Our unit MW output drop by 6-8MW within 1 minutes after the unit go into temperature control. In our opinion, the unit should be able to produce a higher load at baseload instead of partload.

We have done a few recommendations from the OEM but the problem remains there after each of the tests we conducted and not showing any improvement at all. We have done the following:

1. Perform offline compressor wash.
2. Perform calibration on the prformance instrumentations.
3. TTKGVTCDG corrected from 0.65 to 0.35

At one stage the OEM told us that the exhaust Thermocouple lag could be the case. In other words, the TTXM will take some time to pickup the increase in the exhaust temperature and as it picks up the FSRT is reduced accordingly. However, we proved to them that this is not the case. We did the test by raising the MW slowly at 0.5MW/minutes and when we hit the baseload the unit output just dropped by the same amount.

What is puzzling us, there are many department in the OEM and they are giving contradicting answer, some agree that this is abnormal and some say it is normal. We did ask from the OEM if this phenomenon is common accross the fleet and provide us with the information, but they never replied to this specific query...at least until today.

Recently they suggested to change TTKXCOEF_1 from 0.98 to 0.99. We have not done this yet. Also there are some talks I heard from the OEM on the temperature control curves.

Appreciate if anybody can share experience on this. I welcome any suggestion and opinion from all the expert in this forum.

 

Do you have DLN 2.0 or 2.6 or ???

Are you using Water- or Steam Injection for emissions reduction or power augmentation?

Do you have just one unit, or others, that is experiencing this problem? How long has this problem been persisting--did it just start or has it been the case since start-up/commissioning?

What's happening to TTXM just prior to and after reaching Base Load when the power output is decreasing? Have you tracked IGV angle and TTRXGV (IGV Exhaust Temperature Control Reference) with respect to loading and just after reaching Base Load? Have you tracked TNR with respect to loading and just after reaching Base Load? Have you tracked TTRX, TTRXP (Primary Exhaust Temperature Control Reference), and TTRXS (Secondary Exhaust Temperature Control Reference), with respect to loading and just after reaching Base Load? Have you tracked TTRF1 (Calculated Combustion Reference Temperature) during loading and immediately after reaching Base Load? Is the unit using Quaternary Gas Fuel during Premix Steady-State Operation? Are they any Fuel Split changes taking place at the point when the unit is "transitioning" into Base Load? What's happening to your emissions, Mr. Xtranox, when you reach Base Load--are they going up or down from their level just prior to Base Load, when you are loading very slowly (presuming you have DLN combustors)?

Have you tracked CPD and AFPAP and AFPEP and CTDA during loading and after reaching Base Load? Have you tracked FSR and Fuel Flow-rate after reaching Base Load? Is something driving fuel flow-rate down (decreasing FSR) after reaching Base Load besides FSRT? Have you tracked all the inputs to the FSR Min Value "Gate" just prior to and after reaching Base Load to see if anything other than FSRT is driving FSR down after reaching Base Load?

IN GENERAL, when loading a unit up to CPD-biased Exhaust Temperature Control ("Base Load") the unit should load smoothly and reach nominal power output without experiencing the kind of drop you are citing. 6-8 MW out of a nominal 220 MW (or thereabouts for a GE-design Frame 9 F-class gas turbine) is about a 3% drop, and that does seem excessive. T/C lag can usually cause a 0.5-1.0% decrease in power output when first reaching Base Load as temperatures and T/C feedback stabilize.

Again, as a unit is loaded the TTRX and FSRT values should decrease as TNR and FSRN values increase; as FSRN and FSRT approach each other and FSRN exceeds FSRT the unit should go on CPD-biased Exhaust Temperature Control (NOT MW- or FSR-biased exhaust temperature control--i.e., TTRXS should always be just slightly above TTRXP). The "transition" from Droop Speed Control to Exhaust Temperature Control should be smooth and "bumpless." (Again, T/C lag can cause a small decrease from the value first observed when reaching Base Load.) TNR and FSRN should continue to increase after reaching Base Load until a preset difference between FSRN and FSRT is reached, and then FSRN is adjusted to maintain that difference as ambient conditions change. The idea is to keep FSRN "up and out of the way" of FSRT so that when the unit is being operated at Base Load the unit is not always going in and out of Droop Speed Control and Exhaust Temperature Control.

