Start-up of NREL 5MW

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Paul.Schünemann
Posts: 28
Joined: Wed Jul 13, 2016 7:46 am
Organization: University Rostock
Location: Germany

Start-up of NREL 5MW

Postby Paul.Schünemann » Thu Jun 25, 2020 11:49 am

Hello,

I am trying to simulate a start-up event for the landbased NREL 5MW baseline wind turbine. For this purpose I adapted the NREL 5MW Baseline Controller following the advice given here: https://wind.nrel.gov/forum/wind/viewtopic.php?f=4&t=1326&hilit=TPCon

More precisely I added a simple override pitch maneuver to the dll code (linear decrease of pitch angle from 90° to a low value depending on wind speed) and reformulated the torque controller a bit in the way that it not always computes the torque "as if it were in Region 3 whenever the previous blade-pitch-angle command was 1° or greater" (Otherwise it demands a negative torque when the generator is activated during start-up where the blade-pitch-angle generally is greater than 1°).

After the start-up procedure is finished I would like to let the baseline pitch-controller take over control over the pitch-angle. However, this does not work as expected. Instead the pitch angle is going crazy and starts to oscillate enormous at high amplitudes.

To debug the problem, I first tried to zero the integrator of the PI-Pitch-Controller at the time this controller become active, but that doesn't help.
Then, I simplified the problem and just simulated the NREL 5MW wind turbine at steady wind (12 m/s) with
a) using the unchanged baseline pitch controller from the beginning of the simulation and
b) enable the unchanged baseline pitch controller only after 5 sec (TPCOn = 5 in ServoDyn).

Interestingly, here one can see a similar behaviour. If the pitch controller is used from the very beginning everything looks fine, but if the pitch controller is disabled for the first couple of seconds the pitch angle shows again high oscillations after activation (see attached figures).
Does someone have an idea what could be the reason for this behaviour? Is there anything else besides the integrator of the PI-Pitch-Controller one should reset before activating the pitch controller?

Additionally, I wonder about the values of the pitch angle which oscillate between -90° and +180°. As far as I understand, there is a pitch angle limit of [0°, 90°] set inside the controller dll and there is no pitch actuator dynamics modelled in OpenFAST (meaning the actual pitch angle is instantaneously set to the pitch command from the controller as described in this forum post: viewtopic.php?f=4&t=2249#p12751). If that's true, how could the pitch angle reach values beyond the limits of [0°, 90°]? Does someone have an idea?

Best regards,
Paul Schünemann
Attachments
BldPitch for Simulation a) TPCon = 0.PNG
BldPitch for Simulation a) TPCon = 0.PNG (56.65 KiB) Viewed 103 times
BldPitch for Simulation b) TPCon = 5.PNG
BldPitch for Simulation b) TPCon = 5.PNG (20.1 KiB) Viewed 103 times

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Start-up of NREL 5MW

Postby Jason.Jonkman » Thu Jun 25, 2020 1:25 pm

Dear Paul,

My guess is the problem is a related to lack of proper initial conditions that keeps the controller from being initialized properly. When TPCOn = 0, you likely give the simulation reasonable initial conditions of rotor speed and blade-pitch angle, but when TPCOn = 5, the rotor likely overspeeds for a bit and when the controller is enabled 5-s in, the initial conditions to the controller are poor and it can't recover. I've seen a similar problem if you set TPCOn = 0 and provide poor initial conditions on rotor speed or blade-pitch angle. More rigorous controls logic could probably be implemented to resolve this, but is not something I've looked into.

Regarding the blade pitch output exceeding the 0-90-deg range, my guess is this is related to the extrapolation of module-level inputs used within the software. BlPitch is an output of the ElastoDyn module, which time-integrates from n to n+1 before other modules, using extrapolated inputs (including blade-pitch) from ServoDyn to have inputs at n+1. If the input time history is not continuous, the extrapolation is poor. My guess is the controller is commanding large jumps in the pitch angle, resulting in a noncontinuous blade-pitch angle time history. You can reduce the effect of the extrapolation by adding correction steps (NumCrctn >0), but it is better to ensure that the controller outputs smooth (continuous) controller commands.

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

Paul.Schünemann
Posts: 28
Joined: Wed Jul 13, 2016 7:46 am
Organization: University Rostock
Location: Germany

Re: Start-up of NREL 5MW

Postby Paul.Schünemann » Wed Jul 01, 2020 3:16 am

Dear Jason,

Thanks for your immediate reply.

Yes, you are right, it seems to be related to the initial conditions. I thought about that before, but just wasn't aware how sensitive the controller is on poor initial conditions. Animated by your post, I run a simulation with TPCOn = 0, a steady wind of 12 m/s and an initial rotor speed of 12.1 rpm - and everything went fine. However, even with a slight change of the initial rotor speed to e.g. 12.25 rpm, the pitch controller becomes unstable resulting in unreasonable pitch angles.
Thus, I agree, that implementing a more rigorous controls logic should resolve the issue. However, at the moment I am not shure, what exactly to change in the controller. Have to think about it a bit. If someone has an idea, anything is appreciated ;-)

Regarding the blade pitch output, you are right, too. After setting NumCrctn = 1, the blade pitch angle output doesn't exceed the 0-90-deg range anymore. Thanks, for the explanation on the integration scheme and extrapolation. Something else that I've learned. That's why I love this forum. :)

Best regards,
Paul

Jason.Jonkman
Posts: 4864
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Start-up of NREL 5MW

Postby Jason.Jonkman » Wed Jul 01, 2020 6:23 am

Dear Paul,

I'm curious if the ROSCO Toolbox-developed controller for the NREL 5-MW baseline wind turbine has a similar sensitivity to initial conditions: https://github.com/nrel/rosco? This is something I don't know the answer to.

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

Paul.Schünemann
Posts: 28
Joined: Wed Jul 13, 2016 7:46 am
Organization: University Rostock
Location: Germany

Re: Start-up of NREL 5MW

Postby Paul.Schünemann » Thu Jul 02, 2020 1:32 am

Hi Jason,

Thanks for pointing out the ROSCO controller. That's definitely worth a try.
I will check that out in the next weeks and let you know.

Best regards,
Paul Schünemann


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