Nanotechnology provides a new way to improve the insulating properties of traditional dielectric materials. In this study, three types of mineral oil based nanofluids were prepared by suspending Fe3O4, TiO2 and Al2O3 nanoparticles all of which were surface modified by oleic acid. The inception voltage, stopping length and propagating velocity of streamers in the nanofluids under positive lightning impulse voltage were experimentally studied. It is found that nanoparticles can restrain the initiation and propagation processes of positive streamers in transformer oil depending on the types of nanoparticles. In addition, the trap characteristics in pure oil and nanofluids were comparably studied. The relationship between the trap characteristics and mobility of charge carriers in oil samples were then established. The increased trap density in nanofluids diffuses kinetic energy of ionized electrons and converts them into negative ions, resulting in the reduced electrical field strength in front of positive streamer and increased breakdown strength of nanofluids.
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