The main types are mineral oil, natural ester fluids such as FR3, synthetic esters, and silicone oils. Mineral oil is the traditional choice due to cost and proven performance. Natural and synthetic esters offer higher fire points and better biodegradability and can improve paper moisture management. Silicone oils also have high fire resistance and good dielectric properties but are used more selectively due to cost and handling considerations. Selection depends on voltage class, fire safety objectives, environmental requirements, operating temperature, and compatibility with the transformer’s insulation system and design approvals.
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