Variable Speed-Variable Pitch Controllers for Aero-Servo-Elastic Simulations of Wind Turbine Support Structures

Abstract

Site-specific fatigue evaluation of wind turbine support structures has been required by Japanese domestic rule since the revision of the Building Code in 2007. Fatigue loads are strongly affected by wind turbine controllers, as well as by the characteristics of wind and the wind turbines themselves. It is difficult for anyone but expert wind turbine controller engineers to design wind turbine controllers. For this project, blade pitch and generator torque controllers for a typical land-based variable speed- and pitch-controlled wind turbine were formulated using a Loop-Shaping Method, considering wind turbine characteristics such as rotor aerodynamics, tower vibration, and control systems. Controller gains were calculated by defining the gain cross frequency and phase margin for pitch control as well as the peak gain and damping ratio for generator torque control, without introducing control design tools such as the Bode diagram or the Nyquist diagram. Controllers that are compatible with the fatigue loads of towers and blades as well as pitch rate were investigated through case studies of three kinds of towers. Fatigue loads of the support structures were shown to depend not on gain cross frequencies themselves but on those frequencies first normalized by tower mode-bending frequencies. The normalized gain cross frequencies of pitch controllers were recommended to be about 20%. Furthermore, gain scheduling, which reduces pitch commands to filtered pitch angles, was shown to be effective in reducing fatigue loads with improved characteristics.