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7FA Baseload Issues During 1x1 Operation
Gas turbine not going into base load during 1x1 operation. The plant is a 2x1 combined cycle.

I have scoured the site search with no luck. My question is in regards to our gas turbine not going into base load during 1x1 operation. The plant is a 2x1 combined cycle w/ GE 7FA's & D-11 ST. Control system is Mark VIe which was installed last year. We currently have a unit down for maintenance and have been running our online unit in preselect to follow dispatch. If we put the unit in "Baseload" from the Mark VIe, TNR will wind up and keep going passed 104.3%, FSR will go to 100% and will never go into Temp Control. When the FSR hits 100% turbine status will say "n/a". Again this only happens when we are 1x1, normally we run both units in "External Load" control and let our ADS dispatch control plant load through the DCS.

I have dug into the logic a bit but I'm having a hard time. If any more info is needed please let me know.

Fifty7Chevy,

Is that a two-door, Bel Air, four-door, pick-up or Cameo?

Has the unit been running properly before this outage?

Does the load seem to be higher than normal?

What is the load when FSR gets to 100%?

What are the gas control and SRV positions when FSR gets to 100%? (I'm presuming the unit has DLN 2.6 combustors.?.?.?)

What are the values of TTRX and TTXM?

Are you certain the CDP (96CD-1, -2 & -3) and AFPAP (96AP-1, and -2 & -3, if so equipped) transmitter(s) are calibrated and working correctly?

What are the individual CPD transmitter readings when the unit is at or near 100% FSR? What are they when the unit is FSNL? At 25% load? At 50% load?

What is the P2 pressure (usually FPG2) when synchronized?

What is the P2 pressure reference (usually FPRG)?

What is the SRV position at FSNL, 25% load, 50% load, and when FSR is at 100%?

I loathe asking this, but are you certain of the gas control valve LVDT calibrations (other than the SRV)? (I'm presuming the LVDT calibrations were done recently, or that they haven't been done in a while.?.?.?)

Are the GT emissions (before any stack emissions reduction (SCR) treatment) normal/within normal range?

What position are the IGVs at when the FSR is at or near 100%?

Is IBH operating correctly (at higher loads, the IBH control valve should be fully closed--check visually)?

Are you certain the compressor bleed valves are all closed? (Not just by the limit switch indications... but by visual verification.)

What Process- and Diagnostic Alarms are present on the Mark VIe (all of them please--even if they don't seem relevant)?

You said a couple of different things.... First you said:

>We ... have been running our online unit in
>preselect to follow dispatch.

Then you said:

>If we put the unit in "Baseload" from the Mark VIe,
>TNR will wind up and keepgoing passed 104.3%,
>FSR will go to 100% and will never go into Temp Control.

That's two different things. If you are operating the unit in Pre-Selected Load Control and you put in a load setpoint higher than is possible, it's conceivable the unit could go past 104% TNR and yet still be in CPR-biased exhaust temperature control. The Status Field should say "EXHAUST TEMP CONTROL" but there may be a problem with Enumerated Data Display (there are lots of problem with GE Mark VIe HMI displays....).

If you select "BASE LOAD" from the 'Load Control' section of the Start-up Display the unit should normally stop at about 104% TNR when TTXM equals TTRX and the unit is in CPR-Biased Exhaust Temperature Control--and the Status Fiel of the display should also say "EXHAUST TEMP CONTROL" (or TEMPERATURE CONTROL). But, again, there may be some problem with the Enumerated Data display configuration.

When you're running in Pre-Selected Load Control is the operator inputting Pre-Selected Load Setpoints from the HMI, or is it being done from an external signal (Remote Load Control, such as AGC, like you mentioned before)?

When TTXM is equal to TTRX AND the IGVs are at maximum operating angle (CSGV = CSKGVMAX) then the unit IS in exhaust temperature control--either CPR-biased, or Back-up. And if it's in Back-up Exhaust Temperature Control then there should be a Process Alarm to that effect. Again, there may be some issue with the Enumerated Display that is not providing the proper indication. But, when TTRX and TTXM are equal WHEN THE IGVs ARE FULLY OPEN, then the unit IS on exhaust temperature control--even if the display doesn't say so.

About the only way I could envision a normal unit with properly calibrated and properly operating CPD and AFPAP transmitters going to 100% FSR WITHOUT hitting exhaust temperature control (meaning that insufficient fuel was getting into the unit to reach CPR-biased exhaust temperature control) would be if there was some kind of fuel restriction--and that would most likely be evident in a full open SRV (at or very near 100%), and a P2 pressure that was either at or below the P2 pressure reference. And, in this case TTXM would be less than TTRX.

I suppose if the unit was passing through the IBH control valve, and/or the compressor bleed valves then it might not hit CPR-biased exhaust temperature control--but the load would be lower than normal, and so would CPD?

Lots of questions--but lots of unknowns. Again, most important: What are the values of TTRX and TTXM when the unit starts going above approximately 104% TNR and when it gets to 100% FSR?

Please write back with answers and data!

Fifty7Chevy,

The definition of Base Load (primary- (CPR-biased) or secondary exhaust temperature control) is when the IGVs are full open (at their maximum operating angle, CSKGVMAX) AND the actual exhaust temperature (TTXM) equals the exhaust temperature control reference (TTRX).

TTRX is the lesser of TTRXS or TTRXP, the secondary exhaust temperature control reference and the primary exhaust temperature control reference. The primary exhaust temperature control reference is biased by CPR (Compressor Pressure Ratio), and the secondary exhaust temperature control reference (for GE-design F-class heavy duty gas turbines with DLN combustors) is usually FSR bias. An F-class unit should, theoretically, never be on secondary exhaust temperature control.

Now, having said all of the above, for GE-design F-class heavy duty gas turbines with DLN combustors the IGVs are used to keep the exhaust temperature very close to the maximum allowable (the exhaust temperature control reference--CPR-biased exhaust temperature control reference) during part-load operation. So, it won't be unusual to find TTXM as being close to TTRX at part load. But, if CSGV (the actual IGV position) is equal to CSKGVMAX AND TTXM is equal to TTRX, then the unit is technically on exhaust temperature control--or at Base Load. Even if the display doesn't indicate that.

I just wanted to clarify the TTXM vs. TTRX thing--TTXM is quite often, at part load, equal to TTRX, but the unit is NOT at Base Load. Base Load is defined to be when the IGVs are the maximum operating angle AND TTXM equals TTRX.