Performance of axial magnetic field vacuum interrupters at various transient recovery voltages and contact gaps

The primary application of vacuum interrupters (VI) is to interrupt short-circuit currents during faults on electrical power systems. One common VI design uses an axial magnetic field (AMF) to force the arc to remain diffuse, thereby allowing the interruption of very high short-circuit currents. In this work, VI's with 62 mm diameter AMF contacts were short-circuit tested in a synthetic circuit at rated voltages ranging from 17.5-38kV, and with contact gaps ranging from 7-19 mm. Despite the wide range of test parameters, the key parameter determining the success of the interruption attempt was the transferred charge during arcing Q. For Q <; 275 A · s, the VI's interrupted with a high degree of success at the first current zero; and for Q > 275 A · s the interruption performance dropped sharply, with a narrow transition region. This performance was observed over the full range of transient recovery voltages (TRV) and contact gaps tested, which represent most of the medium voltage application range. These results suggest that the transferred charge during arcing is a key characteristic for AMF contact designs.