ABSTRACT This work investigates and evaluates local liquid dielectric for insulation of power transformers. Considering the importance of transformer insulation the investigation is borne out of the need to develop the capacity for local production of transformer dielectric. Natural esters of vegetables origin have been found to have the suitable dielectric properties for them to be used as replacement dielectric fluids for mineral oil. However vegetable oil has poor resistance to oxidation, hence the need for inhibiting the vegetable oil for use as insulation fluid. In this work inhibited locally extracted soybean oil (soybean oil treated with antioxidant) is subjected to accelerated ageing in order to determine its suitability as dielectric fluid. The properties of the soybean oil is investigated against standard and compared with those of mineral oil. The investigated soybean meets acceptability specifications since the measured flash point is 2570C, the pour point is -150C, the acidity is low at 0.0027mgkoH/mg, its dissipation factor after ageing

is 12.11 x 10-3 (which is within recommended units) and showed a high dielectric breakdown voltage (59.08kV). Results obtained showed that the inhabited soybean oil has properties comparable to those of mineral oil and is suitable for use as transformer dielectric fluid.

CHAPTER ONE INTRODUCTION 1.0. Background of the Study Electrical faults that occur in power transfers accounts for over fifty percent of transformer failure’s expenditures . Fifty percent of transformer failure’s expenditure is caused by insulation, dielectric, and oil-related faults [1]. The need for better dielectrics and transformer oils for insulation is unequivocal. In a liquid-filter transformer, the insulator liquid plays an important function by providing both the electrical insulation and the means of transforming the thermal losses to the cooling system. Insulating oil in a transformer must ensure the transfer of heat. This function is realized both by thermal conductivity and convection [2]. Nowadays, transformers can be filled with three basic types of insulating liquids: (i) mineral oils, (ii) synthetic oils or (iii) natural esters [3]. The use of each type is fortified by the application. However, in the face of increasing demand for the use of environmentally friendly products in the industry, more and more companies are working towards developing the use of esters and specifically natural esters for use in the majority of their products. Transformers have been filled with mineral oil for more than one century. This type of oil is a petroleum based product, essentially composed of hydrogen and carbon atoms. Carbon and hydrogen are assembled in different structures: napthenic (CnH2n), paraffinic (CnH2n+2) and aromatic (CnH2n-6)[3]. The distribution of carbons in napthenic and paraffinic structures define

the type of mineral oil. This distribution is controlled by the crude oil and the refining processes used. Because of its wide availability, good properties and low cost, mineral oil is the fluid most used in the electric power transformer industry. New mineral oils have to be in accordance with the IEC 60296 or ASTM D3487.

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