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Electricity is generated as it is used. There is very little ability to store electricity. Because of this instantaneous nature, the electric power system must constantly be adjusted to ensure that the generation of power matches the consumption of power. On continental U.S. power grids, roughly 150 Control Area Operators serve this function by using computerized control centers to dispatch generators as needed.
 Electric Control Area Operators — Continental United States, 1998. (Source: Energy Information Administration, Office of Coal, Nuclear, Electric and Alternate Fuels. Based on data contained in Form EIA-861, "Annual Electric Utility Report.") |
To help the Control Area Operators, electrical generators are divided into three categories:
- Baseload power plants, which are run all the time to meet minimum power needs
- Peaking power plants, which are run only to meet the power needs at maximum load (known as "peak load")
- Intermediate power plants, which fall between the two and are used to meet intermediate power loads.
Nuclear plants, for example, are nearly always operated as baseload plants because they are most stable at full power. Intermediate plants are well-suited to changing power loads (called "load following"); gas turbines can be used as intermediate plants. Peaking plants are generally the most expensive plants to operate. In many cases, these are small, older coal- or oil-fired plants, although gas turbines can also be used as peaking plants.
Although Control Area Operators run the grid within their control areas, on a larger scale, the responsibility for electric grids has traditionally rested with electric utilities. Each utility has responsibility for the operation of the electrical grid within its service area (coordinating the efforts of the Control Area Operators in its service area) as well as investment in new lines, maintenance, and control of access to the grid.
However, with competition in wholesale power markets and increasing competition in retail power markets, utility control of the grid has been viewed as a conflict of interest. Some states have moved to pass the control of the grids to independent system operators, or ISOs. For example, the California ISO controls the transmission grid for California. ISOs also exist in Texas and New England. Ownership of the transmission and distribution systems may be retained by the utilities or be passed off to independent transmission companies ("TransCos"), in which case the utility effectively becomes a distribution company ("DisCo").
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