Machine Tripping on Over Speed

The steam turbine installed at our site currently facing tripping of machine on over speed.

Sequence of tripping start from load side transmission lines voltage becomes zero resulting GCB opened due to over frequency. As GCB opened, load rejection scheme initiated but Turbine tripped on Over speed.

The load rejection scheme initiated successfully on low load approximately 25% of the total load and machine successfully shifted to FSNL however load rejection scheme failed at full load and tripped on over speed.

What are the reason of tripping of machine on over speed at full load?

What options available to avoid tripping of machine on over speed and load rejection initiate successfully at full load?
 
The steam turbine installed at our site currently facing tripping of machine on over speed.

Sequence of tripping start from load side transmission lines voltage becomes zero resulting GCB opened due to over frequency. As GCB opened, load rejection scheme initiated but Turbine tripped on Over speed.

The load rejection scheme initiated successfully on low load approximately 25% of the total load and machine successfully shifted to FSNL however load rejection scheme failed at full load and tripped on over speed.

What are the reason of tripping of machine on over speed at full load?

What options available to avoid tripping of machine on over speed and load rejection initiate successfully at full load?
Good day all,
Engrahmed,

1-Could you tell us the model /type of the installed controls system?

2-Also did you get a "event record" or "trip log report" from the controls system ( Turbine/generator/grid or Line control/protection system)??

3-When was the last time that you performed "load rejection " at 100 %Load and it get succesful ?

4-When was the last time that "overspeed test(s)" has been succesfully performed?

5-Did you get "undervoltage " protection alarms /trip before /during this event?

Without be able to check the software of the installed control system , it is difficult to get better overview on the issue.

Also how Line voltage dropped suddenly to become "zero"??

There should be protection selectivity, in the installed system that may worked during the event, without be able to see the datas we cannot state on anything till now.

Would be nice to get these kind of informations , to try to assist you with this issue.

Controlsguy25
 
Engrahmed,

Most people don't realize it but when a steam turbine is tripped while the generator is producing electrical power (the GCB (Generator Circuit Breaker) is closed and supplying amperes to a load) the first thing that should happen is that the steam turbine stop valves should close, and then the generator circuit breaker should open very shortly after that. WHY?

Because steam turbine-generators are very "light" (as in weight) machines and it takes very little steam to get them up to rated speed (FSNL, Full Speed-No Load). When the generator breaker is closed and the turbine-generator is producing electrical power and supplying a load that load is actually acting somewhat like a brake (not break, brake!) to try to slow down the turbine-generator.

So, let's consider what happens if the generator breaker (GCB) opens before the steam turbine stop valves close. When the turbine-generator is producing power (again, the GCB is closed) the steam flow-rate is higher than the amount required to just keep the unit spinning at FSNL (rated speed). If the GCB opens suddenly--and before the steam turbine stop valves close--the amount of steam flowing into the steam turbine doesn't change and now that braking action of the load it was previously supplying when the GCB was closed is suddenly gone. So, the unit almost always goes to overspeed--because the unit is so light and it takes so little steam flow to just maintain FSNL that the additional steam which is flowing causes the speed to increase VERY rapidly, almost always leading to some kind of speed increase and most frequently leading to an overspeed condition.

For this reason when the steam turbine-generator needs to be tripped if the steam turbine stop valves are closed (suddenly) before the GCB is opened there is the period of time when the GCB is still closed and supplying power to the load that acts like a brake to keep the turbine-generator from overspeeding. The steam flow-rate is shut off, the steam pressure in the lines between the steam turbine stop valves and the 1st stage turbine nozzles has a chance to decrease and the unit speed is prevented from increasing because the GCB is closed and is acting like a brake on the turbine-generator shaft(s). (Also, the speed of the turbine-generator can never increase above synchronous speed nor decrease below synchronous speed as long as the generator circuit breaker remains closed.) [NOTE: I am presuming the breaker that connects the output of the generator circuit breaker to the grid/load is closed!]

Again, if the GCB is opened before the steam turbine stop valves are closed that means the steam flow-rate is not decreasing--and it needs to decrease VERY quickly--usually more quickly than it can be decreased because of the time it takes for the steam turbine stop valves to close! So, without the load of the generator (connected to the grid or whatever the unit is powering) if the steam isn't extremely quickly reduced the speed of the turbine-generator will increase VERY fast. And, because most steam turbine stop valves can't close quickly enough to prevent an overspeed if they are closed at the same time as the GCB is opened the normal practice is to trip the steam turbine-generator by closing the steam turbine stop valves FIRST and then opening the GCB.

