ABSTRACT Modern high‐voltage electrical equipment operates in complex environments where surface conditions are subjected to multifaceted influences from electric fields, thermal gradients, contaminants, irradiation and other extreme environmental factors. Under high‐voltage conditions, surface flashovers frequently initiate at the gas–solid interface of insulators, jeopardising the operational reliability of industrial systems. To address flashover challenges in practical electrical equipment, this review systematically analyses the impact of environmental variables—including gaseous media, voltage waveforms, contaminants and high‐energy irradiation—on surface flashover characteristics. Subsequently, we comprehensively synthesise the mechanisms, contributing factors, suppression techniques and enhancement strategies for surface flashover in four critical equipment categories prone to this phenomenon: high pulsed power systems, spacecraft operating in vacuum environments, gas‐insulated switchgear/transmission lines (GIS/GIL) and outdoor insulators. The environmental dependencies of flashover behaviour in these systems are rigorously examined. Furthermore, to advance flashover prevention in next‐generation industrial applications, this study identifies 25 fundamental scientific questions and 25 technical questions to guide future research directions. By establishing an integrated theoretical framework and methodological foundation for engineering‐oriented flashover studies, this work provides actionable insights and mitigation approaches to enhance the surface insulation of modern industrial infrastructure.

Mechanical loss critically limits the efficiency and stability of piezoelectric devices operated under resonance conditions. In this study, BaTiO3-based piezoceramics doped with La were processed through a sequential reducing-oxidizing thermal treatment to induce piezoelectric hardening without altering the nominal composition. Structural analysis reveals that the oxidation treatment drives a transformation of the major phase symmetry and promotes the formation of a potential secondary phase, indicating a redistribution of doping element during redox processing. As a result, the treated piezoceramics exhibit an approximately 160% enhancement in mechanical quality factor Qm accompanied by suppressed polarization and strain responses under large electric fields. These characteristics suggest effective inhibition of domain-wall motion induced by the redox-driven microstructural evolution. The proposed approach provides an alternative hardening strategy for BaTiO3based piezoceramics with potential for high-power and resonance-driven applications.

Epoxy/BaTiO3 nanocomposites with varying filler contents of BaTiO3 were prepared and characterized for flexible DC insulation applications such as IGBT. Their breakdown strength under DC, AC, and 10 kHz voltage, tensile properties, dielectric response, surface potential decay, temperature-/electric field-dependent conductance, and field grading capability were investigated. Results show that loading BaTiO3 increases the dielectric constant and alters loss behavior due to enhanced interfacial polarization and modified charge transport. However, breakdown and tensile strengths decrease monotonically with filler content, which is attributed to interfacial heterogeneity and local field distortion. Shallow-trap density rises while trap energy level declines with higher BaTiO3 loading, promoting charge trapping-detrapping. Electrical conductivity of epoxy/BaTiO3 nanocomposites increases with both electric field and temperature, while simulation of electric field distribution in the triple point of IGBT encapsulation reveals that the increased permittivity and conductivity with BaTiO3 content can reduce the maximum local electric field by up to 6.7% and 13.7% for the two kinds of typical structure of triple points, respectively. Thus, nano-BaTiO3 effectively tailors dielectric response and charge transport but introduces interfacial complexity that degrades breakdown and mechanical performance. However, a trade-off between intrinsic insulation, tensile strength, and field grading capability can be obtained. This work offers experimental insights for designing epoxy-based encapsulation materials with tunable electrical properties for flexible DC systems.

