Modal properties of a vertical axis wind turbine in operating and parked conditions

Modal frequencies and damping coefficients are crucial quantities to characterize and predict the structural behaviour of wind excited structures. Wind turbines in parked conditions can be treated as telecommunication or lighting towers, for which a fair amount of experimental studies is available. However, natural frequencies and damping coefficients may change dramatically in operating conditions. In this case, experimental investigations are quite few and the literature is almost lacking in damping ratio measurements for the vertical axis typology. This paper uses the results of a long-term monitoring campaign over a vertical axis wind turbine to investigate natural frequencies and modal damping coefficients under different wind and operating conditions. After selecting structural acceleration records in different conditions of the wind and of the turbine spin and also considering a non-stationary thunderstorm event, the study describes an extended investigation of the modal parameters, paying special attention to damping ratios and including higher modes.