The authors present a fuzzy approach in the determination of the unstable machines in transient stability studies of multimachine electric power systems. This approach is based on the derivation of an index that can identify the unstable machine by using the values of the generator rotor angles and accelerations. The index actually computes the degree of instability of every system machine, when a three-phase fault is established on any power system location, causing no topological changes after its clearance. The unstable machine will be the one having the largest value of instability index. The derivation of this index is based on the principles of pattern recognition, fuzzy sets theory and the Taylor series expansion technique. Two examples illustrating the methods application are given, including the analysis of the interconnected Hellenic power system.
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