HV composite polymeric insulators are being accepted increasingly for use in outdoor installations by the traditionally cautious electric power utilities worldwide. They currently represent -60 to 70% of newly installed HV insulators in Nortb America. The tremendous growth in the applications of non-ceramic composite insulators is due to their advantages over the traditional ceramic and glass insulators. These include light weight, higher mechanical strength to weight ratio, resistance to vandalism, better performance in the presence of heavy pollution in wet conditions, and comparable or better withstand voltage than porcelain or glass insulators. However, because polymeric insulators are relatively new, the expected lifetime and their long-term reliability are not known and therefore are of concern to users. Additionally they might suffer from erosion and tracking in the presence of severe contamination and sustained moisture. This leads to the development of dry band arcing that under certain circumstances could lead to failure of polymer insulators. In this paper a review is presented of the recent performance experience of HV composite polymeric insulators in outdoor service, testing methods, aging, the ranking of the materials, the role of fillers, the role of low molecular weight components present in the insulators, the mechanisms responsible for the loss and recovery of hydrophobicity, one of the most important properties of polymers, the mechanisms of failure, detection of faults, type and quantity of natural contaminants, effects of exposure to rain, hydrocarbons, stationary air and wind, various methods to optimize the electrical performance and a relatively new method for evaluating the performance status of polymeric insulators in the field.

Outdoor HV composite polymeric insulators

A systematic study to understand how alumina tri-hydrate (ATH) and silica fillers improve the erosion resistance of silicone rubber during dry band arcing showed that the thermal conductivity of the resulting composite material is the main criterion governing material erosion. The thermal conductivity of the composite material is dependent on the thermal conductivity, concentration, particle size, and bonding of the filler particles to the silicone matrix. In this context, either filler can be shown to perform better than the other, depending on the formulation, in the ASTM inclined plane tracking and erosion test. Therefore, the industry perception that ATH filler imports better erosion resistance than silica in silicone rubber can be misleading. The release of water of hydration from ATH appears to have a secondary effect that may be more relevant in silicone compositions having a low concentration of a filler.

The role of inorganic fillers in silicone rubber for outdoor insulation alumina tri-hydrate or silica

A hypothesis for aging produced by dry band arcing in silicone rubber material used for outdoor insulation is presented and experimentally proven. Aging is indicated by permanent changes. The analytical techniques used to study the permanent changes are Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), and surface roughness measurement. The authors highlight the fact that there are permanent changes occurring in the material that lead to progressive degradation in the long run, even though there can be a complete recovery of surface hydrophobicity in a short time. >

Aging in silicone rubber used for outdoor insulation

This is a laboratory study on the aging of SIR and hydrophobicity recovery. UV radiation, corona discharges, acid rain, dry-band arcing are employed as sources of the aging. Chemical and morphological analysis is used to detect the surface chemical and structural changes derived from these stresses. The nature of highly mobile low molecular weight (LMW) chains achieving a quick recovery of the hydrophobicity is carefully examined. Their generation and extinction caused by these stresses are investigated. From this study, it is shown that oxidation that induces crosslinking, branching, interchanging and a formation of silanol groups are the most dominant chemical reactions during the aging of SIR. Furthermore, silanol groups that are byprodncts of oxidation restrict the diffusion of mobile LMw chains, which decreases the recovery speed of hydrophobicity and accelerates the aging. A typical aging scenario of SIR from the installation to the end of their life is drawn.

