Non-linear AC properties of filled resins

Previous attempts to introduce a degree of non-linear conductivity into polymeric insulating materials, and thereby to achieve stress relief under non-uniform or impulse conditions, involved the use of zinc oxide as a filler. This technique produced a field-dependent conductivity in the filled composite and was advantageous under DC and impulse conditions. Most solid insulation, under normal service conditions is, however, subject to alternating voltages. Stress relief in this instance is more likely to be achieved by introducing field-dependent permittivity rather than field-dependent conductivity. The author describes a successful attempt at producing a composite polymeric material that exhibits field-dependent permittivity by introducing barium titanate, and its variants, as a filler in polyester resin. The effect of filler concentration levels and the grain size of the particulate filler on the degree of field-dependent permittivity have been investigated.