An adaptive load shedding method based on the underfrequency and undervoltage combined relay

The conventional UVLS (undervoltage load shedding) relay and the UFLS (underfrequency load shedding) relay, designed separately without considerations on load characters and mutual influences of the voltage and the frequency, may result in excessive or insufficient load curtailments. This paper proposes a new adaptive load shedding method based on the underfrequency and undervoltage combined relay. With considerations on influences of frequency and voltage dynamics on the load active power, this method uses real-time measured local responses to calculate the amounts of underfrequency load curtailments and undervoltage load curtailments, respectively, and adopts the larger one as the practical amount of load curtailments. Furthermore, this method transmits the practical amount of load curtailments to the underfrequency and undervoltage combined relay, thus triggering local successive load shedding. Simulation results of IEEE 39-bus system have shown that the new method, compared with the conventional one, can adapt better to load characters and disturbance types, thus guaranteeing the frequency stability and the voltage stability mo re effectively.