Sliding mode control of torque ripple in wind energy conversion systems with slip power recovery

This paper deals with wind energy conversion systems (WECS) with slip power recovery. These systems use a double-output induction generator, which is connected directly to grid by stator and through a static converter by rotor. An appropriate control of the converter allows the WECS to operate at the optimum tip speed ratio, maximizing the efficiency of the energy conversion. Wind turbine generators are characterized by a low frequency mode of oscillation. Random wind fluctuations, wind shear and tower shadow effects, may excite this mode, producing large ripple on drive train torque as well as on the generated electric power. Torque ripple may damage drive train components, and fluctuation of the generated electric power may cause flicker in weak grids. In this paper, a sliding mode control strategy of the static converter is proposed. It forces the system to follow wind speed variations and provides damping of the oscillation mode, resulting in a large reduction of torque ripple and electric power fluctuation. The closed-loop system dynamics are completely robust to uncertainties in electrical parameters of the generator and grid voltages disturbances.