Annually for 132 KV transformer, once in 2 years for the transformer below 132 KV transformer and in 2 years interval for the transformer above 132 KV transformer.removes impurities and restores / improves the electrical, chemical and physical properties of transformer insulating oils. This oil is an integral part of most large power transformers. It serves two major roles, as a cooling agent and as an electrical insulator between internal components.