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.
TTRF1 COMBUSTION REFERENCE TEMPERATURE
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
TTRF1 COMBUSTION REFERENCE TEMPERATUREThis 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 ReferenceThis 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 ReferenceThis 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 CommandThis 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 LimitingThis one I'm not familiar with and have not encountered in any sequencing/application code.
TTRXHD Temp Interpol Value for Lean/Lean thresholdsThis is another one I'm not familiar with and have not encountered in any sequencing/application code.
TTRXP Temp Control Primary Temp ReferenceThis is the CPD- or CPR-biased exhaust temperature control reference.
TTRXS Temp Control Backup Temp ReferenceThis 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 control.com, some many times. Use the 'Search' feature of control.com 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?
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 control.com 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 control.com over the past 15+ years, and all of those threads can be accessed using the 'Search' feature of control.com.
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.
Thanks CSA for very detailed reply.
As I understand from the above reply that TTRX is calculated FIRST and is fed to Exh Temp IGV control to select minimum possible opening.
Can this lead to over firing the unit and degrading the units maintenance factor?
And particularly when axial compressor fouls, CPR/CPD decreases for the same IGV angle at any part load: How does this affect over firing?
TTRX is ALWAYS being calculated, even when the unit is not running. It is fed to BOTH the IGV Exhaust Temperature Control algorithm AND the CPD- (CPR-biased) Exhaust Temperature Control algorithm.
IF IGV Exhaust Temperature control is enabled and active, it will use TTRX to move the IGVs to the position that makes TTXM equal to TTRX. This occurs at PART LOAD, because the definition of Base Load is when the IGVs are at their maximum operating angle. IGV Exhaust Temperature Control is only active at Part Load (or on units that have DLN combustors, and then it is always enabled and only active at Part Load (less than Base Load)).
IGV Exhaust Temperature Control is used for one of two purposes, depending on the type of combustors used on the turbine. Primarily, for non-DLN combustor-equipped units it is used to maximizing exhaust temperature at part load to increase steam temperature above that which would be experienced without IGV Exhaust Temperature Control. It helps to make hotter steam at Part Load when the exhaust temperature would otherwise be lower.
For units equipped with DLN combustors IGV Exhaust Temperature Control is used to control the air flow through the axial compressor and combustors at Part Load so as to maintain flame stability in the combustor. The designers know what the air flows are at various IGV angles (and corresponding exhaust temperatures) and can easily use IGV Exhaust Temperature Control to help with flame stability.
Overfiring is difficult to achieve as long as the CPD transmitters are correctly calibrated and properly maintained and the isolation valves are properly opened. If the unit has DLN combustors, or a Performance Monitoring Package, it will also have atmospheric (barometric) pressure transmitters, and these are used to calculate CPR (Compressor Pressure Ratio) which is a fancy CPD that is used to calculate TTRX. So, these transmitters, if used, must also be properly calibrated, maintained and valved-in. (And, they are usually neglected and not properly maintained or even valved-in during normal operation. Sometimes, the sensing lines are also allowed to fill with dust/dirt, insects and/or condensate and are not regularly cleaned or drained (a manual operation).)
Axial compressor fouling with cause CPD to decrease, and when CPD decreases TTRX will increase--which doesn't change the actual firing temperature. One thing that most people don't understand about TTRX is that the exhaust temperature reference curve (TTRX) represents CONSTANT FIRING TEMPERATURE. So, the actual firing temperature doesn't change as TTRX changes, just the relationship between CPD (or CPR) and TTXM changes.
A lot of damage can be done to hot gas path components by operating the unit with high exhaust temperature spreads for a long period of time without over-firing the unit. Sites have also been known to (improperly) jumper exhaust T/Cs to avoid having to shut down to replace failed exhaust T/Cs. Also, some sites have been found to change Control Constants which affect exhaust temperature spread calculations, or exhaust temperature reference calculations, and this can lead to over-firing.
DLN units, operated for any length of time in Lean-Lean or Extended Lean-Lean modes can also experience hot gas path parts problems. Both of these modes are only intended to be transitory or temporary modes as the unit is designed to operate in Premix Steady-State mode. The high temperatures in the combustor during Lean-Lean and Extended Lean-Lean can cause problems particularly for the combustion liners, but if there are also high exhaust temperature spreads while operating for any length of time in either of these modes that can cause problems for transition pieces and/or turbine nozzles and -buckets, also.
Remember: TTRX represents CONSTANT FIRING TEMPERATURE. Firing temperature is not measured; it is only approximated. A dirty axial compressor and/or IGVs will reduce the air flow through the compressor (and increase the axial compressor discharge temperature), and the turbine control system--with properly calibrated and maintained sensors--will sense this and reduce gas fuel flow appropriately at Base Load when operating on CPD- or CPR-biased exhaust temperature control. The only time fuel flow is affected by reduced axial compressor discharge pressure is when the unit is operating at Base Load, when the CPD transmitters (and the atmospheric pressure transmitters, if so equipped) are feeding the exhaust temperature reference calculation AND that is being used as the setpoint for fuel flow (by flowing enough fuel to make TTXM equal to TTRX). At Part Load when IGV exhaust temperature control is enabled and active reduced CPD will not have much of an effect on gas fuel flow-rate, or firing temperature. At Part Load, fuel flow is a function of Droop Speed Control (FSRN); at Base Load fuel flow is a function of FSRT.
