Silicone rubber insulators for polluted environments part 2: textured insulators

The design principles and prototype manufacture of 11kV silicone-rubber composite textured insulators are described. The moulded surface texture increases the maximum creepage length from 375 mm to 503 mm for the four-shed profile used. The trunk sections of the insulator are patterned with a square-section array of hemispherical protuberances, and the shed undersides with interlaced logarithmic spirals of hemispherical ridges. Both textures are sufficiently shallow to facilitate fabrication, to allow effective washing and to avoid an increased salt deposit density (SDD) in artificial pollution tests. The test procedures which were developed in Part 1 of the paper are applied here to the textured prototypes. These procedures had been shown to quantify reliably the performance of conventional non-textured insulators of the same profile: the variation of flashover voltage and leakage current due to the silicone-rubber formulation, SDD and wetting rate could be measured. For textured insulators, it is found that the increased creepage distance improves the flashover voltage by up to 27%, with a corresponding reduction of pre-flashover leakage current. Comparative tests for two silicone-rubber formulations of different hydrophobicity and pollution performance were performed. Although very severe pollution causes a large reduction in flashover voltage, surface texturing mitigates this reduction effectively. This is particularly relevant to recent concerns expressed in IEC-TS 60815-3 (2008) relating to the selection and dimensioning of high-voltage a.c. polymeric insulators for use in heavily polluted conditions.