Methods of equalizing electric fields in high-voltage devices

Size and weight reduction of high-voltage devices depends on an insulation system which ensures a uniform, or almost uniform, electric field distribution over the surface. This in turn requires large metal backup parts and their monolithic bonding to the dielectric part, a source of high thermoelastic stresses. The problem of reducing these stresses is analyzed on the basis of the equation of elasticity for a monolithic insulator with metallized inside wall and metal mounting endpiece. According to the solution of this equation, the thermoelastic stress between dielectric and metal depends on the difference between their linear thermal expansion coefficients and on the ratio of their Young's moduli. Reliable performance requires that the thermoelastic stress be 20 to 50% lower than the ultimate tensile strength of the dielectric and the ultimate adhesive strength of the dielectric-metal bond, these levels being determined principally by the thermal relaxation characteristics of the dielectric. An evaluation of various materials and manufacturing methods indicates that both equalization of electric fields and minimization of thermoelastic stresses are attainable by using ceramic materials or polymer molding compounds for insulators, with a shielding brass mesh rolled into a wire ring for weakening the edge effect and raising the mechanical strength.