Condition assessment of porcelain and toughened glass insulators from residual strength tests
Transmission and distribution networks of most utilities worldwide have a substantial quantity of porcelain and toughened glass insulators that have been in service for over 30 years; a number that has been traditionally considered as useful life for planning purposes. Many utilities are implementing programs to replace older or defective insulators for improving reliability and also for upgrading their lines. In order to help prioritize line maintenance it is important to identify the condition of insulators and replace those insulators whose performance has reduced significantly from their new condition, hence threatening reliability. In this project, porcelain and toughened glass insulators that were in service for periods ranging from 10-30 years were removed from several locations. Electrical (puncture strength, flashover voltage) and mechanical (tensile strength) tests were performed to assess any reduction in performance. Diagnostic tests such as analysis of bonding agents, identification of internal failure paths were performed to understand failure causes. A sufficiently large number of samples were evaluated and this permitted condition assessment with statistically significant confidence limits
Ageing of transmission line insulators: The past, present and future
Contamination flashover as a result of pollution severity leading to costly outages plagues insulator performance. Apart from the problem of flashover, failure of insulators as a result of ageing has also lead to service disruption. Leakage current (LC) is a good measure of insulator surface condition and thus its magnitude has been used as a parameter for monitoring insulator flashover and pollution severity, but this parameter is not effective in monitoring ageing insulator. Furthermore, field monitoring and diagnosis of ageing insulators has not been exploited. This paper presents an overview of the ageing mechanism of both ceramic and polymeric insulators. Previous studies on the correlations of LC with pollution severity, environmental parameters and insulator performance are discussed. A review of recent studies on relationship between LC with ageing insulators and the possible future areas of research on monitoring and diagnosis of ageing insulators are presented.
Reduction of Surface Flashover of the Beam Screen of the LHC Injection Kickers
The LHC injection kicker magnets include beam screens to shield the ferrite yokes against wake fields resulting from the high intensity beam. The screening is provided by conductors lodged in the inner wall of a ceramic support tube. Operation with increasingly higher bunch intensity, and narrow bunches, now requires improved ferrite screening. This will be implemented by additional conductors; however the good high-voltage behaviour of the kicker magnets must not be compromised by the supplementary screening. Extensive studies and optimisations have been carried out, to better satisfy the often conflicting requirements for low beam coupling impedance, fast magnetic field rise-time, high vacuum and good high voltage behaviour. A new configuration is proposed which reduces significantly the electric field associated with the screen conductors and the secondary electron yield of the surface of the ceramic tube. Results of high voltage test results are also presented.