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.