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Electric power transmission

From Academic Kids

Template:Mergefrom Electric power transmission is the second process in the delivery of electricity to consumers. Electrical energy is generated by power plants and is then sold as a commodity to end consumers by retailers. The electric energy transmission and electricity distribution networks allow the delivery of the generated electricity to consumers. The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the economic infrastructure in most industrialized nations.

 in the New Zealand countryside
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Transmission towers in the New Zealand countryside
Transmission lines in ,
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Transmission lines in Lund, Sweden

The transmission grid allows large generation facilities such as hydroelectric dams, fossil fuel plants, nuclear power plants, etc. run by large public and private utility organizations to produce large quantities of energy and then deliver it to distribution networks for delivery to retail customers for consumption.

Electricity is usually sent over long distance through a combination of overhead power transmission lines (such as those in the photo on the right) and buried cables.

The first large scale hydroelectric generators in the USA (engineered and installed under the technical oversight of Nikola Tesla) were installed at Niagara Falls and provided electricity to Buffalo, New York via power transmission lines.

Contents

AC power transmission

AC Power Transmission is the transmission of electric power by alternating current. Usually transmission lines use three phase AC current. In countries with extensive electric railways, sometimes single phase AC current is used as traction current for railway traction.

Transmission-level voltages are usually considered to be 115 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

History

The first transmission of three-phase alternating current using high voltage took place in the year 1891 on the occasion of the international electricity exhibition in Frankfurt. Between Lauffen at the Neckar and Frankfurt/Main one approx. 175 kilometre long powerline for a voltage of 15 to 25 kV was built in 1891.

Initially transmission lines were supported by porcelain pin-and-sleeve insulators similar to those used for telegraph and telephone lines. However, these reached a practical limit of 40 kV. In 1907 the invention of the strain-type insulators by H. W. Buck of the Niagara Falls Power Corporation and E. M. Hewlett of General Electric allowed practical insulators of any length to be constructed, which allowed the use of higher voltages.

In 1912 between Lauchhammer and Riesa the first three-phase alternating current with 110 kV took place. On April 17th, 1929 there was the inaugauration of the first 220 kV line in Germany running from Brauweiler near Cologne, over Kelsterbach near Frankfurt, Rheinau near Mannheim, Ludwigsburg/Hoheneck toward Austria. The masts of this line were already were constructed for eventual upgrade to 380 kV. However the first transmission with 380 kV took place in Germany on October 5th, 1957 between the substations in Rommerskirchen and Ludwigsburg/Hoheneck. In 1967 the first extra-high-voltage transmission with 735 kV took place on a Hydro-Québec transmission line. In 1982 the first transmission with 1200kV took place in the former Soviet Union.

Bulk power transmission

A transmission grid is made up of power stations, transmission circuits, and substations. Energy is usually transmitted on the grid with 3-phase alternating current (AC). The voltage level on the bulk power transmission system is typically between 115 kV and 765 kV. Energy may also be transmitted using high voltage direct current.

Grid input

At the generating plants the energy is produced at a relatively low voltage of up to 25 kV (Grigsby, 2001, p. 4-4), then stepped up by the power station transformer to a higher voltage for transmission over long distances to grid exit points (substations).

Losses

It is necessary to transmit the electricity at high voltage to reduce the percentage of energy lost. For a given amount of power transmitted, a higher voltage reduces the current and resistance losses in the conductor. Long distance transmission is typically at voltages of 100 kV and higher. Transmission voltages up to 765 kV AC and up to +/-533 kV DC are currently used in long-distance overhead transmission lines.

Transmission and distribution losses in the USA were estimated at 7.2% in 1995 [1] (http://climatetechnology.gov/library/2003/tech-options/tech-options-1-3-2.pdf), and in the UK at 7.4% in 1998. [2] (http://www.powerwatch.org.uk/energy/graham.asp)

In an alternating current transmission line, the inductance and capacitance of the line conductors can be significant. The currents that flow in these components of transmission line impedance constitute reactive power, which transmits no energy to the load. Reactive current flow causes extra losses in the transmission circuit. The fraction of total energy flow (power) which is resistive (as opposed to reactive) power is the power factor. Utilities add capacitor banks and other components throughout the system (such as phase-shifting transformers, static VAR compensators, and flexible AC transmission systems) to control reactive power flow for reduction of losses and stabilization of system voltage.

