What does dielectric dissipation factor of transformer oil indicate?

Dielectric dissipation factor (often called tan delta or power factor) reflects dielectric losses caused by polar contaminants and aging byproducts. A low value typically indicates clean, dry oil with good insulating behavior. Rising dissipation factor often correlates with moisture, oxidation products, and contamination, and it can signal reduced resistivity and higher leakage currents. Utilities trend this parameter over time and use it with acidity and interfacial tension to judge oil aging.