The optimization of undervoltage load shedding strategy

The conventional undervoltage load shedding device is designed separately with the underfrequency load shedding device, thus failing to protect the power system from voltage instability and frequency instability. From the first-order model of the induction motor, this paper derives a novel F-index for transient voltage stability assessment with consideration on the change of load reactive power and the degree of transient voltage drop. This F-index physically represents the variation of the equivalent load susceptance. During severe disturbances, F-index of the induction motor rises to a much higher value than those of the constant power load and the constant current load, while F-index of the constant impedance load is always zero. Therefore, this index not only measures the degree of the transient voltage instability of different load models effectively but also reflects the proportion of induction motors in the load. Furthermore, this paper employs the F-index to allocate load curtailments at critical load buses with low voltages, thus providing a frequency and voltage dynamic based adaptive undervoltage load shedding scheme. Compared with the classical adaptive load shedding scheme, this scheme can guarantee transient voltage stability and frequency stability more effectively.