Compressor Discharge Temperature limit (gas Turbine)


My plant has 2 gas turbines with a single-shaft one on one configuration with the same machine. I have a problem with No.1 gas turbine combined cycle load can't reach base load. I noticed the compressor discharge temp of gas turbine No.1 was limited by 510 deg-C and IGV position lower than No.2 gas turbine.

first I need to know the purpose of CTD limit control.
second, why does No.1 not increase IGV position for reach base load.

Ps. No.2 gas turbine IGV position open more than No.1 gas turbine.

The purpose of Compressor Discharge Temperature (CTD) limit control is to ensure that the temperature of the air leaving the compressor does not exceed a safe threshold, preventing potential damage to the gas turbine components. Gas turbine No.1 may not increase its Inlet Guide Vane (IGV) position to reach base load due to various factors such as operational constraints, equipment limitations, or control system settings. Further investigation is needed to determine the specific cause in this scenario.

Firstly, I've never seen a GE-design heavy duty gas turbine with CTD limit--doesn't mean there isn't such a thing, just that I have no experience with it.

So, I gotta believe this is used on one of the new HA machines, for which GE Belfort seems to have responsibility for the control schemes used on these machines (so it's probably a 9HA). It's possible that limiting the air flow through the machine by restricting the IGV opening will reduce the CTD--but one would think that there would be some kind of alarm or indication to the operator(s) that an operational limit had been reached. (CPR-biased exhaust temperature is not alarmed--but it IS noted in the STATUS area of the Main Display.)

Having just written the above, if I recall correctly there is no alarm for Anti-Icing Control--which limits machine output--so maybe the thinking is similar: no alarm is required. BUT, I can tell you for 100% certain, when Anti-Icing Control is enable and the machine output is limited with no obvious indication or alarm Plant Managers and owner's accountants get VERY angry when something is limiting the machine's ability to generate revenue--again, with no obvious indication. (In my mind, it IS worthy of an alarm to alert the operator and his supervision the function is enabled and is the cause of the reduced MW output.)

And, if this is happening on one of the new HA machines, one would certainly think the GE M&D (Monitoring & Diagnostic) Center would want to know this and would be alerting the operators and their supervisors of this condition (CTD Limit is active and responsible for limiting machine output). Because it would seem GE would definitely want to know this condition is occurring because it's obviously something they don't want to occur because if the machine is under warranty they are "on the hook" for repairs if something serious or catastrophic happens.

This may be part of MBC (Model-Based Control), or what GE calls ARES (Adaptive Real-time Engine Simulation). Which GE considers proprietary and does not provide much, if any, information about. ARES uses a LOT of extra instrumentation and it's possible that one or more sensor(s) is(are) not working properly. I would be looking at Diagnostic Alarms, and other possible Process Alarms, to see if one or more sensors are not healthy--and if these sensors are related to CTD Limit.

AND, it's pretty certain if it's one of the new HA machines that GE has a virtual connection to the control systems and probably has a CSA (Control Services Agreement) with the owners/operators of the equipment and can log in to the control system to see what's going on and report their findings--as part of the CSA. Even if it's not an HA machine, it's still very likely GE can log in from a remote center and have a look at things--if they're not continuously connected.
Thank you for your answer.

Certainly, It's an HA machine. and I have never seen a CTD limit before as you mentioned.
For GT-1 has a combustion dynamic problem,
OEM slightly increased OTC to solve this problem. After that, we found this CTD issue in the first outage OEM has changed the CTD guide tube but that can't solve the CTD temp. high problem.

I would like to know how to investigate the root cause of the problem. Whether it's caused by CTD instrument issue or a compressor itself.

Details. Details. Details. You are working on a very technical piece of equipment, and you are asking for help for what is a relatively new machine on the market and there probably ISN'T a lot of experience with the HA machines out there just yet.

Please--what is OTC? (You say the OEM increased it in response to the combustion dynamic problem.)

What are the readings of the redundant axial compressor discharge temperature thermocouples?

WHAT ARE ALL THE PROCESS ALARMS & DIAGNOSTIC ALARMS PRESENT ON THE MACHINE? (You can use the camera on your smartphone to take photos of the WorkstationST Alarm Viewer pages. Then you can post CLEAR photos of the entire width of the pages to this thread. You can obscure the site name if you prefer. This is probably a LOT easier than posting individual alarms to this thread.)

WHAT HAS THE OEM SAID ABOUT THE PROBLEM? (Again, it's probably pretty certain the OEM can remotely access the control systems and that the site has regular communications with the OEM about the condition of the machines. It should be relatively easy for someone (probably in supervision or management) to open a support case with the OEM to get help with this problem--IF the OEM hasn't already provided some sort of troubleshooting procedure.)


HAVE YOU LOOKED AT THE HMI IGV CONTROL PAGE TO SEE IF IT PROVIDES AND INDICATION ABOUT WHAT'S HAPPENING (it probably shows the "median" of the CTD thermocouples, which is usually derived from the redundant thermocouple values). If you use ToolboxST you can look up the values of all of GT1's CTD thermocouples and compare them.

If GT1 has combustion problems, it's entirely possible that either the control system, or the OEM, has limited GT output to protect the machine. You say the OEM has made some adjustments/modifications to the control system--this may have been one of them.

