Can any one explain in detail by referring with some case. When machine is operating at Base load either in Isch or Droop mode by increasing just 1MW load at base load does GT trips..?

How the GT trips if we increase the load either by over exhaust Temp trip or by under frequency

 

As we have said so many times before on these forums, if you need some help, please give us something to work with. What alarms did you get? what did the trip history show? Something caused the trip, there has got to be some evidence of the cause.

Please post as much data as you can and we will try and help

 

If Base Load is enabled and active, then you shouldn't be able to increase load at all.

So, the real question, because you have the answer<b>--you just want it to be something other than what it is</b>--is how are you increasing load when Base Load is enabled and active?

The answer you are ignoring is the Process Alarm that is being annunciated when the trip occurs.

And, if the unit is being operated below rated frequency then the power output will be less than rated at Base Load so if you're trying to increase load by forcing logic (and you are, aren't you?) then it's probably going to trip on exhaust over-temperature. And that's most likely the alarm you're ignoring, isn't it?

This question reflects a serious lack of understanding about how gas turbines operate, and how GE-design heavy duty gas turbines operate.

 

Cryptic questions

well, i think he is asking a hypothetical question. "mark-v student" you seem to have a few concepts amiss. you cannot operate the machine in base load in Isoch mode. Base load operation is possible in parallel with gird or at least one more generator.

In base load operation you cannot increase the load further in any manner as the fuel selection is a min selector gate and the temp control will be active at that time.

In independent operation, if the machine is running close to base load (ie the max possible load under the condition) and you increase the load, the machine will sink , ie it will gradually reduce in frequency and the generator will trip on under frequency.

 

I wish to give link to the trip report of the one the unit. The trip behavior is same on all running units and all are under the ILS load sharing mode. I will also give the alarms faced before trip.
http://www.sendspace.com/file/0k11oy

 

Unfortunately this is only what looks like some data from Historian (but not much), really not enough to analyze anything other than it tripped. Can you get the Trip History log from the machine? This would help a lot more.

 

> Unfortunately this is only what looks like some data from Historian (but not
> much), really not enough to analyze anything other than it tripped. Can you
> get the Trip History log from the machine? This would help a lot more.

The report i added is the trip log from one of my machine which is converted from Mark-VIe. I wish to add alarms also this time here. I understand that as such we can't increase the load on a machine running in Base Load. I want to know if the machine got over loaded by 1% of 9FA machine base load is 2.5 MW what will it happens. My colleague says it trips even if we increase the 1MW, which I can't believe as we can sacrifice some rated speed and can run the still. If we can't run how this this will happen. http://www.sendspace.com/file/pu5nxy[/URL]

 

IMD,

Ahh; okay. This is basically a theoretical question.

And you are questioning something you were told by someone else.

The implication was that you had actually experienced trips under the conditions stated in your original post. And while you may have experienced trips under similar conditions, please read further.

If a GE--design heavy duty gas turbine-generator is operating at Base Load (on CPD- or CPR-biased exhaust temperature control) at any speed (grid frequency), it is not possible for an operator to increase the power output of the generator (the "load" of the generator)--or even to change the load <b>on the grid</b>.) This regardless of whether or not Droop or Isochronous control mode is selected.

[NOTE: It's <b>EXTREMELY</b> important to distinguish between the load on the turbine-generator, and the load <b>on the grid</b>.]

Let's start with the case where Droop Speed Control mode is selected. If the unit is operating at Base Load on exhaust temperature control, it <b>***IS NOT***</b> technically operating on Droop Speed control (though Droop Speed Control mode may be selected). So, if the load <b>on the grid</b> increases, and there is no spinning reserve on the grid then the grid frequency will decrease, which will cause the turbine and generator speed to decrease. (Actually, it will cause the speed of EVERY turbine and generator which is connected to that grid to decrease!)

Because the turbine in this case is a GE-design heavy duty gas turbine, the frequency decrease and corresponding turbine speed decrease and generator speed decrease and axial compressor speed decrease will cause the power output of the turbine to <b>DECREASE</b>. This is because as the speed of the axial compressor decreases the air flow through the machine will decrease.

