Electrical equipment is always subjected to various of operating conduction and voltage waves which cause insulation failure on the surface of pressboard (PB). In this paper, pre-breakdown streamers of surface discharge on the interface of PB in vegetable oil based Fe3O4 nanofluids (NFs) were observed by the shadowgraph method under lightning impulse voltage. The images indicate that streamers of NFs impregnated PB show more branches than that of pure oil impregnated PB. The stopping length of streamers propagation is calculated by shadowgraph images for NFs and pure oil. Results suggest that the stopping length of NFs impregnated PB is shorter under the same extra voltage. Secondary reverse streamer is generated at the dissipation process by the reverse electric field, which is caused by residual space charge imparted by primary streamer. Results indicate that the Fe3O4 nanoparticles have reduced the length of secondary reverse streamer. The positive and negative lightning breakdown voltage of Fe3O4 NFs impregnated PB is increased by 24% and 12% at 50mm gap, respectively. In addition, nanoparticles have effectively changed the electric field distribution resulting in the alleviation of streamer concentrated along the parallel direction of PB, and increased the lightning impulse breakdown voltage.
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