Electric power transmission

Transmission lines in Lund, Sweden

BC Hydro transmission towers and lines in Coquitlam, British Columbia.

File:DSCN2643 PGE 500kV lines.JPG

PG&E's Path 15 500 kV power lines in the California Central Valley.

Power lines near Helsinki, Finland

Twisting pylon of power line for single phase AC traction current (110 kV, 16.67 hertz) near Bartholomä in Germany

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 (110 kV or above). Electricity is usually transmitted over long distance through overhead power transmission lines (such as those in the photo on the right). Underground power transmission is used only in densely populated areas (such as large cities) because of the high cost of installation and maintenance and because the power losses increase dramatically compared with overhead transmission unless superconductors and cryogenic technology are used.

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 A New System of Alternating Current Motors and Transformers, 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 (embodying the patents of Nikola Tesla) 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 (and so are only used for special high voltage links) while single phase AC links suffer from oscillations in the power transmitted (very bad for the smoothness of motors and generators) and the inability to directly generate a rotating magnetic field. Other phase orders of polyphase systems are possible but two phase (90 degree separation) 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 (hot weather in the Southwest portion of the United States might cause many people there to turn on their air conditioners), 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.

Power plants in North America

Power plants in North America

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 (Grigsby, 2001, p. 4-4), then stepped up by the power station transformer to a higher voltage (138 kV to 765 kV AC, ± 250-500 kV DC, varying by country) for transmission over long distances to grid exit points (substations).

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 [2], and in the UK at 7.4% in 1998. [3]

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 ratio of real power (transmitted to the load) to apparent power is the power factor. As reactive current increases, the reactive power increases and the power factor decreases. For low power factors losses will increase. Utilities add capacitor banks and other components throughout the system—such as phase-shifting transformers, static VAr compensators, and flexible AC transmission systems (FACTS)—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. 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.

$P_{loss}=RI^{2}$

For a system which delivers a certain amount of power, P, over a particular voltage, V, the current flowing through the cables is given by $I={\frac {P}{V}}$. Thus, the power lost in the lines, $P_{loss}=RI^{2}=R({\frac {P}{V}})^{2}={\frac {RP^{2}}{V^{2}}}$.

Therefore, the power lost is proportional to the resistance and inversely proportional to the square of the voltage. 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 (33 kV to 115 kV, varying by country and customer requirements) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (100 to 600 V, varying by country and customer requirements).

As of 1980, the longest cost-effective distance for electricity was 4000 miles (7000 km) (see Present Limits of High-Voltage Transmission

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:

• Microwaves
• Power line communication
• Optical fibres

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 (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.

110 kV double circuit power line of EnBW AG near Leonberg, Germany with an aerial cable mounted like a garland on the ground conductor

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 towers in the New Zealand countryside

Transmission is a natural monopoly and there are moves in many countries to separately regulate transmission (see New Zealand Electricity Market). In 2003 in the USA, the Federal Energy Regulatory Commission (FERC) issued a Standard Market Design (SMD) that would guide the establishment of Regional Transmission Organizations (RTOs). The first RTO in North America is the Midwest Independent Transmission System Operator (MISO) [4]. MISO's authority covers parts of the transmission grid in the United States midwest and one province of Canada (through a coordination agreement with Manitoba Hydro). MISO also operates the wholesale power market in the United States portion of this area. Other RTOs have followed MISO: PJM Interconnection in the Mid-Atlantic States, ISO New England, the Southwest Power Pool in the South Central States. In addition, three single state Independent System Operators exist in California, New York and Texas. Altogether over 70% of the US economy is covered by organized transmission grids.

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 (REE) [5] and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía - Polo Español, S.A. (OMEL) [6]. Spain's transmission system is interconnected with those of France, Portugal, and Morocco.

Merchant transmission

Merchant transmission is an arrangement where a third party constructs and operates electric transmission lines through the franchise area of an unrelated utility. Advocates of merchant transmission claim that this will create competition to construct the most efficient and lowest cost additions to the transmission grid. Merchant transmission projects typically involve DC lines because it is easier to limit flows to paying customers.

The only operating merchant transmission project in the United States is the Cross Sound Cable from Long Island, New York to New Haven, Connecticut, although additional projects have been proposed.

There are five merchant transmission interconnectors between five states in Australia: the DirectLink, MurrayLink and Southern Link between New South Wales and South Australia and BassLink between Tasmania and Victoria.

A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from the facility so that the beneficaries will pay the toll. Also, it is difficult for a merchant transmission line to compete when the alternative transmission lines are subsidized by other utility businesses.

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 Power Lines and Cancer FAQ. The issue is also discussed at some length in Robert L. Park's book Voodoo Science.

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.

Future of the power grid

High-temperature superconductors promise to revolutionize power distribution by providing lossless transmission of electrical power. The development of superconductors with transition temperatures higher than the boiling boint of liquid nitrogen has made the concept of superconducting power lines commercially feasible, at least for high-load applications.

Wide-scale use of superconductive transmission lines may also improve reception quality of the AM and shortwave radio bands, presently interfered with by conventional power lines due to EMF losses.

See also

• AC power flow model
• Common utility duct
• Dynamic Demand (electric power)
• Demand response
• Energy conservation
• Energy intensity
• Load profile
• Superconductor
• Wheeling (electric power transmission)
• For radio power transmission between transmitter and antenna, Radio frequency power transmission

External links

• United States: Transmission Lineworker Community Website
• North America: Transmission Maintenence Jobs
• Elbert: Iowa man has powerful idea for nation's grid
• Japan: World's First In-Grid High-Temperature Superconducting Power Cable System
• A Power Grid for the Hydrogen Economy: Overview/A Continental SuperGrid
• Global Energy Network Institute (GENI) - The GENI Initiative focuses on linking renewable energy resources around the world using international electricity transmission.