Study of polarization and depolarization current measurements for assessment of aging state of oil-paper insulated transformers

Polarization and depolarization current (PDC) analysis is a non-destructive dielectric test method for determining the aging state of insulation materials of power transformers. The main influence factors are studied after describing the basic theory of PDC in this paper. The insulation structure, conductivity, moisture level and aging state have great influence on the test results of PDC method. The initial period of the polarization/depolarization current curve is found to be governed by the oil properties and the long time values of these currents are primarily determined by the insulation structure and paper/pressboard condition. Field test shows that the conditions of both the oil and paper can be separately estimated through analyzing the PDC measurement results.