Thermal Modeling of Power Transformers with Unbalanced Loading

Abstract

The weakening of transformer insulation is linked to the thermal stress, which is mainly determined by the loading conditions. Most faults cause changes in the thermal behavior of the transformer. Thus, a comprehensive thermal model and accurate estimation of hot spot temperatures are necessary for transformer condition assessment. A thermal-electrical analogous model based on the effects of solar irradiation and unbalanced transformer loading in each phase to estimate transformer top oil and hot spot temperatures is presented in this paper. Other variables considered in the model are the effects of the varying environment temperatures and the nonlinear thermal resistance of the transformer oil. The model is validated by using data collected from an in-service mineral oil immersed transformer. The simulated results of the proposed thermal models are in close agreement with the on-site measured values, with good accuracy especially in the case of hot spot temperatures. This indicates that the proposed thermal model is satisfactorily suitable for transformer top oil temperature and hot spot temperature calculation