| Summary: | Perfluoroisobutyronitrile (C<sub>4</sub>F<sub>7</sub>N)/CO<sub>2</sub>/O<sub>2</sub> has been considered as a potential eco-friendly gas insulating medium to replace SF<sub>6</sub> due to its unique dielectric properties. Nowadays, few studies on the thermal stability and decomposition properties of C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub>/O<sub>2</sub> gas mixture were reported, which needs to be evaluated comprehensively. Herein, for the first time, we explored the effect of oxygen and temperature on thermal decomposition characteristics of C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub>/O<sub>2</sub> gas mixture. It was found that CF<sub>4</sub>, C<sub>3</sub>F<sub>8</sub>, C<sub>3</sub>F<sub>6</sub>, CO, COF<sub>2</sub>, CF<sub>3</sub>CN, C<sub>2</sub>F<sub>5</sub>CN and C<sub>2</sub>N<sub>2</sub> are the main by-products of C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub>/O<sub>2</sub> gas mixture upon decomposition. The addition of 2%-10% O<sub>2</sub> in the C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> gas mixture yielded decreased of C<sub>3</sub>F<sub>6</sub> and C<sub>2</sub>N<sub>2</sub> while the increase of other by-products. The yields and effective formation rates of C<sub>3</sub>F<sub>8</sub> and COF<sub>2</sub> accelerated when the content of O<sub>2</sub> was higher than 8%. In terms of practical application, adding 4-6% O<sub>2</sub> to C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> gas mixture is appropriate and COF<sub>2</sub>, C<sub>2</sub>N<sub>2</sub>, C<sub>2</sub>F<sub>5</sub>CN can be used as the characteristic products to reflect the severity of partial overheating fault of C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub>/O<sub>2</sub> gas insulated equipment.
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