Generator Breaker Cannot Open

P

Thread Starter

POOM

I would like to know as below

1. gas turbine Ge flame 6 tripped, excitation not trip. What is effect to turbine and generator?

2. gas turbine trip excitation tripped. What is effect to turbine and generator?
 
POOM,

Hmmm....

The Subject of this thread is: "Generator Breaker Cannot Open" and the Summary refers to the generator breaker--which is presumed to be the three-phase, high-voltage circuit breaker that connects the generator output to a grid/load. And yet your two questions below refer to the excitation system.... Hmmm.... Are you really referring the generator excitation breaker?

Here are the answers to your two questions:

>1. gas turbine Ge flame 6 tripped, excitation not trip.
>What is effect to turbine and generator?

IF the generator breaker (as described above) is still closed and the excitation breaker is open ("trips") there is no effect to the turbine--except that the speed will likely increase somewhat. The loss of excitation when the generator breaker is closed changes the generator from a synchronous generator to an asynchronous, or induction, generator--which it was <B>NOT</b> designed to be. This causes rapid and excessive heating of the generator rotor which can cause damage to the rotor windings and rotor retaining rings-which can cause the retaining rings to expand sufficiently to come in contact with the stator, causing catastrophic mechanical damage to the machine. The rotor windings and laminations will also be physically damaged because of the heating. It's NOT a very good thing to happen--not at all.

There is supposed to be a working loss of excitation relay to prevent this condition from occurring.... Its device number is usually 40. And it should open the generator breaker--and depending on the site, it may also trip the turbine, but sometimes it doesn't.

A generator can probably withstand a few seconds of asynchronous operation without a field--but not much more than that. The heating due to loss of excitation is fast and destructive. The turbine is not really affected--other than the speed change because of loss of synchronism with the grid.

>2. gas turbine trip excitation tripped. What is effect to
>turbine and generator?

I don't really understand this question. If the turbine is tripped and the excitation is tripped, that's good. If the turbine is tripped and the excitation is NOT tripped, well that's not really good for the generator. As the speed decreases, if the excitation system remains in "AUTO" mode, it will be steadily increasing the excitation to try to maintain generator terminal voltage. This causes excessive current to flow through the generator windings, and because the voltage is high with respect to the speed (frequency) the stator windings can also be damaged (physically) (due to a phenomenon known as "excessive volts/Hz (volts-per-Hertz), which can occur at low speeds as well as high speeds, or high voltages at rated speed). And there is generally protective sensing/relays to protect against excessive V/Hz by tripping the turbine and excitation simultaneously.

If a turbine is tripped (by cutting off the flow of fuel), what happens is that the generator breaker will trip on reverse power (this is the normal scenario)--and the excitation will usually be shut off ("tripped") at some point after the generator breaker opens (usually a function of turbine/generator speed).

If the generator breaker does not open on reverse power, then the unit will remain at synchronous speed (rated speed) because the generator actually becomes a motor and spins the turbine at rated speed. This requires a LOT of power from the grid and is not really bad for the turbine, but is bad for the generator (high current flows--"in" from the grid). And, as long as the excitation remains on the effects on the generator will be "minimal."

If a turbine is tripped and the excitation is also tripped--but the generator breaker does not trip (open), then, again, the synchronous generator will become an asynchronous motor (an induction motor) and all the bad things described above will happen. And just as quickly. The speed will be slightly less than rated, but the newly-created induction motor will spin the turbine at slightly less than rated speed. (And, this probably isn't really good for the reduction gear, either--because the forces will be in the opposite direction of normal.)

I hope I've covered all the different possible scenarios. It's important for the excitation to remain on whenever the generator breaker is closed--whether the turbine is providing torque to the generator, or the grid is motoring the generator--to prevent fast and destructive heating of the generator rotor. And, the only real "effect" of all of this on the turbine is a change in speed (if excitation is lost before the generator breaker is opened ("tripped").

