Hi everyone,
I have encountered some difficulties regarding the linearization at region 2.
I am using OpenFAST in its latest version 3.1. and I am conducting a series of linearization of the NREL 5MW wind turbine. The following considerations have been taken:
 Operating points considered: 3:1:11 (wind velocities at region 1.5, 2 and 2.5)
 Modules activated: ElastoDyn, ServoDyn, InflowWind and AeroDyn v15
 TrimCase = 2: Internal torque controller recommended for region 2
 CalcSteady = True: In order to enable an automatic stationary state identification
 TrimTol = 0.001
 TrimGain = 1000
 VSCntrl = 1: As it is recommended in FASTv7 manual to conduct linearization
 VS_RtGnSp = 9999.9E9
 VS_RtTq = 43093.55
 VS_Rgn2K = 9999.9E9
 VS_SlPc = 9999.9E9
After the linearization, we extracted the state matrices using fx_getMats.m Matlab’s function and analyze the temporary signals, as well as the stationary values of the parameters.
When analyzing the outputs at a 7 m/s linearization, we found that the parameter GenPwr did not correspond to the expected one at that velocity. Apparently, when the GeneratorTorque controller is activated (VSCntrl=1), it seems that GenPwr is being calculated using the VS_RtTq nominal torque.
After linearizing the model at operating points between 3 and 11 m/s and analyzing the steady outputs of the system, we were expecting to obtain a pattern such as the red signal on the right (GenSpeed·GenTq). However, values of GenPwr did not match our expectations.
Is there something wrong in our input files (attached) or did somebody find a similar problem in its simulations?
On another note, has the LIDAR module already been added to OpenFASTv3?
Thank you in advanced.
Input Files
1 Like
Dear @Daniel.Aldaz,
Just a couple comments:
 The FAST v7 User’s Guide only recommends setting constant torque for
TrimCase
= 3 (linearization of blade pitch in Region 3). For TrimCase
= 2 (linearization of generator torque in Region 2 with fixed blade pitch), the generator torque is assumed to be calculated via the trim controller rather than with an internal torque controller in ServoDyn. So, effectively, you want ServoDyn to return zero torque. This can be achieved by setting in ServoDyn GenTiStr
= True and TimGenOn
> TMax
, where TMax
is the maximum simulation time set in the OpenFAST primary (.fst) input file.
 While the trim controller will calculate the generator torque needed to reach a steadystate solution, it is not tracking generator power. So, the ServoDyn output GenPwr will be calculated using the internally selected generator model (GenPwr = 0 when
GenTiStr
= True and TimGenOn
> TMax
) regardless of the actual generator torque set by the trim controller. So, I wouldn’t rely on ServoDyn output GenPwr being accurate when enabling a trim solution.
I’m aware of the LiDAR module being implemented in OpenFAST, but the pull request has not been issued yet.
Best regards,
Dear @Jason.Jonkman,
First of all, thank you for your fast response.
I have been checking all the comments you made and considering its effect in the linearizations we are running. I followed your recommendation regarding the trim controller of region 2 linearizations, and I will probably consider HSShftPwr
as a more reliable output of the Generator Power when enabling the trim solution.
Given that, we have found some unsual behaviors when studying the bode diagrams of the different outputs.

The low frequencies of the bode representation between the wind speed and the generator speed show an abnormal peak behaviour, which reminded us to the bode diagram of a closed loop simulation.
Besides, if we compare it with the same bode representation of a linear model obtained with FASTv7, the results were quite different for the lower frequencies.

On the other hand, when computing the bode between the wind speed input and the blade pitch, we were not expecting to obtain any result at all. FASTv7 solution did give us an empty output between these two variables.
Even though it is very subtle (around 220dB), it makes us think that the linearization is being computed for a closed loop system of the wind turbine and not for the open loop itself (as this problem appears for all of the operating points considered in the linearizations).
We would be pleased if you could take a look to both problems and tell us what you think about them.
I attach the linear model (.mat) for all of the wind speeds considered as well as the input files (.fst and .dat) of the linearizations. Files
Thank you so much in advanced and best regards,
Dear @Daniel.Aldaz,
I don’t have much experience reviewing Bode diagrams, so, I can’t comment much on those.
Looking briefly at your OpenFAST model, I see that you have specified constant torque in ServoDyn, which is what we recommend using with TrimCase
= 3. But for TrimCase
= 2, I had recommended setting TimGenOn
> TMax
, which I don’t see set.
I only see that you are linearizing twice (NLinTimes
= 2). Are you applying MBC3 and azimuth averaging over 2 azimuths? I would recommend increasing this to 36 linearizations (every 10 degrees of azimuth).
Also, I’m not sure how to interpret your MATLAB data. It appears that you have computed the linearized system for 23 different wind speeds; is that correct? Unfortunately, the MATLAB data structure doesn’t seem to store the state matrices, so, I can’t review those.
Best regards,
Dear @Jason.Jonkman,
Sorry for the misunderstanding. I forgot to update the new files (.fst and .dat) into the Google Drive, even though I had already followed your recommendations and used them in my current linearizations.
I have also increased the number of linearizations to 36 in order to consider a greater number of azimuths when applying MBC3. However the bode diagrams seemed to show the same trend as stated before.
I was computing a linearization for every wind speed in the 3:25 range.
In order to make it clearer, I have changed the MATLAB data of the Google Drive with the results obtained from the matlab function fx_mbc3 (MBC3). It contains two files with the linearizations at 9m/s and 12m/s (while NLinTimes
= 36). Besides, the input files can also be found there.
Could you take a look at it? Thank you so much for all your help.
Best regards,
Dear @Daniel.Aldaz,
Well, I’ve reviewed your files a bit, and don’t see any obvious issues. I’m not sure what to comment on. Can you clarify your question?
Best regards,
Dear @Jason.Jonkman,
First of all, I am sorry for the late reply. I have been taking a close look at the linearization process conducted with OpenFAST v3.1.
My issue is related with the linear models obtained from the .lin after linearizing the system.
For example, in this case I will focus on Region 3 and compute a linearization with the following characteristics:
 Operating point: 16 (wind velocity at region 3)
 Modules activated: ElastoDyn, ServoDyn, InflowWind and AeroDyn v15

