Flexible AC transmission system

Another class of devices used to control the voltage is operated using powered electronic switches to continuously adjust the capacitance and/or inductance in a substation to keep the voltage at precisely the voltage desired. These devices are relatively new in deployment, having been developed with the advent of inexpensive and robust power semiconductor components. These devices are part of a group broadly known as FACTS (Flexible AC Transmission System) devices and include static var compensators, static synchronous compensators, and dynamic voltage restorers. 

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