Dear Rana,
Ah, I see now. The negative damping is showing up for a zero-frequency i.e. rigid-body mode. The damping of the rigid-body mode does not necessarily indicate that the system is unstable. As described in this forum topic: Learizing Baseline 5MW Wind Turbine with FAST, rigid-body modes show up in MBC3 as a pair of zero-valued (or near-zero-valued) frequencies with +/- inf damping (i.e., eigenvalues with real values only). That is, each rigid-body mode will introduce an additional mode beyond the number of enabled DOFs and the damping is unphysical.
Best regards,