The conductivity of hydrocarbon transformer oil containing water and solid conducting particles

The d.c. resistivity test is used to determine the quality of transformer oil in service. It is found that water droplets in oil oscillate between the measuring electrodes in a d.c. field, transferring charge and giving rise to a high conductivity. As the test temperature is raised, oil dissolves more water and the quantity held in suspension decreases with consequent decrease of conductivity. This results in an inverted V resistivity-temperature characteristic. Solid conducting particles such as carbon give rise to high conductivity and a resistivity-temperature characteristic parallel to, but of lower level than, new oil. The conductivity is proportional to the square of the electric stress, the quantity of conducting matter and inversely proportional to the viscosity.