What properties are required for high voltage transformer oil?

High voltage transformer oil must deliver high dielectric strength, low dielectric dissipation factor, high resistivity, controlled moisture content, and good oxidation stability to resist sludge formation. It needs suitable viscosity for circulation and heat transfer over the expected temperature range, plus adequate flash and fire points for safety. It also must be compatible with paper insulation, metals, gaskets, and paints, and meet corrosive sulfur and gas generation behavior limits defined by utility or manufacturer specifications.