Pollution flashover poses a severe threat to the secure operation of power transmission systems. Composite insulators with excellent hydrophobicity have become the mainstream choice, but their reliability still faces the challenge of pollution flashover under harsh natural conditions. Existing flashover monitoring methods have limitations in practical application due to poor correspondence with flashover voltage. To address this issue, this paper proposes a flashover monitoring method for hydrophobic surfaces based on the ratio of fundamental to total leakage current. First, the flashover characteristics of hydrophobic surfaces under different wetting states are analyzed, revealing that the intermediate wetting stage is the key for monitoring. Then, a characteristic parameter Kh10 (arithmetic mean of the 10 lowest Kh values within a certain time interval) is defined, where Kh is the ratio of the maximum fundamental component to the maximum total leakage current per second. Experimental verification is conducted on specimens with different hydrophobicities, and the results show that Kh has good applicability on hydrophobic polluted surfaces. Kh10 effectively suppresses random noise while retaining the physical evolution of surface conductivity. Further analysis demonstrates an approximately linear relationship between Kh10 and the flashover voltage gradient, indicating that Kh10 can reflect flashover strength and the associated surface condition of hydrophobic polluted surfaces to a certain extent. This method provides a basis for flashover condition assessment on hydrophobic polluted surfaces and shows potential for further application to composite insulators.

Alumina-based porcelain insulators are widely used in power systems and have become one of the most essential insulating materials in the grid. However, the issue of zero-value insulators—those that have lost their insulating function—remains a major challenge, posing serious risks to the safe and stable operation of power systems. The formation of zero-value insulators is closely linked to the presence of internal cracks within the ceramic body. In this study, the influence of internal cracks on the power frequency breakdown voltage of alumina ceramics is investigated. Simulated crack configurations of varying lengths and bending geometries were analyzed to examine their effect on breakdown behavior. The results show that the breakdown voltage decreases progressively with increasing air-gap length within the crack. In contrast, the degree of crack bending has minimal impact on breakdown voltage. Overall, the breakdown voltage is determined primarily by the total air path length, rather than the shape of the crack. These findings provide important insights for understanding failure mechanisms in porcelain insulators and guiding the identification and prevention of zero-value degradation.

Unusual flashovers of composite insulators with low pollution levels under rainfall conditions have been observed in recent years, yet their mechanisms remain unclear. This study investigates the triboelectric charging of hydrophobic silicone rubber surfaces under simulated rainfall. Using a custom-built water dripping platform, the charge of sliding water drops was measured by both a Faraday cup and a fine tungsten-wire probe connected to a current amplifier, while the surface potential distribution was obtained through electrostatic voltmeter scanning. Results show that the charge of water drops changes from negative to positive along the sliding path, with the maximum positive potential appearing near the lower end of the specimen. This polarity separation arises from the competition between solid-liquid triboelectric charge transfer and surface charge retention mechanisms on the silicone rubber surface. The transient charging process is governed by a balance between triboelectric charging and surface charge dissipation, which is well described by an equivalent RC model. Additionally, the charge dissipation experiment confirms the exponential decay of both positive and negative surface potentials. A supplementary test reveals that water dripping for 2 min reduces the contact angle of silicone rubber by approximately 10 degrees, indicating a decrease in hydrophobicity that may promote continuous conductive film formation and increase flashover risk under service conditions. These findings provide insights into the coupling of triboelectric charging, surface charge retention, and hydrophobicity degradation, contributing to the understanding of flashover mechanisms for composite insulators under wet conditions.

The inherent molecular structure of epoxy composites make it difficult to synergistically enhance their heat resistance and mechanical performance. Precise molecular structure design provides an effective approach to solving this problem. By introducing entanglement structures into epoxy composites, this study successfully addresses this trade-off issue. Small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) confirm the entanglement network induced by the long carbon chain molecular structure in the epoxy composites. The inclusion of rigid benzene rings facilitates the synergistic improvement of thermal stability and mechanical performance in epoxy composites. Compared with pure epoxy resin (EP), the glass transition temperature, tensile strength, elongation at break, and impact strength of modified epoxy resin (EP-BTDA-10) are increased by 9.00%, 21.70%, 14.50%, and 24.97%, respectively. The constraints on molecular motion from entanglement at high temperatures, coupled with long carbon chains, result in increases of 109.48% in tensile strength and 211.62% in elongation at break for EP-BTDA-10 at 90 degrees C. The entanglement network enhances the cross-linking density of epoxy composites, leading to an 11.84% increase in alternating current breakdown field strength and an order-of-magnitude improvement in resistivity. This study provides a new insight for the design of high heat resistance and high toughness insulating materials with customizable designs for electrical applications.

