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Generator Differential Trip Mystery

Generator Differential Trip Mystery

Generator Differential Trip Mystery

(OP)
I was recently called out to a hydro site that had an 8 MW, 6900V unit trip on A,B,C differential (Basler BE1-87G).  

Within the neutral and phase CT's is a tap to a Static Exciter Transformer.  Outside of the CT's, but still on the generator side of the Generator breaker is a surge pack, arresters, and PT's.  On the buss side of the Generator breaker is a breaker for the Station service transformer(50/51 protection), a breaker for the tie to the grid through a 6900-11500v step up xfmr, and also a feeder breaker feeding a small camp load and aprox 5 miles of line to a headgate(50/51 protection).

All loads were being fed from the grid while generator was offline.  

Assuming a generator problem since it tripped on differential, I tested the generator, CT's, and CT loops back to the relay.  All was good.  

Attempts were made to sync on to the buss, however it tripped instantly on 87, all three phases.  It was then decided to try shedding all the load and disconnecting from the grid and energizing just a dead buss.  This worked, and we were able to energize station service, and sync to grid.  However when the feed to the small camp load and 5 miles of line was energized, the unit tripped on 87 all three phases.  

I guess my question is how could a fault on this line trip all the way back to the differential protection of the generator?  Also why were no problems apparent when this feeder was being energized from the grid.

Thanks for any ideas.

RE: Generator Differential Trip Mystery

Hi.
1. Are somebody provided stability test to 87 (diff)protection on commissioning time?
2. Are this generator first time connected to grid?
3. If isn't first time, maybe is after some maintanace?
4. You need test also this relay with secondary injection.
Not so much options for this type of trip :
1. Reversed connection of star point of one of CT's set.
2. Relay fault.
3. one of CT's set with secondary circuit shorted.
Regards.
Slava

RE: Generator Differential Trip Mystery

(OP)
No this unit was online when it tripped, has been in service for years.

I tested the  Diff Relay and it tested fine, none of the CT's are shorted, loop checks were performed on all CT circuits, and the CT's were ratioed and sauration tests performed.

RE: Generator Differential Trip Mystery

Yeh, it start as mystery.
Try check this relay directly from CT's.
Something starnge or  I'm missing something.
Are You check loop also with megger?
Regards.
Slava

RE: Generator Differential Trip Mystery

About line trip.
If CT's not choiced right, possible unwanted trip by outzone fault. What is CT size?
Regards.
Slava

RE: Generator Differential Trip Mystery

(OP)
Ct's on the differential zone are 800:5, relay set at 0.4.

CT's on the feeder are 200:5, relay set at 1.5 amps sec trip level.

RE: Generator Differential Trip Mystery

Is it a numerical relay or not?
I am looking for internal disturbance recorder information. Do you have any?

RE: Generator Differential Trip Mystery

Hi.
It's IEEE type or IEC type? You wrote A,B,C and PT, I think you at the ANSI land. What is CT: C100, C200, C800?
What is a distance between CT and protection panel, what is a wiring size?
Setting is 8%, is OK.
Regards.
Slava

RE: Generator Differential Trip Mystery



Quote:

I guess my question is how could a fault on this line trip all the way back to the differential protection of the generator?  Also why were no problems apparent when this feeder was being energized from the grid.

Seems to me that it's not that there is a fault on this remote line but when it is connected there is enough current to exceed the relay setting with one set of CTs not seeing any current.  You could apply slightly less load  (maybe station service) and measure all CT currents and phase angles.  If they are all equal with correct phase angle then I would say, since you already tested the relay,  you might have a generator internal fault


RE: Generator Differential Trip Mystery

But my guess is that if you do measure the CT currents you will find some wiring or CT problem.

RE: Generator Differential Trip Mystery

(OP)
This relay does not have event recording capability, the Diff CT's are C100, the distance between the CT's and protection panel is ~10 meters, and the CT wire size is 10AWG.

RE: Generator Differential Trip Mystery

(OP)
Hey GT

The unit is now running with rated load on it.  It was just when this line was in the circuit that the trip occured.

RE: Generator Differential Trip Mystery

Hi Dino
Well in that case, you're right it should not have tripped when you connected the remote load.  Still,  if you attempt to do it again maybe you could connect the load in increments while measuring CT currents

RE: Generator Differential Trip Mystery

BTW... sucks to work on Sunday doesn't it?

