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Enhanced plasma injection (EPI) driven by the electrical explosion of a metal wire in a microcavity has been proven to be an effective method for triggering 10-cm-magnitude high-pressure SF6 gas gap at very low working coefficients and provides a potential technical routine for ultrafast bypass switches (UFBPSs). In this study, aiming to further promote the EPI strength and improve its triggering ability, the EPI actuator is optimized, and several vital parameters of the actuator structure, including diameter D and length L of the microcavity and nozzle shape, are investigated. EPI with a maximum height of ∼10 cm and an initial velocity of ∼1000 m/s can be realized in 0.5 MPa SF6 when a 600 μm aluminum wire is exploded by a ∼1000 J trigger pulse. The EPI strength vs D curve has an inverted V shape, and 2 mm D results in the most intense EPI, due to the competitive effect of the vent congestion inside a thin capillary and parasitic volume in a fat capillary. As L increases, the EPI strength increases to saturation and then decreases. A 20–23 mm L can yield suitable EPI strength with limited trigger energy. The nozzle shape has a remarkable influence, and a divergent-type nozzle leads to a much stronger EPI than the shrink-type or straight-type nozzle, generating an EPI with a maximum height of 12 cm and an initial velocity of 1600 m/s. This study provides a useful guidance for the EPI actuator design for triggering megavolt UFBPSs at very low working coefficients.

Operating experience has shown that vibration can cause pressure fluctuations and initiate bubbles at the oil gap, which seriously threatens the insulation properties of oil-immersed power equipment. The main aim of this study was to determine the characteristics and mechanism of bubble initiation at the transformer oil gap under vibration in this paper. The results show the existence of the vibration-dependent bubbling phenomenon at the oil gap. Interestingly, bubbles initiate during the process of gap expansion, and the evolution of bubble morphology was further analyzed. The threshold characteristic of the bubble initiation process was found, and the relevant influencing factors were experimentally obtained. In the range considered in this study, higher gas content, higher temperature, and lower moisture content are conducive to bubble initiation at the oil gap under vibration, while vibrational frequency does not affect the threshold characteristic of bubble initiation process. The vibration-dependent bubbling phenomenon can be attributed to the vibration-induced cavitation process caused by the decrease in pressure at the oil gap. The influence mechanism of the above factors on the threshold characteristics of bubble initiation was further discussed. This study contributes to providing a reference for risk assessment of abnormal vibration of oil-immersed power equipment.

The high density and controllability of radio frequency (RF)-biased inductively coupled plasma offer significant potential for diamond-like carbon (DLC) deposition on alumina. However, the effects of power and atmospheric parameters on DLC deposition have been understudied due to multiple constraints, such as discharge conditions and carbon source requirements. In this study, considering the dual requirements of ion flux and ion kinetic energy for deposition, an RF-biased inductively coupled plasma-enhanced DLC deposition method was proposed. A total of 273 deposition experiments designed by response surface methodology were performed over wide parameter ranges to explore the influence of atmospheric parameters, such as gas pressure and Ar/H2 auxiliary gas fractions, on feasible power. The results showed that there was a power threshold for DLC deposition on alumina, which increased with gas pressure and slightly decreased with auxiliary gas addition. The effect of auxiliary gas on DLC deposition exhibited duality due to changes in the dominance of ion etching and methane dissociation. The feasible power region for DLC deposition in multicomponent atmosphere was jointly constrained by the power threshold and etching action. This research contributed to the DLC modification of alumina substrate in batches.

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 t-Normalized N that can be used to distinguish single/multi-particles is determined first. Second, each particles neighboring collision signals are extracted from raw data by the established physical relation between adjacent collision signals. Finally, the free metal spherical double-particles mass and density values are calculated as per the numerical connection between adjacent collision signals. 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.

A reliable and repeatable triggering technology for a megavolt gap switch with a low working coefficient η is an urgent need and a research focus. In this study, a novel method of hybrid plasma injection (HPI) driven by pulsed discharge inside a capillary was first proposed. The HPI actuator adopted a metal-polytetrafluoroethylene (PTFE)-stacked capillary, in which severe ablation could generate a hybrid plasma containing gas and metal vapor ionized component ejected outward from the nozzle. The HPI actuator could perform repeatedly with an extremely strong plasma injection and triggering ability and, thus, provided a solution for megavolt ultrafast bypass switches (UFBPSs). The evolution and the trigger properties of the HPI actuator were investigated, and the influence of the stacked material (Al, Zn, and Sn) and its proportion (3/15, 7/15, and 10/15) was studied, followed by the performance degradation in multi-shot. It was found that stacking chemically active and low-ionization-energy aluminum in a proportion of 7/15 strongly enhanced the HPI, with an initial velocity of 1200 m/s and a maximum height of 7.5 cm in 0.5 MPa SF

