Gas Bubble Studies and Impulse Tests on Distribution Transformers During Loading Above Nameplate Rating

The paper describes the results of studies on gas bubble evolution in model coils, and impulse breakdown tests on full-size distribution transformers during loading above nameplate rating. Radio noise measurements and visual observation were used to determine thermally-induced gas bubble formation in model coils during two different one-hour load cycles which resulted in final hot spot temperatures of 145 and 2250C, respectively. Impulse tests were performed on full-size transformers, rated 25 kVA, 95 kV BIL, of core-form and shell-form design as well as of a modified shell-form construction. The full-wave impulse surges were applied while maintaining a severe overload of 260 to 285% of rated current for one hour. This resulted in a hot spot temperature excursion from 45°C to 225°C which, according to model tests, resulted in heavy gas bubble activity. Tests were made on unaged and aged model coils and transformers to determine the effect of insulation degradation on bubble evolution and on the impulse strength during loading. The results are discussed in comparison with the results of breakdown tests under no-load conditions. The work described was sponsored by the Electric Power Research Institute.