Methods include:

Mechanical Filtration: Using fine filters to remove solid contaminants.
Vacuum Filtration: Combining filtration with vacuum processes to eliminate moisture and gases.
Centrifugal Separation: Employing centrifugal force to separate particles based on density.
Adsorption Filtration: Using materials like activated alumina to remove dissolved impurities.
Electrostatic Precipitation: Applying electric fields to capture fine particles suspended in the oil.
These methods effectively restore oil purity and maintain its insulating properties.