On underfrequency load shedding schemes

The author discusses some of the factors which must be considered for testing an underfrequency load shedding scheme. Fault simulation and system separation due to faults or out-of-step conditions must be represented. This is necessary for analysis of generator behavior within the system during the abnormal conditions. The program must be capable of simulating the behavior of distance and out-of-step relays under abnormal operations. Characteristics of these relays are briefly described. It is shown that the traditional approximate models may not always be capable of predicting the system behavior for a given load shedding scheme and cascading system outages may result through undesired tripping of the generator underfrequency relays. Several factors which influence the behavior of a power system under a given underfrequency load shedding scheme are described