Operational modal identification of offshore wind turbine structure based on modified stochastic subspace identification method considering harmonic interference

Xiaofeng Dong; Jijian Lian; Min Yang; Haijun Wang
May 2014
Journal of Renewable & Sustainable Energy;2014, Vol. 6 Issue 3, p1
Academic Journal
The structural modal parameter information can be generally obtained from different vibration responses triggered by unknown excitations using the classic modal identification methods which are based on the assumption that the input to the structure is exactly the same or close to the stationary random white noise. For the actual projects such as the offshore wind turbine structure, however, the imposed loads cannot be considered as the pure white noise excitation because the harmonic components emerge obviously in vibration responses due to periodic rotation excitations of the rotor. When the created harmonic components have greater energy and their frequencies are close to any natural modal frequency of the structure, the classic methods can no longer separate harmonic modes from actual structural operational modes under this strong harmonic disturbance, thus false modes may be generated and the identification accuracy may be badly affected. To identify the actual structural operational modes accurately under strong harmonic excitation, a modified stochastic subspace identification (SSI) method considering harmonic interference called the harmonic modification SSI (HM-SSI) method was proposed in this paper assuming the input harmonic frequencies are known and time-invariant. Then the effectiveness and accuracy of the HM-SSI method were verified through a simple numerical model of cantilever beam excited by various harmonic inputs superimposed on random loads. Besides, the superior robustness and better accuracy of identification results affected by the level of noise and the proportion of harmonic energy were reflected. Finally, the modal parameter information under different operational conditions was obtained and the safety assessment of an operational wind turbine was made based on the measured data from one offshore wind turbine test prototype at high running speed.


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