I. Introduction
Electrical transformer reliability and lifespan strongly depend on cooling conditions. In a typical case, the windings and core are cooled either in transformer oil or, using a fan, by the air circulating inside the tank. However, there are also environments in which natural air cooling is the only option. For example, in underground coal mines, ANAN devices (natural air cooling both inside and outside a tank) are placed in hermetic tanks. For such special units, the desired electrical device parameters are more important than a cost reduction policy. Furthermore, the tank space is often limited because of a small size of underground corridors. In addition, electricity demand constantly grows in those applications. As a result, units with larger power outputs are needed for tanks that have the same volume. Therefore, power losses and hot-spot temperatures should be minimized in the transformers used in these applications.