Charge Injection and Multiplication in SF6 and Vacuum Under DC Electric Fields

In order to evaluate the impact of accumulated surface charges on dielectric surfaces in gaseous insulation, the source of such charges was determined experimentally. Injected charges were measured in a coaxial electrode setup having two different surface roughnesses. Measurements were performed at low electric DC fields (< 2 kV/mm) in both vacuum and pressurized SF6. This allowed to separate gas-dominated from electrode-dominated effects. Measurement results show that charge emission at negative polarity starts at rather low values of the electric field, i.e. 0.3 to 0.7 kV/mm With the proposed setup, it is possible to resolve natural ionization currents in SF6.