Why transformer insulation resistance decrease when filled with insulating oil? 1

[Mr zezo]

hi ; there
This question was asked by someone:"It turns out that this is normal, so what is the reason for this?"
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

 

[Latango]

I'm going to hazard a guess here and say that when a liquid is acting as the dielectric, there is a massive difference in the number of atoms sitting between conductors and earth than when there's a gas there, thus more paths for energy to move.

Also, if it wasn't oil they filled it with, and instead was FR3 / ester type fluids, then you already have to factor in an order of magnitude drop in expected IR readings.

 

[prc]

Oil's resistivity is less than that of air. The IR ratio in an air-to-oil-filled condition was 1:20. But with modern oils of high resistivity, this is now around 1:10.