How is oil filled in a hermetically sealed transformer?

Filling a hermetically sealed transformer is done under controlled procedures to avoid trapping air and moisture. The unit is typically evacuated to remove air, then filled with dry, qualified oil through designated valves using a filtered filling rig. Vacuum filling helps ensure oil penetrates insulation and eliminates bubbles that reduce dielectric strength. The final fill level is set to allow thermal expansion within the sealed tank design. After filling, leak checks, insulation tests, and sometimes oil sampling are performed to confirm dryness and dielectric integrity before commissioning.