The expected service life of high voltage insulators has become increasingly important now that the biggest electrification campaign since development of alumina porcelain insulators is underway. The world is moving to sustainable energy sources and this will significantly impact energy generation and distribution in coming years. At the same time, many existing transmission lines are reaching an age where replacement is going to be required sooner rather than later.
Estimated residual life of installed insulators has been studied for many years. But recently, more and more research is being devoted to this, often in close collaboration with energy supply utilities. For example, Korea Electric Power Corp.’s Research Institute and Sungkyunkwan University in South Korea have recently jointly published studies of different mechanisms behind insulator ageing. Insulators ageing and replacement strategies have also been studied by an IEEE Working Group.
Four different ageing mechanisms have been defined: a) expansion of the cement; b) corrosion of metal parts; c) mechanical and electrical stresses on insulator cores; and d) drying out of the bitumen. Resistance of a ceramic insulator to mechanical and electrical stresses is related mostly to its microstructure.
These days, most high voltage ceramic insulators are made of grades of C-120 alumina porcelain and C-130 high strength alumina porcelain, as specified in IEC 60672. By contrast, cristobalite and quartz porcelain are used as materials for insulators used in distribution applications below 36 kV. Unfortunately, IEC 60672 specifies grades only by their mechanical strength when new, right after production. Ageing, fatigue resistance and microstructure are not even mentioned. In fact, this is one of the major reasons why studies into insulator service life are now becoming so important.
The mechanical strength required of C-120 and C-130 porcelain bodies can be achieved using various mixes of raw materials. But this has resulted in different microstructures and also different service life expectations among competing manufacturers.
Importance of microstructure and its impact on expected service life has been known for a long time due to past publications by experts such as Dr. Johannes Lieberman. He proposed that the ideal microstructure for C-130 should contain ≥ 40 % corundum, ≤ 15 % mullite and a residual quartz content of < 1%. In fact, although a maximum acceptable content of quartz is not included in IEC 60672, many OEMs have placed this requirement into their technical specifications.
For example, a past study by A. Rawat and R. S. Gorur tested 30-year old insulators and established a clear relationship between microstructure and degradation in mechanical and dielectric strength. Their findings confirmed Dr. Lieberman’s ideal microstructure in the Power Frequency Puncture Test since the failed samples in this research all had quartz crystals with size > 50 μm and a high quartz content in general. Moreover, Keekeun Kim et al. demonstrated the role of corundum content as a major element increasing resistance to ageing and were able establish a model relating corundum content to strength. Indeed, using this model they were able to predict the tensile strength of 43-year old cap & pin insulators within an accuracy of ± 2.5%.
Attend the 2022 INMR WORLD CONGRESS in Berlin where porcelain R&D expert, Markku Ruokanen, will review findings of a recent study of porcelain microstructure containing intentionally placed quartz particles as calibrated defects. The objective of this project was to evaluate impact on resistance against mechanical stresses and to quantify the number and particle size of critical residual content.