Effect of electrical polarity on dielectric breakdown in a soft magnetic fluid

Abstract

In this paper, we focus on a dielectric breakdown in transformer oil and three magnetic fluid samples. The magnetic fluids contain iron oxide nanoparticles sterically stabilized with oleic acid. The nanoparticle volume concentration ranges from 0.05 vol% to 0.35 vol%. The breakdown tests are carried out in the needle-sphere electrode geometry with the high negative electrical potential applied once to the needle and then to the sphere. The breakdown voltage was found to increase not only due to the increasing nanoparticle concentration, but also due to the increasing voltage rise rate. The experiments revealed a remarkable polarity effect on the dielectric breakdown voltage of the transformer oil and the magnetic fluids under both above mentioned influencing factors. A significant increase in the breakdown voltage was measured with the negative high potential applied to the needle electrode, as compared with the high potential on the sphere. To provide the explanation of the observed effect, the electric field distribution, space charge and a potential effect of local magnetic fields in the vicinity of the magnetic nanoparticles are considered.