Control of Vacuum Arcs in High-Voltage Vacuum Interrupters by Suitable Stroke Trajectories of Opening AMF Contacts

Vacuum circuit breakers switch off high short-circuit currents during faults on electrical distribution and transmission systems. Vacuum interrupter (VI) contacts suitable for large contact gaps, as demanded for high-voltages (HV), often use axial magnetic fields (AMF) to force the switching arc to remain in a diffuse mode. Because the AMF amplitude decreases significantly with increasing contact gap, the constriction threshold of the vacuum arc and thus the interruption capability of a HV VI are sensitively influenced by the time dependence of the opening stroke of the movable contact piece. This work is devoted to the influence of the time-stroke curve of separating AMF contacts (CuCr) on the mode characteristics of the switching arc for contact gaps of up to several 10 mm as required for rated voltages exceeding 72.5 kV (rms). The spatial AMF distribution in the contact gap is varied during a 50 Hz current half-cycle by means of well controlled contact travel curves for currents ranging up to several 10 kA (rms). The resulting arc evolution is studied with a high-speed, high-resolution CCD video camera using a demountable vacuum chamber with optical access. The experimental data are compared to transient 3D numerical simulations of the vacuum arc plasma based on magneto-hydrodynamic modeling. Opening stroke trajectories of the contact are identified that enable minimum AMF amplitudes needed to suppress arc constriction and anode spot formation. The investigations demonstrate that the contact travel curve is a key characteristic of large-gap AMF contact designs enabling high-current interruption performance at high voltages.