Saha and Purkait: Investigation of Polarization and Depolarization144 Investigation of Polarization and Depolarization Current Measurements for the Assessment of Oil-paper Insulation of Aged Transformers

Moisture and ageing strongly influence the dielectric properties of oilrpaper insu-lation system of power transformer. Moisture measurement in oil sample generally gives inconclusive information since oilrpaper moisture equilibrium is tempera-ture dependent and takes a long time to be in equilibrium. Direct moisture mea-surement of paper sample is not practicable for in-service transformers. The mea-surement and evaluation of the ‘‘dielectric response’ ’ and conductivity is one possi-ble way of diagnosing a transformer insulation condition. In a recent research project, polarization and depolarization current measurement has been used for assessing the condition of oilrpaper insulation. The polarization and depolariza-()tion current PDC analysis is a non-destructive dielectric testing method for de-termining the conductivity and moisture content of insulation materials in a trans-former. On the basis of this analysis it is possible to take further actions like oil-refurbishment, drying or replacement of the winding of the transformer. This pa-per presents a description of the PDC technique with the physical and mathemati-cal background and some results of PDC measurements on several transformers. Analyses and interpretation of the field test data are also presented in this paper