discrepancy between power produced (as reported by power plants) and power sold to endcustomers; the difference between what is produced and what is consumed constitute transmission and distributionlosses, assuming no theft of utility occurs.As of 1980, the longest cost-effective distance for direct-current transmission was determined to be 7,000 km(4,300 mi). For alternating current it was 4,000 km (2,500 mi), though all transmission lines in use today aresubstantially shorter than this.[]

In any alternating current transmission line, the inductance and capacitance of the conductors can be significant. Currents that flow solely in 'reaction' to these properties of the circuit, (which together with the resistance define the impedance) constitute reactive power flow, which transmits no 'real' power to the load. These reactive currents however are very real and cause extra heating losses in the transmission circuit. The ratio of 'real' power (transmitted

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to the load) to 'apparent' power (sum of 'real' and 'reactive') is the power factor. As reactive current increases, thereactive power increases and the power factor decreases. For transmission systems with low power factor, losses arehigher than for systems with high power factor. Utilities add capacitor banks, reactors and other components (such asphase-shifting transformers; static VAR compensators; physical transposition of the phase conductors; and flexibleAC transmission systems, FACTS) throughout the system to compensate for the reactive power flow and reduce thelosses in power transmission and stabilize system voltages. These measures are collectively called 'reactive support'.

SubtransmissionSubtransmission is part of an electric power transmission system that runs at relatively lower voltages. It isuneconomical to connect all distribution substations to the high main transmission voltage, because the equipment islarger and more expensive. Typically, only larger substations connect with this high voltage. It is stepped down andsent to smaller substations in towns and neighborhoods. Subtransmission circuits are usually arranged in loops sothat a single line failure does not cut off service to a large number of customers for more than a short time. Whilesubtransmission circuits are usually carried on overhead lines, in urban areas buried cable may be used.There is no fixed cutoff between subtransmission and transmission, or subtransmission and distribution. The voltageranges overlap somewhat. Voltages of 69 kV, 115 kV and 138 kV are often used for subtransmission in NorthAmerica. As power systems evolved, voltages formerly used for transmission were used for subtransmission, andsubtransmission voltages became distribution voltages. Like transmission, subtransmission moves relatively largeamounts of power, and like distribution, subtransmission covers an area instead of just point to point.[6]

Transmission grid exitAt the substations, transformers reduce the voltage to a lower level for distribution to commercial and residentialusers. 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 andcustomer requirements—see mains power systems).

High-voltage direct currentHigh-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or forinterconnections between asynchronous grids. When electrical energy is to be transmitted over very long distances,the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead ofalternating current. For a very long transmission line, these lower losses (and reduced construction cost of a DC line)can offset the additional cost of the required converter stations at each end.HVDC is also used for submarine cables because over about 30 kilometres (19 mi) lengths AC cannot be supplied.In these cases special high voltage cables for DC are used. Submarine connections up to 600 kilometres (370 mi) inlength are presently in use.HVDC links can be used to control problems in the grid with AC electricity flow. The power transmitted by an ACline increases as the phase angle between source end voltage and destination ends increases, but too large a phaseangle will allow the systems at either end of the line to fall out of step. Since the power flow in a DC link iscontrolled independently of the phases of the AC networks at either end of the link, this phase angle limit does notexist, and a DC link is always able to transfer its full rated power. A DC link therefore stabilizes the AC grid at eitherend, since power flow and phase angle can then be controlled independently.As an example, to adjust the flow of AC power on a hypothetical line between Seattle and Boston would require adjustment of the relative phase of the two regional electrical grids. This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the remaining working grid system. With an HVDC line instead, such an interconnection would: (1) Convert AC in Seattle into

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HVDC. (2) Use HVDC for the three thousand miles of cross-country transmission. Then (3) convert the HVDC tolocally synchronized AC in Boston, (and possibly in other cooperating cities along the transmission route). Such asystem could be less prone to failure if parts of it were suddenly shut down. One example of a long DC transmissionline is the Pacific DC Intertie located in the Western United States.

