Experimental investigation of the heterogeneity of the double layer in transformer oil

The effect of the double layer on the frequency spectrum of the dielectric properties of a fluid was verified experimentally by measuring the dielectric properties of transformer oil with three interdigital electrode structures with spatial wavelengths of 5 mm, 1 mm, and 50 μm. The electrodes were coated with a 5-μm-thick dielectric layer which provided a passive boundary between the electrodes and the fluid. The frequency spectrum contains dispersion characteristics that can be attributed to space-charge polarization in a thin region above the dielectric layer. For electrode structures whose characteristics length scale is large compared with the Debye length, an accurate model for the bipolar medium consists of an insulating region, whose thickness is equal to the Debye length, and semi-infinite ohmic region, whose dielectric properties are equal to the bulk properties of the medium