WT Load Measurement Uncertainty: Load-Based Versus Analytical Strain-Gauge Calibration Method

For load measurements conducted on current wind turbines according to technical specification IEC 61400-13 load-based as well as analytically determined calibrations are possible. The uncertainties involved in load measurements due to the employment of these two methods may vary from application to application, which should be documented by the accredited laboratory and taken into account by the wind turbine designer∕manufacturer. To this end, a comparative study is conducted to define the measurement uncertainties when applying load-based or analytical calibration. Examples are presented for the resulting uncertainties and both techniques are assessed with respect not only to the uncertainties introduced but also to the specific requirements of each application. Although the analysis of the uncertainties involved especially in the analytical calibration regarded the primary uncertainty sources, from the results it is shown that the mechanical calibration should be preferred to the analytical one wherever possible. However, since the wind turbines and their structural components are becoming larger the mechanical calibration might not always be possible. Moreover, given that the problem of imposing loads of large magnitude, that is, of magnitude comparable to the ultimate loads expected to be applied on the structure will become more difficult in the future due to the continuous enlargement of the wind turbine and its components, probably more reliable results can be attained by use of a combination of both methods.