Now, I have zero experience with what is happening with the grid voltage going to zero while the GCB is closed and the turbine-generator is producing power. In my feeble mind, I can't even envision how amperes can flow when there is no voltage (voltage is the "pressure" which makes amperes flow--in my feeble mind). And in my feeble mind if amperes were flowing (when there was no voltage) that would prevent the over-frequency condition.... So, I'm afraid I have no frame of reference (experience) with the scenario you are describing.

You mention something about load shedding. Load-shedding (in my feeble mind) won't work if the GCB is open--because once the GCB is open the turbine-generator isn't supplying any load, the load has effectively already been shed and is at zero. To my feeble mind, again--this is where the steam turbine stop valves have to be closed BEFORE the GCB is opened.

The last thing I want to say is this: There are usually tests that need to be performed weekly (at a minimum) on the steam turbine stop valves to ensure they close as quickly as they can in the event of a problem like this. (Silica can build up on the stems of the steam turbine stop valves to the point that when the stop valves try to close they can't--so these tests do two things. First, they exercise the stop valves (while the unit is running and steam is flowing) to remove the silica from the stop valve stems. And, second, they are used to detect if the stop valves are sticking (because of the silica, usually, or mechanical binding), telling the plant personnel that there is a very serious situation developing or occurring and the unit needs to be shut down and the stop valves need to be examined and cleaned or repaired. Is the plant doing these tests regularly? (Because all to many plants don't seem to be....)

Strange. Very strange. My feeble mind is shot already this morning from trying to imagine how all of what you say is happening can be happening.

Best of luck! Please write back with answers and details and let us know how you progress
 
Controlsguy25,

Yes; there are a LOT of things about this scenario that are not clear or understandable.

And, yes--you are right. Knowing the turbine manufacturer and turbine control system is kind of important (as is the size/rating of the unit).

And about the only way I could think of that line voltage could drop to zero while the generator circuit breaker was closed and the turbine-generator was supplying power to the grid is if there was a significant three-phase fault--in my experience and from the stories I've heard (not all of which I always believe). Even if the exciter suddenly dropped to zero output (with a brushed or brushless exciter) the unit would most likely have tripped on under-excitation or or something similar.

And the question, "What are the reason of tripping of machine on over speed at full load?" is totally not clear. The unit might have been at full load before the event started but if the GCB (Generator Circuit Breaker) opened the load was NOT at rated.

This is just pretty strange. Often, I attribute some of these really strange questions and scenarios to an unfamiliarity with the English language, and I would say there is some of that going on with this original poster.

I sincerely hope the original poster can supply some kind of actionable data (from some kind of data historian or data archival and retrieval system (it would have to be high-speed data)), and will try to answer at least some of the questions you and I posed.

Strange.

Very strange, indeed.
 
Controlsguy25,

Yes; there are a LOT of things about this scenario that are not clear or understandable.

And, yes--you are right. Knowing the turbine manufacturer and turbine control system is kind of important (as is the size/rating of the unit).

And about the only way I could think of that line voltage could drop to zero while the generator circuit breaker was closed and the turbine-generator was supplying power to the grid is if there was a significant three-phase fault--in my experience and from the stories I've heard (not all of which I always believe). Even if the exciter suddenly dropped to zero output (with a brushed or brushless exciter) the unit would most likely have tripped on under-excitation or or something similar.

And the question, "What are the reason of tripping of machine on over speed at full load?" is totally not clear. The unit might have been at full load before the event started but if the GCB (Generator Circuit Breaker) opened the load was NOT at rated.

This is just pretty strange. Often, I attribute some of these really strange questions and scenarios to an unfamiliarity with the English language, and I would say there is some of that going on with this original poster.

I sincerely hope the original poster can supply some kind of actionable data (from some kind of data historian or data archival and retrieval system (it would have to be high-speed data)), and will try to answer at least some of the questions you and I posed.

Strange.

Very strange, indeed.
CSA,

Yes, you right when you saying that this is just strange!

I hope also that original poster, can supply some kind of actionable datas.

I wish you a good evening and kind regards,

Controlsguy25
 
Dear All,

Thanks for response.
Sorry some information was missing.

Please note line voltage zero mean fault in transmission line due to any reason while the generator circuit breaker was closed and the turbine-generator was supplying power to the grid and therefore GCB opened due to the over frequency due to the sudden unavailability of load.

Then load rejection scheme initiated upon feedback of GCB but STG often end up on over speed tripping.
 
The steam turbine installed at our site currently facing tripping of machine on over speed.

Sequence of tripping start from load side transmission lines voltage becomes zero resulting GCB opened due to over frequency. As GCB opened, load rejection scheme initiated but Turbine tripped on Over speed.

