A Statistical Aspect of the Coordination of Two Gaps

Many problems of insulation coordination are concerned with the protection of an air gap by means of another air gap: protection of the external insulation of a bushing by means of a rod gap, protection of line-to-tower insulation by means of the ground hardware of an insulator string, protection of the open gap of a switch by means of the line-to-ground insulation of the stack, etc. When dealing with stresses caused by switching surges, the inherent dispersion of the dielectric characteristics of the air gaps, i.e., the probability of sparkover and the times-to-sparkover, is such that these problems must be approached with statistical methods. Furthermore, since the designer is interested in the probable number of sparkovers per year and not in the absolute probability of sparkover, the number and the distribution of the stresses which the electric system applies to the insulation considered must be taken into account. This paper intends to provide a tool to the designer and to show how it works by applying the suggested method to the coordination of two rod gaps. In the case examined, the necessary quantitative knowledge will be determined by laboratory tests or taken from previous system studies.