On the criterion for the design of oil-cellulose structures

Large oil/cellulose structures, such as transformers, are designed based on a cumulative stress criterion derived from experimental tests at power frequency. However, such structures are often limited by impulse failure for which the industry has tried to establish an equivalence factor to permit power frequency cumulative stress methods to be used. Since the mechanisms of failure differ, there would seem no good reason to suppose that a universal equivalence factor is appropriate. Tests are reported using a 2.3 MV generator to document impulse failure of a number of bulk, creep and hybrid structures to establish the nature of this relationship through statistical comparisons with the established 50/60 Hz methods. Factors varied from 1.94 to 3.34, depending on the configuration. The methodology is described and the results discussed in the context of the design of oil-cellulose structures, having regard to complicating factors such as waveshape and electrode covering. The study permits some speculation about failure under mixed situations (i.e. failure paths involving both creep and bulk liquid).