Modern transformers use mineral oil, natural ester oil, synthetic ester oil, and silicone oil. Mineral oil remains the most common due to cost efficiency and good dielectric performance. Natural esters are biodegradable and fire resistant, making them suitable for environmentally sensitive areas. Synthetic esters offer higher thermal stability and moisture tolerance. Silicone oil is used in fire safe applications due to high flash point. Oil selection depends on voltage class, fire safety requirements, and environmental regulations.
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