Additional Steps For Polymer Flashover and Effects of Aging
Author:Summer   time:2/18/2016 2:46:22 AM  read:523times

Compared with porcelain, additional Steps for Polymer Flashover and Effects of Aging is analyzed in this paper in different factors, such as environment and properties.

The analyses of laboratory data and literature surveys suggest several additional steps that occur in the flashover of a hydrophobic polymer insulator housing.
1. Contamination deposition – same as for porcelain.
2. Wetting – high humidity, fog, dew or light rain deposit moisture on the surface which forms droplets because of the hydrophobic properties. Due to gravity, droplets roll down sloped areas. Where gravity does not encourage droplet movement, discrete droplets remain. Salt and/or conductive pollution dissolves in the water droplets, increasing the liquid conductivity.
3. The residual dry surface pollution is slowly wetted by the droplet migration. This forms a high resistance conductive layer, and changes the leakage current from capacitive to resistive.
4. Ohmic heating – same as for porcelain.
5. Electric field effect on hydrophobic surface – the applied electric field causes closely spaced droplets to join together into a larger single drop, known as a filament. Flashover tends to take longer for a hydro phobic surface because of the time to form a conductive path with filaments. The local electric field has to be sufficiently high to form filaments as well.
6. Spot discharges on hydrophobic surface – filaments reduce the distance between housing terminals, increasing the electrical field between adjacent filaments. When the stress is sufficient, surface discharge activity can occur.
7. Reduction in hydrophobicity –Discharge consumes the thin polymer layer around the droplets and reduces the hydrophobicity by rotation or breaking of the polymer chains. Loss or reduction of surface hydrophobicity results in roplet dispersion and the formation of a continuous conductive layer in a high stress area, allowing elevated leakage current flow.
8. Dry bands form under the same process as porcelain. The resultant activity causes surface erosion whose rate depends upon the specific material formulation discharge-free and promotes aging.
9. Full or partial recovery of hydrophobicity may be possible if the material is discharge free for a sufficient period of time. Recovery ability will depend upon the specific material, formulation, housing design and service environment.
10. Repetition of the aging cycle causes further erosion of the surface, which is enhanced by chemical reaction and local temperature rise. Local hot spots can be of the order of 400°C during heavy discharge activity.
Other aging, such as UV damage, can cause surface crazing which traps and holds contaminants which can promote leakage current flow during wetting.
11. Flashover ultimately occurs along the same process as porcelain. The surface becomes hydrophilic, wets out, dry bands form and the discharge propagates to bridge the housing terminals.
Hydrophobic surfaces (see Figure 4) present a higher resistance to leakage current flow than hydrophilic surfaces and require higher leakage current and commensurate energy dissipation to initiate flashover. This is why polymer insulators have higher flashover voltages than conventional porcelain insulators. As with porcelain, all of the above steps must take place sequentially for flash over to occur. If the process is interrupted, such as a change in wetting conditions or with surface hydrophobicity recovery, flash over does not take place. Thus, visible activity does not always result in flash over.


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