CapacityThe amount of power that can be sent over a transmission line is limited. The origins of the limits vary depending onthe length of the line. For a short line, the heating of conductors due to line losses sets a thermal limit. If too muchcurrent is drawn, conductors may sag too close to the ground, or conductors and equipment may be damaged byoverheating. For intermediate-length lines on the order of 100 km (62 mi), the limit is set by the voltage drop in theline. For longer AC lines, system stability sets the limit to the power that can be transferred. Approximately, thepower flowing over an AC line is proportional to the cosine of the phase angle of the voltage and current at thereceiving and transmitting ends. Since this angle varies depending on system loading and generation, it isundesirable for the angle to approach 90 degrees. Very approximately, the allowable product of line length andmaximum load is proportional to the square of the system voltage. Series capacitors or phase-shifting transformersare used on long lines to improve stability. High-voltage direct current lines are restricted only by thermal andvoltage 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 themaximum applicable current load was usually set as a compromise between understanding of operation conditionsand risk minimization. The availability of industrial distributed temperature sensing (DTS) systems that measure inreal time temperatures all along the cable is a first step in monitoring the transmission system capacity. Thismonitoring solution is based on using passive optical fibers as temperature sensors, either integrated directly inside ahigh voltage cable or mounted externally on the cable insulation. A solution for overhead lines is also available. Inthis case the optical fiber is integrated into the core of a phase wire of overhead transmission lines (OPPC). Theintegrated Dynamic Cable Rating (DCR) or also called Real Time Thermal Rating (RTTR) solution enables not onlyto continuously monitor the temperature of a high voltage cable circuit in real time, but to safely utilize the existingnetwork capacity to its maximum. Furthermore it provides the ability to the operator to predict the behavior of thetransmission system upon major changes made to its initial operating conditions.

ControlTo 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 thetransmission system are designed to provide cost effective performance for the customers.

Load balancingThe transmission system provides for base load and peak load capability, 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 airconditioning and heating loads have an effect on the overall load. They are typically highest in the late afternoon inthe hottest part of the year and in mid-mornings and mid-evenings in the coldest part of the year. This makes thepower requirements vary by the season and the time of day. Distribution system designs always take the base loadand 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

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involves synchronization of the generation units, to prevent large transients and overload conditions.In distributed power generation the generators are geographically distributed and the process to bring them onlineand offline must be carefully controlled. The load control signals can either be sent on separate lines or on the powerlines themselves. Voltage and frequency can be used as signalling mechanisms to balance the loads.In voltage signaling, the variation of voltage is used to increase generation. The power added by any systemincreases as the line voltage decreases. This arrangement is stable in principle. Voltage-based regulation is complexto use in mesh networks, since the individual components and setpoints would need to be reconfigured every time anew generator is added to the mesh.In frequency signaling, the generating units match the frequency of the power transmission system. In droop speedcontrol, if the frequency decreases, the power is increased. (The drop in line frequency is an indication that theincreased load is causing the generators to slow down.)Wind turbines, vehicle-to-grid and other distributed storage and generation systems can be connected to the powergrid, and interact with it to improve system operation.

Failure protectionUnder excess load conditions, the system can be designed to fail gracefully rather than all at once. Brownouts occurwhen the supply power drops below the demand. Blackouts occur when the supply fails completely.Rolling blackouts (also called load shedding) are intentionally engineered electrical power outages, used to distributeinsufficient power when the demand for electricity exceeds the supply.

