Non-invasive measurement of metal vapor parameters and an improved CTM for post-arc sheath growth in intermediate-frequency vacuum switch

The number density of metal vapor in the post-arc contact gap of intermediate-frequency (IF) vacuum circuit breakers has significant influence on the growth of the post-arc sheath but measurements are scarce. Image processing techniques including edge detection and multi-target tracking have been applied in the present work to arc images to derive metal vapor pressure and number density. Through three-dimensional construction of the metal droplet ejection process from the contact surface, a velocity of 10–20 m/s and acceleration of 3.5–6.2 × 10⁵ m/s² were obtained for the droplets. Inside the vacuum chamber, the maximum local pressure is around the droplets in post-arc phase, with the value of 1–5 MPa. Accordingly, the maximum metal vapor number density reaches 10²⁵ m⁻³. The continuous transition model (CTM) has been modified in consideration of the effect of metal vapor ionization on sheath growth and it is capable of predicting that when the metal vapor number density increases from 10¹⁸ m⁻³ to 10²³ m⁻³, the sheath expansion time varies from 2 μs to 5 μs, which demonstrates that metal vapor is a significant factor affecting the post-arc dielectric recovery in high-current IF vacuum arc.