Assessment of the Dynamic Overload Capability of Distribution Transformers

In this paper, a dynamic thermal model for calculation of the top oil temperature in distribution transformer is presented. In the modelling process, the heat transfer phenomena from the transformer winding to oil in the radiator, from radiator to the transformer station and from the transformer station to the ambient are modelled. All electrical losses are considered as an input of equally distributed heat and assumed to be the sum of the short circuit and no load losses. Furthermore, the material properties and the temperature gradients in oil and air are taken into account. To determine the thermal behaviour of a specific transformer, the model uses several empiric factors which are estimated based on measurements during the normal operation of the transformer. In order to validate the…