Method of calculating temperature rise in self cooled transformers

The calculation of temperature rise in self-cooled transformers is one of the most important calculations to determine longevity of a transformer. It is also one of the toughest concepts for many transformer Engineers to understand. The elusiveness of the concept originates because transformer designers are predominantly Electrical Engineers and most of the skills needed to evaluate temperature rise are mechanical in nature. This paper will discuss the major modes of thermal transfer in self-cooled coils. Models will be developed and procedures explained to apply these techniques to numerous core and coil geometries. The models in this paper can be applied to a broad size range of units from small PC board mount all the way in excess of 5000 KVA. The author has used these methods to predict temperature rise in units as small as a 1/2" cube, as large in diameter as 84" and as long as 48".