significant participation in the overall signal. The eigenfrequencies are relatively easier to be determined but corresponding damping ratios require long measurement durations to be used in the identification process. 4.3 Changing Wind Speed and Direction The difficulty in acquiring data series with sufficient length is not in measuring or storing the data but finding a suitable period where the blade pitch angle, the wind speed, rotor speed or dominant wind direction (more critical for the tests performed at parked turbines) remains unchanged. Hansen, et al. reported that about 3 months of measurements were scanned to find 1–3 h long periods of low standard deviations of wind and rotor speeds to improve the assumption of a time-invariant system in the SSI analysis. The wind speed and direction changing during the measurements is an important issue to be considered for the tests performed both on rotating and parked turbines. The tests conducted at the parked condition also provide very useful information for tuning and validating numerical models. Since these tests are done at parked condition during very low wind speed periods the identified system parameters are assumed to be mainly structural and not affected by the aerodynamic factors. However, even very low wind speeds may have an effect on the extracted dynamic properties, especially on damping ratios. James, et al. (29) reported that a drag phenomenon similar to that experienced by a flat plate oscillating normal to a strong wind can cause some changes in the estimated damping ratios and showed that a 1.2 % (in terms of critical damping ratio) damping increase can be seen for the first flapwise modes of a VAWT due to the above mentioned aero-drag effects. The changing wind direction is another important factor affecting the results of the tests performed on parked wind turbines. Since the turbine is kept in a fixed orientation at parked condition, the instantaneous wind direction directly affects the excitation direction. The relative angle between the wind direction and the turbine can be used to check how the effective excitation direction changes during the measurements. The change in this angle also affects the aerodynamic damping component by causing a different aerodynamic coupling between the structure and the air flow. 4.4 Insufficient Excitation Level This problem is also more critical for the tests applied on parked turbines. Besides the unexpected scatter in damping due to the drag phenomenon mentioned above, some wind loading is needed to excite the turbine to be tested. If the wind speed is very low, the level of the ambient excitation may not be sufficient to excite the turbine modes and unstable results can be obtained 4.5 High Amplitudes of Harmonics As mentioned before an important assumption of OMA (Operational Modal Analysis) is steady state random excitation. Although turbulence is close to being random, the resulting excitation from the aerodynamic forces will contain significant components on all P-harmonics (the harmonics of turbine rotational frequency P) on the rotor frame and on the 1P, 3P, 6P, etc. 266
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