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.
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) 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.
A transmission grid is made up of power stations, substations, and transmission circuits. Energy is usually transmitted as a 3-phase alternating current (AC). At the generating plants the energy is produced at a relatively low voltage of 10-15 kV, then stepped up by the power station transformer to a high voltage (220 - 500 kV) alternating current for transmission over longer distances to grid exit points (substations).
It is necessary to transmit the electricity at high voltage to reduce the percentage of energy lost. The higher the voltage, the lower the current, which reduces the size of the conductor needed and the amount of energy wasted. 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 , and in the UK at 7.4% in 1998 .
When electrical energy is required to be transmitted over very long distances, it becomes more efficient to transmit using direct current (HVDC) 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 the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried. HVDC is used to transmit very high quantities of energy over very long distances, for undersea cables over ten kilometers or so (as undersea cables have a very large capacitance), or between asynchronous grids. Since the energy flow across a 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.
In an alternating current transmission line, the inductance and capacitance of the line conductors can be significantly large. The currents that flow in these components of the transmission line impedance result in the generation of reactive power. Reactive flows represent a back-and-forth motion of electrical energy. Reactive flows transmit no energy from the generator to the load, but they consume energy within the grid because they cause extra current above that employed for energy transmission. 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 measures throughout the system to control the flow of reactive power to maintain stable voltages for consumers and to reduce transmission losses.
Substations are used to step the voltage down 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 energy is transformed to low voltage (100 - 600 V, varying by country and customer requirements).
For more details see the article on electricity distribution.
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.
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 Operators .
Spain was the first country to establish Regional Transmission Operators. In that country transmission is controlled by a single company. Source: "http://www.ree.es/ingles/i-index_quien.html"
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. 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 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. 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. 
Special Transmission Grids
Transmission grid for railways
In some countries where electric trains run with AC of decreased frequency (e.g. 16.7 Hertz) there are separate single phase AC grids operated by the railway companies of these countries. These grids are fed by separate generators in some powerstations or by traction current converter plants from the public three phase AC network are exclusively used for the power supply of the railway systems. The used transmission voltages are almost 66 kV, 110 kV and 132 kV ( Traction power network )
Last updated: 06-02-2005 13:55:47