Validated transient heat-transfer model for underground transformer in rectangular vault

A new thermal model for underground transformers is proposed in this paper. The model takes the following important characteristics of the transformer installation into account: rectangular shapes; coil and core arrangement; orientation-based convection models for air (vertical, horizontal-upward, and horizontal-downward heat flows); and turbulent or laminar flow regime. The resulting coupled set of differential and algebraic nonlinear equations is solved simultaneously, providing a robust and fast solution that can help design transformers. The model has been validated against three transformers with different dimensions installed in different vaults with onsite measurements. The average absolute difference between the simulated and measured temperatures over several months is typically less than 4°C. A parameter sensitivity study shows the critical importance of the proper estimation of the full-load heat loss and the ambient soil temperature.