Load Fluctuation on Steam Turbine Generators

H

Thread Starter

Hochelaga

We have 3 GE Steam turbine generators with 25 MW max for each. Actually 2 units are on line with 15 MW each; the third one is in standby.

Sometimes a suddenly load fluctuation happen from 15WW to 20MW and then to 9MW. I cannot understand for which reason. this is for about 30 Sec and then became normal (Steam turbine are not connected to the public network, they are used to produce power for gas plant only).

Please can anyone have an idea how to solve the problem?
Thanks a Lot
 
Hochelaga,

We would need a LOT more information to be of any help.

1) When did this problem start?

2) What has been done to troubleshoot the problem?

3) What were the results of the troubleshooting?

4) What happens to the generator frequency output during these events?

5) What modes are each of the steam turbine governors being operated in? Isochronous? Droop? Some kind of Isoch Load Sharing?

6) Is there some kind of external Load Control or Plant Load Management System in operation?

7) How is the steam generated? With fossil fuel-fired boilers or from gas turbine exhaust heat recovery steam generators?

8) Is thee some large load motor or load that starts and/or stops when these events occur?

9) What kind of steam turbine control systems are used for each of the three steam turbines?

Each of these questions is equally important as the others. None is irrelevant. If you want help, you need to provide as much information as possible.
 
Before answering your question here below some details regarding our plant.

Really we have 3 GE gas turbines 5001 and 3 GE steam turbines NG63/50. Iin normal situation 2 gas turbines and 2 steam turbines are on line and all on Droop mode. Schneider load sharing system is available and acting only on gas turbines, it means if there is more or low consumption of load the load sharing acts only on gas turbine while the steam turbine remain at the same load.

> 1) When did this problem start?
- The problem started from long time ago and appeared time by time. The second to last happened 6 months ago and the last was yesterday

> 2) What has been done to troubleshoot the problem?
- Nothing special has been done apart the simulation of the governor and found working well

> 3) What were the results of the troubleshooting?
- According to trends recorded, we observed that during load fluctuations, while the active power increase the command to the governor was decreasing which is not proportional and is not normal, (Should be proportional), I guess.

> 4) What happens to the generator frequency output during these events?
- Stable

> 5) What modes are each of the steam turbine governors being operated in? Isochronous? Droop? Some kind of Isoch Load Sharing?
- All turbine were on Droop mode, Schneider load sharing system is available to share the load

> 6) Is there some kind of external Load Control or Plant Load Management System in operation?
- Yes Schneider load sharing system and Schneider electrical control system ECS

> 7) How is the steam generated? With fossil fuel-fired boilers or from gas turbine exhaust heat re
- With fossil fuel-fired boilers.

> 8) Is thee some large load motor or load that starts and/or stops when these events occur?
- No things was started or stopped during the fluctuation

> 9) What kind of steam turbine control systems are used for each of the three steam turbines?
- Woodward 505 speed governor

> Each of these questions is equally important as the others. None is irrelevant. If you want help,
> you need to provide as much information as possible.

I can send to you trends during the load fluctuation, if you send to me your e-mail address.

Thanks
 
Oh, how the plot thickens, doesn't it?

If you send trends to any single person, no one else can see them. The point of a forum like this is for everyone to learn from other's problems and experience. Many people post information to free web-hosting sites, like tinypic.com or www.speedyshare.com, or similar. Many people can then download and look at the information and propose different scenarios and/or come up with better ideas and solutions.

I'm still very unclear about many things. What is the difference between the Schneider Load Sharing System and the Schneider Electrical Control System? Are the STs connected to the ECS?

<b>**What is happening to the GT loads when the ST power increases in this unwanted condition?**</b>

If the steam turbines are in droop <b>speed</b> control mode (not droop <b>load</b> control mode) and not connected to the GT Load Sharing System, then if the plant frequency dropped the steam turbine governors should increase load while the frequency is low. If, after the frequency drop there was a slight frequency decrease ("overshoot") while getting the plant frequency back to nominal the steam turbine output would drop while the frequency was high.

But you said the frequency was okay. So, the above doesn't make any sense.

Are the GTs connected to the plant, and also isolated from the grid? So, are the GTs and STs all contributing to the plant load without exporting any power to the grid? And, is the Load Sharing System being used to control plant frequency by varying the GT output(s)?

