Medium voltage load current interruption in presence of ablating polymer material

Abstract

The expected abolishment of SF6-gas in medium voltage switchgear has prompted a search for replacements for the switch types that depend on SF6 to obtain the required capability, reliability, compactness and cost. Ablation of polymers can enhance the arc-quenching capabilities in certain electrical switch designs. Several aspects, like electric arc plasma composition, gas flow and wall stabilization must play together to obtain a successful interruption. In this paper, open and closed switch geometries and two nozzle materials (PP and PTFE) are studied in medium voltage interruption experiments. Fundamental differences in the arc-quenching process for PP and PTFE are shown. In the case of PTFE, the current interruption performance depends on gas flow resulting from the pressure built up under the high current period. In the case of PP, the gas flow plays a lesser role for the arc-quenching capability, as the interruption capability is large even in an open design with no forced gas flow around current zero. Current chopping happens a few tens of microseconds before voltage zero crossing, thereafter, a remnant conductance in the contact gap is observed, lasting for up to several milliseconds. This can be explained by an arc-to-glow transition in presence of hydrogen.