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

Crosslinked polydimethylsiloxane exposed to oxygen plasma studied by neutron reflectometry and other surface specific techniques

Water repellency, high surface resistivity, vandalism resistance, low density and good processability have made silicone rubbers based on polydimethylsiloxane (PDMS) very attractive materials in housings for outdoor insulation. Their ability to recover hydrophobicity after oxida. tion or contamination is of paramount importance and this is the topic of this review. A critical evaluation of the chemical and physical mechanisms responsible for hydrophobicity loss and recovery is presented.

Hydrophobicity changes in silicone rubbers

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

Room temperature vulcanizing (RTV) silicone rubber coatings are applied to electrical insulators to improve their subsequent insulation strength, particularly under wet conditions. Under prolonged wetting the hydrophobicity of the coating is reduced temporarily and the insulator protection is lost. After a dry period, a recovery of the hydrophobicity takes place. To investigate this phenomenon, the surface was subjected to various wetting conditions with and without electrical stress. Measurements of the contact and the sliding angles were used to determine the state of the surface. Using water, the surface free energy of the RTV was calculated from the contact angle measurements as a function of time of exposing the surface to salt-fog. The surface free energies due to London dispersion and hydrogen bonding forces on the surface of RTV coatings were calculated from the contact angle measurements using both water and methylene iodide. Good agreement was obtained with the literature value for a virgin RTV surface. The chemical changes of the surface are shown to be responsible for the loss of hydrophobicity. >

Hydrophobic behavior of insulators coated with RTV silicone rubber

Ethylene propylene diene monomer (EPDM) is extensively used in outdoor composite polymer insulators. The effects of water salinity and temperature on the hydrophobicity of EPDM rubber were studied The hydrophobicity was determined by measuring the contact angle using a droplet of distilled water. The specimens were immersed in saline water solutions for up to 576 h. The salinity levels used were 5, 10/sup 3/, 10/sup 4/ and 10/sup 5/ /spl mu/S/cm. The aging temperatures were 0, 25, 50, 75 and 98/spl deg/C. The weight was also measured to determine the amount of water absorption in the polymer as might occur during a heavy rain. The change in the contact angle is related to the amount of water absorption, the salinity level of solution and the temperature. The absorption of saline water into the bulk is larger at higher temperatures and decreases with increasing salinity. At 25/spl deg/C and higher the absorption of saline water at all salinity levels was saturated during the aging period Generally, after long immersion the contact angle decreased. At 0/spl deg/C the decrease of contact angle was larger at higher salinity. At high temperatures (/spl ges/75/spl deg/C), the sample surface gradually changed its color from the original dark gray to white.

Effect of water salinity and temperature on the hydrophobicity of ethylene propylene diene monomer insulator

The aging of crosslinked polyethylene (XLPE) is often initiated by a process of thermal oxidation that gradually degrades the insulation until failure occurs. Oxidation on the surface causes the loss of hydrophobicity which is accelerated by increasing temperature and moisture. After the hydrophobicity on the surface has diminished, water can permeate into the bulk more readily. To investigate the process of wettability of the surface and its dependence on the level of calcium carbonate filler, XLPE specimens were immersed in saline water solutions at levels of salinity from 5.5 to 10000 /spl mu/S/cm, at temperatures from 0 to 98/spl deg/C and durations from 5 to 150 h. It was found that at a fixed salinity level the contact angle decreased with increasing temperature and was more pronounced for 15 and 25% filler levels than when no filler was used. The contact angle decreased slightly with increasing salinity of the water from 5.5 to 1000 /spl mu/S/cm and rapidly from 1000 to 10000 /spl mu/S/cm for all three filler levels. The contact angle decreased rapidly with increasing time of immersion up to 50 h and slowly thereafter up to 150 h for all filler levels. The contact angle was found to be constant at 86/spl deg/ after exposure to air for 50 h at five temperatures from 0 to 98/spl deg/C. On the other hand after immersing in a saline solution in the range 5.5 to 10000 /spl mu/S/cm for 50 h, the contact angle decreased with increasing time of immersion and increasing the temperature from 0 to 98/spl deg/C for all filler levels. The recovery of hydrophobicity depends on filler concentration, temperature and salinity level of the aqueous solution. Mechanisms and reactions of oxidation responsible for the observed phenomena are suggested.

Hydrophobic property of XLPE filled with calcium carbonate

The recovery of hydrophobicity of nylon insulators after aging by long exposure to a stress of saline water at different temperatures is investigated. The hydrophobicity is determined by measuring the contact angle of a droplet of distilled water on nylon. The aging of nylon was performed by immersing it for up to 336 h in saline water solutions in the range 5.0-10/sup 5/ /spl mu/S/cm. The aging temperatures were 0 to 98/spl deg/C. After aging, the specimens were kept in air at room temperature for up to 4500 h during which the recovery of hydrophobicity and weight changes were measured. Specimens were also kept in high vacuum for 20 h to determine the changes in the contact angle and in the weight with the absence of air.

Recovery of hydrophobicity of nylon aged by heat and saline water

The authors present the results of fog chamber tests performed to compare the contamination performance of 25 kV class outdoor polymeric cable terminators. The experiments were conducted in high-conductivity (>or=1200 mu s/cm) fog. The effects of weathershed material and design, method of stress relief, and experimental conditions on the contamination performance are discussed. It is shown that the weathershed material and design have a greater influence on the contamination performance than the method of stress relief. The results indicate that the method of stress relief does not improve contamination performance under normal operating stress. For terminators using good track-resistant materials, satisfactory performance can be obtained even with a 50% reduction in the leakage distance provided. >

Polymeric cable terminators under accelerated aging in a fog chamber

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