It is about the "standard droop speed control" and how it can be affected by the FPRG or a bad calibration of the gas valves...etc.
Compared to the CONSTANT SETTABLE DROOP SPEED/LOAD CONTROL, The STANDARD DROOP/SPEED CONTROL has "no FSR value feedback" in its regulation loop. This mode of governor control changes FSR in proportion to speed error (droop) directly under the following equation:
FSRN = FSKRN1 + FSKRN2(TNR - TNH)
where FSKRN1 is the Initially set full speed no load fuel reference as calculated, and FSKRN2 is the ratio of percent change in fuel flow reference to percent change in speed/load reference.
OK! at full speed no load TNR-TNH would be equal to 0. and the FSRN value would be equal to FSKRN1, initially caclulated by the OEM for a particular fuel flow rate. And this value is the opening command of the GCV, so for this particular fuel flow rate it depends on the fuel heating value! also of the P2 pressure refrence FPRG!
Lets have an example: suppose FSKRN1 = 30%, when the machine finishes its start-up sequence normally the TNH (HP shaft speed) would be equal to TNR(turbine speed reference). The FSRN = FSKRN1 = 30%, as the machine was firstly commissioned for sure at this stage everything would be stable, because the 30% value was enough to make the machine stable at FSNL 100% speed, for example with an FPRG (gas ration valve control pressure reference) = 13bar.
Lets suppose we have a problem with the P2 pressure either the FPRG constant value or the SRV valve or even the P2 transmitter. This problem causes the P2 pressure to be 10bar when the machine at FSNL. So when the machine reaches FSNL and FSRN = 30% > we won't be having enough energy to make it stable, and TNH would to tend to drop. When it drops, TNR-TNH would be higher than 0 which will make FSRN higher than 30% to reach the FSNL speed again. We will be having a non stable state of the machine fluctuating at FSNL
Myself I'm having a problem with this mode and how it is vulnerable to any misbehave from the gas valves or P2 transmitter, the P2 set point (FPRG), and even a problem with the fuel injectors because we do not have an FSR feedback value in the loop.
NOTE 1: This enquiry is the same for the base load GCV opening value, at base load. And if the droop value is 4%, so TNR-TNH = 4% and 4% * FSKRN2 + FSKRN1 = FSRN, so there is no FSR Feedback value in the loop, at any load point we will be having an particular opening command of the GCV NO MORE!
NOTE 2: i am comparing this regulation loop to all other load regulation loops for single or double shafts where the FSR feedback value is always present. So we will be adding a certain value the actual FSR until we reach the load reference. So the for a certain load point we won't be having a particular pre-calculated value of FSRN which will make the system control adapt the FSRN value to the present conditions and parameters.
example: load output = 100MW, FPR2 = 12bar, FSRN = 40 %
if the the P2 drops for whatever the reason we will be having - load output = 100MW, FPR2 = 9bar, FSRN = 60 %
So the control system rises the FSRN value to maintain the load output.
Let's just step back and look at this rationally, shall we?
The Expected Fuel Characteristics data is just an estimate--NO ONE EVER CHECKS THIS AGAINST THE ACTUAL RUNNING DATA DURING COMMISSIONING. (Unless there is a serious problem.)
GE-design heavy duty gas turbines don't have fuel injectors, they have fuel nozzles. The fuel nozzles have orifices, and the fuel flows through the orifices. The fuel nozzles are supplied by common manifold(s) (depending on the type of combustion system, conventional or DLN). Orifices can become plugged ("choked") and block fuel flow, BUT this is reflected in the exhaust temperature spread. Also, some fuel nozzles have been shown to leak causing more fuel than would be desirable to flow into the combustor--which would ALSO show up in the exhaust temperature spreads.
You are correct--there is NO FSR feedback in Droop Speed Control. None. Zero. Zilch. Nada. Niente. There doesn't need to be any.
The only time load comes into the "equation" when the unit is operating on Droop speed control is if Pre-Selected Load Control or some form of external load control is active (for units with "pure", straight Droop speed control). And, then, FSRN will be whatever is required to maintain the load setpoint, regardless of P2 pressure or GCV LVDT calibration(s).
The entire reason that P2 pressure is controlled and held constant during rated speed (which implies rated frequency!!!) operation is that by holding the fuel pressure upstream of the GCV(s) constant the flow-rate through the GCV(s) is proportional to stroke (or at least it was before the advent of independent gas control valves as used on DLN combustion-equipped units). That made it possible to have a value of FSR that was proportional to GCV stroke (position). DLN units still have FSR, it's just that FSR is split into what's required for the different fuel nozzle orifices in order to maintain emissions requirements (which is ENTIRELY open loop in most cases).
