Anomalous breakdown in PMMA under switching surges superimposed on direct voltage

The results of experimental investigations on anomalous breakdown in polymethylmethacrylate (PMMA) immersed in transformer oil and subjected to switching surge voltages superimposed on direct voltage are reported in this paper. The anomalous breakdown voltage under such combined stresses was found to be much lower than that under direct voltage stresses alone. It is also affected by the relative polarities of the two voltages: When they are same, it increases considerably with increase in direct prestressing voltage; when they are opposite, it decreases a little. The polarity of the resultant applied voltage was also found to have effect on the distance of the anomalous breakdown location from the h.v. electrode. The duration of the superimposed switching surge (1.2/50 ?s to 1000/10000 ?s) also was found to affect the anomalous breakdown volt age. An attempt is made to explain such anomalous behaviour in modern solid synthetic insulating materials.