The dielectric strength of oil in an oil-filled transformer is a direct measure of what?

The dielectric strength of oil in an oil-filled transformer is a direct measure of the breakdown voltage. This characteristic refers to the maximum electric field that the insulating oil can withstand without experiencing electrical breakdown, which is when the oil becomes conductive and can no longer provide the necessary insulation. High dielectric strength indicates that the oil can effectively resist electrical stress, ensuring reliable operation and safety of the transformer.

Thermal conductivity, viscosity, and electrical conductivity pertain to different properties of the oil. While thermal conductivity relates to the oil's ability to conduct heat, viscosity describes its thickness or flow characteristics, and electrical conductivity is a measure of how well the oil can conduct electric current. However, none of these properties directly correlate with the oil's ability to prevent electrical breakdown, which is what dielectric strength specifically measures.