Electric power transmission

Electric power transmission, a process in the delivery of electricity to consumers, is the bulk transfer of electrical power. Typically, power transmission is between the power plant and a substation near a populated area. Electricity distribution is the delivery from the substation to the consumers. Electric power transmission allows distant energy sources (such as hydroelectric power plants) to be connected to consumers in population centers, and may allow exploitation of low-grade fuel resources that would otherwise be too costly to transport to generating facilities.

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. Underground power transmission is used only in densely populated areas due to its high cost of installation and maintenance, and because the high reactive power produces large charging currents and difficulties in voltage management.

A power transmission system is sometimes referred to colloquially as a "grid"; however, for reasons of economy, the network is not a mathematical 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. However, in some places the gaming of a deregulated energy system has been disastrous, such as that which occurred during the California electricity crisis of 2000 and 2001.

Task 3. Read the text on the AC and DC transmission. What are the advantages of each method?

AC (or alternating current) transmission refers to the transfer of energy from a source to one or more main receiving stations by electric current that reverses (alternates) its direction at regular intervals. DC (direct current) transmission involves the transfer of energy from a source usually to one receiving station by electric current that flows in one direction.

Key advantages of AC transmission technology are its flexibility and widespread use by electric utilities. AC facilitates future additions of intermediate stations, acting like exit and entrance ramps on an interstate highway, to serve local load centers and/or provide transmission access for new generation that may locate along the way. The ease of AC connections can encourage siting of fuel-diverse, newer technology and environmentally-friendly generators. Also, the use of AC technology enables expansion into a high-capacity transmission overlay that can be readily integrated with the existing systems. Integration can be achieved using commonly available step-down autotransformers or generator step-up transformers, offering the benefits of enhanced operating flexibility and reduced system losses.

By contrast, traditional DC technology is best suited for specialized applications, such as point-to-point transmission traversing unpopulated areas or where the systems being connected do not operate in synchronism. Examples include underground/undersea cables, long-distance overhead transmission serving as outlets for generating stations, and asynchronous links between two systems that have either no ties or very weak ties. While DC offers freedom from line charging currents and simpler line design with only one (monopolar) or two (bipolar) power conductors, it requires DC/AC conversion at each terminal to integrate with the existing system.

Дата добавления: 2015-08-03