Dielectric polymers have been extensively applied in electronic devices, circuits and electric power systems owing to the excellent capacitive energy storage performance. However, their performance deteriorates rapidly at elevated temperatures, resulting in low energy density, poor discharge efficiency and restricts their applications under extreme conditions. In this work, the all-organic structural design of Polyetherimide (PEI) has been constructed by introducing rigid alicyclic structures into the PEI chains, which effectively increases the glass transition temperature $\left(\boldsymbol{T}_{\mathbf{g}}\right)$ of modified PEI. Additionally, the rigid alicyclic structures can enlarge the bandgap $\left(\boldsymbol{E}_{\mathbf{g}}\right)$ of the modified PEI, which effectively limits the charge transport and greatly enhances the breakdown strength. The experimental results demonstrate that the modified PEI exhibits improved dielectric performances and excellent energy storage properties compared to traditional PEI at extremely high temperatures. At 200 °C, the modified PEI achieves a discharge energy density $(\boldsymbol{U}_{\mathbf{e}})$ of 3.71 $\mathbf{J} / \mathbf{c m}^{3}$ and a charge-discharge efficiency $(\boldsymbol{\eta})$ exceeding 90%, which is 76.7% higher than that of traditional PEI and superior to the most of current polymer nanocomposites and novel synthesized polymers.

This study employs molecular dynamics simulations to analyze and compare the structures of traditional bisphenol a epoxy resins and cycloaliphatic epoxy resins, demonstrating the feasibility of using cycloaliphatic epoxy resins in blending to enhance the electrical and thermal properties of traditional bisphenol A-based resins. Subsequently, six epoxy resin materials with different base resin ratios were prepared, and their glass transition temperatures and dielectric properties were tested to identify the optimal base resin ratio. The results indicate that when both glass transition temperatures reach high values, the cured resin mixture with an 8:2 mass ratio of bisphenol a epoxy resin to cycloaliphatic epoxy resin achieves the highest dielectric strength, exceeding the performance of pure bisphenol A resin systems by over 20

Partial discharge (PD) detection and diagnosis represent the key techniques for evaluating the insulation condition of hydrogenerators. This article reviews recent significant research findings on PD in hydrogenerator stator insulation, focusing on insulation deterioration, PD detection, and pattern recognition issues. First, the stator insulation structure of hydrogenerators, typical insulation defects, and failure mechanisms are summarized. Second, the PD detection methods for hydrogenerators are discussed, including those based on the high-voltage terminal, neutral point, and electromagnetic wave detection, with a focus on analyzing the advantages and disadvantages of each detection technology. Subsequently, interference suppression approaches for stator insulation PD detection are systematically analyzed, identifying various sources, characteristics, and processing methods while comparing the advantages and disadvantages of different approaches. Furthermore, the PD pattern recognition methods are discussed in detail, examining both traditional machine learning and deep learning-based intelligent algorithms and emphasizing their respective advantages and limitations. Finally, this article indicates the future challenges and prospects of PD detection in hydrogenerators, aiming to provide a foundation and basis for the construction of large-scale hydropower engineering.

Synergistically enhancing toughness, breakdown strength (Eb), and glass transition temperature (Tg) of epoxy resin (EP) remains challenging. To address this, we introduce a reactive toughening agent of carboxyl-terminated butadiene-acrylonitrile rubber (CTBN) and small-molecule polyvinyl carbazole (PVK) into the epoxy network to construct all-organic epoxy composites. Results show that tensile and flexural strengths first increase and then decrease with the rising CTBN content, peaking at 5 wt. % and 10 wt. % CTBN, corresponding to increases of 27.8% and 5% over neat EP, respectively. CTBN monotonically reduces Tg slightly, but further addition of PVK partially recovers Tg without affecting thermal stability. With increasing CTBN content, DC and AC Eb are both first increased and then declined, whereas high-frequency Eb shows little variation; meanwhile, dielectric loss and permittivity rise significantly. Introducing PVK further enhances DC and AC Eb by up to 15.4% and 15.7% over neat EP, respectively, and notably reduces dielectric loss and permittivity of EP/CTBN composites, especially at elevated temperatures, by hindering molecular chain motion. Mechanistically, CTBN introduces deep traps and PVK increases trap density, both of which improve Eb, which can be validated by phase-field simulation. This study provides theoretical and experimental support for developing all-organic epoxy composites with high toughness, high Eb, and high Tg.