When the unit is operating on exhaust temperature control, the Speedtronic is trying to put as much fuel into the machine as possible to try to keep the exhaust temperature as high as possible. If the air flow through the machine decreases <b>and the fuel flow remained the same</b> then the exhaust temperature would increase. But, as the exhaust temperature starts to increase as the unit speed decreases and the air flow through the axial compressor decreases the Speedtronic will decrease the amount of fuel to lower and limit the exhaust temperature, and so the amount of torque being produced by the turbine will decrease. And when the the amount of torque being produced by the turbine decreases then the generator output will decrease.

NOTE: The load <b>on the grid</b> isn't changing; only the portion of the load <b>on the grid</b> that's being supplied by the turbine-generator is changing--and that's being done automatically by the Speedtronic in response to changes in exhaust temperature. The operator can't change the load on the unit, or the load <b>on the grid</b>.

And, on a grid where the frequency is already decreasing because the generation can't support the load <b>on the grid</b>, if the gas turbine power output decreases then the grid frequency will decrease even more!

Again, the operator can't increase or decrease the load <b>on the turbine-generator</b> because it's already on exhaust temperature control (Base Load), which is a function of fuel flow, and air flow, and hence speed.

Now, for the case where the turbine is operating in Iscochronous control mode. (And it should almost NEVER be in Isochronous control mode when it is being operated on a large grid with many other prime movers and generators!) The Speedtronic control system will adjust the fuel as the load <b>on the grid</b> changes to try to maintain the frequency. That's what Isochronous speed control mode does: It adjusts the prime torque output in response to changes in speed, because speed is directly proportional to frequency. The the frequency of a grid is inversely proportional to the load <b>on the grid</b>, meaning that if the load <b>on the grid</b> goes up the grid frequency will tend to decrease, and if the load <b>on the grid</b> goes down the grid frequency will tend to increase.

When a unit is operating in Isochronous control mode an operator <b>***CANNOT***</b> control the power output (load) of the unit. Clicking on SPD/LD RAISE or SPD/LD LOWER will not change the power being produced by the unit--it will, however, change the frequency setpoint of the unit (and, hence, it's speed).

The load being produced by the turbine-generator when the prime mover is operating in Isochronous control mode is strictly a function of the load <b>on the grid</b> to which the unit is connected. As the load <b>on the grid</b> goes up, the grid frequency would tend to go down but the Speedtronic will increase the fuel to keep the frequency and unit speed constant. If the load <b>on the grid</b> goes down the frequency would tend to go up but the Speedtronic will lower the fuel to keep the frequency, and unit speed, constant.

And this is true <b>only as long as the unit operating in Isochronous speed control mode is: 1) below Base Load (exhaust temperature control), and, 2) above zero MW ("minimum" load).</b>

If the load <b>on the grid</b> increases to the point that the turbine reaches exhaust temperature control ("Base Load") then the Speedtronic can't increase the turbine's power output any more. Full stop. (And, neither can the operator increase the load on the turbine-generator!)

If the load <b>on the grid</b> continues to increase the Speedtronic control system operating in Isoch control mode cannot increase the fuel any further. So, as the load <b>on the grid</b> increases the frequency will decrease, which will cause the turbine-generator speed to decrease, which will cause the axial compressor speed to decrease, which will cause the air flow through the machine to decrease, which will tend to cause the exhaust temperature to increase if the fuel flow remains constant. But, the Speedtronic will reduce the fuel to limit the exhaust temperature--so the load being produced the the unit operating in Isochronous control mode will actually decrease as grid frequency decreases!

NOTE (Again): The load <b>on the grid</b> isn't changing; only the portion of the load <b>on the grid</b> that's being supplied by the turbine-generator is changing--and that's being done automatically by the Speedtronic in response to changes in exhaust temperature. The operator can't change the load on the unit, or the load <b>on the grid.</b>

So, if a GE-design heavy duty gas turbine is operating at Base Load an operator can't change the load on the unit. Full stop. Period. Whether it's in Isoch mode or "Droop" mode. (And it shouldn't be in Isoch mode when being operated on a large grid in parallel with many other prime movers and generators!).

