Hello,
I am translating to OpenFAST a HAWC2 model provided by DTU of a 25kW upwind, geared, 3 bladed turbine with asynchronous generator and 13m rotor diameter. I am seeing strange behaviour in the power output when I turn on the DrTrDOF. See the figure below for wind stairs from 5 m/s:
The blue curve is with DrTrDOF turned off, and the red is when I turn it on with the values I calculated, and the black is when GenIner is 100x higher than my calculated value. First of all the response is very slow for some reason for my calculated values of DTTorSpr and DTTorDmp. Secondly the power level seems to scale with GenIner.
I am wondering if it is something about the combination of parameters? I have calculated the GenIner as follows:
GenIner = GenIner_LSS/GBRatio^2 = 190.81/23.25^2 = 0.353
and DTTorSpr, and DTTorDmp according to the equations in the post here: viewtopic.php?t=940
K=G J/L
where
K:DTTorSpr
G: Modulus of rigidity for the shaft material
J=pi*d1^4/32 and
L=l1+(GBratio)^2*l2*(d1/d2)^4
d1 and d2 are diameters of LSS and HSS, respectively;
l1 and l2 are lengths of LSS and HSS, respectively.
G [N/m2] 7.93E+10 for stainless steel
d1 [m] 1.20E01
d2 [m] 6.00E02
l1 [m] 7.00E01
l2 [m] 5.00E01
gbratio 23.25
J= 2.04E05
L = 4.33E+03
DTTorSpr (K)= 3.73E+02
DTTorDmp can be calculated as follows:
DTTorDmp =Cc*ζ
where,
Cc is critical damping coefficient =2*sqrt*(K*rotor inertia);
ζ is fraction of critical damping or the percentage of damping.
Rotor inertia = 3448.517 from ED.sum
Cc = 2.27E+03
ζ = 0.6
DTTorDmp = 1.36E+03
fixedfree fn (in Hz) = SQRT( DTTorSpr/RotIner )/(2*pi) = 0.05 Hz
The resulting fixedfree natural frequency also seems to be very low, although I don't have any reference for what it should be for this size and type of turbine, I guess it should be more like ~2 Hz?
Thanks in advance.
Best Regards,
Drew Gertz
Northwind Engineering OÜ
www.northwindengineering.com
DrTrDOF impacting power output
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Re: DrTrDOF impacting power output
Dear Drew,
Yes, I would guess your drivetrain torsional spring constant (DTTorSpr) is too low by several orders of magnitude (and DTTorDmp is too low accordingly). From the FAST/OpenFAST certification/regression tests, DTTorSpr for the UAE Phase VI turbine (20 kW turbine) is 199000 Nm/rad and DTTorSpr for the AOC15/50 turbine (50 kW turbine) is 600000 Nm/rad. I would guess your DTTorSpr value should fall in (or near) this range.
Best regards,
Yes, I would guess your drivetrain torsional spring constant (DTTorSpr) is too low by several orders of magnitude (and DTTorDmp is too low accordingly). From the FAST/OpenFAST certification/regression tests, DTTorSpr for the UAE Phase VI turbine (20 kW turbine) is 199000 Nm/rad and DTTorSpr for the AOC15/50 turbine (50 kW turbine) is 600000 Nm/rad. I would guess your DTTorSpr value should fall in (or near) this range.
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
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

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 Joined: Tue Sep 12, 2017 4:30 am
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 Contact:
Re: DrTrDOF impacting power output
Hi Jason,
Thanks for your reply. I clearly have to revisit the method I used to calculate DTTorSpr .
I tested some higher DTTorSpr values within the range you provided. The following combination turned out to be acceptable:
6e5 DTTorSpr
5.5e4 DTTorDmp (ζ = 0.6)
The resulting fixedfree natural frequency is 2.1 Hz which seems reasonable as well.
My GenIner value of 0.353, however, results in drivetrain resonance. I had to increase it to 1.5 to solve it, but overall I consider the problem to be solved.
Thanks!
Best Regards,
Drew Gertz
Northwind Engineering OÜ
http://www.northwindengineering.com
Thanks for your reply. I clearly have to revisit the method I used to calculate DTTorSpr .
I tested some higher DTTorSpr values within the range you provided. The following combination turned out to be acceptable:
6e5 DTTorSpr
5.5e4 DTTorDmp (ζ = 0.6)
The resulting fixedfree natural frequency is 2.1 Hz which seems reasonable as well.
My GenIner value of 0.353, however, results in drivetrain resonance. I had to increase it to 1.5 to solve it, but overall I consider the problem to be solved.
Thanks!
Best Regards,
Drew Gertz
Northwind Engineering OÜ
http://www.northwindengineering.com
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