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Fr9E Exhaust Temperature Controlling Curves
Understand different exhaust temperature control parameters/ Limits

As a real novice in GT control, Would like to understand different exhaust temperature control parameters and how they act during acceleration and during normal part/base load operation.

These are

TTRX Temperature Control Reference
TTRXGV IGV Temp Control Reference
TTRXGVB Comp Op Lim Prot Biased IGV Exhaust Temp Command
TTRXH Set Point Interp Before Isothermal Limiting
TTRXHD Temp Interpol Value for Lean/Lean thresholds
TTRXP Temp Control Primary Temp Reference
TTRXS Temp Control Backup Temp Reference

Would like to have a detailed understanding of these as I am really novices in GT control.

1 out of 1 members thought this post was helpful...

This is the calculated value of the hot gases passing through the first stage turbine nozzle. It is used for switching combustion modes on DLN combustor-equipped GE-design heavy duty gas turbines.

TTRX      Temperature Control Reference
This is the calculated exhaust temperature reference for Base Load and for protection of the gas turbine hot gas path parts, including the exhaust diffuser.

TTRXGV    IGV Temp Control Reference
This is the calculated value of exhaust temperature reference used to prevent the IGVs from closing too much.

TTRXGVB   Comp Op Lim Prot Biased IGV Exhaust Temp Command
This is a biased value of exhaust temperature used for protecting the axial compressor when using IGV exhaust temperature control.

TTRXH     Set Point Interp Before Isothermal Limiting
This one I'm not familiar with and have not encountered in any sequencing/application code.

TTRXHD    Temp Interpol Value for Lean/Lean thresholds
This is another one I'm not familiar with and have not encountered in any sequencing/application code.

TTRXP     Temp Control Primary Temp Reference
This is the CPD- or CPR-biased exhaust temperature control reference.

TTRXS     Temp Control Backup Temp Reference
This is the secondary, or back-up, exhaust temperature control reference. It might be FSR-biased, or it might be MW-biased.

Have you tried looking in the Operation & Service Manuals for written descriptions of these terms? You should also investigate the Control Specification document supplied with every GE-design heavy duty gas turbine control system.

If you have specific questions about one at a time, you can try asking them here. Many of the above signals have been described before on, some many times. Use the 'Search' feature of to find lots of information.

Thanks CSA for your explanation.

At this time I would like to ask that in a part load condition, which of the temperature reference is being used mostly?

My simple understating is that control system selects the lowest fuel flow comparing all the control references.

I would like to know how the FSR % is calculated/decided from any of the below control references. Can we simply see these control temperatures in trends in MKVI? Is there a scheduled/linearized constants or array built in the software?

1 out of 1 members thought this post was helpful...


Mark* turbine control systems use a MIMIMUM SELECT function to choose which scheme is used to control the fuel being admitted to the turbine. And, none of the signals you asked about are used to directly control the fuel flow-rate.

FSR (Fuel Stroke Reference) is the output of the MIN SEL block that chooses between the various fuel control references. FSRN ((Droop) Speed Control FSR), FSRT (Exhaust Temperature Control FSR), FSRACC (Acceleration Control FSR), FSRSU (Start-up Fuel Control FSR), FSRSD (Shutdown FSR), FSRMAN (Manual Control FSR) and FSRMIN (Minimum FSR) all feed into the MIN SEL block, which chooses the least of all the values and that becomes FSR which is the amount of fuel to be admitted to the turbine.

Unfortunately we can't draw (very easily) in responses to threads, nor can we attach sketches or drawings or photos. So, while it would be easier if I could attached a sketch or drawing I can't.

When a unit is on Part Load (some load between zero load and Base Load) the lowest FSR input to the FSR MIN SEL block is usually FSRN. Droop speed control is how fuel is controlled between FSNL (100% rated Speed) and Base Load. The turbine speed reference, TNR, is raised or lowered when the operator clicks on RAISE- or LOWER SPEED/LOAD (and even when Pre-Selected Load Control is enabled and active), and it is compared to the actual turbine speed, TNH, and the error between the two determines how much fuel will be going into the combustors to produce torque which is applied to the generator rotor. Droop speed control has been covered MANY times and in MANY ways on over the past 15+ years, and all of those threads can be accessed using the 'Search' feature of

There should be a 'FSR Display' on the HMI you are using to monitor and control the turbine. That display has bargraphs of the various FSRs and you can visually see which FSR is in control.

