Setting Manual FSR


Thread Starter


We have 4 units 6551B GE gas turbines which recently being in a processs to be in part of a combined cycle of 210 MW. We are foreseeing a problem of high exhaust temperature may cause our HRSG to trip when GT exhaust temperature (TTXM) would rise due to frequent freequency excursion (from 49.5 to 50.5) in our country. In droop mode, unit running at Base load with FSR temperature control, and if frequency falls too much that time, the TTXM increases.

My questions are:
1. Can we set FSR at manual control to reduce fuel (for a very few minutes) so as to contain TTXM so that HRSG have no effect because of sudden rise in flue gas temperature?
2. I f we do so, is there any other effect on running the unit smoothly in Base load (except reduction in MW output)?
3. If it is permissible to control FSR manually by OEM, how should I do it? We have two target buttons as FSR-GAG PRE-SET & FSR-GAG SET-POINT in the sub menu (Fuel Control) of User Define Display menu.

Any advice will be greatly appreciated.

Could you be more specific? A Combustion Turbine cannot be in both speed/droop control while in temperature control. It is in either speed or temp control. Although frequent load changes can cause the turbine to go to temp control and then return to speed control.

A Combustion turbine in temp control does not respond to frequency excursions.

Perhaps your frequency excursions are due to temp control.


Bob Johnston

As CTTech says, there is something basically wrong in your description of what is happening. Base Load is Base Load, it's derived from Exhaust Temperature irrespective of what the frequency is. If your Exhaust temperature is really rising in your described circumstances, you need to look further to see why this is happening. Using FSR Gag is kind of reverse engineering to resolve your problem. Please keep up the discussion and let us know what happens.

Thanks for reply. A Combustion turbine in temp control does respond to frequency excursions in many ways I think.

Our unit runs at Base load with FSR controlled by CPD bias temperature control. Where TTXM is limited by TTRXB and TTRXB changes on the calculation of CPD, exhaust temp cpd corner & slope. These cpd corner & slope value changes with different logic activation due to frequency change. Those logic conditions (i.e. L83JT0, 83JT1, L83JT2) change the said calculation by selecting different exhaust temp cpd corner & slope.

So, what I found there is when frequency falls the TTRXB is raised by this calculation so as to add fuel to harness falling of output too much (mainly caused by fall of CPD).

I want to know if I set FSR manually (a little lower than the existing FSR value, would there be any other effect on the turbine except power generation?

Dear MMA,

A combustion turbine in temp control doesn't respond to frequency excursions in the same way that a combustion turbine in droop speed control would. Power output goes up and down, but exactly the opposite of how it would react if it was on droop speed control. In fact combustion turbines being run at base load actually make frequency excursions worse--since a decrease in frequency causes power output to go down which is exactly the opposite of what everyone wants to happen and what would happen if the unit were on droop speed control. And, when the frequency goes up a unit running at base load will actually increase its power output which is again the exact opposite of what everyone want to happen and what would happen if the unit were in droop speed control.

Temperature control reference is a function of CPD, and if you look at the generation of the logic signals you listed they should be a function of CPD levels and not frequency.

Of course, to a degree CPD is a function of frequency since speed and frequency are directly related and CPD can be affected by changes in speed. But, the primary exhaust temperature reference is not usually a function of any other variable than CPD.

If you want to know what the OEM would recommend, this isn't exactly the place for that kind of question. I don't see a lot of OEM engineers hanging out here and contributing.

Look at the exhaust temp reference curve; the lower axis is usually CPD, the variable in the exhaust temperature equation. The curve slopes downward to the right from the flat line at the top. That means it has a negative slope, which means that as CPD increases on the bottom axis going to the right the resultant exhaust temperature on the left axis will decrease as it comes down the slope of the curve--the opposite of what happens during loading, but that's the way it works.

That's why when it's hot during the summer the exhaust temperature actually increases even though power output decreases. And why exhaust temperature is cooler in the winter months when power output is higher. CPD is lower during the summer when ambient temperature is cooler, and higher during the winter when ambient temperature is colder.

CPD is a function of several things: compressor speed which should be constant if frequency is constant, but since it's not at your site then CPD will change with grid frequency. Ambient air temperature, or compressor inlet temperature. Cooler air will increase CPD if the compressor is running at a constant speed; warmer air will decrease CPD. And, the amount of fuel being burned will affect CPD since burning fuel causes a pressure increase in the combustor; more fuel, more pressure; less fuel, less pressure. That's why CPD changes when speed is constant and the IGVs aren't moving but fuel is increased or decreased. As has been said before on this site, axial compressors don't behave like most people expect they will.

You have already noted that as frequency decreases exhaust temperature increases--which is because CPD is changing as the speed of the compressor is changing. But, fuel is actually decreasing when CPD decreases when the unit is being operated at base load. Again, it's the opposite of what happens when the turbine is being loaded, but if you have the data which has led you this far, you probably have the data to show that fuel flow decreases when frequency decreases and FSR decreases when frequency decreases when the unit is being operated at base load.

That flat line at the top of the exhaust temperature control curve is the maximum allowable exhaust temperature under any condition, and is usually 1100 degrees F for B-class turbines. Is that higher than your HRSG can handle?
Dear MMA,

When frequency falls CPD is decreased. If CPD falls lower than it is in a certain time, the exhaust temperature would go much higher with the same amount of fuel. So fuel is reduced to maintain the exhaust temperature up to TTRXB value. What is supposed to happen then? Generator output would fall too much, but it actually falls less than that because TTRXB is raised by the calculation [TTKn_I-{(CPD-TTKn_C) * TTKn_S}].
TTRXB value is same as the value of TTRXP value when unit is at Base load. This TTRXP is calculated using value of CPD, and change in TTRXP value is mainly influenced by change in CPD. When frequency falls, CPD also falls, so different CPD Corner value & different CPD Slope value is picked up for the calculation of TTRXP. So in low frequency TTRXB (and also TTXM) is higher. And it is done to avoid generator output to fall that much.

Mr. MMA, you are wrong when you say "to add fuel to harness falling of output". Actually falling of FSR is harnessed a little bit (FSR is not raised) by raising TTRXB so that output does not falls too much.

Mr. CSA is right, FSR is decreased when frequency & CPD is low and increases when frequency & CPD is higher.

M M Ahsan
[email protected]

N Chatterjee

Combustion turbine in temp control does respond to frequency excursions but only when frquency excursion is on the upper side due to which GT unloads as difference between TNR and TNH is shortened. Same is not the case while lower side frequency excursions as GT load can not go beyond the clamping limit of temp control.