Electricity is a deterministic factor in the development of any nation; the same is the case for Nigeria which constitutes a more complex network of power transmission. Hence, there must be a keen assessment approach to ensure better services. The main aim of this thesis is to qualitatively analyze the practical fault statistics and reliability of power transformer systems in Nigeria (a case study of the Transmission Company of Nigeria, Abuja sub-region) based on Fault Tree Analysis and

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... cal Package for Social Sciences, and to demonstrate how this new practical study technique can transform the Nigerian electricity industry through improving the power transformers failures assessment, and hence, enhancing the reliability. The thesis used the qualitative analysis method in the process of gathering data needed to study the problem. The justification for this research work is based on the fact that the techniques of power transformer diagnostic and monitoring have been expanded over the years from inception of this electrical machine in power system; and numerous apparatus abound for the assessment of the state of electrical power transformer, nonetheless there remains a craving necessity for improved diagnostic and monitoring apparatus to assess power transformer state. The causes for the collapses of transformers were practically investigated by the qualitative approach while considering all of the technical and environmental variables, and the assessment yielded the failure statistics of 1196 auto power transformers failures within the period of 6 year in the Transmission Company of Nigeria, Abuja Sub-Region as case study, as well as minimum cut-sets and qualitative importance. The conclusion revealed the main strength and the weakness points of power transformer were core sub-system and coil sub-system, respectively. Summarily, this thesis contributed that the qualitative fault tree technique with practical statistical tool is effortless approach to apply on the power transformer structure, and recommend same to the Electricity Services Providers as a choice technique poised to improving the reliability issue assessment of power transformer to guarantee the functionality and quality of electrical supply of the Nigerian Electricity supply Industry.