Electron scavenging by conductive nanoparticles in oil insulated power transformers

Abstract—Transformer oil-based nanofluids with conductive nanoparticle suspensions have been experimentally shown to have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. A comprehensive electrodynamic analysis of the processes which take place in electrically stressed transformer oil-based nanofluids has been developed and a model is presented for streamer formation in transformer oil-based nanofluids. Through the use of numerical simulation methods the model demonstrates that conductive nanoparticles act as electron scavengers in electrically stressed transformer oil-based nanofluids converting fast electrons to slow negatively charged particles. Due to the low mobility of these nanoparticles the development of a net space charge zone at the streamer tip is hindered suppressing the propagating electric field wave that is needed to continue electric field dependent molecular ionization and ultimately streamer propagation further into the liquid. General expressions for the charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities are derived to show that the trapping of fast electrons onto slow conducting nanoparticles is the cause of the decrease in positive streamer velocity and higher electrical breakdown strength. Index Terms—Electrical breakdown, field ionization, nanofluid, nanoparticle, streamers, transformer oil. I