Interruption in Air for Different Medium-Voltage Switching Duties

Air is an environmentally benign alternative to SF for use in medium-voltage load-break switches. A simple, axisymmetric test switch has been used for empirical studies of the thermal phase of current interruption in atmospheric air. The purpose is to quantify how the pressure drop across the nozzle influences the interrupting capability at a different rate of rise of the recovery voltages (RRRVs) and with different current amplitudes. Tests with pressure drops in the range 0.1-1.1 bar, RRRVs of 40, 80, and 160 V/mu s, and currents of 300, 600, and 900 A were carried out. In general, the current that can be successfully interrupted is proportional with the pressure drop. Likewise, a steeper RRRV requires a proportionally higher pressure drop for the interruption to be successful. For compact air load-break switches for the important 24 kV/630-A class, it seems sufficient to provide a pressure drop of around 0.25 bar lasting for at least 20 ms to comply with the "mainly active load" test-type requirements.