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Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines which facilitate this movement are known as a transmission network. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. The combined transmission and distribution network is known as the "power grid" in North America, or just "the grid". In the United Kingdom, the network is known as the "National Grid".
A wide area synchronous grid, also known as an "interconnection" in North America, directly connects a large number of generators delivering AC power with the same relative frequency, to a large number of consumers. For example, there are four major interconnections in North America (the Western Interconnection, the Eastern Interconnection, the Quebec Interconnection and the Electric Reliability Council of Texas (ERCOT) grid), and one large grid for most of continental Europe.
The same relative frequency, but almost never the same relative phase as ac power interchange is a function of the phase difference between any two nodes in the network, and zero degrees difference means no power is interchanged; any phase difference up to 90 degrees is stable by the "equal area criteria"; any phase difference above 90 degrees is absolutely unstable; the interchange partners are responsible for maintaining frequency as close to the utility frequency as is practical, and the phase differences between any two nodes significantly less than 90 degrees; should 90 degrees be exceeded, a system separation is executed, and remains separated until the trouble has been corrected.