## 1 Introduction
However, compared with another important high-voltage switchgear, high-voltage circuit breaker mechanical defect diagnosis technology is becoming increasingly mature [25; 26; 27; 28], the detection technology of a GIS operation state is still limited to partial discharge fault analysis, and the development of mechanical fault-detection technology is slow. At the same time, related methods in the literature do not consider the influence of different current excitations on the detection system, which has certain limitations. Although the current value can be measured by the CT (current transformer) during the operation of the GIS, in the training process of the diagnostic model, since the data sample cannot cover all the current levels under multiple faults, it is difficult to establish an effective diagnostic model at the full current level. There is an urgent need to propose an intelligent diagnostic method for GIS mechanical defects that eliminates the effects of current. The proposed paper presents a unified vibration feature-extraction method suitable for GIS under different energizing currents applied to a GIS mechanical fault-detection system, which reduces the sensitivity of the detection system to the amplitude of excitation current. This paper analyzes the motivated GIS vibration amplitude-frequency characteristics and non-linear vibration characteristics, combined with Fourier transformation and the non-linear vibration theory, and puts forward the vibration spectrum feature selection standard of GIS and extraction technology for the fault diagnosis process. By comparing the different feature set, current sample, and the type of fault diagnosis results the superiority of the proposed method is demonstrated. The main contributions of this paper can be summarized as follows:  
(1) The GIS vibration information is characterized by a forced response process of 100 Hz electrodynamics on the non-linear system. The GIS vibration spectrum features are expressed at the multiplier frequency of the 100 Hz excitation, that is, the selection criteria of the GIS vibration spectrum features are proposed. (2) The influence of different excitation (current) amplitude on vibration spectrum characteristics and the diagnosis model are analyzed. The GIS vibration information depiction method based on multiplier frequency energy ratio (MFER) is proposed, which effectively reduces the influence of excitation amplitude, improves the difference between different faults, and significantly enhances the practicality of GIS mechanical defect technology based on vibration information. (3) The GIS mechanical defect diagnosis and analysis system based on vibration information are proposed, and the test performance of the system is verified by a large number of tests, which promote the development and application of GIS mechanical state detection technology. The rest of the paper is organized as follows: Section 2 introduces the GIS vibration information acquisition system, experimental data acquisition, and natural frequency analysis of different operation states. Then, the feature-extraction technology of vibration information will be discussed in Section 3, mainly focusing on the force analysis of three-phase common box GIS, the spectral characteristics of vibration information, and the influence of the current amplitude on feature extraction. A unified feature set for improving the applicability of GIS mechanical fault diagnosis model is also designed. In Section 4, experimental data based on different current levels comparing and analyzing the results of the proposed method, highlighting the advantages of this method and effectively improving the accuracy of fault diagnosis. Finally, Section 5 concludes the paper with discussion of future work.