The breakdown and recovery characteristics of the highly repetitive (≤100 kHz) and short nanosecond (∼10 ns) pulsed discharge were investigated. It was found that if one pulse of 40 kV is applied, the air gap between two conical electrodes for a reliable breakdown is only 1 mm; however, if 1000 same pulses are repetitively applied, the longest gap that can be broken down dramatically increases with the increase in the repetitive rate and it reaches 90 mm at 100 kHz. During the preceding pulses that do not cause the breakdown of the gap, long streamers develop from both conical electrodes and nearly bridge the gap, which provides the residual and decaying plasma channels for the rapid breakdown of 90-mm gap in the following pulses, which was experimentally proved to be unique in the extremely non-uniform electric field. The voltage recovery was determined with the two-pulse method and found that the physical processes of the voltage recovery after the short nanosecond pulsed breakdown are similar to that after the microsecond pulsed breakdown, but the time needed for a full recovery in the gas density, in between 50 and 100 μs, is shorter by three orders than that after the microsecond pulsed breakdown. The faster recovery in the gas density may be understood from the small energy of 36 mJ in one nanosecond pulse, which does not induce a high perturbation in the gas density. The evolution in the discharge mode in a 15-mm gap, from corona to diffuse and finally to multichannel spark, with the applied voltage and the repetitive rate was observed. The formation of the multichannel spark is due to the limited energy per pulse that is not enough to heat the primary channel to high conductivity for the collapse of the gap voltage.

The initial stage of the gas breakdown with the generation of runaway electrons was investigated using particle-in-cell/Monte Carlo collision simulations. The parameters of the solved problem are a 1 mm long atmospheric air gap between tip-plane electrodes applied with a nanosecond pulse voltage. The pulse is 5.2 ns in rising time (10%–90%), 10 ns in pulse width (FWHM) and 40 kV in amplitude. The cathode is a cone-shaped electrode, the tip of which is defined by the elliptic equation (the major axis is 4 mm and the minor axis is 1 mm), and the minimum radius of curvature is 0.125 mm. As it is found in the simulation that the development of the discharge channel from the cathode to the anode only takes about tens to hundreds of picoseconds, especially at high overvoltages with runaway electrons, it is assumed that the gap voltage applied in such a short time is nearly constant. Depending on the voltage at the breakdown, different behavior of the energetic electrons is observed. When the voltage is low, about 12 kV, energetic electrons are only produced in the tip cathode layer, where the electric field is the highest; no runaway electrons leading the discharge channel are observed. When the voltage is higher, about 15 kV, the energetic electrons begin to run away at the head of the discharge channel, where the electric field is high enough. When the voltage is even higher, the energetic electrons run away, even at the beginning of the discharge, and from the cathode to the anode. Pre-ionization of the gas ahead of the discharge channel by the runaway electrons is observed, which may play an important role in the fast breakdown of the gas under the nanosecond short pulse.

In this study, the gas-sensing mechanism of ZnO, TiO2, and Ag2O modified SnS2 toward dissolved gases (CO, CH4, C2H2, and C2H4) in transformer oil were studied based on the density functional theory. The modified structure, adsorption structure, band structure, adsorption energy, charge transfer, deformation charge density, density of states, and molecular orbit were analyzed. The results show that the conductivity of SnS2 monolayer is improved to a certain extent after surface modification with ZnO, TiO2, and Ag2O. And the adsorption of CO, CH4, C2H2, and C2H4 on the intrinsic SnS2 monolayer is weak physical adsorption. Compared with the intrinsic substrate, the adsorption energies of MO-SnS2 (M=Zn, Ti, Ag2) monolayer for the four target gases have been increased to different degrees, except that the adsorption energy of TiO2-SnS2 for C2H4 molecule has decreased slightly. With deformation charge density, density of states and molecular orbital analysis, it can be concluded that ZnO-SnS2 and TiO2-SnS2 have good gas sensitivity to CO, C2H2, and C2H4 molecules, and Ag2O-SnS2 also has strong adsorption capacity to C2H2 and C2H4 molecules. This work provides a theoretical basis for the application of gas sensors used in oil-immersed transformers.

The surface pollution of transmission line insulators is still an important factor affecting the safety and stability of the power grid. Regular cleaning has become the most widely used anti-pollution flashover measure in the power system. However, because it can not obtain the current pollution state of the insulator surface without contact, easily and quickly, or it is susceptible to be interfered by the ambient noise and the accuracy is not high enough, the cleaning cycle can only be determined in advance. Therefore, it will cause insufficient or excessive cleaning. In this paper, infrared images of insulators under different pollution levels, ambient temperature and relative humidity are collected by artificial pollution test, and based on infrared image characteristic parameters probabilistic neural network assessment method are established and verified. The results show that the infrared image is denoised by bilateral filtering, and the half-shed surface of insulator is selected as the research area by improved watershed algorithm, it is effective to extract the characteristic parameters of infrared image of insulator. The higher the pollution level and the higher the relative humidity, the more obvious the heating phenomenon of the insulator, and the higher the temperature rise and the dispersion of the temperature distribution. The ambient temperature has no obvious effect on insulator heating phenomenon. The identification precision ratio of pollution level Ⅰ, Ⅱ, Ⅲ and Ⅳ were 93.5%, 83.9%, 89.8% and 91.8%, respectively. The research has some reference for insulator pollution level assessment and pollution flashover prevention.

Electrothermal instability and its effect on the uniformity of discharge channels during underwater electrical wire explosions at different energy deposition rates have been investigated. According to the liquid–vapor transition mechanism, we classified underwater electrical explosion into “phase explosion mode” and “supercritical mode.” Two sets of experiments corresponding to the two modes were conducted, and the exploding wires were observed by a shadow imaging system. Results showed that a bamboo-shaped discharge channel was formed in the phase explosion mode, whereas the discharge channel in the supercritical mode was a uniform cylinder. Self-consistent magnetohydrodynamic simulation results indicated that exploding wires transfer into liquid–vapor coexistence phase in the phase explosion mode, accompanied by a rapid increase in the density perturbation and the formation of bamboo-shaped structures. Moreover, a small perturbation model was derived to analyze the electrothermal instability in different thermodynamic states. It was indicated that the density perturbation is easier to grow in the liquid–vapor coexistence state than the supercritical state, which provides a theoretical explanation for the differences between the two electrical explosion modes.

Effects of photoionisation on the development and electron runaway of the initial discharge in atmospheric air under nanosecond pulse voltage were studied via two‐dimensional particle‐in‐cell/Monte Carlo collision simulations. It was found that photoionisation has little effect at the beginning of the initial discharge. However, as the discharge channel gradually develops towards the anode, photoionisation shows greater impacts on the morphology of discharge but has little influence on the velocity of discharge development. Photoionisation does not appear to have a decisive effect on the growth trend of the highest energy of runaway electrons, but it does affect the change rate of the highest energy and overall distribution of electron energy, resulting in a higher proportion of energetic electrons. The difference in the distributions of the electron energy between the two cases, with and without considering photoionisation, can be attributed to the impact of photoionisation on the discharge morphology, which in turn distorted the electric field. The spatial density distributions of the electrons produced by photoionisation further explained the differences. The authors’ results explicitly demonstrate the influence of photoionisation on the development and the electron runaway of the initial discharge under nanosecond pulse voltage, which provides more comprehensive knowledge for the atmospheric air gap nanosecond pulse discharge physics.