Improved modal dynamics of wind turbines to avoid stall-induced vibrations

Stall-induced edgewise blade vibrations have occasionally been observed on three-bladed wind turbines over the last decade. Experiments and numerical simulations have shown that these blade vibrations are related to certain vibration modes of the turbines. A recent experiment with a 600 kW turbine has shown that a backward whirling mode associated with edgewise blade vibrations is less aerodynamically damped than the corresponding forward whirling mode. In this article the mode shapes of the particular turbine are analysed, based on a simplified turbine model described in a multi-blade formulation. It is shown that the vibrations of the blades for the backward and forward edgewise whirling modes are different, which can explain the measured difference in aerodynamic damping. The modal dynamics of the entire turbine is important for stability assessments; blade-only analysis can be misleading. In some cases the modal dynamics may even be improved to avoid stall-induced vibrations. Copyright (C) 2003 John Wiley Sons, Ltd.