Turbine question 4

[heavyCurrent]

I was in the interview and I was asked this question. "When the generator is tripped but the turbine is still in sync with the system, what would you do?" I said "trip the turbine" and they smiled. What would be a correct answer?

 

[davidbeach]

Probably depends on the plant and the practices at that plant. If you've tripped the generator for something unrelated to the generator/prime mover (it is still safe to operate the unit), you can maintain speed-no load and go for a resynch when conditions allow. At some plants you would have had the right answer.

 

[electricpete]

I'm not sure what the meaning is of turbine in sync with grid even though the generator is tripped.

In the large steam turbine plants that I'm familiar with, the turbine will trip itself upon generator trip in order to prevent overspeed.



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(2B)+(2B)' ?

 

[electricpete]

For plants I mentioned above, if generator trips at low power level, the turbine will not trip.
In that case, if you find yourself in steady operation with tripped generator but not tripped turbine, I can understand you would proceed as per davidbeach.

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(2B)+(2B)' ?

 

[heavyCurrent]

Is it possible that the governor on that turbine is not on the droop mode (perhaps frequency bias mode)?

 

[DRWeig]

I might have answered it with, "I would follow the established procedure, which might entail leaving the turbine to run while investigating the generator trip, or tripping the turbine first. What exactly is the arrangement that you'd like me to analyze?"

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[ScottyUK]

My opinion would be to allow the generator to close back on to the system if it was determined that the reason for it having disconnected in the first place didn't place either the machine or the system in danger.

A second possibility, depending on how the poorly-phrased question is interpreted, is that the machine is still on the bars following a trip command to the generator breaker. Would you then trip the turbine or keep steam to the machine? Personally if the machine refused to disconnect from the system I would try to keep it at minimum forward power to avoid motoring the set and ensure that cooling steam was available for the LP blading. Once a means of disconnecting the set was established - for example disconnecting it by tripping the whole substation off the system - then trip the turbine as well. I have seen this twice, and we transferred the other machines onto the reserve busbar from the main busbar prior to isolating the main busbar further back. Might have been a transfer onto the main bar and isolating the reserve - can't remember in that much detail.

 

[crshears]

I think the question "When the generator is tripped but the turbine is still in sync with the system, what would you do?" is a @#$%^&* trick question, and I HATE trick questions!!!

I've been around the mulberry bush enough times that I would have asked them to clarify their inquiry, and let the chips fall where they may. Anybody that wants to screw around with words like that would not be a person I would be comfortable submitting to the authority and discipline of; at the very least, either the hairs on the back of my neck would stand up or my alarm bells would go off, I'm not sure which. A slippery customer like that suggests to me that you might want to watch your back if you take a job under them; they could either be Teflon-coated, meaning they'd see to it that nothing bad would ever stick to them and as a consequence it would stick to you, or, which might be even worse, they don't know what they're talking about and could drop you deep, deep into the dung-heap.

To say "the generator is tripped" can mean many things, but in my view it cannot mean anything other than that the unit's output breaker or breakers [in the case of a ring bus or breaker-and-a-half scheme] is/are open and the unit is disconnected from any load, network or grid.

And what the *&^%$#@! does "the turbine is still in sync with the system" mean, anyway? Generators and alternators are synchronized with systems; turbines are the prime movers that drive electrical devices, and as such are not 'in sync' with anything in the electrical sense.

...then again, maybe I'm over-reacting, and the question was merely a bullsh*t filter the interviewers were using to see if the interviewee was paying attention and would call them on on it; I've seen that happen, too.

CR

 

[byrdj]

"When the generator is tripped but the turbine is still in sync with the system, what would you do?"

Are you sure that was the phrase? if so, I would reply to them they are not very knowledgable of the operation of a turbine generator and just repeating phrases they have heard and misquoted.

expaination...the system would refer to the electrical grid. the turbine can not sync to the grid, only the generator

I would then aske them if they ment to say "When the turbine is tripped but the generator is still in sync with the system, what would you do?"
This IS a valid question and answering it wrong in actual operation HAS wrecked a few units

 

[burnt2x]

Good point, @byrdj. a star for you.
But grsnting that the question was really about a tripped generator main breaker and the turbine still turning at synch speed, I would still trip the turbine or it will go to a runaway condition after losing its load.

What pete said, most new generators have interlocks with the turbine such that if the generator breaker trips, the generator lockout activates, tripping the turbine at the same time.

 

[byrdj]

If the generator breaker opened while at high load, You have 1 SECOND to respond before the steam turbine accelerates to emergency trip speed.