The only time an operator can increase or decrease the load on a GE-design heavy duty gas turbine is when it <b>***IS***</b> operating on Droop Speed Control and it <b>***IS NOT***</b> operating at Base Load, on exhaust temperature control. Full stop. Period.

Now, the Speedtronic is <b>usually</b> able to make changes to the fuel flow-rate when the unit is operating on exhaust temperature control (Base Load) to be able to respond to most frequency changes without causing the unit trip on exhaust overtemperature or even under-frequency. Notice I said "usually" because sometimes the grid frequency fluctuations are severe and extreme and when the unit is already operating on exhaust temperature control the exhaust overtemperature alarm is only 25 deg F above the current exhaust temperature and the exhaust overtemperature trip setpoint is only 40 deg F above the current exhaust temperature. So, sudden changes in load <b>on the grid</b> (and, hence, grid frequency) could make it very difficult for the Speedtronic to change fuel quick enough to avoid tripping on exhaust overtemperature.

As for tripping on under-frequency, well that's between the site and the utility or the grid regulator. One would hope that someone (site operators or grid regulators or the utility) would shed some load on the grid before the frequency dropped so low as to trip the turbine-generator. But, sometimes they don't catch it fast enough, or the severity of the excursions are just too extreme to be able to shed load quick enough to raise the grid frequency.

These are the facts for almost every GE-design heavy duty gas turbine operating everywhere in the world. There are some units that have some special code to prevent dropping load when operating at Base Load when the grid frequency drops, or to prevent increasing load when operating at Base Load and the grid frequency increases. These code changes have limits, both in magnitude of response and in the period of time they can remain active. (In other words, they can't remain at an elevated output when operating at 98.4% speed indefinitely or for even an hour at a time; usually only minutes are available to allow the grid operators to try to get the grid frequency back to nominal.)

And, in fact, this isn't even that specific to GE-design heavy duty gas turbines, because most gas turbines of any manufacturer all operate similarly at rated load when frequency/speed excursions are experienced.

I hope this answers your questions. Please note again: There is a difference between the load being produced by the turbine-generator, and the load <b>on the grid</b>. The turbine-generator, when operating on a large grid in parallel with other prime movers and generators, is only participating ("sharing") in providing some of the power necessary to support the load <b>on the grid</b>. Turbine-generator operators <b>HAVE ***NO*** CONTROL</b> of the load <b>on the grid</b>. And they only have control of the load on the unit when it's operating in Droop Speed Control Mode below Base Load on exhaust temperature control.

So, your post was quite unclear that it was a theoretical question, for the most part, and it was completely incorrect in that operators can increase (or decrease) load on a unit when it's at Base Load (regardless of whether it's in Droop or Isochronous speed control mode). When you say "...increasing by just 1MW load at Base Load..." (which you said in your original post) you were speaking incorrectly.

You may have been trying to say, "If the load <b>on the grid</b> increases by just 1 MW when a 9FA is operating at Base Load in Droop or Isochronous control mode, what will happen?"

But that's not what you said. So, we have something of a failure to communicate. But, now you have the information necessary to answer your own question!

And, remember: A turbine-generator operator cannot control the load <b>on the grid</b>, and can only control the load on the unit (which is a part of the total load on the grid) when the unit <b>is in Droop Speed Control mode AND exhaust temperature control is not active (it's NOT on Base Load).</b>

 

That is one of the ugliest Alarm Logs I've ever had the displeasure of seeing.

If the grid frequency was low enough to trip the breaker (not necessarily the turbine, but the generator breaker was "tripped" or opened) on underspeed (which is usually 14HS dropout, TNK14HS2), then that unit is taking a BEATING!

It also looks like there is some kind of load-sharing (Isoch or otherwise) that we weren't made aware of (and which isn't typical on most GE-design heavy duty gas turbines).

