Influence regularity of O_(2)on dielectric and decomposition properties of C_(4)F_(7)N-CO_(2)-O_(2)gas mixture for medium-voltage equipment

Fluorinated nitrile(C_(4)F_(7)N)gas mixture has been introduced as the most promising candidate to replace sulfur hexafluoride using in gas-insulated equipment.In this study,the authors explored the influence of oxygen on the dielectric and decomposition properties of C_(4)F_(7)N–CO_(2)–O_(2)gas mixture.The authors found that the dielectric strength of the C_(4)F_(7)N–CO_(2)–O_(2)gas mixture with 2,4,6,8 and 10%O_(2)was increased by 4.85%,6.49%,7.70%,3.21%and 2.74%compared with C_(4)F_(7)N–CO_(2).The addition of 2–6%O_(2)to the C_(4)F_(7)N–CO_(2)gas mixture could effectively reduce the content of most of the decomposition byproducts such as CF4,CO,C_(2)F_(6),C_(3)F_(6),C_(3)F_(8),CF_(3)CN,C_(2)F_(5)CN,(CN)2.While high content of oxygen(>6%)results in higher decomposition of C_(4)F_(7)N,which has a negative effect on the stability of C_(4)F_(7)N gas mixture.Generally,it is recommended to add 2–6%O_(2)in the C_(4)F_(7)N–CO_(2)gas mixture to improve its insulation properties as well as inhibit the decomposition of C_(4)F_(7)N in the discharge for medium-voltage engineering application.

Arc decomposition behavior of C_(4)F_(7)N/Air gas mixture and biosafety evaluation of its by-products

At present,reports on the arc interruption and decomposition behavior of a C_(4)F_(7)N/air gas mixture especially the biosafety of its by-products are limited.Herein,a 57%C_(4)F_(7)N/43%air gas mixture was employed as an arc-quenching medium to conduct 200 breaking tests on a load switch(630 A/24 kV).The arc interruption and decomposition behavior of the gas mixture were evaluated,and the biosafety of the decomposition products was assessed based on an acute inhalation toxicity test.It was found that the average arcing time of the 200 breakings was about 10.1 ms and that breakings were basically completed at the zero-crossing of the second or third current half-cycle.The gas mixture after breaking decomposed dramatically(the C_(4)F_(7)N content decreased by 75.6%),generating a large number of gaseous by-products(CO,CF_(4),C_(2)F_(6),C_(3)F_(8),CF_(3)CN,C_(4)F_(8),CO_(2),C_(4)F_(10)N_(2),C_(3)F_(6),C_(2)N_(2),etc.)as well as solid by-products(carbon particles).The lethal concentration of 50%(LC50,4 h)of C_(4)F_(7)N/air after 200 breakings was 31 ppm(male mice)and 34 ppm(female mice),respectively,and the toxicity was classified as Grade 1.In general,C_(4)F_(7)N has an arc interruption capability,albeit weaker than that of SF_(6)under the same conditions.After breaking,the C_(4)F_(7)N gas mixture has a relatively high acute inhalation toxicity,and relevant safety protection measures for research or operation personnel are strictly needed.