After several operating years of power transformers, the oil is aged resulting in deterioration in oil properties. This deterioration is attributed to the effect of aging byproducts represented by acidity and moisture content. From this view point, this study aims to mitigate the impact of aging byproducts and enhance the aged oil properties with the aid of nanofillers to be reused in the transformer. To simulate actual operating period of the transformer, accelerated aging test is carried out in the laboratory. This test mainly depends on heating the oil in an oven at 120 °C for different number of days. Through these conditions, one day in the oven is considered to simulate one year in the field. Aged oil-based nanofluids are obtained through direct mixing and dispersion of nanoparticles with the aged oil. Titanium oxide is selected as the filled nanoparticles through this study. After the preparation, the samples of aged oil and aged oil-based nanofluids are tested. Different concentrations of nanoparticles are considered. The breakdown voltage (BDV) for the prepared nanofluids is measured and compared to that of pure aged oil. Also, dielectric constant, dielectric dissipation factor (DDF) and conductivity have been measured. Nanofluids exhibited significant enhancement regarding BDV. Also, DDF and conductivity enhanced to some extent. With the aid of acidity, moisture content and particle size measurements, the physical mechanisms behind the obtained enhancement have been discussed considering the effect of aging, the role of nanofiller and the effect of weight fraction. It is found that nanoparticles alleviate the effect of moisture through adsorbing moisture species and forming hydroxyl groups around the surface of nanoparticles.

Pattern synthesis is widely used in many radar and communication systems and received great interest. So, this paper proposes a new beamforming strategy based on a hybrid combination between grey wolf optimizer (GWO) with L2-norm called proposed GWO. This approach is applied to synthesized uniform linear arrays (ULA), Chebyshav arrays, and shaped pattern arrays. Moreover, it is utilized for side lobe level (SLL) and size reduction of antenna elements. In this strategy, the GWO is utilized to optimize the element spacing to adjust the half-power beam-width (HPBW) to save it the same as desired pattern. Furthermore, the excitations of the antenna elements are optimized via the L2-norm minimization problem. The proposed GWO has low complexity (fewer iterations and computing time) compared to other algorithms. In addition, it has a very accurate approximation of the original radiation pattern. As well, the computer simulation technology (CST) microwave package is utilized to achieve the practical validation of the proposed methodologies. As an application of the proposed GWO, it is employed to create a proposed hybrid beamforming (PHB) structure for Multi-input Multi-output (MIMO) systems. Consequently, the BS transmitting antennas are synthesized for gain maximization while utilizing the current amount of antenna elements. This results in considerable savings in antenna components and associated radio frequency (RF) chains which reduces system complexity. Furthermore, array gain maximization will increase the received signal-to-noise ratio (SNR). In addition, the SLL reduction scenario will decrease the interference from undesired users which in turn will also increase SNR. Hence, the performance of the system in terms of spectral efficiency (SE) and power utilization will be improved.

When mineral oil is exposed to aging processes during their operation, oil properties significantly deteriorate. This necessitates reclamation of the oil or replacement with a new one. Both solutions are not preferred due to their high cost. From this viewpoint, this study aims to enhance aged oil properties by using nanofillers to be suitable for reusing in power transformers. Accelerated aging test is carried out through heating oil at 120 °C in an oven for different periods of days. Under this condition, accelerated aging for one day is considered to be equivalent to aging for one year in the field. Nanofluids are prepared by direct mixing of nanoparticles with degraded mineral oil. The considered nanoparticles in this study are titanium oxide nanoparticles. The breakdown voltage for prepared nanofluids are measured and compared to that of pure aged oil. Also, dielectric constant and dielectric dissipation factor have been measured. Nanofluids exhibited significant enhancement either in breakdown voltage or in dielectric dissipation factor. The physical mechanisms behind the obtained enhancement have been discussed considering the effect of aging and the role of nanofiller. It is found that nanoparticles alleviate the effect of moisture through adsorbing moisture species and forming hydroxyl groups around the surface of nanoparticles.

Magnesium oxide (MgO) nanoparticles are used for nanofluid preparation in an effort to investigate their dielectric response and thermal properties. The matrix oil for the prepared samples is natural ester oil. Lightning impulse voltage (1.2/50 μs) is used to investigate the electrical performance of the different concentration samples. In addition, broadband dielectric spectroscopy is used for the analysis of the dielectric constant and electrical conductivity through a temperature range of 30 °C -90 °C and a frequency spectrum of 0.1 Hz - 1MHz. Thermal properties are studied by means of Laser Flash analysis and Differential Scanning Calorimetry. Thermal conductivity is calculated and the understudied results are evaluated. Results reveal an improvement in different properties for the prepared nanofluid samples, including an enhancement by 15.15 % and 12.63 % of the lightning impulse breakdown voltage and the thermal conductivity, respectively.

Nowadays, nanoparticles are used in insulating oils to enhance their dielectric and thermal properties significantly. On the other hand, natural ester oil is considered one of the most environmentally friendly types of insulating oil. In this study, magnesium oxide nanoparticles with their excellent physical and thermal properties, are used to prepare natural ester oil-based nanofluids. Their electrical performance is investigated by subjecting samples of different nanoparticles concentration to lightning impulse voltage (1.2/50 µs) by following the step-up method. For the generation of the pulses, a one-stage impulse voltage generator was used, while measurements from 20 pulses per voltage level were applied to a needle-to-plane gap so as to estimate the relevant breakdown probabilities. Furthermore, relative permittivity and electrical conductivity are measured for a range of frequencies (0.1 Hz - IMHz) and a range of temperatures (30 °C – 90 °C) by means of broadband dielectric spectroscopy. The thermal properties are studied based on thermal diffusivity measurements by LFA (Laser Flash analysis) for different concentration values and a wide range of temperatures (30 °C – 90 °C). Results reveal the improved performance regarding the dielectric and thermal properties for the prepared natural ester oil-based nanofluids. A 15.15 % improvement in lightning impulse breakdown voltage, more specifically of the U50% voltage, is observed. Additionally, results show a 12.63 % improvement for thermal diffusivity.

Lately, gas-to-liquid (GTL) oil usage has a great trend as cooling and dielectric medium in power transformers due to their good properties, represented by its purity, chemical uniformity, and better performance than conventional oils. By the way, the study of GTL oil effect in comparison to conventional mineral oil (MO) on the electrical properties of the insulation paper under thermal aging is presented in this paper. Thermal aging samples were obtained based on an accelerated thermal aging test in the laboratory, that simulates the thermal stress in the service. Two different aging durations (i.e., 3 and 6 days) are applied to the two different oil types. Pure oils and aged oil samples were tested for dielectric losses and dielectric constant for a range of frequencies from 20 Hz to 2 MHz. In addition, insulation papers were impregnated in the same samples to check their effect by thermal aging. The results indicated that aging has a lower impact on the properties of GTL oil and GTL oil-paper insulation under aging conditions compared to that of MO.