RE: Generator Differential Trip Mystery

(OP)
Well I was able to fly out yesterday when we got it back on line, but off again tomorrow morning!

RE: Generator Differential Trip Mystery

Quote:

Attempts were made to sync on to the buss, however it tripped instantly on 87, all three phases.  It was then decided to try shedding all the load and disconnecting from the grid and energizing just a dead buss.  This worked, and we were able to energize station service, and sync to grid.  However when the feed to the small camp load and 5 miles of line was energized, the unit tripped on 87 all three phases.
When the unit tripped, was the camp load still energized from the grid?  If so, what was the load?  If not, can it be energized from the grid with the unit off line?

Can you determine whether there was a fault on the camp line or what the load current was?

Maybe there was fault current or inrush that was sufficient to cause a trip because of CT wiring errors.

RE: Generator Differential Trip Mystery

(OP)
Yes when the unit tripped, the camp feed stayed energized via the grid.  The load on the feeder was less than 50 kW.  There is overcurrent protection on this feeder, but nothing operated.

RE: Generator Differential Trip Mystery

C100 is about 25VA 10P20 ( sorry, I must translate it for myself),you must check Basler's requerements.
It seems as CT errors.
Regards.
Slava

RE: Generator Differential Trip Mystery

Sorry, I back again after speedy read of Basler manual.
with min setting 0.4A you must check CT performance.

RE: Generator Differential Trip Mystery

(OP)
Hi Slava

Thanks for the input, I did test the ratio and saturation curves on the CT's, with no apparent problems.

RE: Generator Differential Trip Mystery

Hi Dino.
You check according to design data, but possible that design was not correct, this is a problem.
It's not first time.
Regards
Slava

RE: Generator Differential Trip Mystery

dinocu--

If the saturation curves are good, then the next thing to look at is the measured circuit burden.  It's possible that you have problems in the circuit that put the CT's into saturation.

old field guy

RE: Generator Differential Trip Mystery

(OP)
Hey old field guy,

When testing the ratio of the CT's using the voltage method via a ratio meter, I had removed the leads right off the relay case and tested the whole secondary circuit back to X1-X2 of the CT's.  I asumed any increased burden in the circuit should have shown up on my ratio.

RE: Generator Differential Trip Mystery

From all the replies and your rebuts, it is clear that most of  the things have been done. Ratio checks, magnetisation curves for the line and neutral CTs. Since you are are able to put a minimum load and only trips when you put the camp load. It looks to me that you could have a loose connection which is only kicking in at high currents. Try to do primary injections again but increase the current to something appreciable and tighten all the CT terminations.
Aubrey.

RE: Generator Differential Trip Mystery

Other possibilities:-
1. Trips coincident with energising a step up transformer on site?
2. Saturation of CT's due to transient DC offset? High X/R of connected circuit?
3. Problem with static exciter transformer within protected zone?

Regards
Marmite

RE: Generator Differential Trip Mystery

Is there anything out on the camp line that may be generating RF? Something like a failing insulator or transformer, or proximity to a radio transmitter. Possibly a very high harmonic content.
On a high impedance grounded system, high frequencies may shunt to ground through the internal capacitance of the generator and so will only be seen by the line side differential CTs.

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

RE: Generator Differential Trip Mystery

(OP)
Hey Waross,

It is quite possible that may be the case.  THere are transformers, reclosers, arrestors etc. on this line and there are radio communications in the area too.  This seems like the best answer I have gotten so far, as I do not think it is an issue with the CT's or wiring.  As the unit is running at full load now, the client does not want to investigate any further at this time, however I will pass the info on to them.

Thank you

RE: Generator Differential Trip Mystery

Hi. I see we back to this issue.
Bill posted good idea. Don't see any problem with reason of unwanted trip.
But , why diff protection operated. Isn't important reason, diff protection must be insensetive to high harmonic and faults  in out of zone.
So, problem or with CT ( see Marmite poste) or with protecton relay, must check with Basler. No fault of relay, this type of relay in this type of application
Yes Dino, you check all, is OK, I again back to my point, problem is design!!!!!! If system work OK with nominal load isn't say always OK with diff protection
Regards.
Slava

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