Condition Assessment of Oil Impregnated Paper Transformer Insulation using DC Conduction Measurements.

Abstract

The objective of this thesis has been to study how DC conduction measurementscan be used as a diagnostic tool for condition assessment of oil impregnated paper.In this context, the DC conduction measurements were performed in an insulation study. Especially the effect of paper degradation and moisture content in paper hasbeen looked into, but also the effect of voltage and insulation thickness were studied.

The DC condition measurements were conducted using a Megger, which has the ability to applied voltage onto the paper samples as well as measured the leakage current. The measurements were performed at both oil free and oil impregnatedpaper under controlled temperature and relative humidity condition inside two different climate chambers. The temperature and relative humidity inside the climate chambers was measured using sensors. Included in the setup is also an electrode arrangement with guard and a high voltage electrode that weights on the measuring electrode.

To investigate the effect of paper degradation, paper has been aged in air at 130◦C for different amount of time. The DP of this paper was measured by following the recommendations in the standard IEC 60450 [1]. To achieve the target moisture content in the paper relative humidity was adjusted according to relative humidity vs moisture in content curves. When studying the effect of moisture, Karl Fischer Titration was performed at paper with different moisture content. Several DC conduction measurements were performed when studying each of the factors mentioned previously. The DC conduction measurements were used to calculate both DC conductivity and tan δ of the paper. To calculated tan δ, the Hamon approximation was used. The results obtained in this study shows that DC conductivity oil impregnated paper increases exponentially as the moisture content in paper increases. This indicates that moisture in oil impregnated paper can be detected using DC conductivity. On the other hand, paper degradation could not be detected using DC conductivity. However tan δ was dependent on paper degradation in spite of using new oil. It was also observed that the DC conduction measurements of paper with approximately 4 % moisture content tested at four layers of paper and 500 V got an logarithmic shape.

The results obtained in this study therefore shows that DC conduction measurements could be used as a diagnostic tool for condition assessment of oil impregnated paper in transformers. However, a exact evaluation criteria could not be obtained.