Static Testing Using OFDR-based Quasi Continuous Fiber Optic Strain Sensing on the WiValdi Wind Turbine Rotor Blade

Abstract

The German research wind farm WiValdi was taken into service in 2023 and enables researchers and industry to conduct full-scale experiments. It currently consists of two highly instrumented wind turbines and multiple wind measurement masts. In one of the six wind turbine rotor blades, four Fiber Optic Strain Sensors (FOSS) are installed [1].The strain which a wind turbine rotor blade exhibits under load is a key factor in the design, the certification and the assessment of its remaining service life as well as its further development and optimization. The aerodynamic optimization of wind turbine rotor blades leads to longer, and increasingly slender rotor blades and therefore increasingly relevant static and dynamic structural mechanical challenges.Usually the strain of wind turbine rotor blades during static deflection tests is measured with electrical strain gauges (SGs) or Fiber-Bragg-Gratings (FBGs). In the present study, quasi-continuous Fiber-Optic-Strain-Sensors (FOSS) are used in parallel with FBGs. The underlying technology is called optical frequency domain reflectometry (OFDR) and since only glass-fibers are used, it is inherently lightning protected [2]. Besides that, it is commercially available and delivers strain results at a spatial resolution of 2.6 mm. For this study, there are four 50-meter-long glass fibers bonded to the wind turbine rotor blade on suction and pressure side of each main spar of a 57 m rotor blade. This contribution shows how the FOSS measurements are post-processed and validated. The validated data is then used to obtain the relation between strain and introduced force over the length of each FOSS. With the accurate prediction of root-bending-moments, one of many possible use-cases of the FOSS results, is presented.Besides the computational model updating of the corresponding Finite-Element model in future investigations, the results will also be used to judge the current structural mechanical state of the wind turbine rotor blade at recurring service inspections. In that, results of the then to be acquired state at a given time and the shown base-line state will be compared. This will lead to high resolution insight into the deterioration of the structural integrity of the wind turbine rotor blade over time.Considering that standard telecommunication glass fibers can be used as sensors, the cost per sensor (i.e. fiber) is relatively low (circa 1500 € for 50 m). Despite the high cost of the required measurement system, the low prices of the fibers make FOSS measurements not only a scientifically interesting but also a potentially economically viable solution for measuring strain on wind turbine rotor blades.