One of the key control problems associated with variable speed wind turbine systems is maximization of energy extraction when operating below the rated wind speed and power regulation when operating above the rated wind speed. In this paper, we approach these problems from a nonlinear systems perspective. For below rated wind speeds we adopt existing work appearing in the literature and provide further insight into the characteristics of the resulting equilibrium points of the closed-loop system. For above rated wind speeds, we propose a nonlinear controller and analyze the stability property of the resulting equilibria. We also propose a method for switching between the two operating regimes that ensures continuity of control input at the transition point. The control laws are verified using a wind turbine model with a standard turbulent wind speed profile that spans both operating regimes.

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