Major Inspection was carried out on one of a frame F9001 gas turbine. This Unit will stay on Full Speed No Load before synchronization. If for any reason the generator breaker opens the turbine trips along on flame out. This was noticed after the major overhaul. Under frequency relay also operates.The control is GE Mark V.

Please assist with any possible solution.

 

When you say that for any reason GCB opens it does not remain FSNL and flames out, I think it should flame out if you do NOT have proper logic. Do you mean that you are at min load and then GCB Opens or you are at Higher load and GCB opens? If you are at high load and if GCB opens, like a load rejection and it will try to flame out if you do NOT have proper logic to maintain enough fuel flow.

Check your logic.

 

There is not enough information to make an appropriate comment.

My first question is: Did the unit behave as you expected <b>before</b> the major inspection? If so, tell us what Process Alarms/conditions caused the generator breaker to open without tripping the unit leaving the turbine at FSNL ready for re-synchronization?

The second question is: What fuel is being burned when this undesirable behaviour is occurring? Also, is the unit single fuel or dual fuel or multi-fuel (gas and distillate/heavy, for example)? Also, does the unit have conventional or DLN combustors?

Third, describe any work that was performed on the fuel valve(s) during the major inspection, including any LVDT calibration(s), etc.

Fourth, during the start-up, was it necessary to make any changes to Firing- or Warm-up- or Minimum FSR?

Fifth, how is the unit being operated? Is it connected to a large or infinite grid in Droop Speed Control Mode? Or is being operated in island mode in Isochronous Speed Control Mode, with a limited number of other generators and prime movers?

Sixth, what is the load of the unit when the generator breaker is being opened?

Lastly, I'm very curious about the operation of the under-frequency relay. Are you saying the under-frequency relay operates before the generator breaker opens or after the generator breaker opens?

There are usually only a very limited number of conditions that result in an opening of the generator breaker without a unit trip. <b>Please provide the Process Alarm message(s) of the conditions that you believe should only result in an opening of the generator breaker and not a unit trip.</b>

What you are describing, a breaker open event that does not result in a turbine trip, is typically called a "load rejection". If there was a contractual requirement during the original installation and commissioning for the unit to demonstrate an ability to quickly re-synchronize after a load rejection (breaker open event), then testing and tuning of the Minimum FSR Control Constant array would have been done to ensure the unit did not flame out after a breaker open event.

Frame 9Es are pretty big machines. If the load rejection/breaker open event occurs at or near Base Load, sometimes it can be very difficult to tune the control system to prevent overspeeding while at the same time maintaining flame in order to quickly re-synchronize the machine.

If the unit has DLN combustors, that makes it even more difficult to tune the unit to maintain flame during a load rejection/breaker open event.

Again, it's important for us to understand if the unit behaved "correctly" before the maintenance outage, and if so, help us to understand what changed during and after the maintenance outage.

Load rejections (breaker open events that do not occur as a result of a unit trip condition) that keep the unit running and ready for quick re-synchronization are not common events to occur (at most sites), and are usually only required for certain contractual conditions and require tuning of the control system to achieve.

 

1. The Unit did not behave like this before CI.

When the Unit is on Governor Control, and the frequency of the grid drops to 48.5hz, the under-frequency relay operates and the generator breaker opens. The turbine speed will now be dropping until 14HS relay drops out and the unit will trip on flame out.

2.Natural gas. The Unit is on single fuel. The Unit has conventional combustors.

3.The work done was Combustion Inspection. LVDT's were calibrated.

4.We didn't change the FSR.

5.The Unit is connected to an infinite grid.

6.The load varies with the frequency.

Please assist.

 

Good information, but you still didn't provide the alarm messages. AARRGGHH!!!! Alarms are important to troubleshooting; they aren't just nuisances to be ignored! (Contrary to popular tribal knowledge and mythical lore.)

And, of course the load is going to vary with frequency if the unit is being operated at Part Load. If the axial compressor speed is not steady, then the mass flow of air through the machine is not steady and gas turbines are mass flow machines that burn fuel. Also, if the unit was at Part Load during the frequency excursion Droop will kick in to try to increase the load to try to support the grid but if frequency is dropping then, well, it's kind of losing proposition for a gas turbine. If the air flow drops too much for the load the unit will be on exhaust temperature control and it's power output at that point is limited.

If the "infinite" grid frequency is dropping to 48.5 Hz, that equates to 97% of rated speed. I presume 14HS drop-out is around 94.5%. When the breaker opens what usually happens is that TNR gets reset to 100.3%. But, there is some "extra" fuel between the gas control valve and the fuel nozzles that needs to be dissipated (into the combustor) and that usually causes some slight overspeed initially. The IGVs should be adjusted to minimum operating position (typically 57 DGA) to assist with controlling the air flow through the machine.

When the fuel is cut back, sometimes there is a little underspeed, as well. Typical sequencing initiates a fired shutdown if the speed drops below 14HS drop-out. Again, if the unit is already below rated speed and close to the 14HS dropout setpoint, this could be part of the problem. But, we still don't have enough information.

If the fuel gets cut back too much and FSRMIN isn't adjusted properly then the flame can be blown out.

