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Comparison of Down-Regulation Strategies for Wind Farm Control and their Effects on Fatigue Loads

With the growth of wind energy worldwide, an increased interest in wind farm control has become visible, with Active Power Control (APC) and Active Wake Control (AWC) being two primary examples. Both these methods rely on the down-regulation (i.e., operation using sub-optimal power settings) of wind turbines in order to provide such services. Apart from these services, down-regulation also affects the loads acting on a wind turbine. Hence, it is important to analyze the effects on the lifetime of wind turbine components, e.g., the tower, blades and rotor shaft. Earlier research on APC for wind farms has resulted in several down-regulation methods which were shown to reduce fatigue loads for some wind turbine components. One of these methods is called the percentage reserve method, which makes it possible for the wind turbine to generate a desired percentage of the available power at every wind speed. In this paper, different down-regulation strategies using the percentage reserve method are assessed on their capability of reducing fatigue loads. The performance of the different control strategies is compared using aeroelastic simulations and by comparing the Damage Equivalent Loads (DELs) of several components for the whole range of operational wind speeds. The fatigue lifetime is analyzed by combining the DELs with a wind speed distribution for the turbine specific wind class. The results show that all down-regulation strategies are capable of achieving significant lifetime fatigue load reductions for some wind turbine components. Whichever strategy provides the best performance, depends on the user's wishes as well as the environmental conditions and the wind turbine in question.