An experimental approach to the cooling of transformer coils by natural convection

The paper describes the experimental work and results of a study of the heat-transfer properties of cooling ducts in transformer coils under natural convection. The data for layer windings of various vertical duct sizes and coil heights are generalized in terms of empirical copper-to-oil temperature gradients. Consideration of the vertical coil leads to a method of treating an assembly of disc coils individually, and it is shown that the individual disc-coil variables are represented conveniently by a correlation type of formula. Finally, it is indicated how the data may be used to predict the hot-spot temperature of a transformer in service.