As transmission voltage levels continue to rise, surface streamer discharges along solid-dielectric/air interfaces have become a critical factor affecting the long-term reliable operation of gas-insulated switchgear (GIS) and cable terminations. This paper investigates the influence of the PTFE solid-dielectric length at the streamer inception position on surface streamer discharge characteristics in composite gaps. The results show that increasing the dielectric length raises the steady propagation field strength required for stable streamer propagation. The propagation speed of the surface component increases markedly with dielectric length, whereas the air component is only weakly affected. In contrast, the photoionization intensity associated with the surface component decreases as the dielectric length increases, while that of the air component exhibits little variation. When the dielectric is short and the applied electric field is either too high or too low, the surface component cannot sustain stable propagation. The analysis indicates that a longer dielectric enhances the vertical electric field, promotes surface charge accumulation and secondary electron emission, and thus facilitates surface streamer propagation. Conversely, for a short dielectric, the ionization capability of the surface component is insufficient and stable propagation is difficult to maintain. These findings provide useful theoretical and practical guidance for material selection, dimensioning, and structural design of solid-dielectric/air insulation systems in high-voltage equipment.

Porcelain insulators with internal defects pose a significant risk to power system safety. However, conventional detection methods are susceptible to environmental interference and rely on the assumption that internal defects always manifest as reduced insulation resistance. To overcome these limitations, we developed a contact detection device for power outage conditions that applies high-level impulse voltage to enhance detection accuracy. In this study, the device was evaluated on porcelain insulators with rated mechanical strengths of 70 and 550 kN. Defective insulators exhibited significantly lower voltage waveform amplitudes compared to defect-free units. Experimental results show that the high resistivity of cement can mask internal defects, leading to missed detection by conventional insulation resistance meters. Preliminary energized tests further validate the method's potential for live-line inspection. The proposed device maintains stable performance under relative humidity up to 90%, positioning it as a practical alternative to conventional insulation resistance method.

Abstract During the manufacturing of gas-insulated switchgear (GIS), basin insulators are susceptible to void defects that may escape conventional non-destructive testing. These defects can deteriorate mechanical integrity and promote local electric-field distortion during long-term operation. In this paper, an electro-thermo-mechanical coupled finite-element model incorporating the fracture phase-field method is developed to investigate the failure behavior of GIS basin insulators containing void defects. In the model, the cumulative thermal effect associated with repeated partial discharge activity is represented by an equivalent localized thermal load at the void wall, enabling a parametric evaluation of the influence of local heating on crack initiation and propagation. The results show that void defects significantly intensify local stress concentration. With increasing defect size, the maximum first principal stress increases, whereas the critical failure load decreases continuously. The localized thermal load reconstructs the internal thermal stress field and induces a distinct spatial shift in the crack-initiation site. Simplified three-point bending tests on epoxy specimens with artificial void-like defects further support the predicted defect-size effect and crack-initiation shift. Moreover, a comprehensive parametric sensitivity analysis reveals that crack propagation exhibits pronounced heterogeneity governed jointly by defect size, temperature-rise intensity, and spatial location, identifying critical thresholds that trigger failure-mode transitions. The proposed model and findings provide theoretical support for defect assessment, structural optimization, and reliability improvement of GIS basin insulators.

Interface defects represent a critical weak point in silicone rubber/epoxy resin (SIR/EP) composite insulation structures. Active infrared thermography (AIT) has shown great potential for detecting interfacial air-gap defects. However, the influence mechanism of air-gap thickness in SIR/EP has not been systematically investigated, and the detectable thickness limit of this method remains unclear. To address these issues, this study investigates the propagation characteristics of thermal waves in SIR/EP interfaces containing air-gap defects. First, time-domain and frequency-domain multilayer thermal-wave models incorporating interfacial air gaps were developed to elucidate the effect of defect thickness on temporal and spectral features. Finite element simulations were then performed to quantitatively analyze the influence of air-gap thickness on the thermal-wave amplitude, phase, and maximum temperature difference. The results indicate that thicker air gaps lead to larger amplitudes and temperature differences, but smaller phase values. Finally, experimental studies using SIR/EP samples with air-gap defects were conducted under both pulsed and laser excitations to validate the theoretical and simulation findings. The results demonstrate that both phase and maximum temperature difference exhibit strong potential for quantitative characterization of air-gap thickness. Based on time- and frequency-domain features, interfacial air-gap defects with a thickness of 0.2 mm or greater can be detected by the AIT method at the SIR/EP interface beneath a 3-mm-thick SIR layer. This work provides theoretical and experimental guidance for quantitative detection and evaluation of interfacial defects in SIR/EP insulation systems.

