Appropriate realization of gain-scheduled controllers with application to wind turbine regulation

Power regulation of horizontal-axis grid-connected up-wind constant-speed pitch-regulated wind turbines presents a demanding control problem with the plant, actuation system and control objectives all strongly nonlinear. In this paper, a novel nonlinear control strategy is devised which, in some sense, optimizes performance across the operating envelope. In comparison with linear control, the nonlinear strategy achieves a substantial improvement in performance. The realization adopted is crucial in attaining the required performance. An extended local linear equivalence condition is introduced which provides a basis for the selection of an appropriate realization. This is an important, and general, issue in the design of gain-scheduled systems and generic realizations, which satisfy the extended local linear equivalence condition, are derived for SISO systems scheduled upon an internal plant or controller variable. For the wind turbine nonlinear controller, realizations which satisfy the extended local linear equivalence condition provide a substantial improvement in performance in comparison with linear control and realizations which do not satisfy this condition.