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Applications offuzzy logic controllers have proved that a tuning algorithm must be added to improve its perfomance against the system disturbances. The tuning methodr represent an additional costs and complexiry for implementation. This paper, presents a novel method which implies a self-tuned c o n t r o h capable of damping the system oscillations whatever the disturbance changes. Moreover, this controller is robust to a degree that permits more than 2500% parameters uncertainties to be considered. The idea of this approach is to combine two control signals, one from a conventional stabilizer, e.g. lead lag compensator, and the other from a fuzzy logic controller. The stabilizer is designed based on modal techniques for the worst case of oscillations and is independent on the system disturbance. On the other side, the ranges of membership firnetions requiredfor the design of fuzzy logic controller are selected based on the worst experience of the input/output behavior. Thefinal controller is testedfor diflerent disturbances and system parameters and the results show a satisfactoory perfomance and robustness. Referring to Fig.(l), one can understand Fuzzy Logic (FL) as a logical system, which is an extension of multivalued logic. It is almost synonymous with the theory of fuzzy sets, a theory which relates to classes of objects with entrap boundaries in which membership is a matter of degree [I]. The basic concept underlying FL is that of a linguistic variable, that'is, a variable whose values are words rather than numbers. In effect, much of FL may be viewed as a methodology for computing with words rather than numbers. Although words may be less precise than numbers, their use is closer to human intuition. Furthermore, computing with words exploits the tolerance for imprecision and thereby lowers the cost of solution. Another basic concept in FL, which plays a central role in most of its applications, is that of a fuzzy if-then rule or, simply, fuzzy rule.

Thls paper presents a novel trend toward retirement of the strong oscillations which may exist if the power system is exposed to Subsynchronous Resonance(SSR) phenomenon. Since these harmful oscillations are mainly due to the oscillatory modes of the turbo-generator shaft, the idea of this trend is to suggest an approp~iate design of the shaft-parameters such that the SSR phenomenon may be fundamentally avoided. The effect of shaft parameters like the spring constant and the inertia constant on the shape and location of these oscillatory modes is extensively studied. Modal analysis, eigenvalue analysis, and nonlinear time simulation are used as tools to design a turbo-generator shak which achieve minimum oscillations. Practical constraints are raken In consideration in a way that the proposed design can be agreeably manufactured.