SF 6 decomposition characteristics directly reflect the generation, development, and evolution of internal faults in gas-insulated equipment (GIE). However, establishing the DC-GIE internal insulation condition comprehensive division criterion and fault diagnosis method based on SF 6 decomposition characteristics remains unsolved. Therefore, partial discharge (PD) and SF 6 decomposition characteristics of typical insulation defects from initial discharge to the breakdown process are discussed in this paper. The average discharge magnitude in a second Q sec is selected as an electrical feature to characterize the PD condition, and the PD state is divided into three levels, namely, mild PD, medium PD, and severe PD. SF 6 decomposition experiments are conducted under different PD conditions of each defect, and the SF 6 decomposition characteristics under different failure conditions are obtained. On this basis, we construct five feature sets of 36 characteristic quantities for GIE PD fault diagnosis and analyze the validity. A minimum-redundancymaximum-relevance-based feature selection algorithm is used to optimize the characteristic quantities, and an optimal feature set is selected, i.e., {C(SO 2)/C(T), C(SOF 2)/C(CT), C(SO 2 F 2)/C(ST)}. The characteristic quantities of SF 6 decomposition component, which characterizes PD condition, are reconstructed, and the component characteristic criterion of PD condition division is proposed. Thus, a decision tree evaluation model for the PD condition of the typical insulation defect inside DC-GIE is constructed. This paper lays a foundation for the insulation condition evaluation and fault diagnosis of DC-GIE.

C 5 F 10 O gas insulating medium has attracted wide attention in the power industry for its excellent environmental potential and insulation performance. When C 5 F 10 O gas is applied in power industry engineering, it will inevitably face the over thermal fault which often occurs inside the equipment. Under the long-term effect of the local over thermal fault, whether C 5 F 10 O itself can maintain stability is an important factor to determine its engineering application. However, the thermal stability characteristics of C 5 F 10 O and its mixture are still unknown. Therefore, through systematic experimental research on the existing experimental platform of partial over thermal of the gas insulating medium, this paper explores the initial decomposition temperature of 5% C 5 F 10 O and 95% CO 2 mixture, the formation conditions of decomposition components, the relationship between the generation characteristics of decomposition components and fault temperature, and so on. The results show that when the pressure of 5% C 5 F 10 O and 95% CO 2 is 0.2MPa, the mixture begins to decompose at about 400 • to form C 3 F 8 and C 3 F 6 . C 2 F 6 and CF 4 will be produced at above 500 • . The process of C 3 F 6 formation is the endothermic reaction, while the process of C 3 F 8 , C 2 F 6 and CF 4 formation is an exothermic reaction, which is very beneficial to the formation of C3F6 un-der superheated conditions. As a result, the product concentration shows as C 3 F 6 > C 3 F 8 >C 2 F 6 >CF 4 , and the rate of product formation is positively correlated with the over thermal fault temperature. The results of this study lay a foundation for the application of C 5 F 10 O gas in the electric industry.INDEX TERMS Environmental friendly insulating medium, C 5 F 10 O, thermal stability, over thermal decomposition characteristics.

The environmentally friendly insulating gas C5-PFK has the potential to replace SF 6 in medium and low voltage gas insulated electrical equipment due to its good environmental performance and insulation strength. However, since C5-PFK molecule has rotatable C-C single bonds, the geometry will change when collided with each other or obtained external energy, which may have an impact on the stability and reactivity of C5-PFK molecule. In this paper, the geometry of C5-PFK molecule is optimized by the density functional theory-B3LYP-D3(BJ) method under the triple-zeta polarization basis set and high convergence criterion. The two C-C bonds connected with the carbonyl group (-C = O-) in C5-PFK molecule are rotated to scan its potential energy surface. Then, we obtained four possible conformers that might exist in C5-PFK molecule. The molecule structure, molecule energy, Mayer bond order, and energy levels (HOMO, LUMO orbital included) of frontier molecular orbital were analyzed to study the four conformers' stability and reactivity, which will pave the way for follow-up research work such as thermostability, transition state, reaction pathways, and adsorption behavior of C5-PFK.

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