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The traditional model of electric power generation and delivery is based on the construction of large, centrally located power plants. "Central," in this case, ideally means that the power plants are located on hubs surrounded by major electrical load centers. For instance, a power plant may be located close to a city to serve the electrical loads in the city and its suburbs, or it may be located in the midpoint of a triangle formed by three cities.
In reality, many other factors influence the siting of power plants, including the availability of fuel resources, the need for a cooling water source, environmental considerations, and other factors. In the West, for instance, several large coal plants are located close to coal mines to minimize the cost of shipping coal. And most of the power for the Pacific Northwest comes from hydroelectric plants that are located far from the cities that depend on their power.
Regardless of where power plants are located, their power must be brought from the plant to the users, and that's the purpose of the electricity grid. The grid consists of two infrastructures: the high-voltage transmission systems, which carry electricity from the power plants and transmit it hundreds of miles away, and the lower-voltage distribution systems, which draw electricity from the transmission lines and distribute it to individual customers.
High voltage is used for transmission lines to minimize electrical losses; however, high voltage is impractical for distribution lines. The interface between the two is the electrical substation, which features transformers that "step down" the transmission voltages—ranging from 138 kV to 765 kV—to lower voltages for the distribution systems. Transformers located along the distribution lines further step down the voltage to 120 V or 240 V for household use.
Substations also include electrical switchgear and circuit breakers to protect the transformers and the transmission system from electrical failures on the distribution lines. Circuit breakers are also located along the distribution lines to locally isolate electrical problems (such as short circuits caused by downed power lines).
Select one of the following for more in-depth information:
The following documents and Web sites provide additional background information on the electricity grid:
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