Porcelain insulators are essential for the mechanical and electrical reliability of overhead transmission lines, especially in Ultra High Voltage (UHV) systems. Manufacturing defects such as pores, impurities, and microcracks can threaten their integrity under stress. Hydrostatic test is widely used as a quality control method, but its parameters are often based on experience, with limited understanding of its effectiveness in detecting specific defects. This study quantitatively evaluates the sensitivity of hydrostatic test to typical defect types in large-tonnage porcelain insulators. Finite element simulations were conducted to analyze how defect size, shape, and material properties influence stress distribution. The results provide insights into the defect elimination mechanisms of hydrostatic test and support the development of the more standardized and effective testing procedures.

ABSTRACT In response to the current challenge of accurately determining the specific heating status of composite insulators with localised temperature rise using infrared detection technology, this paper proposes a novel diagnostic method. This method involves applying a disturbance to monitor the surface temperature rise data of composite insulators and establishing a relationship between the characteristic parameters of the temperature rise curve and the heating type of composite insulators. By leveraging the fact that the surface temperature rise rate of composite insulators is influenced by the heat source’s position, this approach aims to effectively distinguish between the two types of heating that cause localised temperature rise phenomena. The effectiveness of this method is validated through simulations and experiments, and relevant recommendations for engineering applications are provided.

Epoxy-metal interfaces are critical structures in power equipment but are highly susceptible to cracking under coupled thermal and mechanical stresses, posing serious threats to equipment reliability. In this study, a cycloaliphatic epoxy-based composite insulator end-seal structure was employed as a representative case. A thermo-mechanical phase-field fracture model was established to describe crack evolution at tensile/shear mixed-mode epoxy-metal interfaces, in which an interfacial state variable distinguished the matrix from the interface, and the Benzeggagh-Kenane criterion captured the transition between tensile and shear failure modes. The synergistic effects of thermal cycling and external loading on interfacial crack growth were examined, along with the influences of applied load, interfacial strength, and structure. Results revealed that the maximum interfacial stress occurred at the lowest temperature. Compared with either thermal cycling or external loading alone, thermo-mechanical coupling significantly increased the interfacial stress, promoting crack initiation and its progression into interfacial debonding, matrix damage, and complete penetration, consistent with experimental observations. Lower cooling rates and core-rod constraints suppressed instantaneous crack instability, leading to gradual damage accumulation. Increasing the tensile load accelerated crack evolution, while higher interfacial tensile toughness GIC inhibited early-stage cracking and influenced the competition between interfacial and matrix cracks. Interfacial shear toughness GIIC governed crack deflection during tensile/shear transitions. Inclined interfaces, though unable to prevent cracking, significantly delayed full penetration. This work elucidates the selective crack evolution mechanism of mixed-mode epoxy-metal interfaces under thermo-mechanical coupling and provides theoretical guidance for interfacial design and reliability enhancement in power equipment.

The UHV dc transmission lines under construction and planning have long distances, and most of them have to cross high-altitude and low-pressure areas. In order to make the external insulation of the equipment operate normally in a high-altitude and low-pressure environment, the influence mechanism of air pressure on the flashover along the insulation surface has always been a hot issue in the field of high-voltage external insulation. This article studies the influence of the air pressure on the streamer discharge in the air. A three-electrode arrangement is used to generate a uniform electric field to measure the streamer discharge characteristics, and a fluid simulation model is acquired to analyze the streamer discharge mechanism. The relationship between the air pressure and the streamer discharge characteristics has been researched deeply. The streamer stable propagation field is positively correlated with air pressure, and the streamer propagation velocity is negatively correlated with air pressure. The relationship between the streamer channel diameter and the air pressure is established, which provides a theoretical basis for the accurate simulation calculation of streamer discharge characteristics under low pressure.

