Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Diagnostic Methods for Outdoor Polymeric Insulators

Material samples of silicone rubber with known differences in their composition, i.e. different filler content and extra silicone oil added, have been aged at the Anneberg field station on the west coast of Sweden. ac or dc voltage supplied to cylindrical samples at stress levels of 50 or 100 V/mm. The work includes laboratory examination of material changes together with on-site, visual observations and leakage current measurements. Material samplings for the laboratory tests were made after 18 months of electrical aging, which went on for one more year in order to gather further information on the long-term electrical performance of the material. The dominant aging factor was the level of the applied stress, independent of ac or dc voltage. The dc stressed samples showed a higher leakage current and exhibited larger surface degradation compared with samples exposed to ac voltage. The material parameter, an addition of extra silicone oil, initially led to an increase in adhesion of pollutants, whereas the overall performance was improved by the higher suppression of the leakage current related to oligomer diffusion. Samples with lower levels of alumina trihydrate (ATH) exhibited a delayed onset of degradation, but once damaged they degraded more rapidly than the specimens with a higher ATH content. Infrared spectroscopy showed that the ATH was completely consumed at the eroded surface regions. The aging of the surfaces was further assessed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The low molar mass siloxanes present in the pollution layer were extracted and analyzed by size exclusion chromatography and gas chromatography-mass spectroscopy. The results indicated that the main degradation factor was thermal depolymerization activated by electrical discharges. Oxidative crosslinking of the silicone rubber, usually attributed to surface close corona discharges, appeared to have played a minor role.

Aging of silicone rubber under ac or dc voltages in a coastal environment

Among the new insulating materials widely used in high voltage outdoor insulation sys- tems, silicon rubber materials also called polymeric materials are very important; during last 20 years they have been applied for manufacturing outdoor insulators. These materials are organic in nature; therefore they are prone to decomposition under different environmental conditions; the most important among these conditions for consideration of w.r.t. of high voltage electrical insula- tion is the hydrophobicity. Hydrophobicity is treated as the resistance to formation of conducting water tracks that increase leakage current, chances of flashover, and other deterioration effects. The superior contaminant performances of polymer outdoor insulating materials are regarded as the contribution of their hydrophobic surfaces. In other words, it is the formation of water beads on surface, which resist the flow of water in continuous conducing track. Hydrophobicity theory, its importance for polymeric silicon rubber insulators, methods of hydro- phobicity measurements - especially by leakage current, low molecular weight component chains responsible for hydrophobicity recovery, chemistry of hydrophobicity loss and recovery, hydropho- bicity recovery related with temperature, humidity, UV radiation, corona, electric field and dry band arcing are overviewed in this paper.

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Hydrophobicity of silicone rubber used for outdoor insulation (an overview)

This paper presents the results of investigations on the performance of nonceramic insulators in tropical field conditions. Six different types of 33 kV insulators (silicone rubber, EPDM composite, porcelain, and RTV coated porcelain) were installed at six different test sites exposed to marine, industrial and clean environments in Sri Lanka and Tanzania. The visual scrutiny, hydrophobicity class (HC) and surface conductivity were used for the evaluation of the field performance. The silicone rubber and RTV coated insulators preserved the hydrophobic properties although some biological growth was found on them. The EPDM insulators showed distinct surface changes. At the same time, damage due to dry-band arcing appeared on the porcelain insulators exposed to marine pollution.

Performance of nonceramic insulators under tropical field conditions

The aging (biological deterioration) is a major problem of composite insulators now-a-days. The main thing in aging is to predict how, when and with what speed it occurs and under what conditions it can lead to failure and what overall average expected life of a composite insulator is. For this a lot of researches have been done. This review summarizes the methods of artificial field testing (aging), natural testing, standards the developed for aging, techniques of analysis, results achieved until now about various parameters from various locations, handling guidelines and a conclusion on what is further needed.

Composite insulators and their aging: An overview

The long-term performance and the material state of polymeric insulators were examined from December 1987 to February 1997. To a large extent the study was conducted at Anneberg field station, on the west coast of Sweden. The project comprised a great number of commercially available polymeric insulators from several prominent manufacturers. Each type of insulator was energized with high voltage alternating current (HVAC) as well as high voltage direct current (HVDC). In addition, of each type of insulator two were non-energized: one was exposed to the environment only and one was stored indoors.
 The silicone rubber (SiR) insulators maintained a high degree of their initial hydrophobicity and with respect to leakage currents performed better than the porcelain insulators. The obtained results show that heavily stressed SiR insulators with specific creepage distances in the order of 8.2 mm/kV to 9.3 mm/kV had leakage currents exceeding 80 mA during a salt-storm in January 1993. However, after that occasion they showed relatively low leakage currents indicating that the SiR has the ability to recover its high surface resistivity and good performance. The measurements indicate that, at light levels of pollution, it is possible to reduce the creepage distance of the SiR insulator compared to that of a ceramic one.
 Under severe field conditions the ethylene-propylene-diene monomer (EPDM) rubber insulators performed worse with respect to leakage currents and flashovers compared to the porcelain insulators with the same electric stress. Visual observations verified that the surfaces of most of the EPDM rubber insulators had eroded. The surface erosion included cracking and chalking due to environmental exposure and leakage current activity. The material aging of the EPDM rubber resulted in a degraded performance of the insulators under contaminated conditions. In sum, the results suggest that the application of a higher electric stress of the EPDM rubber insulator compared to that prescribed for the ceramic one is not advisable.

Torbjörn Sörqvist

Papers

Polymeric Outdoor Insulators - A Long-term Study