Dear @Nitin.Sivakumar,
If I understand correctly, you are comparing the rotor inertia computed by the structural model of FAST v7 against the results from ElastoDyn in OpenFAST. I’m not exactly sure I can pinpoint why this change exists, but regardless, the difference is about 0.2%, and so, is not too concerning either.
Best regards,
Dear Jason sir,
I want to calculate the drive train damping coefficient for 5 MW wind turbine through openfast linearization. I got from the forum that I need to activate the GenDOF and DrTrDOF and linearize . But I don’t understand how to identify and get the value exactly from the linearized matrices. Please excuse me if the doubt is trivial and asked before.
Thanks and regards
Dear @Nitin.Sivakumar,
The damping coefficient of the drivetrain torsion model is actually an input to ElastoDyn (DTTorDmp). If you are referring to the damping ratio of the free-free mode of the drivetrain with rigid rotor, you can calculate this analytically based on the knowledge of the generator inertia expressed relative to the low-speed shaft (GenIner*GBRatio^2), the rotor inertia (RotIner), and the drivetrain torsional stiffness (DTTorDmp). That is:
natural frequency of drivetrain free-free mode = SQRT( DTTorSpr/RotIner + DTTorSpr/( GenIner * GBRatio^2 ) )/( 2 * pi )
damping ratio of drivetrain free-free mode = DTTorDmp*SQRT( 1/( DTTorSpr * RotIner ) + 1/( DTTorSpr * GenIner * GBRatio^2 ) )/2
Of course, DTTorSpr, DTTorDmp, RotIner, and GenIner * GBRatio^2 will also appear in the linearized state matrix “A” that you can obtain through an OpenFAST linearization analysis, and you can obtain the same natural frequency and damping ratio by computing the eigenvalues of this matrix.
Best regards,
Thanks for your quick reply.
Best Regards