Surface Insulation Performance Research Articles

Surface flashover in 50 years: II. Material modification, structure optimisation, and characteristics enhancement

AbstractSurface flashover is a gas–solid interface insulation failure that significantly jeopardises the secure operation of advanced electronic, electrical, and spacecraft applications. Despite the widespread application of numerous material modification and structure optimisation technologies aimed at enhancing surface flashover performance, the influence mechanisms of the present technologies have yet to be systematically discussed and summarised. This review aims to introduce various material modification technologies while demonstrating their influence mechanisms on flashover performances by establishing relationships among ‘microscopic structure‐mesoscopic charge transport‐macroscopic insulation failure’. Moreover, it elucidates the effects of chemical structure on surface trap parameters and surface charge transport concerning flashover performance. The review categorises and presents structure optimisation technologies that govern electric field distribution. All identified technologies highlight that achieving a uniform tangential electric field and reducing the normal electric field can effectively enhance flashover performance. Finally, this review proposes recommendations encompassing mathematical, chemical, evaluation, and manufacturing technologies. This systematic summary of current technologies, their influence mechanisms, and associated advantages and disadvantages in improving surface insulation performance is anticipated to be a pivotal component in flashover and future dielectric theory.

Revelation of the high surface insulation performance of the electron-delocalized group through molecule self-assembly on alumina insulator

Surface modification of vacuum insulators is a promising way to improve their surface insulation performance but commonly used surface modification technologies in surface insulation area such as fluorination and plasmas treatment are unable to control the surface chemical components at the molecular level, which makes it hard to explore molecular groups with high surface insulation performance. In this paper, molecule self-assembly is introduced to control the surface molecular structure precisely and vacuum surface insulation performance of a special molecular structure electron-delocalized group is studied. Hexagonal phenyl group containing delocalized electrons and cyclohexyl group with the same hexagonal molecular structure but without delocalized electrons are grafted onto the surface of alumina insulators through molecule self-assembly. Secondary electron emission yield test shows that phenyl group endows alumina insulators lower SEY than cyclohexyl group. Correspondingly, phenyl group grafted alumina insulators show higher surface insulation performance than cyclohexyl group. The results indicate that group containing delocalized electrons can endow insulators higher surface insulation characteristics.

Silane modified Cr2O3/polyimide nanocomposite films with excellent surface insulation performance for space applications

The exploration of deep space significantly increases the probability of spacecraft failures due to surface electrostatic discharge, which imposes higher vacuum insulation protection requirements on polyimide (PI), the external insulation material of spacecrafts. To address this challenge, this study proposes using silane coupling agent KH550 for organic grafting treatment of Cr2O3nanoparticles, which are then used to dope and modify PI to enhance the vacuum surface insulation of PI films. The KH550 grafting improves the interface strength between the fillers and the matrix, allowing the fillers to be uniformly dispersed in the matrix. Compared to pure PI films, the prepared PI-Cr2O3@KH550 composite films exhibit significantly enhanced vacuum surface flashover voltage, improved surface/volume resistivity, and dielectric properties. The results demonstrate that PI composite films with 0.8% by mass of Cr2O3@KH550 show the most notable performance improvement, with the DC flashover voltage and impulse flashover voltage in vacuum increasing by 20.7% and 27.8%, respectively. The doping of chromium oxide nanoparticles introduces more deep traps into the PI films and reduce the surface resistivity. The higher deep trap density inhibits charge migration, thereby alleviating secondary electron emission and surface electric field distortion. Simultaneously, the lower surface resistivity facilitates dissipating surface charges and improves the surface insulation. These findings are of significant reference value for promoting the enhancement of aerospace insulation performance.

Enhancing Surface Insulation Performance of Ceramic for Spark Plug by APPJ Treatment

Enhancing Surface Insulation Performance of Ceramic for Spark Plug by APPJ Treatment

Regulation mechanism of porous structures on flashover performances: combined effect of hindering collision ionization and corona stabilization

The microscopic morphology is recognized as one of the key factors affecting the surface flashover strength, but the effect of nanoscale morphology is rarely investigated. In this paper, a novel strategy, namely, porous structural material was presented to improve the flashover strength, and porous nylon membranes with pore diameter from 100 nm to 5000 nm were used to explore the influence of nano-scale porous morphology on the flashover strength. The regulation mechanism of porous structures on flashover performances were explored through the analysis of potential scanning, partial discharge measurement, corona and flashover optical photos. The results indicated that the flashover strength under both AC and DC voltages could be significantly improved by the porous morphology, with a maximum increment of higher than 100%. It is believed that the improved flashover strength is attributed to the combined effect of hindering collision ionization and corona stabilization of porous structures. The present research provides a new insight for improving the surface insulation performance.

