Tow-Level Hierarchical Fuzzy Control and its Application in PSS

This paper proposes a new method for large scale control systems. This approach has a hierarchical structure and uses the idea of Fuzzy set and optimal tem, stabilizer in control theory. This method has been developed to design a power sys a multi machine power system. The overall control problem is reduced to some kinds of optimal control subproblem with low dimension. The solutions of these subproblems constitute the local controllers. This makes the first level of hierarchical control structure. The local controllers are then coordinated by the second level controller called coordinator. In fact, the second level controller the prediction the interactions between subsystems and transfers them back to the first level controllers.This will be done during each coordination cycle. The prediction of each interaction is done by implying Fuzzy logic theory. To do so we are require to assign appropriate Fuzzy set to each state variable of overall system and then developed the ne cessary Fuzzy rules to predict the future states and interactions between subsystems. The number of Fuzzy sets and their domains are Mvaried during the control process The two most advantages of the proposed method are parallel processing property and its low computational complexities.At the end we apply this technique to design a two-level power system stabilizer for three area power system. The simulation results are very encouraging, specially when we compare them with the results obtained by centralized and decentralized controllers.