There are lots of unknowns here and several presumptions (since this author doesn't like to assume things (assumption makes an ass- of -u- and -me, ass-u-me, pun intended!)): Droop Speed Control is being used to load the unit; the unit is NOT operating on Back-up Exhaust Temperature Control at any time; there is no active external load control signal on the unit(s); all fuel splits have been properly calculated (by the OEM and/or its representatives) during emissions testing; there are no unusual Exhaust Temperature Spreads or a high number of failed- or jumpered exhaust T/Cs; the unit is operating on natural gas fuel; lastly, there are no "unusual" control schemes being used (some kind of fuel mixing/calorific control algorithm, compressor extraction, etc.).

Another thing which comes to mind is: Is the grid frequency stable--i.e., does it not vary by more than 0.1% or so? (Even that's a "large" amount if we're talking about a very large, "infinite" grid; if the grid is much smaller and some kind of AGC (Automatic Governor Control) is being used to control frequency, then it should be visible in the TNR/FSRN variations at Base Load.)

Lots of questions here, and lots of unknowns. Are there any Diagnostic Alarms? Don't be overwhelmed by the questions; try to provide the information as best as you can, telling us what you've done and what you haven't.

What we're trying to determine is: Is something happening after reaching CPD-biased Exhaust Temperature Control that is causing FSRT and TTRX to decrease--which would cause the power output to decrease? This author has seen several sites which did not have properly calculated CPD-biased and MW- or FSR-biased Exhaust Temperature Control Constants (this is presuming the unit has MW or FSR Back-up Exhaust Temeprature Control).

Lastly, what turbine control system is in use? (The presumption has been that it's a GE SpeedTronic turbine control panel with typical control signal database pointnames...)

markvguy

 

Mr Markvguy, thanks for the reply on this. I try to answer as accurate as I can but let me know if you need more input from me. I have marked each question as Q: and my reply as A: . Hope this can make it easier for everybody to follow the issue. TQ.

Q: Are you using Water- or Steam Injection for emissions reduction or power augmentation?

A: We are not using the water or steam injection for emission reduction or power augmentation, our machine is Single Shaft Combined Cycle MS9000FA with DLN2.0+ combustion system. The rated output is 225 MW for GT and 125 for ST.

Q: Do you have just one unit, or others, that is experiencing this problem? How long has this problem been persisting--did it just start or has it been the case since start-up/commissioning?

A: We just have 1 unit and we never have feedback of similar problem anywhere else. We just noticed the problem 2 years after commissioning. Exactly when is unknown and the historian does not capture “baseload” status which make it difficult for us to find the transition points. In addition, during the first 2 yrs of operation, the machine rarely hit the baseload at 350MW. For info, due to how they structure the power purchase agreement, we only dispatch the machine to our declared available capacity at 350MW – producing more than that is not economical for us. As the machine degraded (degradation curve is 4% /yr for combined cycle), we are more frequently hitting the baseload and now at even lower output than 350MW.

Q: What's happening to TTXM just prior to and after reaching Base Load when the power output is decreasing?

A: TTXM was at 617degC and increased to 621degC within 30s after the switching to FSRT. 15 s later the TTXM drop to 620degC and maintained at this point throughout the test.

Q: Have you tracked IGV angle and TTRXGV (IGV Exhaust Temperature Control Reference) with respect to loading and just after reaching Base Load?

A: No change to IGV opening. IGV was maintaining max open at 86dga.

Q: Have you tracked TNR with respect to loading and just after reaching Base Load?

A: Starting at transition point, the TNR increased by 0.1% from 104.04% to 104.14% in 30s and maintained at 104.14% +-0.03 thereafter. The LK90MAX is set at 390MW.

Q: Have you tracked TTRX, TTRXP (Primary Exhaust Temperature Control Reference), and TTRXS (Secondary Exhaust Temperature Control Reference), with respect to loading and just after reaching Base Load?

