Milad Akbari

Temperature distribution in the power transformers is investigated in this study under Geomagnetically Induced Current (GIC) conditions. Thermal stress can significantly reduce the insulation life, and in the case of excessive hot spot temperature (HST), catastrophic failure of transformers is likely, as happened in the past Geomagnetic Disturbance (GMD) events. Although a few reports emphasize the impact of GIC on the local heating within the transformers, especially in the structural parts, the effect of GIC on the thermal condition of transformers has not been investigated profoundly. This article studies the power transformer HST during the GIC. Since finding stray losses under GIC conditions is challenging, a hybrid approach, including a topological transformer model and 3D finite element method (FEM), is implemented. The detailed topological transformer model is utilized in the EMTP time-domain simulations to determine the harmonic currents at different GIC levels. Additionally, FEM is employed to calculate the temperature distribution within the transformer. The simulation results reveal that the structural parts are saturated with low GIC magnitudes, resulting in high stray losses and local hot spot heating in those areas. Furthermore, the tank can reach high temperatures at mid-GIC levels. These results clearly show that the transformer structural parts are highly vulnerable under severe GIC situations.

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.

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.

Zero sequence characteristic and parameters play a vital role in some transformer transients involving unbalance or saturation conditions such as inrush, ferroresonance, and Geomagnetically Induced Current (GIC). An important parameter of the zero-sequence characteristic is the inductance of the air gap between the core and the tank. Based on a Finite-Element Method (FEM), this paper determines the air gap inductance, considering fringing flux effects. The comparison of the simulation results with an analytical method shows a good agreement for small air gaps but reveals a significant difference for relatively large air gaps. Thus, an inductance enhancement factor (IEF) is introduced to correct the analytical results and facilitate the transformer modeling parameter estimation from the design data. Furthermore, the significance of the accurate calculation of the air-gap inductance is demonstrated in the time-domain simulation of different saturation levels of the transformer core, when the transformer is subjected to GIC.