Assessment of dielectric insulation condition of power transformers using Dielectric Absorption Ratio (DAR) and Polarization Index (PI)

The operational reliability and service life of power transformers significantly depend on the conditionof their solid dielectric insulation. Moisture ingress, thermal aging, and electrical stress alter the dielectricproperties of insulation, reducing its electrical strength and accelerating degradation processes. Thispaper examines two widely used diagnostic indicators—the Dielectric Absorption Ratio (DAR) and thePolarization Index (PI)—which are employed to assess insulation moisture content and aging degree.The diagnostic criteria, interpretation of measured results, and significance of these indicators inpredictive maintenance are presented.

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