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Before putting into operation, ultra-high voltage transformers need to undergo strict handover tests to ensure their safe and reliable operation. The grading capacity of the grading ring on the transformer will affect the accuracy of the handover test. When there are problems with high altitude environments or grading ring structures, their grading ability will change. It is particularly important to study and design grading rings that can have good grading ability in different situations. This article first uses COMSOL software to build a three-dimensional model of the withstand voltage test circuit of a 500 kV transformer. Secondly, the influence of changes in the pipe diameter and ring spacing of the equalizing ring on the equalizing capacity of the equalizing ring is studied. Finally, the variation of equalizing capacity of the equalizing ring in a rough state is studied. It is found that as the diameter of the equalizing pipe and the ring diameter increase or the distance between the rings decreases, the equalizing ability of the equalizing ring will be enhanced, with the greatest impact brought by the change in pipe diameter. When the grading ring has a rough surface, its grading ability will significantly decrease. Among them, the grading ability decreases most significantly when adhering to conical particles, and decreases least when concave, indicating that the smoother the surface of the grading ring, the better the grading ability. This study can provide some assistance for the design of grading rings for high altitude ultra-high voltage transformers.

Temperature plays a crucial role in influencing the mechanical properties of materials. Accurately measuring the temperature of devices is essential for understanding the evolution of their mechanical properties under stress and evaluating their health and lifespan. However, traditional methods encounter challenges in measuring transient temperatures and lack sufficient time-resolution capability, particularly when it comes to the rapid temperature changes at the solder interface during the switching process of power devices. In this paper, based on the close correlation between the intensities of the characteristic spectral lines of the laser-induced elements and the temperatures, a method of measuring the surface temperatures with the time-resolved capability of the order of microsecond is proposed, and a quantitative relationship between the surface temperatures of the sample and the spectral characteristics is established. The findings demonstrate that an increase in the surface temperature of the material results in enhanced intensity and signal-to-noise ratio of laser-induced plasma spectra. This enhancement is influenced by the spectral acquisition delay and gate width. To establish a quantitative relationship between surface temperature and spectral properties, back propagation-artificial neural network (BP-ANN) and partial least squares (PLS) are employed for fitting and calibration. The fitted models can achieve linear correlation coefficient indexes exceeding 0.99. Notably, the BP-ANN fitted model exhibites a small fitting bias, with a root mean squared error (RMSE) of 2.582 and a correctness rate of 98.3%. The method provides an effective means for transient temperature measurement of objects and gives a strong support for the assessment of the health status of the soldering interface of power devices.

Grafting nanomaterials on the carbon fiber (CF) surface is considered an effective strategy for enhancing the interfacial properties of CF-reinforced polymer composites (CFRPs). However, the mechanical properties of the CFs are often compromised during treatment. A new method for in situ growing silver nanoparticles (AgNPs) on the CF surface is proposed in this study. The CFs are first immersed in a low-viscosity silver nitrate solution to form a thin liquid film on the surface. Subsequently, using the abundant active particles in the plasma, the silver ions are reduced to silver atoms and grown into AgNPs on the CF surface. The tensile strength of CF@Ag was 38.74% greater than that of untreated CF, potentially due to the reparative action of AgNPs on defects in CF. The CF showed an evident improvement in surface wettability because of the AgNPs. Furthermore, the interfacial properties were noticeably improved, with the interfacial shear strength of CF@Ag increasing to 91.59 MPa, which was about twice that of pristine CF. Thus, the mechanical properties of composites are significantly improved (flexural strength increased by 190.74%). This study presents a non-destructive and convenient method for growing nanoparticles onto CF to establish a robust interface in CFRPs. [ABSTRACT FROM AUTHOR]

We studied positive streamers with a 5 mm gap under 20–101 kPa pressure and 1%–31% O2 concentrations conditions using a 2D axisymmetric fluid model based on local field approximation. As the pressure decreases from 101 kPa to 20 kPa, the axial reduced electric field, the mean electron energy and the electron diffusion coefficient increase, which leads to the acceleration of the streamer propagation velocity and the increase of the streamer channel radius. The opposite change of ionization cross section and gas molecular density caused by the decrease of pressure leads to the non-monotonic change of the peak of net ionization rate. At medium-low pressure, there is a wider ionization region at the streamer head. As the O2 concentration decreases from 31% to 1% in N2/O2, the streamer propagation velocity decreases. When the O2 concentration drops to 1%, the streamer velocity decreases with a descent gradient of nearly 4 times, compared to 11% O2. Based on the space charge effect and chemical reaction rate, a possible mechanism is proposed to explain the abrupt change in the streamer velocity. [ABSTRACT FROM AUTHOR]

