# Gas Interstate Pressure Control by SRV

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#### niazbibiyana

Hello,

We have DLN 2.0+ technology with 9fa.03 GT (mark Vie speedtronic). from last few days, I am trying to find out P2 pressure reference generating & stroke control of SRV valve.

In manual I have found that total 3 factors ( median one) are controlling p2 pressure reference value, which is input of SRV reference. 3 signals are FPRGCRIT, FPRGSUP, and the minimum of FPRGN and FPRGTAMB.

its clear that, during start up "FPRGN" contributes to generate P2 reference (Derived from TNH and offset and gain). During shutdown "FPRGCRIT" contributes to generate P2 reference (Derived from CPD).

I want to know what about entire load condition (from 0MW to 100% load)? in this large load range, which factor is maintaining P2 reference? because, in load condition ofcourse SRV stroke changes. how this change occurs? which factor is responsible?

Here, the GCVs control reference values are generated from FSR and DLN scheduling or/and others factors.

#### CSA

niazbibiyana,

So, it seems there is some new SRV control scheme underfoot, but the basic function of the SRV is unchanged: It's purpose is to control the pressure upstream of the Gas Control Valves (called the interstage pressure, or the P2 pressure). And, if it's going through a MEDian select block, that means that the middle of the three values is the one that's going to become the SRV control reference.

It would seem that when the unit is on-line and generating power that FPRGN would be the one that would be in control, doing it's normal thing. Here's what I believe the formula for FPRG (FPRGN) is:

FPRG=FPRGN=(TNH*FPKGNG)+FPKGNO

(Which everyone should recognize is the formula for a straight line.) When the grid frequency is at or near rated and is stable, TNH is stable. And when multiplied by FPKGNG and then FPKGNO is added to it, it, FPRG, or FPRGN, is a stable value.

As the Gas Control Valves open when the unit is being loaded what would tend to happen (if the SRV remained in a fixed position) is that the P2 (interstage) gas pressure would go down. BUT, the formula doesn't allow for that--and as the GCVs open and the P2 pressure started to decrease the SRV will open to maintain FPRG (which is stable and not changing when the grid frequency is stable and not changing).

Conversely, as the unit is unloaded and the GCVs are closing, the P2 pressure would tend to increase (if the SRV remained at a fixed position). So, the formula tells the SRV to close to keep the P2 pressure constant (as long as the grid frequency is stable and not changing.

The other two are probably just for over-riding FPRGN if conditions warranted.

Hope this helps!

N

#### niazbibiyana

CSA,

""<i>As the Gas Control Valves open when the unit is being loaded what would t end to happen (if the SRV remained in a fixed position) is that the P2 (interstage) gas pressure would go down. BUT, the formula doesn't allow for that--and as the GCVs open and the P2 pressure started to decrease the SRV will open to maintain FPRG (which is stable and not changing when the grid frequency is stable and not changing).

Conversely, as the unit is unloaded and the GCVs are closing, the P2 pressure would tend to increase (if the SRV remained at a fixed position). So, the formula tells the SRV to close to keep the P2 pressure constant (as long as the grid frequency is stable and not changing.</i>""

Thanx for your appreciable help. But confusion in one point. Imagine a scenario, when, Machine in running in "FSRN" fsr, means in load/speed control, for drooping characteristics, when frequency drop, demand increases, machine TNH also must must drop as its synchronized. So, if we think the linear relation by the thumb equation, <b> FPRG=FPRGN=(TNH*FPKGNG)+FPKGNO </b>, P2 pressure reference should decrease. But practically its different, when load demand increase, P2 reference must increase, which turn srv stroke increase. SO, its clear that, <b> FPRG=FPRGN=(TNH*FPKGNG)+FPKGNO </b> this equation may not be always applicable in transient grid event.

So, I want to know, at grid frequency fluctuation, which parameter takeover the controlling of P2 pressure reference?

