Polymeric Outdoor Insulators - A Long-term Study
Doctoral thesis, 1997
The long-term performance and the material state of polymeric insulators were examined from December 1987 to February 1997. To a large extent the study was conducted at Anneberg field station, on the west coast of Sweden. The project comprised a great number of commercially available polymeric insulators from several prominent manufacturers. Each type of insulator was energized with high voltage alternating current (HVAC) as well as high voltage direct current (HVDC). In addition, of each type of insulator two were non-energized: one was exposed to the environment only and one was stored indoors.
The silicone rubber (SiR) insulators maintained a high degree of their initial hydrophobicity and with respect to leakage currents performed better than the porcelain insulators. The obtained results show that heavily stressed SiR insulators with specific creepage distances in the order of 8.2 mm/kV to 9.3 mm/kV had leakage currents exceeding 80 mA during a salt-storm in January 1993. However, after that occasion they showed relatively low leakage currents indicating that the SiR has the ability to recover its high surface resistivity and good performance. The measurements indicate that, at light levels of pollution, it is possible to reduce the creepage distance of the SiR insulator compared to that of a ceramic one.
Under severe field conditions the ethylene-propylene-diene monomer (EPDM) rubber insulators performed worse with respect to leakage currents and flashovers compared to the porcelain insulators with the same electric stress. Visual observations verified that the surfaces of most of the EPDM rubber insulators had eroded. The surface erosion included cracking and chalking due to environmental exposure and leakage current activity. The material aging of the EPDM rubber resulted in a degraded performance of the insulators under contaminated conditions. In sum, the results suggest that the application of a higher electric stress of the EPDM rubber insulator compared to that prescribed for the ceramic one is not advisable.