The electric field distributions along gas-solid interfaces determine the reliability, lifetimes and sizes of gaseous insulated switchgears/pipelines (GIS/GIL), which also affect the reliability of power systems. In this study, the characteristics of steady and transient electric field distributions were first introduced, followed by the concept of functionally graded materials (FGMs). The development histories of FGM applied in electrical engineering and the related optimisation methods were described in detail. The field regulation effect and fabrication technology of different FGM insulators were also compared. To overcome the limitations of traditional FGM insulators, the design of surface FGM (SFGM) was proposed, together with their preparation methods and electrical performance. Furthermore, the future development prospects of FGM and SFGM insulators for compact GIS/GIL were summarised.

Element doping is an effective method to improve the performance of ZnO varistors. Previous studies mainly focused on the variation of microstructures and Schottky barriers. In this study, the effects of Co dopant on electrical properties are investigated from the aspect of multiscale defect structures, including intrinsic point defects, the heterogeneous interface of depletion/intergranular layers, and interface states at grain boundaries. Combining with analysis of phase composition and energy dispersive spectroscopy, it is found that Co tends to dissolve into ZnO grains when slightly doped. It substitutes Zn²⁺ with the same valence and affects little on densities of donors. Segregation of Co at grain boundaries would result in the formation of spinel phase Co(Co_4/3Sb_2/3)O₄ and transformation of the intergranular phase from α-Bi₂O₃ to δ-Bi₂O₃. Meanwhile, densities of point defects are indirectly affected by oxygen ambient during sintering, resulting in abnormal variation of grain resistivity. And interface states are enhanced, leading to improved barriers at grain boundaries. Therefore, reduced leakage current, enhanced grain resistivity, and improved non-linear coefficient in Co-doped ZnO varistor blocks are understood from the underlying multiple defect structures. This presents a potential approach to explore short-term performance and long-term stability of ZnO varistors from the aspect of defect responses.

Polypropylene (PP)-based nanocomposite is a promising insulation material for recyclable high-voltage direct current (HVDC) cables, where the coupling agent plays an important role. In this study, four silane coupling agents with different alkyl chain groups (methyl, propyl, octyl, and octadecyl) were used to surface-modify magnesium oxide (MgO) nanoparticles. The surface-modification effect on the electrical properties of PP/MgO nanocomposites was investigated. The results show that surface-modified nanoparticles introduce quantities of deep traps, whose quantity increases as the alkyl chain length increases. The similar tendency also occurs on DC volume resistivity. All these nanocomposites show remarkable space charge suppression ability and improved DC breakdown strength. Among them, the nanocomposites with octyl-modified MgO show the best electrical properties, which could be attributed to the introduction of a large quantity of deep traps. The work may give a reference for the selection of coupling agents in PP-based nanocomposite insulation material for HVDC cable.

Fluorinated nitrile (C₄F₇N) gas mixture has been introduced as the most promising candidate to replace sulfur hexafluoride using in gas-insulated equipment. In this study, the authors explored the influence of oxygen on the dielectric and decomposition properties of C₄F₇N–CO₂–O₂ gas mixture. The authors found that the dielectric strength of the C₄F₇N–CO₂–O₂ gas mixture with 2, 4, 6, 8 and 10% O₂ was increased by 4.85%, 6.49%, 7.70%, 3.21% and 2.74% compared with C₄F₇N–CO₂. The addition of 2–6% O₂ to the C₄F₇N–CO₂ gas mixture could effectively reduce the content of most of the decomposition by-products such as CF₄, CO, C₂F₆, C₃F₆, C₃F₈, CF₃CN, C₂F₅CN, (CN)₂. While high content of oxygen (>6%) results in higher decomposition of C₄F₇N, which has a negative effect on the stability of C₄F₇N gas mixture. Generally, it is recommended to add 2–6% O₂ in the C₄F₇N–CO₂ gas mixture to improve its insulation properties as well as inhibit the decomposition of C₄F₇N in the discharge for medium-voltage engineering application.