Thank you so much

#### Isulamu

niazbibiyana,

If you allow to me contribute! and based on your scenario, so the machine if loaded and when we have a frequency fluctuation the TNH would drop, so the FPRG too. YES you are right. However, in the same time when the TNH drops, the FSRN would tend to increase, because the error between the turbine speed refrence TNR and the TNH has increases (TNR is constante and TNH dropped). the GCV will open, and like Mr CSA said the GCV opens, the P2 will start to decrease under its new FPRG. And as a result of that, the SRV will follow and open, and in this case if everything is working correctly we will have more fuel flow rate to let the TNH gain its initial load again.

N

#### niazbibiyana

Isulamu,

<b>If you allow to me contribute! </b>.. Ha ha . its open forum discussion. Anybody is allowed. <b> Thanx Mr. CSA and also you</b>. I got my point, I have opened trender and checked all parameters thoroughly.

I noticed, while frequency as well as TNH drop, load demand high, GCVs are in more opening, causes actual P2 pressure drop (feedback). But due to small amount of TNH drop turns P2 pressure (reference) drop (but very small amount) in the mean time due to VGCs opening, Actual P2 pressure (feedback) drops more. To recover new P2 reference (FPRG), SRV opens.

Thank you all

#### CSA

Thank you, Isulamu.

I think, though, niazbibiyana is trying to ascribe some perceived problem to P2 pressure control without explaining the entire problem without bias (personal bias).

Because of the control scheme he has described it is really impossible to say with any degree of certainty what is happening when the grid frequency is off normal. niazbibiyana could calculate the change in P2 pressure using the Control Constant values from the United and the change in TNH when the grid frequency is off normal to see that the change in P2 pressure will not be large. So, as long as the gas fuel supply pressure and flow-rate is unrestricted AND the SRV is not already at or near 100% open, and the load is sufficiently below Base Load then the unit should be able to respond to the grid frequency disturbance.

The problem is most people don't understand what should happen when there is a grid frequency disturbance and because the unit doesn't respond in the way they think (perceive) it should respond they want to blame the turbine control system for the perceived problem.

Frequency disturbances can also be erratic or "stable" (meaning the frequency can increase or decrease to some value and be relatively stable at that point until the grid operators can respond appropriately to return the frequency to normal). We don't know what the nature of the disturbances are.

And most people believe their unit(s) should be stable during any grid frequency disturbance, producing a stable power output regardless of the grid frequency fluctuation(s) (which is NOT correct at all and is actually contributing to the grid frequency instability!).

So, while I mostly agree with your explanation I don't believe it will satisfy niazbibiyana AND we really need to be able to see the application code running in the Mark VIe at his site AS WELL AS to understand the conditions during off-frequency operation. He may also be experiencing combustion mode changes during the grid frequency disturbances that we have yet to hear about. (DLN combustors-equipped units are not well-suited for most off-frequency operations.)

I sincerely believe there are some serious misunderstandings about droop speed control and off frequency operations at niazbibiyana's site, which are causing the questions posed in this thread. We would need much more information in order to be able to sort out what is happening and what should be happening, and even then if that didn't match the site perceptions it probably would not be accepted.

Thanks for trying though. Best to just let this thread die without a LOT more information from niazbibiyana.

#### Isulamu

CSA

I want to grab the opportunity to discuss with you a droop control subject. I was about to post a thread about it later this week.

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.

The standard droop control has no FSR value feedback. 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

OK! at full speed no load TNR-TNH would be equal to 0. and the FSRN value would be = FSKRN1, initially caclulated by the OEM for a particullar fuel flow rate. And this value it depends on the fuel heating value! also of the P2 pressure refrence FPRG!

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 injection nozzles.

#### CSA

Isulamu,

Since this really is a separate question not entirely related to this thread, it would be a good idea to start a new thread on the subject (for future reference).

Are you saying that the unit is having problems maintaining rated speed when it gets to near rated speed? Because problems with either GCV LVDT calibration OR fuel make-up (heating value) are about the only things that cause this problem. And, when Auto Synch is selected and enabled then the loop closes on actual speed anyway--though sometimes this causes a longer than normal wait for the generator breaker to close when the turbine speed has to be adjusted to get it close to 100.3% or so if it's much lower than 100% or much higher than 100%.

Other than that, problems with fuel make-up and/or GCV LVDT calibration kind of get "ignored" once the generator breaker closes.

