The conventional undervoltage load shedding (UVLS) and the underfrequency load shedding (UFLS) are designed separately, lacking considerations of mutual impacts of the voltage and the frequency on load characteristics. Thus it may cause excessive or insufficient load curtailments. To deal with the problem, this paper proposes an underfrequency and undervoltage combined load shedding method. It takes account of the combined effects of frequency and voltage on load dynamic characteristic. Only the local information of loads are required to determine the load shedding strategy. Simulation results of IEEE 39-bus system have shown that this method, compared with the conventional UFLS/UVLS, can maintain the frequency stability and the voltage stability more effectively.

Frequency stability,voltage stability,load characteristics,local responses,load shedding method