Mechanical behaviour of wind turbines operating above design conditions

Improving production of electric power from renewable sources is fundamental in order to decrease the use of fossil fuels. A crucial aspect for future development of renewable sources is to guarantee competitive prices to end users and profitably economic returns to those who invest in these technologies. This can be reached by constantly surveying plants performances, optimizing the operative conditions and evaluating the possibility to implement upgrades. For what concerns wind turbines, there are many possibilities to increase power production. This study is focused on analyzing the HWRT (High Wind Ride Throughout) cut-out strategy: it is a method that allows extending the power curve of a wind turbine above the cut-out wind speed (25 m/s, typically) at which the wind turbine is abruptly shut down for structural integrity issues. The HWRT instead is a particular generator and pitch control strategy that maintains the turbine productive for higher wind speeds (up to at least 30 m/s) through a soft cut-out strategy. Starting with a reverse engineering approach, this study aims at creating a mathematical model of a real wind turbine operating with the HWRT control, and then evaluating the effects of this control strategy on stresses and structural vibrations. The point of view of this study fills a lack in the way this kind of issues is commonly approached in the wind energy practitioners community: actually, wind turbine power capture optimization strategies are typically assessed mainly by the point of view of the energy balance and insufficient attention is devoted to the mechanical aspects and the possible consequences on the wind turbine remaining useful lifetime. The research is structured in the following steps. At first, the wind turbine model is constructed and the characteristic dimensions, blade shapes and natural frequencies are found. Subsequently, with this information, aeroelastic simulations through the FAST (Fatigue, Aerodynamics, Structures and Turbulence) software are implemented and validated against operation data. Finally, conclusions are drawn about the impact of the soft cut-out strategy on structural health and fatigue.