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

The multi-Megawatt wind turbines have long, slender, and heavy blades that can undergo extremely wind loadings. A good understanding of the modal dynamics of these large machines is of great priority. In this work, the modal dynamics of the NREL 5 MW wind turbine is investigated. For this aim, the FAST software is implemented. The vibration characteristics of blades, tower, and whole wind turbine machine are extracted. In order to examine the effects of wind velocity, two operating conditions of machine are considered, namely the normal operating condition at rated wind velocity and rated rotor speed, and the other, the parked condition with fixed rotor at the wind velocity equal to rated wind velocity. The blade root bending moments (both in plane and out of plane) and tower bending moments (both longitudinal and lateral) are extracted. The frequency spectrum of the results is utilized as a tool to study the effects of each vibration mode on the wind turbine dynamics in each aforementioned operating condition. It is shown that tower vibration during normal operation is highly influenced from the blade edge-wise bending mode. On the other hand, during the 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.