How to simulate special events?

Hi, everyone,

According to IEC61400 and GL standards, we should calculate the ultimate/fatigue load in special load cases such as:
DLC2.1 Normal turbulent wind with control system fault or grid loss.
DLC2.2 NTM wind with internal electric fault.
DLC2.3 EOG wind with grid loss or other electric fault
and so on.

I wonder what does grid loss/generator shortcut mean to our simulation settings. Shall I simply deactivate generator torque/lock the pitch during the simulation? And another question is about LVRT. When short time low grid voltage happens, what will it cause to our load simulation?

Does FAST has special function/tricks for simulating such load cases? If I use ADAMS with Aerodyn, it is easier to do this by using ACF commands.

Thanks a lot!

1 Like

Dear Wei Ni,

Please see the “Simulating Special Events” section of the “Controls” chapter of the FAST User’s guide.

Best regards,

Dear Jason,

Thank you for your hint. I will read the users’ manual

Dear Jason Jonkman,

First of all congratulations for this forum, it is very useful. I work at IKERLAN, a research center, and I am studying the deflection failure on the wind turbine blades. We want to analyze the limit behavior of the 5 MW NREL Offshore blade. To develop this simulation we used the files from the “Definition of a 5-MW Reference Wind Turbine for Offshore System Development” article in FAST code. We would like to know your opinion about the options we choose to carry on these calculations and listen to any suggestions you could make.

After computing the different load cases based on the guidelines in IEC standards, we selected the worst extreme wind cases for flapwise and edgewise directions.

We selected two extreme winds, which are the boundary conditions of service:
• Flapwise: EWM50 will be the worst case
• Edgewise: ECD-R-2 will be the worst case

EWM50
For this case, we have assumed that the extreme situation would be when the blade is parked and the blade pitch angle is 0º, flat to the wind. We assume 0º pitch angle because it is worse than if the blade was feathered, 90º pitch angle. To achieve these conditions, we start with an initial zero rotor speed and mechanically actuated brake. The most significant changes in the FAST input file will be these ones:
PCMode 0
TimeGeOn 9999.9
THSSBrDp 0
TiDynBrk 0
TTpBrDp(1,2,3) 0
TBDepISp(1,2,3)0
BlPitch(1,2,3) 0
RotSpeed 0
HSSBrDT 0

Like you suggest in the NWTC forum, we also changed the Aerodyn input:

StallMod= Steady
IndMode=None

We would like to know if you think that these parameters have the correct value or term to achieve the condition of EWM50.

ECD-R-2
On the other hand, we choose the ECD-R-2 load case. At this extreme event due to a strong gust of wind of 26 m/s, the Vcut is exceeded. We have assumed that this event is short so the wind turbine operation will be normal, this way we want to obtain the most extreme case.

We are not sure if our hypothesis is correct, therefore, we would like to know your opinion about how to deal with this extreme event.

Thank you very much for your time,

Best regards,

Eneko Aritz

Dear Eneko,

Your settings for modeling a blade flat into the wind under EWM50 sound reasonable, although there is no point activating the dynamic brake or tip brakes (i.e., TiDynBrk, TTpBrDp, and TBDepISp need not be set to zero). Please note that if you plan to look at loads in the tower, it is general overly conservative to assume all blades are stuck flat into the wind under EWM50.

I also agree with your assumption that the turbine will likely not shut down due to short duration gusts that cause the wind speed to sligthly exceed cut-out wind speed (which is 25 m/s for the NREL 5-MW turbine).

FYI – We’ve performed several IEC-style loads analyses on the NREL 5-MW turbine using FAST, in both land-based and offshore floating configurations. While we haven’t run every case in the IEC design standard, we have run many of them. Chapter 5 of my PhD thesis-turned NREL report explains how we carried out these load cases: nrel.gov/docs/fy10osti/45891.pdf.

I hope that helps.

Best regards,

I thank you for your reply, it was very useful.

