I have a question concerning the calculation of the rotor rotational dynamics in FAST.
I am simulating a reduced-order, dynamic wind turbine model (as described in Bianchi: Wind Turbine Control Systems) where I implemented the rotor torque and thrust calculations via look-up tables of the Cq- and Ct-coefficients extracted from the NREL 5MW reference turbine.
For the sake of comparison, I also implemented a full BEM-calculation (including the usual corrections) in the dynamic simulation, which yielded almost the same results. Running a closed-loop simulation using turbulent wind as input, the deviations in the aerodynamic rotor torque around the mean value are quite high and therefore also in the rotor rotational speed.
By contrast, running a simulation with FAST (using the same pitch controller), rotor torque and the rotor speed are much more concentrated around the mean value with less deviations. These deviations between FAST and my simulation can also be observed when only the rotor rotational degree of freedom is switched on.
My question is: How is the rotor rotational speed calculated in FAST?
Other than the aerodynamic rotor torque, are there additional terms in FAST that lead to an averaging effect and how are they calculated?
For a stiff drivetrain model, I would assume the differential equation to be
d/dt om_rotor = (M_A – M_g)/(J_r + J_g)
(om_rotor: rotor rotational speed, M_A: aerodynamic torque, M_g: generator torque, J_r, J_g: rotor and generator inertias).
Am I missing something here?