Electrical characterisation of ZnO microvaristor materials and compounds

One of the most challenging design aspects of high voltage equipment is to ensure adequate electrical field distribution and controlled magnitudes within the insulation regions of the equipment to ensure integrity and long term satisfactory in-service performance. Electrode geometry can be designed to improve distribution and lower peak magnitudes of electrical fields. However, this is not sufficient in many cases. In order to influence the distribution of the field within the insulation medium and its surroundings, a number of techniques were introduced for bushing, cable terminations and electrical machines. These include semi-conducting and specialist grading materials. In recent years, the development of zinc oxide (ZnO) microvaristors has allowed a further opportunity for field control, in particular, in outdoor insulators and cable terminations applications. This paper gives an overview of the fabrication process of ZnO microvaristors loaded grading compound materials for electrical field control and details an experimental approach to characterize the material. Further analysis was carried out to obtain the electrical properties of the material, such as conductivity and permittivity as a function applied voltage. An application to outdoor insulators is proposed and modelled.