P2 pressure is set to be high enough so that small fluctuations in P2 pressure won't adversely affect unit operations. In other words, if the gas fuel supply pressure/flow is restricted such that the SRV reaches 100% stroke and still can't maintain the P2 pressure reference (FPRG), then what will happen is that the load will eventually start to drop. (I'm presuming grid frequency is stable at all times in this response.) If the unit is on pure Droop speed control--without Pre-Selected Load Control OR external load control (sometimes called AGC)--the unit will lose load. Because the fuel flow-rate for the given FSR is not sufficient to maintain load because the gas fuel supply is restricted. It's not that P2 pressure is high or low--it's because the gas fuel supply pressure/flow is restricted and is insufficient to maintain normal operating parameters (namely, P2 pressure).
IF the unit is operating at Part Load (meaning it will be on Droop speed control)--AND it is being operated with Pre-Selected Load Control or external load control active--that means that TNR will be adjusted to make the actual load equal to the load setpoint. And THAT will cause FSRN to change to maintain load. If P2 pressure were to drop because the SRV was unable to maintain FPRG because the gas fuel supply pressure was restricted, then load would start to drop and Load Control (Pre-Selected or external) would start increasing FSRN to try to maintain the load control setpoiont. BUT, if the gas fuel supply is restricted there's only so much that the Mark* can do to maintain load.
If gas fuel supply pressure were much higher than normal, that would cause the SRV to close more than normal--but as long as it was able to maintain P2 equal to FPRG there would be no operational problems as far as maintaining load, unless the gas fuel supply pressure was so high that the SRV had to go very nearly closed, which would represent a restriction of gas fuel flow-rate.
Constant-settable Droop Speed Control does, in fact, use load in it's calculation of FSRN. BUT, when operating parameters are normal (that is gas fuel supply pressure/flow are normal and fuel nozzles are not choked (restricted)) it operates virtually identically to straight droop speed control--except that it's reference is a load-biased TNR (TNRL if I recall correctly). This was done to prevent load swings during DLN combustion mode transfers, and it has just been carried over to the majority of new units regardless of combustion system. It works to try to help stabilize load during temporary, transient conditions.
And, if a unit with Constant-settable Droop is being operated with some kind of load control active (Pre-selected or external), then it's going to respond primarily to the load setpoint, trying to make actual load equal to the load control setpoint by adjusting TNR as necessary.
Now, with regard to GCV LVDT calibration. The very fact that FSR is NOT used in Droop speed control feedback means that inaccurate LVDT calibrations will not (generally) adversely affect Droop speed control operation. Let's consider the case where the LVDTs were calibrated such that the indicated position is 10% less than the actual position. The speed error is still going to result in the same change of FSRN, which is going to result in the same change in the gas control valve position(s). Same goes for if the LVDTs were indicating 7% less than actual position. The speed error is going to result in the same change in FSR.
If you can describe the problem you are experiencing with unit operation, we can probably try to help understand what might be happening. If you are not experiencing unit operational problems but are having trouble understanding the control scheme(s) be sure you are NOT including the effects of load control (Pre-Selected or external) when you considering data.
And, if you're worried about the effects of grid frequency changes on P2 pressure (because frequency is directly proportional to speed and speed is used to calculate FPRG), you shouldn't be--as long as the gas fuel supply pressure/flow is not restricted. In other words, as long as FPG2 equal FPRG and the SRV is not at 100%, then the unit is going to operate just fine with small changes in P2 pressure caused by grid frequency deviations. If you are experiencing something different, please describe what you are seeing. Actually, post some trend data to a sharing site and post the link to the data here.
>Lets suppose we have a problem with the P2 pressure either
>the FPRG constant value or the SRV valve or even the P2
>transmitter. This problem causes the P2 pressure to be 10bar
>when the machine at FSNL. So when the machine reaches FSNL
>and FSRN = 30% > we won't be having enough energy to make it
>stable, and TNH would to tend to drop. When it drops,
>TNR-TNH would be higher than 0 which will make FSRN higher
>than 30% to reach the FSNL speed again. We will be having a
>non stable state of the machine fluctuating at FSNL
This could happen, but, I don't think unit operation would be "non stable." FSRN would just try to increase to make TNH equal to TNR. And, at FSNL a drop of 2 barg in P2 pressure is NOT going to make TNH less than it should be. P2 pressure is calculated to provide proper pressure/flow at Base Load, not FSNL. Gas fuel flow-rate at FSNL is MUCH less than at Base Load. So a small drop, even 2 barg, in P2 pressure at FSNL isn't going to have any appreciable effect on TNH at FSNL.
>NOTE 1: This enquiry is the same for the base load GCV
>opening value, at base load. And if the droop value is 4%,
>so TNR-TNH = 4% and 4% * FSKRN2 + FSKRN1 = FSRN, so there is
>no FSR Feedback value in the loop, at any load point we will
>be having an particular opening command of the GCV NO MORE!
