some questions when fixing pitch angle

[Wangzhenyu12]

  I am studying the aerodynamic imbalance caused by the presence of pitch angle error in a single blade. 
  My OpenFAST version is v3.4.1.  The model is 5MW onshore wind turbine and I use Beamdyn.
  I use the method of modifying the parameters in #1035,  make PCMode = 0 in ServoDyn and BlPitch(1)=3   BlPitch(2)=0    BlPitch(3)=0   in ElastoDyn.
  Since I don't control rotor speed , GenDOF is still True and CompServo =1.

The wind speed time-series is a turbulent wind with an average wind speed of 9 m/s. The blades do not pitch in normal operating condition at this wind speed. And the calculations give strange results like these two figures. I also decrease pitch angle error, when BlPitch(1)=0.5, the results seem reliable. But when I make BlPitch(1)=BlPitch(2)=BlPitch(3)=3, there is no pitch angle error ,the results are still strange and similar to these two figures. Please tell me what's wrong and how to deal with it. Thanks!

[jjonkman]

Dear @Wangzhenyu12,

My guess is that your operational torque controller is getting confused. The baseline DISCON controller for NREL 5-MW baseline wind turbine includes a combination of pitch and torque controller, where the torque control is influenced by the current pitch angle. In your case, you are using this DISCON controller for generator torque, but ignoring the commands from the pitch controller (or rather overriding these commands with your user-speceified fixed pitch angles, including pitch imbalance). I haven't fully thought through the logic, but I could foresee this approaching causing problems with the torque controller.

I suggest switching from VSContrl = 5 to a different torque controller, such as VSContrl = 1 with appropriate settings for the simple variable-speed torque control parameters.

Best regards,

[Wangzhenyu12]

Dear jjonkman

Thanks for your quick reply. According to your advice, I make VSContrl = 1 and set simple variable-speed torque control parameters based on 《Definition of a 5MW Reference Wind Turbine for Offshore System Development》, then I get reasonable simulation results.

And I have another question, if I want to simulate a condition that the pitch angle of blade 1 is always larger than the other two blades by a fixed value during the pitching process, what should I do?

Best regards,

[jjonkman]

Dear @Wangzhenyu12,

To simulate a fixed blade-pitch angle offset error, you have a few options:

• You can define the pitch offset by adding a fixed offset to the aerodynamic and structural twist in the AeroDyn blade input files and the ElastoDyn (or BeamDyn) blade input files (using different files for each blade), respectively;
• if active pitch control is disabled, you can define the offset via the fixed pitch angles set in ElastoDyn (through inputs BlPitch(1-3)); or
• If active pitch control is enabled, you can define the pitch offset directly within the controller.

Best regards,