The Overload Assessment of Power Transformers by Thermal Monitoring

Abstract:
This paper describes an approach for determination of loading of oil-cooled transformers depending on their ambient temperatures. For this purpose, a simplistic, empirically based model was developed that can predict changes of the top oil temperature with high accuracy. According to the ideas of the so-called One-Body-Model, the entire transformer is considered as a homogeneous tempered body with a specific heat capacity. In contrast to previous approaches, the heat exchange with the environment is mapped through a complex function, which depends primarily on the temperature difference between the transformer and the surrounding environment. Furthermore, time-dependent parameters, such as the applied load, the current temperature level and the associated temperature gradients as well as temperature-dependent material properties that are influencing factors for the determination of the heat exchanges are considered. Based on the obtained results from each examined transformer, each transformer’s linear function characteristic can be derived, which indicates the maximum overload capacity in relation to ambient temperature.