A prediction of deposition of contaminants on insulator surface with and without grading rings

The deposition of contaminants on a HVDC insulator is primarily governed by the inwardly-directed component of the electric field at the insulator surface. This component is calculated for a string-type insulator with and without grading rings. The results seem to explain satisfactorily the formation of a clean (?contaminant-free?) ring surrounding the insulator pin as well as around the insulator edge. An optimum ratio of the diameter of the grading ring to the insulator disc occurs for various insulator lengths. The contamination pattern on the surface of the insulator in the absence of wind forces can therefore be predicted to a certain degree of accuracy. The results are presented and discussed in the light of theoretical models.