How does resistivity reflect transformer oil insulation quality?

High resistivity means the oil strongly resists current flow, supporting low dielectric losses and better insulation margins. As oil ages, moisture, acids, and contamination reduce resistivity, which can increase leakage currents and raise the risk of partial discharge or surface tracking. Resistivity is temperature dependent, so measurements are referenced to standard temperatures and trended over time. A declining resistivity trend alongside rising tan delta or acidity typically signals deteriorating oil condition and the need for dehydration, filtration, or regeneration.