If there is something happening to cause the Woodward 505s or the ST control valves to cause more steam to flow into the STs and the load was stable and all of the Ts (GTs and STs) are all connected together, then if the frequency is stable the GT output must be decreasing by the same amount as the sT output is increasing. If the plant load and plant frequency is stable, there is no other explanation--unless there's something more that we don't know.

If the steam turbine output increases that means the control valves have to open to let more steam flow in, and this would typically cause a drop in steam turbine header pressure which would typically cause the boiler(s) to be "fired" harder (you did say the steam source was fossil fuel-fired boiler(s)) to maintain the steam header pressure. Does the fuel flow to the boilers increase during these events?

If the steam turbine control valves are hydraulically controlled, is the ST hydraulic system pressure stable during these unwanted sT power excursions?

I have seen Woodward 505 control valve outputs connected to servo driver modules that were powered by sources other than the 505s, but in those cases when the source was lost the control valves closed, rather than opening.

I don't have a lot of experience with 505s, and I don't have access to a 505 manual at this time. But, as I recall, they are pretty basic controllers with droop speed control, but they also have some "cascading" features or some other possible contro modes that can be enabled and selected based on discrete inputs or panel display buttons.

Again, if the plant is truly isolated from the grid and all the GTs and STs are connected in parallel with each other, then if the power output of any prime mover and its generator increases without a system load increase (motors starting, etc.) then one of two things will or should happen. The immediate (re)action would be for the plant frequency to increase. If nothing is done to reduce the power output of any prime mover then the frequency will remain higher than desired. If the Load Sharing System and/or the ECS is working properly and sensing the frequency increase, then it should correspondingly decrease the power output of one or more other prime movers and their generators to maintain the desired frequency.

So, either the frequency of the plant increases because one or more prime movers has increased its power output without a corresponding decrease in the output of the other prime movers, or the Load Sharing System/ECS decreases the power output of one or more prime movers it is controlling to maintain the desired frequency.

I could imagine a scenario where the GT droop control would respond by itself to the increased frequency by reducing its active power output, but I would suspect the Load Sharing System and/or the ECS would be the "primary" control element and direct the connected generation to reduce output to maintain frequency.

This is an interesting problem, but probably because there's a lot we don't--and probably can't--know.
 
The question put by CSA is very relevant, i.e. the load condition of GTs during load fluctuations in STs, i beleive there will be similar load fluctuations in GTs also as all the M/c are in parallel.
I have not worked on GTs, however i have experiance in STs with Woodward Governor control, the problem source needs to be identified first. If the source is ST then we can think many possibilities.

1) We need to understand the signal condition ( 4 - 20 ma) sent by WWG to I/H convertor, some times due to cable problem there may be furious signals going to I/H converter, hence normally WWG recommends to introduce isolators.
2) There may be sticky operation of servocylinder or pilot piston which is directly related to oil quality
3) Disturbance in PID values of WWG
4) If i am correct, WWG has feed back signal from Tie breaker by which it senses about the parallel source, if there is any loose contact in this wire, can also cause hunting,

Please check these issues and post the developments on this site

Regards,
Rangacharya
 
There is no difference between Schneider Load Sharing System and the Schneider Electrical Control System. It is the same system

When the load fluctuation occurred on Steam turbine unit C, the 2 gas turbines which were on line also fluctuated but steam turbine unit A was steady because the load sharing acts only on GT.

Steam and gas turbines were on Droop load control

No one unit connected to the public grid, the power generated used only in the actual plant.

Reviewing trends attached, the green curve is the command to the governor not the feedback as written, during the first increasing of the load, the command was decreasing which is not normal, I guess!!!

http://speedy.sh/YuAEG/Trend1-STG-C-05.08.12.bmp
http://speedy.sh/Cz9db/Trend2-STG-C-05.08.12.bmp
http://speedy.sh/Sx4Vj/Trend3-STG-C-05.08.12.bmp
http://speedy.sh/R5wKg/Trend4-STG-C-05.08.12.bmp
http://speedy.sh/GZWAU/Trend5-STG-C-05.08.12.bmp
http://speedy.sh/bph9H/Trend6-STG-C-05.08.12.bmp
http://speedy.sh/jF74M/Trend7-STG-C-05.08.12.bmp
http://speedy.sh/gqfwE/Trend8-STG-C-05.08.12.bmp

Note: Steam turbine always loaded manually from the ECD (electrical control desk) and the load should be unchanging unless the plant shutdown.

