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PG6111FA M-VIe control IGV hunting in Preselect Load
IGV hunting during only on preselect load operation.

Dear Sir,

We have two fram 6FA units (6111FA). Usually we used to run both machines on base load operation (averagely both will reach 71MW). But recently we have been instructed to maintain on 65MW with preselect mode. we observed the IGV is varying during the preselect load operation from 80deg to 86deg angle at the time CSRGV also varying. We don't have any alarm for the IGV variations. however it is following the reference but we would like to know why the CSRGV hunting on preselect mode? we tried to read back the logics but could not get the correct reasons.

please help us.

1 out of 1 members thought this post was helpful...

Dear STLTLS,

Does the unit have DLN combustors, and if so, which version of DLN combustion system does it have?

How fast is the IGV reference changing from 80 to 86 DGA? Every second? Every two seconds? Every five seconds?

What is TNH (or TNH1) doing during these IGV oscillations?

Usually, CSRGV is the output from a MIN SEL (Minimum Select block) which means there are several inputs being selected from. What are those inputs and which one, or which ones, are oscillating at the same rate as CSRGV?

What is DWATT doing during these IGV oscillations?

What is TTXM doing during these IGV oscillations?

What is TTRF1 doing during these IGV oscillations?

What is L70R doing when Pre-Selected Load Control is active?

What is L70L doing when Pre-Selected Load Control is active?

CANCEL PRE-SELECTED LOAD CONTROL (by clicking ONCE on RAISE- or LOWER SPEED/LOAD). And monitor the above-mentioned signals and tell us what how they are responding. (It's entirely possible Pre-Selected Load Control is not tuned properly.) If necessary, use the RAISE- or LOWER SPEED/LOOAD buttons to manually adjust the load to something between 64 and 66 MW (approximately 65 MW) and then observe and report the information on the requested signals. This is probably the most important part of the troubleshooting exercise, and the unit load is NOT going to be unstable or drift wildly or aimlessly during the test, which should only take approximately five (5) minutes to complete and observe and report the requested information.

Looking forward to your response.

If you could create a Trend Recorder file, and use it for the same test while on and off Pre-Selected Load Control, that would be best. And if you could post screenshots of the two tests to a website and then post links to the website in your response that would be the best.

>Dear CSA,

Thanks a lot for the quick response against our query.

Please find the data observed during the IGV variations. The rest exercise as you advised we will simulate and will get back to you soon with the collected datas...

>Does the unit have DLN combustors, and if so, which version
>of DLN combustion system does it have?

Reply: Yes the unit is having DLN2.6+ combustion system.

>How fast is the IGV reference changing from 80 to 86 DGA?
>Every second? Every two seconds? Every five seconds?

Reply: Actually the variations are not happening on constant timings. Some time varying 0.1 deg every 3 secs. Some times varying 0.5secs every 2 secs. Totally it takes 2 to 3 mins approx for the 80 to 86 deg variations.

>What is TNH (or TNH1) doing during these IGV oscillations?

Reply:TNH is varying between 99.7% to 100.3% during the IGV variations.

>Usually, CSRGV is the output from a MIN SEL (Minimum Select
>block) which means there are several inputs being selected
>from. What are those inputs and which one, or which ones,
>are oscillating at the same rate as CSRGV?

Reply:The CSRGV driving from CSRGV_AUX signal which is coming from CSRGVV3_2 Block. There are several Inputs which is coming from some more calculations which really confusing us to get the source.

>What is DWATT doing during these IGV oscillations?

Reply:Yes. around 4MW hunting.

>What is TTXM doing during these IGV oscillations?

Reply: Yes. around 22DegF hunting.

>What is TTRF1 doing during these IGV oscillations?

Reply:Yes. around 38DegF hunting.

>
>What is L70R doing when Pre-Selected Load Control is
>active?

Reply:Yes for the load raising to reach the PS load SP.

>What is L70L doing when Pre-Selected Load Control is
>active?

Reply:Yes for the load lowering to reach the PS load SP.

Looking forward further advice...

1 out of 1 members thought this post was helpful...

