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Electric-Field Limits for the Design of Grading Rings for Composite Line Insulators

A major consideration for utilities applying composite insulators on overhead lines and substations is the aging of the polymer materials used for insulator housing. Historically, a primary driver for composite insulator development was to achieve a good contamination of flashover and aging performance. However, the application of composite insulators is not only limited to highly contaminated areas since large numbers are now also installed in locations with low contamination severities. The study of the long-term aging and life expectancy of composite insulators in clean environments is, therefore, of fundamental importance. Recent work at the Electric Power Research Institute (EPRI) and STRI has identified water-induced discharge activity as an important aging mechanism in these environments. This paper summarizes the findings of the extensive research carried out by these two organizations to determine a practical limit for the permissible E-field on insulator surfaces for design purposes. The initial work by the EPRI to determine e-field thresholds for water-induced corona, published in 1999, is expanded on the basis of small- and full-scale tests by both organizations to refine threshold levels and determine the influence of insulator parameters, such as hydrophobicity and shed profile. The influence of water-induced corona on the aging performance of insulators is also investigated on the basis of in-service inspections, accelerated aging tests, and natural exposure tests. The implications of this work on the need and dimensioning procedures for grading rings are also discussed.