Determination of opening velocities for vacuum circuit breakers at transmission voltage

The objective of this paper is to propose a quantitative method to determine opening velocities for VCBs at transmission voltage. A couple of 2/3 coil-type axial magnetic field (AMF) contacts, which are used in a 126 kV vacuum interrupter, are arranged in a demountable vacuum chamber. The movable contact is opening with three different kinds of opening travel-curves. Each curve is adjusted by two opening velocities, v1 and v2, respectively. The v1 is an average opening velocity within 1/3 full contact gap, which is with 1.8 m/s, 2.4 m/s and 2.7 m/s, respectively. The v2 is an average opening velocity within 2/3 full contact gap, which is with 2.4 m/s, 2.7 m/s, and 3.0 m/s, respectively. The test current ranges from 4 kA to 40 kA, stepped by 4 kA. When a VCB is used to interrupt a short-circuit current, the preferred opening velocity is determined by reducing the arcing interval tI for the intense arc mode and increasing the arcing interval tII until the formation of high-current anode mode (HAM). The preferred v1 is higher than a threshold value v1_th, at which there is a peak critical contact gap dI_peak, for the evolving from intense arc mode into diffuse arc mode. The preferred value v2 is lower than another threshold value v2_th, at which there also exists a peak critical contact gap dII_peak, for the evolving from diffuse arc mode into the footpoint mode and anode spot mode.