Fixed-structure H∞ control design for linear Individual Pitch Control of two-bladed wind turbines

In this paper, a fixed-structure Individual Pitch Control (IPC) design method for two-bladed wind turbines is presented. IPC is an active load reduction technique designed to lower wind turbine loads, which are caused by continuously varying wind conditions. Based on load measurements, the once-per-revolution (1P) loads and harmonics of this frequency (2P, 3P, etc.) can be reduced by rotating (pitching) the blades along their axes. Instead of using the typical Multi-Blade Coordinate (MBC) transformation for IPC, a linear coordinate transformation specifically for two-bladed wind turbines is considered. The advantage of using this transformation is that only a single simple Single-Input Single-Output (SISO) controller is required to reduce the odd harmonic loads (1P, 3P, etc.). A second SISO controller can be used to reduce the even harmonic loads (2P, 4P, etc.). Moreover, the coordinate transformation allows for fixed-structure controller design, which makes it highly suitable for practical application. With nonsmooth H_∞-controller synthesis, the optimal controller parameters for the new IPC design method can be found. The performance of the resulting controller is evaluated by means of a high-fidelity simulation of a two-bladed wind turbine.