ABB UCGDN Tap Changer Drying Process

And your question is?

The diverter switch was shipped from the factory to the field and assembled. This assemble process was carried out 4 times and the breakdown voltage of the insulating oil in the switch fell down to 16 kV in 3 months. Finally, the switch was dried and assembled near the power transformer and the result was taken.

Every piece of material to be placed in the transformer must be transported to be completely free from air and moisture.

I wanted to share this experience.

Normal. That is why one needs to cycle the oil after build thorough an oil purification system, normally portable units, to dry out the oil, increase the dielectric of the oil, filter out contamination during the previous fill operation, and once completed, the tfx is enegrised, off load, to stay dry and warm.

It seems you have not followed the complete process after build, or followed the procedure when filling the tfx with oil.

Just hire in a small trailer mounted unit and purify the oil. It should take about 12-24hrs depending on the moisture content, to bring the oil back to around 53kV breakdown, if the tfx is small. I recall the breakdown voltage should be around 63kV but check you specs.