Transformer oil is widely used in electric transformers serving the dual functions as insulating medium and cooling material. However, safety and stability of power system is interrupted due to the operation failure of electric transformers in certain case. Therefore efforts should be focused on improving the property of transformer oil for the stable operation of insulating system. The purpose of this paper is to obtain a new type of nanomodified transformer oil with improved dielectric and thermal properties. Boron nitride (BN) nanoparticles with high thermal conductivity were dispersed into transformer oil to form the nanooil. Dielectric and thermal properties of the samples before and after modification were measured. Dielectric property experiments were performed in accordance with IEC standard. Discharge phenomenon in breakdown process was captured by a high speed CCD camera. In order to investigate the effect of BN nanoparticles on thermal property, temperature change and heat distribution of the samples in heat transfer process were measured by an infrared thermal imager and a temperature sensor. It is found that the nanooil has higher dielectric breakdown strength and lower dissipation factor which indicate better dielectric property compared with nonmodified oil. Obtained results show significant improvement in heat transfer process with increasing the concentration of BN nanoparticles. It is proposed that the nanoparticles create an interfacial region which contributes to the enhancement of dielectric strength. For thermal property the ballistic phonon transport of the nanoparticles acts as the key factor in the remarkable improvement. Furthermore, the improved thermal property of the nanooil has an inhibitory effect on the breakdown process according to the bubble theory.

Liquid insulation plays a significant role in the design and operation of power system. There is good evidence in the literature that the addition of nanoparticles to conventional transformer oils can lead to significant increases both in electrical and thermal characteristics. In this paper, spherical BN nanoparticles with high thermal conductivity were added into transformer oil to form nanofluids, with the aim of enhancing electrical and thermal properties. Electrical properties and thermal characteristics of oil samples before and after modification were measured. It was found that the nanofluids have higher AC breakdown voltage and relative permittivity, lower dissipation factor which indicates better electrical properties compared with pure oil. In order to investigate the effect of BN nanoparticles on thermal properties, temperature changes and heat distribution of the nanofluids with different concentrations of BN particles in thermal conductivity process were measured by an infrared thermal imager and a temperature sensor.Obtained results showed that, under the same experimental condition, the heat dissipation of nanofluids with higher concentration of BN particles was faster. It is concluded that the added BN fillers have a very significant effect on electrical and thermal properties in transformer oil-based fluids.

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