Materials based lifetime assessment of porcelain insulators

A lifetime assessment of porcelain insulators based on the mechanics of brittle materials is currently built up within the research project “lifetime analysis of ceramic insulators (“LeKI”). This uses a procedure based on fracture mechanics. Herein the stress is calculated, under which a given crack will propagate. This is exactly the failure mechanism of brittle materials, so fracture mechanics is in the case of brittle materials generally accepted as more accurate than the still common force balance. In addition fracture mechanics allows predicting sub- critical crack growth, which describes the slow growth of cracks until they reach critical condition and cause “sudden” failure. Two variants exist: static fatigue is based on stress corrosion cracking, while vibrations result in dynamic fatigue. In the given case of insulators mainly static fatigue has to be considered. This procedure has already been demonstrated to successfully predicting the lifetime of other ceramics (like Si3N4 and ZrO2). Within LeKI the lifetime analysis shall be extended to alumina- based porcelain. In a first step approximately 250 insulators have been dismounted after a service life of 40 – 55 years from the participating German utilities TenneT TSO, Amprion TSO and 50Hertz TSO and analyzed with respect to mechanical and fractographic properties. In selected poles force gauges have been installed to conduct a long term monitoring of loads applied to the insulators. In laboratory the laws for sub- critical crack growth for both bulk porcelain and glaze have been determined. Both are currently connected to a sequential law (corrosive media can only reach the inner parts of the insulator by first passing the glaze). Goal of the investigation is a deepened understanding of the mechanisms ruling the fatigue of the insulators. This should allow optimization of the maintenance scheme at the participating utilities and especially the adaption of the lifecycles of insulators to the one of the metallic parts. In this contribution the procedure will be presented as well as the results obtained so far from the ongoing project. These clearly demonstrate the decisive importance of the glaze for the mechanical long-term stability of the insulators.