Base Load Limit of Frame 5 Gas Turbine

Gent's,

I hope this message finds you all well.

We have GE Frame 5 gas turbines with Mark V control system. After major overhaul of one unit and during baseload test, the generator output reached:
- 26.1 MW
- 9.3 MVAR
- 0.942 pf
the turbine variables reached:
- TTXM = 474.6
- CTIM = 18.4
- FSR = 85%
- CPD = 9.3 bar

The generator nameplate mentions that generator rating is 25MVA, 20MW at 0.8pf.

We checked the Mark V constants and found that LK90MAX is 43.40 MW. Normally we do not put turbines on baseload. Every few months we put each turbine on baseload for few minutes to take a record of turbine capability.

Is LK90MAX responsible for limiting MW during baseload ? Why it is set too far from the generator rating (25MW at 1.0 pf) ?
What is the philosophy of limiting generator output ?

Thank you
 
Mohamedibr752,

Good questions--all.

I am extremely surprised at the values you supplied from the generator nameplate. The typical generator supplied by GE or one of its licensed packagers for use with a heavy duty gas turbine has a rating (in MW) which is higher than the turbine nameplate rating. For example, if the turbine nameplate rating was 24.9 MW, the generator nameplate rating might be something like 29.7 MW, or 32 MW. (For a few years, GE used the same generator for both Frame 5 and Frame 6 applications, rated at somewhere around 38 MW or so. It was actually less expensive for them to use the same generator for two different Frame-size machines than to build two different generators, one for each machine. Spreadsheets--you gotta love 'em!)

Anyway, the reason most generators have a slightly higher rating than the turbines they are driven by is that the output of a gas turbine can exceed it's nameplate rating when the ambient temperature is below the turbine nameplate rating, and the compressor is in a new and clean condition, and the turbine inlet air filters are clean, and the turbine clearances are within specifications, and the exhaust back-pressure is not excessive, and the IGV LVDT feedback has been properly calibrated. The lower the ambient, the higher the output of the turbine--which means the generator output increases also when the ambient temperature is below the turbine nameplate rating. So, it's necessary to have a generator coupled to the turbine that is capable of putting out more power than the turbine nameplate rating for the occasions when the turbine is being operated at Base Load AND the ambient temperature is below the turbine nameplate rating AND the machine is in good condition.

It's possible the generator at your site was upgraded to allow it to produce more power, and/or the generator was a replacement for the original generator and the new generator was upgraded (but, typically--if GE performs the generator upgrade, anyway, a new nameplate will be supplied to replace the original nameplate to avoid any later confusion). You did NOT list the turbine nameplate rating (including ambient temperature), which would help us to at least try to understand or explain what might be happening.

Now, to the LK90MAX Control Constant value. It's very sad to say, but, many field service personnel performing commissioning of GE heavy duty gas turbines were not properly trained and do not know to examine a select group of Control Constants as kind of a "sanity" check when commissioning a GE-design heavy duty gas turbine. Along with this situation, the people who performed the factory configuration of GE turbine control systems were also not trained and never became aware of problems with the default Control Constant values which the computers would produce during the initial configuration of GE-design heavy duty gas turbines. So, they didn't check these very same Control Constants for correctness before shipping the software to the site to be downloaded to the turbine controllers. And, at that time (and to this date, as I understand it!) there is no automated program or application that any person can run to perform a very basic sanity check of Control Constants before shipping or commissioning a GE-design heavy duty gas turbine.

During the Mark V production era the very first heavy duty gas turbine which was to be equipped with a Mark V was to be a Frame 6 (but it wasn't, actually!). And so the default Control Constants which were loaded into the database were those for a Frame 6--which the LK90MAX value you listed would be consistent with (for a Frame 6 which was to be installed and operated in a colder, Northern climate). The generator would normally be rated at or near this Control Constant value (but not always).

LK90MAX, when used with the L90LVn algorithm block, does serve to limit the output of the turbine (and generator--because the generator can only produce what the turbine is sending it through the load coupling). But, it only really protects the generator when it's properly set--and I would estimate the value in your case was not properly set.

LK90MAX IS NOT responsible for limiting base load during output. CPD (or CPR) biased exhaust temperature control is responsible for "limiting" turbine-generator output--unless the turbine output would cause the generator to exceed the value of LK90MAX. The presumption is that the IGVs are at the maximum operating angle (usually Control Constant CSKGVMAX--which means the IGV LVDT feedback is properly calibrated), and the CPD transmitter(s) are properly calibrated. (The basic "definition" of Base Load is IGVs at maximum operating angle AND actual exhaust temperature is equal to the calculated exhaust temperature limit based on CPD transmitter feedback.) Again, if the generator output is greater than LK90MAX then the Mark* would limit the gas turbine output to some value below the calculated exhaust temperature limit for the operating condition.

