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Arc ablation occurs when the armature passes over the surface of metal rail. Ablation not only seriously affects the service life, but also may cause the failure to launch. In this paper, the research status of sliding arc ablation technology is summarized from the mechanism of sliding arc ablation, the characterization method of sliding arc ablation degree, and the enhanced design of materials or structures. Three key issues that need to be further solved are listed: the influence mechanism of sliding arc ablation on microstructure and properties of metal materials, the service properties evaluation and quantitative characterization methods of metal material, as well as the modified and enhanced design of materials or structures. In response to those problems, efforts are needed to improve the awareness of sliding arc ablation with the theoretical basis and characterization methods, as well as guide the improvement and strengthening of rail design.

The converter transformer is an important equipment in the bulk power and long-distance power transmission projects, and the oil-pressboard insulation at the rectifying valve side, which mainly deals with combined ac–dc voltage, requires more study of discharge and its influence to ensure equipment safety. In this research, an oil-pressboard insulated experimental platform with needle and plate electrodes, which is similar to the sharp defects of components, such as the corner ring and the gasket of converter transformer, is established, and the photographs of partial discharge (PD) are used to analyze the influence of PD under the above experimental conditions; then, the generation and evolution of various PDs and their by-products can be researched in detail, which can influence the electrical aging process of oil-pressboard insulation. Results indicate that the main by-products brought by PD are bubbles and paper fibers, which can affect the proportion and development of subsequent PDs. These results can provide experimental support to research the internal mechanism of the electrical aging phenomenon of converter transformer, especially at its rectifying valve side.

The oil-pressboard insulation on the valve side of the converter transformer is subjected to the steep rising edge pulse voltage caused by commutation in actual operating conditions. The steep rising edge pulse voltage will aggravate insulation degradation and easily lead to insulation failure. However, the effect of voltage waveform steepness on insulation degradation is rarely considered directly in the current research. In this article, the actual voltage waveform of the main insulation on the converter transformer valve side is analyzed, and its harmonic content is determined. The experiments under steep rising edge pulse voltage and superimposed harmonics voltage are set up, and partial discharge pattern and space charge distribution are collected. The steeper the voltage waveform is, the narrower the phase distribution of the discharge pulse is, and the larger the maximum discharge quantity is. There is obvious charge accumulation inside the oil-impregnated pressboard under the steep rising edge electric field. The increase of waveform steepness makes the charge not easy to migrate under the externally applied electric field, and the retention effect is enhanced, resulting in an increase in charge accumulation. Moreover, the steepness of the voltage waveform becomes larger, and the charge injection is accelerated under the pulse voltage. The collision, trapping, and detrapping of high-energy electrons release more energy, which is more likely to cause insulation damage and form more traps. The increase of traps leads to charge accumulation, which results in electric field distortion and further promotes the generation of partial discharge. Based on the above analysis, the influence of voltage waveform steepness on oil-pressboard insulation failure is obtained, which provides a theoretical basis for insulation structure optimization and condition assessment of converter transformer.

Power transformer may generate bubbles in the transformer oil due to various factors, such as internal insulation winding vibration and pressure change. Bubbles distort the spatial electric field, which makes the discharge breakdown along the gas channel and accelerates the discharge breakdown of insulating medium. Therefore, in order to prevent the arc discharge induced by microbubbles inside the transformer from increasing the risk of transformer combustion damage, it is necessary to conduct in-depth research on the dynamic law of suspended bubbles before insulation breakdown from the source by combining experiments and simulations. In this article, the bubble dynamic model of a needle plate electrode under the multicoupling physical field of transformer is established in terms of the migration and deformation of suspended bubbles in liquid phase. By comparing the migration trajectories of single suspended bubbles obtained from experiments and simulations, the reliability of the model is verified. The variation of velocity component of voltage amplitude and the change of shape stretching of single suspended bubble in power frequency period are simulated, and the fluctuation law of pressure formed by electrostriction force inside and outside the bubble during migration is analyzed. The capacity absorption process of multibubbles in the strong field region near the needle electrode was simulated. The changes of the surrounding liquid pressure and the spatial electric field distribution during the formation of the gas channel were analyzed, and the internal relationship between the gas channel and the formation of the discharge breakdown was revealed.

Partial discharge in oil paper insulation is an important reason for insulation breakdown. The study of partial discharge in transformer oil is of great significance for ensuring the reliability of transformers. In this paper, a comprehensive observation platform of partial discharge with needle plate electrode and oil-paper insulation is built, and the original shape image set of partial discharge is collected and established. Through multi-channel convolution processing of the true color image, gray level equivalent distribution of 3D section, segmentation stretching of threshold interval and binary neighborhood search, a general technical method suitable for processing a series of low brightness discharge images with background interference is finally obtained.

The DC bias may affect the transient transfer characteristics of the current transformer (CT), threatening the correct action of the relay protection and the safe operation of the power system. Compared with the high voltage direct current (HVDC) transmission in the unipolar-earth loop operation mode and geomagnetically induced current (GIC), the amplitude of the stray current is smaller, and the randomness and fluctuation are larger and accompanied by sudden changes. Combining the characteristics of stray current and the CT equivalent model considering the eddy current loss of the iron core, the mechanism of local transient saturation under the influence of bias magnetization is explained, and the CT saturation phenomenon caused by stray current and its effect on line distance protection are simulated and analyzed. The research shows that the influence of low amplitude and high duration of stray current will make the magnetic flux of CT grow and affect the transmission of steady-state and transient-state so that the measurement impedance appears amplitude-phase shift, which leads to under-range rejection of distance protection.

