Issue Running Linear System Model


I am attempting to extract and run a linear model of a full wind turbine system. I have had success already linearizing and running a model that did not include the turbine (just floater and dummy tower/nacelle mass). I am having issues running the turbine model. For reference I am using OpenFAST v3.5.1.

To be specific, running the turbine model in a wave-only environment works fine, but trying to run in a wind environment the linear model accelerates to infinity. I have attached an image that shows what I mean. Running the linear model with the same wind input as the linearization point, the linear model does not regulate.

The OpenFAST simulation for linearizing was done with a fixed pitch angle so I am running the linear model the same. My linearization process is:

  1. Enable linearization settings to linearize at 36 time steps during steady-state conditions. (ran simulation, computed steady-state, and prepared time steps to get 1 full rotation)
  2. Run MBC3 to perform rotor averaging of the system matrices (note that the code indicates no rotating frame states found (there are only outputs) so sets blades to 0 and does not do transform)
    3)Store A,B,C,D as matlab files for use.

The inputs to my linear model are:

IFW Wind Speed: Constant value
ED Collective Pitch Command: Constant value (0.02967 rads/ 1.7 deg)
All other inputs (AD User Props, IFW power law/propogation, Yaw moment, individual pitch commands, & generator torque) are set to zero.

Also of note is that my radiation and excitation modes are set to 0 in HydroDyn to avoid the hydrodynamic states. I have also tried with these set to 2 with no change.

When linearizing, all control modes were set to zero to allow linearization. Here is what I have tried to get the model to perform:

  1. Feed GenTq from OpenFAST output as linear model input
  2. Fed different wind inputs.
  3. Changed operating point of linearization.
  4. Run linearization with all inputs and selected only the ones above.
  5. Changed pitch input to 0, to 2,3,5 times original (only saw change in 5x but just delayed taking off to infinity)
  6. Set constant (high) value for generator torque (just makes performance a little uneven.

The result is always the same, I am unable to achieve a steady-state response of the system.

The only thing I have not messed with yet is the DISCON input file for the controller. Is the linearization impacted by the DISCON input file for the controller (full model uses a bladed-style DLL for controller setup).

Dear @Ian.Ammerman,

Just a few comments:

  • Have you removed the generator azimuth state from linearized model? This topic has been discussed several times on this forum, e.g., see: This topic has been discussed several times on this forum, e.g., see: FAST linearization V7.
  • Are the inputs to your linear model (wind speed, pitch) perturbations about the operating point values?
  • Does the following forum topic which asked similar questions help you: OpenFAST: Linearization of NREL 5MW Onshore Turbine?
  • FYI: OpenFAST does not currently support linearization when a DISCON controller is enabled.

Best regards,

Dear @Jason.Jonkman ,

Thank you for your response. I had neglected to remove the azimuth state from the model. Making this change, the model is operating correctly.


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