And, if FSRMIN isn't adjusted properly then there could also be too much underspeed even if flame isn't lost.

But, I still don't understand--because you haven't provided the chronological list of alarm messages!!!--the exact sequence of events. I'll bet money that the Alarm Printer is either (1), turned off or unplugged, or, (2) out of paper, or, (3) out of ink on the ribbon, so that's why you can't provide a chronological list of alarms.

Is the unit losing flame (indicated by a 'Loss of Flame Trip' alarm) or, is it going into a fired shutdown because the speed is dropping below TNK14HS2 (14HS drop-out)? It's not typical for turbines to trip when the speed drops below 14HS; it's more common for the Speedtronic to initiate a fired shutdown, which will eventually cause the flame to go out (usually around 20% speed on a gas-fired unit which has been properly tuned).

"...the unit will trip on flame-out...." What does that mean? A trip means fuel is immediately shut off--regardless of speed or load. If the unit loses flame because of lack of fuel there should be a "Loss of Flame Trip" alarm--unless there is some other condition tripping the unit, and there should be a Process Alarm for every condition that results in a trip--but, oh, I forgot. We don't have a chronological list of alarms!!!.

If the GCV LVDTs were "calibrated" during the outage, do you have the as-found (before) and as-left (after) calibration information? Because FSRMIN is open-loop; it's just a position, not a fuel flow. If the "calibration" resulted in a change to the actual position of the valve with respect to the indicated position that causes the fuel flow now to be less than it was before the "calibration" then this could be causing the new, undesired behaviour.

Please don't pick and choose what information you provide. Not every problem can be solved easily. You may or may not have the information requested; but tell us if you don't. It's very difficult to provide concise answers without good information.

 

Thanks a lot.The alarm printer in this site is defective.

The alarm that comes are:

1. under frequency
2. Generator breaker trip
3.Loss of flame trip
4.Hydraulic protective trouble

Note: immediately the generator breaker opens, the turbine speed will be decreasing at a very fast rate until the unit trips.

The LVDT was left at 0.69 for GCV and SRV.
Again how can we check FSR logic.

 

Defective. Hadn't considered that possibility. The fact remains: There is no working alarm printer that can be used to obtain chronological information--as it the case on so many sites, so I'm not singling this one out just trying to point out the undeniable fact that printed alarm logs are useful and important tools in the struggle to troubleshoot and understand problems.

If you "calibrated" the LVDTs, that does not require changing the zero stroke voltage. The range for zero stroke voltage is 0.700 VAC RMS, +/-0.020 VAC RMS, or from 0.680 VAC RMS to 0.720 VAC RMS. 0.69 VAC RMS falls in the range.

However, if the calibration was not done properly or resulted in a significant change to the zero- and 100% stroke values (used by the Mark V to calculate offset and gain values to scale the LVDT feedback), then that could introduce a problem.

<b>Based on the information provided, </b>I suspect there are other issues at work here. You say the unit has a Mark V. Configure the VIEW2T tool to capture data the next time there is a frequency disturbance that results in a breaker open event. You could use L52G or L52GX transitioning from logic "1" to logic "0" as the trigger, and you should record turbine speed (TNH or TNH1), load (DWATT, usually), the Master Protective Logic signal (L4), the Master Protective Trip Logic signal (L4T), the state of L14HS, the value of FSR and the value of FSRMIN, and the Gas Control Valve position feedback (FSG, usually), the value of P2 pressure (FPG2, usually), and the Stop/Ratio Valve position (FSGR, usually). You could also add the values of the flame detector logic signals, L28FDA, L28FDB, L28FDC, and L28FDD.

If you're not familiar with VIEW2T, get to a command prompt on the operator interface and type VIEW2T /? and then press ENTER, and you will be presented with a Help screen with the parameters you will need to supply. I believe if you put all of the signals in a file in the proper order, and supply the filename as one of the parameters to VIEW2T it will save a lot of typing.

Unfortunately, I don't have a PC that is configured to run IDOS or TCI any longer, so that's about as much help as I can provide.

I suspect that during the frequency excursions the P2 pressure reference is fluctuating (as it would if the speed is changing) and that's not going to help the situation, either.

If servos were replaced recently and the polarity of the currents to each of the coils wasn't verified properly, that can cause problems as well.

So, there are many possibilities--not just one.

It is unfortunate, though, that this "infinite" grid you are connected to isn't better controlled. But, again, without actionable data, there's not a lot more we can do to help you.

If you succeed in obtaining data using VIEW2T you will need to post it to a sharing/hosting site for others to be able to download it and evaluate it. A couple that have been successfully used here on control.com are tinypic.com and speedyshare.com. Or, there are others that offer free, limited-time posting of files.

Hope this helps! Get the defective printer fixed or replaced. Contrary to tribal knowledge and popular myth, dot matrix printers are still available in the market (as are ribbons for them!). You just need one that is IBM-compatible. I believe the spec's for the printer are listed in the Mark V Maintenance Manual, GEH-5980.

 

Thank you very much CSA. I will carry out all you advice and get back to you. I appreciate.