Because of their availability and low cost, mineral oils have been widely used for a long time in power transformers to allow their insulation and cooling. However, their low fire safety and low biodegradability potential have made it necessary to look for other insulating liquids as an alternative to this mineral oil used in high voltage electrical equipment. This work presents an experimental study to compare between the physicochemical characteristics of mineral oil, olive oil, sunflower oil and different oil mixtures. In order to determine mainly the breakdown voltage and the electrical field intensity of electro-convection, oils insulating should be mixed in precise amounts. All tests have been realized in accordance with the standard test procedures: IEC 60156, IEC 60245 and IEC 61125. The obtained results of testing new as well as aged oil samples concerning the resistivity, the dissipation factor tgδ, the conductivity, the viscosity, the breakdown voltage, the increase of the water content and the flexibility of the oil to the movement in an electrical field, show that a half mixture of naphthenic oil and olive oil could be a potential liquid for the insulation of electrical devices and especially power transformers mounted in areas which have a non-cold climate.

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