Dear expert Friends

I hope this message finds you well.

I would like to bring to your attention an operational issue we have observed with our GE turbine. The turbine is transitioning out of the preselect mode when the gas pressure drops below 22.5 Bar. This change in operating mode is causing concern as it may affect the efficiency and performance of the turbine, especially under conditions where the gas supply is not optimal.

After a thorough assessment, we would like to seek your expertise on the possible solutions to mitigate this issue. Specifically, we are interested in understanding the potential adjustments that can be made to prevent the turbine from exiting preselect mode under low pressure conditions. Could you advise on the appropriate corrective actions or modifications to the system that would help maintain the turbine in the preselect mode, even if the gas pressure temporarily falls below 22.5 Bar?

Furthermore, we are considering the option of simulating the deactivation of the non-disengagement factors that currently trigger the transition from preselect mode. However, we are concerned about any potential consequences that could arise from removing these factors. Could you please clarify what risks or operational problems might emerge from this simulation? Specifically, we would like to understand the impact on the turbine’s safety, efficiency, and long-term performance, as well as any other system-wide effects that may need to be considered.

We appreciate your prompt attention to this matter and look forward to your insights and recommendations on how to address these concerns effectively.

Thank you for your support.

Best regards,
[Abed Akbar]


[[email protected]]

 

@abed.a,

You haven't told us enough about the machine (Frame size; type of combustors (conventional; DLN; or ???), the turbine control system, and what alarms are being annunciated prior to and after the machine "aborts" Pre-Selected Load Control. We also don't know what the speed of the machine is doing when the gas supply pressure is less than optimal, and what the frequency of the grid/system the machine is synchronized to is doing--is it stable, is it unstable, is it fluctuating a lot above and below rated.

The ability of the machine to produce electrical power is directly related to the fuel supply--in this case natural gas. If the gas supply pressure is low then that's going to limit the machine's ability to produce electrical power, because as the gas fuel supply pressure drops below a setpoint--sometimes it's the setpoint of one or more pressure switches, sometimes it's the value of gas fuel supply pressure input to the turbine control system from one or more pressure transmitters.

If the turbine control system sensed low gas fuel supply pressure and alarmed the condition on the operator interface it's incumbent on the operator to take some kind of action. If the gas fuel supply pressure continued to drop at some point the machine load (electrical power output) is going to start decreasing, and the Stop-Ratio Valve (SRV) is going to start opening excessively. If the machine is operating on Pre-Selected Load Control at some point the gas control valve(s) will also start to open excessively to try to maintain load. If, for some reason, the gas fuel supply pressure were to suddenly increase it's entirely possible the machine would trip on exhaust overtemperature. If the gas fuel supply pressure were to continue decrease, the load would continue to decrease and at some point there won't be enough fuel to maintain flame in the combustors and the machine will trip on LOSS OF FLAME.

It's entirely understandable the machine might be programmed to drop out of Pre-Selected Load Control to protect the machine from either of the conditions above. If an operator took no action after the turbine control system dropped out of Pre-Selected Load Control and the gas fuel supply pressure continued to drop it's still possible the machine might trip on LOSS OF FLAME.

But what should be obvious is: The machine can't continue to make power, whether or not the machine is in Pre-Selected Load Control if the gas fuel supply pressure won't support maintaining load. Even if the machine remained in Pre-Selected Load Control if the gas fuel supply pressure won't support that load, the machine can't produce that power. Full stop. Period. The generator converts torque into amperes to supply electrical power to the grid/system it is synchronized to (and we are presuming the machine is synchronized to some kind of grid or system with other machines--you haven't told us if it is or isn't). The torque comes from the gas turbine, produced by burning a combustible fuel. If there's not enough fuel there won't be enough torque and there won't be enough amperes. And, if the fuel supply pressure gets too low, the machine is going to lose flame--and trip on LOSS OF FLAME.

From your description and questions it seems you are trying to force the machine to produce electrical power when there's not enough gas fuel to do that. We don't have enough information to make any real kind of assessment, much less a thorough one. We would also need to see the programming in the turbine control system to ascertain what precisely is happening. Without alarm information and some other very important information there's not a lot more we can offer. If you want to make electrical power, you need to have a proper source of gas fuel. You could force the turbine control system NOT to reccognize low gas fuel pressure--but that's not going to allow the machine to produce as much electrical power as it could, regardless of the Pre-Selected Load Control setpoint. The turbine control system is programmed and configured with the presumption that the gas fuel supply pressure is a per the macine's specifications (spelled out in the Control Specification supplied with a Mark* turbine control system in Sect. 05.02.nn). If the fuel ain't there, the turbine control system can't make up for lack of fuel. Even if it's operating in Pre-Selected Load Control.

Your best course of action if the machine must produce nearly rated electrical power is to resolve the issue of low gas fuel supply pressure. Or, explain to the purchasers of the electrical power that there's nothing that can be done if the gas fuel supply isn't as per specification. The turbine control system can't make fuel. All it can do is control the flow of fuel. And if the fuel supply won't support desired loads when the fuel control system is essentially "wide open" (a dangerous condition, by the way) then there's nothing to be done. You can simulate, force, beg and cajole the turbine control system--but if there ain't no fuel there ain't gonna be much electrical power. The effect of fooling the turbine control system could be catastrophic, and dangerous. And depending on the machine and the combustion system it could cause some serious damage.

