Ionisation and irradiation effects in high-voltage dielectric materials

A study of the chemical changes produced by exposure of dielectric materials to high-energy ionising radiation has shown that the effects are similar to those involved in electrical deterioration during stress aging and when ionisation has taken place in the dielectric. The principal effects observed in hydrocarbons and polymeric materials after exposure to radiation are: the crosslinking of the molecules to form an infusible gel structure; the evolution of hydrogen and low-molecular-weight fragments; molecular degradation by main-chain scission; the formation and annihilation of unsaturated groups; and, in the presence of air, processes of oxidative degradation. The corresponding electric stress and ionisation phenomena are: wax or `cheese¿ formation in solid-type cables; the gas evolving characteristics of paraffinic oils; the embrittlement of insulating paper; the formation of double bonds in aged cable oils, as evidenced by increased absorption in the ultraviolet region; and the oxidation effects with air resulting in an increase in power factor. While the mechanisms of radiation-induced change are not yet completely understood, they should throw light on the corresponding electrical stress and ionisation phenomena. Some of the methods used to protect organic systems against the effects of high-energy radiation are also satisfactory in reducing electrical effects, and a study of these protective mechanisms should be beneficial in the field of high-voltage dielectrics.