The impulse design of transformer oil-cellulose structures

Transformer oil/cellulose structures are often designed based on a cumulative stress criterion derived from experimental tests at power frequency. However, such structures must also meet stringent impulse requirements defined by a Basic Insulation Level (BIL). The industry has tried to establish an equivalence factor to permit power frequency cumulative stress methods to be used to estimate impulse withstand strength. Since the mechanisms of failure differ substantially under surge conditions. 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 impulse design under hybrid situations (i.e. failure paths involving both creep and bulk liquid).