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Structural vibration control in wind turbines

2019
Koçan, Çağrı
Wind turbines are exposed to many different sources of loads which cause significant vibrations during their lifetime. Vibrations in the wind turbine system reduce efficiency and decrease the fatigue life. Although vibrations cannot be completely eliminated, they can be mitigated or converted to the other forms of energy by using various vibration control approaches which then increase the lifetime of the wind turbine system. Lowering the vibration response of the wind turbine provides stability under normal and extreme conditions, lesser amount of noise, maintain the high performance and manufacturability. Vibration control approaches usually aim to increase the damping characteristics of the system by using viscoelastic materials as well as additional external active or passive systems that include damping in themselves. In this thesis, dynamic response and fatigue load analyses are presented for a 5MW reference wind turbine, which has developed by United States National Renewable Energy Laboratory (NREL) as a reference model, by using aeroservoelastic simulation tool FAST. For both onshore and offshore cases, the effect of structural damping on the wind turbine in terms of vibration response and fatigue life is observed. Each structural tower and blade modes that induce significant vibrations are taken into account under different environmental conditions such as normal and extreme turbulence model and transient events such as gusts. The parked and operating conditions are investigated under different mean wind speeds. The effect of blade pitch angle to the fatigue damage is observed. By examining the analyses, it is aimed to guide for damping treatment approaches to mitigate vibrations and increase fatigue life of the structure. In this aspect, in order to identify how much structural damping increase can be achieved, finite element model of the NREL 5MW wind turbine is created. Viscoelastic link treatment is applied to the finite element model by a parametric approach and investigated that remarkable vibration mitigation can be obtained by using viscoelastic links.