Correlation between Lidar measured wind speeds and aerodynamic loading

The IEC standards prescribe an inflow wind field based on models with empirical assumptions to perform load calculations. The use of preview wind speed measurements using nacelle-mounted Lidars allows mitigation of structural loads by suggesting appropriate control action. This relationship is affected by uncertainty in site conditions and the dynamic control of wind turbine in different operation regimes. Although efforts have been put to derive the relation between the aerodynamic loading and the wind speed, there is scope to explore this relation using measurements. Deriving the load statistics based on the inflow wind field is therefore necessary to establish the correct control strategies. This study focuses on two aspects: firstly, the effect of variability in the inflow conditions due to wind speed fluctuations, turbulence and wind evolution on loading statistics and secondly, the effect of different wind turbine operation modes and Lidar measurement ranges on loading statistics. By understanding the effect of these two aspects on aerodynamic loading, a suitable control strategy can be designed by establishing correlation and coherence based look-up tables with inflow-loading relationship for each operation regime. The study defines a correlation between the Lidar measured wind speed and aerodynamic loading for three Lidar measurement ranges at below and above rated operation modes. Strong correlations are observed at above-rated operation, while uneven correlations at below-rated operation are observed. Strong correlations are observed for turbulence intensities higher than 12%. The preview distances of 50 m and 110 m provide the high correlation estimates The study of coherence between these two signals provides useful insights on the evolution of wind, the length scales present and the frequencies. The coherence remains high for wavelengths upto 200m for above-rated and below-rated operations, the time scale was found to be around 2-2.5 s and the decay parameter ranges from 2 to 7.