Estimation of Hot-Spot Heating in OIP Transformer Bushings Due to Geomagnetically Induced Current

This paper studies the temperature distribution in the transformer bushing in the presence of Geomagnetically Induced Current (GIC). The thermal stress can significantly affect the transformer bushing reliability and lifespan. However, the existing research works have focused on thermal modeling of transformer bushing due to loading, and the impact of GIC on this valuable apparatus has not been investigated yet. In this paper, the hot-spot temperature (HST) in an oil-impregnated paper (OIP) bushing is studied during the GIC. The harmonic currents at various GIC levels are obtained from the EMTP-RV time-domain simulations using a detailed topological transformer model. Based on a frequency-dependent model of the bushing resistance, the total power loss is calculated by taking into account the GIC, load current, and harmonic currents. Subsequently, the temperature distribution in the modeled bushing is obtained from the finite-element method (FEM), considering all thermal mechanisms, including fluid flow, internal convection, and thermal conduction. In addition, a Thermal Equivalent Circuit (TEC) is developed for the bushing that replicates the FEM results. The simulation results reveal that the bushing HST may exceeds the recommended permissible temperature of the IEEE bushing standard. This situation may result in bushing insulation deterioration and reduction of the bushing lifespan.