Stepwise behavior of free recovery processes after diffused vacuum arc extinction

The objective of this paper is to gain insight into free recovery processes in vacuum interrupters after diffused vacuum arcs extinction. A sub-microsecond voltage impulse, which peaked at 90 kV with a rising time of 150 ns) (rate of rise 480 kV/μs), were electronically controlled to apply upon a switching gap of a vacuum interrupter at a predetermined interval after a power frequency current zero. Based on the breakdown voltages at different intervals after arc extinction, we can obtain the free recovery behavior of the vacuum interrupter. The contact materials under study included Cu, CuCr₂₅ and CuCr₅₀. The contacts were butttype, and the contact diameters were 12 and 25 mm, respectively. The arc current peaked at 2.1 kA with a frequency of 50 Hz and the arcing time was ~9 ms. The results show the free recovery behavior stepped from ~28 kV to ~70 kV at ~4 μs after arc extinction for the three kinds of contact materials with the two contact diameters given. By calculation of the dissipation of metal vapor density and ion density, we conclude that the decay of ions, rather than metal vapor, may be the main reason for the stepwise behavior of the free recovery process after diffused vacuum arcs extinction.