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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.