Comparison of Different Air Flow Concepts for a Medium Voltage Load Break Switch

The research and development work towards a compact SF6-free load break switch for the medium voltage range has led to several design proposals. The interruption capability of three different nozzle and gas flow concepts with atmospheric air as the interrupting medium is compared and assessed. The three test switches are installed in circuits corresponding to the mainly active load and switch-fuse test duties of the 24 kV / 630 A load break switch standard. A pressure tank is used to provide different air flow rates, and the interruption capabilities of the different flow concepts are compared with basis in the tank pressure required to give successful interruptions. 270 current interruption tests were carried out. Air flows directed radially onto the arc or swirling along the arc turn out to result in a substantially better interruption performance than when the air flows straight and parallel to the arc. Air flows corresponding to upstream over-pressures of a few tenths of a bar seem to be sufficient for an air-based load break switch rated for 24 kV / 630 A.