A: The TTRXB is following TTRXP and TTRXP is always lower than TTRXS before and after the baseload.

Q: Have you tracked TTRF1 (Calculated Combustion Reference Temperature) during loading and immediately after reaching Base Load?

A: TTRF1 slowly increase during loading until at 1335degC it switchover to baseload. 6seconds after reaching baseload the TTRF1 increased to 1341degC and maintained at 1341degC +-1degC.

Q: Is the unit using Quaternary Gas Fuel during Premix Steady-State Operation?
A: Yes

Q: Are they any Fuel Split changes taking place at the point when the unit is "transitioning" into Base Load?

A: No. The unit is always running on NG at the transition (we are dual fuel unit) and the we always at Premix Mode from 1232degC TTRF1 (about 180MW Combine Cycle).

Q: What's happening to your emissions, Mr. Xtranox, when you reach Base Load--are they going up or down from their level just prior to Base Load, when you are loading very slowly (presuming you have DLN combustors)?

A: I do not have info on this. I will come back on this when data is available.

Q: Have you tracked CPD and AFPAP and AFPEP and CTDA during loading and after reaching Base Load?

A: CTDA maintained at 409degC. AFPAP maintained at 29.7inHG. There is noticeable drop in CPD by 0.14bar within 37s after reaching baseload (CPD prior to baseload was 14.45bar). AFPEP - I do not have info on this. Will provide when data is available.

Q: Have you tracked FSR and Fuel Flow-rate after reaching Base Load?

A: FSR always follows the FSRN at partload and immediately switch to FSRT at baseload. No info on Fuel Flowrate – Will provide when data is available.

Q: Is something driving fuel flow-rate down (decreasing FSR) after reaching Base Load besides FSRT?

A: No. FSR drop due to FSRT.

Q: Have you tracked all the inputs to the FSR Min Value "Gate" just prior to and after reaching Base Load to see if anything other than FSRT is driving FSR down after reaching Base Load?

A: I did not capture all in view2 but based on the data, the FSR exactly matched with the FSRT.


IN GENERAL, when loading a unit up to CPD-biased Exhaust Temperature Control ("Base Load") the unit should load smoothly and reach nominal power output without experiencing the kind of drop you are citing. 6-8 MW out of a nominal 220 MW (or thereabouts for a GE-design Frame 9 F-class gas turbine) is about a 3% drop, and that does seem excessive. T/C lag can usually cause a 0.5-1.0% decrease in power output when first reaching Base Load as temperatures and T/C feedback stabilize.

Again, as a unit is loaded the TTRX and FSRT values should decrease as TNR and FSRN values increase; as FSRN and FSRT approach each other and FSRN exceeds FSRT the unit should go on CPD-biased Exhaust Temperature Control (NOT MW- or FSR-biased exhaust temperature control--i.e., TTRXS should always be just slightly above TTRXP). The "transition" from Droop Speed Control to Exhaust Temperature Control should be smooth and "bumpless." (Again, T/C lag can cause a small decrease from the value first observed when reaching Base Load.) TNR and FSRN should continue to increase after reaching Base Load until a preset difference between FSRN and FSRT is reached, and then FSRN is adjusted to maintain that difference as ambient conditions change. The idea is to keep FSRN "up and out of the way" of FSRT so that when the unit is being operated at Base Load the unit is not always going in and out of Droop Speed Control and Exhaust Temperature Control.

There are lots of unknowns here and several presumptions (since this author doesn't like to assume things (assumption makes an ass- of -u- and -me, ass-u-me, pun intended!)): Droop Speed Control is being used to load the unit; the unit is NOT operating on Back-up Exhaust Temperature Control at any time; there is no active external load control signal on the unit(s); all fuel splits have been properly calculated (by the OEM and/or its representatives) during emissions testing;

Q: there are no unusual Exhaust Temperature Spreads or a high number of failed- or jumpered exhaust T/Cs;

A: Confirmed, no unusual temperature spreads and no jumpered or failed T/Cs.

Q: the unit is operating on natural gas fuel; lastly, there are no "unusual" control schemes being used (some kind of fuel mixing/calorific control algorithm, compressor extraction, etc.).

