accurately. However, the authors needed measurement durations of 30 minutes (900 cycles of the lowest frequency) to test a rotating VAWT (vertical axis wind turbine) which has a maximum aeroelastic damping ratio of 7 %. Similarly Hansen et al. (27) performed comprehensive measurements and analyses on a 2.75 MW turbine. The researchers utilized data blocks with durations varying between 1 and 3 hours. The data series recorded at different locations on the tower and main shaft were analyzed by using Stochastic Subspace identification which does not require the use correlation functions but the time domain response measurements. However, it was again not possible to extract the symmetric and whirling flapwise modes which have been known to have high aeroelastic damping. The authors conclude that this situation might be caused due to the fact that these modes have too high aerodynamic damping to be identified from the response to turbulence excitation. Therefore, it is a common application to use data series which include at least 500 cycles. A recent work aiming at evaluating how the reliability of the estimated system parameters changes as a function of the measurement duration can be found in (32). In this work, the authors apply two different system identification algorithms (SSI-Stochastic Subspace Identification and LSCELeast Square Complex Exponential) to analyze a simulation data and compare the results of the identification with the known real values. The utilized SSI method is the same as the one which Hansen et al. used in their research study (27). Similarly, LSCE is based on NExT which was developed and used by Carne and James. Figure 8 shows an example of how estimated damping ratio changes with the measurement duration. The authors report that the required number of cycles of the lowest frequency can be as high as 1000 (especially for LSCE) while for a lightly damped system 200 cycles can provide quite satisfactory results. Figure 8 Change of estimated damping value as a function of measurement duration. For wind turbines where low frequency vibrations dominate the overall response, such a limitation requires minimum measurement durations around 30 minutes or longer. It has to be noted that high damping modes are still not guaranteed to be extracted unless they have a 265
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