Scaling SF6 helical arc interrupters for performance gain

Simplistically the debate about the relative merits of vacuum and SF₆ for power electrical engineering applications is often based upon dielectric withstand capabilities of the two media. Thus the exclusive use of SF₆ for EHV applications may superficially derive from its superior breakdown strength when moderately pressurised. There are, of course, other factors which govern technical choices for particular applications. For example the use of hologenic traces in Geiger Muller particle counters rather than vacuum alone was based upon their ionic scavenging action to produce better time resolution for particle detection. In the area of distribution circuit breakers the comparisons which need to be made between vacuum and SF₆ centre around the SF₆ rotary arc circuit breakers. This paper deals with one type of such circuit breaker which utilises the arc in a helical form. It is shown that ideal dielectric strength considerations alone are insufficient on which to base choices but that the helical arc device offers considerable scope for future design development. Hitherto insufficient knowledge and understanding of several fundamental aspects of helical arcing has prevented full advantage to be taken of this type of device. An attempt is made through this paper to describe some of these newer fundamental aspects and the manner in which they may impact on future design considerations for rotary arc circuit breakers