The development of composite materials with excellent thermal conductivity is attracting extensive attention to meet the increasing heat concentration challenges in both power and electronic equipment. Previous strategies for improving the thermal conductivities usually accompanied by the cost of mechanical strength. Herein, we propose a new strategy for developing both mechanically strong and thermally conductive composite films. The aramid nanofibers (ANF) are well prepared with introducing Ag nanoflakes and graphene oxide (GO) via vacuum-assisted filtration and hot-pressing treatment. Results indicate the synergistic effects of low dimension materials have resulted in a tightly arranged phonon network construction. The in-plane thermal conductivity of the prepared ANF/Ag/GO film reaches 9.84 W m−1 K−1, about 371% higher than that of pure ANF. Meanwhile, the strong hydrogen bond formed between the functional group of GO and the amide group of ANFs plays a synergistic strengthening and toughening role at the complex interface, and the tensile strength reaches 231 MPa. We believe that these findings shed some light on the design and fabrication of multifunctional materials for thermal management applications. [ABSTRACT FROM AUTHOR]

Anti‐icing of superhydrophobic materials has attracted a lot of attention. The potential for applications in power electronics, transportation, engineering and other fields is enormous. However, in practical applications, the anti‐icing performance parameters of superhydrophobic materials are still far from meeting the engineering requirements. It is a feasible solution to modify superhydrophobic materials to assist surface deicing by combining the vibration and heat generated by equipment in actual engineering. In this study, conventional superhydrophobic materials are modified by h‐BN. The electrical properties are improved to meet the engineering requirements. At the same time, it reduces the ice adhesion at the interface and improves the thermal conductivity of the material. The modified material has the possibility of assisting in de‐icing in engineering applications. A feasible solution is found for the engineering interface ice‐covering problem. [ABSTRACT FROM AUTHOR]

Pretreatment using O2 plasma is a commonly used method today to increase the surface activity of CFs and enable more functional groups to be grafted onto them. Although the oxidation treatment of the CF surface is considered a key procedure for further grafting with functional groups, few researchers have investigated how the oxidation treatment influences the grafting processes. In this paper, the effects of the degree of oxidation on the grafting of amino functional groups onto CFs were experimentally investigated. The experiment showed that the oxidation process developed in steps including initial oxidation, saturation oxidation, and peroxidation. The results demonstrated that the degree of oxidation was positively correlated with the grafting efficiency within a certain range. Furthermore, the groups (unsaturated C, C-OH, and O=C-OH) produced after oxidation determined the results of grafting with amino (C-N, C-NH2, and O=C-NH2) groups. When the O2 plasma treatment time was increased from 0 min to 3 min, the IFSS of CF@HA increased by 1.07 times and the flexural strength of CFRP increased by 1.26 times. This work proves that the ammonia grafting of CF can be controlled by adjusting its earlier oxidation process. [ABSTRACT FROM AUTHOR]

Thermal defects widely occur in electrical equipment and affect its stable operation. This paper proposes a novel detection method based on derivatives for thermal defects of electrical equipment. First, the equivalent models of the equipment are established to analyze the surface temperature formed by different defects. Second, the theoretical foundation of the derivative-based detection method is described, and numerical calculations on thermal defect detection based on derivative characteristic curves are carried out. The results show that the method can detect the position effectively and calculate the depth with an error of 1.6%. Then, the thermal defects in different situations are analyzed, and the results indicate that the method is effective in all cases. Finally, the experiment is conducted by artificially preparing samples with thermal defects, and the achieved calculation error of the experimental results is within 3.3%, which indicates the validity of this method. [ABSTRACT FROM AUTHOR]

