Zonal thermal model of distribution transformer cooling

The present paper proposes an algebraic zonal model describing the cooling process, via internal oil and external air natural convection, of distribution transformers during a standardised heating test. The conception and adjustment of the presented algebraic zonal model rely on the oil flow and thermal results obtained by a more complex numerical differential model that has been verified and validated by the authors. The domain of the original differential model is divided into several interrelated control volumes or zones where mass and energy conservation laws are applied in conjunction with the necessary boundary conditions. The generated nonlinear algebraic equation system is solved, obtaining in the process the top oil temperature, the internal and external surface temperatures and the heat dissipation distribution. The results of the zonal model have been validated using the previous results from the differential model as well as experimental measurements from heating tests with three different transformers under various power loads. The model developed has proven to be an efficient tool in improving and optimising the thermal design of these devices.