Dear Hamid Reza,
Your equations are effectively what FAST does for a 2-DOF generator + drivetrain model, except that I’m not sure where the "2"s in your equation come from.
For this 2-DOF model, FAST linearization will produce a 4-state model, including Δδg and Δωg corresponding to the generator DOF and Δδtg and Δωtg corresponding to the drivetrain-torsion DOF. As discussed in my Sep 02, 2014 response in the forum topic found here, it is common to remove the azimuth angle perturbation (Δδg) as a state. Then, with a little manipulation, you can transform this 4-state model into your desired 3-state model.
Best regards,