The load rejection scheme initiated successfully on low load approximately 25% of the total load and machine successfully shifted to FSNL however load rejection scheme failed at full load and tripped on over speed.

What are the reason of tripping of machine on over speed at full load?

What options available to avoid tripping of machine on over speed and load rejection initiate successfully at full load?
Good day all,
Engrahmed,

1-Could you tell us the model /type of the installed controls system?
BHGE Steam Turbine with Rx3i Control System

2-Also did you get a "event record" or "trip log report" from the controls system ( Turbine/generator/grid or Line control/protection system)??
Please provide list of variables for required data.

3-When was the last time that you performed "load rejection " at 100 %Load and it get succesful ?
2019

4-When was the last time that "overspeed test(s)" has been succesfully performed?
Annual Turn Around-Jan-2020

5-Did you get "undervoltage " protection alarms /trip before /during this event?
GCB opening on over frequency and then end up with tripping on over speed.

Without be able to check the software of the installed control system , it is difficult to get better overview on the issue.

Also how Line voltage dropped suddenly to become "zero"??
Fault in transmission line due to any reason while the generator circuit breaker was closed and the turbine-generator was supplying power to the grid

There should be protection selectivity, in the installed system that may worked during the event, without be able to see the datas we cannot state on anything till now.

Would be nice to get these kind of informations , to try to assist you with this issue.

Controlsguy25
 
Engrahmed,

It would be a good thing to tell us , the plant configuration :
1-Steam turbine type /load scheme
2-Is there any exctraction or other process steam for any utility??
3-Some parameters like STGload ,ST Control/Stop/Trip Valves trends, Line Voltage, Distance protection, Generator protection/AVR trends?
4-Was the last overspeed /load rejection tests succesfully achieved ?
5-Do you get SLD ?
6- Do you have the Controls blocks ( load rejection/overspeed) checked ?


Also I just found this interesting article called
"Optimized Steam Turbine Governor Controlling Single or Multiple Grid Faults"
( Case studied by Siemens engineers) :
https://www.researchgate.net/publication/267503921_Optimized_Steam_Turbine_Governor_Controlling_Single_or_Multiple_Grid_Faults#pf4


Here some quotes:
Grid Codes do not necessarily assume that power plants
must also be able to control another fault occurring a few
seconds later. Since load rejection to house load can certainly
occur following a short circuit, e.g. during extreme weather
conditions or environmental disasters, it would be of great
advantage to enhance the power plant availability if a multiple
grid fault of this kind could also be controlled. For the multiple
grid fault it is assumed that the power plant described here is
connected to the grid via two transmission lines. During storm
a three-phase short circuit (for example a tree cuts all three phases)
occurs in one of the two transmission lines which is
cleared after 250 ms. Thereafter a two-phase short circuit (for
example two phases contact each other) occurs in the remaining
second transmission line which result in a load rejection to
house load for the power plant. The rejection of electrical load
shortly after a short circuit can give rise to very high
overspeeds however and these will cause tripping by the
dedicated overspeed protection system. An overspeed trip
results in shutdown of the turbine generator and the extended
downtimes associated with it. The main benefit of load
rejection to house load as compared with the shutdown by a
protection system is the immediate re-establishment of
availability, i.e. the power plant could be synchronized with the
grid only a few seconds later.

SINGLE GRID FAULT:
In the event of a single grid fault, a generator must supply
active and reactive power for as long as possible so as to
support the stability of frequency and voltage. If, however, the
generator slips out of synchronism with the grid or if the shaft
train is endangered due to overspeed, for example, the turbine
generator must immediately be disconnected from the grid and
switched to house load operation. The controller monitors the
electrical power on the generator terminals. If specified limits
(load band around zero) are exceeded, fast travel mode is
triggered for the valves and the mechanical load is decreased as
quickly as possible. The valves remain closed for a hold time of
approx. 1.5 s, after which time they are slowly reopened.
It is impossible to distinguish between load rejection to
house load and a grid short circuit on the basis of the active
power of the terminals at the beginning of the event. Unlike
load rejection, electrical active power returns after a grid short
circuit once the fault has been cleared and the valves must then
reopen. The generator then must continue to supply active and
reactive power to the grid, so as to stabilize frequency and
voltage. This means that if the short circuit is active for 250 ms

I finally got the document downloaded!



It is available with access ( after registration) you can try to download it, at least read it it contains precious informations on such event.

It can help you to have a better overview, of what happened and what is possible to do to remediate, on this in the future and fix the issue and get the unit available, for a resynchronization ( reliability , availability improved) during/after such event.

Thats what we can check prior to state, on the event .

