nuisance tripping 1

[mesh]

Hi!

Can anyone help me find the solution of the nuisance tripping of a 51N electromechanical relay. The relay is on the primary of a 10MVA, delta-wye, 69/13.2 kV, power transformer. CT ratio 100:5; 51N relay tap 1A; phase relays tap 5A. The tripping ocurs only during peak loads(Ip is approx. 50A).

Thanks.

 

[Alex68]

Is this your unique protection on the neutral? Does any other protection trip in the same condition?

It seems that during the peak load, you insert also a big unbalanced load (1ph load)or something which contains an earth fault. Is it possible?
You should control the load list and verify each load or line inserted during the peak load, in order to explain the trip during this load.

Anyway what is the neutral current during any other load?
Is it possible that the peak load makes worse an already critical situation?

 

[busbar]

What kind of tests are done after trip, before reenergization? What are the results?

 

[jghrist]

With a delta transformer winding, you should have no current in the 51N under any unbalanced load condition or any low side fault condition, except for CT errors. Tripping under full load indicates an error in the CT connections. Have the ratios been checked?

 

[mesh]

I suspect bad CT connections, but we haven't done any tests yet.How do we test CT connections and CT ratios? Is it possible for bad CT connections that its effect be felt only during peak loads?
Thanks.

 

[busbar]

Based on information furnished and given the cost of a transformer outage and timely repairs, thorough testing of transformer and supporting gear may be past due.

It’s your system and your maintenance budget, but for persons here otherwise unfamiliar, it may be a bit futile {and expensive} to speculate on a cause without some specific and comprehensive test results—best reviewed on site by a qualified person with an understanding of the relationship of power equipment versus protective-device functions.

I would not take hi-side 51N trips casually. “Close and test” can too easily become “burn and learn.” Nuisance tripping may be something else.

 

[bigamp]

mesh,

If you are able to take the feeder/transformer out of service for a time, you could do primary injection tests to confirm CT connections and ratios.

The exact details on how to do a primary injection test depends on how the CT's are connected but is generally as follows:

Put a temporary 3-phase short circuit on one side of the CT's (generally the "downstream" side). The short circuit needs to be short term rated to carry the current you are going to primary inject. In your case with 100/5 CT's you should look to inject 100A and you may be able to achieve this with a secondary test set. Put ammeters in each CT secondary and also in the CT residual circuit. Primary inject into one phase and return via another phase. If all is well, the ammeters in the CT secondaries corresponding to the phases you are injecting should read the same (the current should be injected current divided by CT ratio) and the ammeter in the CT residual circuit should read zero (or very close to it). Do the test for all three combinations i.e. in A out B, in A out C, in B out C. You will know you have found the problem if you get a high residual current reading. It helps if the ammeters are analogue type. The protective relay should remain in the circuit.

Prior to doing any work, be sure that safe and correct isolations are carried out and at the completion of the work be sure to re-instate the system to normal. Also be sure to use the services of a suitably experienced test engineer.

Any errors in CT connection or ratio become more noticeable at higher currents.

Regards

 

[electrix]

I experienced a similar problem some years ago. I found that the relay itself was defective. A replacement relay solved the problem.

 

[mesh]

Thank you very much for all your inputs.

Mesh

 

[RRaghunath]

The following may help:

Keep the trip latch out and measure the currents in the relay using test pin, this way the CT secondary wiring problem can be detected without taking shutdown / going through primary injection test procedure. However, the protection will not be in circuit for the short period while the relay currents are being measured.

 

[RajT]

You stated that the tripping only occures at full load.
At full load the cable from the breaker to the trans primary may also be at its limits if its rating just meets the full load criteria. If that happens and if the cable is old, then it may be that there may be some high imped arcing within the cable to its armour. This arcing fault (it could be anywhere along the length ofcource) would be seen by the sensitive 51N resulting in a trip.
It is easy to check the condotion of the cable first before embarking on transformer fault finding mission.

 

[vandax]

Beside agree with bigamp comment's, I would suggest to consider:
1.Install disturbance recorder in your system, to know what was/is happen in your system. You will reduce time & cost for investigation.
2. Re-check the relay after it has already tripped, to make sure it has worked well.
3. Check all connection (CT-interconnection-connnection-relay-output) to make sure that all protection system is ok.
4. If there is proven that the relay worked well, check all load one by one, to find which one is the root cause.
rgds

 

[Bung]

I tend to side with Busbar on this one. Blaming the relay for tripping at incovenient times is not the answer. If the transformer really is faulty, a lot of smoke is going to escape and it won't be easy or cheap to replace. Spend some lesser $s on hiring a professional test outfit to track down the problem, and watch how they do it. I learned a lot doing that kind of thing, and I'm still prepared to do it.
Bung
Life is non-linear...