Hope this helps! It certainly seems someone is trying to blame the turbine control for some combustion- or hot gas path parts failures, and that would be extremely unusual if everything is normal and properly maintained, and the sensors are in working order and their isolation valves are properly opened during normal operation, and the exhaust T/Cs are working and not (improperly) jumpered out. And, if the unit is not operated for extended periods of time with high exhaust temperature spreads or in Lean-Lean or Extended Lean-Lean for any period of time other than when normally loading/unloading the unit, or when troubleshooting an inability to remain in Premix Steady State combustion mode.
Thanks for your prompt and detailed reply.
QUOTE: "Axial compressor fouling with cause CPD to decrease, and
when CPD decreases TTRX will increase--which doesn't change
the actual firing temperature. One thing that most people
don't understand about TTRX is that the exhaust temperature
reference curve (TTRX) represents CONSTANT FIRING
TEMPERATURE. So, the actual firing temperature doesn't
change as TTRX changes, just the relationship between CPD
(or CPR) and TTXM changes."
Does the above necessitate one to have a back up control curve...? The unit is a mechanical drive assisted by a helper motor when GT reaches base load for to main driven load. Speed is variable.
It is said by control engineers that with fouled axial compressor the calculated TTRX is significantly higher as compared with the TTRX calculated for a clean / pristine engine. And this results in over firing as control allows more fuel at a particular operating conditions.
Can you further explain a single shaft mechanical drive control system with DLN combustors?
If the unit is operating on Base Load (CPR-biased exh temp control), no matter what the exhaust temperature reference (TTRX) is, the firing temperature is constant. Again, the exhaust temperature reference curve represents constant firing temperature.
TTRF1 is a calculated value of firing temperature. If all the associated instrumentation is calibrated, scaled and working correctly the calculated value of firing temperature (TTRF1) should be relatively constant when the unit is operating on CPR-biased exh temp control. That should be easy enough to verify; just trend or monitor TTRF1 when the unit is on CPR-biased exh temp control. As conditions change (ambient temperature; compressor cleanliness; etc.) TTRF1 should remain relatively constant. (REMEMBER: TTRF1 is a calculated value of firing temperature. It isn't a perfect calculation, and its ONLY used to switch combustion modes.)
The helper motor is just that--an additional source of torque for the driven device. The turbine and turbine control system probably don't "recognize" the helper motor as far as any control function (without being able to see the control system configuration and programming it's really impossible to be completely sure about that).
The CPR-biased exh temp control curve in the Mark* is derived from monitoring actual firing temperature in a combustion laboratory and observing the relationship between exhaust temperature and CPD (or CPR). And the relationship between exhaust temperature and CPD (CPR) is constant at Base Load. The exhaust temperature control curve maintains a constant firing temperature when the unit is operating on CPR-biased exh temp control--thats what the curve does.
Compressor cleanliness (and IGV cleanliness) do affect CPD, and therefore CPR. And when they are not clean, CPD, and CPR, are lower. This results in a higher exhaust temperature for the SAME (constant) FIRING TEMPERATURE.
When the unit is at PART LOAD, firing temperature DOES change with load. Just trend TTRF1 as the unit is Loaded or unloaded (below Base Load).
But, at Base Load TTRF1 should remain constant regardless of compressor or IGV cleanliness and ambient temperature or even helper motor status.
That's how I believe it works, and should work. Many people think firing temperature changes as exhaust temperature changeswhen operating on CPR-biased exh temp control. BUT, that's not how CPR-biased exh temp control should work. Again, trend TTRF1 when the unit is operating on CPR-biased exh temp control. It will most likely remain fairly constant.
It is counter-intuitive--that as exhaust temperature changeswhen at Base Load that firing temperature doesn't change. But that's not how the physics of the machine actually work.
It would be really great if you would trend TTRF1 while the unit is loading and unloading, and while it is on CPR-biased exh temp control (Base Load) and report your findings. It should be relatively simple--TTRF1 should go up and down as load changes, and it should remain relatively stable when at Base Load (regardless of the helper motor status).
I may be wrong (I have been wrong many times before), but I don't think so as far as this thread is concerned. Please write back to let us know what you observe. Enquiring minds want to know.
Looking forward to hearing back from you about how TTRF1 changes with load (below Base Load), and how it doesn't at Base Load.
What I've attempted to draw below is a graph, with TTRX (or TTXM) on the left, vertical axis. Exhaust temperature (reference or exhaust) increases vertically, from bottom to top. CPR (or CPD) on the horizontal axis at the bottom. Compressor pressure (ratio or discharge) increases going from left to right. The horizontal line at the top of the graph represents what's called the "Isothermal" exhaust temperature, and that is the maximum allowable average exhaust temperature for the unit. The line on the right side of the graph, the one that slopes down, represents constant firing temperature.
