Analysis and design of gain-scheduling blade-pitch controllers for wind turbine down-regulation*

The use of down-regulation control strategies on turbines offers a means of stabilising the grid or improving the efficiency of the wind farm. Nonetheless, these de-rating strategies are often imposed upon an existing blade-pitch control structure, resulting in sub-optimal or poor performance. In particular, such an approach would potentially deteriorate the robustness of the rotor speed control loop and excite other turbine dynamics. Thus, this work proposes a blade-pitch control design for de-rating operation, where the strategy takes into account the changes in the aerodynamic characteristics of the turbine caused by down-regulation. Most importantly, the proposed controller retained the desired control bandwidth and robustness properties in both nominal and de-rating operations. Numerical simulations showed that an improvement on the pitch activities during down-regulation could lead to better reductions in the fatigue loads of some key turbine components.