Electric power transmission is one process in the delivery of electricity to consumers. It refers to the 'bulk' transfer of electrical power from place to place.
Typically power transmission is between the power plant and a substation in the vicinity of a populated area. This is distinct from electricity distribution which is concerned with the delivery from the substation to the consumers. Due to the large amount of power involved, transmission normally takes place at high voltage (110 kV or above). Electricity is usually sent over long distance through overhead power transmission lines (such as those in the photo on the right). Power is transmitted underground in densly populated areas (such as large cities) but is typically avoided due to the high capacitive and resistive losses incurred.
A power transmission system is sometimes referred to colloquially as a "grid". However, for reasons of economy, the network is rarely a grid (a fully connected network) in the mathematical sense. Redundant paths and lines are provided so that power can be routed from any power plant to any load center, through a variety of routes, based on the economics of the transmission path and the cost of power. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical limit of the line. Deregulation of electricity companies in many countries has lead to renewed interest in reliable economic design of transmission networks. The separation of transmission and generation functions is one of the factors that contributed to the 2003 North America blackout.
AC power transmission
AC power transmission is the transmission of electric power by alternating current. Usually transmission lines use three phase AC current. In electric railways, sometimes single phase AC current is used as traction current for railway traction.
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 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.
Bulk power transmission
A transmission grid is a network of power stations, transmission circuits, and substations. Energy is usually transmitted within the grid with 3-phase alternating current (AC).
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 variable load than to generate it locally. Because nearby loads are often correlated (hot weather in the Southwest portion of the United States  might cause many people there to turn on their air conditioners), imported electricity must often come from far away. Because of the irresistible economics of load balancing, transmission grids 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 one link is disabled.
Long-distance transmission of electricity is almost always more expensive than the transportation of the fuels used to make that electricity. As a result, there is economic pressure to locate fuel-burning power plants near the population centers that they serve. The obvious exceptions are hydroelectric turbines—high-pressure water-filled pipes being more expensive than electric wires. The unvarying portion of the electric demand is known as the "base load", and is generally served best by facilities with low variable costs but high fixed costs, like nuclear or large coal-fired powerplants.
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).
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 thus the resistive losses in the conductor. Long distance transmission is typically done with overhead lines at voltages of 110 to 765 kV. However, at extremely high voltages, more than 2 million volts between conductor and ground, corona discharge losses are so large as to offset the advantage of lower heating loss in the line conductors.
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 (FACTS)—to control reactive power flow for reduction of losses and stabilization of system voltage.
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. One prominent example of such a transmission line is the Pacific Intertie located in the Western United States.
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 (33 kV to 115 kV, varying by country and customer requirements) and distribution (3.3 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).
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:
- power line carrier
- Optical fibres
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, providing 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 had issued a notice of proposed rulemaking setting out a proposed Standard Market Design (SMD) that would see the establishment of Regional Transmission Organizations (RTOs). The first RTO in North America is the Midwest Independent Transmission System Operator (MISO) . 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.
In July 2005, the new FERC chairman, Joseph Kelliher announced the end of SMD efforts because "the rulemaking had been overtaken by the voluntary formation of RTOs and ISOs" according to FERC.
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)  and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía - Polo Español, S.A. (OMEL) . Spain's transmission system is interconnected with those of France, Portugal, and Morocco.
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 . 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. The issue is also discussed at some length in Robert L. Park's book Voodoo Science.
Alternate transmission methods
Hidetsugu Yagi attempted to devise a system for wireless power transmission. Whilst he managed to demonstrate a proof of concept, the engineering problems proved to be more onerous than conventional systems. His work however, led to the invention of the yagi antenna.
Another form of wireless power transmission has been studied for transmission of power from solar power satellites to the earth. A high power array of microwave transmitters would beam power to a rectenna in an unpopulated desert area. Formidable engineering, environmental, and economic problems face any solar power satellite project.
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. 
Special transmission grids for railways
In some countries where electric trains run on low frequency AC (e.g. 16.7 Hz and 25 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)
- Highest transmission voltage (AC): 1150 kV on Powerline Ekibastuz-Kokshetau
- Highest transmission voltage (DC): +/-600 kV on HVDC Itaipu
- Highest pylons: Pylons of Pearl River Crossing (height: 253 metres and 240 metres)
- Longest powerline: Inga-Shaba (length: 1700 kilometres)
- Longest span of powerline: 5376 metres at Ameralik Span
- longest under sea cables: Basslink (under construction, length of submarine/underground cable: 290 kilometres, total length: 357.4 kilometres), Baltic-Cable (length of submarine/underground cable: 249 kilometres, total length: 261 kilometres)
- HVDC, High voltage direct current
- traction current, traction power network, power grids of electric railways
- SVC, Static Var Compensation.
- FACTS, Flexible AC Transmission System.
- Distributed generation
- Electricity market.
- Power line communications (PLC).
- Electricity pylon
- Overhead line crossing
- National Grid
- National Grid (US)
- Electricity distribution
- Electrical power grid
- Overhead powerline
- Submarine power cable
- Union for the Co-ordination of Transmission of Electricity (UCTE), the association of transmission system operators in continental Europe, running one of the two largest power transmission systems in the world
- Non-Ionizing Radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the IARC.
- A summary of the IARC report by GreenFacts.
- Grigsby, L. L., et al. The Electric Power Engineering Handbook. USA: CRC Press. (2001). ISBN 0-8493-8578-4
- Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system". (Transmission of power; polyphase system; Tesla patents)
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