Experimental Investigation of the Influence of Gaseous Defects on Glycerin-Impregnated Multilayer Film Insulation for LTD

Experimental study of high-purity-glycerin-impregnated multilayer polypropylene (PP) film insulation, wound in laboratory ambient temperature and pressure, was performed to examine the insulation's breakdown behavior under high-voltage impulse with wavefront of 160 ns. The experimental data show that with the existence of distributed visible tiny gaseous defects between film layers, for 100% winding coverage factor, the breakdown field tends to decrease linearly from about 500 kV/mm for single film layer, which is very close to the intrinsic breakdown field of the PP film, to about 330 kV/mm for four film layers. However, this falling tendency of breakdown field becomes mild and then nearly staying as the film layer number (n) increases from 4 to 10, with the ten-layer-film insulation withstanding 20 impulses at the field of 275 kV/mm. Furthermore, for 50%-90% covering factors, the average breakdown field is 254 kV/mm with one-sigma deviation of 51 kV/mm when n ranges from 5 to 8, and by lowering the covering factor from around 80% to 50%, the average breakdown field declines about 10%. The experiment also reveals that if tiny bubbles do not gather together into a long gaseous channel between film layers, the lowest breakdown field is 173 kV/mm, and the flashover field along film layer surface is greater than 15 kV/cm.