E.A. Cherney

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

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 loss and recovery of hydrophobicity of RTV silicone rubber insulator coatings

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. >

Hydrophobic behavior of insulators coated with RTV silicone rubber

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. >

Suppression mechanism of leakage current on RTV coated porcelain and silicone rubber insulators

The results of a study on the suppression of leakage current on room temperature vulcanizing (RTV) silicone rubber coated porcelain suspension insulators in a salt-fog chamber are reported. A comparative study of the performance of coatings on different parts of suspension insulators with RTV is reported. The results of attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy are reported. On the basis of these results, a suppression mechanism for leakage current is suggested. A mechanism for the suppression of leakage current on RTV coated porcelain and silicone rubber insulators dealing with the temporary loss of hydrophobicity and its return is suggested. >

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

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. >

Role of the size of particles of alumina trihydrate filler on the life of RTV silicone rubber coating

The paper reports on a study of the influence of the size of the particles of alumina trihydrate (ATH-Al/sub 2/O/sub 3/.3H/sub 2/O) filler on the life of room temperature vulcanised (RTV) silicone rubber coating in a salt-fog chamber. The particle sizes examined include 1.0, 4.5, 13, 17 and 75 /spl mu/m. The optimum size to give the lowest leakage current and the longest time to failure of the coating is determined. The particle size affects the roughness of the coating. This is determined by a high resolution surface roughness tester and a scanning electron microscope (SEM) examination. The roughness is enhanced after prolonged testing in salt-fog. The leakage current affects the amount of silicone fluid on the surface. The amount of silicone fluid present on the surface after exposure to dry-band arcing in salt-fog is a function of the particle size. Measurements of surface roughness, the amount of silicone fluid on the surface and the leakage current combined with theoretical analysis of the heat of conduction lead to identification of the mechanisms by which the size of the ATH particle imparts resistance to tracking and erosion. >

A Comparative Study of Polymer Insulating Materials Undern Salt-Fog Conditions

This paper describes the performance of the widely used polymeric insulating materials for outdoor insulation - high temperature vulcanized (HTV) silicone rubber, ethylene-propylene rubber (EPR) and epoxy resins with a commonly used inorganic filler, alumina trihydrate, under salt-fog conditions. The role of the filler and the effect of the filler concentration in imparting track resistance to the materials is investigated. Various methods to describe the surface aging of polymer materials such as the peak and the average of the leakage current, the cumulative charge, the weight loss and the flashover voltage have been investigated. The results show that at moderate concentrations of filler, 105 to 130 parts per hundred (pph) of polymer, there is little difference in the performance of silicone rubber and EPR materials. At low concentrations of filler (30 pph), the EPR samples track whereas the silicone rubber samples erode. The epoxy materials exhibit inferior properties when compared to silicone rubber and EPR. The results also indicate that monitoring the weight loss, the reduction of the flashover voltage, and the peak of the leakage current are better methods to characterize aging of insulators than the average of the leakage current or the cumulative charge.

Performance of Polymeric Insulating Materials in Salt-Fog

The paper presents the results of a study conducted on polymeric insulating materials in a salt-fog chamber. The materials examined include high temperature vulcanized (HTV) silicone rubber, ethylene propylene rubber (EPR) and cycloaliphatic epoxy. The influence of inorganic filler, type, concentration and dispersion and the magnitude of the electric stress on the tracking and erosion properties of the materials are reported under high salinity conditions. The presence of filler in EPR material is shown to enhance leakage current. Studies of filler dispersion by an energy dispersive X-ray analysis (EDAX) show non-uniformity in filler concentration where erosion or tracking is initiated. The mechanisms by which the filler operates to impart improved resistance to tracking or erosion is discussed through measurements of surface temperature, released gases, weight loss and leakage current.

Porcelain insulator maintenance with RTV silicone rubber coatings

The selection, application and performance of RTV (room-temperature vulcanizing) coatings for the maintenance of porcelain insulators are examined. The ability of coating systems to suppress leakage current, thereby eliminating the cause of wood pole fires and flashover, is discussed. The conditions associated with the temporary loss of coating hydrophobicity and its return are also discussed. Coating life expectancy is considered in terms of the pollution conditions, the frequency of maintenance cleaning, and the formulation of the RTV coating system. It is concluded that RTV coating systems perform by virtue of the fact that they are able to suppress leakage current. This enables insulation systems to attain increased flashover performance of about 30% over uncoated insulations. Under severe conditions of pollution, the effectiveness of the coating systems is temporarily lost, showing only some improvement over uncoated insulation. However, after maintenance cleaning, the coating systems return to their initial state with only a slight loss in their ability to suppress leakage. >

Influence of thickness, substrate type, amount of silicone fluid and solvent type on the electrical performance of RTV silicone rubber coatings

Room temperature vulcanizing (RTV) silicone rubber coating is increasingly being used by power utilities to overcome the contamination problems encountered in ceramic and glass insulators under wet conditions. The formulation of the RTV coating system is currently undergoing an intensive development in an effort to optimize its performance and in particular to extend its lifetime. While there are many forms of RTV, those used for insulator coating purposes invariably use a base of polydimethylsiloxane. The amount of RTV used in the service application is also important as it impacts on both the cost of the protection offered and on the electrical performance. This paper reports on a study of the influence of the coating thickness, different substrates, addition of silicone fluid to the coating formulation and different carrier solvents on the leakage current, pulse current count rates and the lifetime of the RTV coatings. It has been found that all these parameters affect the electrical performance under wet and contamination condition in a salt-fog. The optimum coating thickness has been determined for a fixed substrate. Mechanisms by which these four parameters affect the performance are suggested and discussed.