Fault Diagnosis and Condition Division Criterion of DC Gas Insulating Equipment Based on SF6 Partial Discharge Decomposition Characteristics

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SF6 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 SF6 decomposition characteristics remains unsolved. Therefore, partial discharge (PD) and SF6 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 $Q_{\text {sec}}$ 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. SF6 decomposition experiments are conducted under different PD conditions of each defect, and the SF6 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-redundancy-maximum-relevance-based feature selection algorithm is used to optimize the characteristic quantities, and an optimal feature set is selected, i.e., {C(SO2)/C(T), C(SOF2)/C(CT), C(SO2F2)/C(ST)}. The characteristic quantities of SF6 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.