On the design of metal-oxide varistors for dynamic insulation of high voltage systems

The author describes the performance of metal-oxide varistors as applied to dynamic insulation of high-voltage power systems. A mathematical model for a dynamic insulator is constructed, which takes into consideration the effects of the surface contamination conditions as well as the arc development and propagation on the flashover behavior of outdoor insulators. The nonlinear I-V characteristics of metal-oxide elements are utilized to both suppress system overvoltages and improve the flashover performance of these insulators. A method for designing the optimum configuration (i.e. the one that corresponds to best transient and flashover performance) of metal-oxide units of given characteristics is developed. Experimental data of zinc-oxide varistors are used to illustrate the design procedure. It is shown that the optimum zinc-oxide configuration depends mainly on the system voltage level, geometry of the insulating surface, and atmospheric contamination level. The new design procedure is used to develop a simple dynamic insulator unit resulting in an improvement of over 14% in the flashover performance under uniform contamination conditions. The concept of manufacturing solid insulator units with built-in dynamic insulation properties is suggested