Graphite matrix/copper composites have found significant potential uses in numerous areas. Until now, a high ambient gas pressure has to be applied to overcome the non-wettability between the copper melt and graphite matrix, which has restricted the technological and economic efficiencies, as well as the composite performance. In this paper, we have proposed a pressureless infiltration strategy to realize the preparation of highly conductive graphite matrix/copper composites. A tungsten carbide network is first in situ constructed in the graphite matrix, transforming the capillary force from the resistance mode to the impetus mode. Results show that the contact angle of graphite/copper decreased from 138.5° to 23° due to the constructed tungsten carbide network. The composites exhibit good interfacial bonding, and the electrical conductivity reaches 15.1 × 105 S/m, far higher than the requirement of the industry standard. [ABSTRACT FROM AUTHOR]

Aluminum/graphite composites are receiving much attention due to the unique features and diverse possibilities in different fields. Generally, the aluminum melt into the inner pores of the graphite matrix requires an ambient prompting with high temperature and gas pressure. The infiltration dynamics of aluminum melt, which highly depends on the ambient temperature, would have a significant effect on the final composite's performance. In this work, we have studied the temperature effect of aluminum melt during the infiltration process into the porous graphite matrix based on numerical simulation and experimental verification. Results show that the moderate temperature increase would effectively promote the infiltration of aluminum melt, achieving a higher filling rate with a shorter filling time. However, the promotion effects of the temperature increase would come to a certain limit due to the opposing effects of capillary porous and residual gas. The detailed influences of the temperature on the infiltration would be presented, and the final composite performance, including mechanical strength and thermal management capability, would be examined and discussed. [ABSTRACT FROM AUTHOR]

A method based on LIBS technology for measuring transient surface temperatures with a time resolution of several μs was proposed in this study, which holds great significance for fast sliding friction processes in linear electromagnetic propulsion, gun barrels, and high-speed trains. The effects of surface temperature on laser-induced plasma emission spectra were studied, and it was found that both the spectral intensity and signal-to-noise ratio increased with increasing temperature. The mechanism of the surface temperature effect was revealed by interferometry and spectral analysis techniques, which were mainly related to the processes of plasma ablation and surface air heating. Finally, three algorithms including SPF, PLS, and BP-ANN were used to predict the surface temperature, and the results showed that BP-ANN performed best in the 1 μs, 2 μs, and 3 μs exposure time. Under an exposure time of 3 μs, the regression coefficient (R2) and root mean square error (RMSE) could reach 0.9992 and 2.466, respectively. This research would provide a new measurement technology for transient temperature in high-speed physical processes. [ABSTRACT FROM AUTHOR]

Because of the versatility of superhydrophobic materials, they have attracted a lot of attention even in power electronics, transportation, engineering, and other fields. The volume fraction of fluorinated silicon oxide nanoparticles in superhydrophobic materials is one of the most important factors. Increasing the volume fraction will decrease the stability between the coating and the hydrophobic surface. Especially, the flashover voltage of the coating gradually decreases from 10 to 35 vol.%. Meanwhile, the flashover voltage dispersion of the coating increases drastically after 30 vol.%. In order to improve the electrical properties of the superhydrophobic coating, self‐assembly of surface energy differences strategy is proposed in this work. A binary filling phase of the coating is introduced by 2D boron nitride nanosheets and silicon oxide nanoparticles. Although Hexagonal boron nitride with high surface energy and low roughness, it will be spontaneously assembled and wrapped by silicon oxide nanoparticle based on surface energy differences, which forming a low surface energy filled phase. Experiment results prove that the flashover voltage of the superhydrophobic coating is optimized by the binary filling phase coating. This method offers new ideas for the selection of filling phase and application of superhydrophobic materials. [ABSTRACT FROM AUTHOR]