CommunicationsOperators of long transmission lines require reliable communications for control of the power grid and, often,associated generation and distribution facilities. Fault-sensing protective relays at each end of the line mustcommunicate to monitor the flow of power into and out of the protected line section so that faulted conductors orequipment can be quickly de-energized and the balance of the system restored. Protection of the transmission linefrom short circuits and other faults is usually so critical that common carrier telecommunications are insufficientlyreliable, and in remote areas a common carrier may not be available. Communication systems associated with atransmission project may use:• Microwaves•• Power line communication• Optical fibersRarely, and for short distances, a utility will use pilot-wires strung along the transmission line path. Leased circuitsfrom common carriers are not preferred since availability is not under control of the electric power transmissionorganization.Transmission lines can also be used to carry data: this is called power-line carrier, or PLC. PLC signals can be easilyreceived 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. Thesecables are known as optical ground wire (OPGW). Sometimes a standalone cable is used, all-dielectricself-supporting (ADSS) cable, attached to the transmission line cross arms.Some jurisdictions, such as Minnesota, prohibit energy transmission companies from selling surplus communicationbandwidth or acting as a telecommunications common carrier. Where the regulatory structure permits, the utility cansell capacity in extra dark fibers to a common carrier, providing another revenue stream.

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Electricity market reformSome regulators regard electric transmission to be a natural monopoly and there are moves in many countries toseparately regulate transmission (see electricity market).Spain was the first country to establish a regional transmission organization. In that country transmission operationsand market operations are controlled by separate companies. The transmission system operator is Red Eléctrica deEspaña (REE) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía – PoloEspañol, S.A. (OMEL) [7]. Spain's transmission system is interconnected with those of France, Portugal, andMorocco.In the United States and parts of Canada, electrical transmission companies operate independently of generation anddistribution companies.

Cost of electric power transmissionThe cost of high voltage electricity transmission (as opposed to the costs of electricity distribution) is comparativelylow, compared to all other costs arising in a consumer's electricity bill. In the UK transmission costs are about0.2p/kWh compared to a delivered domestic price of around 10 p/kWh.[8]

Research evaluates the level of capital expenditure in the electric power T&D equipment market will be worth$128.9bn in 2011.[9]

Merchant transmissionMerchant transmission is an arrangement where a third party constructs and operates electric transmission linesthrough the franchise area of an unrelated utility.Operating merchant transmission projects in the United States include the Cross Sound Cable from Shoreham, NewYork to New Haven, Connecticut, Neptune RTS Transmission Line from Sayreville, N.J., to Newbridge, N.Y, ITCHoldings, Inc. transmission system in the midwest, and Path 15 in California. Additional projects are in developmentor have been proposed throughout the United States.There is only one unregulated or market interconnector in Australia: Basslink between Tasmania and Victoria. TwoDC links originally implemented as market interconnectors Directlink and Murraylink have been converted toregulated interconnectors. NEMMCO [10]

A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from thefacility so that the beneficiaries will pay the toll. Also, it is difficult for a merchant transmission line to competewhen the alternative transmission lines are subsidized by other utility businesses.

Health concernsSome large studies, including a large United States study, have failed to find any link between living near powerlines and developing any sickness or diseases such as cancer. One old study from 1997 found that it did not matterhow close one was to a power line or a sub-station, there was no increased risk of cancer or illness.[11]

The mainstream scientific evidence suggests that low-power, low-frequency, electromagnetic radiation associatedwith 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 nearpower lines. No adverse health effects have been substantiated for people not living close to powerlines.[12]

There are established biological effects for acute high level exposure to magnetic fields well above 100 µT (1 G). 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 µT (3 mG) to 0.4 µT (4 mG). These levels exceed average

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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.Tree Growth Regulator and Herbicide Control Methods may be used in transmission line right of ways[13] whichmay have health effects.

