Dielectric Characteristics of SF6 Gas for Application to HVDC Systems

Dielectric problems of SF6 gas insulated DC apparatuses were investigated. The DC breakdown voltages of the SF, gas gap become lower with larger electrodes. The decrease can be estimated fairly well for electrode areas of 101 to 104 cm2 on the basis of the statistical distribution of the initial breakdown field strength for small electrodes. Preceding the gap breakdown large dark currents start to flow at the field of 5 MV/m, which are caused by metal particles on the anode. However, this current can be eliminated by sufficient cleaning of the anode. The DC dielectric strength of spacers supporting high voltage conductors is decreased by metal particles, but is only as low as that for the lightning impulse voltages. Moreover, breakdown voltage of spacers are not affected by DC polarity reversal tests when done under contamination with metal particles or when clean. As a result of these investigations most of the technical difficulties that arise in the gas insulation of HVDC power apparatuses could be eliminated.