When the unit is operating at Base Load, the reference for the GCV is FSRT. And the Mark* will be trying to flow as much fuel as it can to make TTXM equal to TTRX, and FSRT will be adjusted to keep TTXM equal to TTRX. In this case, the control loop is CLOSED--because the reference is TTRX (exhaust temperature reference) and the feedback is TTXM (actual exhaust temperature). FSRT will be changed to whatever is required to make TTXM equal to TTRX. And load will be whatever it is.
>NOTE 2: i am comparing this regulation loop to all other
>load regulation loops for single or double shafts where the
>FSR feedback value is always present. So we will be adding a
>certain value the actual FSR until we reach the load
>reference. So the for a certain load point we won't be
>having a particular pre-calculated value of FSRN which will
>make the system control adapt the FSRN value to the present
>conditions and parameters.
Yes. Again, FSKRN1 and FSKRN2 are calculated values based on expected fuel characteristics and reasonably good calibrations of GCV LVDTs and a new and clean turbine and ambient conditions matching machine nameplate rating.
I have seen units with poorly-calibrated GCV LVDTs that reached "FSNL" and were not exactly at 100.3% speed (which is the typical TNR for generator drive GE-design heavy duty gas turbines). The unit might have been at 99.87% or 100.45%, BUT when auto-synch was selected TNR was adjusted to make TNH change so that the speed-matching permissive of auto-synch was achieved. Then after that, the value of FSR was not as per the expected fuel characteristics. And, sometimes the unit would be at 103.21% when it reached Base Load, or 104.98% when it reached base load (again because either the GCV LVDTs weren't calibration properly, or the actual fuel didn't match the expected fuel, or because the fuel nozzle orifices weren't the same as when the unit was new)--but the unit operated just fine in all conditions and at all loads, Part Load and Base Load. In fact, you will find that the majority of GE-design heavy duty gas turbines (without ARES, or MBC (Model-Based Control)) have FSRs that don't work out exactly to 4% Droop. And, they run just fine. They aren't 100% perfectly tuned and operating, but can you say that about any car or truck? (Actually, most modern vehicles have O2 sensors in the exhaust and they operate MUCH closer to design that most heavy duty gas turbines, because the O2 sensor feedback isn't used to calculate fuel flow or performance--only emissions.)
Again, small errors in P2 pressure aren't going to impact FSR or GCV position(s). It's when the SRV is fully open and it still can't maintain P2 equal to FPRG, or when it's nearly closed while trying to maintain P2 eaual to FPRG, that problems with unit operation are going to occur. And, that is really only going to happen with the unit is at higher loads, not at FSNL (unless gas fuel supply pressure/flow is severely restricted or excessively high). So, don't worry about P2 pressure differences unless the SRV is at or near full open or is at or near full closed while the unit is at higher loads (probably 40-50% load or more).
Droop speed control is proportional-only control. There is nothing that causes the control system to make the actual speed equal to the speed reference. It is recognized there will be an error, and the error is vital to how the control scheme works. If other parameters aren't all perfec, well, then, Droop speed is not going to be perfect.
>example: load output = 100MW, FPR2 = 12bar, FSRN = 40 %
>if the the P2 drops for whatever the reason we will be
>having - load output = 100MW, FPR2 = 9bar, FSRN = 60 %
>So the control system rises the FSRN value to maintain the
In this example you've given, you are making at least two assumptions--that the unit is operating on load control (Pre-Selected or external), while it's on Part Load (not at Base Load). If the unit is using Constant-settable Droop, then, FSRN may be adjusted to maintain load without any load control. But, you didn't state if the unit was on "pure" ("Straight") Droop speed control, or on Constant-settable Droop speed control. AND, you have made some assumptions about how P2 pressure affects load. P2 pressure is only going to affect load if the SRV is fully open (100% stroke) and fuel gas supply pressure/flow is restricted such that total fuel flow-rate to the unit is also restricted. And, even then, it may not be able for the GCV to maintain load. What usually happens in these cases is that the unit loses flame because the fuel pressure at the nozzles isn't high enough to maintain sufficient fuel to maintain flame. Recall, that there is three-to-five times more air flowing through a gas turbine than is required to combust the fuel. It's a very delicate balance maintaining flame, especially in DLN combustors where the amount of air entering the combustion section of the combustor is very high (for pre-mixing and keeping flame temperature, and NOx emissions, low).
I think you're just making too many assumptions about inter-relationships between operating parameters. We don't know a lot about the units at your site (you say you have some two-shaft machines) and we don't know a lot about the fuel(s) being burned, or how much they might vary over time (some units burn waste gases from some nearby process (steel mills; refineries; etc.)). 30% FSR for FSNL is pretty high for a typical natural gas fuel, unless it's pretty bad gas (in my experience).
So, try to look at real data, using comparable terms ("straight" droop and Constant-Settable Droop are similar, but use different terms), and DON'T include any operating data when any kind of load control (Pre-Selected or external) is active when at Part Load. And, Base Load is very different from Droop speed control (it's closed exhaust temperature control, not proportional speed control).
Hope this helps!