United States government policyHistorically, local governments have exercised authority over the grid and have significant disincentives to takeaction that would benefit states other than their own. Localities with cheap electricity have a disincentive to makinginterstate commerce in electricity trading easier, since other regions will be able to compete for local energy anddrive up rates. Some regulators in Maine for example do not wish to address congestion problems because thecongestion serves to keep Maine rates low. Further, vocal local constituencies can block or slow permitting bypointing to visual impact, environmental, and perceived health concerns. In the US, generation is growing 4 timesfaster than transmission, but big transmission upgrades require the coordination of multiple states, a multitude ofinterlocking permits, and cooperation between a significant portion of the 500 companies that own the grid. From apolicy perspective, the control of the grid is balkanized, and even former energy secretary Bill Richardson refers to itas a third world grid. There have been efforts in the EU and US to confront the problem. The US national securityinterest in significantly growing transmission capacity drove passage of the 2005 energy act giving the Departmentof Energy the authority to approve transmission if states refuse to act. However, soon after using its power todesignate two National Interest Electric Transmission Corridors, 14 senators signed a letter stating the DOE wasbeing too aggressive.

Special transmission

Grids for railwaysIn some countries where electric locomotives or electric multiple units run on low frequency AC power, there areseparate single phase traction power networks operated by the railways. These grids are fed by separate generators insome traction powerstations or by traction substations from the public three phase AC network.

Superconducting cablesHigh-temperature superconductors (HTS) promise to revolutionize power distribution by providing losslesstransmission of electrical power. The development of superconductors with transition temperatures higher than theboiling point of liquid nitrogen has made the concept of superconducting power lines commercially feasible, at leastfor high-load applications. It has been estimated that the waste would be halved using this method, since thenecessary refrigeration equipment would consume about half the power saved by the elimination of the majority ofresistive losses. Some companies such as Consolidated Edison and American Superconductor have already beguncommercial production of such systems.[14] In one hypothetical future system called a SuperGrid, the cost of coolingwould 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.[15]

Electric power transmission 13

HTS transmission lines[16]

Location Length (km) Voltage (kV) Capacity (GW) Date

Carrollton, Georgia 2000

Albany, New York[17] 0.35 34.5 0.048 2006

Long Island[18] 0.6 130 0.574 2008

Tres Amigas 5 Proposed 2013

Manhattan: Project Hydra Proposed 2014

Essen, Germany[19] 1 10 0.04 Proposed

Single wire earth returnSingle-wire earth return (SWER) or single wire ground return is a single-wire transmission line for supplyingsingle-phase electrical power for an electrical grid to remote areas at low cost. It is principally used for ruralelectrification, but also finds use for larger isolated loads such as water pumps. Single wire earth return is also usedfor HVDC over submarine power cables.

Wireless power transmissionBoth Nikola Tesla and Hidetsugu Yagi attempted to devise systems for large scale wireless power transmission inthe 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 kmvertically using a ground-based laser transmitter. The system produced up to 1 kW of power at the receiver end. InAugust 2010, NASA contracted with private companies to pursue the design of laser power beaming systems topower 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. Ahigh power array of microwave or laser transmitters would beam power to a rectenna. Major engineering andeconomic challenges face any solar power satellite project.

Security of control systemsThe Federal government of the United States admits that the power grid is susceptible to cyber-warfare.[20][21] TheUnited States Department of Homeland Security works with industry to identify vulnerabilities and to help industryenhance the security of control system networks, the federal government is also working to ensure that security isbuilt in as the U.S. develops the next generation of 'smart grid' networks.[22]

Records• Highest capacity system: 6.3 GW HVDC Itaipu (Brazil/Paraguay) (±600 kV DC)•• Highest transmission voltage (AC):

• planned: 1.20 MV (Ultra High Voltage) on Wardha-Aurangabad line (India) - under construction. Initially willoperate at 400 kV.

