Fault Location in SF6 Insulated Conductors Using Direct Fluxgate Magnetometry

Gas insulated substations (GIS) are becoming more popular for substation design largely due to reduced space requirements as well as environmental benefits. Isolated phase designs generally consist of a single cylindrical conductor enclosed in a coaxial aluminum tank (or sheath). Pressurized SF6 is usually used for electrical insulation. For increased reliability, instrumentation methods have been developed to detect the location of power arcs which occur within the enclosed containers. Under EPRI funding, we have developed a new technique in which the location of a fault is detected by directly measuring the axial component of the 60 Hz magnetic field which appears outside the sheath during the flashover. The instrument is a specialized form of "fluxgate" magnetometer modified to allow long life using inexpensive batteries. The magnetometer output is coupled directly to a switching circuit which operates a "magnetic disc" indicator. When a fault is registered, the disc indicator exhibits a bright color which can be easily read from a distance of 10-15m. The instrument is proposed for use at areas within the substation which are more prone to flashovers, especially near dielectric spacers which support the inner conductor every few meters. Flashover measurements indicate that the sensor reliably detects a fault current of 4 kA or larger under typical substation conditions. This sensor is attractive for retrofitting existing gas-insulated systems in that installation does not require shutting down the system.