Mass Imbalance Identification for a Wind Turbine Rotor Based on Higher-order Modal Dynamics

Previous studies have shown the effectiveness of modal analysis-based mass imbalance methods to detect and diagnose rotor imbalances in three-bladed horizontal-axis wind turbines through the use of first-order flapwise modes and their related theories. However, the usage of first-order edgewise and higher-order flapwise modes for imbalance detection, identification, and prognosis has not yet been explored. This study reports promising results that the modal dynamics and linearly proportional correlation between frequency difference and degree of mass imbalance of first-order flapwise modes can be applied to the first-order edgewise and second-order flapwise modes of a simple balanced and imbalanced three-beam rotor system as well. We begin with a series of modal analysis calculations and perform mesh convergence studies of the higher-order modes to establish the mass-balanced finite element model. From the modal analysis of the first nine modes, including the first-order flapwise, first-order edgewise and second-order flapwise bending mode families, related anti-symmetric first-order edgewise and second-order flapwise mode shape pairs were found to have the same frequencies for the balanced three-beam rotor system and a quantifiable frequency difference versus imbalance index for the imbalanced three-beam rotor, replicating the pattern of first-order flapwise modes demonstrated in prior work. First-order edgewise and higher-order flapwise modes can provide further applications in modal analysis-based mass imbalance methods by leveraging the theories and applications of the preceding theory based on first-order modes.