• worldwide: 1.15 MV (Ultra High Voltage) on Ekibastuz-Kokshetau line (Kazakhstan)• Europe (under construction): 750kV (Extra High Voltage) on the Rivne NPP/Khmelnytskyi NPP to Kyivska

Substation route (Ukraine)[23]

• Largest double-circuit transmission, Kita-Iwaki Powerline (Japan).• Highest towers: Yangtze River Crossing (China) (height: 345 m or 1,132 ft)

Electric power transmission 14

• Longest power line: Inga-Shaba (Democratic Republic of Congo) (length: 1,700 kilometres or 1,056 miles)• Longest span of power line: 5,376 m (17,638 ft) at Ameralik Span (Greenland, Denmark)•• Longest submarine cables:

• NorNed, North Sea (Norway/Netherlands) – (length of submarine cable: 580 kilometres or 360 miles)• Basslink, Bass Strait, (Australia) – (length of submarine cable: 290 kilometres or 180 miles, total length: 370.1

kilometres or 230 miles)• Baltic Cable, Baltic Sea (Germany/Sweden) – (length of submarine cable: 238 kilometres or 148 miles, HVDC

length: 250 kilometres or 155 miles, total length: 262 kilometres or 163 miles)•• Longest underground cables:

• Murraylink, Riverland/Sunraysia (Australia) – (length of underground cable: 180 kilometres or 112 miles)

ReferencesNotes

[1] Hans Dieter Betz, Ulrich Schumann, Pierre Laroche (2009). Lightning: Principles, Instruments and Applications. (http:/ / books. google. com/books?id=U6lCL0CIolYC& pg=PA187& lpg=PA187& dq=Spatial+ Distribution+ and+ Frequency+ of+ Thunderstorms+ and+ Lightning+in+ Australia+ wind+ gust& source=bl& ots=93Eto3OuyQ& sig=nB7VACqDBK7xJDGHijfCdny7Ylw& hl=en&ei=DFkLSt2lKJCdlQeTyPjtCw& sa=X& oi=book_result& ct=result& resnum=3#PPA203,M1) Springer, pp. 202–203. ISBN978-1-4020-9078-3. Retrieved on May 13, 2009.

[2] Bureau of Census data reprinted in Hughes, pp. 282–283[3] Hughes, pp. 293–295[4] American Electric Power, Transmission Facts, page 4: http:/ / www. aep. com/ about/ transmission/ docs/ transmission-facts. pdf[5] California Public Utilities Commission (http:/ / www. cpuc. ca. gov/ environment/ info/ aspen/ deltasub/ pea/

16_corona_and_induced_currents. pdf) Corona and induced currents[6] Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers (15th Edition) McGraw-Hill, 2007 ISBN

978-0-07-144146-9 section 18.5[7] http:/ / www. omel. es[8] What is the cost per kWh of bulk transmission (http:/ / www. claverton-energy. com/

what-is-the-cost-per-kwh-of-bulk-transmission-national-grid-in-the-uk-note-this-excludes-distribution-costs. html) / National Grid in the UK(note this excludes distribution costs)

[9] The Electric Power Transmission & Distribution (T&D) Equipment Market 2011–2021 (http:/ / www. visiongain. com/ Report/ 626/The-Electric-Power-Transmission-and-Distribution-(T-D)-Equipment-Market-2011-2021)

[10] http:/ / www. nemmco. com. au/ psplanning/ psplanning. html#interconnect[11] Power Lines and Cancer (http:/ / www. abc. net. au/ rn/ talks/ 8. 30/ helthrpt/ stories/ s175. htm), The Health Report / ABC Science -

Broadcast on 7 June 1997 (Australian Broadcasting Corporation)[12] Electromagnetic fields and public health (http:/ / www. who. int/ mediacentre/ factsheets/ fs322/ en/ ), World Health Organization[13] Transmission Vegetation Management NERC Standard FAC-003-2 Technical Reference Page 14/50. http:/ / www. nerc. com/ docs/

standards/ sar/ FAC-003-2_White_Paper_2009Sept9. pdf[14] 600m superconducting electricity line laid in New York (http:/ / www. newscientist. com/ article/

dn11907-superconducting-power-line-to-shore-up-new-york-grid. html)[15] Superconducting cables will be used to supply electricity to consumers (http:/ / www. futureenergies. com/ print. php?sid=237)[16] Superconductivity's First Century (http:/ / spectrum. ieee. org/ biomedical/ imaging/ superconductivitys-first-century/ 3)[17] Albany HTS Cable Project (http:/ / www. superpower-inc. com/ content/ hts-transmission-cable)[18] High-Temperature Superconductors (http:/ / www-03. ibm. com/ ibm/ history/ ibm100/ us/ en/ icons/ hightempsuperconductors/ )[19] High-Temperature Superconductor Technology Stepped Up (http:/ / www. powermag. com/ business/