Actually like i made the statement in a previous thread, i am working at compression station.
recently i was noticing and making the comparison of two machines running at the same load reference (84% LP speed). however the GCVs were not at the same opening (can't remember the data exactly and i am not on site while writing this post). but i can confirm that this is due to different P2 pressure (the one with higher P2 had a lower GCV opening), note that the FSRN loop have the FSR feedback.
In the same time i have 5001P turbo-generator simulator which i use frequently, and i had the scenario of having the same condition (different P2). i wanted to know how the machine would react. So this led me to study the standard droop regulation loop and this was the origin of my enquiry.
My biggest concern was about reaching the FSNL speed with a P2 lower than expected and without an FSR feedback? i couldn't simulate this in the simulator this is why i posted this thread.
Also the majority of time during loading conditions i was in part load with manual control (imagining the operator is setting the load according to the grid demand).
I agree with you when the unit is being operating at Part Load. it is being operated with Pre-Selected Load Control or external load control that TNR will be adjusted to make the actual load equal to the load setpoint (even if we have a P2 problem). but in the same time we must not forget that TNR is limited with the TNKR3 MAX constant value, and even if we would not reach this value (while adjusting TNR), we may have a disagreement between the droop value and the output load (4% droop will not equal to 100% load).
Also i found this in a control specification OEM document about the settable constant droop control. "This method of speed/load control is applied to units where the fuel stroke reference (FSR) is not predictable as a function of the gas turbine output power due to varying fuel heating values or where fuel is switched between different combustion system injection nozzles.”
combined with what you said "Constant-settable Droop Speed Control does, in fact, use load in it's calculation of FSRN. BUT, when operating parameters are normal (that is gas fuel supply pressure/flow are normal and fuel nozzles are not choked (restricted)) it operates virtually identically to straight droop speed control--except that it's reference is a load-biased TNR (TNRL if I recall correctly). This was done to prevent load swings during DLN combustion mode transfers, and it has just been carried over to the majority of new units regardless of combustion system. It works to try to help stabilize load during temporary, transient conditions."
Everything seems to be clear to understand now.
After reading your reply you helped giving more insights about the issue, thank you very much
The plot thickeneth! I completely missed that you wrote you were working at a compressor station; I do remember you writing there were single- and two-shaft machines at the site--and wondered, but didn't specifically ask. My bad. But, at least now I'm beginning to understand (I thinketh!).
Anyway, so, it's been MANY years since I worked on a two-shaft machine (as in a couple of decades), but I DO remember that once the LP speed gets to near rated the HP shaft speed is NOT controlled when the unit has variable second-stage nozzles. LP speed is the primary control loop, and the HP is "just along for the ride." The second-stage nozzles control the "split" of the energy being produced by the HP shaft, some of which goes to drive the axial compressor (which is connected to the HP shaft), and the remainder goes to the LP shaft--to produce pressure or flow, whatever the reference is that's coming from the compressor control scheme (even if it's in the Mark VI/VIe). I don't recall is FSRN is HP shaft speed or LP shaft speed, but it's certainly not Droop speed control. That's really only for generator-drive applications. Compressor-drive applications are either controlling the gas compressor discharge pressure or the gas compressor discharge flow-rate, or some value which is biased by one or the other (a "hybrid" reference of sorts).
The SRV is STILL controlling P2 pressure as a function of HP speed, which is NOT fixed, but variable in a two-shaft machine. It's variable in a range, because the axial compressor (driven by the HP shaft) has a range of speed where it can safely and efficiently operate. And the P2 pressure reference offset and gain (FSKRN1 and FSKRN2) are calculated to produce sufficient P2 pressure to make sure there is flow into the combustors (the fuel gas pressure downstream of the GCV is higher than CPD AND there is sufficient flow to maintain flame and produce torque--enough torque to drive the axial compressor, and also to drive the LP shaft which is driving the gas compressor).
The second-stage nozzles are modulated (controlled) to keep the HP shaft operating in the proper range when the LP is being controlled by the plant operating reference (gas compressor discharge pressure and/or flow).
The FSR when the LP shaft is being controlled by the plant reference is derived from the amount of fuel required to make the required plant variable (pressure and/or flow) equal to the plant reference. And, the second-stage nozzles are modulated to keep the HP shaft speed in the proper range. (Yes; fuel DOES affect HP speed, too, but in the typical GE-design two-shaft control scheme it's secondary because the REAL output of the unit is the gas compressor pressure and/or flow.)
So, the P2 pressure of a two-shaft machine IS going to be variable--while the P2 pressure of a single-shaft machine at rated speed/frequency is NOT going to be variable, it's going to be constant (as long as grid frequency is stable and at rated).
