Affection of power system relay cooperation to voltage sag and sensitive equipment

When the power system component faults, the voltage sag duration depends on the cooperation time of protection system. Many literatures have analyzed the influencing factors of voltage sag magnitude, such as fault types, fault location, pre-fault voltage, transformer connections, and fault impedance. But the affection of protection cooperation to sag duration is not considered in detailed. A probabilistic modeling of the cooperation of the protection system is proposed based on the concept of Markov state space to estimate the sag duration. After that, the voltage sag frequency and sensitive equipment trip probability are estimated by utilizing the configuration and setting value of the protection system. The proposed method was applied to the IEEE 57-bus test system, and the simulation results show that the proposed method is practical, simple and adaptive, and the study of this paper has both theoretical significance and engineering application value for voltage sag frequency assessment.