How is transformer mineral oil purified to extend operational life?

Purification removes moisture, gases, and solid particles that reduce dielectric strength and accelerate aging. Common methods include vacuum dehydration, degassing, and fine filtration. Regeneration goes further by using adsorbents such as activated alumina or fuller’s earth to remove oxidation products, acids, and sludge precursors. After treatment, oil is retested to confirm restored properties. Purification is often performed with the transformer in service or during outages, depending on equipment, contamination level, and safety constraints.