Distribution of cathode current density and breaking capacity of medium voltage vacuum interrupters with axial magnetic field

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Abstract
A high-speed photographic technique incorporating computer-aided reconstruction of the cathode current density was applied to butt solid contacts with an external axial magnetic field (AMF) at relatively low current density j<2 kA/cm/sup 2/. In the present work, we have further developed this technique. This advanced technique is applicable for the analysis of the current density distribution on the complicated electrodes of commercial vacuum interrupters (VIs) carrying currents up to I=50 kA at current density up to j/spl sim/4 kA/cm/sup 2/. The experiments have been carried out for three types of electrode designs generating AMFs with different configurations. The results obtained by this new technique proved a previously derived conclusion that current density tends to distribute evenly across that part of the contact surface where the AMF induction fits the inequality B/sub z//sup (1)/<B/sub z/<B/sub z//sup (2)/, where B/sub z//sup (1)/, B/sub z//sup (2)/ are characteristic induction fields defined in our previous work. Results of the analysis of erosion tracks on the electrodes are also in agreement with this conclusion. Slots on the electrode surface lead to substantial nonuniformity of current density distribution even if the AMF is properly configured. Comparison of the breaking capacity and distribution of current density for different electrode designs proves that interruption failure occurs at peak current density /spl sim/3.5 kA/cm/sup 2/. This value exceeds by at least 50% previous estimates of maximum current density interruptible by a commercial VI. We have also found that changing the configuration of the AMF may substantially improve breaking capacity. In our experiments, a slight modification of the AMF distribution provides a 12% increase of the breaking capacity.