Breakdown strength of transformer oil modified by TiO2 nanoparticles under AC-DC combined voltage

The dielectric strength of transformer oil can be greatly improved by the modification of nanoparticles under AC, DC and positive impulse voltage, respectively. In comparison with the certain voltage, the combined voltage is more complicated and important for the safe operation of the converter transformer. However, the modification effect of nanoparticles on the breakdown characteristic of transformer oil is still not clear under the AC-DC combined voltage. In this paper, Titania (TiO
₂
) nanoparticles were prepared and used to modify the mineral transformer oil. The breakdown characteristics of transformer oil before and after modification were measured under the AC-DC combined voltage with a needle-sphere electrode configuration. In order to systematically study the modification effect of TiO
₂
nanoparticles, the influence of proportion of AC to DC was also investigated. The results showed that TiO
₂
nanoparticles have an improvement on the breakdown strength of transformer oil under combined AC-DC voltage. It can be found that the positive DC breakdown voltage of the nanofluid is similar to that of pure oil. While as the pre-applied AC voltage increases from 10 kV to 20 kV, the breakdown strengths of the nanofluid are increased by 12.9% and 21.8% respectively, in comparison with that of pure oil under combined voltage.