Identification of overheating in transformer solid insulation by polarization depolarization current analysis

This article shows how the on-site off-line electrical diagnostic technique, Polarization Depolarization Current (PDC) analysis, identifies and locates overheating / thermal fault in transformer solid insulation. The method is particularly useful for transformers using coated paper of any kinds since the determination of Degree of Polymerization (DP) in paper and the Furanic compounds in oil become impossible. In order to tell more precisely how severe the solid insulation is thermally aged, the Depolarization Index (dP.I.) is here introduced and discussed.