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

Hydrophobicity Recovery of Polydimethylsiloxane after Exposure to Corona Discharges

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 review of the current state of understanding of dielectric mixture properties, and approaches to use numerical calculations for their modeling are presented. It is shown that interfacial polarization can yield different non-Debye dielectric responses depending on the properties of the constituents, their concentrations and geometrical arrangements. Future challenges on the subject are also discussed.

Dielectric mixtures: electrical properties and modeling

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

Under wet and contaminated conditions dry band arcing occurs on the surface of HV outdoor insulators coated with room temperature vulcanizing (RTV) silicone rubber. The thermal impact of the arcing on the surface of the RTV causes chemical changes. The gradual loss of hydrophobicity due to dry band arcing is of considerable concern because it results in a reduction of the withstand voltage and subsequent failure of the insulator and power outage. In this study the RTV coatings were subjected simultaneously to salt-fog and electrical stress. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and electron spectroscopy for chemical analysis (ESCA) were then employed to investigate the chemical changes. Possible chemical reactions and processes occurring under dry band arcing are suggested. The results of ATR-FTIR and ESCA indicate the formation of low molecular weight polydimethylsiloxane (PDMS), the reduction in the surface density of CH3 groups and crosslinking of the PDMS backbone chains at the surface. Chemical derivative analysis was also carried out to further elucidate the chemical changes on the surface of RTV coating. >

Chemical changes at the surface of RTV silicone rubber coatings on insulators during dry-band arcing

The results of a study on the loss and recovery of hydrophobicity of RTV (room-temperature vulcanizing) silicone-rubber insulator coatings in a salt-fog chamber are reported. The results complement those previously reported on the ability of the coatings to suppress leakage current and insulator flashover. The temporary loss of hydrophobicity caused by dry-band arcing and the subsequent recovery are studied in depth. The gradual loss of hydrophobicity as determined from the leakage current and the contact-angle measurements is shown to be related to the physical changes to the coating brought about by dry-band arcing. >

The loss and recovery of hydrophobicity of RTV silicone rubber insulator coatings

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

The ability of room temperature vulcanizing (RTV) silicone rubber insulator coatings to suppress leakage current, thereby preventing power outages due to contamination, was studied. The RTV coatings were applied to porcelain suspension and line post insulators for a comparative study of the performance of coated and uncoated insulators. A study of the role of alumina trihydrate (ATH) which is used to impart tracking and erosion resistance to RTV coating is reported. A study of the diffusion of silicone fluid from the bulk to the surface of the RTV silicone rubber having various ATH filler levels using attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy is also reported. It has been found that the diffusion rate of the silicone fluid from the bulk to the surface of the coating after extensive dry band arcing decreases with increasing filler level. >

E.A. Cherney

Papers

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

Chemical changes at the surface of RTV silicone rubber coatings on insulators during dry-band arcing

The loss and recovery of hydrophobicity of RTV silicone rubber insulator coatings

Hydrophobic behavior of insulators coated with RTV silicone rubber

Effects of filler level in RTV silicone rubber coatings used in HV insulators