Modelling of oil-paper insulation layers in the frequency domain with cole-cole-functions

Abstract

Modern impedance analyzers offer the possibility to measure the frequency related conductivity and the complex permittivity epsiv(omega) of a sample from a few mHz up to some MHz. Impedance measurements are relatively easy to accomplish and deliver very precise results over a wide range of conductivity and capacitance. Using the measured spectra from oil-paper-confining-layer, inference can as well be made on the mobility of ions & molecules and also regarding the transport-mechanisms. It could be proved that impedance measurements with dry and moist oil-paper-confining-layer-systems show a distinctive permittivity function epsiv(omega). For modelling the dielectric performance of pure oil and pure paper systems the cole-cole-function approach for dielectric systems was applied. The modification of the relative permittivity is on one hand caused by polarization processes within the confining-layer-system and on the other hand by lamination on the boundaries between oil and paper. The moisture and temperature have a great impact on the results. One reason is the high relative permittivity of water, the other the heat convection which disturbs the lamination and the polarization. This paper supplements the theoretical model with experimentally measured data permitting better understanding of complex processes in insulation systems. The model of oil-paper insulation layers in the frequency domain could be used for transformers diagnosis