Investigation of H∞ -Tuned Individual Pitch Control for Wind Turbines

Abstract

Large wind turbines experience amplified asymmetrical loads at particular harmonics of the rotational frequency. Individual pitch control (IPC) has emerged as a potential controls solution to this problem. H ∞ control, which facilitates robust, multivariable controller synthesis in the frequency domain, is a candidate approach to IPC design for several reasons. Firstly, the objectives of asymmetrical load attenuation are best described in the frequency domain. Secondly, the IPC control signals and loads in orthogonal directions are coupled, which necessitates a multivariable controller approach. Thirdly, H ∞ synthesis is a method that can explicitly impose constraints on the robustness of the closed-loop system. A downside of IPC is the significant increase in blade-pitch travel incurred, which introduces additional loading on the blade-pitch bearings over time. We investigate strategies to constrain the blade-pitch travel in the controller tuning procedure. A comprehensive study is thus presented for a range of H ∞ -synthesized IPCs to attenuate asymmetrical loads on large rotors at harmonics of the rotational frequency while mitigating blade-pitch travel. All developed controllers are validated and compared using a 25 MW fixed-bottom offshore wind turbine model via linear analysis of the robustness of the closed-loop system to input and output disturbances and nonlinear analysis via the study of structural load power spectra, damage-equivalent loads (DEL), and the actuator duty cycle (ADC) of the blade-pitch actuator.