Influence of H2O molecules on electrical tree initiation in silicone rubber

Silicone rubber (SiR) has various excellent characteristics, and is widely used in various apparatus as insulating material. However, tree initiation processes and degradation phenomena in SiR are not clear. The investigations of tree initiation processes at high electric field region are very important. In this paper, the effect of H₂O on electrical tree initiation voltage will be examined. Three kinds of samples were tested; untreated SiR, SiR treated by vacuum heating, and SiR impregnated with liquid water after vacuum heating treatment. The tree initiation voltage of SiR treated by vacuum heating was higher than that of untreated SiR. It is considered that the tree initiation voltage is increased by removing gaseous H₂O molecules. However the tree initiation voltage of SiR impregnated with liquid water after vacuum heating was higher than that of SiR treated by vacuum heating. These results seem to contradict each other. We consider that the tree initiation voltage is increased by presence of liquid H₂O. It is suggested that the states of H₂O molecules included in the free volume have a large effect on ac tree initiation in SiR.