Transformer Problem

We have a transformer of 3.6MVA, 33KV/415,4800 Amp, Volt S Delta/Star , DY-11, in running condition.

But 1.5 years ago it came in idle condition so we stop the 33KV input supply .

6 months ago we found leakage from LT Bushing. And the oil become drain out up to bushing level for 15-20 days

For maintenance of bushing we drain the oil up to the level from where we can replace the bushing & gourmet of the bushing safely.

After compliance of job we fill up the oil with filtration.

Checked

BDV = 60 KV in 2.5 mm

Insulation resistance LT- Earth= 0 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= 0 M-Ohm (1000 V Meager)

Insulation resistance HT- LT= 0 M-Ohm (1000 V Meager)

Filter OIL for 4 circulations Oil Temperature 70 deg C

BDV = 60 KV in 2.5 mm

Insulation resistance LT- Earth= 0.5 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= 10 M-Ohm (1000 V Meager)

Insulation resistance HT- LT= 100 G-Ohm (1000 V Meager)

Filter OIL for circulations 7 days Oil Temperature 75 deg C

BDV = 65 KV in 2.5 mm

Insulation resistance LT- Earth= 0.5 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= Infinitive (1000 V Meager)

Insulation resistance HT- LT=Infinitive (1000 V Meager)

Applied 415 Volt Supply in 33KV side

Measured LT Voltage-5 Volts

Connect the LT Terminals in a salt-water solution to pass the current 80 amp in all three phases for one day.

Next day Insulation resistance remain unchanged

Increase the LT Current up to 600 amp and kept for 4 days

Result as follows

Insulation resistance LT- Earth= 1.5 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= Infinitive (1000 V Meager)

Insulation resistance HT- LT=Infinitive (1000 V Meager)

Remove the LT supply from HT side, drain total oil, disconnect all LT bushing from winding check insulation resistance of all bushings found all are infinitive but one FRP strip used for support all 3 phases and neutral with transformer body is 0 to 10 M-OHM in difference of FRP strip.

Insulate the FRP strip

Result as follows

Insulation resistance LT- Earth= 150 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= Infinitive (1000 V Meager)

Insulation resistance HT- LT=Infinitive (1000 V Meager)

Riffle the transformer oil after filtration

Insulation resistance LT- Earth= 10 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= 100 (1000 V Meager)

Insulation resistance HT- LT=100 (1000 V Meager)

Please suggest me what farther I can do?

With Thanks & Regards

Amit Nandi

9826009245