Finding ideal profiles for porcelain and glass suspension insulators has proven a challenging process, as demonstrated by a succession of many different designs introduced over the past 100 years. All aimed to achieve some optimal combination of pollution flashover performance and self-cleaning properties. As a result of this evolution, there is now a broad range of different profiles of ceramic disc insulators in service on transmission networks worldwide. Whatever preferences have emerged for one design over others have been due to factors such as local climate, topography and tradition. In China, for example, most power companies have come to prefer double or triple-shed aerodynamic profiles.
Double (shown here) or triple shed aerodynamic profiles
are preferred by Chinese power utilities when specifying
ceramic line insulators.
By contrast, the main factors affecting shed profile in the case of composite insulators have traditionally been related to production. Depending on the manufacturing technology used, most sheds are smooth and not overly complex so that they can easily be removed from the mold cavity during production. The variations that do exist tend to be in shed size, spacing and angle of inclination. Yet there has still not been a large body of research on how shed profile impacts pollution performance of these types of insulators. Some experts believe specific creepage distance plays the decisive role while others disagree.
In China, past co-operation between Tsinghua University and the China Southern Power Grid was aimed at evaluating shed profiles for composite insulators in terms of relative pollution flashover performance. Supported by local manufacturers, the research assessed 36 different shed profiles. All were classified into 4 main categories, depending on number of different shed diameters within each repeating unit:
1. one large, one small;
2. one large, two small;
3. one large, one medium, two small;
4. one large, one medium, four small.
Insulators used on ±400 kV line to Tibet see one large
shed one medium shed and two small sheds in each
repeating unit.