NEW ANALYTICAL METHOD FOR CALCULATING THE FEREQUENCY DECREASING IN LOAD SHEDDING SCHEME

When a power system is in stable operation at normal frequency, the total mechanical power input from the prime movers to the generators is equal to the sum of all connected loads, plus an real power losses in the system. Any sigintficant upset of this balance causes a frequency change. If the power system disturbance is due to excess of load (or outage of Gen.) the remaining generating units will be overloaded, thereby causing the frequency to drop, sometimes to levels that may cause permanent damage to steam turbines. To avoid this kind of damage, load shedding relays are often applied throughout the system to provide a means of helping balance the load to the remaining generation in the system. This paper presents an analytical method to design the under frequency protection system in such a way that the final frequecny of system can be estimated very easily in each overload condition.