State clean energy programs can be influenced by the design and operation of the U.S. transmission system. As a result, states have a strong interest in the system, even though they do not have a direct responsibility for the operation of the system. This background document provides some basic information about how the electricity transmission system operates, how it is designed and regulated, and the interrelationship between the operation and reliability of the electricity grid and state clean energy policies and programs. I. Electricity Grid Basics: How does the Transmission system work? Transmission lines are the interstate highway of the electricity delivery system. They carry high voltage power (138 kilovolts (kV) and above) from power plants over long distances to substations. There are more than 150,000 miles of interconnected transmission lines across the country and additional transmission that connects the U.S. to power plants and load centers in Canada and Mexico. Once the power reaches the substation, it is “stepped down ” in voltage and delivered to customers through the distribution network. Some basic physical properties of electricity influence the design and operation of the transmission system
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