Abstract
In order to reveal the effects of nanoparticle materials on prebreakdown and breakdown properties of transformer oil, three types of nanoparticle materials, including conductive Fe3O4, semiconductive TiO2 and insulating Al2O3 nanoparticles, were prepared with the same size and surface modification. An experimental study on the breakdown strength and prebreakdown streamer propagation characteristics were investigated for transformer oil and three types of nanofluids under positive lightning impulse voltage. The results indicate that the type of nanoparticle materials has a notable impact on breakdown strength and streamer propagation characteristics of transformer oil. Breakdown voltages of nanofluids are markedly increased by 41.3% and 29.8% respectively by the presence of Fe3O4 and TiO2 nanoparticles. Whereas a slight increase of only 7.4% is observed for Al2O3 nanofluid. Moreover, main discharge channels with thicker and denser branches are formed and the streamer propagation velocities are greatly lowered both in Fe3O4 and TiO2 nanofluids, while no obvious change appears in the propagation process of streamers in Al2O3 nanofluid in comparison with that in pure oil. The test results of trap characteristics reveal that the densities of shallow traps both in Fe3O4 and TiO2 nanofluids are much higher than that in Al2O3 nanofluid and pure oil, greatly reducing the distortion of the electric field. Thus, the propagations of positive streamers in the nanofluids are significantly suppressed by Fe3O4 and TiO2 nanoparticles, leading to the improvements of breakdown strength.
Highlights
Breakdown characteristics of transformer oil have been widely investigated for its important significance in the safe operation of power equipment [1,2]
Naphthenic transformer oil (25# Karamay) was used as the base oil, which was filtered in order to remove the impurity particles and meet the demand of pure oil defined by CIGRE working group 12.17 [17]
It is worth noting that the improvements for breakdown voltage of Fe3 O4 and TiO2 nanofluids are up to 41.3% and 29.8% respectively, much higher than that for Al2 O3 nanofluid of 7.4%
Summary
Breakdown characteristics of transformer oil have been widely investigated for its important significance in the safe operation of power equipment [1,2]. Breakdown phenomena in transformer oil are caused by the initiation and propagation of charged gaseous channels called “streamer” under the high local electric field [3]. Positive streamers have been proved to initiate at lower applied voltages and propagate faster and further than negative streamers [4,5,6], which constitute a greater risk to oil. Sci. 2018, 8, 601 insulated high voltage electrical equipment than negative streamers. It has been found that the positive impulse breakdown strength of transformer oil depends on the prebreakdown phenomenon [7,8,9]
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