The interfacial cracks generated during the high‐temperature carbonization process, caused by the mismatch of thermal shrinkage rate between the fiber and matrix, limit the huge application potential of C/C composites. In this study, pre‐oxidized carbon fiber (OPF) prepared from pitch, as a reinforcement, was introduced into the pitch‐based carbon matrix to realize the co‐carbonization of OPF and pitch. The results showed that the mechanical and thermal properties of the OPF‐reinforced C/C composites were significantly improved. The compressive strength and flexural strength were 136.97 and 46.71 MPa, respectively, both were about 2.5 times higher than those of pristine C/C composites. And OPF‐C/Cs had a carbon residual of about 30% at 800°C, which was 2.8, 4.4, and 2.1 times higher than that of Pristine‐C/Cs, CF‐C/Cs, and MCF‐C/Cs, respectively. The rough surface and abundant active sites of OPF could promote the growth of the graphite microcrystalline structure, resulting in an excellent fiber/matrix interface. Moreover, the adaptive expansion of OPF occurred during the co‐carbonization process with pitch, which inhibited the generation of cracks at the fiber/matrix interface. Owing to its excellent affinity with pitch, OPF is believed to have a wide range of promising applications in high‐performance C/C composites that require high‐temperature treatment. [ABSTRACT FROM AUTHOR]

Carbon-carbon composites are widely used in thermal management, photocatalysis, aerospace, high-speed railway, and nuclear energy applications, owing to their low density, high temperature stability, corrosion resistance, and excellent self-lubrication performance. Improving the weak combination interface of carbon-carbon composites has always been challenging. Prior to the present study, there was no effective method for stimulating the interaction between the reinforcing phase and the carbon matrix. In this study, pre-oxidized carbon fibers with abundant active functional groups were introduced into ultrafine carbon powder, for significantly improving interfacial structural integrity in carbon-carbon composites. The effects of pre-oxidized carbon fiber on the composites were systematically studied using metallographic microscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy methods. The results show that chemical bonds are formed between pre-oxidized carbon fibers and the carbon matrix, increasing the interfacial bonding strength. The pre-oxidized carbon fibers expanded adaptively after sintering, with the diameter increasing by 19.47%. The fiber expansion process inhibited cracks, decreasing the composites' porosity by 21.6%. Compared with carbon fibers, the pre-oxidized carbon fibers improved the flexural strength, impact resistance, electrical conductivity, and thermal conductivity by 31.8%, 53.3%, 18.58%, and 14.06%, respectively. [ABSTRACT FROM AUTHOR]

In graphite/copper composites, the intrinsic non-wettability of graphite and copper makes it difficult for the copper melt to realize effective infiltration into the porous graphite matrix. The resulted void defects have reduced the final performance of the composites. It is of great significance to improve the saturation of copper melt in the graphite skeleton, which requires a well understanding of the infiltration behavior of copper melt. In this paper, a two-phase flow finite element model was developed, and verified by previous published experimental data. Effects of the external pressure on the infiltration behavior has been studied, and then an optimization method based on WC particle doping was proposed. Results indicated that effective transfer of displacement pressure at the liquid flow front can reduce the void defects of composite materials. WC doping has reduced the capillary resistance, and effectively improved the copper saturation in the infiltration process from 80.4% to 90.7%. This work provided a new method to reduce the void defects of composites, which may be helpful for the development of high performance copper/graphite composites. [ABSTRACT FROM AUTHOR]

When the high-speed train starts to raise the bow at the station, the cars themselves are in a stationary state. At this time, arc generated by the air breakdown between the pantograph and the contact network (referred to as "bow network") may stay for a long time to ablate the contact wire, the thermal erosion of the arc may seriously affect the service performance of the contact wire, and, in serious cases, may even break the contact wire. Therefore, the study of temperature distribution characteristics of the contact wire in the process of arcing caused by bow raising is of great significance to reduce the ablation of the contact wire on account of the arc. This paper uses the four electromagnetic thermal laminar flow modules of COMSOL multi-physics field simulation software to establish a two-dimensional axisymmetric arc dynamic model, based on the bow network arc burning MHD (magnetohydrodynamics) simulation model, and employs the dynamic grid technology to simulate arc burning process. The effect of speed on the arc characteristics and the temperature field of the contact wire is studied. Simulation and experimental results find that: the arc length decreases continuously during the bow raising process, and the high temperature arc area shrinks gradually; the arc pressure is influenced by the bow raising speed; the bow raising speed can effectively reduce the maximum temperature of the contact wire and the arc burning time, which can reduce the material erosion of the contact wire by arc heat. [ABSTRACT FROM AUTHOR]

