Control design for a two-bladed downwind teeterless damped free-yaw wind turbine

Abstract

In this paper, a control architecture for a two-bladed downwind teeterless damped free-yaw wind turbine is developed. The wind turbine features a physical yaw damper which provides damping to the yawing motion of the rotor-nacelle assembly. Individual Pitch Control (IPC)11List of abbreviations: Individual Pitch Control (IPC); Collective Pitch Control (CPC); Linear Individual Pitch Control (LIPC); Multi-Blade Coordinate (MBC); Multi-Blade Coordinate (MBC); Out-of-Plane (OoP); In-Plane (IP); Variance Accounted For (VAF); Damage Equivalent Load (DEL); Extreme Direction Change (EDC); Proportional Integral (PI).List of symbols: Ttrq: Demanded generator torque for speed regulation; Tdtd: Demanded generator torque for drivetrain damping; Tgen: Demanded generator torque; Ωrated: Rated generator speed setpoint; Ωgen: Generator speed control setpoint; θcol: Collective blade pitch angle; θcol: Collective blade pitch angle; θ1: Individual blade 1 pitch angle; θ2: Individual blade 2 pitch angle; Θ1: Blade 1 pitch angle; Θ2: Blade 2 pitch angle; θtilt: Non-rotating blade pitch setpoint for tilt coordinate; θyaw: Non-rotating blade pitch setpoint for yaw coordinate; My, 1: Blade 1 Out-of-Plane root bending moment; My, 2: Blade 2 Out-of-Plane root bending moment; Mx, 1: Blade 1 In-Plane root bending moment; Mx, 2: Blade 2 In-Plane root bending moment; Mtilt: Rotor tilt moment; Myaw: Rotor yaw moment; ψ: Rotor azimuth angle; ψoff: Azimuth angle offset; ϕref: Yaw setpoint angle; ϕ˙yb: Yaw bearing angular velocity; n: n’th harmonic of load; ϕyaw: Yaw misalignment (between turbine yaw position and wind direction); ϕ: Yaw error (controller input); Lθ: Low-pass filter for individual pitch angles; Lϕ: Low-pass filter for yaw misalignment signal; HIPC: High-pass filter for individual blade pitch signals; Lyb: Low-pass filter for yaw bearing angular velocity signal; fIPC: Cut-off frequency of high-pass filter for individual pitch control κ: Gain of yaw model; τ: Time constant of yaw model; Td: Time delay in yaw model. is employed to obtain yaw control so as to actively track the wind direction and to reduce the turbine loads. The objectives of both load and yaw control by IPC are conflicting and therefore two decoupling strategies are presented and compared in terms of controller design, stability, and turbine loads. The design of the different controllers and the physical yaw damping are coupled and have a large impact on the turbine loads. It is shown that the tuning of the controllers and the choice of the yaw damping value involve a tradeoff between blade and tower loads. All results have been obtained by high-fidelity simulations of the state-of-the-art 2-B Energy 2B6 wind turbine.