So If the generator lost sync (tripped), you should notice the turbine running near rated speed (not synch speed) or the speed settling back to rated once the govenor performed its function.

So if the question was "When the generator is tripped but the turbine is running near rated speed, what would you do?" to answer would require knowing what was the desired condition prior and the current expected condition. If it were "On Line". then determine the reason for the generator seperation, If that was knowm and no corrective action required, then re synching could be the answer. If the reason for breaker open was not known and expected correction would be needed, I would not keep a hot turbine running at no load. I would look a log to see when last over speed trip test was performed and if needed, take advantage of the tripped generator and perform OST then return the unit to turning gear while the generator problem is resolved.

Most over speed event (wrecks) start with the operators statement, "When the turbine was tripped, the generator was still in sync with load, so I..." Expaination (for most utility logic in the US), when the turbine is tripped, valves close and stop steam flow so generator load quickly reduces. once the reverse power relay times out, the generator breaker is open. IF one of the steam valves fails to close when the turbine is tripped, the generator will stay synched, producing the power from the steam flow through the failed valve.


unfortunately when this situation occurs, the wrong answer is trip the generator breaker, wrecking the unit. the correct answer depends upon station design. Do you have boiler block valves, if not how can you dump the boiler pressure QUICKLY and during this process keep a close eye on excitation to ensure the generator does NOT trip till steam flow is reduced to reverse power. Remember, overspeed trip is no longer operational since a steam valve is failed open so even with minimun load, the turbine will slowly accelerate and possibly wreck (for really low load breaker opening, the turbine could reack stall before damaging speed)

 

[waross]

Maybe not a trick question so much as a question to determine the applicants understanding of turbine generator systems. The turbine turns the generator and the generator establishes and maintains sync with the system.
When the generator breaker trips the unit generally immediately loses sync. Even at no load, even an error of 1/10 of a Hz will result in dangerous loss of sync in one second or less.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GTstartup]

I think I would have answered. "you cannot trip a generator". You can only trip what's driving it or disconnect it from the load. Then I think they would have to clarify the question. Personally I think they interposed generator and turbine and the question should have been "When the turbine is tripped but the generator is still in sync with the system, what would you do?" which is a legitimate question.

 

[davidbeach]

There's also the issue of what kind of turbine. A high speed steam turbine or a low speed hydro unit?

 

[heavyCurrent]

Here is the exact wording:

"A differential relay trips a generator but the turbine remains at sync speed. What do you do"

 

[GTstartup]

That's completely different. Shut down and investigate.

 

[ScottyUK]

Definitely take it off, a stator diff relay operating is a sign of very bad things. Nothing good will come of keeping it running, and you can't do any meaningful tests at sync idle.

 

[davidbeach]

All that and still at synch speed, you've got one heck of a governor.

 

[waross]

My thought also David.
A differential trip indicates internal issues. There may be internal arcing which may quickly destroy the generator core. Remove the field and stop the turbine as fast as possible.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[powerjunx]

Hi heavyCurrent,

Your first post is quite vague however the second re-phrase is definite. Question: "A differential relay trips a generator but the turbine remains at sync speed". Your answer: "trip the turbine" headshot, you got it right!, see them smiled. Your knowledge of the machine and its behaviour during normal and abnormal conditions has been proven.

Here's common methods of isolating the generator from service during abnormal condition, TRIPPING MODES as describe in IEEE Guide in Gene protection.. as follows:

(A). Simultaneous Trip: provides fastest means of isolating the generator, used for all internal generator faults and severe abnormalities within the protected zone. Isolation is carried out by tripping at the same time of circuit breaker, field breaker and shutdown of prime mover.
>>>>>>> this is your case <<<<<<<<

(B.) Generator Tripping: provides isolation trips of main generator and field breakers. It trips the machine during system disturbance rather then internal faults of generator. In these scheme, the prime mover is not shutdown and is used where prompt remedial action had corrected the abnormal condition the machine can be re-connected on-line.

(C.) Unit Tripping: a variation of generator tripping method is where one main generator circuit breaker is opened. The application is suitable when it is desirable to maintain the unit auxiliary load connected to the generator. Immediate re-connection to the system is achieve with minimal time delay.

(D.) Sequential Tripping: applicable to steam turbo-generator where tripping delay has no detrimental effect on the generating unit. Generally, used to trip the generator for prime mover problems where high speed response tripping is not a requirement. For instance, the turbine has been tripped, tripping of generator followed by field breaker is initiated.






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