As the unique power entrance of high-speed trains, the pantograph–catenary system (PCS) plays the critical role of power transmission for high-speed trains. The pantograph achieves the electrical power based on the sliding electrical contact between the pantograph strip and the high-voltage contact line. However, when the environmental temperature is relatively low especially during the winter, the contact line is covered by ice or snow easily. The ice or snow adhering on the surface of contact line may lead to poor electrical contact between the pantograph strip and the contact line, which may trigger the pantograph–catenary arc appearing. Accompanying with mechanic shock and friction heat, the off-line arc may deteriorate the quality of power transmission and meanwhile erode the contacting surface of the pantograph strip and the contact line to some extent. In order to solve this technique bottleneck, a novel rotatable circular-shape pantograph strip is proposed to improve the overall performance of the traditional pantograph strip. Compared with the traditional strip, the circular pantograph strip owns many advantages, such as less friction, better thermal dissipation, and better shock resistance. The thermal dissipation performance of both the traditional and circular pantograph strip is evaluated in this article, with the consideration of suffering off-line arc under the icing conditions. [ABSTRACT FROM AUTHOR]

Using Fe reaction products to overcome the poor wettability between copper and carbon, C/Cu-Fe composites with excellent properties were prepared by gas pressure infiltration technique. The elemental Fe, being distributed between carbon and Cu, reacted with carbon to form Fe3C layer and effectively improved the wettability of the C/Cu system. The contact angle decreased from 124° to 21°. Compared to composites without Fe doping, the C/Cu-Fe composites exhibit significantly higher compressive and flexural strengths, as well as electrical resistivity reaching six times better than the industry standard requirement. Microstructure analysis indicates that the continuous tight Fe3C interlayer with moderate thickness at the C/Cu interface has a bridge effect on the interfacial bonding enhancement. [ABSTRACT FROM AUTHOR]

Carbon/copper composites are widely used in collector brushes, pantograph sliders, collector shoes and other fields due to their excellent mechanical friction, wear, thermal conductivity and electrical conductivity. However, the interfacial bonding force of composites is weak because of the natural non-wetting of copper and carbon graphite matrix, which restricts its comprehensive mechanical and electrical properties. In this paper, the nickel element was doped and introduced in the preparation process of the carbon graphite matrix, and the carbon/copper composites were further prepared by the pressure impregnation method. The results showed that the modified carbon matrix doped with nickel was denser, and the nickel element could evenly distribute in the carbon matrix. The porosity of the modified carbon/copper composites was significantly reduced and the interfacial bonding method was changed from the original simple mechanical interlock to solid solution bonding so that the interfacial state was obviously improved. The effect of nickel content on the mechanical and electrical properties of composites was systematically studied. It was found that when the nickel content was 3 wt%, the compressive strength and flexural strength were increased by nearly 52% and 58%, and the electrical conductivity was increased by nearly 35%. [ABSTRACT FROM AUTHOR]

We investigate the evolution of the species from both the target and the air, and the plasma parameter distribution of the nanosecond laser-produced plasmas in atmospheric air. The technique used is spatially resolved optical emission spectroscopy. It is argued that the N II from the air, which is distributed over a wider region than the target species in the early stages of the discharge, is primarily formed by the shock wave. The ionized species have a larger expansion velocity than the excited atoms in the first ∼100 ns, providing direct evidence for space-charge effects. The electron density decreases with the distance from the target surface in the early stages of the discharge, and both the electron density and the excited temperature variation in the axial direction are found to become insignificant at later stages. [ABSTRACT FROM AUTHOR]

Laser produced copper plasmas of different spot sizes in air were investigated using fast photography and optical emission spectroscopy (OES). The laser energy was 33 mJ. There were dramatic changes in the plasma plume expansion into the ambient air when spot sizes changed from ∼0.1 mm to ∼0.6 mm. A stream-like structure and a hemispherical structure were, respectively, observed. It appeared that the same spot size resulted in similar expansion dynamics no matter whether the target was located in the front of or behind the focal point, although laser-induced air breakdown sometimes occurred in the latter case. Plasma plume front positions agree well with the classic blast wave model for the large spot-size cases, while an unexpected stagnation of ∼80 ns occurred after the laser pulse ends for the small spot size cases. This stagnation can be understood in terms of the evolution of enhanced plasma shielding effects near the plasma front. Axial distributions of plasma components by OES revealed a good confinement effect. Electron number densities were estimated and interpreted using the recorded Intensified Charge Coupled Device (ICCD) images. [ABSTRACT FROM AUTHOR]

