Encyclopedia
- Power line redirects here. For the telecommunication technology, see Power line communication.
- Power grid redirects here. For the board game, see Power Grid .
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. This is distinct from
electricity distribution, which is concerned with the delivery from the substation to the consumers. Due to the large amount of power involved, transmission normally takes place at high voltage . Electricity is usually transmitted over long distance through overhead power transmission lines . Underground power transmission is used only in densely populated areas because of the high cost of installation and maintenance.
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. Deregulation of electricity companies in many countries has led to renewed interest in reliable economic design of transmission networks. The separation of transmission and generation functions is one of the factors that contributed to the
2003 North America blackout.
AC power transmission
AC power transmission is the transmission of
electric power by
alternating current. Usually transmission lines use
three phase AC
current. In electric
railways,
single phase AC current is sometimes used in a
railway electrification system. In urban areas, trains may be powered by DC at 600 volts or so.
Today, 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 not insulated, so design of these lines requires minimum clearances to be observed to maintain safety.
History
In an AIEE Address, May 16, 1888,
Nikola Tesla delivered a lecture entitled
, describing the equipment which allowed efficient generation and use of
alternating currents. Tesla's disclosures, in the form of patents, lectures and technical articles, are useful for understanding the history of the modern system of power transmission. Ownership of the rights to the Tesla patents was a key commercial advantage to the Westinghouse company in offering a complete alternating current power system for both lighting and power.
The first transmission of three-phase alternating current using high voltage took place in the year 1891 on the occasion of the international electricity exhibition in
Frankfurt. In that year, a 25 kV transmission line, approximately 175 kilometres long, was built between Lauffen at the
Neckar and Frankfurt.
The rapid industrialization in the
20th century made electrical transmission lines and grids a critical part of the economic infrastructure in most industrialized nations. Initially transmission lines were supported by porcelain pin-and-sleeve
insulators similar to those used for
telegraph and
telephone lines. However, these reached a practical limit of 40 kV. In 1907 the invention of the disc insulator by Harold W. Buck of the Niagara Falls Power Corporation and Edward M. Hewlett of
General Electric allowed practical insulators of any length to be constructed, which allowed the use of higher voltages. The first large scale hydroelectric generators in the USA were installed at
Niagara Falls and provided electricity to
Buffalo, New York via power transmission lines. A statue of Tesla stands at Niagara Falls today in tribute to his contributions.
Voltages used for electric power transmission increased throughout the 20th century. The first three-phase alternating current power transmission at 110 kV took place in 1912 between Lauchhammer and
Riesa,
Germany. On April 17, 1929 the first 220 kV line in
Germany was completed, running from
Brauweiler near
Cologne, over
Kelsterbach near Frankfurt, Rheinau near
Mannheim,
Ludwigsburg-Hoheneck near
Austria. The masts of this line were designed for eventual upgrade to 380 kV. However the first transmission at 380 kV in Germany was on October 5, 1957 between the substations in Rommerskirchen and Ludwigsburg-Hoheneck. In 1967 the first extra-high-voltage transmission at 735 kV took place on a
Hydro-Québec transmission line. In 1982 the first transmission at 1200 kV took place in the
Soviet Union.
Bulk power transmission
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.
Efficiency is improved by increasing the transmission voltage using a step-up
transformer, which has the effect of reducing 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
Ohms Law, the losses are proportional to the square of the current, halving the current results in a four-fold decrease in transmission losses.
A transmission grid is a network of
power stations, transmission circuits, and substations. Energy is usually transmitted within the grid with
three-phase AC. DC systems suffer from the fact that voltage conversion is expensive while single phase AC links suffer from oscilations in the power transmitted and the inability to directly generate a rotating magnetic field. Other phase orders of polyphase systems are possible but two phase still needs either 3 wires with unequal currents or 4 wires and higher phase order systems need more than 3 wires for marginal benefits.
The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the variable load than to generate it locally. Because nearby loads are often correlated , imported electricity must often come from far away. Because of the economics of load balancing, transmission grids now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow even if one link is disabled.
