الفهرس 
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Abstract
Development of power transformers with small size and high voltage rate is one of the challenge points in transmission and distribution networks. To achieve this point,
transformer oil with high dielectric strength should be introduced. Therefore, it is important to enhance dielectric properties of transformer oil. In this thesis, the breakdown strength of nanofilled transformer oil is evaluated. Different factors affecting breakdown strength of nanofilled transformer oil are considered. These factors are oil temperature, type, concentration levels and size of nanofiller. Therefore, breakdown strength of nanofilled transformer oil is
experimentally evaluated at 35, 70, 100 and 130 oC to simulate the real operating conditions from no load to heavy load. Accordingly, preparation of nanofilled transformer oil samples is carried out considering different nanofiller material types and concentration levels. The adopted nanofiller material types are SiO2
, TiO2 , and ZrO2 considering the same particle size of 25 nm. The selected material types have different relative permittivities of 4.3, 70 and 100 for SiO2 , ZrO2 , and TiO2 , respectively. The studied nanofiller concentration levels are 0.05, 0.1, 0.2, and 0.4
g/L. Also, the effect of particle size on breakdown strength is studied considering TiO2 nanoparticles of 25 nm and 100 nm particle sizes at the adopted concentration levels. The breakdown strength test is carried out on all prepared samples according
to ASTM D1816 standard for all prepared samples. The evaluation of breakdown strength is based on average breakdown voltage. On the other hand, a proposed breakdown mechanism of nanofilled transformer oil is also presented. The proposed mechanism is based on electric filed distortion results from charge accumulation
over the surfaces of nanoparticles when exposed to external electric field. Electric field distortion in the nanofilled transformer oil is validated using finite element
analysis. Finally, the obtained results show that concentration levels, type, and size of nanofiller as well as oil temperature have a significant effect on breakdown
voltage of nanofilled transformer oil.