I feel sorry for the turbine and generator and auxiliaries. That's not good for a machine--especially a 9FA.

 

Further to the review of the provided Alarm Log, there is no indication that the turbine tripped. Only that the generator breaker was opened (sometimes referred to as "tripping" the breaker).

But the title of this thread as well as the original post and subsequent posts indicates that the <b>turbine</b> tripping, and turbine tripping is very different from generator breaker tripping (opening).

A turbine (GT) trip is an emergency shutdown that involves a fast shut-off of fuel to the combustors, an opening of the generator breaker, and coasting down of the unit shaft speed by inertia to Cooldown. A turbine trip is a thermal stress on the hot gas path hardware because of the sudden loss of flame, so when you speak of a GT trip, please be most specific.

A shutdown, such as an operator-initiated STOP, is an orderly reduction in fuel flow, a generator breaker opening, and a ramp down of fuel to decelerate the unit to as low a speed as possible so as to reduce the thermal stresses on the hot gas path components. This is very different from a trip.

From the information provided in the Alarm Log, it does not appear that the GT tripped, only that the generator breaker was opened, and that is usually what happens when a turbine underspeed condition occurs as is shown in the Alarm Log.

 

Actually, I have mis-spoken on one aspect.

When the Speedtronic is in Droop Speed Control mode and if the unit is operating at Base Load on exhaust temperature control, the operator can <b>LOWER</b> the load <b>on the generator</b>. The load <b>on the generator</b> cannot be raised by the operator when Droop Speed Control is selected and the unit is operating at Base Load, but the load <b>on the generator</b> can be <b>LOWERED</b> below Base Load, exhaust temperature control.

My sincere apologies for any confusion that might have created.

 

CSA,

I have to say that this is the best explanation to the loss of gas turbine load output that I have seen. For the longest time I had difficulty understanding why a machine load output would decrease as frequency decreased. And the secret is the operation of an axial compressor. I hope that this will enlighten others as it did for me!

 

MIKEVI,

Thanks very much. I just hope the original poster found the information useful and helpful. And that we'll hear back one way or the other.

And by the way--your absence has not gone unnoticed. We could use the benefit of your experience and knowledge.

Jump in! The water's fine!

 

I'd just like to add that often the terminology is vendor specific. For instance, in as CSA has mentioned about GE gas turbines, "base mode" may be different than what a steam plant considers base mode.

In a steam plant, "base mode" (or sometimes called "load mode") often means just put the unit at a fixed MW output and it will pump out that MW regardless of what the frequency is. This doesn't matter so much on an infinite grid, but it does make a difference on a smaller system.

Droop mode (or speed mode) and base mode in some plants are impossible to use at the same time.

Isoc is pretty much the same across the board.

 

Kindly refer my case, here tripping means got opened the breaker. Power failure to auxiliary lead to low lube oil and finally on that reason GT got tripped. From the trip report I understood for breaker open reason is only under speed. I like know more about a gt on base load with FSRT as minimum FSR. on a small grid let us say @1000MW running with only four 9FA gts in island mode, when all machines are in base load and all are in FSRT control, suddenly 1MW extra load increased on island than generation capacity, will all machines experience same under speed and get breakers opened for all machines? or they can observe some extra load and sacrifice the speed and will continue to run?.

 

CSA has written laboriously, which part of it did not clear your query. here is the simple explanation. yes in case all the machines are running in a independent grid and all the machines are running near the full load(base load) conditions, even a 1 MW increase will cause a speed reduction. due to speed reduction the power output capability of the GT decreases thus coupled with a increase in load and a reduced power output capability the machine sinks and trips in under frequency.

usually as the frequency reduces the section load also reduces (due to reduced motor power output). this may compensate for a reduced power output. if it does not compensate as it is in your case , the machine will trip.

This is the first time i am seeing such a tripping indecent usually most of the independent island operations have a under-frequency load shedding scheme. 4 GT's tripping is a very serious issue. what you first need to do is to implement a load shedding/load restoration scheme.

