Effects of nanoparticle charging on streamer development in transformer oil-based nanofluids

TLDR

Nanofluids containing conductive nanoparticles exhibit higher positive voltage breakdown levels and slower streamer velocities than pure transformer oil, contrary to conventional expectations. The enhanced breakdown performance is due to electron trapping by charged nanoparticles, which converts fast electrons into slow, negatively charged carriers with mobility reduced by about 10⁵, and a comprehensive electrodynamic model captures the resulting electric field, electron trajectories, and charging dynamics. Case studies show that nanoparticles act as electron scavengers, slowing streamer propagation, reducing heating and vaporization, and thereby improving high‑voltage equipment performance and reliability.

Abstract

Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1×105. The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the charging characteristics of the nanoparticles as a function of time and dielectric permittivity and conductivity of nanoparticles and the surrounding transformer oil. This charged nanoparticle model is used with a comprehensive electrodynamic analysis for the charge generation, recombination, and transport of positive and negative ions, electrons, and charged nanoparticles between a positive high voltage sharp needle electrode and a large spherical ground electrode. Case studies show that transformer oil molecular ionization without nanoparticles cause an electric field and space charge wave to propagate between electrodes, generating heat that can cause transformer oil to vaporize, creating the positive streamer. With nanoparticles as electron scavengers, the speed of the streamer is reduced, offering improved high voltage equipment performance and reliability.