Dear sir,

We are having Frame-V (MARK-V control system) supplied by M/s Thommossen.

Presently the machine running at 18.5 MW. At this load the speed of the machine is varying between 5100 - 5115 rpm. Few months before the variation used to be 5 rpm. Slowly the same getting increased to 15 rpm.

We have observed the following variations regard to FSR / SRV / GCV & Hydraulic Oil Pressures.

______ FSR variation is from 51.52 - 54.77,
______ SRV variation is from 42.11-45.43 &
______ GCV from - 50.01 - 54.25.
______ 78.0 - 76.5 bar.

All the above variations are cyclically happening and SRV/GCV are hunting in field in the same frequency. Inter stage fuel gas pressure and GCV gas out let are also having fluctuations.

We have a doubt the whether Hydraulic pressure is the reason or any other points to be addressed. Is it possible to increase the Hydraulic pressure on line when machine is running? If so how to adjust?

You are requested to suggest us the probable reasons for the speed & other variations and remedial course of action to be taken to control the variations.

 

NVS,

Is the Mark V a SIMPLEX or TMR turbine control panel?

Is the generator synchronized to a large electrical grid, or is it synchronized to a smaller electrical grid ("island")? If it's a smaller electrical grid ("island"), is this unit operated in Droop- or Isochronous Speed Control? Does the unit get load signals from some external source (PMS--Power Management System; or, AGC--Automatic Governor Control: or from a DCS--Distributed Control System; or ???)

<b>What Diagnostic Alarms (all of them) are annunciated and active at this time?</b> (Even if the Diagnostic Alarms don't seem related, provide a comprehensive list of all active, annunciated Diagnostic Alarms.)

What Process Alarms are annunciated and active at this time? (Even if the Process Alarms don't seem related, provide a comprehensive list of all active, annunciated Process Alarms.)

And, most importantly, when did this problem start? You say a few months ago everything was relatively stable; was there a maintenance outage performed since the operation was stable? If a maintenance outage was performed, what was done to the SRV/GCV and hydraulic system?

Is the unit operating at Part Load (say, on Preselect Load Control), or at Base Load?

Does the unit exhaust into an HRSG, and if so, is IGV Exhaust Temperature Control enabled and active and are the IGVs unstable also?

Is it possible for you to tell if the gas fuel supply pressure (upstream of the SRV) is stable or is it changing?

This is a difficult problem to troubleshoot. The SRV pressure reference (P2 pressure reference) is a function of speed, and if the speed is not stable then the P2 pressure reference will not be stable and the SRV will not be stable.

Also, if the GCV is not stable then the pressure between the SRV and GCV will not be stable and that will cause the SRV to move to try to maintain a stable P2 pressure--but in this case the speed is also unstable which is making control of P2 pressure more difficult, if the GCV is not stable.

The speed of the turbine-generator is controlled by the grid when it is synchronized with other turbine-generators. So, if the speed is unstable it is most likely because the grid is unstable. If this is a small grid ("island") and the frequency control is not good (by whatever method--a single large machine being operated in Isochronous Speed Control; or multiple machines being operated in Droop Speed Control and being "managed" by an external controller to maintain frequency) then the SRV is going to swing because the speed is swinging because the frequency control is not good.

If this unit is the Isochronous unit on a small grid (island), then the problem could be that the load is changing much more than previously, or it could be hydraulic problems, or worn hydraulic actuators.

Since you said this is a Thomassen unit, it's probably an older machine. Have the SRV/GCV actuators and valves ever been refurbished/or repaired?

Since this is a Frame 5 it probably does not have a hydraulic accumulator, which would probably help with hydraulic pressure fluctuations. At this point, I would estimate the hydraulic pressure fluctuations are the result of the speed changes causing the SRV & GCV to change requiring higher/lower hydraulic flows. If the hydraulic filters are dirty (high differential pressure) then quickly changing flow requirements for the changing valve references may be hampered because of low flow through the dirty hydraulic filters.

