What properties must electrical transformer oil meet for insulation and cooling?

The oil must maintain high dielectric strength, low moisture content, low acidity, high resistivity, and low dissipation factor to support insulation. For cooling, it needs appropriate viscosity, good thermal conductivity, and stable behavior across the expected temperature range. Oxidation stability is critical to prevent sludge and maintain heat transfer. Low gassing tendency and chemical compatibility with copper, paper, and sealing materials reduce long term risk. For safety and environment, fire point, biodegradability, and toxicity classification may be specified depending on site requirements and regulations.