Liquid insulation plays an important role in power applications. Transformer oil as an insulation and coolant is a common liquid insulation that nowadays its dielectric and thermal properties can be improved by mixing various nano particles. In recent years, investigators have sought to improve electrical, thermal and physical properties (strength of breakdown voltage, thermal conductivity and viscosity) of the transformer oil only via nanoparticles. The nano silicon dioxide (SiO2) particle is a semiconductive and hydrophilic particle that enhances the dielectric and some of the physical properties of the mineral oil. In this study, new combinations were prepared: mineral oil based on both nano and micro SiO2 particles with different concentrations (nano alone, micro alone and nano micro compositions). The AC breakdown voltage, dielectric loss factor, viscosity, fire, flash and pour points were tested and measured. The AC breakdown voltage (an increasement about 1.68 times more than pure oil) and viscosity have been improved with adding 50% micro 50% nano particles to mineral oil in sample 4. Therefore, the value of breakdown strength for oil, containing both nano and micro SiO2, was found to be much greater than the values for pure oil. Fire and flash points didn’t have a difference in sample 4 in comparison to the others, but cloud and pour points in samples 8 (100% nano with concentration 0.08 g/l) and 4 were acceptable. The viscosity values for samples 0 (pure oil), 2 (100% nano with concentration 0.01 g/l) and 4 were 12.63, 8.54 and 9.33 (mm2 /s), respectively. So adding both micro and nano SiO2 to the transformer oil could improve some of the physical and electrical properties of the oil.
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