wikiHydroTimor -- on hydroelectric related terms -- by HydroTimorElectric Power Transmission
Electric power transmission is one process in the transmitting of electricity to consumers. The term refers to the bulk transfer of electrical power from place to place. Typically, power transmission is between the power plant and a substation near a populated area. Electricity distribution is 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 transmitted over long distance through overhead power transmission lines. Underground power transmission is used only in densely populated areas (such as large cities) because of the high cost of installation and maintenance and because the power losses increase dramatically compared with overhead transmission unless superconductors and cryogenic technology are used.
A power transmission system is sometimes referred to colloquially as a "grid"; however, for reasons of economy, the network is rarely a true grid. 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 or thermal limit of the line.
A transmission grid is a network of power stations, transmission circuits, and substations.
At the generating plants the energy is produced at a relatively low voltage of up to 30 kV, then stepped up by the power station transformer to a higher voltage (138 kV to 765 kV AC, ± 250-500 kV DC, varying by country) for transmission over long distances to grid exit points (substations). AC power transmission is the transmission of electric power by alternating current. Usually transmission lines use three phase AC current. Single phase AC is used only for distribution to end users.
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. Overhead transmission lines are uninsulated wire, so design of these lines requires minimum clearances to be observed to maintain safety.
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 breakers, switches and transformers. Transmission efficiency is improved by increasing the voltage using a step-up transformer, which reduces the current in the conductors, whilst keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Joule's Law, the losses are proportional to the square of the current, halving the current makes the transmission loss one quarter the original value. However, at extremely high voltages, more than 2000 kV between conductor and ground, corona discharge losses are so large that they can offset the lower resistance loss in the line conductors.
The current mainstream scientific view is that power lines are unlikely to pose an increased risk of cancer or other somatic diseases.
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wikiHydroTimor -- on hydroelectric related terms -- by HydroTimor