Improved Performance of Cylindrical Solid Insulators with Concave Curved Edges in Vacuum

The withstand voltage capability of a cylindrical solid insulator can be improved considerably by incorporating a concave geometry. in the insulator ends at the contact region with the conductors. It is found that the higher the value of the angle ¿between the curved edgÿof the insulator and the ambient field the lower is the value of the tangential field at the triple junction. The influence of the relative dielectric constant of the material and the length of the insulator on the field at the triple junction is found to be negligibly small. Measurements of the. flashover voltage in. ultrahigh vacuum (< 10-8 Torr) are conducted for cylindrical rods made of glass-ceramic with concave curvature at both-ends. ¿is varied in the range 0-65°. The DC, AC and 1.2/50 Ps lightning impulse flashover voltages are between 20 to 30% higher than the respective voltages of a straight cylindrical insulator of the same thickness. The DC pre-stress has negligible influence (< 10%) on the subsequent combined DC + impulse and DC + AC flashover voltages, of a concave insulator made of Macor glass-ceramic.