Smooth Sliding Control Applied To Power Optimization Via Extremum Seeking In Variable Speed Wind Turbines

This paper addresses the application of extremum seeking control (ESC) in an outer loop to perform the maximum power point tracking (MPPT) or the maximum torque regulation, depending on the available feedback information, in wind energy conversion systems (WECS). For the case when power measurements are not available, a torque estimation scheme is proposed. We also design a nonlinear robust controller for the inner loop. The robust part of the controller, which maintains fast transient response, is based on sliding mode control that features a smooth control signal, free of chattering, previously designed for linear plants. In this sense, this work presents the first generalization of this controller for the class of nonlinear plants representing the WECS dynamics. Moreover, the proposed robust inner loop control does not require measurements of the stator flux nor any flux observer. The controller is designed by using an inherit stability property of the WECS dynamics demonstrated in this paper, named, the input-to-state stable w.r.t. the turbine angular velocity and its time derivatives. The complete stability analysis is provided and the effectiveness of the proposed scheme is supported by analysis and simulation results.