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Abstract: This paper extracts a comprehensive reliability model for a transformer with oil natural air forced (ONAF) cooling system, which is the most common type of power transformers. The transformer is first classified into three subsystems and a reliability model is thereafter developed corresponding to each subsystem. Markov process and frequency/duration approaches are applied to analyze the extracted models and achieve their equalized representations. Merging subsystem models leads to the transformer entire reliability model consisting 17 states. The model is then reduced to an 11-state one which is more computationally manageable. Numerical analyses and sensitivity studies are conducted to demonstrate the performance of the proposed model and the results are thoroughly discussed. [Copyright &y& Elsevier]

• Evaluate the effect of different insulator profiles on Flashover Voltage (FOV). • Analyze the risk of the insulators utilization under different ambient conditions. • Effect of different pollutions and humidities variations on the reliability of the aged Silicone Rubber (SiR) insulators. • Reliability of the aged Silicone Rubber (SiR) insulators has been calculated. In this paper, the effect of different pollution and humidity variations on the reliability of aged Silicone Rubber (SiR) insulators are investigated. Firstly, four different types of 20 kV SiR insulators have been selected to evaluate the effect of different insulator profiles on Flashover Voltage (FOV). Then artificially accelerated aging tests with 1000 h duration have been conducted for the aging of insulators. Numerous FOV tests have been carried out on the aged SiR insulators under different humidity and pollutions. Next, the risk of insulators utilization under different ambient conditions have been calculated by using the experimental test's data. Finally, the reliability of insulators under different conditions have been calculated and a new mathematical model for the reliability in different humidity has been proposed. The effect of SiR insulator profiles on FOV has been studied and the results have been shown that lower FOV is related to insulators with thicker profiles as well. [ABSTRACT FROM AUTHOR]