Periodic stability assessment of a flexible hub connection for load reduction on two-bladed wind turbines

The periodic stability of an innovative load reduction system for two-bladed wind turbines is investigated using Floquet theory. The load reduction system introduces an additional cardanic degree of freedom between the hub mount and the nacelle carrier flange. A reduced rotor model is considered to analyze the time-variant behavior as a function of rotor shapes and design parameters. Aerodynamic excitation of the rotor is neglected in this study. The linearized system equations of motion of this system indicate a dependance of the rotor shape and the periodic system stability. Due to the time-variance of the asymmetric rotor the Floquet multipliers are derived to determine system stability and the dominant periodic coefficients. It can be shown that the rotor shape has a significant impact on the stability of gimbal-hinged rotors. Gyroscopic effects are the reason for a stabilization of disc-like rotors, whereas a cylindrical, two-bladed rotor assembly is in general unstable. The introduction of an additional spring-damper coupling is a possibility to stabilize such configurations, but results show that resonance phenomena have to be accounted for.