Investigation of vacuum arcs between with varying gaps up to 40 mm

As one of the most common circuit breaker technology, the vacuum circuit breaker (VCB) is widely used in medium grid voltages. A main task is the interruption of high short circuit currents, where the VCB outperforms other approaches due to low arc voltages during breaking operations. These low voltages are the result of the special kind of plasma produced as the contacts are separated. The applied current evaporates the contact surface and with the lack of a gaseous atmosphere, a metal vapor plasma is ignited. To minimize evaporation on a stationary point axial or transversal magnetic fields are applied to move the plasma over the contact surface. This plasma constricts due to the pinch effect with rising currents. The constriction behavior for gaps in the range of 5 to 20 mm gaps is well known and sufficient for VCBs in medium voltage grids. Higher voltage levels require lager gaps to withstand dielectric stress. With growing gaps, the question of the constriction behavior of vacuum arcs arises.To investigate this task pin electrodes are used to create a reproducible reference for different gaps. To achieve long contacts gaps a programmable frequency converter with servo drive is used. Different opening curves with final gaps up to 40 mm are performed. With a 35 Hz resonance frequency of a synthetic test circuit achieve long arcing times are produced with peak currents up to 7.25 kA. The breaking operations are observed using a high-speed camera. This test setup is combined with a computer-aided evaluation tool for reproducible analysis of arcing behavior. The description and discussion of the behavior with rising gaps shows a polarity and velocity dependence of the arc greater than 15 mm. The optimum of the arc length and controllability is essential of vacuum interrupters. This contributes to the feasibility of VIs in higher voltage levels.