HVDC

High voltage DC (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 can be more economical to transmit using direct current instead of alternating current. For a long transmission line, the value of the smaller losses, and reduced construction cost of a DC line, can offset the additional cost of converter stations at each end of the line. Also, at high AC voltages significant amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried. Since the power flow through an HVDC link is directly controllable, HVDC links are sometimes used within a grid to stabilize the grid against control problems with the AC energy flow.

Grid exit

At the substations, transformers are again used to step the voltage down to a lower voltage for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (34.5 to 115 kV, varying by country and customer requirements) and distribution (4.6 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (100 to 600 V, varying by country and customer requirements).

Communications

Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protection relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section. Protection of the transmission line from short circuits and other faults is usually so critical that common carrier telecommunications is insufficiently reliable. In remote areas a common carrier may not be available at all. Communication systems associated with a transmission project may use:

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

Sometimes there are also communications cables using the transmission line structures. These are generally fibre optic cables. They are often integrated in the ground (or earth) conductor. Sometimes a standalone cable is used, which is commonly fixed to the upper crossbar. On the EnBW system in Germany, the communication cable can be suspended from the ground (earth) conductor or strung as a standalone cable.

Some jurisdictions, such as Minnesota, prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications common carrier. Where the regulatory structure permits, the utility can sell capacity in extra "dark fibres" to a common carrier, providng another revenue stream for the line.

Electricity market reform

Transmission is a natural monopoly and there are moves in many countries to separately regulate transmission (see New Zealand Electricity Market). In the USA the Federal Energy Regulatory Commission has issued a notice of proposed rulemaking setting out a proposed Standard Market Design that would see the establishment of Regional Transmission Organizations (RTOs). The first RTO in North America is the Midwest Independent Transmission System Operator (MISO) [3] (http://www.midwestmarket.org). MISO's authority covers parts of the transmission grid in the United States midwest and one province of Canada (through a coordination agreement with Manitoba Hydro). MISO also operates the wholesale power market in the United States portion of this area.

Spain 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 (REE) [4] (http://www.ree.es/ingles/i-index_quien.html) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía - Polo Español, S.A. (OMEL) [5] (http://www.omel.es). Spain's transmission system is interconnected with those of France, Portugal, and Morocco.

Health concerns

It is argued by some that living near high voltage power lines presents a danger to animals and humans. Some have claimed that electromagnetic radiation from power lines elevates the risk of certain types of cancer. Some studies support this theory, and others do not. Most studies of large populations fail to show a clear correlation between cancer and the proximity of power lines, but a 2005 Oxford University study did find a statistically significant elevation of childhood leukaemia rates [6] (http://www.hpa.org.uk/hpa/news/articles/press_releases/2005/050603_childhood_cancer_voltage.htm). Recent studies (2003) connect DNA-breakage with low level AC magnetic fields.

The current mainstream scientific view is that power lines are unlikely to pose an increased risk of cancer or other somatic diseases. For a detailed discussion of this topic, including references to a variety of scientific studies, see the Power Lines and Cancer FAQ (http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html). The issue is also discussed at some length in Robert L. Park's book Voodoo Science.

Alternate transmission methods

There is a potential for the use of superconducting cable transmission in order to supply electricity to consumers, given that the waste is halved using this method. Such cables are particularly suited to high load density areas such as the business district of large cities, where purchase of a right of way for cables would be very costly. [7] (http://www.futureenergies.com/print.php?sid=237)

Special transmission grids for railways

In some countries where electric trains run on low frequency AC (e.g. 16.7 Hz) power there are separate single phase traction power networks operated by the railways. These grids are fed by separate generators in some power stations or by traction current converter plants from the public three phase AC network. Sample transmission voltages include:

  • 25 kV (United Kingdom)
  • 25 and 50 kV (South Africa)
  • 66 and 132 kV (Switzerland)
  • 110 kV (Germany, Austria)

Records

See also

External links

References

  • Grigsby, L. L., et al. The Electric Power Engineering Handbook. USA: CRC Press. (2001). ISBN 0-8493-8578-4

Further reading

  • Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system". (Transmission of power; polyphase system; Tesla patents)de:Stromnetz

es:Red de transporte de energía eléctrica fi:Sähköverkko fr:Réseau électrique it:Trasmissione di energia elettrica nl:Hoogspanningsnet ja:送電

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