Thanks for your insight @Derek.Slaughter.
There is a slight abnormality in Generator Torque as linked in the below discussion. After that dicussion, I have updated the interfaces also to 3.5.3, but still the transient response shows same heavy fluctuations.
GenTq behavior for 5MW turbine - FAST 3.5.3 coupling.
Could this be related to version incompatibility ? Actually I found it hard to map the compatible versions for Rosco, OpenFAST releases and the corresponding examples release. Right now we go by the dates of corresponding releases and believe that they must be compatible. However it would be really helpful if a table is maintained regarding the compatible versions in the documentation at-least from FAST 3.0.0.
Thanks,
Ashok.