 

Mark-V guy (since you're no longer a student),

If the four units are already at Base Load--presumably at 50.0 Hz--what would you expect would happen if the load on the grid being supplied by the gas turbines increases?

The four Speedtronics can't increase the fuel.

The synchronous generators (alternators) are keeping all the units running at the same speed (frequency).

What do you think would happen, Mark-V guy?

Or, more precisely, what do YOU think should happen?

Inquiring minds want to know.

 

After this incident, we every one becoming experts in ILS logics and other load shedding scheme experts.

Kindly correct me, if I am wrong for your above question...

As already in BL further increase in the load cause, as its small island condition, the machines under go over load and hence there will be sudden dip in speed. due to this compressor o/p reduces at same fuel flow, causes ttxm will start raise. due to this fsrt will decrease in spite of little TTRXB raise and hence further speed down will lead to Back-up curve will come into pic and on further fsrt down will slowly experience under frequency and get 52G opened. I hope these machines got same experience. However as replied earlier we are under through our logics for ILS where I got major bug and requested GE to go through. Let's hope everything will be smooth. Thanks for your patience reply on every corner of my various posts here and closing this thread.

 

CSA

I've read many of your posts, I'm curious (if you don't mind me asking) do you work for GE?

Chris

 

First of all how we a going to increase 1MW demand? We on a 1MW motor. This motor has potential difference. Therefore additional current will be drawn from the syste. This motor is infinitely small when compared to the grid system. For a while the grid hasn't notice that this motor actually drawing additional current. Why is that so? System voltage hardly change since this motor can't produce measurable change in system voltage. And so thus its frequency (for a while).

Next question-We know that all the prime movers do not produce any additional torque since the maximum fuel limits have been met. But the motor draws measurable additional torque via additional current being drawn from the grid. Where does the motor get this additional torque?

I think that is what you mean Mr CSA. I have answered this question probably more than a year ago. It was quite a long post. I don't want to repeat it again. At least not now. Let me provide a brief about it.

At 50.00 Hz synchronizing frequency there are there important forces to be recognized. (1) Energy inputs to the grid (From all prime movers (2) Energy consumed by the electrical loads and (3) Stored energy of the grid! The third force i.e. stored energy of the grid to many of us is not obvious since it is not directly measured.

Almost all of us here have recognized the existence of stored energy without really aware that we are actually talking about it. As long as you have mentioned about system frequency (Hz) you are by default the believers of third force!

Back to CSA question. If we add 1MW demand to the already "saturated grid" then we will get the 1MW demand at expense of our system frequency. The real meaning is that the stored energy of our grid system will reduce. If we keep that additional 1MW for very long time, the rest of the system will collapse due to under frequency even if we are assuming all our your prime movers can hold the same outputs at lower system frequency.

Here are approximate values for some of the stored energy of the prime movers

(1) Nuclear powered steam turbine generator = 11MJ/MVA @ 60Hz

(2) Conventional thermal steam turbine generator =8-10MJ/MVA

(3) Hydro turbine generator = 2-4MJ/MVA @ 60Hz.

GT? Probably around 5-6MJ/MVA.

(See William D. Stevenson Jr "Elements of Power System Analysis" 4th Edition, McGraw-Hill for the detail.

Assuming our grid consists of 10 units of 100MW, 120MVA conventional steam turbine generators with rated stored energy of 8MJ/MVA at 60Hz. Our grid system is rotating at 60Hz while all the prime movers are operating at their maximum outputs. Then stored energy from our generators + prime movers is

Stored energy = 10* 8 * 120 =9600MJ @ 60Hz

If we add 1MW demand, we can calculate approximately frequency decay for the first 5 seconds

Total stored energy consumed = 5s X 1MW= 5MJ

Remaining stored energy = 9600-5=9595MJ

New system frequency = 9595*60/9600=59.96875Hz.


Every time the frequency reduces that mean stored energy reduces. Most of us here only aware about frequency change rather than stored energy change although both of them have similar meaning. It is important to note that for great frequency variation the relationship between stored energy consumed and system frequency decay is not exactly linear as given by the above example.