THE INFLUENCE OF A CYLINDRICAL SHIELD ELECTRODE ON THE DIELECTRIC STRENGTH OF LARGE VACUUM GAPS

The investigations are carried out with replicas of switching electrod es with gap spacings up to 20 mm. The electrodes are surrounded by an insulated metallic shield, which resembles the abstract model of a vac uum interrupter. The shield diameter of 100 to 120 mm results in an ad ditional cylindrical gap of 12.5 to 22.5 mm between the shield and the outside of the switching electrodes. The influence of this additional gap on the breakdown voltage of such three-electrode arrangements wil l be demonstrated. The model electrodes are made of stainless steel, c opper or aluminum alloys, respectively. Before stressing the vacuum ga p with 1.2/50 mus impulse voltages, the electrodes are conditioned by applying high ac voltages which are slowly rising until the current re aches a certain value. To avoid electrode damage the current is limite d by a 1 MOMGEA resistor. After this conditioning procedure the breakd own voltage is determined by a kind of 'up and down' method. The prebr eakdown behavior of the three-electrode arrangement is similar to that of a homogeneous two-electrode gap. The shield electrode gets a poten tial of approximately 45% of the applied voltage. The time-dependent s hield potential can be measured by a capacitive probe. The results of these investigations demonstrate that diameter as well as material of the shield electrode influence the dielectric strength of the entire e lectrode arrangement in vacuum. A comparison of these results with tho se of the breakdown voltages of homogeneous vacuum gaps shows a signif icantly lower increase of breakdown strength with increasing gap spaci ng or decreasing shield diameter.