The purification of transformer oil only removes dissolved gases and water out of the oil. The water captured in the paper insulation cannot efficiently be removed by only circulating dry oil around the active part of the transformer. Even the water captured in the inner layers of the cellulose material will be completely unaffected. If the insulation material of the transformer core reaches a critical high concentration of water, the transformer needs to be repaired. With a Hot Oil Spray process (HOS), the transformer can be dried inside its housing