Tailoring Organic/Inorganic Interface Trap States of Metal Oxide/Polyimide toward Improved Vacuum Surface Insulation.

High-voltage and high-power devices are indispensable in spacecraft for outer space explorations, whose operations require aerospace materials with adequate vacuum surface insulation performance. Despite persistent attempts to fabricate such materials, current efforts are restricted to trial-and-error methods and a universal design guideline is missing. The present work proposes to improve the vacuum surface insulation by tailoring the surface trap state density and energy level of the metal oxides with varied bandgaps, using coating on a polyimide (PI) substrate, aiming for a more systematical workflow for the insulation material design. First-principle calculations and trap diagnostics are employed to evaluate the material properties and reveal the interplay between trap states and the flashover threshold, supported by dedicated analyses of the flashover voltage, secondary electron emission (SEE) from insulators, and surface charging behaviors. Experimental results suggest that the coated PI (i.e., CuO@PI, SrO@PI, MgO@PI, and Al2O3@PI) can effectively increase the trap density and alter the trap energy levels. Elevated trap density is demonstrated to always yield lower SEE. In addition, increasing shallow trap density accelerates surface charge dissipation, which is favorable for improving surface insulation. CuO@PI exhibits the most remarkable increase in shallow trap density, and accordingly, the highest flashover voltage is 42.5% higher than that of pristine PI. This study reveals the critical role played by surface trap states in flashover mitigation and offers a novel strategy to optimize the surface insulation of materials.

Methyl and carbon–carbon double bond in anhydride molecules modulated surface charge trap depth of epoxy materials

Epoxy insulators in gas insulated switchgear and gas insulated transmission lines tend to accumulate surface charges, leading to insulation flashover. Improving the surface trap characteristics of epoxy materials, which can accelerate the surface charge dissipation of epoxy insulators, is a promising method to improve the surface insulation performance. The surface trap characteristics of epoxy materials are strongly influenced by the chemical groups in the acid anhydride molecules. In this work, by quantum chemical calculations and isothermal surface potential decay tests, taking six organic anhydrides that differ only in the methyl and carbon–carbon double bonds, we find the modulation laws of methyl and carbon–carbon double bonds on the charge trap depth within and between molecular chains. The regulation mechanism is revealed from the microscopic perspectives of electron energy structure and electron cloud offset. The changes of surface charge trap depth of epoxy materials are primarily attributed to the changes in the spatial distribution of the electron cloud density between and on the valence bonds caused by the interaction between the electron-donating methyl group and the electron-absorbing carbon–carbon double bond.

An Advanced Ceramic Material for Internal Insulation Used in Gas-Insulated HVDC Apparatus

The traditional polymer insulating material has encountered a bottleneck in improving the surface insulation performance in gas-insulated high voltage direct current (HVDC) apparatus. In this article, an advanced ceramic insulating material is proposed, which has great potential to be utilized in HVDC apparatus. The short-time and long-time surface insulation properties of the ceramic material are researched and compared with those of epoxy composite widely used at present. The conductivity and temperature dependence of the ceramic material is significantly lower than the epoxy composite. The surface charge accumulation and trap evolution characteristics of the ceramic can improve the surface electric field distribution. The long-time test further verifies its reliability, where the failure probability of the ceramic is 74% lower than that of the traditional material. This work may provide a basis for the application of the ceramic material in gas-insulated HVDC apparatus and may be helpful in understanding the surface insulation failure mechanisms in electro-thermal conditions.

High Performance Epoxy Composites Containing Nanofiller Modified by Plasma Bubbles.

