Measurement of mechanical loads in large wind turbines: Problems on calibration of strain gage bridges and analysis of uncertainty

Abstract

The paper discusses the complexity of calibration of strain gage full bridges applied to measure mechanical loads in large wind turbines, when direct application of calibration loads is not feasible. In particular, at first, it presents a generalized static‐dynamic mechanical model which allows to calibrate the strain gage full bridges using its own unbalanced masses to generate known reference inputs. Then, the paper discusses the uncertainty associated to such a calibration, according to the ISO/IEC Guide 98‐3:2008 “Guide to the Expression of Uncertainty in Measurement”. The uncertainty of the reference input and the following calibration is discussed, which is often larger than the target set by the standard IEC‐61400‐13 used for wind turbine type certification. The paper comments on the attainable range of calibration which is rather limited with respect to expected load range in operation. Even if calibration should take place in isothermal effects, this is not always the case in real world practice. Therefore, the thermal effects on strain gage bridges are also discussed, putting into evidence its influence on calibration uncertainty both for full bridges in T configuration and in parallel configuration.

Real-Time Monitoring of Wind Turbine Blade Alignment Using Laser Displacement and Strain Measurement

“…Bezziccheri et al [19] go into more detail about different external effects on the calibration and accuracy of strain gauges on wind turbine blades and outline the difficulty in applying a known load on large-scale WT blades for accurate calibration as well as describing a model to provide reference loads using unbalanced structural masses. The extent of temperature differentials is highlighted by solar imaging of WT blades as well as temperature sensors near the gauges on the blades showing differentials of up to 20 deg.…”

Regression-based Main Bearing Load Estimation

“…To overcome these problems a load-based calibration can be performed (Lekou and Mouzakis, 2009). But this is only possible for a load range, which is rather limited with respect to the expected load range in operation (Bezziccheri et al, 2017). Additionally, classic strain gauges suffer from signal drift (Berg et al, 2014;Verbruggen et al, 2012) and have a limited lifetime of typically a few years (Verbruggen et al, 2012), which makes this technology inappropriate for a long-term application with a 20 year target life time.…”

Wind Turbine Geometrical and Operation Variables Reconstruction from Blade Acceleration Measurements

“…A proper knowledge of the rotor geometrical properties, namely the angle between the rotor rotation axis and the horizontal (tilt) and the blades' angle with respect to a plane orthogonal to rotor rotation axis (cone angle), is important for many applications in wind turbine numerical modelling and structural health monitoring, as they impact the perceived loads in both the blades and the tower, but not always known with adequate uncertainty levels. In fact, recently proposed strain gauges calibration improved procedures try to explicitly include the impact on the rotor tilt and blade cone angles, as well as the blades pitch angle, in the internal forces originated at the tower basis, when a nacelle untwist event occurs [1]. More recently, a new method for computing the forces and bending moments on operating wind turbine blades using strain measurements at the blades' root has also been presented [2].…”