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

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

Tracking and erosion of high temperature vulcanizing (HTV)-silicone rubber (SIR) and the suppression mechanism of alumina trihydrate (ATH) filler were investigated in the present study. The tracking and erosion resistance of HTV-SIR filled with 0 to 60%wt ATH was evaluated by employing an IEC 587 inclined plane (IP) tracking and erosion test, during which leakage current pulses on HTV-SIR were counted. Surface temperature distributions and the occurrences of thermal spots >400/spl deg/C also were observed by means of an infrared thermovision study. We employed thermogravimetry (TG)-differential thermal analysis (DTA)-mass spectrometer (MS) to observe the thermal degradation of unfilled and filled HTV-SIR in both air and argon. The test results indicate that 40%wt is a critical ATH level and whether tracking and erosion is allowed in the IP tracking and erosion test. Highly filling ATH (>40%wt) reduces the number of low unit silicone oligomer precursors which promote dry-band arcing as well as the presence of residual carbon which leads to carbonization. Chemical modifications of water vapor liberated from heated ATH to methyl groups, which occurs at the thermal decomposition temperature of silicone rubber, were found to result in the above process. The protection mechanisms of ATH for the tracking and erosion of HTV-SIR are formulated herein.

Tracking and erosion of HTV silicone rubber and suppression mechanism of ATH

Silicone rubber samples having various concentrations and mean particle sizes of either alumina tri-hydrate or silica filters, prepared by room temperature and heat cured under pressure (hot pressed), are tested for erosion resistance in the ASTM D2303 inclined plane tracking and erosion test. Their corresponding thermal conductivities are determined using a transient temperature technique in which an infrared laser is employed as the heat source and a thermal imaging camera as a temperature detection device. Scanning electron microscope observations show greater filler bonding to the silicone matrix in the hot pressed samples than in the room temperature vulcanized samples leading higher thermal conductivity and increased resistance to erosion, for both ATH and silica filled samples. The correlation study shows a strong relationship between the erosion resistance and the thermal conductivity of the tested samples, highlighting the importance of an outdoor insulating material to have high thermal conductivity in order to withstand dry band arcing. The results can be used to provide guidance on filler selection for silicone rubber compounding for outdoor insulation applications.

Thermal conductivity of filled silicone rubber and its relationship to erosion resistance in the inclined plane test

Aging of polymeric and composite materials is reviewed mainly from the standpoint of their interfacial performance. Insulating materials can be divided into simple polymers and composites. Polymers for power cables, transformers, insulators and rotating machines consist of thermoplastics such as PE, PET and PPS, elastomers such as silicone, EPR and EPDM, and thermosets such as epoxy. Composites for GIS, rotating machines and insulators comprise epoxy/glass, epoxy/silica/alumina, and epoxy/mica systems. Aging processes are complicated in general, and take place under simultaneous multiple stresses such as electrical, thermal, mechanical and environmental stresses. Some of the phenomena covered in this paper are associated with the degradation by tracking and erosion and the loss of hydrophobicity in case of surface properties, and with PD, electrical treeing, water treeing and combined phenomena. The quality of cable insulation such as XLPE has been improved extensively from the standpoint of design electric strength. Interfacial problems will emerge for modification of cable joints. It is expected that polymers for outdoor use and filled epoxy resin systems should be improved from their environmental stability and from their design stress enhancement, respectively.

Aging of polymeric and composite insulating materials. Aspects of interfacial performance in aging

Silicone-based materials for outdoor insulators have the advantage that low molecular weight (LMW) components migrate through the material and coat the surface, thereby restoring hydrophobicity over a period of hours. By measuring the IR absorption of siloxane migrating to the silicone surface through a thin carbon coating, the time constant for migration was calculated. According to the time dependence of IR-absorbance, the time constant for migration ranged from 1.6 to 5.3 h depending on alumina trihydrate (ATH) filler concentration.

Diffusion of low molecular weight siloxane from bulk to surface

Analysis using gas chromatography/mass spectroscopy (GC/MS) was performed for evaluation of surface degradation of silicone insulating materials. Silicones are used as coatings for porcelain insulators and shed material for high voltage composite insulators. A comparison between virgin silicone rubber and aged silicone rubber samples, which were aged either on actual power lines or during a field exposure test, was made by GC/MS analysis. The GC/MS spectrum of siloxane in silicone rubber has a series of peaks which corresponds to the number of dimethylsiloxane units in the molecule. We found that the aged samples had a larger concentration of low molecular weight siloxane species than the virgin samples. The top shed surfaces generated more low molecular weight siloxane species than the bottom shed surfaces. Since GC/MS analysis can determine the molecular weight distribution of polymer insulating materials, evaluation of the degree of surface degradation and estimation of the remaining life of insulators may be possible.

Evaluation of surface degradation of silicone rubber using gas chromatography/mass spectroscopy

This paper reports on the effects of silicone fluids, used as processing agents, on the loss and recovery of hydrophobicity. The recovery of hydrophobicity was evaluated by the measuring the contact angle, the surface electrical resistance and the leakage current on a silicone rubber (SIR) surface. Two types of silicone fluids (A and B) having different chemical structures were selected from consideration of hydrophobicity and processability. SIR specimens were exposed to corona discharges in air and were analyzed using contact angle and surface resistance measurements. It was observed that the contact angle and the surface resistance increased gradually with increasing test time. The recovery of the hydrophobicity expressed as the contact angle and the surface resistance increased with increasing ratio of fluids A/B. The tracking and erosion resistances of SIR were investigated using the rotating wheel dip test. The leakage current of SIR decreased with increasing ratio of fluids A/B.

Effects of added silicone oils on the surface characteristics of silicone rubber

Dynamic contact angle measurements based on the Wilhelmy method were performed to evaluate the time variation of hydrophobicity of silicone rubber. The technique can determine both advancing and receding contact angles of a plate sample and evaluate the change of hydrophobicity in a short time. By changing the immersing time of silicone rubber into water, some aspects of overturn and/or re-orientation of polar groups at the surface were observed as a change of the contact angle. Also, the contact angle of silicone rubber was compared with that of fluorinated silicone rubber.

Evaluation of time variation of hydrophobicity of silicone rubber using dynamic contact angle measurement

Thermogravimetric analysis (TGA) was used to evaluate the surface degradation of polymer insulators designed for outdoor high voltage applications. Such insulators are usually formed as a composite of silicone matrix and inorganic materials, such as aluminum trihydrate (ATH) filler and/or fumed silica. However, the surface of insulators is subject to long-term degradation during outdoor use. This contribution describes an analytical technique which can be used to investigate surface degradation phenomena of filled silicones and provide a quantitative evaluation of the surface degradation. Comparison of the TGA curves for virgin silicone and field-aged silicone removed from an outdoor insulator indicated depolymerization-induced loss of the siloxane matrix must occur during field aging.

Toshiyuki Kuroyagi

Papers

Diffusion of low molecular weight siloxane from bulk to surface

Evaluation of surface degradation of silicone rubber using gas chromatography/mass spectroscopy

Effects of added silicone oils on the surface characteristics of silicone rubber

Evaluation of time variation of hydrophobicity of silicone rubber using dynamic contact angle measurement

Evaluation on surface degradation of silicone rubber using thermogravimetric analysis