Dry Type Transformer Winding Failure After Energizing

[Warrengarber]

We had a tech go out last week to test three 125Kva dry type transformers. He did an insulation resistance test and a TTR test on the used tap setting for each transformer. They all tested good PI above 1.0 all insulation resistance readings passed and the ratios were all within NETA specs. One of the transformers failed after it was energized. I was not there but the customer called me Saturday night and said the B phase winding was "smoked". So Monday morning I went out to the site to see what had happened. He was right you can see that the B phase winding was burnt on the bottom and it appears to have come from the middle of the winding out from the bottom. Can it be that because the megger puts out voltage but very little amperage so the test results were good but when they energized it with real current it failed? I was told that the panel this transformer feeds only had three single pole circuits in it so there could not have been much inrush. Does anyone else have any experience with this happening? I will say that the transformer were not build very well in the last photo you will see that the cable mounts are supported by flimsy plastic standoffs.

[Kalbi_Rob]

We had a tech go out last week to test three 125Kva dry type transformers. He did an insulation resistance test and a TTR test on the used tap setting for each transformer. They all tested good PI above 1.0 all insulation resistance readings passed and the ratios were all within NETA specs. One of the transformers failed after it was energized. I was not there but the customer called me Saturday night and said the B phase winding was "smoked". So Monday morning I went out to the site to see what had happened. He was right you can see that the B phase winding was burnt on the bottom and it appears to have come from the middle of the winding out from the bottom. Can it be that because the megger puts out voltage but very little amperage so the test results were good but when they energized it with real current it failed? I was told that the panel this transformer feeds only had three single pole circuits in it so there could not have been much inrush. Does anyone else have any experience with this happening? I will say that the transformer were not build very well in the last photo you will see that the cable mounts are supported by flimsy plastic standoffs.

Sounds like you had a similar issue:
We had a customer call us after we finished testing, and they were starting to energize. They stated smoke was being emitted from C-phase, and temperature of the windings was double on C-phase versus the other two phases. We went out to site, and they energized the xfmr. After a few minutes, C-phase began to smoke. We performed TTR and PI test on the transformer and it passed with flying colors. It turns out, metal shavings fell into the transformer, and due to the baked insulation or paper insulation, neither ttr or pi testing could find the issue. Once the transformer was energized, the magnetic field heated up the metal slag, and caused it to start burning the insulation causing smoke. Manufacturer and customer resolved the issue without any further input from our office.

[Warrengarber]

This one smoked and then tripped the Main Breaker ground fault. The whole building went black. I will pass the information along to the contractor so they watch were the shavings go. Thank you

Sounds like you had a similar issue:
We had a customer call us after we finished testing, and they were starting to energize. They stated smoke was being emitted from C-phase, and temperature of the windings was double on C-phase versus the other two phases. We went out to site, and they energized the xfmr. After a few minutes, C-phase began to smoke. We performed TTR and PI test on the transformer and it passed with flying colors. It turns out, metal shavings fell into the transformer, and due to the baked insulation or paper insulation, neither ttr or pi testing could find the issue. Once the transformer was energized, the magnetic field heated up the metal slag, and caused it to start burning the insulation causing smoke. Manufacturer and customer resolved the issue without any further input from our office.

[SecondGen]

We had a tech go out last week to test three 125Kva dry type transformers. He did an insulation resistance test and a TTR test on the used tap setting for each transformer. They all tested good PI above 1.0 all insulation resistance readings passed and the ratios were all within NETA specs. One of the transformers failed after it was energized. I was not there but the customer called me Saturday night and said the B phase winding was "smoked". So Monday morning I went out to the site to see what had happened. He was right you can see that the B phase winding was burnt on the bottom and it appears to have come from the middle of the winding out from the bottom. Can it be that because the megger puts out voltage but very little amperage so the test results were good but when they energized it with real current it failed? I was told that the panel this transformer feeds only had three single pole circuits in it so there could not have been much inrush. Does anyone else have any experience with this happening? I will say that the transformer were not build very well in the last photo you will see that the cable mounts are supported by flimsy plastic standoffs.

Is this a Square D transformer? I've had similar issues. One in particular I remember was less than 100kVA and had a PI over 1.0 on the 480 side to ground but the readings were very low. Nothing was visible but my suspicion was metal shavings down in the winding or faulty interwinding insulation. This was acceptance so my recommendation was not to energize and I think the mfg ended up replacing it.

[Warrengarber]

This one was a GE and the manufacture replaced it. The readings were all good when it was tested before being energized. Very cheaply made and flimsy.

Is this a Square D transformer? I've had similar issues. One in particular I remember was less than 100kVA and had a PI over 1.0 on the 480 side to ground but the readings were very low. Nothing was visible but my suspicion was metal shavings down in the winding or faulty interwinding insulation. This was acceptance so my recommendation was not to energize and I think the mfg ended up replacing it.

[TestingProToBe]

Sounds like you had a similar issue:
We had a customer call us after we finished testing, and they were starting to energize. They stated smoke was being emitted from C-phase, and temperature of the windings was double on C-phase versus the other two phases. We went out to site, and they energized the xfmr. After a few minutes, C-phase began to smoke. We performed TTR and PI test on the transformer and it passed with flying colors. It turns out, metal shavings fell into the transformer, and due to the baked insulation or paper insulation, neither ttr or pi testing could find the issue. Once the transformer was energized, the magnetic field heated up the metal slag, and caused it to start burning the insulation causing smoke. Manufacturer and customer resolved the issue without any further input from our office.

This leads me to the question- what testing could have been done ahead of time to uncover the presence of metal shavings, I’ve been thinking this over a lot and all I can come up with is to apply a low voltage to the primary side and slowly ramp it up. Then maybe use some ultrasound testing to listen for crackling? This hardly feels like a conclusive approach and may end up damaging the unit anyway.
Any thoughts?

[Kalbi_Rob]

This leads me to the question- what testing could have been done ahead of time to uncover the presence of metal shavings, I’ve been thinking this over a lot and all I can come up with is to apply a low voltage to the primary side and slowly ramp it up. Then maybe use some ultrasound testing to listen for crackling? This hardly feels like a conclusive approach and may end up damaging the unit anyway.
Any thoughts?

The insulation around the windings prevents normal insulation testing from picking up this issue. No non-destructive testing will cause the metal to heat up due to the lack of power produced to create a sufficient magnetic field. Without the magnetic field, it is impossible to cause inductive heating, or cut the lines of magnetic flux needed to identify. There is one test that I've performed once, that could be your best bet within the costs willing to be paid for by the end user. Transformer Surge tests are essentially a hi-pot for a transformer, and is not required by any standard at the moment. It is rarely ever used outside the factory due to this. I don't think it would actually produce the intended results you are looking for, but it is the closest test I know of without energizing the transformer.

https://electrominst.com/test-technology/surge-tests/