I find it utterly incomprehensible--and completely stupid--that operators and their supervisors and plant managers will put up with 4 MW swings while the turbine is operating with Pre-Selected Load Control active. If the setpoint is 65 MW, and the unit is continually bouncing between 63 and 67 MW while Pre-Selected Load Control is supposed to be holding the load CONSTANT is acceptable to anyone. Anywhere. And I've seen more excessive load swings that were NOT of any concern to anyone in the plant because Pre-Selected Load Control was active and "controlling" the load.

But amazingly it is. And yet operators, their supervisors and plant managers absolutely REFUSE to run the unit without Pre-Selected Load Control--for fear the load will change or the unit will trip or some other catastrophe will occur. And, yet a 4 MW swing WHILE PRE-SELECTED LOAD CONTROL IS ACTIVE is acceptable. That is just nonsense, illogical, and completely indefensible--yet it is ("defended") all the time. Not just at your site, STLTLS, but at MANY sites around the world.

I'm going to suggest--based on the information provided and my experience--that Pre-Selected Load Control IS NOT tuned correctly. When Pre-Selected Load Control is active, and grid frequency is relatively stable (which it may not be based on the TNH deviations you provided!), L70R and L70L should not be switching, alternately, from "0" to "1," very often. And, any load swing--again as long as grid frequency is relatively stable--should be less than plus-or-minus 1 MW. And that is achievable by tuning Pre-Selected Load Control.

Now, if the grid frequency (reflected in the TNH values) is bouncing around, AND if the grid frequency oscillations are seen to be "in phase" with the IGV oscillations AND the load swings, then it could just be that the grid frequency in your part of the world (or the application--islanded load???) is not as stable as it could or should be. That may or may not be "fixable" and it may be acceptable (seems to be).

If it's not fixable and is acceptable, it's still EXTREMELY worthwhile to try canceling Pre-Selected Load control for 15 minutes or so (I PROMISE: The sky WILL NOT FALL!!!), and observing the parameters suggested. Then, before the operators, their supervisors and plant managers die of excessive heart rate and high blood pressure waiting for a catastrophe of epic proportions to inevitably occur because Pre-Selected Load Control is not active, click on Pre-Selected Load Control and re-enable it--so the 4 MW swings which are acceptable for automatic load control can begin again and all will be good in the world and everyone's job is safe and they can return to eating biryani and reading the newspaper or surfing the World Wide Web, confident in the knowledge that Pre-Selected Load Control will save their jobs and the plant.

(I'd be willing to bet SERIOUS MONEY that operators, supervisors and plant management is adamantly opposed to canceling Pre-Selected Load Control, and will most likely NOT allow this data to be gathered.)

Anyway, I need to get off my soap box and stop preaching to the unwilling and the close-minded. I've been trying for the better part of three decades, and I have been pretty universally rejected and ignored. (You'd think I would learn by now. But, I'm really an optimist at heart--even in the face of human nature and repeated personal experiences.)

I hope you will get the data requested with Pre-Selected Load Control disabled for a few minutes (one quarter of an hour!). Best of luck.

Dear CSA,

Sure we will get the data by today and will post once gathered all the data as you said.

But regarding the load swing is not often which can observe by trend, Hope it is due to the IGV variations (as i mentioned in my earlier posts).

One more thing i have to mention here which i missed in my earlier posts that we observed that the IGV variations during startups or shutdown timing where the IGV variations is no more on less load with Preselect load. during the shutdown sequence unit was on preselect load and gradually load reduced and we observed the IGV not hunting. The same observed during startup also (IGV not hunting till it reached to base load).

After reaching base load if we want to maintain the unit load on 65MW by selecting preselect load, then we are facing this hunting issue.

On lesser MW with preselect load IGV not hunting during shutdown or startup sequence. (i.e. FSNL upto 55MW)

Thanks in advance...

1 out of 1 members thought this post was helpful...

STLTLS,

>Sure we will get the data by today and will post once
>gathered all the data as you said.

How is the data acquisition coming?

One of the issues with DLN combustion systems is that there can be (and very often are) load ranges where continuous operation is not a good idea for several reasons-- including stability (of IGVs and/or load, or other parameters).

