Hybrid DC Current switching in Vacuum under the effect of External Magnetic Field

We have investigated the switching performance of a capacitor-based hybrid DC current breaker using vacuum interrupter technology (VI) as a main mechanical switch. To increase the switching capability despite the low arcing voltage in vacuum we applied an external static transverse magnetic field through the VI by means of two permanent magnets to obtain a magnetic flux density of 35-60 mT between the contacts. The first consequence was increased current switching capability by 50% at the initial arc jump. But the most fascinating result was obtained for the high current switching >1kA. After the expected unsuccessful commutation associated with damped oscillation with the initial arc, amplified current oscillations have appeared after few ms, until the current zero crossing. This phenomenon is normally observed only in gas or in air hybrid DC circuit breakers where parallel HV capacitors are used for current commutation, but was never observed in vacuum so far. The reason is that, unlike in air or in gas, the vacuum arc in normal conditions has no apparent negative dynamic resistance. Explanation of such phenomenon in vacuum based on electrical and finite element Multiphysics simulations and synthetic experimental observations in this work and from the literature is presented and discussed.