Effect of Space Charge on Electric Breakdown of Sulfur Hexafluoride in Nonuniform Fields

In a nonuniform field, the positive direct and 60-cycle breakdown voltages of sulfur hexafluoride (SF6) as a function of pressure pass through a pronounced maximum. In the region of this maximum, corona is present at a much lower voltage than the breakdown voltage, and the impulse ratio may be less than one. In fact, the positive impulse, i.e., 1¿x40-¿sec (microsecond), breakdown voltagemay be only one half of the positive direct or 60-cycle breakdown voltage. This breakdown characteristic is contrary to the more usual behavior of gases which are less electron-attaching. For example, nitrogen and hydrogen have impulse breakdown voltages which are greater than the direct or alternating breakdown voltage. In this investigation, a positive impulse voltage was superimposed on a positive direct voltage applied to a point electrode. In the presence of corona, a space charge was established which caused the breakdown voltage versus positive direct voltage bias characteristic of SF6 to have a slope opposite to that obtained for air. Below corona onset in SF6, the breakdown voltage does not vary with bias. At high pressures where corona onset and breakdown are nearly coincidental, the breakdown voltage of SF6 does not vary with bias. The experimental evidence indicates that the occurrence of the maximum in the breakdown voltage versus pressure characteristic of SF6 is caused by the ability of these electronattaching gases to form space charges which greatly enhance the direct or alternating breakdown voltage.