The thermal conductivity of polymer materials is a fundamental parameter in the field of high-voltage electrical insulation. When the operating frequency and power for electrical equipment or electronic devices increase significantly, the internal heat will increase dramatically, and the accumulation of heat will further lead to insulation failure and serious damage of the whole system. The addition of filler with high thermal conductivity into polymer is a common solution. However, the interfacial thermal resistance between filler and bulk materials is the major obstacle to improve thermal conductivity. Herein, in order to reduce the interfacial thermal resistance, nanofillers are modified by plasma technology. The surface modification of nano-Al2O3 is carried out using plasma bubbles with three atmospheres (Ar, Ar+O2, air) as well as coupling agent. The situation of surface grafting before and after the modification is characterized using FTIR, XPS, and SEM. The effect of the mechanism of modification on the thermal conductivity and reaction pathway is investigated. The results showed that the thermal conductivity after plasma modification is increased significantly. Especially, the thermal conductivity is increased by 35% for the sample modified by Ar+O2 atmosphere. This results because more hydroxyl is introduced on the filler surface by the plasma bubbles, which enhance the interface compatibility between filler and epoxy. In addition, surface insulation performance for the modified samples also is enhanced by 14%. This is associated with the change of surface resistance and trap distribution. These results provide potential support for the development of fabrication for high performance epoxy composites.

Plasma Fluorinated Nano-SiO2 Enhances the Surface Insulation Performance of Glass Fiber Reinforced Polymer.

With the extensive application of glass fiber reinforced polymer (GFRP) in the field of high voltage insulation, its operating environment is becoming more and more complex, and the surface insulation failure has gradually become a pivotal problem affecting the safety of equipment. In this paper, nano-SiO2 was fluorinated by Dielectric barrier discharges (DBD) plasma and doped with GFRP to enhance the insulation performance. Through Fourier Transform Ioncyclotron Resonance (FTIR) and X-ray Photoelectron Spectroscopy (XPS) characterization of nano fillers before and after modification, it was found that plasma fluorination can graft a large number of fluorinated groups on the surface of SiO2. The introduction of fluorinated SiO2 (FSiO2) can significantly enhance the interfacial bonding strength of the fiber, matrix and filler in GFRP. The DC surface flashover voltage of modified GFRP was further tested. The results show that both SiO2 and FSiO2 can improve the flashover voltage of GFRP. When the concentration of FSiO2 is 3%, the flashover voltage increases most significantly to 14.71 kV, which is 38.77% higher than that of unmodified GFRP. The charge dissipation test results show that the addition of FSiO2 can inhibit the surface charge migration. By the calculation of Density functional theory (DFT) and charge trap, it is found that grafting fluorine-containing groups on SiO2 can increase its band gap and enhance its electron binding ability. Furthermore, a large number of deep trap levels are introduced into the nanointerface inside GFRP to enhance the inhibition of secondary electron collapse, thus increasing the flashover voltage.

Enhancing surface insulation of epoxy composite by constructing nanofiller‐skeleton charge channel

AbstractWith the increase of voltage level, the surface flashover of epoxy composite insulation material becomes an important issue, which restricts the development of UHV (Ultra High Voltage) transmission technology. In this article, a method of using SiC skeleton to construct charge channel was proposed, which can enhance the surface insulation performance of epoxy composite. In addition, we investigated the influence of SiC skeleton structure on charge dissipation and flashover voltage. At the same time, we explored the synergistic influence mechanism of SiC and Al2O3 nanofillers and SiC skeleton on the surface flashover characteristics of epoxy resin. The results showed that the SiC skeleton effectively enhances the surface insulation performance of epoxy resin. When Al2O3 nanofiller and SiC skeleton act simultaneously, the flashover voltage enhancement reached 13.65%. Combining with finite element simulation, the flashover voltage enhancement mechanism of epoxy resin after modified by nanofiller‐skeleton method was analyzed. It showed that the internal charge channel formed by the SiC skeleton plays an important role in suppressing the local field distortion. The results of the study provided a new idea for modifying epoxy composite to improve the flashover voltage.

Surface modification of silicone rubber by CF4 radio frequency capacitively coupled plasma for improvement of flashover

The flashover performance of insulating materials plays an important role in the development of high-voltage insulation systems. In this paper, silicone rubber (SIR) is modified by CF4 radio frequency capacitively coupled plasma (CCP) for the improvement of surface insulation performance. The discharge mode and active particles of CCP are diagnosed by the digital single-lens reflex and the spectrometer. Scanning electron microscopy and x-ray photoelectron spectroscopy are used for the surface physicochemical properties of samples, while the surface charge dissipation, charge accumulation measurement, and flashover test are applied for the surface electrical characteristics. Experimental results show that the fluorocarbon groups can be grafted and the surface roughness increases after plasma treatment. Besides, the surface charge dissipation is decelerated and the positive charge accumulation is inhibited obviously for the treated samples. Furthermore, the surface flashover voltage can be increased by 26.67% after 10 min of treatment. It is considered that strong electron affinity of C–F and increased surface roughness can contribute to deepening surface traps, which not only inhibits the development of secondary electron emission avalanche but also alleviates the surface charge accumulation and finally improves the surface flashover voltage of SIR.