I'm fairly confident the Operations & Service Manuals provided with the two-shaft machines have an explanation of how the control scheme works for a two-shaft machine. And, it's not exactly the same as for a single-shaft generator drive application, which uses Droop speed control for Part Load operation (from FSNL to Base Load). Again, the high-points of GE-design heavy duty two-shaft gas turbine control are that the primary fuel control is used to control the LP shaft speed which is driving the gas compressor which is producing pressure and flow, and the second-stage nozzles are used to control HP shaft speed. Both do use the same P2 pressure control formula (FPRG=(TNH*FPKGNG)+FPKGNO), but because the HP shaft speed of a two-shaft machine is NOT FIXED P2 pressure WILL NOT be constant during loaded unit operation, as it is for a single-shaft generator drive. (NOTE: Some two-shaft GE-design heavy duty gas turbines are also used for generator drive applications. I worked on one even before I worked on compressor-drive two-shaft machines, but I could only spell Droop speed control and didn't really have any clue how it worked or why and I was more interested in how the unit got up to speed (the start-up sequence) than how it controlled LP shaft speed and load and frequency. (And, the generator was driving the main propulsion engine of a ship, so it more than likely operating in Isochronous speed control, anyway...!)
Hope this helps! All GE-design heavy duty gas turbine control schemes are NOT the same (especially when we're talking about single- and two-shaft machines and generator- versus mechanical- (compressor-)drive units). But, I don't recall ANY GE-design heavy duty gas turbine that didn't have an SRV for gas fuel control that used HP shaft speed as the reference for P2 pressure.
One more thing before I sign off this thread: It's entirely possible that the LP shaft speed of two compressor-drive units of the same Frame size can be identical, but the FSRs and HP shaft speeds can be different (they shouldn't be drastically different, but they can and most likely will be different). That's because of a LOT factors: time since last maintenance outage; condition of turbine inlet air filters; cleanliness of axial compressor; condition of gas compressor; type of gas compressor; calibration of second-stage nozzle LVDTs; calibration of IGV LVDTs; calibration of GCV LVDTs; and so on. Every two-shaft machine isn't going to produce exactly the same amount of torque at exactly the same amount of speed for exactly the same amount of fuel flow at exactly the same ambient conditions. Same goes for two generator-drive units of the same Frame size....
I'm trying to find the right tack to make this work for you.
Droop speed control is about how much the energy flow-rate into the prime mover changes for a change in the error between the prime mover's speed reference and the actual speed.
FSR is a position--a position that is proportional to the valve position (for units with conventional combustors, and units with combined SRV/GCV assemblies). The people who configure the units have a program, called, oddly enough, The Gas Program, that takes the expected fuel characteristics and chooses the correct valves, or valve internals as the case may be, and determines what the flow-rate through the valve(s) will be for a given position. So, you can really say that FSR, while the unit is running at rated speed (or thereabouts, anyway) that the position reference is really a flow-rate reference. And, if the LVDT is even remotely properly calibrated the feedback to the servo-valve output regulator is valve position (stroke). So, the position reference is really a flow-rate reference, and the feedback is similarly equatable to flow-rate.
Now, Droop speed control is proportional control only. And, all it does is change the energy flow-rate reference based on the error between the prime mover's speed reference and its actual speed. Look at the engineering units for FSKRN1 and FSKRN2. The gain's engineering units are % FSR/% Speed, and the offset's engineering units are % FSR. So, the speed error is in % Speed (TNR and TNH are both in percent speed), and when multiplied by the gain (which is % FSR/% Speed) the % Speed "cancels out", and you're left with % FSR. And when you add the offset, which is also in % FSR, you get: % FSR (or just FSR, for most of us).
Droop speed control is about how much the energy flow rate changes for a given change in the speed error. The offset (FSNL FSR) is how much energy it theoretically takes to maintain approximately 100% rated speed. And when TNR is 100.3% (which is usually the off-line speed setpoint) and TNH is 100.3% (which it will usually be when the generator breaker is open, so before and during synchronizing), the error is zero. Zero plus FSNL FSR equals FSNL FSR. When auto synch is enable, then the process of making the generator speed a little greater than grid frequency begins--and when the generator breaker closes: WHAM! The unit instantly slows down to grid frequency. BUT, TNR remains the same (for an instant) and because it's higher than TNH is (when the generator breaker closes and slows the unit down) the difference (the speed error) gets multiplied time the gain and that extra fuel--which was making the unit spin a little faster than grid frequency immediately gets converted to amperes, which is how the unit takes on load (produces electrical power).
Now, when the operator wants to increase the load, he watches the MW meter BUT he clicks on RAISE SPEED/LOAD. Those words mean: Increase the turbine speed reference, and if the unit if off-line (the generator breaker is open) the actual speed will increase (there's nothing stopping it from increasing in speed!). OR, if the unit is on-line (the generator breaker is closed) the fuel increase can't make the unit speed increase (the grid frequency is holding it from increasing) but the generator senses the increase in torque coming from the increased fuel and converts the torque to amperes--which is what makes the MW meter increase its reading. (The opposite happens when the operator clicks on LOWER SPEED/LOAD.)
