Stability analysis of small horizontal-axis wind turbines using Takagi-Sugeno fuzzy models

In modern wind turbines the output power for wind speeds above the nominal wind speed is limited by the adjustment of the rotor blade angles. The adjustment is performed with the pitch system. For cost reasons, in small wind turbines simple mechanical pitch systems are used actuated by centrifugal force and thus depending on the rotor speed and the blade mass. With the help of the converter the generator torque, and as a consequence also the rotor speed, is controlled, so that the constant regulation on nominal power output above the nominal wind speed is enabled. The main difficulty using low cost centrifugal force regulators in pitch systems are determined by a variable structure mathematical description, the manufacturing tolerances, and age-related changes of dynamic properties. The nonlinear dynamics of such systems has not yet adequately been studied. In particular, no stability was verified considering the foregoing model uncertainties.