I am simulating design load case 2.3 based on IEC 61400-1 (which caters to finding loads pertaining to potentially significant Extreme Operating Gust, EOG, while turbine is producing power power and comes across a grid failure) for NREL 5 MW reference machine using FAST. I am using the control file DISCON.dll (prepared by Jason Jonkman) for the same.

From my calculation, the perpendicular distance from yaw axis to blade tip for this machine is ‘7.738 m’ (based on overhang of 5.019 m, tilt of 5°, precone of 2.5° and rotor radius of 63 m). Here, I find some results which I fail to understand:

Why does the pitch system respond in a way its depicted here (as I see there is some kind of lag in its actuation time)?

Why does tower clearance give so high values as I assumed it should be less than 7.738 m during the gust (I understand some values are so high as they are calculated from blade tip to yaw bearing when blade tip is above yaw bearing, but still…)? There was no warning of tower strike during the run!!

I thought tower clearance would more or less show opposite trend than out-of-plane deflection (OoPDefl1) when blade tip is below yaw bearing. Shouldn’t it be the case??

Is there something I am missing fundamentally especially with regards to tower clearance or are my calculations erroneous? Would appreciate, if you can provide any kind of insight to this?

Thanks a lot,
Neelabh

PS: I used a time of 30 s to eliminate computational transients and at 35.2 s, generator is switched off to simulate grid loss!

I don’t know anything about the controller to answer this question.

The clearance gets large when the blade points up because it is the distance from the tip to some point (not sure exactly where - check the manual) where the blade would hit the tower when the blade points down. You should only look at the tower clearance when the blade azimuth puts it near the tower.

Maybe 2) answers this?

When we run MExtremes to try to see what is the closest the blade gets to the tower, we set up some calculated channels to see if we got too close to the tower:

"Azimuth3" "(deg)" "mod(240 +FileInfo.Time(:,$Azimuth$ ),360)"
"Azimuth2" "(deg)" "mod(120 +FileInfo.Time(:,$Azimuth$ ),360)"
"TipClr1" "(m)" "filter_using_bounds(FileInfo.Time(:,$TipClrnc1$ ),FileInfo.Time(:,$Azimuth$ ),[175 185],NaN)-2.95-4.5"
"TipClr2" "(m)" "filter_using_bounds(FileInfo.Time(:,$TipClrnc2$ ),FileInfo.Time(:,$Azimuth2$ ),[175 185],NaN)-2.95-4.5"
"TipClr3" "(m)" "filter_using_bounds(FileInfo.Time(:,$TipClrnc3$ ),FileInfo.Time(:,$Azimuth3$ ),[175 185],NaN)-2.95-4.5"
It sets the clearance to NaN if the azimuth is not within 5 deg of straight down. If I remember correctly, the 2.95 is the minimum distance the standard allowed the tip to get to the tower circumference (a fraction of the undeflected distance). The tower radius at the clearance location was 4.5m. This made it so that a negative value for the minimum would flag a standards violation. You could leave out the 2.95 if you just want to know how close it gets to the tower circumference. A negative value would mean a tower strike, which would be sad…

To add to the answers Marshall’s already provided:

The operational pitch-control system provided with the NREL 5-MW turbine has the goal of regulating generator speed to rated speed. The pitch controller only uses the generator speed error (the difference between the actual and rated generator speeds) as input; the pitch controller doesn’t know anything about the wind speed, so, there is naturally a lag relative to the gust. More information on the control system for the NREL 5-MW turbine is given in the specifications report: nrel.gov/docs/fy09osti/38060.pdf. Regardless, please note that it is not common for the operational pitch controller to remain active when the generator is taken offline; instead, the safety system will likely pitch the blades to feather to shut down the turbine.

2 and 3) I think you calculated the blade tip-to-tower clearance of the undeflected NREL 5-MW turbine incorrectly. Your overhang, shaft tilt, precone, and blade radius values are correct, but the precone and shaft tilt angles sum, such that the clearance (not including tower radius) equals: (5.019 m)*COS(5 deg) + (63 m)*SIN(5+2.5 deg) = 13.223 m. Your results show that blade 1 clearly drops below this value.

Marshall: Although, I was not able to run MExtremes, but with your answer I was able to filter out clearances for a specific ‘Azimuth’ after receiving the FAST output from *.out file. Meanwhile, could you please refer me to the standard that sets values for tower-tip clearance values?

Jason: You are correct, I made a mistake with my tower clearance calculation as I forgot to include the distance advantage, we have, due to shaft tilt. And that’s a lot. Now the plot seems to make more sense.
Secondly, could you please tell me if there is a provision in FAST where we can kick in Safety system after generator trips? And if not, can the loads generated (specifically, the forces/moments on rotor blades) after grid loss (generator power =0) be relied upon, based on the fact we still have pitch control ON?

Keeping a clearance between the blade and tower is discussed in Section 7.6.5 of the 3rd edition of the IEC 61400-1 wind turbine design standard.

FAST is set-up so that users can implement their own logic for the turbine control, safety, and protection functions of a wind turbine. If you do not wish to implement your own logic, you can use the simple control functions to mimic basic behavior (e.g., you could use the override pitch maneuvers to mimic the safety system shutting down the turbine by pitching the blades to feather). Obviously, the output from FAST would not be reasonable if the control actions are not close to the behaviour of a real turbine.

Thanks Jason, I will incorporate the safety system as you mentioned.

Meanwhile, I forgot to add, for Extreme operating gust @25 m/s (hub height velocity), the tower clearance value is in range of 14 m-18 m for azimuth of 175 -185 degrees for blade 1 (can be seen in the plot above). And this is before the generator is turned off. So I have a feeling there is a tower strike for this scenario (even though I didn’t receive any warnings) ??

In Section 6.2.4.1 of Germanischer Lloyd’s “Guideline for the Certification of Wind Turbines Edition 2003 with Suppl 2004,” it states that: “If the deformation analysis is performed by dynamic and aeroelastic means, at no time may the clearance be less than the minimum of 30% for the rotor turning.” By 30%, they mean 30% of the minimum distance between the rotor tip when undeflected and the circumference of the tower.

If the tower clearance is 14 - 18 m, why do you expect a tower strike? The blade will only strike the tower if the tower clearance output falls below the tower radius (the tower clearance output does not include the effect of the tower radius). Regardless, FAST will not issue a warning even if an actual tower strike occurs (this is not something that that FAST checks for – it is up to you as the user to track the tower clearance output and check for possible tower strikes).

Marshall has retired from NREL since posting that response. I’ve not heard of “filter_using_bounds”, but it appears to be a user-written function that allows the value of the first argument (TipClrnc*) to be replaced with the fourth argument (NaN) if the second argument (Azimuth*) is outside the range of the third argument ([175 185] degrees, i.e., when the blade is near the tower). You could create this function yourself if you wish to filter the tip clearance output the same way within MExtremes.