What does a tan delta test of transformer oil indicate?

A tan delta test, also called dissipation factor or power factor test for oil, indicates how much dielectric loss the oil has under AC stress. Low tan delta means the oil behaves like a good insulator with minimal dielectric heating. Higher tan delta often points to contamination or aging such as moisture, oxidation byproducts, sludge precursors, or dissolved polar compounds that increase conductivity and dielectric losses. The result is used alongside moisture, acidity, and BDV to judge whether oil needs drying, filtration, regeneration, or replacement, especially before stressing the transformer at high load.