Pollution Design of Composite Insulators is performed according to an related standard. The contamination performance of polymeric insulators is greatly dependent on the weathershed material performance.

Pollution Design of Composite Insulators:
Aging, which leads to loss of hydrophobicity, tracking and erosion, and eventually to flashover is still one of the main problems with polymeric insulators. Although the leakage current is a cause of aging, it can be used as an indicator of surface degradation. Thus, the parameters of leakage current waveforms have usually been used to evaluate polymer insulators performance. Several attempts have been made to correlate leakage current parameters to the contamination level and to the surface degradation like loss of hydrophobicity, dry- band arcing, and tracking and erosion of insulators. Hence, it is important to predict the level of leakage current before the occurrence of the loss of hydrophobicity and housing damage.

The performance of polymeric insulators is evaluated mainly based on IEC 61109. However, since the pollution design method for polymeric insulators has not yet been established, the conventional design method for porcelain insulators is adopted. In overseas countries, pollution design is performed according to an international standard, IEC 60815, based on pollution levels such as light (creepage distance 16 mm/kV), medium (20 mm/kV), heavy (25 mm/kV) and very heavy (31 mm/kV).

Polymeric materials such as SIR and EPDM have been in use as weathersheds on outdoor insulators for about 40 years. Contamination- related power outages caused by dry-band arcing is a serious limitation with porcelain and glass. Standard laboratory tests, such as the salt-fog and clean-fog test, which evaluate their performance under simulated outdoor contamination conditions have been developed by the IEEE, CIGRE (International Conference for Large Electric Networks) and IEC (International Electrotechnical Commission). As there is no such standard test for polymeric insulator performance, they are being evaluated by methods developed for porcelain and glass.

The contamination performance of polymeric insulators is greatly dependent on the weathershed material performance. There are two important factors, which have been demonstrated in service that should be considered in the material evaluation. The first is due to the relatively poor thermal stability of polymers, dry band arcs can cause weathershed degradation in the form of tracking or erosion even before insulator flashover. The second factor is that SIR resists the formation of a continuous electrolytic film, thereby limiting leakage current, much better than EPDM and porcelain [83]. This has been the reason for the superior service performance of SIR insulators [84]. In order that a test be considered suitable for polymers, correlation with results obtained from service has to be demonstrated. The insulators made from polymers cannot be evaluated meaningfully using the same contamination test conditions which have been standardized for porcelain and glass.

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