Suitable power transformer insulating oil is defined by dielectric strength, low moisture, low acidity, high resistivity, and low dissipation factor. It must have good oxidation stability to resist sludge formation and maintain interfacial tension. Viscosity and thermal conductivity must support cooling and circulation. Additional requirements often include low gassing tendency, acceptable corrosive sulfur behavior, and compatibility with sealing materials and cellulose insulation. Utilities usually specify acceptance values for new oil plus maintenance thresholds for in service oil, ensuring consistent performance across the fleet.
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