  This paper addresses the problem of controlling a variable-speed wind turbine
with a Doubly Fed Induction Generator (DFIG), modeled as an
electromechanically-coupled nonlinear system with rotor voltages and blade
pitch angle as its inputs, active and reactive powers as its outputs, and most
of the aerodynamic and mechanical parameters as its uncertainties. Using a
blend of linear and nonlinear control strategies (including feedback
linearization, pole placement, uncertainty estimation, and gradient-based
potential function minimization) as well as time-scale separation in the
dynamics, we develop a controller that is capable of maximizing the active
power in the Maximum Power Tracking (MPT) mode, regulating the active power in
the Power Regulation (PR) mode, seamlessly switching between the two modes, and
simultaneously adjusting the reactive power to achieve a desired power factor.
The controller consists of four cascaded components, uses realistic feedback
signals, and operates without knowledge of the C_p-surface, air density,
friction coefficient, and wind speed. Finally, we show the effectiveness of the
controller via simulation with a realistic wind profile.
