Electric power transmission

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) and 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.

Bulk power transmission

A transmission grid is made up of power stations, transmission circuits, and substations. Energy is usually transmitted on the grid with 3-phase alternating current (AC). The voltage level on the bulk power transmission system is typically between 115 kV and 765 kV.

Grid input

At the generating plants the energy is produced at a relatively low voltage of up to 25 kV (Grigsby, 2001, p. 4–4), then stepped up by the power station transformer to a higher voltage for transmission over long distances to grid exit points (substations).

Losses

It is necessary to transmit the electricity at high voltage to reduce the percentage of energy lost. For a given amount of power transmitted, a higher voltage reduces the current and resistance losses in the conductor. 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 [1], and in the UK at 7.4% in 1998 [2].

In an alternating current transmission line, the inductance and capacitance of the line conductors can be significant. The currents that flow in these components of the transmission line impedance result in the generation of reactive power, which transmits no energy to the load. Reactive current flow causes extra losses in the transmission circuit. 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 ( such as phase-shifting transformers, static VAR compensators, and flexible AC transmission systems) to control reactive power flow for reduction of losses and stabilization of system voltage.

HVDC

High voltage DC (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it can be more economical to transmit using direct current 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 corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried. Since the power flow through an 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.

Grid exit

At the substations, transformers are again used to step the voltage down to a lower voltage for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (34.5 – 115 kV, varying by country and customer requirements) and distribution (4.6 – 25 kV). Finally, at the point of use, the energy is transformed to low voltage (100 – 600 V, varying by country and customer requirements).

Communications

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"

Health concerns

It is argued by some that proximal living to high voltage power lines presents a danger to animals and humans. Some have claimed that electromagnetic radiation from power lines elevates the risk of certain types of cancer. Some studies support this theory, and others do not. Studies of large populations fail consistently to show a clear correlation between cancer 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 an increased risk of cancer or other somatic diseases. For a detailed discussion of this topic, including references to a variety of 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. [3]

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 )

Records

• Highest transmission voltage (AC): 1150 kV on Powerline Ekibastuz-Kokshetau
• Highest transmission voltage (DC): +/-600 kV on HVDC Itaipu
• Highest pylons: Pylons of Pearl River Crossing (height: 253 metres and 240 metres)
• Longest powerline: Inga-Shaba (length: 1700 kilometres)
• longest submarine cables: Basslink (under construction, length of submarine/underground cable: 290 kilometres, total length: 357.4 kilometres), Baltic-Cable (length of submarine/underground cable: 249 kilometres, total length: 261 kilometres)

See also

• AC Power Transmission, transmission with AC current
• HVDC, High voltage direct current
• traction current, traction power network, power grids of electric railways
• SVC, Static Var Compensation.
• FACTS, Flexible AC Transmission System.
• Distributed generation
• Electricity market.
• Liberalization
• Lineman
• Power line communications (PLC).
• Pylon
• Overhead line crossing
• Submarine cable

External links

• Union for the Co-ordination of Transmission of Electricity (UCTE), the association of transmission system operators in continental Europe, running one of the two largest power transmission systems in the world
• Non-Ionizing Radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the IARC.
• A summary of the previous report by the industry lobbying group GreenFacts.

References

• Grigsby, L. L., et al. The Electric Power Engineering Handbook. USA: CRC Press. (2001). ISBN 0–8493–8578–4