ECN publication
Reducing Wind Turbine Loads with Down-Regulation
Published by: Publication date:
ECN Wind Energy 7-9-2017
ECN report number: Document type:
ECN-E--17-032 ECN publication
Number of pages: Full text:
19 Download PDF  

With the growth of wind energy worldwide, an increased interest in wind farm control has also 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, downregulation also affects the loads acting on a wind turbine. Hence, it is important to analyse their effect on the lifetime of certain wind turbine components. Earlier research on down-regulation for wind farm control has resulted in several methods which were shown to reduce the fatigue loads of 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 work, different downregulation strategies using the percentage reserve method are assessed based solely on their load reduction capabilities. There exist several strategies for the down-regulation of wind turbines using the percentage reserve method. In partial load, the aerodynamic efficiency of the blades can be reduced by increasing or decreasing (until stall) the optimal Tip-Speed Ratio (TSR), or by increasing the pitch angle. In full load, down-regulation is achieved by reducing the rated generator torque or the rated rotor speed. This leads to a total of six down-regulation strategies which are implemented in ECN’s controller. Besides the loads, the axial induction is also analysed for each down-regulation strategy. It was observed that decreasing the TSR and increasing the pitch angle in partial load result in a significant decrease of the axial induction factor, which is crucial for pitch based AWC. The performance of the different control strategies is compared using aeroelastic simulations of the 10MW Innwind turbine and by comparing the Damage Equivalent Loads (DELs) of several components for the whole range of operational wind speeds. It is observed that decreasing the TSR results in increased fatigue loads at the tower for low wind speeds caused by prolonged operation at the cut-in rotor speed, where the 3P frequency excites the tower frequency. The strategies incorporating a higher TSR show a very positive effect on the tower fatigue loads. However, due to the higher rotor speed at very low wind speeds, blade root fatigue loads increase significantly in this region. The two strategies based on increased pitch angles show very stable behaviour over the whole range of wind speeds and result in a reduction of fatigue loads for both tower and blade roots. Additionally, it has to be noted that reducing the rated rotor speed results in an increase of the tower loads at higher wind speeds. This is caused by a decrease in aerodynamic damping resulting from operation at a lower rotor speed and increased pitch action. However, the fatigue loads of the blade roots decrease as a result of operation at a lower rated rotor speed. Finally, lifetime fatigue load effects are analysed by combining the DELs that were computed for different wind speeds with a given wind distribution. The results show that all down-regulation strategies are successful in reducing the lifetime fatigue loads for some of the wind turbine components, with load reductions of up to 25% being achieved for some components when a 20% down-regulation level is selected.

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