Identifying Transformer Core Overheating Problem

I need assistance with identifying the cause of a transformer core overheating problem. I am not an electrical engineer but I have 20
years of experience in transformer repair.
The unit in question is a 2500/3333 KVA GTE/Sylvania dry type transformer. The voltage is 13800 Delta - 4160Y/2400. The temperature rise is 80C at 2500 KVA and 150C at 3333 KVA. The age of the unit is approximately 20 years old.
My customer sent two of these units (identical) to me for rewinding. According to the customer, there was nothing wrong with the units but because they were in an area where consistent, reliable power was necessary, they wanted to have them rewound to extend the life.
We rewound both the primary and secondary windings and sent the first unit back to the customer. After having the unit energized for 24 hours with no load, they noticed a haze in the air. They checked the temperature gauge which was reading about 140C. Using a thermal gun, they read the core temperature at around 175C.
We assumed there was a core ground. After the unit had cooled, we asked the customer to remove the copper strap that grounds the core to the core frame and take a continuity reading. The reading showed that the core and core frame were grounded. They discovered that the bolts which hold the top core frame together had shifted during shipment and were touching the end of the core laminations. At that point I thought we had found the problem. They loosened the bolts and moved the core frame over enough to break the connection between the bolts and the laminations. Re-energizing the transformer resulted in the same overheating problem.
We were in the process of reassembling the second unit and we took a continuity reading. It also showed grounded. We disassembled the transformer down to the core legs and the bottom core frame. We were still getting a grounded reading. We did not disassemble the bottom core frame and core legs.
After the heating problem was discovered and the second unit was disassembled, we began to look for clues. One thing we discovered was "bluing" of the core laminations at the bottom and the top, near the center core leg. The unit had apparently been very hot before it was sent to us. The customer said the max temperature prior to them sending the unit to us was 120C. He said his max load was 170 amps. The full load current of the transformer is 463 amps. That's a load of only 37%. At 120C, the transformer was overheating at 37% load. The FULL LOAD temperature rating of the unit is 80C plus the ambient temperature.
Despite all of this, I am still not sure that the grounding of the core to the core frame is the problem. This is for a utility power plant that is supposed to start back up tomorrow (March 28). Now both transformers are down and we don't know where to go from here. Any help you can provide would be greatly appreciated. I also have additional information (and photos) about the transformers that cannot be posted here. Thank you.
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<snip long discussion about problem>
Despite all of this, I am still not sure that the grounding of the core to the core frame is the problem. This is for a utility power plant that is supposed to start back up tomorrow (March 28). Now both transformers are down and we don't know where to go from here. Any help you can provide would be greatly appreciated. I also have additional information (and photos) about the transformers that cannot be posted here. Thank you.
Whew, cutting things close on this outage ;-)
I would agree that it seems the units were overheating before you got them ('blueing' of the iron is a good clue). As to *why*, well that's a bit harder. You've already looked at core grounding. With the secondary wye connected, there isn't much chance of an incorrect connection (now, if it had been delta.....)
Core heating can be caused by lots of harmonics, but as you mentioned a power plant, this thing probably is used to power a lot of pumps/motors and harmonics their wouldn't seem likely.
The only other thing I can think of is saturation from applying the wrong voltage. But if their 13.8kv delta connected, I really don't see how that could be (unless they were meant to be 13.8kv wye, but I doubt they could have lasted this long like that).
Frankly, that's all that comes to mind. I've worked/rewound a lot of motors and DC stuff, but not that many transformers.
One last thing, is this a common core for all three phases? If so, would a large phase current imbalance be something to look at?
daestrom
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probably it's too stupid to suggest, but did you look at the open circuit inductance of these things? If there are shorted turns that will find them.
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Tony wrote:

I think we have isolated and identified the problem. It seems there was a ground between the core frame and core at the bottom of the transformer. This caused the core to become very hot at the ground point which caused the insulating material on the laminations to burn off in that area. Then the situation was several individual laminations acting as a solid piece of steel.
Now the efficiency of the core has been compromised. To compensate for this, it looks like we will be rewinding the coils again with more turns on the primary and secondary to reduce our flux density to a more desirable level.
Tony, one of the most frustrating things about this is that the transformer passed every electrical test with flying colors. TTR, insulation resistance, phase displacement, excitation, etc. If the turns had been shorted, we would have found it in either the TTR values or the full voltage test.
Thanks to everyone for the great suggestions and insight.
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Has anybody suggested shuffling the core iron to distribute the bad pieces and eliminate the shorts? Be sure all the burrs are facing the same way, i.e., don't turn any laminations around.
--
Fred Lotte
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----------------------------

---------------------------------- You still will have a compromised core and excessive eddy currents. Your problem is the 1 turn coil due to the shorted laminations. More turns may aggravate this by making it N+:1 rather than N:1 I would suggest that the best fix would to take the core apart - smooth out burrs etc and revarnish the laminations. This is a lot of work but may be not much worse than rewinding and would get at the root of the problem.
--

Don Kelly @shawcross.ca
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I have to agree with Don on this. If you have shorted laminations, you have higher eddy-current losses. I would doubt the unit will last another 20 years like this. Remember, the temperature you're seeing on the unit is on the outside surface. There could be higher temperatures inside the core and if the heat causes more varnish to fail, well it will only get worse.
What you're doing may be acceptable for a short term fix to get the unit back on line, but what's the cost of doing it right, versus having it fail in the middle of a hot summer season when those MW's of unit output are worth a lot more than now (springtime).
At least consider 'de-rating' the thing to avoid some future work putting a lot more load on the unit.
Of course, you're a vendor/contractor for the customer and you have to weigh that against everything. If it is the customer's recommendation, they assume the liability. If it's yours, well how good is your insurance ;-)
daestrom
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and
I think your on to something here. Since the old transformer showed the same signs. I would do a quick harmonic check of the building. Old 6 steppers, VFD's were notorious for harmonic's back into the system.
I would also ohm the transformer ground. It would not be the first primary transformer in an old faciltiy that was not properly grounded or floating. At least not for me.
Let us know if you find the culprit

motors
a
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