I've also been giving this perceived problem a good deal of thought.... You say that GT1's power output is "limited," as opposed to GT2's power output (which I presume is at Base Load). You say the IGV angle of GT1 is less than the IGV angle of GT2. This goes along with the idea that the IGV angle of GT1 is less than the IGV angle of GT2--not as much air is entering the axial compressor of GT1, meaning less work is being performed by the axial compressor of GT1, therefore less heat is being generated because the axial compressor discharge pressure of GT1 is lower. BUT, you haven't told us what the CTD of GT1 and GT2 are under this condition ("CTD Limit"). You seem to be saying that the CTD of GT1 is lower than the CTD f GT2 and that therefore GT1's output is being limited by something restricting the CTD of GT1 (that is, that something in the control system is keeping the CTD of GT1 lower than GT2--when GT1 IS NOT operating at Base Load, and GT2 IS operating at Base Load). And, as I wrote above, per my experience CTD increases as IGV angle increases so it makes sense to me that if the IGV angle of GT1 is lower than that of GT2 the CTD of GT1 will be lower than that of GT2--without any "limit" being imposed on CTD.

I don't know about these new-fangles HA machines, but in the past several FSRs (Fuel Stroke References--which is another way of describing the called-for fuel flow-rate reference (valve position was always proportional to fuel flow-rate) were ALL being CONTINUOUSLY calculated ALL the time (whether the machine was running or not!) and these all fed into what was called a Minimum Select function (block or gate) and the LOWEST value of all of the inputs was selected to be in control of the fuel control valve positions (which controls the flow-rate of fuel into the combustors of the machine). If I was on site, I would be using ToolboxST to be looking for a similar block/function to find out what is the lowest value of fuel flow-rate reference and then find out why it is the lowest value of fuel flow-rate reference (it may be called FSR in the HA machines, but knowing the penchant of GE Belfort for changing things (ANYTHINGS) which have been used for decades and have been working fine just because it wasn't invented in France the machine fuel flow-rate reference (or valve position reference) may be called something entirely different. Just because they can (change it).

It may all be hidden in the ARES (MBC) block(s)--who knows.

I'm now of the opinion there IS no such thing as "CTD Limit" over-riding control of the machine. It seems that someone has just looked at the CTDs of the two machines--operating at DIFFERENT setpoints--and decided because the CTD of one is lower than the CTD of the other that "CTD Limit" is active. I find it difficult to believe that CTD is so critical to the operation of the machine that it's necessary to have a special control function to keep it from exceeding some value--that may be a huge mistake on my part, again--I have no experience with HA machines so I may be (I probably am) wrong. But in my career (and in my life) I am always trying to understand things by using my experience and knowledge to try to understand how things are done. Sometimes, I can find a YouTube video (for example, showing how crinkle-cut french fries are made...!), or find some reference material or white paper on the World Wide Web. (I often read instruction manuals, too--I know; most people don't read the instruction manual UNTIL there is a problem and find out the equipment has been configured properly or is being misused. I want to know that BEFORE I commission the equipment--silly me.)

Anyway, @totogt, I still want to know how you determined that something in the turbine control system is limiting CTD and causing an inability to select Base Load or manually load the machine above it's current operating condition. I really want to know how you arrived at this conclusion. I may be wrong, but I am unafraid to be wrong, because I always learn something when I'm wrong (often I learn many things when I'm wrong). And I like learning things. (It makes me less wrong in the future!)
Thank you for your advice.

GT can't reach load demand set point from the dispatching center
and here I have some data to share on how I point to the CTD limit.



IF GE Belfort followed standard practice with the algorithmic blocks the signal names on the right are outputs of the block. The MBC in the signal names you have highlighted lead me to believe it's something to do with MBC (Model-Base Control). MANY times the signals at the right of the block are logic signals indicating something has "happened"--such as CTD Limit, and those are used to either trigger a Process Alarm or some indication in the Status field of the HMI Main Display (a box with text descriptions of things like, Part Load or Base Load, or Sequence in Progress, Temperature Control or Cooldown--things like that).

But, you're digging into the application code in ToolboxST and that's the place to dig and follow signals around. It's Greek to me because they seem to be using some new mnemonics--and, with so many signal names redacted and these damn "tile" block representations (which don't graphically depict what's happening in the blocks) are maddening to me, as well and without being able to see any Longname descriptions for the signals I can only make SWAGs (Scientific Wild-Arsed Guess) at what the signal is or does.

I'm sure you already know this, but you can right-click on a block and a windows will open and one of the choices should be 'Item Help' or 'Block Help'. If the selection is NOT grayed-out then it means if you right-click on the selection some kind of explanation of the block's functionality will be displayed. Sometimes there is useful information, and even graphical representations of what's happening inside the block. Other times, it's all gobbledy gook and one can't discern anything useful from it.

If the selection is grayed-out, then it means there is no block help available (there's FAR TOO MUCH of that going on in Mark* VIe these days!!!). Sometimes it can mean the block is actually a macro, and by clicking on the name of the macro in the left window pane of ToolboxST the macro will miraculously open and you can then see the internals of the block--but, one then has to be prepared to do a lot of sketching and tracing back to inputs and outputs to see what each signal does. But, the information is there; it just takes a little more time and effort (okay; a LOT more time and effort). But, it can be sorted--if it's a macro. If it's not a macro and no one wrote a description for the block, they you are completely out of luck in understanding what's happening.

Without a lot more information, that's me. I'm done. I can't offer anything more. I wish you the best of luck in your endeavor. The OEM don't make it easy. And since MBC is considered proprietary they publish very little, if anything, about the actual inner workings of the application code.