Ageing Assessment of Transformer Paper Insulation through Polarization-Depolarization Current Measurement

Condition assessment of insulation systems is essential for reliable and safe operation of power transformers. Polarization-depolarization current (PDC) measurement is a non-destructive technique that has been applied for assessing the ageing condition of insulations. Modelling the dielectric response of insulations can provide better understanding of correlations between measured properties and dielectric processes occurring inside insulations. In this paper, a circuit model is proposed for condition assessment of liquid-impregnated Kraft paper based on PDC measurement. The model can represent the insulating materials separately and thus it can reflect the conditions of insulating liquid and Kraft paper. The model was applied on depolarization currents of aged Kraft papers of different degree of polymerization (DP) impregnated with fresh synthetic ester liquid. While calculated conductivity showed an inconsistent trend with ageing, the R-C parameters associated with large time constants deduced from the proposed model followed a trend with the ageing of paper insulation.