Theoretical Investigations on the Decomposition Characteristic Gases of Fluoronitriles/CO2 Mixture After Arc Interruption

Abstract

Since sulfur hexafluoride (SF6) has been subject to various regulations to reduce the emissions, the eco-friendly alternative gases or gas mixtures have attracted considerable attentions for the electric applications in high voltage transmissions and substations. The mixture of fluoronitrile, i.e., heptafluoro-iso-butyronitrile, (CF3)2CFCN, with CO2 is an effective technique as a candidate to replace SF6. However, such a fluoronitrile/CO2 mixture is lack of the self-healing ability unlike SF6 that it recombines quickly after being dissociated by an electric arc or fault. Therefore, it is important to monitor the decomposition products of the fluoronitrile/CO2 mixture for the purpose of insulation failure alert. Mechanisms for the reactions of (CF3)2CFCN with atomic oxygen due to dissociation of CO2 were calculated using the high-level ab initio methods. It is revealed that the reaction of O(³P, ¹D) with (CF3)2CFCN proceeds predominantly via the successive C-O addition/elimination pathways and the singlet-triplet intersection to form C3F7 and NCO radicals as the nascent products. Various ketones, i.e., CF2O, CF3C(O)CF3, FC(O)CN, etc., alkanes, i.e., CF4, C2F6, C4F10, C6F14, etc., and alkenes, i.e., C3F6, are produced via the secondary reactions. To mimic the decomposition details after arc breaking, the reactive molecular dynamics simulations were carried out by mean of an extensively optimized reactive force-filed for the C/H/N/O/F system. It was found that the arc decomposition of the fluoronitrile/CO2 mixture produces the NOx (e.g., NO, NO2) species. In the presence of water impurity, some acidic byproducts including HF, FNCO, FCOOH, etc., can be generated. Theoretical calculations provide an a priori method to detect arc failure of the fluoronitrile/CO2 insulation using the characteristic gases and shed new lights on the operation and maintenance of the electric equipment for health and safety measures.