Dear Ankang,
I don’t agree.
In a one DOF model with only the rigid-body generator rotation DOF enabled, then there is not drivetrain torsion and the 6215000 N-m/(rad/s) drivetrain damping of the NREL 5-MW turbine is not used.
Instead, Kr in your model represents the aerodynamic damping (i.e. the change in aerodynamic torque with rotor speed) and Kg in your model represents the generator damping (i.e. the slope of the generator torque-speed curve). You can get both through a FAST linearization analysis.
Best regards,