An assessment of the reliability of in-service gapped silicon carbide distribution surge arresters

Although gapped silicon carbide arresters are no longer purchased by electricity utilities, they still form the majority of distribution arresters in service in Australia and in many other countries. Their numbers are large and so they represent a considerable capital investment. Most of the arresters of this type are now over ten years old and many are much older. So the question must be asked - what is to be done with this ageing and outdated class of arresters? Earlier extensive Australian studies in the 1960s had revealed that internal deterioration resulting from inadequate seals was the predominant cause of degradation and failure of gapped silicon-carbide arresters. Since then, manufacturers have improved the design and testing of arrester seals, but utilities still report small but appreciable failure rates for in-service distribution arresters. This paper describes the results of a recent investigation into this problem. Its aims are: • To determine the condition of typical gapped silicon-carbide distribution arresters now in service in Australia, • To assess the causes of degradation and of failure of such arresters, and • To make recommendations to management concerning the eventual replacement of such arresters by metal- oxide arresters. With the cooperation of 8 electricity utilities in Australia, nearly 400 arresters were recovered from the field after about 10 years or more of service. The arresters were subjected to electrical diagnostic tests (insulation resistance, power frequency withstand and sparkover voltages, and impulse sparkover voltage). The electrical tests were supplemented by seal tests on 42 percent of the arresters and all of these (and some others) were dismantled for inspection of the condition of internal components. The full paper details the results of the electrical testing, the seal tests and of the inspection of the internal components of the dismantled arresters. Comparisons are made with the results of similar investigations conducted in North America about fifteen years ago. The significant findings from the present investigations are summarized in the following Figure (Figure 2 of the full paper) which associates the condition of the arresters (as characterized by the test failure rate) with years of service. The electrical test results showed that after about 10 years of service, there is a marked upturn in the number of arresters with unsatisfactory insulation resistance, and after about 13 years of service, a marked upturn in the number of arresters will) reduced power frequency sparkover level. Inspection of the internal components of dismantled arresters confirmed that the likelihood of significant degradation increased markedly with years of service, and was evident in almost 75 percent of arresters with 13 years or more of service. The authors therefore recommend that all gapped silicon carbide arresters with 13 or more years of service be progressively replaced by modern metal oxide arresters