Dear Jason,

I found some particularity linearizing the 5 MW reference on-shore wind turbine (FAST v7). The A matrix of the state space model obtained after linearization at different wind speeds in region 3 (making a pitch trim) differs if the torque characteristic is used as explained in the FAST manual (making VSContrl=1, VS_RtGnSp=1E-9, VS_RtTq=Tn, VS_Rgn2=1E-9, VS_SlPc=1E-9) or leaving as a “standard curve” (i.e. VSContrl=1, VS_RtGnSp= 1173.7, VS_RtTq=Tn, VS_Rgn2= 0.0255764, VS_SlPc=10). The linearization points result the same (torque, rotational speed, pitch angle, etc), but the difference appears to be in two coefficients corresponding to the First derivatives of Variable speed generator DOF (internal DOF index = DOF_GeAz)and Drivetrain rotational-flexibility DOF (internal DOF index = DOF_DrTr) with respect to the state Variable speed generator DOF (internal DOF index = DOF_GeAz (rows 7 and 8, and column 7 in the pictures below).

For example:

---------------------- FEATURE FLAGS ------------------------------

False FlapDOF1 - First flapwise blade mode DOF (flag)

False FlapDOF2 - Second flapwise blade mode DOF (flag)

False EdgeDOF - First edgewise blade mode DOF (flag)

False TeetDOF - Rotor-teeter DOF (flag) [unused for 3

True DrTrDOF - Drivetrain rotational-flexibility DOF

True GenDOF - Generator DOF (flag)

False YawDOF - Yaw DOF (flag)

True TwFADOF1 - First fore-aft tower bending-mode DOF (flag)

False TwFADOF2 - Second fore-aft tower bending-mode DOF

True TwSSDOF1 - First side-to-side tower bending-mode DOF

False TwSSDOF2 - Second side-to-side tower bending-mode DOF

True CompAero - Compute aerodynamic forces (flag)

False CompNoise - Compute aerodynamic noise (flag)

Linearization point:

Wind speed= 23.568 m/s,

Gen speed= 1173.7 rpm,

Pitch angle= 21.497 °,

Torque= 44205

For “standard curve” values

For Fast manual values

The difference is for 1: Run FAST and for 3: do both.

Therefore, my question is: do you know what could be the reason and which one is correct?

Thank you very much in advance.

Best regards,

Bernabé Ibáñez