Hi All
I am studying the statistical properties of rotor speed for a wind turbine using FAST v8 simulations (as OpenFAST does not run smoothly on my computer, I use FAST v8 for wind turbine modeling). However, I have encountered a perplexing issue: For the 5MW reference wind turbine, how are the rotor inertia, the generator inertia relative to the highspeed shaft, and the drivetrain inertia cast to the lowspeed shaft calculated in FAST v8?

On pages 2021 of Ref. 1, the following Equation 1 is provided:

In FAST v8, I simulated Test 18 (NREL 5 MW  Landbased wind turbine) with the following modifications:
The flexibility of the drivetrain is ignored

Since the torsional flexibility of the drivetrain is neglected, and assuming the gearbox has no frictional losses (page 14, Ref. 1), the rotor dynamics simulation results in FAST should conform to Equation 1. Therefore, the physical quantities in the Equation 1 correspond to the simulation results in FAST.
T_aero (in Equation 1) = RotTorq (in FAST)
T_gen = GenTq
N_gear = 97 [1]
Omega = RotSpeed•2•pi/60
t = TimedOmega/dt is calculated in MATLAB using the following code:
dOmega_dt = gradient(RotSpeed•2•pi/60, Time)

From the FAST v8 simulation results, the relationship between RotTorq  N_gear•GenTq and dOmega/dt is depicted in the attached figure.
This suggests that in Equation 1,
I_rotor + N_gear^2•I_gen = I_drivetrain = 5025•1000 (kg•m2) = 5.025•10^6 (kg•m2) 
However, my calculation of the moment of inertia of the three blades about the rotor axis is significantly higher, approximately 38.7•10^6 (kg•m^2), which contradicts the result in point 4. I am unsure how the value of 5.025•10^6 (kg•m^2) is derived. I am confused. I have two suspicions:The first is the value of 5.025•10^6 (kg•m^2) is obtained by multiplying the generator inertia about the highspeed shaft (534.116 kg•m^2) by N_gear^2(97^2). However, this seems to exclude the moment of inertia of the blades relative to the rotor axis.
The second is the value of 5.025•10^6 (kg•m^2) is given on page 36 of Ref. 2, but the same document also states, “For comparison; the rotor rotational inertia is 36.34 × 10^6 kg•m^2”. Why doesn’t FAST use this value for I_rotor?
Please forgive me for any errors in my English writing. I hope someone can clarify my confusion, point out errors in my calculations, or identify any logical or knowledge gaps in my understanding. Thank you!
Best regards!
Yangyudong.Liu
[1] Jonkman, J. M. , Butterfield, S. , Musial, W. , & Scott, G. . (2009). Definition of a 5mw reference wind turbine for offshore system development. office of scientific & technical information.
[2] Lindenburg, C. , & Winkelaar, D. . (2003). Aeroelastic modelling of the dowec 6 mw predesign in phatas.