High frequency dielectric response of paper/oil insulation

Dielectric response of oil impregnated kraft paper has been measured in the frequency range 8 Hz to 2 MHz in a custom designed test cell. Temperature was varied between -17 and 60degC, allowing master curves for 20degC condition to be established for the frequency range 10 mHz to 10 MHz. Paper types and moisture was varied. The dielectric response showed losses in the low frequency region dominated by conduction and in the high frequency region, by a loss peak located around 10 MHz. The activation energy for the two relaxation mechanisms was found to be different: about 0. 9 eV for the conduction influenced part and between 0.4 and 0.7 for the part influenced by the high frequency peak. The low frequency part of the dielectric response is very dependent on the water content, while the high frequency part seems quite unaffected by water.