Ionization, electrode surfaces and discharges in SF6at extra-high-voltages

The influence on DC electrical performance of SF6gas from small protrusions on electrode surfaces has been investigated experimentally in a system of moderate size over the pressure range from 1/2 to 15 atm. abs. The protrusions used were steel or aluminum spheres with 0.079cm radius or a rod 10 times higher than its tip radius of 0.039cm. Over the complete voltage range, 100 kV to 1500 kV, good agreement was found between measured values and those calculated using a simple ionization development model for discharge initiation. This simple model was applied to calculate the effect of other protrusion shapes. In each case when the product of gas pressure times protrusion height above a flat electrode exceeded 80 atm-microns their presence decreased the breakdown stress. Scanning microscope views of the sphere tips after sparking showed microscopic protrusions which account for the deterioration found after the first spark.