This paper deals with leader breakdown characteristics of SF₆ inhomogeneous gaps under positive lightning impulse voltages. The high voltage electrode is a thin aluminum sheet whose thickness is 100μm, the grounded electrode is a plane electrode, and gap length is 50mm. To control the electric field at edges of the sheet electrode, we used the electric shields that are four types.
It is found out that breakdown voltages can be estimated by a supposition of space charge which acts like the electrodes around the edges of the sheet electrode. We named this space charge "Imaginary Electrode", When the electric field of Imaginary Electrode surface reaches the critical field, a leader breakdown occurs. With the present experimental electrodes, electric shields don't affect the radius of Imaginary Electrode, but gas pressure affects the radius of Imaginary Electrode, which is 0.25mm at 0.5MPa, 0.5mm at 0.3MPa, and 3.0mm at 0.1MPa. This gas pressure dependency of the radius of Imaginary Electrode is influenced by the structure of inhomogeneous electrodes.