Deterioration of insulating materials by internal discharge

Various round-robin tests have been made to investigate degradation phenomena of insulating materials by corona discharges which take place in a cavity inside specimens composed of three thin films adhered with each other. For specimens made of several kinds of materials such as polyethylene, ac voltages of 3 to 8 kV are applied under conditions of different frequencies and film adhesion methods, and life versus voltage characteristics are measured and deterioration behaviors are also observed. Although there exist considerable dispersions between lifetimes measured, degradation grades by internal discharges can be clarified to a certain extent among different materials and the order of corona endurance natures of several materials may agree fairly well with those measured already by other experimental procedures. Measured lifetime distributions can also coincide well with values calculated by the Weibull analyses modified by the Committee. Meanwhile, sheet specimens of 2 mm in thickness with a cylindrical hole are put between electrodes and examined tentatively with regard to breakdown behaviors by discharges and it can be understood that most of the thick specimens made of three kinds of materials are broken down through degraded channels orginating from small pits which take place at bottom edges in the holes.