If the SRV/GCV actuators and valves have never been refurbished then the actuators and/or valve stems might be worn and could be causing or making the swinging worse.

I noted that you didn't say the MW was changing, so it would seem the unit is likely being operated at Part Load in Preselect Load Control. The speed/frequency swings may be made worse by the over-riding action of Preselect Load Control trying to keep MW stable when they should be changing as the frequency is unstable.

I think the problem is primarily related to frequency, and hence turbine speed, instability. Dirty hydraulic filters, and/or worn SRV and/or GCV hydraulic actuators and/or valve stems may also be adding to the problem. If IGV Exhaust Temperature Control is also enabled and active and the IGVs are moving then the lack of a hydraulic accumulator might also be making the problem worse. Many times problems like this are the result of two or more conditions acting together to make a small problem worse.

Again, turbine speed--which is a function of grid frequency--is the reference for P2 pressure which is the reference for the SRV regulator. If the P2 pressure is unstable because the turbine speed is unstable because the grid frequency is unstable then the GCV will moving opposite to the SRV to try to keep the MW constant if Preselect Load Control is active while the frequency is unstable.

If the grid frequency is stable (not likely), then there is a problem with turbine shaft speed pick-ups. And this would be alarmed--with both Process- and Diagnostic Alarms.

You could monitor the P2 pressure reference, usually signal name FPRG, and actual P2 pressure, usually signal name FPG2, and turbine speed, TNH or THN1, and GCV position (which should be relatively equal to FSR--and is almost per the information you provided). You should see that as the speed changes the P2 pressure reference changes and so the P2 pressure changes. As the P2 pressure increases the GCV position (and FSR) should decrease, and as the P2 pressure decreases the GCV position (and FSR) should increase.

Please write back with the answers to all of the questions above and to let us know what you find. Many people read these threads on control.com--both now and in the future--and are interested in the answers to all of the questions, as well as the resolution to the problem(s).

 

Dear CSA,
Thank u for ur response

Mark V - TMR panel
Running under - Isochronous Speed Control
Unit exhaust to HRSG
Last MI during 2008

MW also got varied (Peak load - 21MW, Present load - 18MW, Variations 0.1 to 0.2MW)

No diagnostic or process alarms observed during this variations, same variations were observed from past 6months

As per mechanical dept suggestion checked, replaced lube oil filters and observed same variations.

Checked lube oil level in lube oil console and found half the tank got empty, so top-upped and observed the same variations.

Removed GCV/SRV lube oil console plate and observed mist (dust/mud) coming from vent port, so will this mist lube oil choke servo valve filters and causes variations?

One more help/request from my side regarding spares or drawings required for refurbishing gcv/srv during shutdown as we don't find any drawing and list of internal spares to be procured.

 

NVS,

You said you replaced the lube oil filters. While the same fluid is also used for the hydraulic system, there are almost always additional filters for the hydraulic system. Were those replaced also?

Yes; certainly dirty lube oil/hydraulic oil will cause problems. I'm not aware of any "vents" under the combined gas valve assembly that might spew dust/mud; that's the first time I've heard of any dust/mud in that area. If the lube oil tank level was allowed to get so low then it's certainly likely that the lube oil cleanliness is also questionable as it doesn't sound like periodic lube oil testing/analysis has been done.

As for gas valve spares, you need to find the drawing for the combined gas valve assembly in the Parts Section of the manuals provided with the turbine-generator. You should have actuators (2, one for each valve), valve stem seals, valve plugs and seats (for both valves) on hand at all times. The mechanical department should be able to look at the drawing and identify the parts most likely to require replacement.

Please write back to let us know how you fare.

 

Dear CSA,

After make-upping oil also we observed same variations, so we are going for GT shutdown for refurbishing GCV, SRV and for complete changing of oil.

and thank u a lot for your helpful suggestions posted..