Electric power transmission
Electric power transmission is the second process in the delivery of electricity to consumers. Electricity is generated by power plants and is then sold as a commodity to end consumers by retailers. The electric power transmission and electricity distribution networks allow the delivery of the generated electricity 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.
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) 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. Power is usually transmitted as a 3-phase alternating current (AC). At the generating plants the power is produced at a relatively low voltage of 10-15 kV, then stepped up by the power station transformer to a high voltage (220 - 400 kV) alternating current for transmission over longer distances to grid exit points (substations).
Losses
It is necessary to transmit the electricity at high voltage to reduce the percentage of power lost. The higher the voltage the lower the current that flows, which reduces the size of cable needed and the amount of energy wasted. 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 [1] (http://climatetechnology.gov/library/2003/tech-options/tech-options-1-3-2.pdf), and in the UK at 7.4% in 1998 [2] (http://www.powerwatch.org.uk/energy/graham7x.htm).
When power is required to be transmitted over very long distances, it becomes more efficient (and hence economical) to transmit using direct current (HVDC) instead of alternating current. This is because smaller insulators are required for a DC line than an AC line carrying a given amount of power. Also, at high AC voltages significant amounts of power are lost due to the capacitance between phase lines or, in the case of buried cables, between a phase line and the soil or water in which the cable is buried. HVDC is used for very high quantities of power over very long distances, for undersea cables over ten kilometers or so (as undersea cables have a very large capacitance), or between asynchronous grids. Since the power flow across a HVDC link is directly controllable, HVDC links are sometimes used within a grid to stabilize the grid against control problems with the AC power flow.
Current can flow in an AC line even when the voltage is zero. Typically, this flow is due to inductance at the load or within the AC distribution line itself, and is called a reactive flow. Reactive flows transmit no power from the generator to the load, but they burn power within the grid. The fraction of total power which is resistive (as opposed to reactive) power is the power factor. Utilities add capacitor banks throughout the system to improve the power factor, in order to reduce the associated real power losses.
Grid exit
Substations are used to step the voltage down to low voltage local power lines for distribution to commercial and residential 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 (100 - 600 V, varying by country and customer requirements).
For more details see the article on electricity distribution.
Communications
Transmission lines can also be used to carry data: this is called power-line carrier, or PLC.
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.
Health concerns
It is argued by some that living in proximity to a high voltage power line presents a danger to the animals and humans. Some have claimed that electromagnetic radiation from power lines causes elevated risk of certain types of cancer. Some studies have purported to identify a risk, while others have not. Studies over larger populations have consistently shown no clear correlation between health effects and the proximity of power lines.
The current mainstream scientific view is that power lines are unlikely to pose any increased risk of cancer or other somatic diseases. For a detailed discussion of this topic, including references to many of the scientific studies, see the Power Lines and Cancer FAQ (http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html). 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.[3] (http://www.futureenergies.com/print.php?sid=237)
See also
- HVDC, High voltage direct current.
- SVC, Static Var Compensation.
- FACTS, Flexible AC Transmission System.
- Distributed generation
- Electricity market.
- Liberalization
- Power line communications (PLC).
External link
- Scientific Facts on Power Lines (http://www.greenfacts.org/power-lines/index.htm) - A summary by GreenFacts of a scientific consensus report published by the International Agency for Research on Cancer (IARC) on health effects of ELF electromagnetic fields from power lines.
- Union for the Co-ordination of Transmission of Electricity (UCTE) (http://www.ucte.org/), the association of transmission system operators in continental Europe, running one of the two largest power transmission systems in the world
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