Detecting the space charges within micro-regions (less than several mu m) at metal/insulation interface induced by high-frequency voltages is challenging for the design and stability of power electronic devices. This study fabricated a novel embedded metal-epoxy insulation micro configuration using a mask magnetron sputtering method to simulate the interface structure under square-wave electric field. An improved Kelvin probe force microscope with probe vibration mode optimization and lateral pressure electrode was employed to detect the interface charges in the interface microregion. A high electric potential occurs at the insulation side of the interface after high-frequency square-wave field excitation. A transition zone of potential distribution is found at the interface within 5 mu m, which contributes to the electric field distortion at the interface. Interestingly, significant charge accumulations occur at the interface region less than 10 mu m. It decreases initially and then increases with the voltage frequency. It is indicated that the competition of charge injection and the charge recombination at the interface cause the reduction of charges and electric field less than 5 kHz. The study is useful for guiding the design and application of insulation in high power density electronic devices.

This paper systematically investigates the failure characteristics and mechanisms of insulating materials in DC voltage dividers under combined high-frequency voltage and high-temperature conditions via simulations and experiments. The results showed that high-frequency harmonics severely degrade the insulation strength of polypropylene/paper/polypropylene (PPLP) at 10 kHz, in which the bulk breakdown strength of PPLP decreases by over 50%. Furthermore, the surface flashover voltage in oil is reduced by 17.7% under high-frequency voltage alone, and by as much as 51% when white flocculent substances are present in the oil. The dielectric properties of PPLP strongly depend on frequency and temperature, which aggravate the heat accumulation of the divider under high-frequency voltage. Furthermore, the multilayer structure of PPLP introduces deeper trap levels due to interfacial states, which reduce the breakdown strength and flashover voltage of PPLP. Electro-thermal coupling induces a rapid temperature rising to 98 °C at 25 kHz caused by dielectric loss, leading to oil turbidity and white precipitation, consistent with finite element simulations. Consequently, a failure mechanism is proposed as follows: prolonged electro-thermal stress causes chain scission in styrene-containing materials, releasing monomers that repolymerize into white polystyrene deposits. Their porous structure and dielectric mismatch distort the interfacial field, trigger partial discharge, and aggravate surface flashover.

Zinc oxide (ZnO) varistors play an important role in transient voltage surge protection for power equipment. B2O3 doping was reported to be able to improve the nonlinearity of the I-V behavior of ZnO varistors, which is the figure-of-merit for arrestor applications. The present work unravels the mechanism of B2O3 doping effects on the microstructure and electrical properties from a perspective of defect structure. The low-melting-point of B2O3 prompts a uniform distribution of the spinel intergranular phase, which hinders the grain growth of ZnO. Combining experiment and calculation, it is revealed that the B2O3 doping reduces the donor concentration, and increase the interface states, leading to an enhanced Schottky barrier. A moderate B2O3 doping (≤ 1.0 mol%) can enhance the nonlinear coefficient and the potential gradient and reduce the leakage current, while a heavy doping (> 1.0 mol%) may deteriorate the electrical properties, resulting from an over-sintering during the liquid-phase sintering.

The development of smart grids and new power systems has imposed higher requirements on the dielectric performance of polypropylene (PP) in oil-impregnated capacitors. Herein, the dielectric properties of PP films are successfully improved by adjusting their crystalline characteristics. The results demonstrate that under the optimal impregnation process, the capillary effect enhances impregnation efficiency, thereby inducing superior compatibility between the PP films and oil. This facilitates the phase transformation of beta-crystals into alpha-crystals with a more perfect crystal structure during high-temperature impregnation. The resultant increase in the alpha-crystal content is accompanied by a corresponding enhancement in overall crystallinity. It consequently increases the dielectric constant of PP, while retaining an extremely low dielectric loss. Notably, the phase transformation also hinders the mobility of PP molecular chains, thereby weakening the charge energy accumulation process. These synergistic effects lead to a breakdown strength of up to 537 kV/mm, representing an 11.0% increase compared with the unimpregnated PP films. This study proposes a promising strategy for enhancing the dielectric performance of PP via phase regulation, which paves the way for its application in high-performance capacitors.

