Transformer Bushing Thermal Model for Calculation of Hot-Spot Temperature Considering Oil Flow Dynamics

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Abstract
Thermal stress plays a prominent role in reliability of bushings and contributes to reliability of power transformers, especially during overload conditions. Thus, exploring the temperature distribution in the bushings is essential. This paper proposes a new thermal model to estimate the hot spot temperature (HST) of oil impregnated paper (OIP) bushings based on modified thermal-electrical analogy model. The proposed model is developed based on finite element method (FEM) to accurately model all fluid flow and internal convection as well as the thermal conduction mechanism. To do so, convection thermal resistances are defined, and its non-linear characteristic is calculated for different overloading condition. The proposed approach is applied on a 245kV, 800A OIP bushing to analyze not only the normal loading condition but also short-term overloading beyond the rated current. In addition, the oil flow condition with different load currents and transformer top oil temperatures (TTOT) are investigated. The results show a consistent temperature rise with the typical test conditions with no identified problematic situation. However, during overloading, the temperature rise exceeds beyond the permissible limits recommended by IEEE C57.19. Hence, the overloading of transformers may deteriorate the bushing insulation and reduce its lifetime.

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