2009 ELECTROSTATICS JOINT CONFERENCE - PAPER 1.1 1 Electron Scavenging by Conductive Nanoparticles in Oil Insulated Power Transformers J. George Hwang, Student Member, IEEE, Markus Zahn, Fellow, IEEE, Francis M. O’Sullivan, Leif A. A. Pettersson, Olof Hjortstam, and Rongsheng Liu, Senior Member, IEEE (Invited Paper) 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. I NTRODUCTION T HE widespread use of transformer oil for high voltage insulation and power apparatus cooling has led to exten- sive research work aimed at enhancing both its dielectric and thermal characteristics. A particularly innovative example of such work is the development of dielectric nanofluids. These materials are manufactured by adding nanoparticle suspensions to transformer oil, with the aim of enhancing some of the oil’s insulating and thermal characteristics. Segal et al. investigated transformer oil-based nanofluids, using magnetite nanoparti- cles [1]–[3]. The research aimed to explore if a transformer oil-based magnetic nanofluid could be used to enhance the cooling of a power transformer’s core. J. G. Hwang and M. Zahn are with the Department of Electrical Engineering and Computer Science, Laboratory for Electromagnetic and Electronic Sys- tems, Massachusetts Institute of Technology, Cambridge, MA, 02139 USA e-mail: ghwang@mit.edu, zahn@mit.edu. F. M. O’Sullivan is with the MIT Energy Initiative, Massachusetts Institute of Technology, Cambridge, MA, 02139 USA e-mail: frankie@mit.edu. L. A. A. Pettersson and R. Liu are with ABB Corporate Research, Power Technologies, S-72178 V¨ aster˚ as, Sweden e-mail: leif.a.pettersson@se.abb.com, rongsheng.liu@se.abb.com. O. Hjortstam is with ABB AB, Metallurgy, S-72159 V¨ aster˚ as, Sweden e- mail: olof.hjortstam@se.abb.com. Manuscript received April 17, 2009. Electrical breakdown testing of magnetite nanofluid found that for positive streamers the breakdown voltage of the nanofluids was almost twice that of the base oils during lightning impulse tests. The lightning impulse withstand re- sults obtained by Segal et al. for two common transformer oils (i.e., Univolt 60 and Nytro 10X) and their associated nanofluids are summarized in Table I. Also, the propagation velocity of positive streamers was reduced by the presence of nanoparticles, by as low as 46% for Univolt-Colloid Nanofluid. The results are significant because a slower streamer requires more time to traverse the gap between electrodes to cause breakdown. This allows more time for the applied impulse voltage to be extinguished. These results are very impor- tant in that it indicates that the presence of the magnetite nanoparticles in the oil samples inhibits the processes which lead to electrical breakdown. The results found by Segal et al. are in direct conflict with conventional wisdom regarding the breakdown of dielectric liquids, which suggests that the presence of conducting particulate matter in a dielectric liquid will decrease the breakdown strength. A comprehensive electrodynamic analysis of the processes that take place in electrically stressed transformer oil-based nanofluids has been developed. The results demonstrate that conductive nanoparticles act as electron scavengers in electri- cally stressed transformer oil-based nanofluids converting fast electrons to slow charged particles [4], [5]. 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 drive electric field dependent molecular ionization and ultimately streamer propagation further into the liquid. A general expression for the charging dynamics of a nanoparticle in transformer oil with finite conductivity is derived to show that the trapping of fast electrons onto slow nanoparticles is the cause of the decrease in positive streamer velocity. This explains the paradoxical fact that nanofluids manufactured from conductive nanoparticles have superior positive electrical breakdown performance to that of pure oil. An electrodynamic model is presented for streamer forma- tion in transformer oil-based nanofluids. The model is analyzed numerically via the finite element simulation package COM- SOL Multiphysics. This study builds upon earlier work that showed that transformer oil stressed by a positively charged electrode leads to molecular ionization of oil molecules into slow positive ions and fast electrons [4], [5]. The fast electrons cause a propagating electric field wave that is the dominant