I am very unclear about the load instability you are attempting to describe. I suspect you may have uncovered something else which might be related to the situation with the data you have collected and that may be the reason for the delay in replying.

One thing you may want to check during the IGV and load "instability" is the hydraulic system pressure. If the hydraulic accumulator is properly charged and the valves are in the proper positions the hydraulic system pressure should be stable. But, if there is a problem with the hydraulic accumulator charge pressure or valve positions that could mean that when the IGVs are fluctuating the hydraulic system pressure is fluctuating which could cause issues with the fuel control valve stability which could cause load fluctuations (instability).

Again, though, it is something that is "in the nature" of DLN combustion systems that on some units at some loads they are just not very stable. Sometimes it's mechanical wear of hydraulic actuators or servo-valves; sometimes it's poor stability of the IBH (Inlet Bleed Heat) control valve--which could also be leaking. Sometimes it's one or more leaking compressor bleed valves. It can be all manner of thibgs--and sometimes it's an unfortunate combination of things, none of which by themselves would result in a problem but when combined with other stimuli and situations can be problematic.

Combustion turbines, especially those with DLN combustion systems, can be very frustrating. The IGVs, for example are used for limiting air flow into the combustors to maintain flame stability and emissions levels--but they can't be closed too far or the actual firing temperature and)or the exhaust temperature will be too high. And sometimes DLN tuning of some units can inadvertently introduce instabilities at some ambient or machine conditions.

It would be great to see the data you have collected and to hear what progress you are making on resolving the original issue, or the one(s) which have become more evident as a result of the data.

You should also be looking at Control Constant differences (particularly DLN-related values) between the two units.

Looking forward to hearing back from you and getting some actionable data to analyze and review.

Dear Sir,

Sorry for the delay response. Our plant was not operating few weeks due to some other reasons. Yesterday started, and I have collected the datas as suggested. Still the IGV angle is hunting. Please find the below observations.

During Part load CSGV was hunting around 5 deg, DWATT Hunting around 3.5MW, TTXM is 12degF, TNH is 0.2 varying, TTRF 15degF varying.

I can share the trend report. But i don't know how to attach here.

Please help us.

1 out of 1 members thought this post was helpful...

STLTLS,

The best way to share files is to post them to a web-sharing or hosting site (some people have used tinypic.com; others have used dropbox.com; and still others have used onedrive.com; and I think others have Google Drive), and then copy and paste the link to the location to a reply to this thread.

I have re-read this entire thread today, and I see you have two GTs at your site. What I can't really determine is: Is this only happening on one of the GTs or both? (Perhaps I missed this clarification; I did only quickly scan the thread.)

If this is happening on both GTs, I would say there's not too much you can do about it. You need to find the block which writes to CSRGVOUT and find the MIN SEL block that feeds CSRGVOUT and look at all the different references coming in to the MIN SEL block. The lowest of all the inputs will be passed to CSRGVOUT--so you want to see which references are the lowest and nearly equal, because what seems to be happening based on the descriptions provided is that there is at least two references that are very close to each other and the MIN SEL function is switching back and forth between them.

Unfortunately, this may be a LOT easier said than done. That's because the people at GE are using more and more macros--which are clever ways of programming that are EXTREMELY difficult to decipher what is happening in the macro. I didn't say impossible, but nearly so. It's not like the "old days" when blocks had Help files to describe what is happening inside and it was "visible" (in basic ASCII text) what was happening. Now, with macros (which can have exactly the same functionality) the way values are passed to the macro and the way the macro handles the values can be maddening to try follow signals through the macro. I guess one reason they are doing this is because with a macro one can actually see the internal calculations and determinations--and one can't see that with a block (only the inputs and outputs, not what goes on between the inputs and the outputs). But, the side effect of this is that while someone who reads and writes macros can do this, those of us who don't do it at all or rarely do it find following signals into, through and out of a macro very difficult and time-consuming.

Anyway, please post the file(s) to the World Wide Web and post the link to the file(s) here. tinypic.com is free, but the files only stay there for a few weeks (I think). Which isn't great for someone reading these threads two or six years from now. dropbox.com, onedrive.com and Google Drive may be able to leave the files there for a longer time, but eventually you or the owners of the site will purge them. Your choice, but without data there's not too much we can do.

