What is transformer oil and what is its primary function?

Transformer oil is an insulating liquid used inside oil filled electrical equipment. Its primary functions are electrical insulation and heat transfer. It provides dielectric strength around windings and tap changers, reduces the likelihood of arcing by filling gaps, and removes heat from copper and core losses to the tank walls and radiators. Transformer oil also protects cellulose insulation by limiting oxygen exposure in sealed designs and by carrying diagnostic information, because dissolved gases and chemical byproducts in the oil reflect developing faults and aging of the paper oil insulation system.