Hope this helps!
 
Dear CSA,
Thank you so much.(sorry for my English language)

Yesterday my gas turbine flame 6 tripped by high vibration, but gen breaker not open. From event I don't sure may effect to gas turbine or generator damage or not, and I want to know for the future:

1. GAS TURBINE TRIP, excitation off BUT Gen. breaker not open. What will happen to turbine & generator?

2.GAS TURBINE TRIP, excitation on BUT Gen. breaker not open What will happen to turbine & generator?

3. GAS TURBINE running,excitation off. What will happen to turbine & generator?

Note. Your reply I will share with my team; we are waiting you.

Hope you reply again,
poom
 
POOM,

> 1. GAS TURBINE TRIP, excitation off BUT Gen. breaker not
> open. What will happen to turbine & generator?

The synchronous generator becomes an induction motor--NOT GOOD for the generator. No damage to the turbine.

> 2.GAS TURBINE TRIP, excitation on BUT Gen. breaker not open
> What will happen to turbine & generator?

Synchronous generator becomes a synchronous motor. Not bad for the generator; no damage to the turbine.

> 3. GAS TURBINE running,excitation off. What will happen to
> turbine & generator?

Synchronous motor becomes induction generator--BAD for generator. No damage to turbine.
 
POOM,

There is an error in my previous post in the following sentence in the response to Question 3:

> Synchronous motor becomes induction generator--BAD for
> generator. No damage to turbine.

It should read:

Synchronous <b>generator</b> becomes induction generator--BAD for
generator. No damage to turbine.

<b>Any</b> time the generator breaker is closed and excitation is not ON, the synchronous generator becomes an induction machine--and synchronous generators are NOT designed to be induction machines (generator or motor) at any time--ever; never. As was explained in my first response, very destructive heating occurs pretty fast when excitation is lost on a synchronous machine when AC current is flowing in the stator windings (whether the machine be operating as a generator or a motor). There is a loss of excitation relay which should detect low excitation and also send a signal to trip (open) the generator breaker. It's a very, Very, VERY, <b>VERY</b> bad way to operate a synchronous machine for even a very short period of time.

The turbine isn't damaged because it continues to rotate in the same direction whether or not fuel is flowing into the combustors and burning. So, if the turbine trips (no fuel flowing; no flame in the combustors) and the generator breaker doesn't open (and hopefully excitation remains on) the synchronous generator will become a synchronous motor using current from the grid (a LOT of it) to spin the turbine at rated speed. Air will still be flowing into and through the turbine in the same directions. However, as I write this I would estimate that if the turbine tripped from load and the generator breaker did not open and the unit continued to turn at rated speed with the IGVs closed, that's not too good for the IGVs or the axial compressor. And the cool air flowing through the turbine section at rated speed probably isn't too good--because of the high temperature differential for a sustained period. So, it's probably not great for the turbine and axial compressor (including IGVs), but it's probably not too bad, either--depending on how long the unit was being turned at rated speed with no fuel flowing.

If the generator breaker at your site failed to open with the gas turbine was tripped because of high vibration, the reason was that the reverse power functions--there are usually two (2) of them failed to open the generator breaker. Mark V and Mark VI and Mark VIe turbine control systems almost always have a reverse power detection function to open the generator breaker when amperes are detected flowing into the generator from the grid, and motorizing the generator (turning it into a motor which is turning the turbine). This reverse power detection function requires the load transducer (the MW transducer) to be working properly.

The second reverse power detection/tripping method involves a physical relay, ANSI device number 32, which senses reverse power (amperes flowing into the generator instead of out of the generator) and trips the generator breaker. So, there is usually redundancy in the two functions--the one in the Mark* control system and the external protective relay (which, in some cases, is part of a digital generator protective relay which may also perform many other generator protective functions).

