Improvement Mechanism of AC Breakdown Voltage of Insulating Oil with Dispersed Nanoparticles

SUMMARY We investigated the fundamental insulation characteristics of insulating oil with dispersed nanoparticles. The AC breakdown voltage of the insulating oil increased by mixing the dispersed hydrophilic nanoparticles, because the hydrophilic nanoparticles absorbed moisture in the oil, which decreased the number of weak points in the oil for breakdown. In contrast, the hydrophobic nanoparticles did not contribute to the increase of AC breakdown voltage, and decreased the AC breakdown voltage. On the other hand, in the case of overdosing of nanoparticles, the aggregation of nanoparticles caused the decrease in the AC breakdown voltage of the oil, which depended upon the volume concentration of nanoparticles regardless of the diameter of nanoparticles. The aggregation of nanoparticles was led by dielectrophoresis phenomenon, which brought the gap length dependence of the AC breakdown voltage of the insulating oil with the dispersed nanoparticles, because the gradient of electric field strength at the gap was smaller in the case of shorter gap length.