They allow large centralized 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 distribution networks for delivery to retail customers for consumption.
Electricity is usually sent over long distance and in rural areas over power transmission lines such as those in the photo on the right.
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.
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2 Grid exit 3 Communications 4 Status 5 External links |
In a typical transmission grid, substations at the generating plants step (transform) the medium voltage electric power (15 - 50 kV) up to high voltage (220 - 400 kV) alternating current (AC) for transmission over longer distances to a grid exit substation.
It is necessary to transmit the electricity at high voltage to reduce the percentage loss of power over long distances: for a given cable the loss of power is proportional to the square of the current, while for a given transported power, this current is inversely proportional to the voltage. Long distance transmission is typically at voltages of 100kV and higher. Transmission and distribution losses in the USA were estimated at 7.2% in 2003 [1], and in the UK at 7.4% in 1998 [1].
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.
Substations step the voltage down and supply electricity to low voltage local power lines for distribution. 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.
Transmission lines can also be used to carry data: this is called power-line carrier, or PLC.
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).
See also:
Grid input
Grid exit
Communications
Status
External links
Potential for superconducting cable transmission