Generator Differential Trip Mystery

[dinocu]

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.

 

[slavag]

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

 

[oldfieldguy]

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

 

[dinocu]

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.

 

[achilufya]

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.

 

[Marmite]

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

 

[waross]

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

 

[dinocu]

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

 

[slavag]

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

 

[slavag]

Hello.
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