On The Origin, significance, and Minimization of Nonuniform Contamination Along HVDC Insulator Strings

it is shown theoretically and experimentally that the electric force on charged airborne contaminants can cause the distribution of contaminant along the length of an insulator string energized by HVDC to be nonuniform. This nonuniformity causes a nonlinear voltage distribution along the string. As a result, some sections of the insulator string may be subject to electrical stresses high enough to initiate insulator failure. Comparative flashover tests between insulators with uniform and nonuniform contamination distributions confirm this hypothesis (i.e. that nonuniform contamination along the insulator string degrades HVDC insulator performance). It is also shown that grading rings can be attached to each end of an insulator string without creating air gaps less than those needed to protect against transient overvoltages. The grading rings modify the electric field in the vicinity of the string and cause the distribution of contamination along the string to be more uniform. This in turn may lead to improved insulator performance