Sensorless Nonlinear Control of Wind Energy Systems with Doubly Fed Induction Generator
The problem of controlling doubly fed induction generators (DFIG) associated with wind turbines is addressed. The control objective is twofold: maximum power point tracking and reactive power regulation in the DFIG. Unlike previous works, we seek the achievement of this control objective without resorting to physical sensors of mechanical variables (e.g. wind turbine velocity and DFIG rotor speed) Interestingly, wind velocity is also not assumed to be accessible to measurements. The control problem is dealt with using an output feedback controller designed on the basis of the nonlinear state space representation of the controlled system. The controller is constituted of a high gain nonlinear state observer and a nonlinear sliding state feedback mode. Using tools from Lyapunov's stability, it is formally shown that the closed loop control system, expressed in terms of the state estimation errors and the output reference tracking errors, enjoys a semi global practical stability. Accordingly, it is possible to tune the controller design parameters so that it meets its objectives with an arbitrarily high accuracy, whatever the initial conditions are. These theoretical results are confirmed by simulations involving wide range variation of the wind speed.