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This paper deals with an experimental study of the influence of conductive (Fe 3 O 4 ) and insulating (Al 2 O 3 ) nanoparticles at various concentrations on the dielectric strength of transformer mineral oil. The method of preparation and characterization of these nanofluids (NFs) through the measurements of zeta potential, the real and imaginary parts of dielectric permittivity as well as the concentration and size of nanoparticles using scanning electron microscope images of nanoparticles powders and energy dispersive x-ray spectroscopy analysis are presented. Experimental findings reveal that these two types of nanoparticles materials significantly improve AC breakdown voltage and the magnitude of this enhancement depends on the nanoparticle concentration, and the size and nature (material) of nanoparticles. For a given type of nanoparticle, the effect is more marked with the smallest nanoparticles. The conductive nanoparticles offer higher enhancement of dielectric strength compared with insulating nanoparticle based nanofluids. With Fe 3 O 4 , the breakdown voltage (BDV) can exceed twice that of mineral oil and it increases by more than 76% with Al 2 O 3 . The physicochemical mechanisms implicated in this improvement are discussed. Energies 2018, 11, 3505 2 of 13This cooling property (heat transfer) was at the origin of NFs' development. Nowadays, NFs are used in several fields, like electronics (chips, electronic circuitry components); transportation (the cooling systems of heavy power machines and heat generation parts of vehicles); heating buildings and reducing pollution; nuclear cooling systems; space and defense (space stations and aircrafts); and solar absorption for heat-transfer performance. Indeed, they have preferable better thermal characteristics (thermal diffusivity, thermal conductivity, convective coefficient of heat transfer) than the base fluids. These strongly depend on properties and the volume fraction of the added nanomaterial [7,8]. Furthermore, for a given particles volume, the contact area of the solid-liquid surface between nano-size particles and the suspension fluid is greater than that for micro-size particles. Therefore, the shape and size of particles has a clear impact on heat transfer and thermal conductivity characteristics [9][10][11].The volume fraction of particles (concentration), their shape and size, and the surface contact area between particles and liquid are major parameters that influence not only the thermal properties as indicated above, but also have a very great influence on the dielectric properties of composite materials.

Abstract:The problem of voltage collapse in power systems due to increased loads can be solved by adding renewable energy sources like wind and photovoltaic (PV) to some bus-bars. This option can reduce the cost of the generated energy and increase the system efficiency and reliability. In this paper, a modified smart technique using particle swarm optimization (PSO) has been introduced to select the hourly optimal load flow with renewable distributed generation (DG) integration under different operating conditions in the 30-bus IEEE system. Solar PV and wind power plants have been introduced to selected buses to evaluate theirs benefits as DG. Different solar radiation and wind speeds for the Dammam site in Saudi Arabia have been used as an example to study the feasibility of renewable energy integration and its effect on power system operation. Sensitivity analysis to the load and the other input data has been carried out to predict the sensitivity of the results to any deviation in the input data of the system. The obtained results from the proposed system prove that using of renewable energy sources as a DG reduces the generation and operation cost of the overall power system.