High-Temperature-Superconductor-Technology-Stepped-Up_4412. html)[20] BBC: Spies 'infiltrate US power grid' (http:/ / news. bbc. co. uk/ 2/ hi/ technology/ 7990997. stm)[21] CNN: Video (http:/ / www. cnn. com/ 2009/ TECH/ 04/ 08/ grid. threat/ index. html?iref=newssearch#cnnSTCVideo)[22] Reuters: US concerned power grid vulnerable to cyber-attack (http:/ / in. reuters. com/ article/ oilRpt/ idINN0853911920090408)[23] Украина начала строительство крупнейшей воздушной ЛЭП в Европе (http:/ / pda. korrespondent. net/ business/ economics/ 1397420)

Further reading

• Grigsby, L. L., et al. The Electric Power Engineering Handbook. USA: CRC Press. (2001). ISBN 0-8493-8578-4• Hughes, Thomas P., Networks of Power: Electrification in Western Society 1880–1930, The Johns Hopkins

University Press,Baltimore 1983 ISBN 0-8018-2873-2, an excellent overview of development during the first 50

Electric power transmission 15

years of commercial electric power• Reilly, Helen (2008). Connecting the Country – New Zealand’s National Grid 1886–2007. Wellington: Steele

Roberts. pp. 376 pages. ISBN 978-1-877448-40-9.• Pansini, Anthony J, E.E., P.E. undergrounding electric lines. USA Hayden Book Co, 1978. ISBN 0-8104-0827-9• Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system".

(Transmission of power; polyphase system; Tesla patents)• The Physics of Everyday Stuff - Transmission Lines (http:/ / www. bsharp. org/ physics/ transmission)

External links• Japan: World's First In-Grid High-Temperature Superconducting Power Cable System (http:/ / www. sei. co. jp/

news_e/ press/ 06/ 06_09. html) - Link broken• A Power Grid for the Hydrogen Economy: Overview/A Continental SuperGrid (http:/ / www. sciam. com/ article.

cfm?id=a-power-grid-for-the-hydr-2006-07)• Global Energy Network Institute (GENI) (http:/ / www. geni. org) – The GENI Initiative focuses on linking

renewable energy resources around the world using international electricity transmission.• Union for the Co-ordination of Transmission of Electricity (UCTE) (http:/ / www. ucte. org/ ), the association of

transmission system operators in continental Europe, running one of the two largest power transmission systemsin the world

• Non-Ionizing Radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002)(http:/ / monographs. iarc. fr/ htdocs/ monographs/ vol80/ 80. html) by the IARC – Link Broken.

• A Simulation of the Power Grid (http:/ / tcip. mste. illinois. edu/ applet2. php) – The Trustworthy CyberInfrastructure for the Power Grid (TCIP) (http:/ / tcip. mste. illinois. edu/ ) group at the University of Illinois atUrbana-Champaign (http:/ / illinois. edu) has developed lessons and an applet which illustrate the transmission ofelectricity from generators to energy consumers, and allows the user to manipulate generation, consumption, andpower flow.

• Map of U.S. electric power generation and transmission (http:/ / www. energy-graph. com/ interactive-map. html)Maps

• Electric power transmission systems (worldwide) based on Openstreetmap-Data (http:/ / www. flosm. de/ en/Power-Grid. html)

Article Sources and Contributors 16

Article Sources and ContributorsElectric power transmission  Source: http://en.wikipedia.org/w/index.php?oldid=595510412  Contributors: 220 of Borg, 2over0, A. B., A. 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