Silicone rubber optical fiber composite insulators introduce interface defects due to embedded optical fibers, and their structural design remains immature, resulting in inadequate interface sealing performance. In actual operation, the combined effects of high electric fields, high humidity and heat, and mechanical loads lead to frequent failures. This study proposes replacing conventional silicone rubber with cycloaliphatic epoxy resin (CEP), which exhibits superior aging resistance, to enhance long-term operational reliability. However, the correlation mechanism between the structural parameters of CEP optical fiber insulators and their electromechanical properties remains unclear, lacking corresponding design basis. Therefore, based on finite element simulation technology, this study systematically analyzed the influence patterns of core rod diameter, fiber implantation method, spiral groove angle, fiber implantation quantity, and voltage equalization ring structural parameters (outer diameter, circular tube radius, shielding depth) on their mechanical and electrical properties. Research findings indicate that in terms of mechanical properties, the helical groove structure with a 40 mm core rod diameter, a groove angle of 135°, and six embedded optical fibers exhibits the lowest optical fiber strain. In terms of electrical performance, the minimum peak electric field strength at the end of the insulator occurs when the equalizing ring has an outer diameter of 370 mm, the circular tube radius is 25 mm, and the shielding depth is 50 mm, reaching only 4.6 kV/cm, which meets the requirements of DL/T 1000.3-2015. This study establishes optimization principles for key structural parameters of CEP optical fiber composite insulators, offering significant engineering value for enhancing the overall performance of optical fiber composite insulators and improving the operational safety of power systems.

Glass insulators are critical components in overhead transmission lines, and their reliability is closely tied to residual stress introduced during tempering. Due to their non-uniform geometry, understanding the surface stress distribution is essential for explaining self-explosion failures. This paper presents a coupled CFD-FEA numerical framework to simulate residual stress in large-tonnage tempered glass insulators. CFD is first used to model fluid and temperature fields during tempering. These results are then input into an FEA model to compute stress distribution. Simulations show that the disc region exhibits surface compressive stress of similar to 170 MPa and maximum internal tensile stress of similar to 73 MPa. At the disc-rib junction, tensile stress reaches similar to 90 MPa, while the rib area shows tensile stress of 85-95 MPa and compressive stress of 90-120 MPa. Stress varies across subregions of the head. The simulation results are validated using SCALP-05 photoelastic analysis, confirming their accuracy. This study provides a foundation for understanding failure mechanisms and optimizing the tempering process to improve insulator performance.

The UHV AC and DC transmission lines across low temperature and cold areas necessitates the adjustment of UHV insulation design to ensure safe and stable operation. The issue of flashover along the outer insulation surface at low temperatures has been extensively discussed, yet there remains a lack of understanding regarding the mechanism of streamer discharge in such environments. The stable control of temperature, humidity, air pressure, and other environmental parameters is achieved in this study through the utilization of a cold box and dry air system. Additionally, the characteristics of streamer discharge in low-temperature environments are measured using advanced equipment such as photomultiplier tube PMT and high-speed camera ICCD. Furthermore, a fluid model is employed to establish a simulation model for streamer discharge in low-temperature environments, with simulation results demonstrating consistency with experimental findings. The experimental and simulation results demonstrate that the progression of streamer discharge is impeded by low temperatures. Moreover, a decrease in temperature necessitates a higher electric field for the advancement of streamer discharge, resulting in a reduced propagation velocity of the streamers. Investigating the mechanism of streamer discharge in low temperature environments is crucial for comprehending and revealing the mechanisms behind surface discharge under these conditions, ultimately leading to improved design standards for external insulation in low temperature environments.

