The variable-speed generator DOF in the Linearization of OpenFAST

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Jason,

I am confusing about a state that was included in the OpenFAST linearization. There is a DOF called the Variable speed generator DOF (internal DOF index = DOF_GeAz). Intuitively, this is the generator’s angular position, so its derivative should be the generator speed. However, for the linearization at a specific operating point (OP), I found that the OP of the derivative of the Variable speed generator DOF (internal DOF index = DOF_GeAz) always equals the value of the rotor speed OP. Does this mean this DOF is the rotor azimuth and its derivative is the rotor speed? If so, how about their corresponding derivatives in the Matric A and C? This may be induced by a possible mistake that I made, so I attached a *.Lin file for your reference.

LinearizationOutputSampleABCD.lin.txt (46.4 KB)

Thank you!

Xianping

2

Dear Xianping,

Yes, the variable-speed generator DOF (DOF_GeAz) state in ElastoDyn is the azimuth angle of the generator and its first-time derivative is the generator speed. Please note that these states in ElastoDyn are expressed relative to the low-speed shaft, and so, you’d have to multiply these by the gearbox ratio (GBRatio) to get the high-speed shaft equivalent values. The rotor azimuth and rotor speed equal the generator states plus the drivetrain-torsion states.

Best regards,

3

Dear Jason,

GOt it! Thank you for your in time reply.

Xianping