The presence of free conductive particles in gas-insulated transmission lines (GIL) is the most significant threat to their insulation. The non-invasive identification of free conductive particle defects in GILs is extremely challenging. Existing identification methods which use the acoustic amplitude-flight time (AAFT) pattern are not often adopted on site due to their faulty theory, non-robust extraction algorithm, and generally inaccurate calibration. In this study, the theory established by L. E. Lundgaard is expanded by deriving estimation equations related to the particle's moving state. A new method for extracting the state information from the raw data of acoustic signals is established and the robustness of the acoustic identification method is enhanced. A convenient calibration method is also developed by building a standard collision signal generator. The proposed methods are validated in a scale GIL model and a real GIL model. The results of this study may represent a workable reference for GIL condition monitoring.

Free metallic particles are the main factor inducing flashover incidents and threatening the safety of gas-insulated switchgear (GIS) and gas-insulated transmission line (GIL) devices. Acoustic diagnosis is widely used to detect free metal particles due to its high sensitivity, but it generally fails under multiple particle conditions because of the overlap of acoustic signals. This paper proposes a new method of acoustically recognizing multiple free metallic particles. The pattern of Δ t -Normalized N that can be used to distinguish single/multi-particles is determined first. Second, each particle's neighboring collision signals are extracted from raw data by the established physical relation between adjacent collision signals. Wire, lamellar, and spherical free metal double-particles’ mass values are calculated as per the numerical connection between adjacent collision signals. The geometry of the particles is assumed to be known, so the density of the spherical particles can be effectively estimated. The proposed method is validated in a scale concentric hemisphere platform. The results of this work may provide a workable reference for GIS free metal multi-particle monitoring.

Gas-insulated switchgear (GIS) might withstand ac/lightning impulse (LI) superimposed voltage caused by lightning intrusion or switching operation during operation. When there are insulation defects, the breakdown probability is greatly increased, which threatens the safety of GIS and needs to be further studied. In this study, the discharge characteristics of two typical insulation defects, including a metallic protrusion on bus bar and foreign matter attached to insulator, under ac/LI superimposed voltage were studied, and the influence of superimposed phase, polarity, and ratio was investigated. It was found that, under a quasi-uniform electric field, the superimposed breakdown voltage ( ${U}_{s}$ ) was the same as the LI breakdown voltage ( ${U}_{\text {LI}}$ ). However, when there were insulation defects, ${U}_{s}$ was significantly affected by the superimposed phase and polarity. For the metallic protrusion, the most critical condition was an −LI superimposed at $\varphi =90$ , with ${U}_{s}$ decreasing by 11% relative to ${U}_{\text {LI}}$ . For the metallic particle attached to insulator, when an +LI was superimposed at $\varphi = 0^{\circ }$ or −LI at $\varphi = 180^{\circ }$ , ${U}_{s}$ decreased most, by as much as 19.6% relative to ${U}_{\text {LI}}$ . Finally, the validation test was performed in a 110-kV GIS with insulation defects and ${U}_{s}$ was 7%–14% lower than ${U}_{\text {LI}}$ . The results showed that, under a proper superimposed phase and polarity, ${U}_{s}$ could be significantly lower than ${U}_{\text {LI}}$ of GIS with insulation defects, indicating the great potential of the ac/LI superimposed voltage test for the high-sensitive detection of GIS insulation defects.

Fast linear transformer drivers (FLTDs) are a popular and potential route for high-power devices employing multiple “bricks” in series and parallel, but they put extremely stringent demands on gas switches. Electrode erosion of FLTD gas switches is a restrictive and unavoidable factor that degrades performance and limits stability. In this paper, we systematically investigated the electrode erosion characteristics of a three-electrode field distortion gas switch under the typical working conditions of FLTD switches, and the discharge current was 7–46 kA with 46–300 ns rise time. A high speed frame camera and a spectrograph were used to capture the expansion process and the spectral emission of the spark channel was used to estimate the current density and the spark temperature, and then the energy fluxes and the external forces on the electrode surface were calculated. A tens of kilo-ampere nanosecond pulse could generate a 10¹¹ W/m² energy flux injection and 1.3–3.5 MPa external pressure on the electrode surface, resulting in a millimeter-sized erosion crater with the maximum peak height R_z reaching 100 μm magnitude. According to the morphological images by a laser scanning confocal microscope, the erosion crater of a FLTD switch contained three kinds of local morphologies, namely a center boiling region, an overflow region and a sputtering region. In addition, the crater size, the surface roughness, and the mass loss were highly dependent on the current amplitude and the transferred charge. We also observed Morphology Type I and Type II, respectively, with different pulse parameters, which had an obvious influence on surface roughness and mass loss. Finally, the quantitative relationship between the electrode mass loss and the pulse parameter was clarified. The transferred charge and the current amplitude were proved to be the main factors determining the electrode mass loss of a FLTD switch, and a least squares fitting expression for mass loss was also obtained.

Free conductive particle is one of the main insulation threats to GIS. However, due to the extremely strong variety and randomness of particle behaviors, it is difficult to reproduce the particle-induced-breakdown in GIS, and the breakdown mechanism with multiple-micro-particle coexisting, are unclear and need further investigations. In this research, the movement pattern of multiple particles in a 1100 kV GIS was firstly analyzed, and found that the slight axial electric field could drive particles to move towards the convex side of the basin insulator, eventually causing an accumulation in a region. “Multi-particle-in-series” (MPIS) phenomenon was observed, with a low probability, but inevitably in the long run. Secondly, the physical model of multiple-particle-induced breakdown was established based on the streamer criteria, and a step-by-step discharge during MPIS process was responsible for the breakdown. Lastly, a 252 kV GIS was employed to act as a scaled model to equivalently simulate the electric field in 1100 kV GIS and experimentally study the particle-induced breakdown properties. A single particle in each state and position was difficult to induce the breakdown, while MPIS triggered several breakdowns, although at a very low working coefficient, which eventually verified the breakdown mechanism induced by MPIS in GIS.

Metal particles are. a great threat to GIS insulation, especially latent metal particles with higher densities, which are difficult to detect and provide early warnings before sudden faults. Thus, the detection of particles in predelivery or handover test stage is necessary and urgent. This paper proposed a new method by applying mechanical vibration to excite latent particles in power-frequency withstand-voltage test (PVT) to improve the detection effectiveness. In a scaled GIS, the charging and moving properties of latent particles under power-frequency voltage and impact vibration were investigated by experiment and simulation. Results showed that impact vibration could dramatically decrease particle lifting field by over 50% and trigger continuous bouncing under a lower electric field, releasing detectable acoustic signals. Validation tests were also conducted in a 330-kV GIS. In routine PVT, the critical detectable thickness of silver disc-type particles was ∼0.1 mm, while an 80 g--20 ms impact vibration could expand critical thickness to ∼0.4 mm, greatly enlarging the detectable parameter domain. Finally, the propagation and attenuation of impact vibration in different GIS components were studied, and the application method was discussed. The proposed new method affiliated to PVT could improve the detection effectiveness of latent particles and thus the insulation reliability of GIS