The development of new materials for use in advanced distribution transformers is required to improve their heat resistance rating, operational safety, and reliability. The combination of polymer and nanoparticles is the preferred approach to improve the thermal stability of materials. In this study, the thermal stability of laboratory-prepared meta-aramid insulation paper modified with polyphenylsilsesquioxane (PPSQ) was evaluated. The results showed that the thermal stability of the modified insulation paper was superior to that of the unmodified paper. Molecular dynamics simulations were used to analyse the internal mechanism of the performance improvement achieved by modification with PPSQ. From a microscopic perspective, the addition of PPSQ provided a relatively larger structural space for the meta-aramid fibre and increased the free volume of the entire system, which tightened the meta-aramid fibre molecular chains, increased the density of the structure of the paper, and enhanced the mutual permeability between meta-aramid fibre chains. Therefore, the stress can be better transmitted, the performance loss such as phase delamination was effectively prevented, and the bonding between the meta-aramid fibres was strengthened.

The properties of novel cable insulation systems will rely critically upon the morphology of the material. Here, a blend of high and low-density polyethylene (PE) was processed in order to generate three sets of samples with different morphologies. The influence of thermo-oxidative ageing at 120°C was then considered. The resulting chemical changes included the introduction of unsaturation and oxygen-containing groups and were determined by antioxidant consumption and oxygen permeability. Such chemical defects were found to be concentrated in the fraction of each system that was molten at 120°C and, consequently, served to inhibit recrystallisation following ageing. The resulting spatial distribution of charge trapping sites was therefore strongly dependent on morphology. The electrical conductivity of each system varied non-monotonically with ageing: short times reduced the conductivity; a rapid increase in conductivity over five orders of magnitude occurred beyond a critical ageing threshold. Despite the pronounced structural differences between the morphologically distinct sets of samples, all exhibited comparable conductivity values beyond this threshold, implying that while charge transport is strongly influenced by chemical factors, crystallinity is relatively unimportant. This experimental finding appears at odds with theoretical studies of the electronic states in crystalline and amorphous PE.

Breakdown failure in insulation material is one of the key problems that threaten the safe operation of high-voltage direct current cable. In this work, the effect of boron nitride nanosheets (BNNSs) concentration, space charge and temperature on DC breakdown strength have been explored. Cross-linked polyethylene (XLPE)/BNNS nanocomposites were prepared by the melt blending method, and the basic characteristics of nanoparticles and composite were characterised. The experimental results indicate that DC breakdown strength of nanocomposite can be effectively improved when a small amount of BN nanosheet is doped into the matrix. The breakdown strength of the sample reaches the maximum value of 407.52 kV/mm when BNNS content is 0.5 wt%, which is about 33% higher than that of pure XLPE. Further, the effect of space charge on the breakdown of nanocomposites has been studied by pre-injecting charges. For the samples with different BNNS contents, all the breakdown strength present ascending trend when the polarity of the applied voltage is the same as that of the pre-injected charges. Besides, it can be found that the breakdown strength of the XLPE/BNNSs composite decreases significantly at 50°C, which is due to more charge accumulation at 50°C. It reaches 2.06 × 10⁻⁸ C which increases by about 2.2 times than the room temperature.

In insulating liquid, a conductive particle becomes charged through the interaction with a conductor exposed to an applied field. Then, it migrates in the action of both electric field and fluid and causes the enhancement of local field when it is in proximity of the conductor with opposite polarity. The enhancement will lead to partial discharge (PD) and may even ignite full gap breakdown in special cases. This study reviews comprehensive researches relevant to these topics. In the first place, several theoretical methods about how to obtain the charges held by a conductive particle with known potential are introduced, so are experimental methods. Then, forces acting on the charged particle in liquid are classified, and its migration characteristics, as well as influential factors, are described. Subsequently, PD and breakdown mechanisms of liquid initiated by conductive particles are presented. The latter involves two cases: electric-field enhancement when the particle concentration is low and bridging phenomenon when it is high. At last, two important, but frequently ignored factors, i.e. dielectric barrier and oil flow, are highlighted, and the authors’ suggestions for future work are put forward.