And, that's that.

 

Thank you for your attention. The turbine in question is a Mark V Frame 9F DLN 1. It operates in synchronous mode and is currently in DROOP mode.

My concern pertains to a gas pressure drop within the range of 21 to 22 Bar. While I understand the relationship between power generation, gas flow, and flame-off, the issue is the pressure drop of 1 to 1.5 Bar, which causes the turbine to exit the preselect mode and enter a floating state.

In this floating state, the turbine remains controllable using the Raise and Lower commands. However, my question is how to maintain the preselect mode within this gas pressure range.

Thank you again for your guidance.

 

@abed.a,

First, you should really take up your issue with GE Belfort as they have responsibility for the programming of the turbine control system for GE-design Frame 9 heavy duty gas turbines.

Second, when the turbine control system "aborts" Pre-Selected Load Control mode, what ACTUALLY happens to the load? Does it decrease--if so, by how much?

Does it increase--if so, by how much?

WHAT IS HAPPENING TO THE GRID/SYSTEM FREQUENCY AT THE TIME THE SITUATION YOU ARE DESCRIBING TAKES PLACE?

I don't want to go into how Droop Speed Control works (it's been done so many times on Control.com it should be renamed to DroopSpeedControl.com). But, Pre-Selected Load Control operates "on top" of Droop Speed Control and only adjusts the turbine speed reference if the load drifts from the Pre-Selected Load Control setpoint when Pre-Selected Load Control is enabled and active. Under normal operating circumstances when Pre-Selected Load Control IS NOT enabled and active load DOESN'T drift, or if it does it drifts negligibly and only as system frequency drifts.

Somehow people (power plant operators and their supervisors) have come to believe that the machine should ALWAYS be in Pre-Selected Load Control (enabled and active) or the machine load is going to drift wildly and uncontrollably. And that is simply not true. Full stop. Period.

If, when the turbine control system "aborts" Pre-Selected Load Control the machine load changes negligibly (less than a MW or two for a 9F machine) that's normal. If you and or the power plant operators and their supervisors will just try this simple test you can prove it to yourself. The next time the turbine control system "aborts" Pre-Selected Load Control (for whatever reason) do nothing. That's right--just sit and watch and pay attention to what happens to the machine load. (I am presuming--because you haven't answered the question--that the grid/system frequency is stable and not changing by more than a few hundredths of a Hertz, which is normal for most grids/systems.) It would be best if you would use some data capture method (Trend Recorder; Trender; VIEW2; Historian) to monitor grid frequency and machine speed and machine load when Pre-Selected Load Control is "aborted" for whatever reason so that you have actual data (not verbal, anecdotal data) to review and analyze. Unless there is something very atypical about the machine and its operation/configuration at your installation you will most likely find that the load is very stable--maybe even MORE STABLE than when Pre-Selected Load Control is active! Do this for as long as people can stand the terror and risk of leaving the machine to operate without Pre-Selected Load Control disabled. Unless the installation also provides power to a very large "captive" user nearby that causes large load changes/swings and the grid/system frequeny remains relatively stable you will prove that it's not really necessary to use Pre-Selected Load Control continuously at all times the machine is running. It's as simple as that. If the powers that be--and the operators--can be persuaded to let the machine operate in Droop Speed Control WITHOUT Pre-Selected Load Control enabled and active.

It will never happen, of course, because EVERYONE knows that what I'm suggesting will lead to serious machine damage and possible personnel injury--but worst of all, possibly someone losing their job for actually trying this test for 5 or 10 or 15 minutes. The fear and trepidation of the unknown risks of doing this simple test are just too much for most people. (I'm joking, of course, about the possible outcomes of such a test--but I have argued long and hard many times in many places around the world about just trying this test. And the result is ALWAYS the same: No.)

Grow some balls. Do the test. Give yourself and your plant personnel some new information to make decisions about how to operate the plant.

Finally, if it's true that the turbine control system is "aborting" Pre-Selected Load Control when the gas fuel supply pressure drops by as little as 1 or 1.5 bar, then it seems to me that means the gas fuel supply pressure to the plant is at the very lower limit of the specification. Without understanding the situation and without being able to review the programming and configuration of the turbine control system about the only reason I could offer for automatically canceling Pre-Selected Load Control when the gas fuel supply pressure drops by as little as 1 to 1.5 bar is that it's known and recognized that the gas fuel supply pressure is marginal under normal circumstances and in order to protect the combustion system (which is more than likely some version of DLN (Dry Low NOx) combustion system) the turbine control was programmed to cancel/abort Pre-Selected Load Control.

All that canceling/aborting Pre-Selected Load Control means is that the human power plant operators need to be slightly more diligent in watching the machine and how it is operating. Unless, there are other factors about the installation that we are unaware of--and can't be aware of unless we are told. Operating without Pre-Selected Load control enabled and active is not going to do anything to damage the machine or the combustion system. And, operating with Pre-Selected Load Control continuously IS NOT ABNORMAL--contrary to popular, and entirely false, opinion.

Talk this over with GE Belfort engineers. There's either something unusual about your installation that we don't know about or they have some other reason for this bit of "logic" that we don't know about--and only they can help you if you insist that continuously operating with Pre-Selected Load Control enabled and active is necessary and non-negotiable.

 

Thank alot