LK90MAX is usually set to protect the generator--because generator power output is primarily a function of the amperes flowing in the generator stator winding conductors. And, amperes flowing in conductors (generator stator windings) causes heat--and the rating of a generator is typically the value at which the conductor insulation will not be damaged by the heat generated by the current flowing in the generator stator winding conductors. I have seen the odd case where LK90MAX was set to protect the load coupling between the turbine and generator (or between the Load Gear and generator as the case may be) because the load coupling was not rated as high as the generator (but was higher than the turbine nameplate rating).

Looking further at the data you supplied, I am very surprised at the load being produced by the generator. That's possibly consistent with a Frame 5 in a new and clean condition with upgraded turbine nozzles and buckets and being operated at an ambient temperature below the turbine nameplate rating value. But, in my experience, 9.3 bar is pretty high for a Frame 5, suggesting the CPD transmitter or scaling may not be very accurate. It would be necessary for you to supply the turbine nameplate rating details (all of them--not just the power output, but including the ambient operating conditions as well). And, it would be necessary for you to supply the TTKn_x control constant values, as well as the CPD transmitter scaling values and the CPD transmitter scaling I/O Configurator Constants, as well, for us to be able to make a guess as to what's going on. Also, an FSR of more than 80+% is also pretty unusual, unless the fuel quality is low, or the gas control valve LVDT feedback calibration is not very accurate (I'm presuming the unit was being operated on gas fuel). But, again, if the axial compressor was clean and its internal tolerances were all nearly new or well within specification, AND the IGV LVDT feedback calibration was very accurate, and the turbine inlet air filters were nearly new and clean, AND the exhaust duct back-pressure was well within specification, AND the turbine nozzles and buckets were all well within specification.

I have a final question that is mostly for my own curiosity: When you "put the unit on Base Load," HOW do you put the unit on Base Load? Do you use the BASE LOAD button on the HMI, or do you put in a very high value of load (an impossibly high value to achieve) and let the unit reach CPD-biased exhaust temperature control?

Hope this helps, and thanks in advance for your answer to my question.
 
Dear CSA,

Thank you very much for your reply.

