Abstract: The loading capability of an oil-immersed power transformer is closely related to oil temperatures and hot-spot temperatures (HST). In order to accurately calculate top-oil temperatures (TOT) and HSTs and thus help evaluate the loading capability of the oil-immersed transformer,a thermal-electrical analogy model for the trans- former is improved based on the principle of heat transfer and the circuit laws.In the improved model,first,the oil viscosity is introduced to redefine the nonlinear thermal conductance of the relative parts of the transformer,and the heat transfer between the transformer and the ambient environment is taken into account.Then,a hot-spot temperature node is introduced into the improved model,so that this model can really reflect the general process of the heat transfer in the transformer.Finally,a genetic algorithm (GA) is employed to globally optimize thermal parameters.By taking a 180000kVA oil-immersed power transformer as an example,the calculated results of the HSTs,TOTs and bottom-oil temperatures of the original and improved models are compared with the on-line data and the results calculated by the recommended empirical formulas in the transformer loading guide.It is found that the improved model is more accurate.Then,the improved model is used to calculate the relative daily loss of life,the maximum TOT and the maximum HST of a large oil-immersed power transformer in given working conditions,so as to evaluate the loading capability of the tested transformer and thus provide a reference for the load rating in- crease of the tested transformer.

Key words: transformer thermal model, hot-spot temperature, thermal-electrical analogy, relative loss of life, loading capability