What are the key characteristics of transformer oil used for insulation and cooling?

Key characteristics include high dielectric strength, low dielectric losses, high resistivity, low moisture content, good oxidation stability, suitable viscosity, and safe flash and fire points. The oil must remain chemically stable, avoid sludge formation, and stay compatible with cellulose insulation, metals, and sealing materials. It should also support heat transfer by maintaining predictable viscosity and thermal properties across operating temperatures. Utilities prioritize consistent quality, strong documentation, and proven performance under long term thermal and electrical stress.