Predicitve testing of ferroresonant transformer (cvt)?

[electricpete]

On a 120v:120v dry-type ferroresonant transformer for UPS application, we experienced failure from turn-to-turn short which was believed to be due to thermal aging. We want to figure out what kind of testing we can do (on-line or off-line) to have maximum confidence that the sister unit transformers will operate reliably and we can detect similar degradation ahead of time. What periodic testing of ferroresonant transformers is appropriate if maximum reliability is desired?

Megger testing is a given. 500vdc.

We have in the past tried TTR but I believe results are variable (effective turns ratio changes as input voltage varies and output voltage remains constant). Does everyone agree TTR is not valid test?

Doble test?

* DC Hi-pot or step voltage test? I know this is fairly common on rotating machines but less so I believe on transformers. We did test one to 2kvdc with no failure. However we are not sure whether the test itself may degrade the reliability of the future reliability of the transformer (we’re not worried about failing a transformer during a test, just worried about passing the test and failing later as a result of the test).

Excitation current?
Winding resistance?
I think excitation current and winding resistance pose some challenges because on 3-phase unit we compare phases. In this case we would have to compare sister units or trend over time. Would it be beneficial?
Tear-down visual inspection and re-assembly (somehwat intrusive and may introduce problems).

Infrared?

Any thoughts and opinions at all would be appreciated. I have some experience testing large oil-filled power transformers but none at all on the small transformers. Also interested in any relevant links or standards references.
Thanks in advance!

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[jbartos]

Suggestion similar to the similar postings of yours:
A no-load transformer starting current signature can be compared to subsequent ones taken at regular time intervals. Any deviation in the signatures can be analyzed by comparisons. E.g., the higher current would mean that the parallel branch in the transformer electrical equivalent circuit is somewhat deteriorating. Additionally, transformer leakage current signatures also reveal the transformer insulation deterioration.