I am sure that the article can be an help !

Let us know what you think on it.
Kind regards,
Controlsguy25
 
Controlsguy25 and Engrahmed,

"Please note line voltage zero mean fault in transmission line due to any reason while the generator circuit breaker was closed and the turbine-generator was supplying power to the grid and therefore GCB opened due to the over frequency due to the sudden unavailability of load.

Then load rejection scheme initiated upon feedback of GCB but STG often end up on over speed tripping."

The above statements are ... well, they are ....

A generator set provides electrical power to a load (or loads) when the GCB (Generator Circuit Breaker) is closed and there is a load (or loads) on the load side of the GCB. Usually, when a power plant is connected to a utility grid, there is some kind of breaker which connects the plant to the utility grid (a utility tie breaker). So, for a generator set to supply electrical power to the load (or loads) of a utility grid BOTH the GCB and the utility tie breaker have to be closed.

A generator converts mechanical energy (torque, from a prime mover--in this case a steam turbine) to electrical energy: amperes, primarily. To increase the amount of electrical energy being produced the amount of mechanical energy being applied to the generator has to be increased. In the case of a steam turbine that means opening the steam turbine control valves further.

Load ONLY exists when the GCB is closed, and if there is a utility tie breaker (or air-break switches or multiple breakers) are also closed and there is load on the utility grid. SOME plants can supply their own power (sometimes called "house load") when separated from a utility grid--BUT the GCB has to be closed. There is NO load on the generator set if the GCB is open. None. Zilch. Zero. Niente. Nada.

So, load shedding CANNOT occur if the GCB is open. Not no how, not no way. It just can't happen. Period. Full Stop.

For a steam turbine generator set to "survive" a load rejection (utility grid separation) with or without the GCB open the steam turbine control valves have to react extremely quickly to reduce the steam flow-rate to the steam turbine AND prevent the overspeed which is going to occur from exceeding the overspeed trip setpoint. If the utility tie breaker opens the load (all of it if the generator set doesn't also supply some plant load(s)) is gone--there is no more load (unless the generator set also supplies some plant load(s))--and, usually, that's only a small percentage of the generator set's rated power output). When the load is lost--particularly on a steam turbine-generator set--and the steam turbine steam stop valves do not close before the GCB opens the unit speed is going to go above normal rated speed. The question is: Will it exceed the overspeed trip setpoint and cause the steam turbine stop valves to be closed?

SOME--but not many--steam turbines have a means of quickly re-directing steam flow from the steam turbine to the condenser, or in some cases to atmosphere, in order to reduce the amount of speed increase above normal rated speed.

It was written that the unit 100% load rejection capability was successfully tested in 2019. So, it's possible for this unit to survive a "full load" rejection and be quickly re-synchronized again. The question then is: What is the difference between the 100% load rejection test procedure and what is happening now or recently? It's not clear from the original post if this only happened once, or if it is happening more frequently or every time the unit gets separated from the utility (which is also not really clear from any of the original poster's responses or the original post).

If the unit successfully survived a 100% load rejection test approximately one year ago, then what has changed between now and then?

If the unit successfully survived a 100% load rejection test approximately one year ago, then does the load rejection test properly simulate what is happening now?

If the unit successfully survived a 100% load rejection test and the load rejection test properly simulates what is happening now when the unit is separated from the grid, why aren't the steam valves able to limit the steam flow to prevent the unit speed from exceeding the overspeed trip setpoint?

Either the current conditions are not properly simulated during a load rejection test, or something has changed. The steam turbine control valves may not be closing as fast as they have in the past (presuming the load rejection properly simulates what is currently happening when a load rejection is occurring).

BUT something in the current operating conditions does not match the load rejection test conditions. It could be a logic problem--the way the utility separation is occurring is not the same as the way the load rejection occurs during a load rejection test (a different breaker or air switch is opening now than is being opened during the load rejection test).

Still, load shedding cannot occur when there is no load. It just can't happen. No matter how much one wishes it could, it can't. If Engrahmed thinks that closing the steam control valves when the GCB is open is load shedding, well, it's not. Load shedding, by definition, means reducing the electrical load on the generator in an organized manner--which forces the turbine control system to close the steam control valves in an organized manner. BUT, the GCB is closed when load shedding is occurring.

So, methinks we will never see or know how the turbine steam supply is configured and controlled (except by a Rx3i), and we will never know how the current operating conditions and grid separations differ from the load rejection test(s) that have been successfully performed. So, I will continue to read the responses to this thread, but I'm most likely not going to have anything further to contribute without a LOT more information from the original poster (which we're not likely to get). The original poster is looking for a simple answer to what he believes is a simple problem. And neither the answer nor the problem is simple.

