SF6 breakdown in cylindrical field under dc voltage

Investigations were made in a coaxial cylinder system at high dc-voltages up to 900 kV and at high gas pressures up to 6 bar. The SF6-breakdown-behavior can be classified into three regions according to the following aspects: (1) breakdown after steady corona discharges, (2) validity of the similarity law for the discharge onset voltage, and (3) deviations of the similarity law with a marked polarity effect. Possible explanations are given for the polarity effect and the deviation in region 3. The results can be of importance for a future application of SF6-insulation in HVDC systems.