Why transformer insulation resistance decrease when filled with insulating oil?

Thread Starter

fnv1217

Joined Sep 11, 2019
2
Hi all!
We are in the business of power transformer repair and lately we have this one project where we encounter a problem.
After doing the repair of the transformer, it was shipped overseas without insulating oil. Prior to that, Insulation resistance tests and Insulation Power Factor test was conducted at reduced voltage of 1 KV. Before shipment, all insulation tests yielded a value of about 5 gigaohms between windings, and windings to ground. Insulation Power factor was below 1%. Upon arrival at site, unit was vacuum filled with oil and electrical tests was conducted. Insulation resistance dropped to about 0.5 gigaohms for all tests while the Insulation power factor remains almost the same. Oil was tested for Moisture content and DBV and yielded 19 ppm and 76 kv/mm. We drained the oil and conduct the same tests again and values measured was like before the unit was oil-filled. Anybody have any idea what could be the problem?
 

atferrari

Joined Jan 6, 2004
3,958
Heard of something similar where the test was redone using different levels of oil inside. No idea about the results or conclusions. Sorry.
 

Alec_t

Joined Sep 17, 2013
11,277
The logical conclusion is that the oil is more conductive than the medium (air?) in which the original tests were conducted, assuming the test conditions were otherwise identical.
 

Thread Starter

fnv1217

Joined Sep 11, 2019
2
Heard of something similar where the test was redone using different levels of oil inside. No idea about the results or conclusions. Sorry.
Yes, we have done that test wherein we gradually remove the oil and measure the IR at different levels of oil in the tank. The IR returned to its original high value when almost all of the oil were drawn out. The puzzling fact is that the oil has low moisture content of 19 ppm and a breakdown volatge of 74kv/mm.
 

rsjsouza

Joined Apr 21, 2014
247
One idea: in your resistivity tests are you using DC or AC at the operating frequency?

Not that I am an expert in insulating oil, but I wonder if the impedance of the oil increases significantly with the frequency (just like the primary of a transformer).

Also, the characteristics of the oil vary greatly with temperature. Are you testing the oil at the working temperature of the transformer?
 

dendad

Joined Feb 20, 2016
3,525
I would think that even if the oil resistance is very high, you in effect have many very high restive paths in parallel, so the total resistance is lower.
 

Glenn Holland

Joined Dec 26, 2014
705
My guess is the oil is for thermal conductivity rather than insulation and the oil is providing a weak, but conductive sneak path to the casing.
 
My guess, is that there is a mechanical defect. Picture two spark gaps. One has pointy ends. The other ends with a ball. Same separation distance. The pointy end will break down at a lower voltage.

Trapped water? Not sure if you have to worry about using vented screws and virtual leaks with your transforner. e.g. places where water can hide? Shipped with a dessicant?

Shipping and condensation?

Is temperature acclimation appropriate before filling with oil?
 

atferrari

Joined Jan 6, 2004
3,958
My guess, is that there is a mechanical defect. Picture two spark gaps. One has pointy ends. The other ends with a ball. Same separation distance. The pointy end will break down at a lower voltage.

Trapped water? Not sure if you have to worry about using vented screws and virtual leaks with your transforner. e.g. places where water can hide? Shipped with a dessicant?

Shipping and condensation?

Is temperature acclimation appropriate before filling with oil?
All transformers I recall discharging / loading in vessels were fitted with a battery of some inert gas to keep the interior full and with positive pressure. It is the very first thing you check prior starting. The next is the impact recorder.
 
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