Gas Turbine in Base Load with Droop Speed control


Thread Starter

sriram sandeep

Dear all,

our machine GE 9fa with MK vie controller. Generally Gas turbine in base load operation means it is under cpr/cpd exhaust temperature control (when we give the command Load select as baseload)

But recently by seeing some posts in I have got one doubt how can we operate the machine in base load with droop speed control. as when we are saying base load all the time i used to consider it is under cpr/cpd exhaust temperature control.

Is it in this way. am i right?
Load selection will be in preselect load only, but we will give the MW setpoint near to the base load rating so that it will stay in droop speed control without transferring to exhaust temperature control or is there some other way? kindly clarify...

and in case of isochronous control i understand that the controller tries to maintain the grid frequency which is vice versa to other controls of Gas turbine like exhaust temp control and droop speed control. but my concern is in case of other than isochronous control grid frequency effects the machine operational output frequency and hence the output produced by gas turbine. so, in isochronous control as though the recent posts i understand the machine will adjust the fuel flow to maintain the grid frequency which is very odd to hear (very hard to imagine gas turbine thermal stresses even though grid is some what reasonably stable). is there any grid frequency set point when operating in isochronous control???

Another doubt...

few weeks back our machine operated on baseload that time TTXM is 629 degc.. but where as during preselect load operation the TTXM is maintaining around 648 degc. During base load operation it is following TTRX as TTRXB (speed biased) which is minimum value. but in case of preselect load operation TTRX is not TTRXB (more value) but less value than TTRXB. why?

As in base load and under exhaust temperature control speedtronic tries to put more fuel to maintain the higher exhaust temperatures. but why its not happening here?

I don't know where to start.

When the unit is at Part Load the IGVs are not fully open, so the exhaust temperature can be higher than when the unit is at Base Load--part of the definition of Base Load is the IGVs must be full open, which means the exhaust temperature may be (usually is) lower because the mass flow of air through the machine is higher when the IGVs are fully open.

Have a look at the example of the exhaust temperature control curve in Sect. 03 of the Control Specification. It has a negative slope--which is counter-intuitive. Yes; the exhaust temperature generally increases as load increases, but as the unit nears Base Load the exhaust temperature will decrease as load increases. And, when the unit is at Base Load on exhaust temperature control the exhaust temperature will decrease as load increases and it will decrease as load decreases. Again, it's counter-intuitive but that's the way combustion turbines work. You might doubt that, but that's the way it is.

If you put a load setpoint in that is higher than the unit can achieve (because it goes on exhaust temperature control) then it's not on Droop Speed Control and it is on exhaust temperature control. That's because the FSR Minimum Select function chooses FSRT because it will be less than FSRN because the load setpoint will drive FSRN to be higher than FSRT.

If you want to run the unit on Droop Speed Control, it does NOT have to be in Pre-Selected Load Control. You can just click on LOWER SPD/LD until FSRN is less than FSRT (which is when load will start to decrease) and when you reach the load you want to maintain just stop clicking on LOWER SPD/LD. If the frequency is lower than it should be the unit should be increasing load--not maintaining load which is actually exacerbating the frequency excursion. I don't understand what you're trying to accomplish by keeping the unit in Droop Speed Control near Base Load--unless it's because you want to have the unit increase load when frequency is less than rated.

If the unit is synchronized to a grid with other turbine-generators it should not be operated in isochronous speed control--especially if the grid frequency is less than nominal. And, the unit should not be at or near Base Load when it is being operated in isochronous speed control. Because when the grid frequency decreases and the unit tries to respond by increasing load it will hit Base Load very quickly which means it won't be able to do any more. Worse, when a gas turbine is at Base Load and the frequency decreases the power output will not stay at rated--it will decrease which makes the frequency problem worse.

So, it's not possible to be on both Droop Speed Control and exhaust temperature control at the same time.

Does this help at all?

When a GE-design heavy duty gas turbine is operating on exhaust temperature control as load increases exhaust temperature will decrease.

When a GE-design heavy duty gas turbine is operating on exhaust temperature control as load decreases exhaust temperature will increase.
CSA sir, it is a great answer.

Just want to put some additional information.

I operate fr6 GE with mk6 control system. Because management policy, we set the load 34 MW which is 1 MW lower than the base load power (base load =35). At this condition the load is changing according to the frequency exactly as Mr CSA explained. I can see it clearly on SIGNAL TREND application. It requires you to pause and zoom the graphics before you find it, pretty small fluctuation happens there. Mr CSA may called it negative gradient, but I call it negatively correlated. As frequency increase, the load (and FSR, and temp exhaust) will decrease. Vise versa.

But around 5 months from now, the exhaust temperature will increase due to compressor fouling (common problem on gt). at this situation the load will change according to the temperature exhaust. At the same load, but due to compressor fouling, the temp exhaust increase and turbine control no longer controlling the frequency. As the temp increase the load decrease. The negative correlation is now between temperature and load (no longer between the frequency and load).

Its all because of the minimum value selector applied there. Which one is minimum, FSRN or FSRT, will be chosen as the final FSR. Strictly said that all the gas turbine control manufacturer i have dealt with applied the same philosophy (Siemens, Trisen, etc). Because it ensures safety operation of gas turbine.

I can say that FSRN is droop mode and FSRT in temperature control mode. They can not be at the same time as Mr CSA said.

Hope it helps