Breakdown sequence in SF6 at pressures up to 8 bar in a geometryrepresentative of a circuit-breaker

Abstract

Breakdown sequence in SF6 of an electrode arrangement whichrepresents the contacts of a circuit-breaker is investigated at pressures upto 8 bar under positive impulse voltages. The Schlieren frames point out therole played by the cathode as a source emitting electrons by field effectemission. Photomultiplier records demonstrated, at Pd = 2 bar, that thedischarge's formative time strongly decreases: from 1 µs at pressure lowerthan Pd, down to 150 ns at pressure higher than Pd. A change in thedischarge regime is associated with this transition. An experimentalinvestigation, using photomultipliers and Schlieren devices, allows us todescribe both breakdown sequences. At low pressures, there is a criticalvolume at the anode. Nevertheless, it is a negative discharge which isresponsible for the breakdown although the geometrical electrical field at thecathode boundary is about 60 kV cm-1. At high pressures, there is nocritical volume. However, a positive discharge develops across the gap.Physical considerations taking into account the electron energy are proposedto explain the mechanisms involved in both types of discharge.