Analysis of Two Interpolation Methods of H∞ Controllers for Robust Wind Turbine Control

This paper presents the design process of two collective pitch angle controllers based on different interpolation methods of three Linear Time Invariant (LTI) H∞ controllers to improve the generator speed regulation in the above rated power production zone. The control dynamics vary according to a selected parameter, which changes in the different operating points of the above rated zone, as for example the collective pitch angle in blades. One interpolation method develops a gain-scheduling of the controller’s state space matrices by polynomials approximations, where the controller is represented by Linear Fractional Transformation (LFT). The other interpolation method solves complex a Linear Matrix Inequalities (LMI) system, which guarantees the closed loop stability during the transitions. The selected wind turbine non-linear model to design these controllers has been developed in GH Bladed v4.00 and it is based on the offshore 5 MW wind turbine defined in the Upwind European project. The two gain scheduled controllers are validated in GH Bladed and an exhaustive analysis is developed to calculate the load reduction rate in wind turbine components compared to a baseline robust control based strategy previously defined