Power line
Electric power transmission is part of the process in the delivery of electricity to consumers. Electricity is generated remotely by power plants and is then sold as a commondity to end consumers by retailers. The electric power transmission and electricity distribution networks allow the delivery of the generated electrity 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.
They allow large centralized generation facilities such as hydroelectric dams, fossil fuel burning steam turbine plants, nuclear power plants, etc. run by large public and private utility organizations to produce large quantities of energy and then deliver it to remote 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, and power is transmitted as a 3-phase (usually) alternating current. At the generating plants the power is created at a relatively low voltage of 10-15 kV, then stepped up by the power stationtransformer to a 'medium' voltage level (15 - 50 kV) and then by a network operator to high voltage (220 - 400 kV) alternating current (AC) for transmission over longer distances to a grid exit substation.
Losses
It is necessary to transmit the electricity at high voltage to reduce the percentage of current, while for a given transported power, this current is inversely proportional to the voltage. 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 , and in the UK at 7.4% in 1998 .
When power is required to be transmitted over very long distances, it becomes more efficient (and hence economical) to transmit power as direct currentinstead of alternating current. This is not often done 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 ammount 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 home 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).
In the U.S.A. the most commonly supplied power from distribution lines to small end users is via single phase or three phase as defined by standards such as the National Electric Code.
Communications
Transmission lines can also be used to carry data: this is called power-line carrier, or PLC.
Legal status
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 proximity to a high voltage power line presents a danger to the animals and humans living nearby. Some have claimed that electromagnetic radiation from high-tension lines causes elevated risk of certain types of cancer. Some studies have purported to identify the risk, while other studies have not. Studies over larger populations and with better methodology have consistently shown no clear correlation.
The current mainstream scientific view with regard to the health concerns voiced about living in proximity to transmission lines is that they are unlikely to pose any increased risk with regard to cancer and other somatic diseases. For a detailed discussion of this topic, including references to many of the scientific studies, see the . The issue is also discussed at some length in Robert L. Park's book Voodoo Science.
See also
