Thermal current interruption in CO₂-based mixtures: II. Influence of flow conditions

Abstract A comprehensive experimental investigation into the switching behavior of CO 2 -based SF 6 alternative gas mixtures under short-line fault-like conditions has been performed at ETH Zurich. The first part of this study introduced the experimental methodology and showed the dependence of the interruption limit in these mixtures on the blow pressure and arc length at current zero. The second part presented here will explore how modifications to the flow conditions through nozzle shape parameters can influence the interruption performance, and extend conclusions drawn in Part I regarding the insensitivity of the interruption performance to arc length. Measurements using different nozzle expansion angles show that this insensitivity applies more broadly to all conditions downstream of the nozzle throat, including the location of the shock in the diverging region of the nozzle. The influence of nozzle erosion was also investigated, where it was found in a severely eroded nozzle that the hot-dielectric performance deteriorated while the thermal performance remained similar when the same <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>p</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">u</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>10.8</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mtext>bar</mml:mtext> </mml:mrow> </mml:math> blow pressure was maintained. The experimental results further demonstrate that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>l</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">u</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , the length of a nozzle’s upstream acceleration zone, is a key parameter that should be limited in order to reduce hot-dielectric failures. The investigation was aided by a one-dimensional arc model introduced herein, that was used to inform nozzle design choices and better understand experimental results. The modeling results reveal that extending <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>l</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">u</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> reduces flow acceleration, limiting the convective cooling in the stagnation zone, potentially offering a physical explanation for the experimentally observed decline in hot-dielectric performance with increasing values of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>l</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">u</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> .