questions about reference control input

This forum if for discussing controls. Questions about how to implement controls in FAST are more appropriate to the CAE Tools forum.

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Jason.Jonkman
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Re: questions about reference control input

Postby Jason.Jonkman » Mon Feb 12, 2018 8:12 am

Dear Cheng,

I'm sorry, but I'm not sure I understand enough about what you've have done to know what you have done wrong.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
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Cheng.Zhang
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Re: questions about reference control input

Postby Cheng.Zhang » Mon Feb 12, 2018 8:33 am

Dear Jason,

Sorry about my English. :oops:

If there has rotating states,
1.I can use mbc3.m get MBC_AvgA, MBC_AvgB, MBC_AvgBd, MBC_AvgC, MBC_AvgD, MBC_AvgDd.
2. I need to transform input blade pitch angle θ1, θ2, θ3. to θ0, θc, θs.
3. In order to compare with the FAST's reslut. I need to inverse transform outputs y0, yc, ys to y1, y2, y3
4. Do I need MBC transform the state vector X(assume this state vector only has 3 states x1, x2, x3, which in rotating frame), transform x1, x2, x3 to x0, xc, xs ?

Best regards,
Cheng

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Re: questions about reference control input

Postby Jason.Jonkman » Mon Feb 12, 2018 11:03 am

Dear Cheng,

Yes, I agree that A_nr = MBC_AvgA.

But I'm confused by what you are asking. By using A_nr = MBC_AvgA in place of the periodic AMat, you've already transformed the states from the rotating to the nonrotating frame. Likewise using B_nr = MBC_AvgB in place of the periodic BMat, you've already transformed the inputs from the rotating to the nonrotating frame etc.

To compare to the nonlinear FAST solution, you can solve/integrate the linear system:

dxdot_nr = A_nr*dx_nr + B_nr*du_nr
dy_nr = C_nr*dx_nr + D_nr*du_nr

and then calculate the global states, inputs, and outputs used by FAST by transforming the nonrotating state, input, and output perturbations to the rotating frame i.e.:

x = x_op + T_1*dx_nr
u = u_op + T_1c*du_nr
y = y_op + T_1o*dy_nr

See the MBC3 documentation for a description of T_1, T_1c, and T_1o. Please note that I'm using dx, du, and dy here to be explicit that the linear solution applies to perturbations of the states, inputs, and outputs about the nonlinear operating point (x_op, u_op, and y_op).

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Cheng.Zhang
Posts: 57
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Organization: ECN
Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Mon Feb 12, 2018 11:32 am

Dear Jason,

Thank you for your patience! It seems that I fell into a wrong thinking!

I thought I need to use dx_nr = inv(T_1)*dx to calculate the dx_nr, actually, it has tramsformed already by MBC_AvgA.

Thank you very much!
Best regards,
Cheng

Cheng.Zhang
Posts: 57
Joined: Tue Nov 14, 2017 6:50 am
Organization: ECN
Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Wed Apr 25, 2018 9:50 am

Dear Jason,

I used FAST linearization got linear model of floating wind turbine, and designed some controller based on linearized model.
Now, I want to design some controllers based on nonlinear model. Could you please tell me where can I find a nonlinear model for floating wind turbine controller design?

I'm also confused about the baseline GSPI control. In regin 3, it is a constant power controller, the generator torque is calculated by T_gen =P_rated/(w_gen*n) . So the output generator power should be a constant value equals P_rated, but after I run the .ddl file with FAST, the result shows that the output power is not a constant value. Why? Is there anything wrong with my understanding ?

Thank you
Best regards,
Cheng

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Re: questions about reference control input

Postby Jason.Jonkman » Thu Apr 26, 2018 1:35 am

Dear Cheng,

FAST is a nonlinear model; the linearization functionality only linearizes the equations of motion about a specific operating condition. What type of "nonlinear" model are you seeking? The nonlinear FAST equations of motion cannot be written out easily.

Yes, the baseline controller for the NREL 5-MW wind turbine uses a constant power torque-control law in Region 3. However, the torque is calculated by the low-pass time-filtered generator speed whereas the actual power is calculated using the unfiltered speed, so, there may be small deviations from the rated power as a result.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Cheng.Zhang
Posts: 57
Joined: Tue Nov 14, 2017 6:50 am
Organization: ECN
Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Fri May 04, 2018 3:57 am

Dear Jason,

Thank you!
I designed a controller for floating wind turbine in region 3(above rated), and I made some simulation on the FAST.
I found that when I increase the wind speed. the rotor speed is increased, but the platform pitch is decreased. Is it normal? If yes, what's the relation between the rotor speed and platform pitch? Is there any dynamic equation between the rotor speed and platform pitch in region 3?

