الفهرس 
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Abstract
With the tendency of the power system to increase the generating capacities
and transmission voltage levels to meet the rapidly growing worldwide energy
demands, associated problems with additional stresses on conventional
insulation systems of high voltage equipment such as power transformer
insulations have become an expected concern. Additional stresses will affect
the lifetime of transformer insulations. Consequently, there is a critical need to
develop new reliable transformer insulation materials to meet these worldwide
challenges and resist the faster the deterioration effect.
As nanotechnology moves forward, nanofluids represent very promising fluids
for applications as transformer insulating liquids, from the viewpoint of their
excellent dielectric and thermal properties. where there are many research
papers stated that nanofluids provide better heat transfer and dielectric
properties than those of base liquids. Nevertheless, it is still a significant step to
move these fluids from the lab domain to high voltage power transformers.
This step still requires more comprehensive studies of dielectric performance of
nanofluids.
In this thesis, the dielectric performance of the transformer mineral oil (MO)
based on these nanofluids developed using conductive Zinc Oxide (ZnO),
semi-conductive Titanium Dioxide (TiO2) and Insulating Silicon Oxide (SiO2)
nanoparticles have been researched. Nanofluids (NFs) were prepared with
various concentrations ranging from 0.01 to 0.1 wt. %. The experiments have
been designed and performed on prepared samples for study of: AC breakdown
voltage, relative permittivity, DC conductivity, lightning impulse breakdown
voltage, acceleration voltage and breakdown time. Studies on the analysis of
the dissolved gas in the presence of nanoparticles under impulse faults have
also been put forward. The experimental results demonstrated improvement of
the AC breakdown strength with ~30 % enhancement for ZnO concentration of
0.06 wt. %, ~22 % for TiO2 concentration of 0.1 wt. % and~15% for SiO2 at
relatively low concentration of 0.01 wt. %. Positive effects on relative
permittivity and the opposite ones on DC conductivity have been obtained for tested nanofluids. For positive impulse voltage, the breakdown voltage of ZnO
nanofluid achieved ~9% enhancement under quasi-uniform field and ~32%
under non-uniform field. For negative impulse voltage, ZnO nanofluid
achieved slight worsening of breakdown voltage by ~9%. Potential
mechanisms behind nanoparticle influence on the dielectric properties of
nanofluid have been discussed and analyzed by using thermally stimulated
current technique.
Moreover, the deterioration behavior of nanofluid in combination with
cellulose insulation has been investigated and compared with that of mineral
oil-cellulose system. Accelerated thermal aging experiments of cellulosic
insulations impregnated in nanofluid and mineral oil were conducted under
laboratory conditions at 120ºC for aging period up to 20 days. Different aging
properties such as tensile strength, breakdown voltage, and dielectric
dissipation factor of impregnated paper/pressboards were monitored and
analyzed throughout the aging period. In addition, properties of oils related
deterioration rate such as breakdown voltage, acidity value, interfacial tension,
viscosity and color are thoroughly investigated in this research work to assess
the degree of deterioration of both nanofluid and mineral oil. It is found that
paper/pressboards aged in nanofluid possessed higher mechanical and dielectric
properties than those in mineral oil. Regarding aged oils, nanofluid exhibited
higher values of aging indicators such as interfacial tension, acidity and
viscosity than mineral oil. The breakdown voltage of nanofluid was superior to
that of mineral oil in the initial aging period, thereafter, showed a lesser
reduction tendency with increasing age.