Core cooling duct
Cores for large power transformers can be quite large items with the approximate dimension limits of up to 9 m in length, 4.5 m in height, and 1.4 m in diameter of the core leg. They are built from steel sheets and are very heavy; their mass can be in the order of up to 180 tons.
As soon as the transformer is energized, no-load losses are dissipated in the core, with their magnitude reaching up to 250 kW. This is a lot of energy in such volume and in order to prevent overheating, the core must be properly cooled. Since individual core sheets are insulated, surface cooling of larger cores is not efficient enough. Hence, they must have cooling channels, ensured by some kind of supports between core sheets.
Requirements for core cooling ducts
Large power transformers are liquid-cooled with mineral oil, synthetic or natural ester, or other types of dielectric liquids being most often used media. As some of the media are chemically extremely aggressive, the selection of materials for providing cooling channels is very limited.
Besides chemical stability which is required from the material for cooling channels, there is also a requirement for thermal stability.
The third requirement is of mechanical nature.