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My suspicions are confirmed. The core must be grounded only in one place to avoid the additional magnetic flux. Something terrible is stalking between the high voltage potentials

I made calculation of the DGA and the result is defect with high temperature 780 degrees celcius and discharges. I didn't understand exactly what bolts were grounded. The bolts that press the core? Something terrible is stalking between the high voltage potentials

Tests: 0. ratio 1. no load 2. resistance 3. isolation 4. I didn't see what are voltage ratings, but tandelta/powera factor test is nessessery 5. short circuit impedance 6. any bushings? - tandelta tests 7. oil probe test Something terrible is stalking between the high voltage potentials

There isn't general criteria for replacement of transformers.We have 45 years 180 MVA trafo, with average load of 10 MVA and it's in perfect condition / after replacement of oil 10 years ago/, but we have 25 years 210 MVA trafo with average load of 80%, several very heavy short circuits nearby...

It's dangerous to use test voltage more that nominal voltage of the transformer. You did't say nothing about the test temperature. The isolation of T2 is very low. Try to clean the bushings,mark the temperature and do the measurements again, but these 3 measurement circuits : HV-LV+G ; LV-HV+G...

Can You recognize the type of the PDs? I believe that You have bad contacts and after some time the contacts are welding, so this is the reason for decreasing of PDs. Can You point where was the position of PDs over one period of voltage? Something terrible is stalking between the high voltage...

where was the cable? what was the lenght? what were the conditions during the measurement? Somethin terrible is stalking between the high voltage potentials

You need few more data before start: 1. Power S? 2. how much phases 3. frequency 4. Primary voltage U1 , secondary voltage U2 and taps? - what is the tollerance for ratio on main tap and the other taps 5. System for voltage regulation 6. Connection 7. Short circuit voltage or...

can you send pictures of the types of te PDs? Somethin terrible is stalking between the high voltage potentials

There is a big difference between these tho measurements. PI=R600/R60 is based on pure polarisation. PF or tandelta /in Europe/ is global macro index. Of course everithing depends of two time constants inside the transformer. One of them - oil constant and the other one - paper constant. In my...