With the development of modern science and technology, a lot of sensitive electric equipments were widely applied. Voltage sags were considered as the mostly reason which are affect these electric equipment running in normal and safety. A new method is proposed based on the analysis of existing variety of voltage sag detection methods and the repeatability of periodic signal, hysteresis bands are used in order to avoid the impact of the sampling error, noise during the voltage of the value approaches zero, simulinks validate real-time of this method. Based on the theory method, algorithm program is compiled, and commissioning work has been carried out on the signal acquisition card, the feasibility of this method is proved furtherly.
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