Transformer Differential Trips
Transformer Differential Trips
(OP)
Looking forward to input!
In summer 2008 we commissioned a 20/27/33 MVA, 115-25 kV, Delta-Wye substation transformer. Subsequently we have had 3 differential trip events on the unit and have yet to determine the cause. Except for thes ethree events the transformer has otherwise been in service, we have had faults on circuits with no differential issues.
First event - Fall of 2009
Don't remember the type of trip. Transformer mfg came in and found several of the stud connections on the feed through bushings very loose. Loose such that some of the ring terminals were not even hand tightened! Some burning on one feed through and it was replaced.
Tests
DGA for the oil
TTR for the transformer
Ratio for the CT's
Review of the diff settings
All tests were acceptable and the unit was returned to service with the thought that CT connections were the problem.
Second event - Fall of 2010
Restrained Diff trip on B phase and lockout of high side circuit switcher. Again mfg was called in. Found all CT connections tight on feed throughs but one exhibited severe burning and was replaced.
Tests Again
DGA for oil
TTR for the transformer
Ratio for the CT's
Review of diff settings
Review wiring for grounds/proper connections, etc.
Diff relay sent to mfg for complete testing....found ok
Diff relay mfg reviewed settings and found no issue
Third event - Fall of 2012
Another restrained trip on phase B and lockout. Diff trip and lockout of the high side circuit switcher for fault on distribution circuit. Owner opened all distribution breakers, reset lockout relay, and closed circuit switcher thus re-energizing transformer! YES a definite NO-NO! Then closed one un-faulted circuit and diff tripped and locked out station. This trip is assumed to be from inrush on a circuit that had approx. 50 to 75 amps of steady state load. Mfg called in again. No CT or feed through issues found.
Test again
DGA for oil
TTR for transformer
Ratio for CT's
Review wiring and settings for diff relay and again checked for inadvertent grounds
Diff relay mfg had 2 engineers review settings....no issues found
Excitation test of CT's this time....ok so far
Awaiting results of oil tests.
We have approx 5,000 amps of available fault duty at the transformer low side.
Differential relay is a Basler BE1-87T with no memory so event history is not available.
High side Ct's are 600/5 MRBCT set on 200 amp tap
Low side Ct's are 1200/5 MRBCT det on 1000 amp tap
Any Ideas....additional tests? I know there are more tests we could do for the health of the unit, but given the issues we have found with the Ct's over time, we have been concentrating on that. Short of shutting down the station in the months of Sept and Oct each year, don't know what else to do. If it is a false trip, it is the first one I have ever experienced.
Would love to get some input!
Thanks
In summer 2008 we commissioned a 20/27/33 MVA, 115-25 kV, Delta-Wye substation transformer. Subsequently we have had 3 differential trip events on the unit and have yet to determine the cause. Except for thes ethree events the transformer has otherwise been in service, we have had faults on circuits with no differential issues.
First event - Fall of 2009
Don't remember the type of trip. Transformer mfg came in and found several of the stud connections on the feed through bushings very loose. Loose such that some of the ring terminals were not even hand tightened! Some burning on one feed through and it was replaced.
Tests
DGA for the oil
TTR for the transformer
Ratio for the CT's
Review of the diff settings
All tests were acceptable and the unit was returned to service with the thought that CT connections were the problem.
Second event - Fall of 2010
Restrained Diff trip on B phase and lockout of high side circuit switcher. Again mfg was called in. Found all CT connections tight on feed throughs but one exhibited severe burning and was replaced.
Tests Again
DGA for oil
TTR for the transformer
Ratio for the CT's
Review of diff settings
Review wiring for grounds/proper connections, etc.
Diff relay sent to mfg for complete testing....found ok
Diff relay mfg reviewed settings and found no issue
Third event - Fall of 2012
Another restrained trip on phase B and lockout. Diff trip and lockout of the high side circuit switcher for fault on distribution circuit. Owner opened all distribution breakers, reset lockout relay, and closed circuit switcher thus re-energizing transformer! YES a definite NO-NO! Then closed one un-faulted circuit and diff tripped and locked out station. This trip is assumed to be from inrush on a circuit that had approx. 50 to 75 amps of steady state load. Mfg called in again. No CT or feed through issues found.
Test again
DGA for oil
TTR for transformer
Ratio for CT's
Review wiring and settings for diff relay and again checked for inadvertent grounds
Diff relay mfg had 2 engineers review settings....no issues found
Excitation test of CT's this time....ok so far
Awaiting results of oil tests.
