Dear Neelabh,
The frequency of osciallation seen in the generator power and generator speed plots seems to be around 0.52 Hz, which is the 3P frequency for a rotor speed of around 10.4 RPM that is consistent with the generator speed of around 1010 RPM (= ( 10.4 RPM rotor speed ) * ( 97:1 gearbox ratio ) ) and makes sense for the NREL 5-MW turbine at a wind speed of 9 m/s. The 3P oscillation results from the asymmetry of the flow due to wind shear and yaw error. (Even without shear in the wind, the NREL 5-MW has a shaft tilt that will induce shear.) (Asymmetry in the flow will cause a 1P oscillation in the rotating frame, which leads to a 3P oscillation in the fixed frame; a 1P oscillation in the fixed frame would only result from an imbalance in the rotor.)
The “odd” square shape of the generator speed time series is likely due to the small round-off error that results from the fact that FAST is only outputting 4-digits of precision based on the setting of OutFmt = “ES10.3E2” in FAST’s primary input file. My guess is that the shape would look more rounded if you added at least one more digit of precision in the output format. This roundoff is also likely the reason why you only see generator speed oscillation in one of the three time series. See the following forum topic for more information on OutFmt: FAST Integrator, step size & precision.
I hope that helps.
Best regards,