Dielectric polymers with high glass transition temperatures (Tg) are widely employed in capacitive energy storage applications for electronic equipment and power systems. However, polyetherimide (PEI) suffers from significant leakage current and a pronounced increase in conductivity loss at high temperatures, which prominently restricts its operational reliability under extreme conditions. In this work, an all-organic strategy is proposed by random blocking the 2, 2-Bis [4-(4-aminophenoxy) phenyl] propane (BAPP) into the PEI backbones. The staggered potential barriers between distinct chains create local electronic trap states and increase the energy barrier for trapped carriers to escape. Furthermore, the incorporation of the BAPP segments effectively decreases the interchain spacing and fractional free volume (FFV). Experimental studies and density functional theory (DFT) calculations reveal that a large number of deep traps combined with a reduced FFV, enhance electron capture capability and restrict charge transport. As a result, the random copolymer with 50 mol% BAPP segments (C50PEI) exhibits superior high-temperature capacitive performance, delivering an exceptional discharged energy density (Ue) of 4.68 J/cm3 with a charge-discharge efficiency (𝜂) exceeding 90% at 200°C. This work provides an all-organic design strategy for the development of dielectric capacitors under extreme high-temperature conditions.

In response to the critical demand for lowtemperature-rise, compact energy-absorbing materials for overvoltage protection in flexible HVDC transmission systems, this paper presents the fabrication of electro-induced phase transition silicoaluminate (SiAl)/titanium dioxide $(\mathbf{T i O}_{\mathbf{2}})$ composites. A systematic investigation was conducted on their dielectric polarization characteristics and pulsed energy absorption temperature rise behavior. Broadband and variabletemperature dielectric spectroscopy revealed that the dielectric loss relaxation peak of the $\text{SiAl} / \text{TiO}_{2}$ composite is concentrated in the intermediate frequency range of 100 Hz to 10 kHz, which is attributed to Maxwell-Wagner interfacial polarization. The corresponding relaxation activation energy is as low as 0.2752 eV, significantly lower than that of pure SiAl and pure TiO ${ }_{2}$, indicating that the interfacial polarization process is more readily excited by an external electric field. Pulsed energy injection tests at 250 J demonstrated a temperature rise of only 6.87 °C for the $\mathbf{S i A l} \boldsymbol{/} \mathbf{T i O}_{\mathbf{2}}$ composite, which is 50.7% of that for pure $\mathbf{T i O}_{\mathbf{2}}$ and 35% of that for conventional energy-dissipating resistors. This study elucidates the mechanism of low-temperature-rise energy absorption dominated by interfacial polarization: the interfacial relaxation loss facilitates the conversion of a portion of electrical energy into non-thermal stored forms such as interfacial binding energy and lattice distortion energy, thereby decoupling energy absorption from temperature rise. These findings provide both theoretical and experimental support for the development of novel overvoltage protection materials for flexible HVDC applications.

Space charge accumulation at the electrode/insulation interface indicates a significant challenge to composite insulation in high-power electric and electronic devices. The present study implements an external magnetic field to fabricate the non-uniform distribution of gamma-Fe2O3@BNNS (boron nitride nanosheets) nanoparticles at the electrode/epoxy nanocomposite interface. The pulsed electro-acoustic method (PEA) technique was utilized to evaluate the space charge accumulation under DC and square-wave voltages. Furthermore, the dielectric and trap parameters of the nanocomposites were explored by wide-band dielectric spectrum and thermally stimulated current (TSC) technique, respectively. The findings indicate that the incorporation of gamma-Fe2O3@BNNS markedly diminished both the average charge density and the maximum electric field. The modified gamma-Fe2O3@BNNS effectively mitigated space charge accumulation at the electrode/sample interface under square-wave voltages. Specifically, the 0.5 wt% modified gamma-Fe2O3@BNNS yields a 53 % decrease in charge accumulation at a frequency of 500 Hz. While, the 0.1 wt% sample demonstrates a 30 % reduction in electric field distortion compared to the neat epoxy at 500 Hz. Hetero-charge accumulation is detected in epoxy nanocomposites. The modified gamma-Fe2O3@BNNS fillers at interface introduced additional deep traps, contributing to reduction in charge injection and increase in charge recombination. Consequently, it improves the space charge characteristics of epoxy resin composites for high-power electrical and electronic applications.