Although nature’s wisdom resides in achieving an exceptional functionality, such as anisotropic properties by constructing intelligently hierarchical architectures, the preparation of bulk biomimetic materials under environmental factors is still a great challenge. Inspired from the transport behavior of ions and water through the directionally aligned channels in trees, we demonstrate a facile, scalable approach to construct an anisotropic 3D biomorphic silicon carbide (bio-SiC) framework in epoxy resin (EP) matrices. The 3D bio-SiC framework has not only the highly dense elongated microchannels in the axial direction, which serves as a phonon “expressway” to facilitate thermal conduction of the whole composites, but also the robust latticed structure in the radial direction restricting the molecular chain motion. Consequently, the novel epoxy composite (bio-SiC/EP) with a nacre-mimetic architecture has a high thermal conductivity (TC) of 10.40 W m− 1 K− 1, an outstanding TC enhancement efficiency of 253 per 1 vol% filler at ~ 22 vol% bio-SiC loading, together with an extremely low coefficient of linear thermal expansion (CLTE) of 12.44 ppm K− 1 (~ 17 vol%), an excellent thermal stability, a remarkable anti-flaming performance, and a high flexural strength of 184 MPa (~ 17 vol%). Our finding gives a promising insight to achieve anisotropically highly thermoconductive polymer-based thermal management materials.

Discharges of different polarities develop in different modes, resulting in different guiding effects by femtosecond laser filaments. Knowledge of the contribution of laser filaments to positive and negative discharges is the basis of the laser-guided long-air-gap discharge technique. This study presents a direct comparison of the inception, propagation, and breakdown characteristics of discharges of both polarities. Long-air-gap discharge experiments under the switching impulses of both polarities are carried out under the same experimental conditions. Discharge modes and phases are also considered. The statistical results show that positive discharge inception voltages are transformed from a Weibull distribution into an exponential distribution under the influence of laser filaments, but there is little effect on the negative discharge inception voltage. The guidance probability of a positive discharge reaches 15% at most during the dark period stage, leading to little effect on the breakdown discharge probability. However, for negative discharges, the guidance probability can exceed 95%. An investigation of the filament contributions to both polarity discharges shows that the different migration directions of photoelectrons lead to a difference in the effects of laser filaments on inception voltages, and the difference in the connection of the two discharge passages leads to a difference in the guidance probability. Through the results of a simulation model, it is speculated that the connection condition for positive discharges is that the positive leader overlaps with the laser filaments, and, for negative discharges, the rod electrode is connected to the laser filaments through bi-directional discharge propagation.

The design of low dielectric-loss epoxy resin (EP) systems plays an important role in the insulation optimization of medium-frequency transformers (MFTs). Due to the diversity of EP and curing agents, the traditional design method inspired by dielectric-loss measurements cannot complete low dielectric-loss EP systems design in plentiful candidate EP systems. In this study, molecular dynamics (MD) computation was introduced to drive the design of a low dielectric-loss EP system for MFTs. By analyzing the relationship between the dielectric loss and molecular motion of the EP systems, two MD results were selected as descriptors to indicate the dielectric loss of EP systems, including mean square displacement and α-transition temperature. Eventually, the low dielectric-loss EP system blending EP, methyl tetrahydrophthalic anhydride, and dodecenyl succinic anhydride was effectively designed according to the descriptors. The rationality of the computationally driven design was verified by broadband dielectric spectroscopy measurement and the finite element method. Compared with previous EP systems, MFT insulated with the designed EP system had not only a 40% lower dielectric loss (9.79 W) but a higher overload capacity. This study provides an effective method for the design of low dielectric-loss EP systems.

Diamond-like carbon (DLC)-modified alumina microparticles are expected to be an ideal filler that can greatly improve the thermal conductivity of epoxy-alumina composites with a high fill factor, while the composites can still maintain high dielectric properties. Plasma-enhanced chemical vapor deposition (PECVD) has been mostly used to prepare DLC film on bulk material, and the ion energy in the plasma shows a crucial influence on the properties of the DLC. However, the properties of the DLC on solid microparticles prepared through the PECVD method as well as the effect of the ion energy are still unclear. In this paper, DLC on alumina microparticles has been prepared through a highly efficient method of repeated pulsed hollow cathode discharge in methane gas, and the characteristics of the DLC on the alumina particles have been studied and analyzed. The morphology and bond composition of DLC on particles have been studied through electron microscopy, Raman spectrometer and x-ray photoelectron spectroscopy. The ion energy distribution in the discharge plasma was diagnosed and used to analyze its effect on the hybrid bond content of the DLC through correlation analysis. The correlation of the ion energies and the hybrid bond content of the DLC shows that hydrocarbon ions in the discharge plasma with an energy of 100–200 eV present the most benefit to the formation of the sp3 hybrid bond content of DLC film on alumina microparticles. In addition, hydrogen ions with an energy of 700–1000 eV are also beneficial to improve the sp3 hybrid bond content of the DLC film.