Example. If our 2 GT are loaded 8 MW each and our ST load at 15 MW each and we shutdown one gas train in the plant which consume 6 MW, the status of the GT and ST will be as follow

GT become 5 MW each and ST remain as they were with 15 MW

I hope i gave some clarification.

regards
 
There is this little alarm message which is interesting:

I/H Converter-Control Oil <b>FAILURE</b>

Also, from the trends it seems there is an oscillation in control oil pressure, and some big swings which precede the disturbance.

I don't think we have enough information to be of much more help. You need to understand the source of the oscillation and subsequent disturbance. It would seem the 505 is trying to respond as it should, but then interaction of the Load Sharing System seems to be causing some problems, because it's maintaining frequency.

So, when the ST load increases, which tends to increase the frequency the Load Sharing System responds by decreasing the load of a GT or GTs to maintain frequency. That is "normal". You need to understand why the ST load is changing.

We don't know what's happening to cause the reversal from 15 MW to 5 MW. I suspect some operator is trying to adjust loads.?.?.?

As Rangacharya says, it could be mechanical binding of the actuator, but that pesky little alarm mentioned above sure worries me. I don't like seeing alarms like that active when problems like this occur, and that one was active for some time it seems....
 
Looking in to the trends especially of control oil pressure, there should not be so much fluctauations, whether you have accumaltor in the oil circuit with nitrogen filled bladder, if yes kindly check the pre charge pressure of nitrogen.

I will post my further observation after studying the trends in detail

Regards Rangacharya
 
> What happened to the problem? whether it is solved any clue you got. please
> mention the forum for latest development so that thread gets closed.

...Had similar experience before in our 330MW
GEC Alsthom ST.

...Please check your Control oil accumulators.
It is possible that the "bladders" are
damaged. If so, replace the bladders and
charge with the correct N2 pressure.

...Please update us all on the latest development.

VVK
 
Q
Hi,
We are facing the same problem.

We have three STGs capacity 27MW/each operated in Droop mode. Total 50MW. When connect to public network at 22kV via a 22kv/22kV Transformer, the output power of the generator fluctuate a lot. Turbine control Woodward 505.

When in droop mode the ECS work as the load sharing panel.
How your problem done?
Can you help me?

Thank you very much
Quang Doan
 
There is no difference between Schneider Load Sharing System and the Schneider Electrical Control System. It is the same system

When the load fluctuation occurred on Steam turbine unit C, the 2 gas turbines which were on line also fluctuated but steam turbine unit A was steady because the load sharing acts only on GT.

Steam and gas turbines were on Droop load control

No one unit connected to the public grid, the power generated used only in the actual plant.

Reviewing trends attached, the green curve is the command to the governor not the feedback as written, during the first increasing of the load, the command was decreasing which is not normal, I guess!!!

http://speedy.sh/YuAEG/Trend1-STG-C-05.08.12.bmp
http://speedy.sh/Cz9db/Trend2-STG-C-05.08.12.bmp
http://speedy.sh/Sx4Vj/Trend3-STG-C-05.08.12.bmp
http://speedy.sh/R5wKg/Trend4-STG-C-05.08.12.bmp
http://speedy.sh/GZWAU/Trend5-STG-C-05.08.12.bmp
http://speedy.sh/bph9H/Trend6-STG-C-05.08.12.bmp
http://speedy.sh/jF74M/Trend7-STG-C-05.08.12.bmp
http://speedy.sh/gqfwE/Trend8-STG-C-05.08.12.bmp

Note: Steam turbine always loaded manually from the ECD (electrical control desk) and the load should be unchanging unless the plant shutdown.

Example. If our 2 GT are loaded 8 MW each and our ST load at 15 MW each and we shutdown one gas train in the plant which consume 6 MW, the status of the GT and ST will be as follow

GT become 5 MW each and ST remain as they were with 15 MW

I hope i gave some clarification.

regards
Hello All,


Hochelaga, Quang Doan,

Can you provide us some feedback, on the issue you have been faced (dicsussed on this thread).

We can try to support you as best as we can!

Regards,
Controlsguy25.
 
We have 3 GE Steam turbine generators with 25 MW max for each. Actually 2 units are on line with 15 MW each; the third one is in standby.

Sometimes a suddenly load fluctuation happen from 15WW to 20MW and then to 9MW. I cannot understand for which reason. this is for about 30 Sec and then became normal (Steam turbine are not connected to the public network, they are used to produce power for gas plant only).

Please can anyone have an idea how to solve the problem?
Thanks a Lot
Please check power generation feedback signal coming to governer.
 
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