Synthetic ester insulation oils are increasingly used in power transformers due to their superior fire safety, biodegradability, and environmental compatibility. However, their degradation behavior and fault diagnostic characteristics under localized thermal stress remain insufficiently understood. This study investigates the dielectric and gas-generation properties of KI50EX synthetic ester oil under localized overheating at $300^{\circ} \mathrm{C}, 400^{\circ} \mathrm{C}$, and $500^{\circ} \mathrm{C}$ for one hour. Experimental results show that the dielectric constant and dissipation factor increase while volume resistivity decreases with temperature, indicating enhanced polarization relaxation and charge transport within the insulation system. The degree of dielectric deterioration is markedly more pronounced at higher temperatures, with both permittivity and loss factor showing sharp increases at $500^{\circ} \mathrm{C}$, reflecting accelerated molecular degradation and intensified conductive losses. The total amount of dissolved gases also rises significantly with temperature, and CO2 and CO are the predominant components under all conditions, suggesting that oxidation is the primary gasgeneration pathway. Conventional dissolved gas analysis (DGA) methods, including the Three-Ratio and Duval Triangle approaches, show deviations when mineral-oil reference zones are applied to synthetic ester oil. The Three-Ratio method consistently indicates low-temperature overheating $(150-300^{\circ} \mathrm{C})$, while the Duval Triangle partially overestimates faults at $500^{\circ} \mathrm{C}$. These findings demonstrate pronounced temperature-dependent deterioration and the limitations of traditional DGA for ester-filled transformers. Future work will focus on extended aging and the development of refined diagnostic criteria for synthetic ester insulation systems.

This study investigates the surface flashover characteristics of epoxy resin-impregnated paper used in dry-type bushings under combined electrothermal stress. The effects of electrode spacing and temperature on insulation performance were examined experimentally. AC surface flashover tests were performed on samples with gaps of 2 mm, 3 mm, and 4 mm at three controlled temperatures: 25 °C, 90 °C, and 120 °C. The breakdown voltage data were analyzed using the Weibull distribution. Results indicate that at a fixed temperature, flashover voltage increases nonlinearly with gap distance, which is attributed to persistent electric field nonuniformity introduced by the finger-type electrode geometry. At larger spacings (3 mm and 4 mm), elevated temperature significantly reduces the flashover strength due to enhanced surface conductivity and increased electron kinetic energy, both of which facilitate discharge initiation and propagation. In contrast, at the 2 mm gap, the electric field dominates the flashover process, and temperature exerts negligible influence within the tested range. This work elucidates the spacing- and temperature-dependent flashover mechanisms in epoxy-impregnated paper under electrothermal conditions, providing insights for condition assessment and reliability improvement of dry-type bushing insulation.

The epoxy-impregnated paper (EPIP) insulation core layer used in dry-type bushings on the valve side of converters is subjected to prolonged exposure to high-frequency harmonics during operation. This exposure can lead to degradation of insulation properties and even breakdown, significantly increasing the risk of equipment failure. To elucidate the influence of high-frequency harmonics on the breakdown strength of EPIP, this study systematically conducted breakdown characteristic tests under various harmonic types, frequencies, and amplitudes. Results indicate that breakdown strength decreases more significantly with higher harmonic frequencies and amplitudes. Compared to sinusoidal harmonics, square-wave harmonics cause a more pronounced reduction in breakdown strength. Subsequently, this paper employed spherical plate electrodes to investigate the partial discharge patterns of epoxy impregnated paper. Findings revealed that under superimposed waveforms, partial discharges exhibited multi-peak discharges primarily concentrated at voltage rise/fall edges. Both discharge phase and discharge frequency increased with the superimposed sine wave amplitude, indicating that superimposing higher-frequency harmonics exacerbates partial discharge degradation.