Insulation oil is an important dielectric in power devices, and many studies on mixed insulation oil have been conducted in recent years to improve the performance of insulation oils. To replace mineral oil directly, the authors previously developed a novel three-element mixed insulation oil successfully; the main parameters of which satisfy the IEC 60296-2012 standard for mineral oil. In the present study, the AC breakdown properties of insulation paper (pressboard) immersed by the new mixed oil and naphthenic mineral oil were compared. For both insulation oils, the increase in temperature cannot significantly reduce the breakdown strength of oil-immersed insulation paper (pressboard) at a low moisture content, and the increment of moisture content cannot reduce the breakdown voltage at a low temperature (25°C). The breakdown voltage decreases only when the two factors increase simultaneously. For the mixed and mineral oils, the AC breakdown voltage of oil-immersed paper (pressboard) with different thicknesses has significant difference. The mineral oil has a high breakdown voltage for the thin insulation paper, whereas the mixed oil has a high breakdown voltage when the thickness of the insulation paper (pressboard) exceeds 0.2 mm. This phenomenon is mainly caused by the breakdown field strength that varies with the increase of dielectric thickness and the different change trends of paper (pressboard) immersed with the mixed and the mineral oils. Moreover, the stack of thin multilayer insulation paper enables the mixed oil-immersed paper to have a higher breakdown strength than the mineral oil-immersed paper. For the AC breakdown voltage of oil-immersed pressboard with an oil-gap structure, the mixed oil is comprehensively superior to the mineral oil due to its larger relative permittivity.

Gliding arc is a mostly used non-equilibrium plasma generation method whose characteristic is affected by the back-breakdown phenomenon. Mode transition induced by back-breakdown of the gliding arc is studied in this work and effects of gas flow rate, applied voltage, electrode parameters on mode transition were studied. Experimental results show that there are two typical modes during the development of the gliding arc. Mode A comprises periodicitical development of the arc from the minimum gap breakdown to the longest extinguishment, while the arc in mode B continues to produce a back-breakdown at the ends of the electrodes. As the flow rate descends, the gliding arc gradually changes from mode A to B. It is the decrease of the arc velocity caused by lower flow rate leads to the occurrence of back-breakdown, which generates mode B. Smaller electrode opening angle, shorter length and wider minimum gap reduce the gliding speed, so that arc is more likely to enter mode B. As the applied voltage is increased, enhancing of the electric field strength on the breakdown path of the back-breakdown and thickening of the arc's diameter allow the gliding arc to enter mode B at a higher arc speed.

In order to improve the arc quenching ability of the multigap system, it is important to know the principle of arc quenching and how the chamber structure affects the arc development process in the multigap system. In this study, the two-dimensional geometric model of the multigap system is established based on the magnetohydrodynamics theory in COMSOL multiphysics simulation platform. The principle of arc quenching in the multigap system is explained in detail by analysing the physical characteristics of the arc in the multigap system. The simulation result shows that the arc is compressed at large scales in the semi-closed chambers of the multigap system, which results in an instantaneous temperature rise of the arc, thus, forming the self-expanding airflow. The strong self-expanding airflow cuts off the arc channel and blocks the energy supply of the arc. The influence of the chamber structure on arc quenching in the multigap system is manifested in the fact that proper improvement of the width and depth of the chamber subserves arc cooling, the reduction of the deflection angle and the increment of the number of the chambers are conductive to arc extinction.