Now, you're probably going to say, "The operators at MY plant never, or rarely, click on RAISE- or LOWER SPEED/LOAD! They always use Pre-Selected Load Control!" And while that may be true, what's really happening is that when the operators enable Pre-Selected Load Control the load control setpoint they enter gets compared to the actual load and TNR gets increased or decreased to make the actual load equal to the load control setpoint (reference). BUT, in the "background", at ALL times, between generator breaker closure (really FSNL) and Base Load TNR is being raised or lowered to change load. Always. Regardless of where the reference is coming from--from RAISE- or LOWER SPEED/LOAD, or Pre-Selected Load Control, or external load control.
And, just because the value that's being calculated is FSRN (Speed Control FSR) doesn't mean that FSR is simply a valve position. It's NOT--it's really a fuel flow-rate reference, that's in the form of a valve position.
Now, when P2 pressure is at rated, or even when it's near rated (it's NOT an exact value, there's really a range of P2 pressure the unit can operate very comfortably in!) the flow through the gas control valves is, for all intents and purposes, proportional to stroke. Normally, a single valve whose stroke (position) is proportional to the flow-rate through the valves is VERY expensive (or at least it was a few decades ago when GE heavy duty gas turbines were first being produced). BUT, by using a valve upstream of the gas control valve to maintain a constant pressure upstream of the gas control valve (in this case the SRV), the gas control valve can be a less expensive valve and still provide the functionality of the flow-rate being proportional to the stroke (position) of the valve.
I'm not going to get sidetracked into "what is the exact range of allowable P2 pressure for a particular turbine or turbines"--suffice it to say that unless the SRV is 100% open and can't open any further, as long as the P2 pressure is nearly equal to the P2 pressure reference then the unit will operate without much problem. If the gas fuel supply pressure/flow is restricted for some reason and the SRV has to go full open and still can't maintain P2 pressure well then at some point something has to give ("fail")--and that is probably going to be flame in the combustors. This is the classic "Loss of Flame" trip--insufficient fuel to maintain flame in the combustors.
However, given that there are parameters which protect the turbine (unless they are defeated!) from serious under-speed (under-frequency) or over-speed (over-frequency) the speed changes which result from grid frequency disturbances are NOT going to cause problems for the gas control valves.
I'm wondering if this isn't your problem that you're struggling with.... When the grid frequency decreases (in other words TNH decreases!) what happens is that turbine load should go up--because of Droop speed control. When TNR is constant (which it will be when the load is constant and stable) and TNH goes down, the speed error increases--which cause FSRN to increase. This should cause the generator load to increase--but MOST plants and plant operators and technicians believe that THEIR turbine-generator load should remain stable when the grid frequency is off rated (in this example it's low). BUT, that's NOT what's supposed to happen--the prime movers and generators synchronized to a grid should ALL (those that are not already at full power output!) increase their load to try to help maintain grid stability. BUT, if anyone is operating their turbine with any kind of load control while it is at Part Load then the load control is going to try to counter (act against) the Droop speed control. Load control (Pre-Selected or External) wants to keep the load constant--regardless of grid frequency. But, in reality, when there is a grid disturbance the prime mover governor SHOULD change the load to help support grid stability. Those units operating in load control actually make the grid frequency problem WORSE!
That's why you hear about this "free governor mode" of operation. Which is nothing more than Droop speed control. GE has something to "allow" load control (Pre-Selected or external) to properly respond to grid frequency disturbances, and this can be considered "free governor mode."
Now, gas turbines have a dirty little secret. When they are operating at Base Load, producing as much power as they can while synchronized to a grid with other prime movers and generators and the grid frequency changes they DO NOT respond the way they should. When the grid frequency decreases the axial compressor speed decreases (and the IGVs are full open at Base Load), and this causes the exhaust temperature to increase. So, the Mark* says, "Hey! I have to reduce fuel to keep the exhaust temperature from increasing!? Which has the effect of reducing the power output of the gas turbine--at precisely the time when it should be INCREASING its output! But, it can't--or it will trip on exhaust over-temperature. (Don't worry; GE has a fix for this, too!) And the opposite happens when the unit is at Base Load and the grid frequency increases. This causes the axial compressor speed to increase, which causes the exhaust temperature to decrease so the Mark* adds fuel to turbine to maintain exhaust temperature--which increases the load being produced at the precise instant it should be decreasing load! (That GE fix I just mentioned also takes care of this aspect of the problem at Base Load, too.)
Remember, from FSNL to Base Load a GE-design heavy duty gas turbine-generator unit operates on Droop speed control. Load is changed by changing the fuel flow-rate, which is done by looking at the difference between the actual speed and the speed reference. And even if the result of the calculation is valve position (FSR), it's really fuel flow-rate--which is what Droop speed control is all about: how much the energy flow-rate into the prime mover changes for a change in the speed error between the speed reference and the actual speed.