Catenary cantilever insulators used on high-altitude electrified railways are simultaneously exposed to low atmospheric pressure, ice accretion, and surface contamination. Unlike conventional vertical insulator strings, their horizontal or 45° inclined installation orientation alters water runoff, icicle-chain continuity, residual air-gap distribution, thereby changing the flashover path and the dominant factors governing flashover voltage. In this study, icing flashover tests were conducted in an artificial climate chamber at ice thicknesses of 5, 10, and 15 mm, icing-water conductivities (γ) of 300, 600, and 900 μS/cm, and atmospheric pressures of 94.36, 74.09, and 56.89 kPa. Single-factor power-law model and multifactor empirical model were established to quantify the individual and combined effects of ice thickness, icing-water conductivity, and atmospheric pressure on the mean AC flashover voltage. The results show that flashover voltage decreased with increasing ice thickness and icing-water conductivity, but increased with atmospheric pressure. The orientation-specific fitted exponents indicate that the horizontally installed insulator is more sensitive to ice thickness, whereas the 45° inclined insulator is more sensitive to atmospheric pressure. High-speed imaging and COMSOL simulations indicate that continuous icicle bridging under horizontal installation intensifies local electric-field distortion, with a maximum distortion rate of 109.09%. These results demonstrate that installation orientation governs icicle morphology and flashover development, thereby distinguishing catenary cantilever insulators from standard vertical insulator strings and supporting external-insulation design for high-altitude icing regions.

Abstract: In this work, liquid-phase-sintered SiC ceramics with Al2O3 and CeO2 additives were prepared by spark plasma sintering (SPS). The upper plunger exposed length (H) was adjusted to 15, 12, and 9 mm by changing the die–plunger position, while the composition and sintering schedule were kept unchanged. The effect of H on resistance evolution, shrinkage behavior, microstructure, and mechanical properties was investigated. When H decreased from 15 to 9 mm, the initial system resistance decreased by about 19%, and the onset shrinkage temperature decreased from 1631 ± 10 °C to 1535 ± 10 °C. The relative density increased from 95.94% to 98.20%. The edge region of S-H15 was too porous for valid indentation, whereas the edge region of S-H9 became dense and showed a Vickers hardness of 22.91 ± 0.09 GPa and fracture toughness of 3.17 ± 0.29 MPa·m1/2. However, fine pores remained in the center of S-H9, reducing its central hardness to 19.75 ± 0.16 GPa. These results show that exposed plunger length is an important tooling parameter for regulating edge densification and microstructural uniformity in SPS-processed SiC ceramics.

Changes in the local dielectric environment near the termination region of streamer propagation can modify propagation characteristics and induce transitions in propagation mode. In this work, polytetrafluoroethylene (PTFE) dielectrics with different lengths were introduced into the termination region of streamer propagation in a three-electrode configuration. Experiments combined with electrostatic field simulations were performed to analyze the steady propagation field strength, propagation velocities of different components, photoionization intensity, and propagation-mode transition. The results show that the terminal dielectric increases the steady propagation field strength, and this effect becomes more pronounced with increasing dielectric length. In contrast, dielectric length has only a limited influence on the propagation velocities and photoionization intensities of different components. When the dielectric is short or the background electric field is low, the streamer mainly propagates as a single air component. With increasing dielectric length and background electric field, two-component propagation with coexisting surface and air components becomes more likely. Simulation results indicate that the terminal dielectric reconstructs the local electric field distribution near the dielectric by enhancing the vertical electric field component and extending its effective range, which may provide favorable conditions for the transition from single-air-component propagation to two-component propagation. These results improve the understanding of streamer component evolution under local dielectric perturbation and provide a reference for the local geometric optimization of insulation structures.

To eliminate the threat posed by cracks in porcelain insulators to the stable operation of equipment, an enhanced imaging method for crack defect detection based on active infrared thermography is proposed. However, the low emissivity of porcelain insulator surfaces limits the effectiveness of active infrared thermography. To address this issue, this paper enhances defect imaging from two aspects: post-processing algorithms and surface coating treatments. For porcelain insulators without room temperature vulcanized (RTV) silicone rubber coating, analysis of surface cracks reveals that the surface glaze creates "hot spots" at the edges of the insulator skirts, resulting in suboptimal imaging of crack defects. By applying first-order differential processing, the characteristic imaging effectively eliminates the influence of non-defect anomalous temperature regions, thereby improving the imaging quality of crack defects. For porcelain insulators coated with RTV silicone rubber coating, the RTV silicone rubber coating increases surface emissivity and suppresses additive noise, significantly enhancing crack defect imaging. The results demonstrate that combining first-order differentiation and characteristic imaging in post-processing can markedly improve the imaging of crack defects in porcelain insulators using active infrared thermography. Furthermore, applying an RTV silicone rubber coating to the porcelain insulator surface further enhances defect visualization.