The breakdown behavior of high-purity-glycerin impregnated multi-layer polypropylene (PP) film insulation, with the existence of individually distributed visible tiny gaseous defects (defects due to gaseous inclusions) between the film layers, was investigated experimentally under high voltage impulse of 160-ns wave-front. The experimental results show that, for 100% winding coverage factor, the sample insulation of 10-layer PP film survived after 20 impulses of interval of several minutes at an electric field of 275 kV/mm. The breakdown field tends to decrease fast and linearly from about 500 kV/mm for 1-layer film, which is very close to the intrinsic breakdown field of the PP film, to about 330 kV/mm for 4 film layers. This tendency of falling of breakdown field becomes mild and then nearly staying as the number of film layers (n) increases from 4 to 10, indicating a flat decay law of n to the (-0.05-th) power. Furthermore, for 50%-90% covering factors, the average breakdown field is 254 kV/mm with 1-sigma deviation of 51 kV/mm when n ranges from 5 to 8, slightly decaying versus the reduction of covering factor by about 10%. The lowest breakdown field is 173 kV/mm in all cases for about 30 samples. The experimental data demonstrate that if visible tiny gaseous defects do not gather together into a long channel between film layers, no flashover will occur along film layer surface at a field of 15 kV/cm, otherwise flashover is inevitably initiated along the film-glycerin interface.
Three-dimensional (3-D) numerical analysis was performed to quantify the electric field in and near a bubble located in single dielectric and one of two dielectrics in series, and especially to have an insight into the quantitative characteristics of the influence of gaseous defects on the field when the void locates in a high-permittivity liquid in series with polymer film. The results show that the field in the bubble predicted by onedimensional (1-D) treatment of the defect in single dielectric will be significantly greater (about 30% to 2 times) than that given by 3-D model; and for a bubble in one dielectric of very large permittivity in series with another, the field by 1-D treatment is dramatically (more than an order of magnitude) higher than the 3-D results. These data indicate that the influence in field of gaseous defects on the partial discharges and/or breakdown behavior of liquid and solid dielectrics may be overestimated from ten's percent to several times, or even more than an order of magnitude when the defects locate in high-permittivity dielectric in series with low-permittivity one.
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