So, you need to confirm the two functions will actually send a trip signal to the generator breaker, because if the turbine tripped and the generator breaker didn't open one possible (but unlikely) cause is both of the reverse power relays failed to operate.

Another possible cause is that there was some problem with the voltage/current in the generator breaker trip circuit--either the fuses were blown, of the trip coil failed, or there was some mechanical problem with the mechanical breaker opening mechanism.

In any case, you should be sure that the reverse power functions both do, in fact, operate properly and send a signal to the generator breaker trip circuit. And, then make sure the generator breaker will, in fact, open when either of the two signals from the reverse power functions initiate a breaker opening.

Hope this helps!
 
CSA...
you keep alluding to the LOE event causing the Alternator to operate as an induction-motor.

That is not the case. It becomes an induction-generator, and it can.remain as such for several minutes!

Regards,
Phil
 
Hi, Phil Corso,

From POOM's second post to this thread:

> 1. GAS TURBINE TRIP, excitation off BUT Gen. breaker not
> open. What will happen to turbine & generator?

Phil, what would you call a synchronous generator operating without excitation and no prime mover while still connected to the grid, drawing near nameplate current?

Remember--the axial compressor of a heavy duty gas turbine requires about 2 of every 3 horsepower produced by the turbine, and when the turbine isn't running (no fuel is being combusted) and the generator breaker is closed the motorized generator will draw nameplate current, or even more, to drive the axial compressor to keep the machine spinning at or near rated speed (depending on whether excitation is on or not, relatively). That axial compressor is a HUGE load on the turbine--or the generator when the turbine isn't burning fuel. Or, the induction motor. And that's a LOT of stator current causing induction heating on the rotor that was never intended to be an induction rotor.

And, I've seen pictures of the effects of a synchronous generator operating as an induction motor for about four minutes on a GE-design Frame 5 synchronous generator--the retaining rings had expanded so much they had scraped the stator windings and the generator vibration was more than 1.8 inches/sec, after only four minutes. Sure, it'll run for a few minutes--but it will never be a synchronous generator again until the rotor is rewound and the retaining rings are replaced. (If I recall correctly, the rotor of the machine in the pictures couldn't be repaired, and a new rotor had to be obtained. And, again--that was after only four minutes of operation.)
 
Hi CSA...

First, I specifically addressed LoE, not LoI (Loss-of-Input).

Secondly your "synch-gen'r operating w/o excitation... w/o prime mover input... while still connected to the grid..." characterizes an induction-motor, not an induction-generator.

Regards,
Phil
 
Phil Corso,

Very good! You're getting the hang of this.

When an electric machine is not being driven by a prime mover and it's connected to a grid--it's a motor. And, when it's being driven by a prime mover and the torque being provided by the prime mover exceeds that required to maintain synchronous speed the electric machine is a generator.

If the electric machine is a synchronous machine and the excitation is present and sufficient and the prime mover is providing torque in excess of that required to maintain synchronous speed the synchronous electric machine is a synchronous generator. And all is good with the synchronous generator.

If the electric machine is a synchronous machine and the excitation is NOT present and the prime mover is providing torque in excess of that required to maintain synchronous speed the synchronous electric machine is an induction generator. And the electric machine's rotor ain't gonna last very long as heating begins pretty quickly.

If the electric machine is a synchronous machine and the excitation is NOT present and the prime mover is NOT providing torque in excess of that require to maintain synchronous speed the synchronous electric machine is an induction motor. And the electric machine's rotor ain't gonna last very long as heating begins pretty quickly.
 
CSA... In all years we have had discussions on this forum I have never, repeat never, impugned, scorned, or ridiculed your knowledge of the 'GT' part of GTGs.

My discussions, based on "light-years" (to me) of education and experience, were only intended to enlighten those interested in improving their electrical knowledge of the 'G' part of GTGs.

Finally, to you especially, and to all forum participants and staff, Have a good life.

Sincerely,
Phil Corso
 
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