Dielectric response of oil-impregnated paper insulation: variation with humidity and ageing level [power transformer applications]

The dielectric response of oil-impregnated Kraft paper with 3 initial humidity levels (0,17%, 1,75% and 3,5%) has been studied in the frequency range 0.1 mHz -1 kHz, before ageing at DP (degree of polymerisation) 1200 and after ageing to DP 900, 600 and 2-300, primarily to investigate if the method can be used for non-destructive characterization of the state of cellulose insulation in electrical power devices, e.g. transformers. Also, because ageing produces acids, known acids have been added to unaged paper to investigate the influence on the dielectric response. Results indicate that the effect of ageing is small compared to the effect of moisture, but the loss peak of the humid samples become more prominent. Light acids (formic, acetic), which are absorbed in the paper, have an effect on the dielectric response, similar to water.