What oil is in a transformer used in modern power grids?

Modern grids commonly use mineral insulating oil for many power and distribution transformers. In fire sensitive or environmentally sensitive areas, natural ester fluids such as FR3, synthetic esters, or silicone fluids may be used. Selection depends on voltage class, fire safety requirements, environmental regulations, ambient temperature, and transformer design. Utilities also consider oxidation stability, moisture handling, and compatibility with existing insulation materials. The chosen oil must meet recognized insulating fluid standards and the transformer manufacturer specifications.