The insulation of ultra-high-voltage in coaxial systems using compressed SF6gas

The DC voltage insulating performance of compressed SF6gas at multimegavolt levels was studied in a metal-clad test facility containing large, 760 mm dia., horizontal coaxial electrodes. Seven different coaxial systems which employed cast epoxy post-type spacers for supports were investigated at gas pressures from 1 to 7.8 atm absolute. Discharge voltages in SF6ranged from 500kV to 4300kV while similar tests with nitrogen and air ranged from 200kV to 1200kV. These results demonstrated that, even in large apparatus at UHV levels, breakdown voltages in SF6increase with increasing system size according to the similarity law when the gradients are moderate and that in such large systems electrode surface features and contaminant particles remain important factors which can reduce insulating performance. Wire particles placed within the test electrodes were found to be more detrimental here than in small systems when particle size is expressed as a percentage of the gas gap length. It is concluded from the tests that compressed SF6can be considered a viable dielectric for UHV equipment of the highest voltage.