Natural ageing of EPDM composite insulators

Long-rod composite insulators, with weather sheds (sheds) made of ethylene-propylene rubbers (EPDM), were exposed for many years to HVAC (high-voltage alternating-current) and HVDC (high-voltage direct-current) under realistic conditions and natural pollution. The changes of their properties with time and their aging were studied. The results show that the insulator shed material undergoes a slow degradation process and loses successively the water-repelling properties that initially make the EPDM composite insulators superior to inorganic glass and porcelain insulators. The outdoor degradation of the shed material depends on the electric stress, on the environmental factors (such as pollution, rain, salt-laden fog, and UV-radiation from sun), and on the materials and fillers used in the construction of the composite insulators. A thorough macroscopic and microscopic study of the EPDM composite insulator sheds illustrates the differences of the surface states of EPDM insulators of different makes in which different basic material compositions and fillers are used. The poor performance of aged EPDM composite insulators compared to inorganic insulators depends both on the design, which often introduces many interfaces, and on the environmental factors that augment electrical aging