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.
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
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
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
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
If CT's not choiced right, possible unwanted trip by outzone fault. What is CT size?
Regards.
Slava
RE: Generator Differential Trip Mystery
CT's on the feeder are 200:5, relay set at 1.5 amps sec trip level.
RE: Generator Differential Trip Mystery
I am looking for internal disturbance recorder information. Do you have any?
RE: Generator Differential Trip Mystery
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
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
RE: Generator Differential Trip Mystery
RE: Generator Differential Trip Mystery
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
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
RE: Generator Differential Trip Mystery
RE: Generator Differential Trip Mystery
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
RE: Generator Differential Trip Mystery
It seems as CT errors.
Regards.
Slava
RE: Generator Differential Trip Mystery
with min setting 0.4A you must check CT performance.
RE: Generator Differential Trip Mystery
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
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
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
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
Aubrey.
RE: Generator Differential Trip Mystery
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
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
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"Why not the best?"
Jimmy Carter
RE: Generator Differential Trip Mystery
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
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
RE: Generator Differential Trip Mystery
I dont know if Dino see this topic, but I hope its intresting data for you. As usually, no mistery at the diff protection.
Please see attached.
Best Regards.
Slava