Anywhere along the sloped line the firing temperature should be 2040 deg F. And the sloped line (along with the horizontal Isothermal line) represent the maximum allowable exhaust temperatures for any given value of CPR (or CPD) at any time--during starting, loading, unloading, whenever.
For the purposes of this example, let's say the design firing temperature for the unit this graph represents is 2040 deg F, and the Isothermal exhaust temperature limit is 1150 deg F. That means that when the unit is operating at Base Load on CPR- (or CPD-) biased exhaust temperature control that for any point on the sloped line the firing temperature should be approximately equal to 2040 deg F.
TTRX |---------- <---Isothermal Exhaust Temperature Limit
(or | \ (1150 deg F)
| \ Exhaust Temperature Control "Curve"
| \ <--- Constant Firing Temperature
| \ (2040 deg F)
|_______________________ CPR (or CPD)
(or | \
| \ Exhaust Temperature Control "Curve"
1109***************\<--- Constant Firing Temperature
| * \ (2040 deg F)
| * \
| * \
| * \
| * \
|____________*__________ CPR (or CPD)
(or | \
| \ Exhaust Temperature Control "Curve"
1084*******************\<--- Constant Firing Temperature
| * \ (2040 deg F)
|________________*_______ CPR (or CPD)
1140*******----- <--- Isothermal Exhaust Temperature Limit
TTRX | * \
(or | * \
TTXM)| * \
| * \
| * \
| * \
| * \
| * \
| * \
|____*___________________ CPR (or CPD)
|---------- <--- Isothermal Exhaust Temperature Limit
| * \
TTRX | * \
(or | * \
TTXM)| * \
| * \
| * \
| * \
|________*_______________ CPR (or CPD)
When the IGVs ARE at maximum operating angle AND the unit is operating on CPR-biased exhaust temperature AND CPR is MORE THAN the value where the sloped line intersects the horizontal Isothermal exhaust temperature limit line, the firing temperature will be equal to the design firing temperature (approximately).
When the IGVs ARE at maximum operating angle AND the unit is operating on CPR-biased exhaust temperature control AND CPR is LESS THAN the value where the sloped line intersects the horizontal Isothermal exhaust temperature limit line, the firing temperature will be LESS than the design firing temperature.
Dirty axial compressors, or dirty IGVs will cause the actual CPD to be less than it would be if the components were clean, and in turn this will cause the exhaust temperature to be slightly higher than it would otherwise be. And when the IGVs are at maximum operating angle AND the unit is operating on CPR-biased exhaust temperature control AND CPR is MORE than the value where the sloped line intersects the horizontal Isothermal exhaust temperature limit line, the firing temperature will be equal to the design value (approximately).
This is how gas turbines work. The sloped line IS the exhaust temperature control line--it just happens to be limited by the Isothermal limit (which is to protect exhaust components (diffusers; HRSGs; etc.)). The sloped line represents constant firing temperature. And it just happens that for a constant firing temperature the relationship between exhaust temperature and CPR is "variable." Ambient conditions (ambient temperature and pressure, mostly), inlet filter differential pressure and compressor- and IGV cleanliness can affect the relationship. But, in no case should the firing temperature exceed the design value (for optimal parts life).
Again, the unit you are working on has DLN combustors. Firing temperature is calculated when the unit has DLN combustors. You can monitor, or trend, firing temperature (TTRF1), load (DW), IGV angle (CSRGV), CPR, average exhaust temperature (TTXM), exhaust temperature reference (TTRX), as the unit is loaded and unloaded. What you should find is that TTRF1 increases as the unit is loaded from synchronization to Base Load. Average exhaust temperature will increase to 1150 deg F (in our example; your unit will likely be slightly different) as the unit is loaded (because the IGVs are being used to limit air flow into the DLN combustor, which causes the exhaust temperature to be higher than it would otherwise be--BUT, the firing temperature is still less than design (less than the sloped line!). As the unit nears Base Load and the IGVs approach maximum operating angle, TTXM may actually start to decrease a little bit until intersection of the actual exhaust temperature and CPR touches the sloped line--and then the firing temperature will be equal to design (approximately).
I hope this helps! It sounds like there are some conflicting opinions at your site--and that's pretty normal. People don't always really understand exactly what's happening, especially since some of it is counter-intuitive. And, since there isn't a lot of written documentation available, and people don't really read what IS available but rely on what they hear or are told without thinking critically about what they hear or are told, there are a LOT of myths and falsehoods out there. Which get repeated, and repeated, and embellished, and repeated.
Do not over-analyze this; it's really NOT that complicated. It's the negative slope of the exhaust temperature reference line that confuses people (actually, they just don't consider the negative slope...).
Thanks for taking the pain to draw the curves in this editor...This explanation has reinforced the understanding.
Intending to collect trend data from control software during loading / unloading/ start up...to get further clarity.
Referring to those nice graphs you wrote "Compressor pressure (ratio or discharge) increases going from right to left". Is it so?