Effects of surface heat distribution on the flashover of insulators

When a potential difference is established between the terminals of an insulator Joulean heat is produced in the insulator surface conductance and to a much lesser extent in the body conductance. As this heat dissipates to the surrounding~ the insulator surface temperature is everywhere raised above the ambient. The surface temperatures which result are never uniform but are determined by the atmospheric humidity, the distribution of surface contamination, the thermal properties of the insulator and the surrounding medium and on the complex means of heat dissipation. The hotter areas of surface are less prone to condensation and therefore tend to have a lower surface conductivity. The voltage distribution over an insulator surface depends in the absence of a surface conducting layer, on the shape of the electrostatic field and this in turn depends on the shape of the insulator. If an insulator can be so shaped that the voltage between its terminals is uniformly distributed over its surface then the breakdown voltage is probably equal to that of the air path between its terminals provided that the surface has no re-entrant curves. A surface conducting layer on an insulator modifies the voltage distribution over the surface. In practice, in a humid atmosphere, this tends to move the areas of highest voltage gradient towards the hottest areas on the surface. Due to heat transfer from the line conductor, the lower insulator unit of a string is normally at a slightly higher temperature than the remainder of the string. In humid atmosphere conditions this worsens the normal mal-distribution over the string. If the whole surface of an insulator is maintained at a temperature substantially above the atmospheric, condensation, in humid atmospheric conditions is prevented and contamination discouraged. Such insulators should in fact have the voltage distribution determined only by capacitance distribution. This voltage distribution is amenable to discipline at the design stage. Considerable waste heat is normally available from a line conductor. Using suitable heat exchangers some of this waste heat may be transferred to a hollow long rod type insulator. A 2-metre length of conductor was found to give an adequate temperature rise to the insulator which supported it.