One more thing, while there is a version of Trend Recorder that is "standalone" (that is, doesn't require a dongle) it's very difficult to find these days. When you have a Trend file open in Trend Recorder it's possible to export the data to a .csv file (which is ASCII text) which can then be imported to MS-Excel and graphed and analyzed. It would be good if you post both versions of the data (in the .trn file format, and in a .csv format as well).

Even with the CSRGV MIN SEL block info, it still just might be that there's nothing which can be done. GE might be able to recommend some kind of Control Constant change which might slow down the oscillations, but maybe not, also.

One thing I did notice from the information in the thread, is that the turbine speed is drifting from 99.7% to 100.3%. If the units are tied to a grid and operating in Droop Speed Control mode (NOT Isochronous Speed Control Mode) that means the grid frequency is not stable--and THAT could be at least partially affecting the IGVs, AS WELL AS A PARTIAL cause of the load swings you have described.

Does the speed also fluctuate when the units operate at Base Load?

At any rate, we need data to be of much more help.

Dear CSA,

Thank you for your valuable suggestions. The IGV hunting is applicable for our both units. The variation of IGV is not stable some time very slow variation accordingly the values also are varying. Pls find the below link for the data's collected.

Trend Data's:

https://www.dropbox.com/sh/266296ihm3lta8g/AAAtq1VNASfQsSR_sYzuOQ3Na?dl=0

Regarding the block will check and will confirm in the next thread..

Thank you again...

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STLTLS,

I hope to return home on Tuesday, and I will be able to download the files and analyze them then.

Please tell us if the grid frequency is as unstable as the data you have provided previously suggests 99.7%-100.3%)?

Are your units synchronized to a "large" national or regional grid, or to a smaller, independent grid with some kind of power and/or load and/or frequency management system? Are there transformers between the units, or are they connected to a common bus which is then connected to a grid via a transformer?

Again, it's entirely possible that the IGV oscillations your units are experiencing are entirely normal for the conditions at your site. If grid frequency is oscillating that will cause two things to happen. First, the air flow through the axial compressor will oscillate, and because the IGVs are being used to control exhaust temperature and combustion reference temperature (TTRF1), and varying air flow will cause varying exhaust temperatures and a varying TTRF1 and the IGVs will be responding to that.

Second, the varying speed will cause Droop Speed Control to try to change load to try to support grid stability. BUT, if Pre-Selected Load Control is active that will fight Droop Speed Control to try to maintain the load setpoint (reference) and cause load swings.

It all will make for a pretty "messy" operation. That's another reason why continuous operation with Pre-Selected Load Control active is a really poor idea (in contrast to false, and very popular belief). That's why you were asked to get data with Pre-Selected Load Control disabled--and while you may have done that I won't be able to see the data files for a couple more days at least.

But, I'm concerned you haven't responded to the question about grid frequency. It makes me think you are of the opinion that it is irrelevant.

I'll let you know when I get a good look at the data.

Dear CSA,

"Please tell us if the grid frequency is as unstable as the data you have provided previously suggests 99.7%-100.3%)?"

Reply: Grid Freq is stable what we understood from the HMI and GCP panel Digital Frequency meter.

"Are your units synchronized to a "large" national or regional grid, or to a smaller, independent grid with some kind of power and/or load and/or frequency management system?"

Reply: It is connected National / State Grid.

"Are there transformers between the units, or are they connected to a common bus which is then connected to a grid via a transformer?"

Reply: No. After the Generator Transformers following to the common bus and to connected to the grid

Thank you in advance.

STLTLS,

Thank you for the information.

>"Please tell us if the grid frequency is as unstable as the
>data you have provided previously suggests 99.7%-100.3%)?"

>Reply: Grid Freq is stable what we understood from the HMI
>and GCP panel Digital Frequency meter.

Frequency and turbine/generator speed are DIRECTLY related. 100.00% TNH (speed) would be equal to 100.00% frequency, and if the grid frequency was 50 Hz then the grid frequency would be 50.00 Hz at 100.00% TNH.

