Thermal rating of transformers. Application of the multiflow principle to naturally cooled transformers

The thermal design of naturally cooled transformers is inherently difficult, because the designer has no direct control over the oil flow, which is determined by convective or thermosiphon circulation. Compared with the positive flow possible, both in amount and direction, with pumps, the circulation is weak and nebulous even under academically simple conditions. In the application of the multiflow method to forced-oil-cooled transformers,1, 2 it was found that, in general, the hottest-spot temperature was well below the assumed standard value. There was therefore margin for considerable economy in utilising this differential.7 The present paper shows that, for naturally cooled transformers, the balance may lie in either direction. The smaller transformers will, in general, be underrated (as for forced-oil-cooled units), and the larger ones overheated so that some reduction in rating or improvement in cooling will then be required for same hottest-spot temperature.