When does the machine changes over from Preselect Load to Base Load??

Our Machine was set at 20MW Preselect Load... The machine came to Temp Control at around 19.2MW... The Status_Fld1 display was switching over between Preselect and Base mode regularly....

What factor will switch the machine from Base to Preselect Load and vice versa..??
What factor should keep the machine in Base Load/Preselect Load constantly...??
I thought, once the machine is on Temp Control Mode, it wud be always in base load... Is it not right??

Ours is a 20MW Frame V machine, MARK V controls, <I> Station display...

au revoir
Rahul

 

First, remember that the FSR Minimum Value Select "gate" will NEVER allow any operation above the TTRX calculated from the current value of CPD (or CPR, depending on the vintage of machine/control). If you enter a PreSelect Load value which would require an exhaust temperature above the current value of TTRX, the unit output will be limited to the output corresponding to the TTRX value.

(NOTE: Base Load control AND PreSelect Load control CANNOT be selected at the same time.)

PreSelect Load control is a form of Droop Speed Control (and we've covered that topic ad nauseum--in other words, please don't take us there again). When you enable PreSelect Load control, the turbine speed reference, TNR, is adjusted to control the unit's power output to the PreSelect Load setpoint currently active in the control system. TNR "drives" FSRN (FSR - Speed Control)--which is one of the inputs to the FSR Min Select "gate." FSRT (FSR - Temperature Control) is another one of the inputs to the FSR Min Select "gate."

You've said your units are nominally rated (or, iso output) at 20 MW. If you set a PreSelect Load setpoint of 20 MW and enable PreSelect Load control, you will be operating very close to exhaust temp control--or Base Load. If the ambient conditions cause the actual exhaust temp to be equal to TTRX, then the Status Field message will indicate Base Load (or, more correctly, Exhaust Temp Control). If the unit is being operated "on the edge" of exhaust temp control with a 20 MW PreSelect Load setpoint, the message field will toggle between 'PreSelect Load' and 'Base Load'--just as you have described.

If you select Base Load, what the Speedtronic turbine control system does is increae FSRN until it is greater than FSRT, and adjusts FSRN to track FSRT as it goes up and down (which why you see the RAISE- and LOWER SPD/LOAD targets flash while the unit is operating at Base Load--TNR is being adjusted to cause FSRN to track FSRT; actually any time TNR is adjusted up or down you will the RAISE- or LOWER SPD/LOAD targets flash).

By keeping FSRN above FSRT, the unit will not toggle back and forth between Droop Speed Control (which is "driven" by PreSelect Load control when PreSelect Load control is enabled).

If it is desired to operate the unit at Base Load, allowing the Speedtronic to maximize power output at all ambient conditions, the unit should be operated with Base Load control enabled. Power output will be whatever it will be when being operated with Base Load control enabled.

Presuming your units are nominally (iso) rate for approximately 20 MW, and it is desired to maintain gas turbine output below 20 MW (or, in other words, not to let gas turbine power output exceed 20 MW), then one can enter a PreSelect Load control setpoint of 20 MW and then enable PreSelect Load control. This will limit power output to 20 MW if ambient conditions were to permit operation above 20 MW, and would maintain Base Load (maximum power output) under all other ambient conditions.

HOWEVER, when operating in such a condition, the Status Field message will toggle back and forth between 'PreSelect Load' and 'Base Load'--which doesn't hurt a thing.

There is usually an "FSR Display" on most <I>s or GE Mark V HMIs which has a series of bar-graphs, one for each of the inputs to the FSR Min Select Gate. You can watch what's happening when you are operating the unit on PreSelect Load control with a PreSelect Load setpoint of 20 MW--the bar-graphs for FSRN and FSRT will be equal heighth and the "active" color will toggle between the two when the Status Field indication is toggling between 'PreSelect Load' and 'Base Load.'

When Base Load is selected, FSRN will be maintained at a constant differential greater than FSRT--and FSRT will always be the "active" value.

You should take the above descriptions and use the Realtime Rung Display and follow the operation of the unit. It is one very good method of learning to read BBLs and the CSP--when you know what's happening and see it/confirm it happening.

markvguy

 

MarkV guy that was a clear explanation of FSR control.

In Frame V machine there is an option of Peak load selection. When it is selected then the max exhaust temperature allowable will be about 20DegC above the normal calculated value. By this for short periods we can run the machine at slightly higher load (>20MW)

My question to MarkV guy - When operating in partload(speed control)the above option of exceeding the normal exhaust temperature control is not given. How to incorporate the requirement?

 

Asrajkumar,

Thanks for the feedback.

Your question is not very clear. Not all GE-design heavy-duty gas turbines have Peak Load capability in the control systems--it's a purchased option, with, as you have stated, clear limitations (it can't be used for extended periods without accelerating the maintenance intervals and the replacement of the hot gas path parts).

To get to Base Load, one has to load the unit using Droop Speed Control until it reaches CPD-biased (Compressor Pressure - Discharge) exhaust temperature control. Peak Load is just another CPD-biased exhaust temperature control--with a slightly higher "firing" temperature limit (which results in a slightly higher exhaust temperature).

When Peak Load is selected, the Exhaust Temp algorithm/BBL (Big Block Language) block inputs are "switched" to a different set of values which raises the temperature control calculations to a higher "curve"--which is also CPD-biased exhaust temperature control, but at a slightly higher output and firing temperature and higher exhaust temperature.

You can't operate a GE-design heavy duty gas turbine at Base- or Peak Load while operating at Part Load. And, to get to Peak Load, the unit has to be loaded "through" Base Load.

You seem to be asking how to "add" Peak Load capability to the unit if it doesn't already have it. You need to contact GE to determine if your unit has the necessary hot gas path components to withstand the elevated firing temperature, and calculate a new set of values and can make the necessary modifications to the control system to select/enable Peak Load if possible. Of course, this is not free....

They will also counsel you about the accompanying affects of running the unit at a higher load--decreased time between maintenance outage intervals and increased degradation of hot gas path parts. Depending on the control system, a "timer" (time "accumulator") will be added to automatically keep track of operating hours at the higher firing rate.

markvguy

 

Thank you for the guidelines "Mark V guy".