There's a lot to think about and comprehend. However, it's NOT rocket science--and there are quite literally tens of thousands of prime movers and generators around the world which use Droop speed control. And there are thousands of GE-design heavy duty gas turbines that also use Droop speed control--and they all work just fine, even if they're not perfect (with their digital turbine control systems). P2 pressure is only about making FSR (valve position) proportional to fuel flow-rate. It's much easier to measure valve position than it is to measure natural gas (or any other gaseous fuel) flow-rate (think about it, you need some pretty sophisticated sensors, or at least some sophisticated calculations (that use square roots!) to calculate gas fuel flow-rate). And when GE heavy duty gas turbines were first being designed those sensors (which are readily available and relatively cheap today) were expensive and required a LOT of calibration adjustments. But, a simple LVDT measuring valve stroke (position) that is proportional to fuel flow-rate was much easier and cheaper to implement.
Constant-settable Droop speed control just uses a load bias to modify TNR. And, it does correct for load variations--which is something I've never really been a fan of (I understand why it was implemented, I just don't think it's necessary today). But, in the end, it still accomplishes the same thing: changing the fuel flow-rate for a change in the speed error (even if one of the components of the speed error is load-biased!). Constant-settable Droop just adds a level of complexity that doesn't seem warranted today (it did when DLN was in its infancy--that's for SURE!). It's also useful for fuels that have a continually varying heat content, or multiple fuels from different sources.
Anyway, it's a lot to think about. The FSR "loop" is, in fact, closed. The servo-valve output regulator looks at the difference between the valve position (for the gas control valve(s)) and the actual valve position(s) and adjusts the current to make the actual position equal to the position reference. And, we know--valve position is proportional to fuel flow--rate (energy flow-rate). The Droop speed control loop is NOT closed--it's not supposed to be closed. It's only function is to determine how much the fuel flow-rate will change for a change in the speed error (that's the function of the gain parameter--the one that's multiplied by the speed error, and the one that determines the Droop setpoint, or Droop characteristic). For units with 4% droop, the fuel flow-rate should change by 25% for each 1% change in speed error. (That's the fuel flow-rate ABOVE FSNL--which is the load-producing region of unit operation!)
If this doesn't do it for you, I don't know how else to try to explain it. It's not rocket science. Sure, there are some complexities (P2 pressure being one of them--but under normal circumstances, or even under allowable grid frequency excursions, P2 pressure changes are NOT going to adversely affect unit power output). Droop speed control is about how much the load changes for a change in the speed error.
And it's that way because back when electricity was first being introduced to the world, the ONLY feedback to the prime mover governor was speed. There wasn't any kind of sensor that could convert watts to spring tension--and governors all used springs back then. There wasn't even any hydraulics in the beginning. And, the real beauty of Droop speed control is that it still works today even with digital control systems (governors). And, it allows today's prime movers and generators to be synchronized with yesteryear's prime movers and governors--because they all "understand" speed! Speed and frequency are proportional (they are directly related)--they always have been and they always will be in an AC power system.
Enough for today (actually for this month!).
I can see that our discussion and your explanation has expanded to make everything clear to anyone who reads this thread.
>Now, when P2 pressure is at rated, or even when it's near
>rated (it's NOT an exact value, there's really a
>range of P2 pressure the unit can operate very comfortably
>in!) the flow through the gas control valves is, for all
>intents and purposes, proportional to stroke. Normally, a
>single valve whose stroke (position) is proportional to the
>flow-rate through the valves is VERY expensive (or at least
>it was a few decades ago when GE heavy duty gas turbines
>were first being produced). BUT, by using a valve upstream
>of the gas control valve to maintain a constant pressure
>upstream of the gas control valve (in this case the SRV),
>the gas control valve can be a less expensive valve and
>still provide the functionality of the flow-rate being
>proportional to the stroke (position) of the valve.
I am already aware of that and we have discussed this subject in a previous thread: https://control.com/thread/1510863192
>I'm wondering if this isn't your problem that you're
>struggling with.... When the grid frequency decreases (in
>other words TNH decreases!) what happens is that turbine
>load should go up--because of Droop speed control. When TNR
>is constant (which it will be when the load is constant and
>stable) and TNH goes down, the speed error increases--which
>cause FSRN to increase. This should cause the generator load
>to increase--but MOST plants and plant operators and
>technicians believe that THEIR turbine-generator load should
>remain stable when the grid frequency is off rated (in this
>example it's low). BUT, that's NOT what's supposed to
>happen--the prime movers and generators synchronized to a
>grid should ALL (those that are not already at full power
>output!) increase their load to try to help maintain grid
>stability. BUT, if anyone is operating their turbine with
>any kind of load control while it is at Part Load then the
>load control is going to try to counter (act against) the
>Droop speed control. Load control (Pre-Selected or External)
>wants to keep the load constant--regardless of grid
>frequency. But, in reality, when there is a grid disturbance
>the prime mover governor SHOULD change the load to help
>support grid stability. Those units operating in load
>control actually make the grid frequency problem WORSE!