Long-distance transmission of electricity is almost always more expensive than the transportation of the fuels used to make that electricity. As a result, there is economic pressure to locate fuel-burning power plants near the population centers that they serve. The obvious exceptions are hydroelectric turbines -- high-pressure water-filled pipes being more expensive than electric wires. The unvarying portion of the electric demand is known as the "base load", and is generally served best by facilities with low variable costs but high fixed costs, like nuclear or large coal-fired powerplants.
Grid input
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 for transmission over long distances to grid exit points .
Losses
It is necessary to transmit the electricity at high voltage to reduce the fraction of energy lost. For a given amount of power transmitted, a higher voltage reduces the current and thus the resistive losses in the conductor. Long distance transmission is typically done with overhead lines at voltages of 110 to 1200 kV. 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 heating loss in the line conductors.
Transmission and distribution losses in the USA were estimated at 7.2% in 1995 , and in the UK at 7.4% in 1998.
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 transmission line impedance constitute
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 which is resistive power is the
power factor. Utilities add capacitor banks and other components 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 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. One prominent example of such a transmission line is the
Pacific Intertie located in the Western
United States.
Power Loss
Electrical power is invariably partially lost during transmission. This applies to short distances such as between components on a
printed circuit board as well as to cross country high voltage lines.
Loss power is proportional to the
resistance of the wire and the square of the current.
Because of this relationship, it is favourable to transmit energy with
voltages as high as possible. This reduces the current and thus the power lost during transmission.
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 and distribution . Finally, at the point of use, the energy is transformed to low voltage .
Communications
Operators of long transmission lines require reliable communications for
control of the power grid and, often, associated generation and distribution facilities. Fault-sensing
protection relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly deenergized and the balance of the system restored. Protection of the transmission line from
short circuits and other faults is usually so critical that common carrier telecommunications is insufficiently reliable. In remote areas a common carrier may not be available at all. Communication systems associated with a transmission project may use:
Rarely, and for short distances, a utility will use pilot-wires strung along the transmission line path. Leased circuits from common carriers are not preferred since availability is not under control of the electric power transmission organization.
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 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 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. Where the regulatory structure permits, the utility can sell capacity in extra "dark fibres" to a common carrier, providing another revenue stream for the line.
Electricity market reform
Transmission is a natural monopoly and there are moves in many countries to separately regulate transmission . In the USA the Federal Energy Regulatory Commission had issued a notice of proposed rulemaking setting out a proposed Standard Market Design that would see the establishment of Regional Transmission Organizations . The first RTO in North America is the Midwest Independent Transmission System Operator . MISO's authority covers parts of the transmission grid in the United States midwest and one province of Canada . MISO also operates the wholesale power market in the United States portion of this area.
In July 2005, the new FERC chairman, Joseph Kelliher announced the end of SMD efforts because "the rulemaking had been overtaken by the voluntary formation of RTOs and ISOs" according to FERC.
Spain was the first country to establish a Regional Transmission Organization. In that country transmission operations and market operations are controlled by separate companies. The transmission system operator is Red Eléctrica de España and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía - Polo Español, S.A. . Spain's transmission system is interconnected with those of France, Portugal, and Morocco.
Health concerns
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 . The issue is also discussed at some length in Robert L. Park's book
Voodoo Science, is a pejorative [i] neologism [i] referring to research that may fall short of the s...
.
It is argued by some that living near high voltage power lines presents a danger to animals and humans. Some have claimed that electromagnetic fields from power lines elevate the risk of certain types of cancer. Some studies support this theory, and others do not. Most studies of large populations fail to show a clear correlation between cancer and the proximity of power lines, but a 2005
Oxford University study did.
One possible response to the dangers of overhead power lines is to bury them underground. When colocated with other utility infrastructure, this creates a common utility duct. In reality, protection from the dangers of electromagnetic fields is seldom the driving concern in burying power lines.
See also
