The most common choice is mineral insulating oil, either inhibited or uninhibited depending on the utility’s specification and climate, because it offers a strong balance of dielectric strength, cost, availability, and cooling performance. For fire sensitive or environmentally sensitive sites, natural ester or synthetic ester fluids are increasingly used because they can provide higher fire point and improved biodegradability, though at higher cost and with different moisture behavior. The selected fluid must match transformer design, sealing system, and standards in the procurement specification, including compatibility with materials and gaskets.
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