Be sure that this is not my problem. i've gained insights about this subject from the simulator which i have and your replies before https://control.com/thread/1475094281.
I was able to contribute and make it clear for others also
https://control.com/thread/1507466583,so do not worry about it and thanks again.
My problem like i mentionned in my previous reply was making the analogy with the LP load control in double shaft heavy duty GT and the generator load control (Standard droop load control) especially when we have some problems with the P2 pressure, and like i said was, now i can confirm that issue is gone.
>Now, gas turbines have a dirty little secret. When they are
>operating at Base Load, producing as much power as they can
>while synchronized to a grid with other prime movers and
>generators and the grid frequency changes they DO NOT
>respond the way they should. When the grid frequency
>decreases the axial compressor speed decreases (and the IGVs
>are full open at Base Load), and this causes the exhaust
>temperature to increase. So, the Mark* says, "Hey! I have to
>reduce fuel to keep the exhaust temperature from
>increasing!? Which has the effect of reducing the power
>output of the gas turbine--at precisely the time when it
>should be INCREASING its output! But, it can't--or it will
>trip on exhaust over-temperature. (Don't worry; GE has a fix
>for this, too!) And the opposite happens when the unit is at
>Base Load and the grid frequency increases. This causes the
>axial compressor speed to increase, which causes the exhaust
>temperature to decrease so the Mark* adds fuel to turbine to
>maintain exhaust temperature--which increases the load being
>produced at the precise instant it should be decreasing
>load! (That GE fix I just mentioned also takes care of this
>aspect of the problem at Base Load, too.)
This is my first time to hear about this, so could you please explain or share the "GE FIX you mentionned"
In the same time after imagining the situation(lets take the case where the grid frequency drops), when the compressor speed decreases means HP shaft speed TNH and the compressor discharge pressure CPD will decrease, which means the speed biased temperature setpoint TTRXB and the CPD bias to isothermal will increase which i imagine will either not reducing fuel flow rate because like you said when the axial compressor drops in speed the exhaust temperature will increase (as TTXM increases still equal to TTRXB) or allowing the turbine to add more fuel flow rate (to make TTXM equal to TTRXB) which will increase load output.
So could you explain more the situation and i hope this is not the fix you were talking about because i am translating one of the recent application code of GE nothing more.
The "fix" that GE sells allows the unit to over-fire when it's at Base Load AND the grid frequency decreases--BUT, they add a timer that keeps track of the time the unit is over-fired and that has to be factored into maintenance outage planning. By over-firing it is meant that the exhaust temperature control is allowed to exceed the Base Load rating for a short period (because it's not allowed to happen indefinitely, either!).
And, the opposite happens when the grid frequency increase--instead of producing more power the exhaust temperature reference is reduced, but this isn't usually counted against maintenance outages, though I think it's also time-limited.
It's just a couple of "tweaks" to the exhaust temperature control scheme that will allow the unit to operate as if it were in Droop speed control in order to help support grid stability during brief grid frequency disturbances.
I'm really at a loss to understand your other question. Most generator drive applications do not use a speed bias, but many two-shaft units do. A LOT of people think TTRXB means "Base Load Turbine Exhaust Temperature Reference", but as you noted it really means "Speed-Biased Exhaust Temperature Reference." When axial compressor speed decreases on a single-shaft generator drive application, the flow through the axial compressor will decrease. This does two things: it reduces CPD, AND it causes the exhaust temperature to increase (if fuel were held constant). When the unit is at Base Load the combination of decreasing CPD AND increasing exhaust temperature will cause the exhaust temperature reference to change. Since the exhaust temperature reference curve has a negative slope, it will cause the exhaust temperature reference to increase but the actual exhaust temperature is already tending to increase (because of the reduced air flow through the unit). So, the net effect is that the power output of the unit will decrease--because of both the decreased air flow (mass flow) through the unit AND because the fuel will also be decreased to limit the exhaust temperature increase and protect against exhaust over-temperature. This is what happens at Base Load, not on Droop speed control. And only for single-shaft generator-drive applications. (The negative slope of the exhaust temperature reference curve can be very confusing for many people; it's counter-intuitive--but that's the way gas turbines work!).
Again, if the fuel were held constant the exhaust temperature would increase simply by virtue of the decrease in axial compressor speed. So, it's necessary to reduce the fuel to limit the exhaust temperature increase, and because the air flow through the machine is decreasing the air flow (mass flow) is decreasing which is also decreasing the power output of the machine.
When ambient temperature increases it decreases the density of the air flowing through the axial compressors (running at a constant speed). This causes CPD to decrease which causes the exhaust temperature reference to increase, but the power output of the machine goes down. The fuel flow also decreases very slightly, but the majority of the power loss is attributed to the decrease air/mass flow, not so much the fuel flow decrease. Again, this only happens when operating at exhaust temperature control.
Hope this helps!