# Questions about controller design for 5MW wind turbine

Dear all,

I am working on the advanced controller design for 5MW wind turbine. And I have some questions about wind turbine operating in the full load region.
Can we permit the wind turbine opearting exceeding rated values, i.e. rated power, rated generator speed? If we can, to what extent can the wind turbine opearte over rated values so that it have less influence on using life? Do there any references account for this problem?

Best Regards
Jinghan.Cui

Dear Jinghan.Cui,

Small deviations above the rated value are expected with any controller, but I’m not sure I can quote what a “reasonable” value is. The deviations about the operating should be determined from some cost-benefit analysis e.g. in terms of actuator authority and load impact.

Best regards,

Dear Jason,

I see. Thank you very much!

Jinghan

Dear Jason,
I encountered another problem while designing a controller for 5MW wind turbine. I would like to take the tower structural fatigue into consideration. But I could not find the expression of thrust force coefficient Ct. Could you give me any information of Ct that I could used in controller design?
Thank you for your time!
Jinghan

Dear Jinghan,

What do you mean by “expression of the thrust coefficient Ct”?

Ct is defined as RotorThrust/(1/2AirDensRotorSweptArea*RotorDiskAverageWindSpeedNormalToDisk^2).

Best regards,

Dear Jason,
Sometimes, we may need a explicit expression of the coefficient to forecast the dynamic of the system. Like the aerodynamic power coefficient Cp could be approximated by
Cp = 0.5176 (116/λi − 0.4beta − 5) e−21/λi + 0.0068λ; 1/λi = 1/(λ + 0.08beta) - 0.035/(beta^3+1);
So is there any approximate method for the thrust coefficient Ct?

Thank you for your time!
Jinghan

Dear Jinghan,

Sorry, but I’m unfamiliar with the source of that approximate expression for Cp (presumably for the NREL 5-MW turbine?) and I’m unaware of whether there is a similar expression for Ct.

Best regards,

Dear Jason,
Yes, I am working on the NREL 5-MW turbine. Do you know how to obtain the look-up table for coefficient Cp and Ct? Many papers present this Three-dimensional graph, but they do not tell the source and the corresponding data.

Jinghan

Dear Jinghan,

You can use the standalone driver for AeroDyn v15 to calculate the surface of power, torque, thrust (etc.) coefficient versus tip-speed ratio and blade-pitch angle. See the following forum topic of more information: http://forums.nrel.gov/t/aerodyn-v15-04-with-parametric-analysis/1765/2 (this assumes a rigid rotor).

Best regards,

Dear Jason,
I am working on drawing the Cp and Ct for the NREL 5-MW turbine using AeroDyn v15. But do I need to download the FAST archive since it presents 'Undefined function or variable ‘ReadFASTtext’ in Matlab when I ran ‘PlotClCdCmCurves_DT.m’. and could you give me a guide for getting more data outputs for different wind speed and different pitch angel?
Thank you very much!

Best regards,
Jinghan

Dear Jinghan,

No, you don’t need to download FAST to use the standalone AeroDyn archive. ReadFASTtext.m is included in the MATLAB Toolbox, which you do need to install and have in your search path if you intend to use PlotClCdCmCurves_DT.m: github.com/OpenFAST/matlab-toolbox.

The combined-case section of the standalone AeroDyn input file allows you to run multiple cases…each with different wind speeds, rotor speeds, pitch angles, etc. You can use this feature to generate complete Cp and Ct surfaces.

Best regards,

I will try this. Thank you so much for your guidance！

Dear Jason,
Sorry to bother you again. The Standalone AeroDyn works well now. But I have a question about the outputs. I fix these parameters (WndSpeed, RotSpeed, Pitch) at constants. And the results show the RtTSR and RtSpeed remain unchanged. But why the RtAeroCp, RtAeroCt and RtAeroPwr change along the time, especially the first two steps? you can refer to the following case. So which value of RtAeroCp and RtAeroCt should I pick corresponding to the Pitch and RtTSR?

Case 1: WndSpeed=7 m/s; ShearExp=0 ; RotSpeed=12.1 rpm; Pitch=0 deg; Yaw=20 deg; dT=0.138 s; Tmax=12.793 s

Time           	RtAeroCp  	RtAeroCt  	RtAeroPwr 	RtTSR     	RtSpeed
(s)           	(-)       	(-)       	(W)       	(-)       	(rpm)
0.0000	  3.706084E-001	  1.002677E+000	  6.181338E+005	  1.217061E+001	  1.210000E+001
0.1380	  3.645007E-001	  1.002645E+000	  6.079472E+005	  1.217061E+001	  1.210000E+001
0.2760	  3.280593E-001	  9.561189E-001	  5.471669E+005	  1.217061E+001	  1.210000E+001
0.4140	  3.280802E-001	  9.560590E-001	  5.472016E+005	  1.217061E+001	  1.210000E+001
0.5520	  3.281090E-001	  9.561003E-001	  5.472492E+005	  1.217061E+001	  1.210000E+001
0.6900	  3.281295E-001	  9.561437E-001	  5.472841E+005	  1.217061E+001	  1.210000E+001
0.8280	  3.281436E-001	  9.561918E-001	  5.473071E+005	  1.217061E+001	  1.210000E+001
0.9660	  3.281998E-001	  9.562582E-001	  5.474005E+005	  1.217061E+001	  1.210000E+001
1.1040	  3.282187E-001	  9.562720E-001	  5.474325E+005	  1.217061E+001	  1.210000E+001
...                   ...

Best regards,
Jinghan

Dear Jinghan,

There may be some time variation due to how the aerodynamics solution is initialized and depending on what functionality you’ve enabled e.g. unsteady airfoil aerodynamics, especially considering that you have a fixed yaw error. If you need constant outputs for a given case, I would recommend azimuth-averaging the solution over one full revolution of the rotor after any start-up transients die out.

Best regards,

Got it! Thank you so much!

Dear Jason,

I have another question about the thrust coefficient Ct. The attachment is the surface of Ct obtained by AeroDyn. From this figure, we can see that when the wind speed is high ,i.e. the wind turbine operates in the full load region, the thrust coefficient Ct may be negative. Do you know how to explain this?

Best regards,
Jinghan

Dear Jinghan,

During normal operation, the pitch angle will increase with wind speed in Region 3 and the thrust coefficient (Ct) should drop, but not go negative. But Ct could well become negative if you pitch the blades further than desired to maintain constant rotor speed in Region 3.

Best regards,