If TNH was 99.7% then grid frequency would be 99.7% of 50.00 Hz--49.85 Hz. And, if TNH was 100.3%, then grid frequency would be 50.15 Hz.

Droop Speed Control would be trying to change load to compensate for speed (frequency) disturbances of 0.3%, and that would cause an interaction with Pre-Selected Load Control and that would lead to load swings (which you have reported occur during the IGV oscillations).

Did you mean 99.97% TNH and 100.03% TNH?

1 out of 1 members thought this post was helpful...

STLTLS,

I have begun my review of the data you provided--there was a LOT of data, including Control Constants. The grid frequency (SFL1), and therefore the turbine speed (TNH), are varying throughout the data. In my very quick review, I have seen SFL1 as high as 50.031 Hz, and as low as 49.872. This is going to have an impact on load--especially when Pre-Selected Load Control is active--and IGV angle (because as compressor speed changes so does air flow through the unit--and that's going to affect a lot of parameters (including TTRF1, TTXM, etc.).

A very large portion of the file is for loads of approximately 54-55 MW.... Not very much data is recorded for loads around 65 MW--which is the point at which I think you were complaining about IGV oscillations.

My first question is: ctifr is usually a measurement of the ambient air temperature entering the inlet filter structure (using an RTD mounted outside the structure. CTIM is usually the median (if I recall correctly) of three T/Cs mounted in the inlet plenum walls near the bellmouth. For most of the data there is a very large discrepancy (10 deg F or so) between the two readings, and it is pretty consistent (ctifr hovers around 81 deg F, and CTIM hovers around 91 deg F).

Usually this means there is some air from the Inlet Bleed Heating system warming the air entering the bellmouth. But, from the data it seems the signal to the IBH Control Valve is, essentially, 0 (CSRIH; csrihout). This is just odd, and while I don't think it has anything to do with the problem you are asking about--it's still odd there is a 10 deg F difference between outside ambient air temperature and the air temperature entering the axial compressor bellmouth. I have seen sites where the ctifr RTD was in the direct sun, and it was hotter than it should have been, and changed readings as the sun went down and then rose. But, CTIM was LESS than ctifr in most cases and at most times of the day.... In this case, ctifr is LESS than CTIM....

Anyway, I also noticed that there were times when csgv was close to 86 DGA and the load was still around 62 MW.... That seems pretty odd. But, I'm still going through all the data (a LOT of it).

Another thing I noticed is that the servo currents being applied to the SRV (Stop-Ratio Valve) are hugely unbalanced--stable, but unbalanced. FAGR_NVR hovers around +6.5, FAGR_NVS hovers around -7.1, and FAGR_NVT hovers around -8.4. This suggests there is some kind of wiring problem with the three servo coil circuits--at least one of them is wiring wrong and that is applying the wrong polarity of servo current to that coil, and the other two are working very hard to try to overcome the incorrect current polarity. This ISN'T affecting the problem you are asking for help with, but it's not a good condition.

Again, I'm working on trying to sort the data and come to some kind of conclusion. Last question: Does this unit have Model-Based Control (sometimes referred to as ARES)?

Dear CSA,

Thank you for your time to time response on our queries which is really making us to feel more strengthen.

Regarding the CTIFR and CTIM difference we are also noticed and we found that CTIM cable is installed with non compensate cable(Non K type T/C extension cable). So already we laid the K type compensation T/C cable till the JB and our team will replace the cable soon for that and we can see the diff between the CTIFT and CTIM.

Regarding the SRV recently during last shutdown we did calibration for SRV and all the servo valves which was found normal (including polarity checks). But still we will also make sure again for the high current reading as you suggested during the next soonest opportunity.

Model Based Control - really i dont understand (sorry I don’t have idea about that what you are pointing about).

Thanks again in advance..

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STLTLS,

Warning on CTIM cable replacement: Make absolutely sure the sensors are thermocouples! Back before I retired, GE was using RTD's for these sensors. RTD's need copper interconnecting wires. They were typically 3-wire RTD's and all 3 wires must be used for accurate measurements.