Disturbances assumptions in contol problem

Wind & Water Controls
1

Hi,
In control problem of floating wind turbine can the following assumptions be true?

asass.jpg
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Assume that the disturbances are statistically independent of the command input signals.
Assume that the disturbances are zero mean.

How to get to above assumptions using FAST?(If it possible)
Note I have made wind data using TurbSim.

Any extra information in these case will be appreciable.

Thanks,

2

Dear Mehdi,

Your image certainly seems reasonable, but I’m not sure I understand your question. FAST intrinsically includes the ability to excite the nonlinear wind turbine with winds and waves, to control the response with blade pitch and generator torque, and to output generator speed.

Are you asking how one could obtain a linearized “plant” model with these disturbances, inputs, and outputs?

Best regards,

3

no I dont want to linearize the model. I just wanted to know that are my assumtions true or can be true?
Assumptions:

  1. the disturbances are statistically independent of the command input signals.
  2. the disturbances are zero mean.

Or if I linearze the model then the assumtions become true.

Regards,

4

Dear Mehdi,

I’m sure you can safely assume that the wind and waves are statistically independent. With linear wave theory, the wave disturbance would certainly have zero mean. The mean wind speed, of course, cannot be zero for a useful wind turbine; whether this means that the wind disturbance has zero mean likely depends on how you define the disturbance.

Best regards,

5

The assumption 1 means that are disturbances(Wind and Wave) statistically independent of the command input signals(Gen Torque and BldPitch) or no?

How we can define the wind disturbance to have zero mean mean?

Regards,

6

Dear Mehdi,

I’m not sure I understand the conditions under which the statement regarding statistical independence applies to answer properly. Perhaps someone with more control expertise should answer. From my understanding, though, if a feedback control law is used to derive the control inputs (generator torque and blade pitch) from the turbine response that is likely to be impacted by the wind and wave disturbances, than I don’t think that you can say that the control inputs are statistically independent of the wind and wave disturbances.

If you define “wind disturbance” as a variation of wind about a given mean wind speed (perhaps including a fixed shear), than the mean of the wind disturbance should be zero. If you define “wind disturbance” as the absolute effect of the wind, than the mean would not be zero.

Best regards,