TTRX is a calculated value that uses some array values and the CPR (Compressor Pressure Ratio) or CPD (Compressor Pressure-Discharge) value to arrive at TTRX--which is the absolute maximum allowable turbine exhaust temperature for any given operating condition. When the unit is operating at Base Load, TTRX is used to determine FSRT, and it is the lowest FSR input value to the MIN SEL block. When operating at Base Load the turbine control is being told to put as much fuel as possible into the unit while still protecting the hot gas path parts and the exhaust diffuser to produce as much torque as possible to produce as much electrical power as possible. And the exhaust temperature reference, TTRX, becomes the limit for the amount of fuel being admitted to the turbine. The Mark* compares TTXM (Turbine Temperature-Exhaust, Median (actually it's an average exhaust temperature)) and TTRX and controls the amount of fuel to make TTXM equal to TTRX.

During part-load operation when the IGVs are being used for either IGV Exhaust Temperature Control OR for DLN combustor air control the IGVs are closed to maximize exhaust temperature--up to TTRX. Maximizing exhaust temperature for units which exhaust into a HRSG (Heat Recovery Steam Generator; a "boiler") maximizes steam production at Part Load conditions which increases the overall plant thermal efficiency (a lower plant heat rate). TTRX sets the limit on exhaust temperature--so TTRX is used to derive the maximum amount the IGVs can be closed during part load operation to prevent TTXM from exceeding TTRX.

When the turbine has DLN combustors the IGVs are used to control the amount of air flowing into the combustors--BUT, that's NOT done by measuring air flow. GE knows, from decades of operating data and experimentation, how much air is flowing for given IGV angles. And, what they do is they modulate the IGVs to control air flow to maintain flame stability--and they do that by keeping the IGVs closed as much as possible. And the effect of closing the IGVs is to maximize exhaust temperature--but TTRX sets the maximum limit on exhaust temperature. So, TTRX is used to derive the IGV angle at Part Load, just like during IGV Exhaust Temperature Control.

(By the way, keeping the IGVs closed as much as possible during Part Load operation decreases turbine thermal efficiency (increases the turbine heat rate)--but if the unit exhausts into a HRSG that's good for the overall plant heat rate and steam production. And for units with DLN combustors, it's absolutely necessary to maintain flame stability.)

Droop speed control is at the heart of everything Part Load--including IGV position. It's a very misunderstood--but very simple and VERY powerful--universal control method for almost EVERY synchronous generator's prime mover (steam turbine; reciprocating engine; combustion (gas) turbine; hydro turbine; etc.). It's NOT a generator function--it's a prime mover function. For GE-design heavy duty gas turbines synchronized to a grid with other generators and their prime movers the unit is loaded and unloaded using Droop Speed Control and FSRN (even when Pre-Selected Load Control is being used, or when a DCS or other control system is providing a load reference to the Mark*.)

The last thing I can think of is that TTRX is actually the MINIMUM SELECTed value of TTRXP and TTRXS. Under normal conditions, TTRXP is always the lower of the two, and the turbine is designed to operate on TTRXP (CPR- or CPD-biased exhaust temperature control). TTRXS is a back-up exhaust temperature control that is meant to be used in the event that CPR or CPD isn't working--BUT, for DLN combustor-equipped machines they MUST have CPR (or CPD) feedback or they will be tripped by the turbine control system. The problem here is that if TTRXS isn't properly configured, or if there are problems with CPD transmitter inputs or AFPAP (Air Flow Pressure-Atmospheric Pressure) transmitter inputs then TTRXS can be less than TTRXP and this will artificially limit the amount of power the turbine being produced. (If you ever see the Process Alarm "Back-up Exhaust Temperature Control Active" or something similar, that's trying to tell the operator to tell the technician that something is amiss with the CPD or AFPAP and the unit is operating on TTRXS instead of TTRXP, and the problem should be corrected as soon as possible.)

Hope this helps!

In the Operations & Service Manual provided with the turbine you should be able to find some very generic and high-level overviews of the turbine control and protection schemes that should help enhance your basic understanding of GE heavy duty gas turbine control and philosophy. It's always best to have a read of those documents in the Manual to get the basics of turbine control and protection.