Anomalous Breakdown Phenomena in PMMA under Combined Direct and Impulse Stresses

The results of experimental investigations on the anomalous breakdown characteristics of polymethyacrylate (PMMA) immersed in transformer oil are reported in this paper. The insulation was subjected to a combination of direct and impulse voltages. The anomalous breakdown voltage was found to decrease when the polarity of superimposed impulse voltage was opposite to that of the direct voltage and to increase when the polarities of both are the same. The polarity of the resultant appled voltage was also found to affect the length of the breakdown path. An attempt is made to explain such anomalous behavior with the help of space charge accumulation in modern synthetic insulating materials. The variation of direct voltage prestress time on the anomalous breakdown voltage are also discussed.