Table 4 frequencies (normalized wrt. tower frequency) and damping ratios identified from standstill measurements 5.2 Operational Modal Analysis of the Rotating Turbine The in-operation vibration data of the turbine was measured by using both the strain data recorded by the gauges and the photogrammetric measurement systems. Due to the technical limitations of the sensors each photographic measurement block consists of 6 cycles of rotation. As mentioned before, it is almost impossible to get stable and reliable identification results by using such short measurement duration. However, the P harmonics dominating the overall response and some important turbine modes can be seen from the PSD plots of the recorded vibration data. Figure 9Figure 10 and Figure 11 show the PSD graphs of the edgewise and flapwise direction blade vibration data, respectively. These figures are presented to provide a 3D frequency distribution which also includes the information related to the measurement location. The X-axis represents the normalized frequencies identified from the analyzed data; the Y-axis corresponds to the marker number (defined in Figure 7) and the Z-axis represents the computed PSD amplitude. The 1P and 2P components, and first edgewise mode can be recognized from Figure 10. 271
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