Transformer Thermal Assessment under Geomagnetically Induced Current Conditions

Power transformers are one of the most critical and expensive pieces of equipment in power systems. The widespread use of the transformer in power grids and its high cost make the lifetime and reliability of this apparatus highly important. Although some factors affect the reliability of transformers, thermal and electrical stresses are the main reasons for transformer failure. As a result, transformer thermal modeling is essential in the design and operation stage to represent the thermal behavior of transformers during normal operation or transient phenomena. However, the multi-physics behavior of transformers and the nonlinear and frequency-dependent parameters make this modeling a challenging task. This thesis aims to develop a more accurate transformer model for representing the thermal behavior of transformers, especially during transient phenomena such as Geomagnetically Induced Current (GIC). To fulfill this goal, it is necessary to perform several tasks in different fields, such as geometry and material modeling, electromagnetic studies, and investigation into computational fluid dynamic (CFD) analysis of transformers. First, the GIC phenomenon and its impact on the transformer are briefly explained. Then, a comprehensive literature review of existing transformer thermal models is performed to find their drawbacks. A 3-phase, 3-leg transformer is then subjected to an electromagnetic-thermal study in both normal and GIC conditions. It is shown that the structural parts, including the tank, clamps, and tank shunts, are saturated with a small amount of GIC. However, the transformer core becomes saturated with larger currents, resulting in additional stray losses in the structural parts. The findings show that the most vulnerable part is the tank, as the hot spot temperature (HST) of the tank approaches 372.14 0C, which is double the permissible limit, under 66.6A GIC per phase. Finally, a new approach is proposed to determine the HST of OIP bushing based on the FEM-modified thermal equivalent circuit (TEC) model. The proposed model can accurately estimate the HST of the bushings under normal and GIC conditions. Furthermore, a detailed thermal analysis is performed to investigate the impact of different parameters such as load, ambient temperature, and top oil temperature on the thermal performance of bushings.