Dear @Muhammad.Sharjil,
The rigid-body mode of the generator state, which has no (or little) stiffness (from aerodynamics), can result in +/- unity damping. You can eliminate this mode by either disabling the generator degree of freedom before linearization (GenDOF = FALSE) or by removing the generator-azimuth state (while keeping the generator speed state) after linearization. This topic has been discussed several times on this forum, e.g., see: Interpretation of state vector - #8 by Judith.Hess.
Best regards,