Finding ideal profiles for porcelain and glass suspension insulators has proven complex with a succession of designs introduced over the past 100 years. All aimed to achieve superior pollution flashover and self-cleaning performance. As a result, there is a relatively broad range of different profiles of ceramic insulators now in service on transmission networks. Whatever preference has emerged from one region to the next has been based on climate as well as local topography and even level of economic development. In China, for example, double or triple-shed aerodynamic profiles are preferred. By contrast, shed profiles for composite insulators have traditionally been dictated by production constraints. Depending on manufacturing technology, most sheds are smooth and cannot be made too complicated or else they could not be easily removed from the mold cavity. Whatever variations do exist tend to be in shed size, spacing and angle of inclination.
There has still not been that much systematic research on how shed profile impacts pollution flashover performance and the level of knowledge on the subject can best be described as ‘fuzzy’. Some believe that specific creepage distance plays a decisive role while others disagree. In China, co-operation between Tsinghua University and the China Southern Power Grid, along with support from manufacturers, has resulted in 36 different shed profiles for composite insulators. All have been classified into one of four categories depending on number of different shed diameters within each repeating unit: 1. alternating one large and one small; 2. one large, two small; 3. one large, one medium and two small; and 4. one large, one medium and four small. Fig. 1 depicts the first of these categories, namely alternating large and small sheds.
Fig. 2 shows the DC pollution flashover voltage obtained for these 36 possible shed design categories using the solid pollution layer method and with test voltage applied by the constant ‘up and down’ method. Values shown represent a proportionate per unit comparison of flashover voltage of each different design versus that of the best-performing geometry, namely the one with alternating one large and one small shed (#22). These findings confirm that shed geometry does have a profound impact on pollution flashover performance of composite insulators, i.e. by up to 22% for units with the same insulation distance. As part of this same research, a total of 19 test specimens of alternating shed design with different shed spacing and radii were compared in terms of relative pollution flashover performance. As per Fig. 3, all 5 designs with different radii have superior flashover performance when the shed spacing is 100 mm. Moreover, it is also obvious that it is not necessarily true that the greater the diameter the higher will be the flashover voltage. Rather, an optimal value exists and flashover voltage was found to be highest with large shed/small shed radii of 90 mm and 66 mm.
Fig. 4 shows the impact of average radius of large and small shed diameters on pollution flashover behavior under 3 different types of spacing of the large sheds. It can be seen that flashover voltage is highest at a spacing of 80 mm. Of course, other factors beside pollution performance also have to taken into account in deciding on the best geometry for composite insulators and these include protection against flashover due to bird streamers or ice bridging. In this regard, there is clearly a need for yet additional research.