I have a question related to the 1st and 2nd natural frequencies of a wind turbine. For the nrel 5mw reference wind turbine, I have calculated the 1st tower Fa and ss as 0,326 Hz and 0,3155 Hz respectively and the 1st drive train torsion as 0,632 Hz. These are out of the 1P and 3P frequency ranges of 0,115 to 0,20 Hz and 0,35 to 0,61 Hz respectively. For a floating offshore wind turbine I would think that again one would want to stay clear of 1P and 3P intervals, but looking into the nrel report for the MIT tlp by Denis Matha, I find that the first platform heave is 0,47 Hz and the 1st tower Fa and ss are 0,63 and 0,57 Hz respectively. Any thoughts on this would be greatly appreciated.
I agree that one should generally avoid locating structural natural frequencies that coincide with 1P and 3P frequencies in the operational speed range of the wind turbine. However, this may be OK if the excitations that occur at 1P or 3P don’t resonate the mode in question. It is also possible for the control system to be modified to add damping to the modes getting excited or to bypass the resonance altogether–e.g. by adding a notch filter to keep the rotor from resonating a structural natural frequency for long.
For the MIT/NREL TLP, we were aware that the compliance of the TLP raised the tower natural frequency so that it crossed 3P instead of falling between 1P and 3P. However, in our study of the MIT/NREL TLP, we did not notice strong resonance of the tower as a result, so, we did not try to modify the tower design or controller to avoid this.
I hope that helps.
Once again thank you very much for your quick and great explanation. That helps me a lot.