Determination of ageing models for the mechanical properties of porcelain Long-Rod insulators based on statistical evaluation

Abstract:
TenneT is a leading European electricity transmission system operator (TSO) with its main activities in the Netherlands and Germany. For many years, TenneT TSO GmbH has used a complex maintenance system to assess the reliability of all system components. Failures in overhead transmission lines (OHTL), such as mechanical failures of high-voltage insulators, can have serious consequences. Besides a smaller proportion of cap & pin insulators of toughened glass, TenneT’s overhead line grid mainly consists of porcelain as well as composite Long-Rod insulators (hereinafter referred to as “Long-Rods”). These insulators are of different ages and have been produced by a variety of manufacturers using different material compositions. The development of a standardised procedure for the operational safety of OHTL components is therefore challenging. In order to set up a customised methodology for the evaluation of aged porcelain insulators in the OHTL grid, a project team from TenneT TSO GmbH and LAPP Insulators GmbH has started the development of a statistical based lifetime analysis for porcelain Long-Rod insulators. The insulators have to withstand severe static and dynamic loads during their envisaged lifetime of 50 years. LAPP, as a manufacturer of all types of porcelain insulators, has set up different services in the past. These include the assessment of old insulator after decades of operation. The aim is to ensure their mechanical integrity despite their age. In consequence, TenneT and LAPP have investigated approx. 1250 Long-Rods based on the procedure described in CIGRE TB 306. These data have now been assessed using statistical tools. In addition to the standard procedure, further investigations were conducted in the research project “lifetime analysis of ceramic insulators (LEKI)” to generate a better understanding of the ageing process of porcelain, used in Long-Rod insulators. The mechanics of brittle materials, i.e. fracture mechanics, served to quantitatively describe sub-critical crack growth as the dominant mechanism of static mechanical ageing. Within the present publication, a statistical data analysis of approx. 1250 porcelain Long-Rods is presented. The influence of crucial parameters such as the insulator age, manufacturer, thermal-mechanical performance and joining material were statistically evaluated. After setting up a multivariate regression, the impact of specific parameters and their combinations could be evaluated. Applying the determined statistical model on the extensive database and adding the experience from sample selection and evaluation, the process described in CIGRE TB 306 and the Thermo-Cycle-Test (TCT), is critically reviewed. The authors concluded that CIGRE TB 306 is a veritable and feasible tool for the assessment of aged insulators, taking into consideration the special requirements of reliability. However, the application and the analysis revealed some potential for optimisation.