Power/energy: Hybrid load shedding is frequency based: Time at a given frequency is also important, bringing stabilization in a matter of seconds even when as much as two thirds of the load must be shed

When an electric system imports large amounts of power from outside its area, it needs a new approach to load shedding. Even the best conventional frequency-coordinated shedding may not prevent a blackout when external power is lost and more than 50 percent of the load must be shed. However, a hybrid load-shedding scheme that includes at least one time-coordinated step in addition to several frequency-coordinated steps can readily shed 70 percent and more of the load. Such a concept has been adopted by the Florida Power & Light Co., the Sierra Pacific Power Co. in Nevada, and the Electric Service Commission (Escom) of the Republic of South Africa. The choice of frequency settings of load-shedding relays, load-shed increments, and time delays is critical; it depends on the circumstances at a given utility. The coordination is aided greatly by a computer program, LOAD SHED, developed expressly for this purpose. Hybrid shedding techniques and the LOADSHED program are reviewed.