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Flashover Mechanism of Composite Insulators

Flashover Mechanism of Composite Insulators is different from that of porcelain insulators, and operating experience has proven that non-ceramic insulators perform better than porcelain in polluted conditions.

The application of non-ceramic insulators is increasing rapidly in the world. A significant percentage of new lines are built with non-ceramic insulators. The main advantages of non-ceramic insulators are the reduced construction time and better contamination performance. The former is particularly important in emergency conditions when the use of non-ceramic insulators permits the fast restoration of service after a hurricane.

The typical problem with non-ceramic insulators is the aging and deterioration of the shed material but not flashover. Most of the flashovers reported in the literature occurred in extremely bad weather, during storms. A survey conducted by EPRI indicates better contamination performance for non-ceramic insulators than for porcelain insulators. Particularly important is the better performance in contaminated conditions. This was discovered during the testing of the first generation of polymer insulators. However, flashover of non-ceramic insulators has been observed in extreme condition, which gives importance to the study of the flashover phenomenon. Laboratory studies indicate that the flashover mechanism of non-ceramic insulators is different compared to porcelain insulators.

Some insulators are built with two shed diameters. The shed diameters alternate along the length of the insulator. The smaller sheds collect less pollution than the larger ones. The pollution is distributed uniformly on the insulator surface but the amount of pollution is different on the upper and lower surface of the shed, also different amounts of pollution were observed on the shank. Figure 2 shows these different areas on the insulator surface.

The flashover between two electrodes on a polluted SIR surface was studied. Electrodes were placed on one shed of a SIR insulator. Both the upper and the lower surface were polluted with a kaolin and salt mixture and were permitted to recover hydrophobicity. The insulator was energized to 10kV (rms) and the pollution was wetted by steam. In nature, the early morning fog or dew wets the insulator in a similar way.

Pollution collecting areas on a non-ceramic insulator

The flashover mechanism of an actual insulator is more complicated because the insulator is divided into four distinct areas shown in Figure 2. The areas are the upper shed, rim, lower shed and shank. The contamination survey showed that the contamination level on each area is different (Figure 3). Also the wetting process depends on the position of the surface. As an example, the water droplets tend to roll of the vertical surface.

Pollution distribution on sheds

Pollution

The actual flashover voltage in natural conditions cannot be measured. However, operating experience has proven that non-ceramic insulators perform better than porcelain in polluted conditions. When non-ceramic insulators replaced the porcelain ones, interruptions due to flashovers stopped. The leakage distance of the non-ceramic insulators was similar to the porcelain insulators. This proves that the flashover voltage of non-ceramic insulators is higher than the porcelain ones. Also, laboratory tests showed significantly higher flashover voltages for both silicone and EPDM insulators compared to glass insulators. Figure 4 compares the flashover voltage of SIR, EPDM and glass insulators. The figure shows that the flashover voltage of an insulators decreases similarly by increasing the pollution level. In general the non- ceramic insulators performed better than the glass insulators. It is important to note that the flashover voltage of a hydrophobic SIR insulator is higher than that of a hydrophilic one, whose hydrophobicity has not yet recovered. Figure 4 also shows that the performance of SIR insulators is better than those of either EPDM or glass. These data can be used only for comparison, because the actual flashover voltage depends on the insulator age, shape, the distance between the sheds, and the shed diameter.

Comparison of pollution performance of aged non-ceramic and cap and pin glass insulators

Karady has described the flashover mechanism of non-ceramic insulators and he has obtained the following results:
1. Non-ceramic insulators collect more pollution than porcelain insulators.
2. Pollution on SIR insulators forms a thin layer consisting of dust, salt and silicone oil mixture.
3. This mixture prevents the release of conductive material when wetted, which results in high surface resistance.
4. The flashover mechanism of non-ceramic silicone insulators is different from the flashover mechanism of porcelain insulators.
5. The actual flashover voltage of non-ceramic SIR insulators is higher than the flashover voltage for porcelain insulators in polluted conditions.
6. Aging slightly reduces the flashover voltage.
7. The results of flashover tests are not conclusive. The salt-fog and clean fog tests can produce greatly different flashover voltages.


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