It would be really great if we could have an on-going dialogue with the original poster to clear up any misconceptions he or we have about what happens and what is happening. But, that is difficult at best in person--and more difficult when English is not the first language of many of the participants. Trying to do that on a World Wide Web forum is nigh impossible, and almost never happens (in my experience). Most original posters learn something from the information provided, and some write back to say so.

Best of luck!
 
Engrahmed,

It would be a good thing to tell us , the plant configuration :
1-Steam turbine type /load scheme
2-Is there any exctraction or other process steam for any utility??
3-Some parameters like STGload ,ST Control/Stop/Trip Valves trends, Line Voltage, Distance protection, Generator protection/AVR trends?
4-Was the last overspeed /load rejection tests succesfully achieved ?
5-Do you get SLD ?
6- Do you have the Controls blocks ( load rejection/overspeed) checked ?


Also I just found this interesting article called
"Optimized Steam Turbine Governor Controlling Single or Multiple Grid Faults"
( Case studied by Siemens engineers) :
https://www.researchgate.net/publication/267503921_Optimized_Steam_Turbine_Governor_Controlling_Single_or_Multiple_Grid_Faults#pf4


Here some quotes:
Grid Codes do not necessarily assume that power plants
must also be able to control another fault occurring a few
seconds later. Since load rejection to house load can certainly
occur following a short circuit, e.g. during extreme weather
conditions or environmental disasters, it would be of great
advantage to enhance the power plant availability if a multiple
grid fault of this kind could also be controlled. For the multiple
grid fault it is assumed that the power plant described here is
connected to the grid via two transmission lines. During storm
a three-phase short circuit (for example a tree cuts all three phases)
occurs in one of the two transmission lines which is
cleared after 250 ms. Thereafter a two-phase short circuit (for
example two phases contact each other) occurs in the remaining
second transmission line which result in a load rejection to
house load for the power plant. The rejection of electrical load
shortly after a short circuit can give rise to very high
overspeeds however and these will cause tripping by the
dedicated overspeed protection system. An overspeed trip
results in shutdown of the turbine generator and the extended
downtimes associated with it. The main benefit of load
rejection to house load as compared with the shutdown by a
protection system is the immediate re-establishment of
availability, i.e. the power plant could be synchronized with the
grid only a few seconds later.

SINGLE GRID FAULT:
In the event of a single grid fault, a generator must supply
active and reactive power for as long as possible so as to
support the stability of frequency and voltage. If, however, the
generator slips out of synchronism with the grid or if the shaft
train is endangered due to overspeed, for example, the turbine
generator must immediately be disconnected from the grid and
switched to house load operation. The controller monitors the
electrical power on the generator terminals. If specified limits
(load band around zero) are exceeded, fast travel mode is
triggered for the valves and the mechanical load is decreased as
quickly as possible. The valves remain closed for a hold time of
approx. 1.5 s, after which time they are slowly reopened.
It is impossible to distinguish between load rejection to
house load and a grid short circuit on the basis of the active
power of the terminals at the beginning of the event. Unlike
load rejection, electrical active power returns after a grid short
circuit once the fault has been cleared and the valves must then
reopen. The generator then must continue to supply active and
reactive power to the grid, so as to stabilize frequency and
voltage. This means that if the short circuit is active for 250 ms

I finally got the document downloaded!



It is available with access ( after registration) you can try to download it, at least read it it contains precious informations on such event.

It can help you to have a better overview, of what happened and what is possible to do to remediate, on this in the future and fix the issue and get the unit available, for a resynchronization ( reliability , availability improved) during/after such event.

Thats what we can check prior to state, on the event .

I am sure that the article can be an help !

Let us know what you think on it.
Kind regards,
Controlsguy25
Engrahmed,

Could you please try to reply to my last post here?

We tried by spending some time and efforts, to get you a better idea/overview, of what happened on your site operation issue.

But you did not get back, to us by writing some notes on my last post.

Please have a serious read on the post, and tell us what is your opinion.

Thank you for your answwer,
Controlguy25
 
Engrahmed,

Could you please try to reply to my last post here?

We tried by spending some time and efforts, to get you a better idea/overview, of what happened on your site operation issue.

But you did not get back, to us by writing some notes on my last post.

Please have a serious read on the post, and tell us what is your opinion.

Thank you for your answwer,
Controlguy25
Ok Looks like I helped/supported you a lot by providing such infos !

BUT you could at least by "courtoisy" try to reply to my post ??
I make effort for supporting people here and some of them are just stonish §!!!

Controls Guy25
 
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