Reliability Modeling of Protective Systems

Electric power systems are comprised of a very large number of interconnected components that are designed for the sole purpose of generating and delivering electrical energy to consumers. Usually, the consumers are free to accept or-reject the available electrical energy at will, suggesting a probabilistic rather than deterministic demand pattern. The system is operated by humans and by automatic control apparatus, both having some probability of failure to perform their function. Moreover, the system physical components are subject to failure in some random way, with each failure often requiring corrective action. Since the system is geographically extensive, it is subject to a large number of natural and man-made hazards. Examples are lightning induced faults and physically damaged components that result from natural or man-made causes. For the purpose of this discussion we classify all of the above as disturbances. Some of these disturbances, such as short circuits, cause severe upsets in system operation and must be somehow removed or isolated. This is the role of the ´protective systems,´ which are installed throughout the power system, to detect and remove hazardous disturbances, which we usually call faults.