The turbine has a rated power of 2.5 MW, a rotor diameter and a tower height of both 80 meters. Detailed information about the technical properties of the wind turbine can be obtained through the website of the manufacturer (2). 3. Measurement Systems Within the scope of the work, three different measurement systems were utilized to monitor the dynamic behavior of the structure. Several data sets were recorded both when the turbine was in operation and at parked conditions. The first 2 groups of measurements were taken by using optical measurement techniques which were based on laser interferometry and photogrammetry. An important advantage of the optical measurement techniques is that no additional preparations such as cable installations for power and data transfer are required inside the blade. However, some reflective markers should be placed on the structure. These markers are made up of a retro-reflective material that is 1000 times more reflective than the background blade material. Since the markers are in the form of very thin stickers (with a diameter of 400mm) they do not have any effect on the aerodynamic performance of the blades. A total of 55 markers were placed on the turbine (11 markers for each blade and 22 markers on the tower). The distribution of the markers throughout the structure can be seen in Figure 1. Figure 1 Distribution of the markers throughout the turbine. The laser interferometry measurements were taken by using a Polytec OFV 505 laser head and OFV 5000 controller with VD 06 velocity decoder. These systems were located in the field at a distance of 200 meters from the turbine. An SLR (Super Long Range) lens which enables an increased measurement range up to 300 meters was also required to take measurements from this distance. The utilized LDV and reflection of the laser beam from the marker on the blade can be seen in Figure 2 and Figure 3, respectively. 258
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