Degradation of insulating materials under electrical stress

An avalanche of physical and physicochemical phenomena, often self-sustained, leads to the end of life of an insulating material. The collapse of the insulating role of a material is mostly due to the electrical field stress. The path leading from the catastrophic situation, the breakdown, back to its origin is followed in the present paper. A defect, pre-existing or created by the in-service stresses, gives rise to an ionization process in a volume bounded by insulating walls. With time the process does not remain identical due to both changes in the material and gas evolution, which makes it difficult to interpret the detected electrical signal. The end of life will occur through microcracks in which new discharges are initiated, activating the propagation of these structural failures. This change of structure of the insulating material when due to the existence of water molecules will lead to the same kind of final degradation. The polymeric insulating materials could contain additives but they lead only to a partial healing. From polymers of low complexity such as the PE used in cables, assuming that there is no pre-existing defect, one endeavors to understand how a microstructure reacts with a local field or accumulated charges. The present paper summarizes the thoughts of the author pertaining to the problems of aging and dielectric breakdown in organic insulating materials, processes often given a concrete form through experiments.