Silicone composites for HV applications based on microvaristors

Microvaristors are small sintered particles of doped ZnO with strongly nonlinear electrical properties similar to those of the varistor ceramics. Composite materials based on a silicone matrix filled with microvaristor powder have a nonlinear current-voltage relation and can be used for field grading applications. The field dependence of the conductivity is determined by the properties of the filler particles, and can be tuned by modifying the formulation of the microvaristors and the sintering conditions as sintering time or temperature. In this contribution, we describe the nonlinear field grading material, and report on the development of a prototype field-grading element for a dry and compact high voltage termination, which has been successfully tested.