An overview of electrical properties for stress gradingoptimization

The scope of the following paper is to present, both from a theoretical and a practical point of view, some aspects involved in stress grading in the general configuration where insulated HV conductors pass through a grounded screen. The difference between capacitive and resistive materials is discussed; particular attention is devoted to the rules governing the potential and the field distribution in both cases. The case of a capacitive varnish with nonlinear permittivity is discussed and a comparison with materials of constant permittivity is made. The experimental behavior of nonlinear semiconducting materials used in practice are examined and a discussion is given on the possible use of resistive materials of constant conductivity. Finally, practical considerations are listed as basic rules for optimizing the grading effect