TrimCase
= 3: Internal pitch controller recommended for region 3

CalcSteady
= True: In order to enable an automatic stationary state identification

TrimTol
= 0.001

TrimGain
= 0.0045

TimGenOn
= 0

VSCntrl
= 1: As, in these cases, it is recommended in FASTv7 manual

VS_RtGnSp
= 9999.9E9

VS_RtTq
= 43093.55

VS_Rgn2K
= 9999.9E9

VS_SlPc
= 9999.9E9

NLinTimes
= 2: In this case I just want to analyze one azimuth to show my issue
After this process, the output file can be found in the following GoogleDrive, as well as the inputs files used for the linearizations.
Even though VSCntrl
= 1 imposses a constant Torque Controller, when analyzing the system between ED Generator torque, Nm
and SrvD GenTq, (kNm)
we were expecting to obtain a constant value of 0.001 (because of the change of units). However, the value of the D matrix, associated to these inputoutput, was completely empty.
This empty bode diagram makes us think that there is no direct relation between the input torque demand and the outputs of ServoDyn, such as GenTq
and GenPwr
(as a similar concept happens with the system and bode diagram between ED Generator torque, Nm
and SrvD GenPwr, (kW)
).
It seems that D’s state matrix has a complete empty row associated with those two outputs and I can not understand why. Besides, by comparing the results with an analogous linearization computed by FASTv7 software, we checked that only D matrix differed that much, as A, B and C matrices had very similar results for these inputoutput systems.
Are we missing some kind of DLL used by the Internal Torque Controller (of VSCntrl
= 1) or is there something wrong in the extraction of D matrices done by the tool?
On another note, when I want to compute a single linearization (NLinTimes
= 1) and CalcSteady
= True, the linearization is conducted at the first time step. Is this done on purpose?
Thank you so much in advanced again for all your help.
Best regards,
Dear @Daniel.Aldaz,
Regarding the linear statespace input ED Generator Torque, Nm
, I think the confusion arises because of how this input is defined. Linear statespace input ED Generator Torque, Nm
represents a perturbation to the generator torque input to ElastoDyn. Normally within OpenFAST, the generator torque is calculated and output from ServoDyn and used as input to ElastoDyn. But in the linear model, the perturbation is performed only on the ElastoDyn input, independent from ServoDyn or the ServoDyn outputs. So, it makes sense to me that the rows associated the ServoDyn outputs SrvD GenTq, (kNm)
and SrvD GenPwr, (kW)
in the D matrix are zero.
Regarding your last question, when CalcSteady
= True, I would only expect OpenFAST to linearize at time zero if the timedomain solution is already in steady state, at least within the tolerance set (TrimTol
).
Best regards,
Dear @Jason.Jonkman,
Thank you for your answer, that makes a lot of sense.
I did not perceive the changes that modularity introduced in the linearizations.
Having that in mind, which inputoutput signals of the other modules would you recommend to obtain a system between the torque demand and the generator torque/power output (similar to GenTq
or GenPwr
from ServoDyn).
Best regards,
Dear @Daniel.Aldaz,
I’m not sure I understand what you mean when you ask abut going from generator torque input and generator torque output. Wouldn’t these be onetoone? Or perhaps you want to look at the influence of the generator torque input on the highspeed shaft torque output (ElastoDyn output HSShftTq
). Likewise, perhaps you want to look at the influence of the generator torque input on the highspeed shaft power output (ElastoDyn output HSShftPwr
).
Best regards,
Dear @Jason.Jonkman,
Yes, I wanted to analyze the influence of the linear statespace input ED Generator Torque, Nm
on the generator torque and power outputs. It is a onetoone relation but it could be helpful to check that it was working properly. I guess the only way will be to study the influence of the ED Generator Torque, (N·m)
on the outputs of ElastoDyn you metioned (HSShftTq
and HSShftPwr
).
What would you suggest in order to represent a generator model so that we could transform ED HSShftTq, (kN·m)
into a signal similar to the electrical SD GenTq, (kN·m)
?
Besides, in simulation we designed a feedback loop controller entering the SFunc through the Generator torque (N·m)
input (electrical generator torque to ServoDyn).
However, due to the modularity of OpenFAST, the only available torque input in the linear model right now is the ED Generator Torque, (N·m)
. I was wondering if this input corresponds to a mechanical or an electrical torque so that we can use the same controller structure or we will need to change it. In the figure shown below, does ED Generator Torque, (N·m)
represents T_HSShft
of T_g
?
Thank you so much for all your answers, they are being extremely helpful in order to progress with our linearizations.
Best regards,
Dear @Daniel.Aldaz,
The input to the linear model, ED Generator Torque
, represents a perturbation to the electrical generator torque (T_g
). Basically, the generator torque operating point is the electrical generator torque commanded by Simulink, and linear input ED Generator Torque
represents the perturbation of this torque. It is just that it linear input ED Generator Torque
does not directly transfer in/out of ServoDyn.
Best regards,