Magnitude and duration are two main features used to characterize voltage sag. Many studies have been done in analyzing the factors that only impact the magnitude of the sag. As another feature of voltage sag, the duration is determined by the coordination of protection system. The fault caused voltage sag is usually assumed to be cleared by the primary protection system simply, the impact of protection coordination on sag duration is not considered by existing studies. In order to estimate voltage sag duration more precisely, this paper firstly uses the concept of Markov state space to establish the probabilistic modeling of protection coordination according to protection configuration, and then estimates sag frequency by utilizing the protection coordination probability and setting value of protection system. In addition, the proposed method was applied to the IEEE 57-bus test system, the simulation results show that the proposed method is simple and can give more accurate answers on voltage sag frequency corresponding to diff erent duration.

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