Effects of gaseous defects on glycerin-impregnated multi-layer film insulation under impulse of 160-nanosecond wave-front

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.