Dear Zichao,
While I coauthored that paper, I did not run the simulations and haven’t used the 750-kW model. I suggest you contact one of the other authors of that report regarding that.
For clarification of how the drivetrain model in FAST works and how output RotTorq is calculated, please see the following forum post: Resistant moment of the rotor and of the electric generator.
Best regards,