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es:Red de transporte de energía eléctrica
ja:送電nl:hoogspanningsnet
Electric power transmission is the second process in the delivery of electricity to consumers. Electricity is generated by power plants and is then sold as a commodity to end consumers by retailers. The electric power transmission and electricity distribution networks allow the delivery of the generated electricity to consumers. The rapid industrialization in the 20th century made electric power transmission lines and grids a critical part of the economic infrastructure in most industrialized nations.
The 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) or 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.
Grid input
A transmission grid, is made up of power stations, substations and transmission circuits. Power is usually transmitted as a 3-phase alternating current. At the generating plants the power is produced at a relatively low voltage of 10-15 kV, then stepped up by the power stationtransformer to a high voltage (220 - 400 kV) alternating current (AC) for transmission over longer distances to a grid exit pointsubstation.
Losses
It is necessary to transmit the electricity at high voltage to reduce the percentage of power lost. The higher the voltage the lower the current that flows, which reduces the size of cable needed and the amount of energy wasted. Long distance transmission is typically at voltages of 100 kV and higher. Transmission and distribution losses in the USA were estimated at 7.2% in 2003 [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/graham7x.htm).
When power is required to be transmitted over very long distances, it becomes more efficient (and hence economical) to transmit using direct current instead of alternating current. This is only done for very high quantities of power over very long distances or for special situations, such as undersea cables, as it requires a large capital investment in power AC/DC inverters.
Power is also lost due to the nature of the loads connected to the grid: this loss is expressed in terms of power factor. If the power factor is low, then a large proportion of power is wasted. Utility companies expend considerable effort to maintain a good power factor.
Grid exit
Substations are also used to step the voltage down and supply electricity to low voltage local power lines for distribution to commercial and residential users. Typically, the electricity is transformed to a sub-transmission voltage (66 - 132 kV) using interconnecting transformers and then transformed to a medium voltage (10 - 50 kV). Finally, in the distribution substation, the power is transformed to low voltage (220 - 330 V).
The universal method of supply from distribution lines to small end users is via single phase or three phase connections.
Communications
Transmission lines can also be used to carry data: this is called power-line carrier, or PLC.
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 (FERC) has issued a notice of proposed rulemaking (NOPR) setting out a proposed Standard Market Design (SMD) that would see the establishment of Regional Transmission Operators (RTOs).
Health concerns
It is argued by some that living in proximity to a high voltage power line presents a danger to the animals and humans. Some have claimed that electromagnetic radiation from power lines causes elevated risk of certain types of cancer. Some studies have purported to identify a risk, while others have not. Studies over larger populations have consistently shown no clear correlation between health effects and the proximity of power lines.
The current mainstream scientific view is that power lines are unlikely to pose any increased risk of cancer or other somatic diseases. For a detailed discussion of this topic, including references to many of the 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.
See also
External links
Potential for superconducting cable transmission
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