Electrical and environmental aging of silicone rubber used in outdoor insulation

Most of the silicone materials used for Hv outdoor insulation are high-consistency, heat cured polydimethylsiloxane (PDMS) elastomers. The unique properties of PDMS that make it suitable for HV applications are reviewed. The surface of these elastomers can be rendered hydrophilic by exposure to discharges. A time and temperature dependent hydrophobic recovery ensues when exposure ceases. A variety of surface characterization investigations have established that corona exposure forms a brittle, wettable, silica-like layer on the surface of most PDMS elastomers. This is consistent with similar effects from oxygen and inert gas plasma treatment. There is still considerable debate as to the relative importance of the two major mechanisms postulated to account for the hydrophobic recovery after corona discharge. The diffusion mechanism invokes migration of low molecular weight species from the interior to the surface, while the reorientation or overturn mechanism envisages a surface reorganization with polar entities such as silanol groups resulting from surface oxidation rotating away and being replaced by methyl groups in the outermost surface layers. In our view, the highly crosslinked silicalike layer cannot reorient readily between hydrophilic and hydrophobic states at the surface, suggesting that diffusion of low molecular weight PDMS components is the more important mechanism of hydrophobic recovery. Recent data obtained on PDMS samples free from lowmolecular- weight diffusible species show that hydrophobic recovey may be due to in-situ depolymerization.

Hydrophobicity loss and recovery of silicone HV insulation

The materials evaluated in fog produced from low (250 mu s/cm) and high (1000 mu s/cm) conductivity water include cylindrical rod samples of high-temperature-vulcanized silicone rubber and ethylene-propylene-diene monomer rubber (EPDM) containing various amounts of either alumina trihydrate or silica fillers, or both. Comparison is made with material performance results obtained with AC, which was reported in an earlier study. In both low- and high-conductivity fog, the time to failure with AC and +DC was very similar, but a reduction by a factor of about four was observed in the time to failure with -DC. For both AC and DC, silicone rubber performed better than EPDM samples in low-conductivity fog, while the order of performance was reversed in high-conductivity fog. A theoretical model to determine the effect of dry band discharges on material is presented. Good agreement between the predicted behavior and the experimental findings is shown. >

The electrical performance of polymeric insulating materials under accelerated aging in a fog chamber

This paper presents an overview of the practices employed by utilities and industries to combat the contamination problem with ceramic insulators. It emphasizes two popular methods, namely, coating insulators with greases and RTV (room temperature vulcanizing) silicone rubber coatings. The experience to date, application methods, user concerns, current theories on how and why the coatings perform, life expectancy, and future direction of the technology, are outlined. The paper also describes research activity in the committee on RTV coatings, which is the state of the art in protective coatings. >

Protective coatings for improving contamination performance of outdoor high voltage ceramic insulators

The authors present a review of surface dielectric behavior of several polymers under high-voltage (HV) outdoor conditions. The scope is limited to materials used to provide weathersheds on non-ceramic insulators and room-temperature vulcanized silicone rubber coating used on ceramic insulators for combating problems related to insulator contamination. The development of arc resistant polymeric compounds is discussed. The discovery and evaluation of water repellent characteristics of solid polymer surfaces are addressed. Key issues regarding dielectric properties that relate to outdoor insulator performance are considered, and developments contributing to the fundamental understanding of some of these issues are discussed. >

Surface dielectric behavior of polymeric insulation under HV outdoor conditions

This paper presents the results of round robin tests performed to evaluate the electrical performance of Room Temperature Vulcanizing (RTV) silicone rubber coatings used for improving the contamination performance of outdoor ceramic insulators. The test, performed in six different laboratories, included several salt-fog chambers and a tracking wheel. Most test facilities were equipped with an on-line data acquisition system for measurement of leakage current activity. Four generic formulations of RTV silicone rubber coatings applied on ceramic rods were evaluated. The results indicate a general consensus in the ranking of the coatings` leakage current suppression capability, despite the variation in the experimental conditions employed. This suggests that guidelines for a laboratory test for evaluating the RTV coating material`s electrical performance under contaminated conditions could be developed.

T. Orbeck

Papers

The electrical performance of polymeric insulating materials under accelerated aging in a fog chamber

Protective coatings for improving contamination performance of outdoor high voltage ceramic insulators

Surface dielectric behavior of polymeric insulation under HV outdoor conditions

Round Robin Testing of RTV Silicone Rubber Coatings for Outdoor Insulation