This study presents the discharge process on rod–rod electrodes under oscillating lightning impulse voltage. For this purpose, a measurement system has been developed to record the discharge currents at both rod electrodes simultaneously and to photograph the spatial propagation of the discharges. The measurement results of the discharge process are shown for an air gap of and for a superimposed frequency of . Like the standardised lightning impulse voltage, three consecutive phases of discharge have been identified before a breakdown occurs. These are streamer discharge phase, channel transition phase and channel formation phase. The three phases are described, interpreted and their correlation is shown. On the basis of the oscillations, the discharge processes recur periodically if the momentary voltage is higher than the mean curve of the oscillating lightning impulse voltage. The streamer discharge emerges foremost at the positive rod electrode and propagates toward the grounded rod electrode. Its propagation depends on the voltage crest value. Arriving at the grounded rod electrode, the channel transition phase begins. There the discharge moves back to the positive rod electrode due to recombination and generation. Then the discharge constricts to the positive rod electrode, which forms a channel. Simultaneously, at the grounded rod electrode, a channel is also emerging. In each period, the channel will build up stepwise. The channel leads to the main discharge of the oscillating lightning impulse voltage.

Flashover of ice-covered insulators seriously affects the safe operation of transmission lines. It is necessary to study the electric field distribution for flashover analysis of ice-covered insulators. The electric field of ice-covered insulators was mostly calculated by a two-dimensional (2D) axisymmetric model that cannot be well corresponded to actual icing situations. In this study, a 3D electric field simulation model of ice-covered suspension insulators under moderate icing condition with applied DC voltage was established. Two characteristic parameters, E _av and E _max were proposed to measure electric field distortion degree. Based on the simulation results, the effects of water film conductivity, icicle length, icicle deviation angle, and icicle distribution on E _av and E _max were studied. The results showed that icicles have a significant influence on the electric field distribution of insulators. E _av increases with the increase of icicle length and decreases with the increase of icicle deviation angle. E _max increases with the increase of the icicle length, icicle spacing and adjacent icicle length difference. When the icicle deviation angle is 45°, E _max is the largest. Lastly, the possible flashover paths were analysed based on the simulation results, which would provide a theoretical basis for building a flashover model of ice-covered insulators.

Current injection circuit breakers consist of a mechanical interrupter (MI) with a current injection and an energy dissipation branch in parallel. The performance of the complete device is largely determined by mechanical operation time and interruption performance of the MI. In the standard configuration, current injection is realised using a pre-charged inductor–capacitor circuit. A higher interruption performance of the MI makes it possible to scale down the resonant injection circuit, and thus have a more economical design. Additionally, the implementation of more complex injection circuits that quickly create zero crossings, while maintaining favourable conditions for interruption, can lead to economic benefits. In this study, the interruption performance of a model gas interrupter as part of a current injection topology is investigated. The results are used to verify a corresponding simulation model and two-dimensional upgrade circuits that influence the injection current to increase the range of interruptible fault currents. On the basis of experimental results, the simulation model is used to investigate the performance of upgrade circuits for the use in high-voltage direct current (HVDC) systems. The results indicate that using improved injection circuits can considerably increase the economic advantage of current injection circuit breakers compared with other topologies.

Frequency response analysis is widely used as a method for the offline diagnosis of winding deformations in power transformers. To apply it to a working transformer, people need to determine how to inject the excitation signal and measure the response signal for windings that bear a rated voltage and current. In this study, a method to obtain the frequency response curve online is proposed. It uses the principle of magnetic field coupling to inject a frequency sweep signal into the windings through a Rogowski coil. Another Rogowski coil sensor placed at the root of a high-voltage bushing is used to measure the response current signal. Experiments on a 72.5 kV bushing show that metal accessories of the bushing have no influence on the injection or the measurement. The feasibility of this method was verified by experiments on charged 110/35/10 kV transformers at a factory and a working 35 kV transformer in a power station. The results show that it is safe to install the Rogowski coil at the root of the high-voltage bushing. The excitation signal can be injected into live windings and the response signal is measurable. Strong electromagnetic power frequency noise can be reduced and the frequency response curve can be measured online.