Real-time thermal model for an oil-immersed, forced-air cooled transformer

A thermal model for predicting the real-time temperatures of an oil-immersed, forced-air cooled transformer is presented. The model consists of a set of three differential equations that are formulated into a computer program. Equations that can be solved for the average real-time temperatures of the core and coil assembly, oil, and tank are developed. Expressions for the convection heat transfer coefficients at all internal oil-solid interfaces, equations for calculation of the incident solar radiation, and correlations for the convective cooling rate from the external tank and cooling tubes are discussed in detail. The thermal model can predict the real-time transformer temperatures for any variation in electrical loading and for any type of ambient thermal environment. Since this model is more detailed than the ANSI/NEMA equations, the results can be expected to predict the transformer temperatures more accurately. The model yielded conservative estimates of the component temperatures when compared with temperatures measured on a distribution transformer. While this test does not verify the accuracy of the model for all types of transformers under all types of loading conditions, it does give initial credibility to the accuracy of the equations and their applicability to transformer heating