High-frequency harmonics (>10 kHz) generated by high-voltage semiconductor devices, combined with extremely cold environments, pose a serious threat to the insulation reliability of power equipment for clean energy utilization. This paper investigates the breakdown characteristics of epoxy resin-impregnated paper (EPIP) used in dry type busing under low-temperature and high-frequency harmonic conditions, with focus on the influence of dielectric loss and the orientation of crepe paper within the EPIP on breakdown strength. Additionally, the breakdown mechanism is investigated in relation to space charge behaviour. The results indicate that breakdown strength decreases significantly under high-frequency harmonic voltages. For example, the breakdown strengths of Sample A and Sample B are 122.48 and 106.77 kV mm(-1) at power frequency, 64.32 and 57.76 kV mm(-1) at 10 kHz, and 51.98 and 46.85 kV mm(-1) at 25 kHz at ambient temperature of 253 K, respectively. The relationship between breakdown strength and voltage frequency follows the inverse power law. It is also found that the proportion of high-frequency voltage components nonlinearly affects the breakdown strength of EPIP under power frequency superimposed with high-frequency harmonics voltage. Moreover, breakdown strength decreases exponentially with temperature in the low-temperature range. Compared with the thermal decomposition temperature of EPIP, the temperature rise caused by dielectric loss under high-frequency voltage is insufficient to directly induce thermal breakdown. Therefore, this paper proposes that the breakdown of EPIP under high-frequency voltage is primarily attributed to space charge injection and recombination, which are thermally assisted by dielectric loss. It is demonstrated that charge recombination is significantly intensified under high-frequency conditions. Furthermore, the shallower trap depth in Sample B facilitates deeper charge injection, resulting in lower breakdown strength. These findings provide both theoretical and experimental support for improving the insulation performance of power equipment under high-voltage and high-frequency operating conditions.

The epoxy resin insulation of the wall bushing plays a critical role in high-voltage air-insulated switchgear (AIS). Breakdown occurs frequently at the short edge of the flange, resulting in serious erosion of AIS. Although accelerated aging and deterioration of epoxy resin have been well investigated in the laboratory, the generation and development of defects, as well as their electrical performance evolution, have not been systematically realized in long-term operational wall bushings, forming obstacles for their accurate deterioration assessment and maintenance. Here in this work, the electrical field distribution under operating conditions and typical fault conditions of internal shielding virtual connection is investigated, and the electric field distortion on the short edge of the flange is analyzed. The extreme electric distortion leads to defect development from blowholes to cracks, characterized by ultrasonic detection on wall bushings with effective insulation, with external burns, and with serious insulation damage, extracted from operating conditions. Combined analysis of ultrasonic and partial discharge results indicates that the deterioration of insulation performance correlates with an increase in defect type and density, as well as discharge capacity. Furthermore, the dielectric properties and breakdown strength are tested, demonstrating a significant increase in permittivity and dielectric loss, and largely reduced breakdown strength with deterioration. This study reveals the dielectric and electrical performance evolution of epoxy resin under long-term deterioration, and provides a reference for accurately evaluating the insulation status of wall bushing in engineering operations.

Thermal ageing is an important factor leading to the deterioration of crosslinked polyethylene (XLPE) cable insulation. In this paper, XLPE insulating material was prepared in the laboratory and treated by thermal ageing at 130 degrees C for durations of up to 40 days. Space charge and DC electrical breakdown characteristics were investigated to evaluate the DC electrical performance of differently aged XLPE. Furthermore, to gain deeper insight into the underlying mechanisms, the charge trap characteristics along with the chemical and crystalline structures were examined. The results show that, during the effective protection period of the antioxidant, the XLPE samples exhibit enhanced DC electrical performance, specifically higher DC breakdown strength and reduced space charge accumulation. This improvement is attributed to the evolution of the microstructure. In the initial ageing stage, the degree of crystallisation increases, whereas physicochemical defects are reduced, accompanied by a more concentrated trap energy distribution. As a result, charge injection and migration are effectively inhibited. In the later stage of ageing, the antioxidant efficacy weakens and molecular chain scission occurs, resulting in the disappearance of spherulitic structure and deterioration of electrical properties. These findings are essential for the understanding of the XLPE cable service performance and the enhancement of the XLPE DC electrical performance.