A long-gap ac arc with a length of more than 10 m (secondary arc) is normally generated at the short-circuit arc channel after a single-phase-to-ground fault. In previous studies, arc breakdowns of secondary arcs have mainly been considered as electrical breakdowns, ignoring the role of heat in the arc channel. Besides, the extinction-reignition theory of secondary arc, i.e., dielectric strength recovery theory, still lacks the support of experimental data. In this study, based on the equivalent experiments performed in the laboratory, the influences of compensation degree of transmission lines, initial recovery voltage gradient of air gap, test current, wind speed, and wind direction on the breakdown characteristics of secondary arcs are studied and statistically analyzed. The laws of the transient recovery voltage (TRV) and of the rate of rise of recovery voltage (RRRV) are also studied by considering the influencing factors mentioned above. The results of this study will provide a more complete experimental basis for the theory of extinction–reignition of secondary arcs and a deeper understanding of the transient characteristics of arc breakdown.

The thermal stress at the interface of different materials becomes distorted during the thermal expansion of a cable terminal, leading to cracking and insulation failure. To give a warning of the ultimate thermal stress of cable terminal, this paper presents a warning method for the ultimate thermal stress of the cable terminal based on tailpipe temperature monitoring. Taking a 110 kV gas-insulated switchgear cable terminal as an example, the heat transfer model of the cable terminal is established under the working condition of a through-flow. The relationship between the temperature of the tailpipe, maximum thermal stress of the epoxy casing, and current-carrying capacity of the cable is established based on a data fitting method, the ultimate stress of the epoxy is taken as a threshold value for the detection and calculate the corresponding current-carrying capacity of the cable and temperature of the tailpipe. The results show that the thermal stress concentration in the cable terminal exceeds the limit when the monitoring temperature at the exposed tailpipe reaches 54.25 °C. If the relationship between the external temperature of the cable terminal and maximum thermal stress is known, warning of the ultimate thermal stress of cable terminal by monitoring the temperature of the tailpipe, providing theoretical support for the regulation of the current-carrying capacity of the cable.

To study the change characteristics of space charge in oil-paper insulation under a slightly nonuniform electric field and a uniform electric field, the existing space charge measuring device was improved, and sphere- and column-plate electrode models were prepared. Oil-paper insulation samples at different aging stages were obtained via electrothermal aging. The real-time waveforms of the space charge distribution in the samples at different aging stages were measured in the process of polarization with a direct-current (dc) voltage and depolarization with a dc voltage, and the effects of different electric-field types on the charge distribution and motion characteristics of oil-paper insulation samples at different aging stages were examined. The results indicated that the existence of a slightly nonuniform electric field accelerated the change in the charge-density peak at the interface and increased the fluctuation range of the charge-density curve in the process of applying a dc voltage. With the aggravation of electrothermal aging, the amount of charge injected into the sample increased, and the slightly nonuniform electric field intensified this process, affecting the accumulation and dissipation of space charge in the sample. Electrothermal aging increased the trap distribution density and energy-level depth. However, for an oil-paper insulation sample at the same aging stage, compared with the uniform electric field, the slightly nonuniform electric field had no obvious effect on the trap energy-level depth, but it increased the amount of trapped charge and changed the trap density distribution in the sample. The results of this study provide a theoretical basis for examining the influence of slightly nonuniform electric fields on the space charge distribution characteristics of oil-paper insulation subjected to ac/dc combined electrothermal aging.

Bubbles in transformer oil can easily lead to partial discharge (PD), which can deteriorate the transformer oil and even breakdown the transformer insulation. To clarify the migration process and the characteristics of bubbles generated in an oil-immersed power transformer exposed to an extremely uneven electric field, we experimentally monitor these phenomena under an extremely nonuniform ac electric field and numerically simulate the migration distance and the migration speed of bubbles with different initial positions and sizes. The results show that the streamer discharge channel formed by a PD in oil is gasified into a bubble channel. After it collides with the surface of the pressboard, its morphology is transformed into approximately spherical bubbles due to the surface tension of the gas–liquid interface. After bubbles are generated in the oil, they move away from areas with a strong electric field due to the electric-field force and gradually approach the oil surface due to the buoyancy force. The experimental results are consistent with the simulation results, which verify the rationality of the simulation model.

As a necessary electrical equipment for the construction of power grid, the safety and reliability of the transformer is the focus that people have always paid attention to. In recent years, the DC bias of the transformer has a serious impact on the safe operation of the power system, which has attracted wide attention of scholars at home and abroad. In view of this phenomenon, combined with the basic principle of transformer excitation, the simulation of the DC bias phenomenon is carried out by using Simulink software in this paper. According to the simulation results, the distortion characteristics of excitation current under DC bias are summarized. Then, based on the variation characteristics of excitation current under over excitation and DC bias, the law of transformer core vibration is deduced, and the cause of transformer abnormal vibration under DC bias is revealed. Finally, by analyzing the data of a transformer substation running under DC bias, it is found that the measured data are basically the same with our simulation results, which provides a basis and reference for further research on the phenomenon of transformer DC bias.