A: Confirmed no fuel mixing, calorific control calculation variable is only based on FTG, the rest are constants as no Gas Chromatography signals being input to Mark V. The gas temperature is good and stable to maintain stable modified wobbie index midway between the allowable wobbie index limits. (KFGW_L = 39.84 < VFGV=42.33 < KFGW_H = 44.03). Please explain what is compressor extraction?

Q: Another thing which comes to mind is: Is the grid frequency stable--i.e., does it not vary by more than 0.1% or so? (Even that's a "large" amount if we're talking about a very large, "infinite" grid; if the grid is much smaller and some kind of AGC (Automatic Governor Control) is being used to control frequency, then it should be visible in the TNR/FSRN variations at Base Load.)

A: Grid does fluctuate about +-0.03% TNH / 0.015Hz most of the time (Ours is 50Hz grid) but this happens all the time and not specific to the moment of reaching baseload. The load does not fluctuate that much after it settled 6-8MW below the highest partload at baseload.

Q: Lots of questions here, and lots of unknowns. Are there any Diagnostic Alarms?
A: Yes we have a few but none can be related to this problem…but no problem to put them in if needed.

Q: Don't be overwhelmed by the questions; try to provide the information as best as you can, telling us what you've done and what you haven't.

A: We have done:
1. Perform offline compressor wash.
2. Perform calibration on the prformance instrumentations.
3. TTKGVTCDG corrected from 0.65 to 0.35
Just recently we change TTKXCOEF_1 from 0.98 to 0.99 and repeat the test and I can not see any improvement.

What we're trying to determine is: Is something happening after reaching CPD-biased Exhaust Temperature Control that is causing FSRT and TTRX to decrease--which would cause the power output to decrease? This author has seen several sites which did not have properly calculated CPD-biased and MW- or FSR-biased Exhaust Temperature Control Constants (this is presuming the unit has MW or FSR Back-up Exhaust Temeprature Control).

Q: Lastly, what turbine control system is in use? (The presumption has been that it's a GE SpeedTronic turbine control panel with typical control signal database pointnames...)

A: We are using GE speedtronic Mark V with HMI.

 

Very good reply; most of the information asked for was supplied!

After reviewing this for some time the following was noted:

1) CPD DECREASES after reaching Base Load, by approximately 0.14 bar which is approximately 2.0 psig (0.14 bar * 14.7 psig/bar = 2.0 psig).

2) TTXM INCREASES after reaching Base Load, which is expected since TTRX would INcrease if CPD DEcreased while operating on Base Load (this is one of the pieces of information which is lacking--how much does TTRXP and TTRX increase after reaching Base Load?). The timing of the decrease of CPD and the increase of TTXM (and most likely TTRXP and TTRX) is very similar--approximately 30-40 seconds.

3) AFPAP is steady, which affects CPR which is what many newer units use to bias Exhaust Temperature Control (CPR, Compressor Pressure Ratio, is a more accurate measure of compressor air flow that simple CPD, Compressor Discharge Pressure). There was no data available on AFPEP, Exhaust Duct Pressure and this can have a measurable effect on unit performance.

4) The missing Fuel Flow-rate Info (usually signal name FQG, Fuel Flow-Gas) will most likely show that fuel flow decreases slightly after the unit reaches Base Load.

5) FSRT drops because TTRX increases; another missing piece of information is how much FSRT/FSR decreases when the unit reaches Base Load.

One value which was not asked for was the position of the Inlet Bleed Heat Control Valve just prior to reaching Base Load and if it changes position after reaching Base Load. If the IBH Control Valve opens slightly after reaching Base Load this will reduce CPD as some of the compressor air is redirected back to the inlet. If memory serves correctly, F-class turbines use a normally open IBH Control Valve which uses air pressure on the actuator to close the valve and keep it closed. So a failure of the actuator or the I/P (current-to-pressure) could cause the valve to open due to the internal spring which would tend to keep it open (or, fail open).