Eneko Aritz

Dear Jason,

I am trying to simulate parked condition by feathering blades. I have read the FAST manual page 34. But I reckon there are other settings I need to change that is not mentioned in page 34 of FAST manual:

In the manual, it is written, I should set BlPitch and BlPitchF to 90 degrees and TPitManS to a time longer than simulation time. I have done it.

Do I need to change below settings as well?

In AeroDyn’s primary input file:
StallMod = STEADY (disable dynamic stall)
IndModel = NONE (disable aerodynamic induction)

And in ServoDyn file:
PCMode = 0 (disable pitch control)
GenTiStr = True
TimGenOn = 9999.9 (a large don’t care > TMax)

Please kindly tell if it is correct. Thank you very much.

Best Regards,

Arash,

Dear Arash,

Yes, see my post dated Mar 30, 20120 in the following forum topic for more information: http://forums.nrel.gov/t/extreme-events/219/1.

Best regards,

Dear Jason,

According to the AeroDyn User’s Guide for an idling or parked turbine the settings should: WakeMod = 0 and AFAeroMod = 1.
But for a special event with pitch system maintenance when one blade is not completely at feathered position I don’t know which are the best settings for AeroDyn. For modelling the two blades at feathered position I should use idling settings in AeroDyn, but for the third blade operational settings would be better. Can you recommend me the settings for this special event or is it the best option to compare both possibilities?

Thank you for your support.

Best regards,
René

Dear René,

I would use WakeMod = 0 and AFAeroMod = 1 even for the case where the rotor is parked/idling with one blade stuck at operational pitch.

Best regards,

Dear Jason,

I am trying to simulate DLC 2.3 of IEC 61400-1:2019, which involves a loss of electrical network, for a land based 5 MW turbine.

I have made use of the guidelines mentioned in the ‘HSS Brake Shutdown after Loss of Grid’ subsection of ‘Simulating Special Events’ from the FAST users guide. I am using the simple built-in HSS brake model.

Could you tell me if there are any recommended or reasonable values for THSSBrDp and HSSBrDt, (i.e. the time when the HSS brake is initially applied (after the loss of grid) and the time taken for the brake to be fully deployed) for the 5 MW turbine?

Sincerely

Abhinav

Dear @Abhinav.Abeendranath,

The high-speed shaft brake in the NREL 5-MW baseline wind turbine is not intended to be deployed to shut down the wind turbine; the brake is only used to park the rotor when not operating for maintenance purposes.

When we simulated DLC 2.3 with the NREL 5-MW baseline turbine, we, instead, shut down the wind turbine after the loss of load by pitching all of the blades to feather (to the maximum pitch of 90deg, at the maximum pitch rate of 8deg/s) after a 0.2-s delay.

Best regards,

Dear Jason,

Thank you for the reply. I am now using a ‘Pitch-to-Feather Shutdown’ to run the DLC 2.3, instead of using the HSS brake. I have made the following changes in my simulation:

TPitManS - changed to 0.2 s after the time of the grid loss
PitManRat - set to 8 deg/s
BlPitchF - 90 degrees
TiGenOn - 0
GenTiStp - False

Could you please tell me if I have missed any other input parameter?

Sincerely

Abhinav

Dear @Abhinav.Abeendranath,

I agree with your settings of TPitManS, PitManRat, BlPitchF, and TimGenOn, but instead of setting GenTiStp = False, I would expect that you’d set GenTiStp = True with TimGenOf = TPitManS - 0.2 s to force the grid loss.

Best regards,

Dear Jason,

Thank you for the reply. I have implemented the simulation for DLC 2.3 (stochastic wind with loss of grid) using the settings mentioned above.

I have attached a plot of the FA bending moment at the tower base, for 2 simulations - one with and the other without grid loss. I have noticed some oscillation about 0, for the latter case, soon after the loss of grid, but it gradually dies down. Could you please comment if this is normal?

Sincerely

Abhinav
dlc_2.3

Dear @Abhinav.Abeendranath,

Yes, this is expected behavior. The thrust quickly drops after the turbine is shut down, and the tower will experience a damped oscillation afterword.

Best regards,

Thanks, Jason.

Sincerely

Abhinav