A new fuzzy theory based principle for distinguishing unstable swing from stable swing in complicated power systems is presented in this paper. In terms of the behavior of electrical quantities, the distinction between stable and unstable swing conditions isn't clear and certain because of the affection of various system parameters in the power system. To solve the problem, the new principle uses the so-called fuzzy comprehensive evaluation to detect out-of-step conditions. Based on the several criteria characterized by fuzzy membership functions, it has many advantages over a conventional principle using a single criterion, such as improved performance and convenience of relay setting etc. Finally, the results of digital simulations are also provided in this paper.
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