Modal Dynamics of NREL 5MW Wind Turbine During Normal and Parked Conditions

Multi-Megawatt wind turbines have long, slender and heavy blades that can undergo extremely wind loadings. Good understanding of the modal dynamics of these large machines is of great priority. In this paper, modal dynamics of NREL 5 MW wind turbine is investigated. To this aim, FAST software has been implemented. Vibration characteristics of blades, tower and whole wind turbine machine is extracted. To examine the effects of wind velocity, two operating conditions of machine have been considered. Namely: normal operating condition at rated wind velocity and rated rotor speed and the other, parked condition with fixed rotor at the wind velocity equal to rated wind velocity. Blades root bending moments (both in plane and out of plane) and tower bending moments (both longitudinal and lateral) are extracted. Frequency spectrum of the results is utilized as a tool to study the effects of each vibration mode on wind turbine dynamics in each of aforementioned operating conditions. It is shown that tower vibration during normal operation is highly influenced from blade edge-wise bending mode. On the other hand, during parked condition the effects of flap-wise bending modes become more dominant. The results are expected to offer better predictions of the vibrational behavior of large wind turbines.