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The hot spot temperature (HST) plays a most important role in the insulation life of the transformer. Ambient temperature and environmental variable factors involved in the top oil temperature (TOT) computations in all transformer thermal models affects insulation lifetime either directly or indirectly. The importance of the ambient temperature in transformer's insulation life, a new semi-physically-based model for the estimation of TOT in transformers has been proposed in this paper. The winding hot-spot temperature can be calculated as function of the TOT that can be estimated by using the ambient temperature, wind velocity and solar heat radiation effect and transformer loading measured data. The estimated TOT is compared with measured data of a distribution transformer in operation. The proposed model has been validated using real data gathered from a 100 MVA power transformer. For a semi-physically-based model to be acceptable, it must have the qualities of: adequacy, accuracy and consistency. We assess model adequacy using the scale: prediction R², and plot of residuals against fitted values. To assess model consistency, we use: variance inflation factor (VIF) (which measure multicollinearity), condition number. To assess model accuracy we use mean square error, maximum and minimum error values of semi-physically-based model parameters to the existing model parameters.

There is an important significance for the design and the life time evaluation of the dry-type transformer based on the hot-spot temperature. At present the methods of obtaining the transformer's hot-spot temperature are as follows: direct measurement and numerical calculation. Direct measurement can obtain winding hot spot temperature through the optical fiber temperature sensor or thermocouple; The numerical calculation mostly uses the finite element or finite difference method, this paper establishes the inversed-heat transfer calculation model of high and low voltage windings. Using the high-precision infrared sensor to acquire the temperature of the high voltage winding. Through calculation the temperature distribution of the low voltage winding has been obtained and the hottest spot temperature of the windings is very closed to the result obtained by the IEEE model in a certain range. It provides a new method for the acquisition of the hottest spot temperature of the dry-type transformer.