Tan Delta (Dissipation Factor) — Transformer Glossary

Measure of dielectric losses in insulation. A high tan delta value indicates deteriorated insulation.

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Tan Delta (Dissipation Factor)

Measure of dielectric losses in insulation. A high tan delta value indicates deteriorated insulation.

New transformer oil should have tan delta < 0.005 at 90°C. Values above 0.1 suggest the oil needs treatment or replacement. Tan delta measures the ratio of resistive current to capacitive current through the insulation — a perfect insulator would have tan delta = 0. Testing is performed per IEC 60247 or ASTM D924. Trending tan delta values over time provides valuable insight into the rate of insulation ageing. A sudden increase indicates contamination (moisture, particles) or thermal degradation.

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DGA (Dissolved Gas Analysis)Diagnostic technique that analyses gases dissolved in transformer oil to detect incipient faults.

Partial Discharge (PD)Localised electrical discharge that does not fully bridge the insulation between conductors. A precursor to insulation failure.