Here are the data you required:
The turbine nameplate is:
Type : PG 5371 (PA)
Nominal power output : 19570 kW - base load, gas fuel (@ 45 C & 0.989 bar)
Nominal speed : 5094 rpm
TTKn_x control constants:
TTK0_C -- Exhaust Temp Control Curve #1 CPD Ref Corner,CPD B psi <76.88 psi>
TTK0_I -- Exhaust Temp Control Curve #1 TX Ref Isothermal deg F <1040 deg F>
TTK0_K -- Exhaust Temp Control Curve #1 TX Base Slope,DW BIA % <45.7 %>
TTK0_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK0_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK0_M -- Exhaust Temp Control Curve #1 TX Ref Slope F/% <3.847 F/%>
TTK0_S -- Exhaust Temp Control Curve #1 CPD Ref Slope F/psi <2.291 F/psi>
TTK1_C -- Exhaust Temp Control Curve #2 CPD Ref Corner,CPD B psi <65.9 psi>
TTK1_I -- Exhaust Temp Control Curve #2 TX Ref Isothermal deg F <1100 deg F>
TTK1_K -- Exhaust Temp Control Curve #2 TX Base Slope,DW BIA % <38.3 %>
TTK1_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK1_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK1_M -- Exhaust Temp Control Curve #2 TX Ref Slope F/% <3.00 F/%>
TTK1_S -- Exhaust Temp Control Curve #2 CPD Ref Slope F/psi <2.101 F/psi>
TTK2_C -- Exhaust Temp Control Curve #3 CPD Ref Corner,CPD B psi <133.88 psi>
TTK2_I -- Exhaust Temp Control Curve #3 TX Ref Isothermal deg F <1050 deg F>
TTK2_K -- Exhaust Temp Control Curve #3 TX Base Slope,DW BIA % <54.120 %>
TTK2_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK2_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK2_M -- Exhaust Temp Control Curve #3 TX Ref Slope F/% <3.282 F/%>
TTK2_S -- Exhaust Temp Control Curve #3 CPD Ref Slope F/psi <1.896 F/psi>
TTK3_C -- Exhaust Temp Control Curve #4 CPD Ref Corner,CPD B psi <0.0 psi>
TTK3_I -- Exhaust Temp Control Curve #4 TX Ref Isothermal deg F <0 deg F>
TTK3_K -- Exhaust Temp Control Curve #4 TX Base Slope,DW BIA % <0.0 %>
TTK3_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK3_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK3_M -- Exhaust Temp Control Curve #4 TX Ref Slope F/% <0.0000 F/%>
TTK3_S -- Exhaust Temp Control Curve #4 CPD Ref Slope F/psi <0.00 F/psi>
TTK4_C -- Exhaust Temp Control Curve #5 CPD Ref Corner,CPD B psi <0.0 psi>
TTK4_I -- Exhaust Temp Control Curve #5 TX Ref Isothermal deg F <0 deg F>
TTK4_K -- Exhaust Temp Control Curve #5 TX Base Slope,DW BIA % <0.0 %>
TTK4_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK4_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK4_M -- Exhaust Temp Control Curve #5 TX Ref Slope F/% <0.0000 F/%>
TTK4_S -- Exhaust Temp Control Curve #5 CPD Ref Slope F/psi <0.00 F/psi>
TTK5_C -- Exhaust Temp Control Curve #6 CPD Ref Corner,CPD B psi <0.0 psi>
TTK5_I -- Exhaust Temp Control Curve #6 TX Ref Isothermal deg F <0 deg F>
TTK5_K -- Exhaust Temp Control Curve #6 TX Base Slope,DW BIA % <0.0 %>
TTK5_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK5_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK5_M -- Exhaust Temp Control Curve #6 TX Ref Slope F/% <0.0000 F/%>
TTK5_S -- Exhaust Temp Control Curve #6 CPD Ref Slope F/psi <0.00 F/psi>
TTK6_C -- Exhaust Temp Control Curve #7 CPD Ref Corner,CPD B psi <0.0 psi>
TTK6_I -- Exhaust Temp Control Curve #7 TX Ref Isothermal deg F <0 deg F>
TTK6_K -- Exhaust Temp Control Curve #7 TX Base Slope,DW BIA % <0.0 %>
TTK6_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK6_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK6_M -- Exhaust Temp Control Curve #7 TX Ref Slope F/% <0.0000 F/%>
TTK6_S -- Exhaust Temp Control Curve #7 CPD Ref Slope F/psi <0.00 F/psi>
TTK7_C -- Exhaust Temp Control Curve #8 CPD Ref Corner,CPD B psi <0.0 psi>
TTK7_I -- Exhaust Temp Control Curve #8 TX Ref Isothermal deg F <0 deg F>
TTK7_K -- Exhaust Temp Control Curve #8 TX Base Slope,DW BIA % <0.0 %>
TTK7_LG -- Exh Temp Control Ref Slope, DW Bias F/MW <0.0 F/MW>
TTK7_LO -- Exh Temp Control Ref Corner, DW Bias MW <0.0 MW>
TTK7_M -- Exhaust Temp Control Curve #8 TX Ref Slope F/% <0.0000 F/%>
TTK7_S -- Exhaust Temp Control Curve #8 CPD Ref Slope F/psi <0.00 F/psi>

We use the base load button on the HMI.

So what do you think the best way to limit the base load of the turbine ? What do you think if we changed the LK90MAX constant to 25MW ? Would it affect turbine operation ? If not, how to calculate the correct value for LK90MAX ?

Thank you in advance.
 
Mahammedibr752,

Well, you didn't supply the CPD transmitter scaling.... For both the transmitter itself, and the input scaling of the Mark*. And, how recently was the CPD transmitter calibration verified to be in specification? (I'm presuming a single CPD transmitter; there may be more than one, for redundancy.)

I see the turbine nameplate rating is at 45 deg C--and you were operating the unit at 18.4 deg C. That could explain some of the high output power at Base Load on the day in question (presuming 18.4 deg C was the compressor inlet temperature on the day the unit was making 26.1 MW). Is this a normal compressor inlet temperature for the unit, or for this time of year (I'm presuming it's cooler where the unit is located at this time of year)? Does the unit have any kind of inlet air cooling (say, an evaporative cooler, or a chiller, or a fogger, or something like that)?

Do you have the Reactive Capability Curve for the generator? Sometimes it's called the "D-curve" because when GE manufactures the generator and provides the Reactive Capability Curve it is shaped like a capital D (or the concentric curves are shaped like a capital D--there is usually three curves, representing different cold gas temperatures (the "cold gas" in this case being the temperature of the air being used to cool the generator stator and rotor).