Improved DC surface insulation performance of epoxy resin by gradient plasma fluorination

In order to improve the flashover voltage of the disc insulator in gas insulated transmission line under DC voltage, this paper considers reducing the electric field intensity on the surface by adjusting the surface conductivity distribution. Based on the joint simulation of improved particle swarm optimization algorithm and finite element, the surface conductivity gradient partition optimization model is constructed to obtain the optimal surface conductivity distribution sequence. The relationship between conductivity and treatment time is obtained by plasma fluorination, and the gradient distribution of the surface conductivity of the sample is realized by adding different ring polytetrafluoroethylene baffles. Microscopic tests are used to characterize the differences between different regions of the sample surface. The two-dimensional (2D) electric potential distribution, 2D charge density distribution, electric field intensity distribution and flashover voltage on the sample surface under pre-charging conditions are studied and compared with the simulation results. The results show that the maximum field strength of the sample with gradient treatment can be reduced by 71% and the flashover voltage can be increased by 45%. The research in this paper provides a good reference and theoretical support for the surface modification of disc insulators, such as the surface parameters with gradient distribution.

Surface charge behavior and flashover performance on epoxy-based spacers by graded conductivity coatings subjected to DC voltages

Surface charge accumulation subjected to direct current (DC) voltages poses hazardous effects on the surface insulation performance of the involved insulators. It is of great significance to investigate surface charge regulation methods applicable to industrialized spacers. In this study, we propose a graded conductivity coating method, with excellent adhesion strength consisting of TiO2/epoxy composites with different TiO2 contents, realizing a gradually decreased conductivity distribution on the spacer surface from high voltage (HV) to grounded (GND) electrodes. Surface charging behavior and flashover performance under DC voltages on this kind of spacer is obtained. A simulation model for surface charge computation is constructed for the sake of better understanding the surface charging mechanism on spacers with graded conductivity coating. The results indicate that the randomly distributed surface charges are obviously suppressed by this coating manner and the ring-shaped homo-charges adjacent to HV and GND electrode dominate surface charge patterns. The main cause lies in the fact that surface charge dominant mechanism changes from the bulk conductivity model to the surface conductivity model, where surface leakage current becomes the primary sources of surface charges, with the increase in coating conductivity. DC flashover performance is enhanced as well due to the improvement of surface charge accumulation and electric field distribution. This study supplies an important reference for designing DC gas insulated transmission line spacers of high reliability as well as inspires novel ideas in surface charge regulation.

Dielectric Surface Flashover under Long-Term Repetitive Microsecond Pulses in Compressed Gas Environment.

As a key component of a high-power microwave (HPM) system, a multi-gap gas switch (MGS) has recently developed insulation failure due to surface flashover. Although design criteria for surface insulation have been put forward, it is still not clear how the insulation in this case deteriorated under long-term repetitive microsecond pulses (RMPs). In this paper, flashover experiments under RMPs were carried out on various dielectric surfaces between parallel-plane electrodes in SF6 and air atmospheres, respectively. Based on tests of the surface insulation lifetime (SIL), an empirical formula for SIL prediction is proposed with variations of insulator work coefficient λ, which is a more suitable parameter to characterize SIL under RMPs. Due of the accumulation effect, the relationship between E/p and ptdelay varies with the pulse repetitive frequency (PRF) and SIL recovery capability decreases with an increase in PRF and surface deterioration is exacerbated during successive flashovers. It is concluded that the flashover path plays a crucial role in surface insulation performance under RMPs due to the photoemission induced by ultraviolet (UV) radiation, signifying the necessity of reducing surface paths in future designs as well as the improvement of surface insulation.

Effect of plasma step gradient modification on surface electrical properties of epoxy resin

In this paper, plasma fluorination is combined with plasma silicon deposition to achieve step gradient modification on an epoxy resin surface. The physicochemical characteristics of samples are investigated and the electrical performances measured. The obtained results show that compared with untreated and single treated samples, the samples treated by step gradient modification significantly improve the flashover performance. According to experiment and simulation, the mechanism explanations are summarized as follows. First, it is found that the step gradient conductivity can effectively optimize the electric field distribution of a needle-needle electrode. Then, step gradient modification suppresses the accumulation of surface charge at the triple junction and makes the charge distribution more uniform. Furthermore, it can accelerate the surface dissipation on a high electrical field region and control the dissipation rate on a low electrical field region. All these results can restrain surface discharge and increase the flashover voltage. The step gradient modification method proposed in this paper provides a new idea for improving the surface insulation performance.

