Transformer life has a considerable economic impact on the operation of an electrical network. The most important parameter in transformers life expectancy is the insulation temperature value. The aim of this paper is to present a new and more accurate top-oil thermal model based on heat transfer theory, application of the lumped capacitance method, the thermal-electrical analogy and a new definition of nonlinear thermal resistances of oil immersed transformers. The model presented in this paper takes into account oil viscosity changes and loss variation with temperature. The changes in transformer time constants due to changes in he oil viscosity are also accounted for in the thermal model. The models are validated using experimental results, which have been obtained from a series of temperature rise tests performed on a range of oil immersed distribution transformers. The experimental results verify the model proposed in this study for oil immersed transformers.
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