Why is transformer oil resistivity an important parameter?

Resistivity reflects how well the oil resists conduction current under electric field. High resistivity helps minimize leakage currents and dielectric losses, supporting insulation integrity and reducing heating in the liquid. Low resistivity often indicates contamination by moisture, acids, or polar aging byproducts. Because resistivity, power factor, and moisture are linked, utilities use them together to judge oil cleanliness, oxidation condition, and the risk of partial discharge or surface tracking.