There isn't much difference in the MW rating of the turbine and generator (at 0.8 PF): 19.7 MW vs 20 MW .... That's odd--in my experience. Again, when the compressor inlet temperature is below the rating nameplate rating of the unit (45 deg C) the power output of the unit will be above the turbine nameplate rating. This is because if the axial compressor inlet temperature is colder than the turbine nameplate temperature rating the density of the air entering the axial compressor will be higher than it would be at the turbine nameplate temperature rating, and that means that for the same IGV angle the mass-flow of the air flowing through the unit will be higher than at the turbine nameplate temperature rating. And, gas turbines are really mass-flow machines--if one can increase the mass flow through the turbine by any means the power output of the turbine will increase. (Hence why inlet air cooling (evaporative coolers, or chillers, or foggers) are effective (in lower humidity environments!). So, if it was expected that the turbine in this thread was expected to operate more frequently at lower compressor inlet temperatures it would seem to me that a generator with a higher output rating would have been supplied. Or, maybe this turbine was expected to be more of a "peaker" unit--a unit that operates at higher ambient temperatures to provide power to supplement demand (such as from air conditioners--which consume a LOT of power) to support high power demand (loads), and wasn't expected to operate much at lower ambient temperatures.

This is why I ask about the Reactive Capability Curve. Whenever one is operating a combustion turbine-generator at or near the rating of the generator one should be monitoring the generator temperatures (stator temperature; cooling medium temperature (air or hydrogen) AND the capability curve. Many present-day HMIs have a Reactive Capability Curve display which can be used to quickly see how the unit is being operated--and if the operation is within the generator's capability to sufficiently cool itself (which is what the Reactive Capability Curve really represents). Any generator can produce just about any amount of power output if the power input is high enough--BUT, how long it can produce high power outputs is dependent on the ability of the heat produced by the current flowing in the generator stator to be effectively removed, thereby protecting the insulation of the generator conductors. Too much heat for too long can damage the insulation and that can cause very bad things to happen to the generator. So, by consulting the Reactive Capability Curve one can see if the unit is being safely operated and take appropriate action if not (meaning, reducing the load--either the real load (MW) or the reactive load (MVAr) or both, as the case may be). If the HMI has a Reactive Capability Curve, it's a good bet it's "animated" and will have a small cross-hair or something like that to show the MVA of the unit at the present time (presuming the generator has both a MW- and MVAr transducer). If the HMI doesn't have a Reactive Capability Curve, the operators should have quick and ready access to one and it should be in a plastic sleeve or laminated in plastic so that it lasts a good, long time--because it's a very important document.

Now, as for setting the "correct" value of LK90MAX in your case.... I don't really have a good answer for you. That's really going to be something the management/ownership of the unit/site is going to have to consider. Because, if you operate your unit at Base Load at the inlet temperature you cited for very long--unless the generator has been updated and the nameplate wasn't replaced (which it should have been)--the generator insulation is probably being thermally stressed. And, when someone--anyone--tells management/ownership they have to limit the output of the unit (output is usually how the site gets paid for the electricity they produce) they get pretty testy--and resistant to any immediate change. That would be limiting the revenue--the monies received for the power being produced--and that's not what they want to have happen. Especially if they're accustomed to the "extra" output, which means "extra" revenue.

Now, we really don't know how you operate the unit when it's NOT being tested at Base Load. And we don't really know how long the unit is being tested at Base Load. And we really don't know what you're trying to protect--by changing the value of LK90MAX. And THAT is the real question that someone has to consider and answer: What are you trying to protect--and when are you trying to protect it?

Because if you NEED that power in an emergency and LK90MAX prevents the unit from producing power, then that's not a good thing, either. The fact that the turbine nameplate rating and the generator nameplate rating are so close to each other (at the turbine nameplate temperature rating) complicates matters--at least in my mind. As currently set (43.40 MW), LK90MAX isn't providing any meaningful protection of the generator at all. And, I would venture to say it's not protecting the load coupling (between the turbine and the Load Gear), which is another consideration which needs to be considered (and I do not know where to tell you to find the load coupling rating value; that would have to come from the original supplier of the unit, or from the load coupling manufacturer). It's even possible that the nameplate rating of the Load Gear is being exceeded at 26.1 MW, and therefore LK90MAX isn't protecting the Load Gear, either.

It's not a simple answer--or decision. It all depends on how the unit is normally operated, how often--and for how long--the unit is tested at Base Load. And, what management/ownership expects the unit to be capable of doing at different times of the year....

But in NO CASE should the generator be operated outside the Reactive Capability Curve for any appreciable period of time (more than a couple of hours) without expecting some kind of degradation of the generator winding insulation (stator and rotor windings!).

Lots to consider, eh?
 
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