Variation in Gas Turbine load (MW)

We have GE frame 6 Gas Turbine (30 MW) initially operating/controlled by Speedtronic Mark-IV control system and later on upgraded to Mark VIe. Generally we operate GT on part load (Preselected mode) as per our steam and power balance. Since last few days we have noticed continuous load variation from 16.5 to 19.1 MW (Machine set on 18 MW preselected load). TNR value is changing continuously though there is less variation in TNH value.
 
dax2172,

Can you please provide better data on both the TNR and TNH variations?

What happens when you click on RAISE SPD/LOAD or LOWER SPD/LOAD (which disables Pre-Selected Load Control but would not results in excessive load swings!)? Disabling Pre-Selected Load Control should result in a stable TNR. (The operators will freak out when you ask to do this, but they have nothing to fear, and Pre-Selected Load Control can be quickly re-enabled at any time during this test (which only needs to last for three or four minutes).)

If TNH is unstable that suggests the frequency of the grid the unit is synchronized to is unstable. And that will (and should!) result in less stable load as Droop Speed Control responds to the grid frequency disturbances. (It's a popular but incorrect belief that when grid frequency is unstable load should remain stable. That's simply not the way AC power systems and Droop Speed Control work.)

Anyway, if you will provide better data, including generator frequency (often signal name SFL or SFL1) we might be able to help. It would also be useful to know what fuel is being used and if the fuel supply pressure is stable, and if fuel supply pressure has changed since the load instability began. Please tell us also if the turbine has a DLN combustion system.
 
dax2172,

Can you please provide better data on both the TNR and TNH variations?

What happens when you click on RAISE SPD/LOAD or LOWER SPD/LOAD (which disables Pre-Selected Load Control but would not results in excessive load swings!)? Disabling Pre-Selected Load Control should result in a stable TNR. (The operators will freak out when you ask to do this, but they have nothing to fear, and Pre-Selected Load Control can be quickly re-enabled at any time during this test (which only needs to last for three or four minutes).)

If TNH is unstable that suggests the frequency of the grid the unit is synchronized to is unstable. And that will (and should!) result in less stable load as Droop Speed Control responds to the grid frequency disturbances. (It's a popular but incorrect belief that when grid frequency is unstable load should remain stable. That's simply not the way AC power systems and Droop Speed Control work.)

Anyway, if you will provide better data, including generator frequency (often signal name SFL or SFL1) we might be able to help. It would also be useful to know what fuel is being used and if the fuel supply pressure is stable, and if fuel supply pressure has changed since the load instability began. Please tell us also if the turbine has a DLN combustion system.
Dear CSA,
Thanks for your reply.
As we run GT on pre-selected mode we don't raise/lower speed/load control switch.
Next as our grid frequency is stable TNH almost remain stable at 100%, occassionally to 100.1%.
TNR vary between 102.0 and 102.3% with GT load variation between 18.6 to 16.9 MW at 18 MW set point.
We use Natural Gas as fuel and it supply pressure is steady. We don't have DLN combustion system.
Do lub. Oil temperature play any role??. Being winter it is 56 to 57 deg. Centrigrade.
Hope this will serve your purpose
 
dax2172,

I know the operators and their supervisors and the instrument department personnel will ALL cringe and be shocked at the suggestion of running the unit without Pre-Selected Load Control active because they are just all CERTAIN that the unit will drift to any load and be "out of control." That's because they ass-u-me that the unit simply MUST be in either exhaust temperature control (Base Load) or Pre-Selected Load Control or the turbine control system will not be able to control the fuel and therefore the load will also be uncontrolled.

It's simply not true that the unit must be in Pre-Selected Load Control or exhaust temperature control (Base Load) or the fuel/load will be uncontrolled. The assumption that the fuel/load will be uncontrolled unless exhaust temperature control (Base Load) or Pre-Selected Load Control is active is just that: an assumption.

When the unit is NOT operating on exhaust temperature control (at Base Load) Pre-Selected Load Control can be used to change load without having the manually click on RAISE- or LOWER SPEED/LOAD continuously until the desired load is achieved. That's all it's really intended for. It just happens that the designers of Pre-Selected Load Control did not make Pre-Selected Load Control automatically disable itself once the desired load setpoint was reached.

I was suggesting--as a TEST--to click once on either RAISE SPEED/LOAD or LOWER/SPEED LOAD to disable Pre-Selected Load Control to see what happens to TNR when Pre-Selected Load Control is not active. I know this is absolute heresy--to disable Pre-Selected Load Control while the unit is operating and producing power--but nothing unexpected will happen. Before Pre-Selected Load Control units ran perfectly and were able to maintain desired load setpoints without much operator intervention or action at all. Then, this poorly designed--and named--function called Pre-Selected Load Control was invented and the world just assumed that it should be active at all times when the unit was not at Base Load on exhaust temperature control. Which is just NOT true. (In fact, in many parts of the world it has become forbidden to use Pre-Selected Load Control without some kind of over-riding control scheme to allow the turbine to properly respond to grid frequency disturbances (but that's for another thread, day, time).)

Fuel flow-rate, and therefore load, changes when TNR changes. Pre-Selected Load Control is a method for changing TNR, same as clicking on RAISE- or LOWER SPEED/LOAD is a method for changing TNR. When TNR is NOT changing the load is stable. By asking to commit heresy by disabling Pre-Selected Load Control I'm attempting to determine if TNR will be stable and if the problem is a control system problem or a mechanical problem (with the SRV or GCV or hydraulic system).

I would really like to know what Process Alarms and what Diagnostic Alarms are active while the unit is running on Pre-Selected Load Control. It would also be helpful if you would tell us if you, or someone at your plant, can use the Trend Recorder function of ToolxboxST to gather data.

I do not understand if you mean the L.O. temperature is 56- or 57 deg C or the ambient temperature is 56 or 57 deg C. L.O. temperature should be approximately 125 deg F, which is about 51-52 deg C, and so 56-57 deg C is not excessive, though it would seem it should be a little lower if it is winter time at the site.
 
Since Pre-Selected Load Control uses feedback from a load (MW) transducer, it's possible the load transducer, or one of its inputs (potential transformer; current transformer) is failing or having a problem.

It would require a shutdown and a person or persons with some sophisticated test equipment to test the transducer and its inputs.

BUT, disabling Pre-Selected Load Control for a few minutes (I know; I'm suggesting heresy yet again) would help with troubleshooting the problem....

I promise--NOTHING (nada; niente; zippo; zilch) bad will happen. No one will lose their job because of negligence if Pre-Selected Load Control is disabled for a few minutes.

[This is like disputing Fake News! It's so damn difficult to get people to think critically and logically and responsibly.]
 
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