Best regards,
Cheng

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Re: questions about reference control input

Postby Jason.Jonkman » Fri May 04, 2018 6:40 am

Dear Cheng,

Yes, in an active blade pitch-to-feather controller, the mean rotor thrust peaks at rated and decreases for higher wind speeds. See e.g. the following paper for more information: https://www.nrel.gov/docs/fy08osti/42589.pdf.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Cheng.Zhang
Posts: 57
Joined: Tue Nov 14, 2017 6:50 am
Organization: ECN
Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Fri May 04, 2018 8:48 am

Dear Jason,
Yes, but the rotor speed is increased.
Does it mean the rotor aerodynamic torque increases, the rotor thrust force decreases?

For floating offshore wind turbines(FOWT), the control objectives are regulating power to rated value(e.g. 5MW) and regulating the platform pitch. What the meaning of regulating platform pitch? For instance, if I linearized the FAST model with a constant wind speed of 14m/s, a rotor speed of 12.1 rpm, then I got a stable state with blade pitch equals 8.6deg, platform pitch equals 1.39deg. So if wind speed changed to a 15m/s stochastic wind, does it mean we need to actuate the blade pitch to regulate the platform pitch to 1.39deg, or regulate it to 0deg?

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Re: questions about reference control input

Postby Jason.Jonkman » Fri May 04, 2018 9:02 am

Dear Cheng,

I would normally expect a Region 3 controller to regulate rotor speed to the rated speed. So, I would not expect the rotor speed to increase in Region 3 (except for oscillations about the rated speed). But you can always run various steady-state simulations to see how the rotor thrust varies with rotor speed, pitch angle, wind speed, etc.

I would say it is not common for the FOWT controller to regulate platform pitch. Of course, you'd likely want to design the FOWT controller so as to prevent it from exciting the platform pitch or to mitigate problematic interactions between platform pitch and power regulation.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Cheng.Zhang
Posts: 57
Joined: Tue Nov 14, 2017 6:50 am
Organization: ECN
Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Fri May 04, 2018 9:28 am

Dear Jason,

Yes, the controller does not work well.

As far as I know, a lot of papers' control objective including reducing platform pitch, the control problem of FOWT is the platform pitch motion.
I can't understand you said it is not common for the FOWT controller to regulate platform pitch. Could you please explain to me more specific?

Best regards,
Cheng

Jason.Jonkman
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Re: questions about reference control input

Postby Jason.Jonkman » Fri May 04, 2018 9:34 am

Dear Cheng,

Yes, the FOWT controller may have on objective of adding damping (or at least not reducing damping) of the platform-pitch mode, but I would not guess it would be common to use the FOWT controller to change the target (mean) value of the platform-pitch angle. If the target (mean) value of the platform-pitch angle is not desirable for a given steady-state thrust, it would be better to redesign the platform than to attempt to correct that with FOWT control.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Cheng.Zhang
Posts: 57
Joined: Tue Nov 14, 2017 6:50 am
Organization: ECN
Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Fri May 04, 2018 9:56 am

Dear Jason,

I'm not sure I understand well.

If my controller is successfully adding the damping of platform pitch. Does it mean the platform pitch acceleration is decreased?
For example, suppose that,
1.in a condition of 14m/s stochastic wind, apply a controller makes rotor speed at rated, and make the platform stable at constant value 1deg.
2in a condition of 16m/s stochastic wind, apply a controller makes rotor speed at rated, but make the platform stable at constant value 2deg.
Could I say the controller performance well? (If yes, let the platform pitch acceleration to aero can be as a control objective)

I'm really stuck in this problem, I still didn't clear about the control objectives of FOWT, thank you for your help.

Best regards,
Cheng

Jason.Jonkman
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Re: questions about reference control input

Postby Jason.Jonkman » Fri May 04, 2018 10:25 am

Dear Cheng,

Adding damping through FOWT control should reduce the oscillations of platform pitch about the mean, not the mean value itself. Changing the mean platform-pitch angle through FOWT control is what I questioned.

Regarding your example. I would expect the mean platform-pitch angle to decrease with increasing wind speed in Region 3 due to the drop in mean rotor thrust with pitch-to-feather control. Of course the oscillations about the mean may increase with wind speed due to the increasing variation in wind speed and associated severity of sea states.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Cheng.Zhang
Posts: 57
Joined: Tue Nov 14, 2017 6:50 am
Organization: ECN
Location: france

Re: questions about reference control input

Postby Cheng.Zhang » Fri May 04, 2018 11:01 am

Dear Jason,

Yes, my example has something wrong, the platform pitch should decrease with increasing wind speed in region 3.
What I want to express is just like what you said, in different wind condition, the objective is to reduce the oscillations of platform pitch about the mean, rather than mean value itself.
Your explanation is very clear, many thanks!
So, in order to reduce the oscillation, I can design a controller that force the acceleration of platform pitch to zero.

Best regards,
Cheng


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