Abstract: The fuzzy H_○∞ controller design problem was studied for steam valve opening of multi-machine power systems which consisting of turbo-generators with mixed time-varying delays. The mixed time-varying delays include the interval retarded-type discrete delay， the neutral-type discrete delay and the time-varying distributed delays. Fuzzy models were established， and the controllers of the systems were designed. Linear matrix inequalities (LMIs)-based H_○∞ stability conditions were derived. Moreover， the results were expanded to different conditions of time-delay， including H_∞ stability condition with the fast time-varying distributed delays (i.e. the derivatives of distributed delay is larger than 1). Therefore， the obtained results are new and less conservative. Finally， simulation results for the steam value opening of three-machine infinite bus systems were provided to verify the effectiveness of the proposed results.

Key words: mixed time-varying delay, multi-machine power systems, fuzzy control, LMI, H_○∞ performance