Dry type transformer core overheating (300F)

[pwrengrds]

There are two idential 550kVA transformers in an industrial plant. Durning a infared scan one core was recorded at 300F. They are 4160 delta/460 delta 220C insulation, 150C rise, dry type transformers. One is at 95% tap, the other at 100% tap. Tests on the transformer (power factor, excitation current, megger, turns ratio) showed simlar, normal results. A visual inspection did not show any burning of the insulation. The core ground was lifted and showed both cores grounded.

My explaination of the high temperature is circulating currents through the core because of the ground, shorting of the core iron causing excessive eddy currents, or heavy loading of the transfomer.

Currently the plant is going to mointor the temperature and see what it does. My questions are

What is the expected temperature of a dry transformer core? How hot is excessive? What is the best course of action. What would cause the excessive rise in temperature? I would appreciate any input.

 

[electricpete]

I don't know the answers. I'm pretty sure there has been a discussion of a very similar question regarding dry transformer core temperature on this forum before (try searching)

The apparent extra core ground path certainly sounds like a plausbile explanation. Vacuuming might help try to alleviate. Also sometimes core grounds can be cleared by applying a voltage to ground and increasing the current to attempt to burn off the ground. Also in your search to localize it I have heard of people applying positive voltage one one end-lamination and negative voltage on other lamination and probe the voltage to ground possible with multimeter at various laminations... look for the one at ground potential. Experiment with voltage at only one end to confirm.

Other possible causes of abnormal core overheating (if this in fact is abnormal?):
High voltage on high side of the transformer (check all three phases).

If the taps were on the high side of the transformer, then a high tap setting (above 100%) increases the flux. You don't have that situation.

I think that harmonic loads can cause extra heating in both the core and the windings.

Out of curiousity, did both cores read that hot or just one?

 

Have you checked the core bolts insulation with a megger?
Shorted core bolts can cause excessive exciting current and excessive heating of the core

delphi

 

[benray]

What is the load on this transformer? what is the ambient temperature? 300ºF core temperature is not unusual when load is about 75 per cent or more.

 

[jbartos]

Suggestion: Try to monitor voltage waveforms for harmonic content. The voltage Total Harmonic Distortion (THD) is supposed to be less or equal to 5% up to 69kV. Above 69kV, it is supposed to be 3%. See IEEE Std 519-1992 and similar IEC Stds.