What Is a Power Factor (tan-delta) Test?

Published 27 Aug 2026Updated 27 Aug 20266 min readIntermediateTesting
Quick answer — The power factor (dissipation factor, tan-delta) test measures the dielectric loss of the insulation by applying an AC voltage and measuring the small phase angle between the applied voltage and the resulting current — the loss angle. A rising tan-delta indicates moisture, contamination or aging in the insulation system.
Contents
What is a transformer power factor / dissipation factor test?

What it measures

An ideal insulation is almost purely capacitive, with a loss angle near zero. Real insulation has a small dielectric loss; tan-delta is the ratio of the resistive to the capacitive current. Higher values mean more loss.

How it is run

A test set applies a known AC voltage (often 10 kV) across the winding-to-ground or winding-to-winding insulation and measures the capacitance and tan-delta. On bushings it is also used with a power-factor tap.

Interpreting results

Compare values against baseline and by phase, and use the trend over time. Moisture in the insulation (from a failed seal or aging) increases tan-delta sharply, as does contamination or conductive particles.

References & standards

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Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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