Why is the specific resistance of transformer oil important?

Specific resistance, often called resistivity, indicates how well the oil resists conducting leakage current. High resistivity supports strong insulation performance, reduces dielectric losses, and helps prevent surface conduction that can lead to partial discharge. Low resistivity can result from moisture, acids, and contamination, and often correlates with higher tan delta. Monitoring resistivity helps detect oil deterioration and guides decisions on filtration or regeneration. It is particularly important at operating temperatures because resistivity decreases as temperature rises.