As one of the most important electrified railway components, the pantograph-catenary (PC) system plays a key role in the energy transfer for the train through the sliding electric contact. However, the performance of the PC system is severely limited by pantograph arcing, and the study on the dynamic characteristics of pantograph arcing is still far away from satisfactory. In this article, a magnetohydrodynamic (MHD) model of pantograph arcing is first established, which focused on the impact of crosswind as well as the input current on the arcing dynamics. Once the model was validated by laboratory experiment, the influences of the crosswind and input current on the arc temperature and voltage are studied in detail. Two parameters, namely the diameter of the catenary and the gap distance between the pantograph strip and the catenary, were varied to see what effects that might have. An interesting periodic phenomenon for both the temperature and voltage fall has been found, which was explained based on the Karman vortex street of fluid theory. [ABSTRACT FROM AUTHOR]

Using an artificial wind tunnel, the three basic preconditions presented in the preceding article by Kang et al are experimentally verified and systematically analyzed. The experimental results confirm that, in a discharge of flowing air, the breakdown path of an air gap deflects at an angle along the direction of the airflow. Some of electrons and ions are blown away by the airflow, and the mean free path of electron increases with increasing airflow velocity. In addition, the physical essences of the effects of flowing air on the discharge are revealed, and the differences in flowing air discharge between the uniform and non-uniform electric fields are discussed. [ABSTRACT FROM AUTHOR]

A physical model of flowing gas discharge is presented. This model extends Townsend's discharge model for static gases to flowing gases and can be applied to a wider range of gas discharge situations and the breakdown laws of flowing gases are analytically solved. The process of modeling reveals that the discharge of flowing gases includes three basic characteristics: the deflection of the major discharge path, the blowing away of some of electrons, and a decrease in gas density. The presented model enables quantitative calculations of the ionization coefficient and breakdown voltage in flowing gases. Comparisons between the present results and the corresponding ones obtained experimentally show good consistency. These results supplement the theory of gas discharge and provide some support for insulation protection in flowing gases. [ABSTRACT FROM AUTHOR]

Currently, the detection equipment for offline arcing on high-speed trains is of great interest because it can evacuate the contact state between the contact wire and the strip. In order to ensure the safety of the train under the effect of the arcing, the self-designed fourth-order Hilbert curve fractal antenna is chosen to detect the arcing, in this paper, based on its electromagnetic radiation signal. Aspects of time-, frequency-, and joint time–frequency-domain analyses of the signals were considered. Meanwhile, the experiments were conducted several times with different circuit currents and different electrode gaps, and the electromagnetic pulses under different moments were studied. Results indicated that the electromagnetic radiation pulses were corresponding to the abrupt variations of voltage during arcing, and all the characteristic frequencies were located at similar values (18 MHz). Mechanisms of using the short-time Fourier transform method to obtain an accurate arcing moment, extinguished time, and arcing duration have been explained in detail. Thus, the characteristics of arcing electromagnetic radiation could become the significant and efficient parameters to detect the pantograph–catenary operation state of high-speed trains. [ABSTRACT FROM AUTHOR]

In recent years, the speed and the current transmission density of high-speed railways are increased. As a result, the rate of the occurrence of pantograph–catenary arc also increases, which directly influences the service performance of the pantograph–catenary system. The detection and identification of the pantograph–catenary arc is of great significance for evaluating the off-line level to ensure the safe operation of the pantograph–catenary system. The currently used conventional optical arc detection method would be distorted by strong sunlight or other environmental lights in the actual operation, which would affect the accuracy of detection. On the contrary, the arc sound signal is not affected by the diurnal variation and can be easily obtained; it can also be distinguished well from other interfering sounds. In this paper, a new method that uses the arc sound is proposed to realize the effective detection of the pantograph–catenary arc. The frequency spectrum and the intensity characteristics of the pantograph–catenary arc sound are analyzed. The results show that the frequency band of the arc sound signal is wide and mainly gets distributed at 5–17 kHz. The curve of the sound waveform is drawn by calculating the short-time average energy, which is used to obtain the arc's starting time, duration, and intensity, for calculating the off-line rate. [ABSTRACT FROM AUTHOR]