TABLE OF CONTENTS All electrical power systems that contain generation are liable to be subjected to a variety of abnormal operating conditions such as network faults, loss of some or all generation, the tripping of circuits within the system and other disturbances that can result in the reduction in the generation capacity available to the system. In these situations a balance between the existing load and the remaining generation should be reestablished if possible before the reduction in frequency produced by the overload affects the turbines and the generation auxiliary equipment, which could eventually result in the total collapse of the system.
To assist in restoring the equilibrium, frequency relays are employed. These disconnect the less important loads in stages when the frequency drops to a level that indicates that a loss of generation or an overload has occurred. This type of scheme is very useful in industrial plants where in-house generation is synchronised to the public grid system. The concepts, criteria and conditions applicable to the design of an automatic load shedding system for industrial plants, based on frequency relays, are set out in this chapter.
When a total or partial loss of generation occurs within the system, the first indicators are a drop in voltage and in frequency. However, given that voltage drops can also be caused by system faults, it is generally recognised that a drop in frequency is a more reliable indication of loss of generation. A sudden loss of...
