What is an oil immersed transformer and where is it commonly used?

An oil immersed transformer is a transformer where windings and core are fully submerged in insulating oil. The oil provides both dielectric insulation and cooling. These transformers are commonly used in transmission networks, substations, power plants, and outdoor distribution systems. They are preferred for high voltage and high MVA applications because oil allows compact insulation design, efficient heat dissipation, and long service life under heavy electrical and thermal loading.