Testing and Transformer oil quality

Transformer oils are subject to electrical and mechanical stresses while a 3 phase step up transformer is in operation. In addition there is contamination cused by chemical interactions with windings and other solid insulation, catalyzed by high operating temperature. The original chemical properties of transformer oil change gradually, rendering it ineffective for its intended purpose after many years. Oil in large polemounted transformers and electrical apparatus is periodically tested for its electrical and chemical properties, to make sure it is suitable for further use. Sometimes oil condition can be improved by filtration and treatment.