Interpretation of Hot Spot Factor for Transformers in OD Cooling Modes

The hot-spot factor (H) is a crucial component of the thermal diagram in IEC 60076-2 to derive the hot-spot temperature from the test data. In this paper, H is expressed as the sum of two separable components, one relating to convection in the fluid domain and the other relating to conduction in the solid domain. Dimensional analyses on fluid flow and heat transfer show that both components are functions of dimensionless winding geometry, loss distribution, Re and Pr. The relationship between H and Re and Pr for a fixed winding geometry with uniform loss distribution is obtained by conducting computational fluid dynamics (CFD) parametric sweeps and multilayer least-square-based correlation. The correlation obtained is verified by the consistency between H from new CFD simulations and those from the correlation. For nonuniform loss distribution, when the hotspot is at the location of the highest power loss, H is in a linear relationship with Q and this linear relationship is coupled with a nonlinear relationship between H and Re and Pr.