An underfrequency load shedding scheme for high dependability and security tolerant to induction motors dynamics

The load shedding scheme is a last resort to prevent power system collapse when the available generation is less than the load demand. The traditional load shedding schemes monitors the frequency and voltage by underfrequency and undervoltage relays and provides load cutoff until the frequency returns to a stable point of operation. The reduction of rotating inertia demands faster load shedding to meet the stability requirements. In some electrical power systems in Brazil, it was found that, in the event of opening a transmission line upstream of the load shedding substation, some under frequency relay may operate improperly. This paper proposes a new method to avoid the unwanted operation of load shedding schemes when the utility substation (source) is out of service. The proposed method monitors negative sequence voltage, frequency and its rate of change, enabling faster load shedding (high dependability) without unwanted (high security) operation when compared with traditional methods. Events were classified by linear discriminant analysis and tested in the IEEE 9 bus test system. The tests consisted of several disturbances such as loss of generation and line outages at different levels of feeder loading, reactive compensation and distributed generation. The proposed method reached a 100% rate of correct classification.