Optical investigation on pre-strike arc characteristics in medium-voltage load break switches
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Medium voltage load break switches (MV-LBS) should pass fault current while closing and be able to re-open for the next operation. Replacing SF6 as a high impact greenhouse insulating gas with air, makes the switch design more challenging because of the higher pre-strike arcing time and energy dissipation between contacts which leads to more contact surface erosion and an even higher possibility of welding. In this paper, a synthetic test circuit is used to emulate stresses applied to MV-LBS during the making of short-circuit currents. Since there are difficulties in accurate direct measurement of arc voltage because of the inherent response of the measurement system, an alternative method using optical emission spectroscopy (OES) is proposed. OES measures the pre-strike arc temperature distribution profile close to the cathode surface at a test voltage of 18 kV and a making current of 17 kA. The arc electrical characterization is achieved using the obtained spectroscopy results, Lowke's model and thermal air plasma transport properties. A maximum arc temperature of 12 500 K while the arc moves from the lower part of the cathode to the center, arc voltage of 30–58 V, and dissipated energy of 79–87 J are calculated for the pre-strike arc considering the impact of copper evaporated from the contact surfaces. Different arc behavior is observed in closing the contacts compared to free-burning arcs, which indicates gas flow blowing the arc caused by the contact movement. This investigation could be used for a better understanding of switching behavior and efficient control of the operation.