FPG2 (P2 pressure) shouldn't affect droop speed control at all--unless there is some problem with gas fuel supply pressure/flow which causes the SRV to be unable to maintain proper P2 pressure. As we've discussed in this thread, as long as the gas fuel supply pressure and flow-rate are not restricted small errors in P2 pressure should not affect turbine operation--unless the gas fuel supply make-up is significantly different than expected. Which would also potentially cause problems for the fuel nozzles--orifice sizing. In this case (a significant change in gas fuel supply make-up from the original expected fuel characteristics), it would be best to contact the OEM or a company which could analyze a very recent fuel analysis and the specifications of the fuel nozzles currently being used in the unit and make recommendations for changing the fuel nozzles and/or the P2 pressure reference. But, I would suspect it would require a pretty significant fuel make-up difference to result in a change in P2 pressure--and I would think only the OEM would really be able to address the P2 pressure issue.

Hope this helps!

N

#### niazbibiyana

Hello CSA & Isulamu,

Thanks both of you as I gained huge knowledge about p2 reference generation and also control. I want to know one more thing called "FPRGCRIT". Actually when this parameter does work? I am uploading here full control block of our mark VIe.

http://i64.tinypic.com/wsr68k.jpg

Here it says that, "required P2 pressure for choked GCVs." But I dont understand when and in which condition, its required maintain GCVs in choked condition.

Thank you

#### Isulamu

CSA,

You are right. A new thread will be better to discuss this. As far as i'm concerned i think i couldn't make it clear for you to imagine the governor mode which i am talking about. I will try to explain it more in the new thread, and we will carry on the discussion there.

thank you

#### Isulamu

niazbibiyana,

In your initial post you said "During shutdown "FPRGCRIT" contributes to generate P2 reference (Derived from CPD)." would you please give us the reference of this statement?

On the other hand, based on my knowledge FPRGCRIT stands for CRITICAL P2 required to maintain chocked GCVs, (chocked GCVs means to let the mass flow rate only dependent of upstream pressure/ temperature of GCVs).

In other words, FPRGCRIT is the minimum limit which the FPRG (gas ration valve control pressure reference) CAN BE!!, like i mentionned before to maintain chocked GCVs, also as FPG2 nears the FPRGCRIT line (for whatever the cause), a Raise Inhibit will prevent the unit from operating at higher loads without the required supply pressure and to avoid the GCVs full open position.

Now back to the application code you shared, it is clear that after the flame has been established the FPRGCRIT = FPRGCRIT_AUX as long as this last is lower than FPRGN (TNH based P2 reference)
And FPRGCRIT_AUX is is composed of two CPD-based curves and it is conditioned with the L28FXM signal (DLN mode control 4).

#### CSA

niazbibiyana,

Two questions:

1) Can you provide the values of CPD, load (DWATT), FPG2, and FPRGCRIT when the unit is running at a stable load?

2) Can you provide the values of all the Control Constants in the block you posted?

N

#### niazbibiyana

Hello CSA and Isulamu,

<b> 1) Can you provide the values of CPD, load (DWATT), FPG2, and FPRGCRIT when the unit is running at a stable load? </b>

Right now GT is running with 200MW at free governing mode, with 109% TNR, (Droop is 10%), IGV 71.6 degree, FPG2 feedback pressure 363 PSI, FPRGCRIT is 330 PSI. Grid frequency 50.5 hz. CPD-183.4 PSI. Right now FPRG is generating from minimum value of FPRGN & FPRGTAMB , which is 359.5175 psig. (machine rating ISO is 248 MW, 9FA.03)

<b> 2) Can you provide the values of all the Control Constants in the block you posted? </b>

I am uploading photo.. U can zoom it. If its not clear, plz ask me again. I will manually put all values.

http://i68.tinypic.com/34sgg8m.jpg

http://i67.tinypic.com/2eelrts.jpg

Isulamu, You have asked me <b>"In your initial post you said "During shutdown "FPRGCRIT" contributes to generate P2 reference (Derived from CPD)." would you please give us the reference of this statement? </b>

No I can not. Who told me, he is not belongs to I&C. Also it was illogical. BTW, many many thanx to explain it.