Presentations

Abstract

Ambitious targets for renewable energies require optimal planning and life-cycle assessment of offshore wind farms. Wakes from wind farm clusters, called cluster wakes, can influence the power of downstream wind farms due to reduced wind speeds in the wake region. Previous studies have shown that cluster wakes adversely impact loads at distances up to 7 km, but cluster wakes have already been shown to persist up to 55 km, even in weakly unstable atmospheric stratification. Our objective is to determine if far-reaching (15 km to 21 km) cluster wakes impact turbine loads in the wind farms downstream and to ascertain any dependence of the loads' effects on prevalent atmospheric conditions. We analyzed highly resolved SCADA data from the Albatros and Hohe See wind farms in the German Bright to determine the load effects in several cluster wake cases. The analysis of load effects on individual turbines requires the separation of effects on turbine loads due to wakes from the load effects due to ambient wind speed variations. Standard load tables in scenarios of free wind and inner farm wakes were thus constructed from SCADA data with high temporal resolution using the fluctuations of the fore-aft nacelle accelerations, which was the load proxy considered in all analyses. We found increased loads (~3%) at the turbines within the cluster wake as compared to the turbines in free wind using the standard load tables. This was, however, much lower than the turbine loads caused by individual turbine wakes within a wind farm. These load effects were also independent of the atmospheric stratification and the upstream mean wind speed. When comparing the absolute values of loads between the same turbines, no load effects were observed due to lower wind speeds within the cluster wake region. We also computed load spectra using 10 Hz SCADA and compared turbines affected by the cluster wake to turbines in free wind. We found no additional structural mode shapes excited by the cluster wake-affected turbines. A limitation of the current analysis is the lower number of cluster wake situations and the absence of load sensors on the turbines. Additional data, also from other wind farms, could help determine the fatigue effects of cluster wakes on offshore wind turbines. Further investigations can improve our understanding and contribute to adapting IEC standards for load validation in cluster wake scenarios. The work demonstrated that analyzing common SCADA data with high temporal resolution is a valuable and relatively cheap tool for analyzing wind turbine operational response under complex inflow and site conditions.