One bushing of 3 phase transformer overheating.

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11bgrunt

11bgrunt

Pragmatist
Location
TEXAS
Occupation
Electric Utility Reliability Coordinator
4200kVA Station type outdoor, exposed bushings, Pri 14.4/24.9kV, Sec 480/277Y three phase transformer.
External cable tray with multiple conductor runs per phase and neutral. Plant peaks at 3500 amps on 480V. Load is relatively balanced within 5%. Load is extruders, formers and lighting with an office on dry type transformer beyond their switchgear. 4-400HP AC motors on VFDs.
Y primary, Y secondary. SEL 735 metering the secondary.
Transformer has been in service 4 years. Plant reports there have been no changes in one year or more.
There is/was no voltage or plant operation's complaint.
Wildlife protection was on the secondary bushings. On X1 that wildlife guard started smoking and thermal showed 420F.
Extensive checks were made to include lifting manhole and looking at oil and connections. No definite cause but secondary bushing was called bad.
Transformer was replaced with similar unit.
24 hours later the plant is up and running normal. X1 on the new transformer is hot.
Today at 7AM the T Delta between X1 and X3 is 128F.
6PM, with 3500 amps, the T Delta is 325F. Bushing shows to be 450F.
SEL shows voltage harmonics under 3% and no voltage sag. PF is 90-92 across the phases.
If the wildlife guard had not smoked and no access to thermal, we would be unaware of any issue with the transformer.
Ther is no operation complaint from the plant. They are running full bore. Loss of production from the transformer C/O and start up time is the complaint now.

If it last until morning, the plan is to start looking for stray currents.
I am looking for a direction.

Thanks,
 
I will connect a PQA to the transformer secondary and check the SEL and watch waveforms.
There has been more than one thermal camera used but another one will go out tomorrow. My transformer SME is stumped.
.
 
In the image from the thermal camera is there any evidence of a more localized hot spot on the bushing or the conductors where they are attached? Perhaps a comparison coulde be made with the thermal image of another bushing to see if there's any difference in how the temperature gradients are distributed at one bushing vs. the other, and then see if that might provide some clues.
 
synchro said:
In the image from the thermal camera is there any evidence of a more localized hot spot on the bushing or the conductors where they are attached? Perhaps a comparison coulde be made with the thermal image of another bushing to see if there's any difference in how the temperature gradients are distributed at one bushing vs. the other, and then see if that might provide some clues.
Click to expand...
 

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I do residential work and have no experience on anything like this. Let me ask, how are the wires terminated? Do they have lugs crimped to them that then get bolted to a plate, or is it more like LV panels where the wire end goes into the lug and a set screw is then tightened? If the lugs are crimped and one or more is bad, changing out the transformer would not solve the problem.
 
I think I would start by measuring the current in each individual wire that is hot and making sure that they are roughly the same. You might have one or more conductors not even connected at one end or the other. Have you checked the temperature at the other end of these conductors?
 
The E60 uses .jpeg files.
You have another camera? Like a 600 series?
If you can zoom in and clear the picture up (focus) and upload that I can put the image in my FLIR tools on my computer and analyze a little better.

if the temperature range can be adjusted dual it up and look again.

From what I see it’s a termination problem.
The wire isn’t hot, and the bushing isn’t hot below the bushing termination area.
Did you put new paddles on the wire when you changed the XF out?
One in particular looks hot all the way up the crimp area

 
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Were the wire terminations replaced when the transformer was replaced? The thermal image looks like a termination issue, but not clear enough to see.
 
don_resqcapt19 said:
Were the wire terminations replaced when the transformer was replaced? The thermal image looks like a termination issue, but not clear enough to see.
Click to expand...
The 6- 750cu jumpers did not get the 2-hole connectors replaced. Connectors were cleaned and reattached.
 
No sign of damage to 750 connectors, no pitting and cleaned up fine.
 
Maybe so.
Crimps on both ends of the 750s. The other end connected to bussbar are all fine.
 
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