Promotion of Epoxy Resin Surface Electrical Insulation Performance and Its Stability by Atmospheric Fluorocarbon Dielectric Barrier Discharge

This study investigates the promotion of epoxy resin (ER) surface electrical insulation and its stability by atmospheric fluorocarbon dielectric barrier discharge (DBD) plasma. Surface physicochemical and electrical properties of untreated and plasma treated samples are evaluated by scanning electron microscopy, X-ray photoelectron spectroscopy, surface and volume resistivity, surface charge accumulation, dissipation measurements, and flashover tests. Analysis shows that dusty-like fluorocarbon granules formed in plasma phase are deposited on ER samples and the concentration of fluorine groups is increased after plasma treatment. The surface withstand strength of epoxy resin in vacuum can be promoted up to 50%. Surface resistivity decreases by two orders of magnitude with treatment time while volume resistivity reduces slightly. The surface charge accumulation on treated ER samples is prohibited and the maximum surface charge density decreases from 77.84 to 1.42 pC/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The surface charge dissipation is accelerated with the central energy levels reducing to below 0.9eV and the density of electron traps increases from ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sup> to ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Furthermore, the stability of surface flashover strength is tested. It is found that promotion of surface flashover strength of treated ER sample is stable after being kept in ambient air for a month. The physicochemical changes caused by plasma treatment prevents electrons from transferring to the surface which leads to a better surface insulation performance.

Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application

This study presents the surface discharge characteristics of insulating gases, including sulfur hexafluoride (SF6), dry air, and N2, under a non-uniform field. Surface discharge experiments were conducted, with the gas pressure ranging from 0.1 to 0.6 MPa, on samples of epoxy dielectrics under an AC voltage. The experimental results showed that the surface insulation performance significantly improved in insulating gases possessing electronegative gases, such as SF6 and dry air. Surface flashover voltages of SF6 were saturated with an increasing pressure, compared to dry air and N2. The surface discharge mechanism is proposed to explain the improvement and saturation of dielectric characteristics of the electronegative gas in complex dielectric insulations, as well as its influence on the surface flashover voltage. As an application, an insulation design method is discussed with regards to replacing SF6 gas in high-voltage power equipment based on the knowledge of the physics behind gas discharge.

Fluorinated SnO2/ER as an antistatic coating for enhancing the surface insulating strength of epoxy insulators

AbstractThe surface coating of epoxy resin has been widely used in the composite materials with micron scale fillers, but the related research on the composite coating filled with nanoparticles is still rare. SnO2 is a common antistatic additive in composite materials, and it is also a semiconductor material, capable of uniform voltage. In this paper, nano‐SnO2 powder was considered as the filler of coating materials. By analyzing the microscopic test results, the flashover voltage, surface charge dissipation, and surface conductivity of the coating sample, the effects of coating treatment and nano‐SnO2 fluorination on the surface insulation properties of epoxy insulators were studied. It is found that the flashover voltage and surface charge dissipation rate of the coating sample can be improved by both coating treatment and fluorination treatment, thus improving the surface insulation performance of epoxy insulators. The modification process and test results have guiding significance for improving the surface insulation performance of epoxy insulators.

The effect of gradient conductivity of doped BiFeO3 as filler on the surface insulation performance of epoxy composite

In this paper, we studied the effect of filler conductivity on the surface insulation of composites. We proposed using different valence element doping as a strategy to prepare filler with gradient conductivity. The pristine BiFeO3 (BFO), Bi0.95Ca0.05FeO3, and Bi0.95Sm0.05FeO3 nanoparticles were prepared to fill into epoxy resin. The Rietveld refinement of the x-ray diffraction patterns was performed to investigate the transition of the lattice structure. According to the Kubelka–Munk function, the energy bandgap was calculated to verify the regulation of the BFO conductivity. Subsequently, we measured the flashover voltage of the composites with three types of BFO in ambient and vacuum atmosphere, respectively. We found the vacuum flashover is more sensitive to filler conductivity, and BFO with higher conductivity as filler is more conducive to surface insulation, whether in the air or vacuum. Finally, the optimal BFO bandgap of 2.1819 eV is determined for enhancing surface insulation of composites. Our research supplies a novel method for constructing high insulation composites for more wide applications and provides a scientific basis on flashover mechanism from the perspective of filler characteristics.