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 80=B0C at 2500 KVA and 150=B0C 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 140=B0C. Using a thermal gun, they read the core temperature at around 175=B0C.
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 120=B0C. 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 120=B0C, the transformer was overheating at 37% load. The FULL LOAD temperature rating of the unit is 80=B0C 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.