R. Hackam

Outdoor HV composite polymeric insulators

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.

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. >

Improvement of NO/sub X/ removal efficiency using short-width pulsed power

Pulsed power has been used to remove nitric oxide (NO) in a mixture of nitrogen, oxygen, and water vapor simulating the flue gases from a power station stack. The effect of the pulsewidth at a fixed applied voltage on NO removal concentration was studied. The dependence of the energy efficiency of the removal of NO at a fixed applied voltage on the pulsewidth, on the removal ratio of NO and on the discharge current was investigated. This removal energy efficiency increases with decreasing pulsewidth and decreasing removal ratio of NO.

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

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. >

Surface flashover of solid dielectric in vacuum

The surface of a solid dielectric insulator becomes electrically charged when subjected to a high‐voltage stress in vacuum. A method for calculating the surface flashover voltage based on the assumption that the discharge occurs in a layer of desorbed gases from the insulator surface is proposed. The electric field strength required to cause surface flashover is calculated by taking into account the secondary electron emission characteristics of the dielectric material. The dependence of the surface flashover field on the insulating material is deduced. A dependence of the flashover voltage on the insulator length to a power law of 0.5 is theoretically predicted. The calculated surface flashover voltage is compared with the previously reported measurements and good agreement is obtained.

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. >

Surface flashover of solid insulators in atmospheric air and in vacuum

The surface flashover of Teflon, plexiglass, quartz, Pyrex glass, Macor glass‐ceramic, and sapphire solid insulators has been measured in vacuum (∼10−8 Torr, ∼10−6 Pa) and in atmospheric air using dc, ac (60 Hz), and 1.2/50‐μsec lightning impulse voltages. The dependence of the flashover voltage on the following parameters is investigated: (1) spacer material, (2) diameter of the spacer, (3) spacer length, (4) number of spacers stacked in series, (5) air pressure in the range 10−6–105 Pa, (6) electrode material, (7) spark conditioning, and (8) the external resistance in series with the gap. At a fixed insulator length the flashover voltage decreases with increasing spacer diameter. The withstand voltage of spacers stacked in series increases with increasing the number of spacers. The dc flashover voltage of different insulating materials is theoretically calculated in vacuum as a function of the length of the insulator and compared with the experimentally obtained results. Good agreement is obtained.

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. >

A new technique for loss reduction using compensating capacitors applied to distribution systems with varying load condition

A new method based on a heuristic technique for reactive loss reduction in distribution network is presented. This method allocates capacitors to certain nodes (sensitive nodes) which are selected by first identifying the branch which has the largest losses due to reactive power. Then, the node therein, which has the largest reactive power is selected. The capacitor rating is determined by differentiating the system losses with respect to the load connected to that node. The compensating capacitors are placed at these optimal locations with appropriate VAr ratings to achieve maximum benefits in dollar savings. The variation of the load during the year is considered. The capital and installation costs of the capacitors are also taken into account. This method is applied to a 38 feeder distribution system of 27.6 kV, 560 MVA of the city of Windsor, Ontario, resulting in annual saving of about CAN

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

145000 after amortizing the capital and installation costs of applying the compensating capacitors, and using a figure for the cost of energy of 1.986 cents per kWh. >