This author has seen many IBH Control Valve position feedback devices improperly calibrated (an LVDT or LVDTs is/are not typically used) and it's best in such cases to have someone standing by the IBH Control Valve to monitor the valve position to ensure there is no movement of the valve stem just prior to and immediately after the unit reaches Base Load. This is pretty critical to proper operation--the IBH Control Valve must not "leak" during Base Load operation.

Some units use compressor discharge extraction air for supplying the plant instrument air (allowing the load on AC motor-driven air compressors to be reduced when the gas turbine is operating). Another usage of compressor discharge extraction air is for the Turbine Inlet Air Filter Self-cleaning System. The Inlet Bleed Heat System uses compressor discharge extraction air, also.

Many units use what is called a "multi-segmented control curve", which means the sloped portion of the Exhaust Temperature Control Bias "curve" is actually a series of straight lines. The intersections of these straight lines must be properly calculated or there will be some discontinuity--and this has caused problems at many sites over the years. These intersections are sometimes referred to as "breakpoints" in the CSP (Control Sequence Program); the signal names are sometimes Lk83BP1, -2, -3, etc., and they are usually expressed in terms of CPR ratio.

Since it's not uncommon for some changes to be made to the Exhaust Temperature Control Curve constants during Emissions Tuning, the breakpoints MUST be recalculated if any changes are made. If the unit is operating at or near one of these discontinuous breakpoints it could cause the kind of problem you are experiencing. This author has also seen breakpoint values supplied by "the factory" which were discontinuous. (This should be one of the checks performed by the commissioning personnel prior to operating any gas turbine at Base Load for the first time.... It is generally part of GE's "Red Flag Review" process prior to start-up, but since the values may be changed during Emissions Tuning they sometimes don't get re-checked as they should be.)

This author is not comfortable with analyzing Exhaust Temperature Control Constants "on-line" in a public forum such as this; if you'd like to take this discussion "off-line" and send your CONST_Q.SRC file for review to mkvguy at yahoo(dot_com) (the email adddress is "disguised" to prevent receiving any more spam than is already being received!) an Excel spreadsheet can be quickly prepared to analyze the breakpoints and plot the control curve to check for any discontinuity(s). No recommendation will be made about the values of any Control Constant; just a simple check of the continuity of the intersections of the breakpoints which could then be passed on to the OEM. Any change to any Control Constant will have to be "made" by the OEM. If you're going to send the Control Constants file, please send the CSP.PRN and CSP_XREF.PRN as well. Please compress (ZIP) these files before sending them, and it's also a good idea to used a unique filename extension when sending these files to a yahoo address or it will block them; use someting like .cmp or .cpd and just indicate what program was used to compress the file in the body of the message.

Lastly, since you indicated that yours was a single-shaft unit there will most likely be another Mark V turbine control panel for the steam turbine. It's not uncommon for problems like this to be the result of more than one issue. If something were to happen to cause the torque contribution from the steam turbine to decrease when the gas turbine reached Base Load, this could have a knock-on effect on other operating parameters in addition to reducing the power output from the generator. So, have you monitored steam turbine inlet steam flow-rate to see what happens just prior to and immediately after the gas turbine reaches Base Load? What does steam turbine inlet pressure do just prior to and immediately after the gas turbine reaches Base Load? Is there any appreciable change in steam turbine control valve position just prior to and immediately after the gas turbine reaches Base Load?

Again, one of the most important things in any troubleshooting exercise is to determine were there any changes prior to the decrease in power output being noticed? In other words, "What has changed?" You've already said the way the unit is being operated has changed and that the performance degradation has caused the unit to be operated at Base Load more recently that previously. Were there any maintenance outages just prior to the drop in power output?

markvguy

 

Mr. Xtranox,

If you've emailed the files, they haven't been received.

([email protected])

In any case, it would be very helpful if you would let us know how this is progressing.

markvguy

 

Hi markvguy,

Sorry for the late reply because I am away from office and did not have email access. I will forward the files requested to your yahoo.

Regards

 

It's probably because during commissioning the PEM derated the unit on baseload control and left the partload curves alone. They do this so they can give you an OpFlex later on.

 

This author wasn't aware GE "gave" anything away.

Did you mean "sell" OpFlex?

markvguy