| Abstract |
In order to expand and upgrade the electrical power grid, the performance
improvement of power transformers is indispensable. Oil-filled transformers are
one of the popular types of power transformers. Transformer oil (Base insulating
oil) provides both thermal cooling and electrical insulation functions. So,
improving its thermal and dielectric properties affects positively the whole
performance of power transformer. Recently, nanotechnology was used as an
effective science in the field of transformer oil development to enhance its
thermal and dielectric properties under the name of nanofluids. In this thesis,
barium titanate nanoparticles were inserted into the base insulating oil by a
concentration of 0.005, 0.01, and 0.02 gram per liter as individual nanofluid
samples. Some measurements were done for these samples. These measurements
are heat transfer coefficient, breakdown voltage, relative permittivity,
conductivity, and dielectric dissipation factor. The dispersion of barium titanate
nanoparticles into the base insulating oil enhances the thermal properties by more
than 20% for all individual nanofluid samples but some dielectric properties were
negatively affected by this dispersion. To overcome this problem of dielectric
properties degradation, other three hybrid nanofluid samples were prepared using
three different types of metal oxide nanoparticles; titania, alumina and silica.
These samples were prepared by adding a concentration 0.01 gram per liter of
each metal oxide nanoparticles together with 0.005 gram per liter of barium
titanate nanoparticles into the base insulating oil. Same measurements were done
for these hybrid samples as same as individual samples. The thermal and
dielectric properties of hybrid nanofluid samples were examined to study the
behavior of nanoparticles hybridization on transformer oil properties. Due to the
different surface charging of barium titanate and titania nanoparticles, the best
enhancement of breakdown voltage and heat transfer coefficient was achieved. Dynamic light scattering technique was used to evaluate the particle size
distribution of nanoparticles into the hybrid samples and to clarify the
corresponding physical interpretation behind the obtained enhancement. On the
other hand, thermal model of 1 MVA, 50 Hz, 22/0.4 kV stepping down immersed
oil distribution transformer was introduced using COMSOL Multiphysics
Software. The COMSOL Multiphysics Software is commercial software that
resolves the heat transfer in fluids based on the finite element method. The liquid
materials that used for the proposed model were the base insulating oil and the
best prepared nanofluids sample. Internal heat energy, temperature gradient, and
temperature distribution were evaluated using COMSOL Multiphysics Software.
The obtained results introduced that; the thermal stress on the base insulating oil
inside transformer can be reduced by about 10 oC due to the presence of the
hybridization of barium titanate and titania nanoparticles. Based on these results,
the filling of distribution transformer by HNFS/TNP oil instead of base insulating
oil doubles the lifetime of the oil in the service and improves the performance of
the transformer.
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