We have approx 5,000 amps of available fault duty at the transformer low side.
Differential relay is a Basler BE1-87T with no memory so event history is not available.
High side Ct's are 600/5 MRBCT set on 200 amp tap
Low side Ct's are 1200/5 MRBCT det on 1000 amp tap
Any Ideas....additional tests? I know there are more tests we could do for the health of the unit, but given the issues we have found with the Ct's over time, we have been concentrating on that. Short of shutting down the station in the months of Sept and Oct each year, don't know what else to do. If it is a false trip, it is the first one I have ever experienced.
Would love to get some input!
Thanks
Alan






RE: Transformer Differential Trips
Regarding your latest trip, if it was inrush, there is a settings problem. I'm not familiar with the Basler unit but generally most are provided with harmonic blocking to restrain the differential elements on inrush.
RE: Transformer Differential Trips
Differential on picking up a feeder circuit can mean one of two things.
The preferred possibility is that the relay is wired wrong and it tripped on a through "fault" where the fault was the cold load inrush.
The non-preferred possibility is that something is loose in the transformer and moves on inrush and you did actually have a differential event.
But in either case, you need a new relay.
RE: Transformer Differential Trips
Please see the SEL paper "Commissioning of Protective Relay Systems" on the instructions and calculations required for the test.
Paper
RE: Transformer Differential Trips
RE: Transformer Differential Trips
Not that I would blame Basler for an errant relay. We also had a problem with an SEL relay, getting stuck with a large value of current on one input of one phase. The event report showed nothing other than the large value of current on one input. The relay was sent back and SEL never could make the problem happen again.
Conclusion: Electronic devices can have hidden damage, and failures that can't be located. Replace them and move on.
Also reconigise some transformers may not produce enough 2nd harmonic noise to pickup the relay harmonic blocking.
Or more recently I found CT's with a low C curve set at half winding to feed EM differential relays.
Another conclusion: Sometimes the problem may not be the relay.
RE: Transformer Differential Trips
vandal06.....giving your post some serious thought....Thanks Just don't understand why we can go 2 years with numerous system faults and not see a diff trip.
Alan
RE: Transformer Differential Trips
Alan
RE: Transformer Differential Trips
If the max load has been 5 MVA on a 33 MVA unit, you might still be below the pickup for the restrained element at this load.
I'd still be suspicious of wiring errors until it's proven correct. There are so many ways to mis-wire a differential relay. I've tried them all, and verified that only one connection is correct.
But arcing on the CT leads seems like a bad thing.....
RE: Transformer Differential Trips
RE: Transformer Differential Trips
RE: Transformer Differential Trips
pwrtran....cabinet heaters are working and all is clean and dry. Most recent trip was on a beautiful dry Sunday morning here in the southeast US.
norcalnewb....We have checked the wiring and grounds 2 or 3 times.
Aside from open or shorted turns in a ct, which should show up on ratio and/or excitation test, does anyone know another test to check a ct? I am concerned about damage from the original loose connections and subsequent arcing. I know we have some instrument transformer experts here. Ct's are all C400.
Thanks to all of you!
Alan
RE: Transformer Differential Trips
RE: Transformer Differential Trips
RE: Transformer Differential Trips
We did use it prior to commissioning and have checked since. Have not input measured values though. Plan for now is to replace the relay and do another complete wiring check and testing.
Thanks
Alan
RE: Transformer Differential Trips
One of my clients had an on-going problem with a transformer tripping at seemingly random times. The transformer, its CT's, its relays, all parts were checked by TWO separate testing companies. Nothing was found. Finally, the manufacturer was called in. They replicated the same tests, then after finding nothing, drained the oil, opened up the top, looked in, found nothing. Pulled a radiator and sent a flexible borescope into the lower regions of the transformer and found the culprit: A core ground wire had come loose and in certain conditions of oil flow (forced oil cooling) it would whip around and touch one of the windings. The transformer would trip off, the pumps would shut down, and the wire would fall back out of contact, so nothing showed up on the test.
Sometimes when you hear hoofbeats in the street and look out the window, it really IS zebras.
old field guy
RE: Transformer Differential Trips
Thanks!!!
Alan
RE: Transformer Differential Trips
RE: Transformer Differential Trips
RE: Transformer Differential Trips
Thanks
Alan