Air-Flow Investigation for a Medium-Voltage Load Break Switch

Replacing SF 6 with air as the arc quenching medium in switching equipment is favorable from an environmental point of view, but may be technically difficult for compact devices. Medium-voltage load break switches have modest current and voltage ratings, typically 630 A/24 kV, and are thus expected to be an earlier application area for air-filled devices than, for example, high-voltage circuit breakers. It is still necessary to understand how the various design parameters affect the interrupting capability. The influence of the air-flow parameters has been investigated through 970 interruption tests with three different contact and nozzle sizes and at three different load currents. The effect of increased air-flow rate versus reduced upstream pressure and air velocity is examined. By increasing the contact and nozzle sizes, lower upstream pressure is required to interrupt the current. According to simple cold gas-flow calculations, increased volume flow rate can, to a certain degree, compensate for reduced air velocity.