Operational experience of load shedding and new requirements on frequency relays

This paper summarises experience and analysis of under-frequency load shedding and power generator islanding. Most power systems can withstand the loss of a single generating unit or a single transmission line. Simultaneous loss of several power system components may cause a severe deficit in active and reactive power. The action of overexcitation limiters on the remaining generators may further aggravate the situation. Such infrequent events may cause severe drop in system frequency and voltage magnitude. Frequency relays for load shedding and power generator islanding must operate correctly even if the system frequency decays and the voltage magnitude decays rapidly at the same time.