Effects of additives on dielectric strength of naphthenic transformer oil

In order to investigate the effects of additives on electrical breakdown strength of naphthenic transformer oil, three types of additives including N, N′-dimethyl aniline (DMA), trichloroethylene (TCE) and TiO2 nanoparticles were used to modify the oil with different concentrations. The breakdown strength and prebreakdown streamer propagation process in the pure and modified oils were investigated under positive lightning impulse voltage. The test results indicate that all additives can improve the breakdown strength of naphthenic transformer oil at certain concentrations, and TiO2 nanoparticles exhibit an optimal modification effect with an increase of 39.4%. Moreover, streamer propagation characteristics of modified oils are significantly affected by the type of additives. The presence of DMA increases the number of discharge branches, while TCE shortens the propagation length of branches without any obvious change in the shape. The positive streamers in the oil modified by TiO2 nanoparticles possess both denser branches and much slower velocities. Based on the analysis of molecule ionization and distribution characteristics of space charge, the improvement in breakdown strength and the change of prebreakdown process are mainly attributed to the significant influence of additives on the formation and transportation of charge carriers in the naphthenic transformer oil.