Aging Characterization Parameters of Oilpressboard Insulation Based on Repetitive PDC Current Characteristics

The polarization/depolarization current (PDC) method serves as an effective means for assessing the aging state of oil-pressboard insulation in converter transformers. However, variations among tested samples lead to difficulties in establishing unified aging evaluation criteria. To address this issue, this pressboard prepared oil-impregnated pressboard samples with varying aging levels and investigated them using the repetitive polarization current method. Results indicate that during the initial polarization phase, the second polarization current curve is significantly higher than the first, subsequently decaying until the two curves intersect, after which it remains consistently lower. The degree of non-coincidence between the two curves decreases progressively with advancing aging of the pressboard. Based on this observation, a new aging characteristic parameter <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$K_{\tau 1}$</tex> is proposed, defined as the ratio of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{K}_{12}$</tex> (the proportion of charge from the dominant time constant branch to the total absorbed charge during the second polarization) to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{K}_{\mathbf{1 1}}$</tex> (the corresponding proportion during the first polarization). This parameter exhibits an increasing trend with the reduction in the pressboard's degree of polymerization (DP), approaching unity as aging intensifies. The proposed characteristic parameter eliminates the influence of sample variations, offers a novel approach for extracting features from repetitive polarization tests, and provides a practical solution for aging assessment of oil-pressboard insulation.