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Charge accumulation on the surface of insulation dielectric used in high voltage DC equipment is one of the important causes of surface flashover. Surface charge accumulation is closely related to surface trap characteristics. In this paper, the surface trap energy level distribution of polyimide (PI) was calculated and analyzed by the corona charging method and surface potential decay method. Besides, electron and hole trap characteristics were studied by changing the polarity and amplitude of the charging voltage. Furthermore, the influence of surface charge polarity and quantity on the flashover voltage was researched. Experimental results indicate that the energy level of traps corresponding to the maximum trapped charge density ranges around 0.94 eV. In addition, the energy level of the hole trap is slightly higher than that of the electron trap. The maximum charge density captured by the electron trap and hole trap is about 1.17 × 1021 and 1.03 × 1021 eV−1 m−3, respectively. The polarity of the surface charge can influence the conductivity characteristics of PI, the surface heterocharge reduces the volume conductivity by about 80% while the surface homocharge improves the volume conductivity of PI by about 6.4 times. Besides, both negative and positive surface charges can improve surface conductivity. The polarity of surface charge influences the flashover characteristics of PI, the flashover voltage increases by about 37.8% with the increase of homocharge density and decreases by about 23.8% with the increase of heterocharge density, which is related to the change of electric field near the cathode caused by surface charge accumulation. [ABSTRACT FROM AUTHOR]

The proliferation of urbanization and the integration of new energy sources have stimulated the development of gas-insulated transmission lines and switchgear (GIL/GIS). In particular, the compact DC GIS in offshore converter platforms will significantly reduce footprints for DC switchyards, exhibit exceptional climatic resistance, and facilitate the cost-effective connection of remote offshore wind farms and submarine links. Nevertheless, insulators used in GIS/GIL always suffer from surface charge accumulation under DC stress, which could distort and enhance the local electric field and thus trigger a flashover at the gas–solid interface if it exceeds certain magnitude levels. This susceptibility becomes a major concern affecting the reliability of DC gas-insulated systems. Beyond these engineering-related challenges lie fundamental physics problems involving mechanisms of charge accumulation and charge-induced flashover which still require exploration. To this end, this paper presents an overview of recent advancements on this topic whilst highlighting relevant issues to be addressed. Specifically, the surface charge accumulation phenomena under DC fields are reviewed, and the charging mechanisms are summarized from macroscopic to microscopic perspectives. Further, the correlation between surface charge and surface flashover is discussed. Moreover, recent developments in tailoring methods for surface charging are also presented. Finally, perspectives are given on current research progress and future needs. [ABSTRACT FROM AUTHOR]

Surface flashover across an insulator in a vacuum is a destructive plasma discharge which undermines the behaviors of a range of applications in electrical engineering, particle physics and space engineering, etc. This phenomenon is widely modeled by the particle-in-cell (PIC) simulation, here the continuum and kinetic simulation method is first proposed and implemented as an alternative solution for flashover modeling, aiming for the prevention of unfavorable particle noises in PIC models. A one dimension in space, two dimensions in velocity kinetic simulation model is constructed. Modeling setup, physical assumptions, and simulation algorithm are presented in detail, and a comparison with the well-known secondary electron (SE) emission avalanche analytical expression and existing PIC simulation are made. The obtained kinetic simulation results are consistent with the analytical prediction, and feature noise-free data of surface charge density as well as fluxes of primary and SEs. Discrepancies between the two simulation models and analytical predictions are explained. The code is convenient for updating and to include additional physical processes. The possible implementations of outgassing and plasma species for the final breakdown stage are discussed. The proposed continuum and kinetic approach are expected to inspire future modeling studies for the flashover mechanism and mitigation. [ABSTRACT FROM AUTHOR]

The micron metal particles are inevitably produced in long-term operation condition of high voltage direct current SF6 insulated apparatus, which could frequently cause the surface discharge. Degradation of long-term surface discharge will be a severe threat to the insulation strength. Aiming at this problem, the surface charge accumulation and flashover voltage of epoxy itself before and after discharge degradation by particles were compared in this paper. The composition after degradation was measured by EDS and FTIR. The microstructure was observed by SEM. The roughness was measured as well. The trap distribution was measured by SPD. The mechanism of degradation and its effect on insulation characteristics were analyzed. The results show that the surface charge accumulation increase three times after degradation. Additionally, flashover voltage decreases 19% after degradation. The smoothly surface after degradation decrease the trap energy level for 14%, which might be the change reason of surface charge accumulation and flashover voltage. This paper can offer as a reference for the effect of micron metal particles on insulation degradation in SF6 gas insulated equipment. [ABSTRACT FROM AUTHOR]

In this article, the influence of polishing orientation on surface charge accumulation and flashover characteristics of the basin spacer in a downsized dc-gas insulated transmission line (GIL) are explored by experiments. Results show that the surface charge distribution of the polished spacers is different from the untreated one. Heteropolar charges can be observed in the polished area and outward expansion area under prestressing condition. The flashover voltages of all polished spacers are decreased. The radial polishing (RP) has the greatest impact on the flashover voltage, and the flashover voltage of the radially polished spacer is decreased by 11% under positive prestressing condition. The metal dust adsorptions of the spacers are also measured, which shows that the radially polished spacer is easier to absorb metal dust and the metal dust diffuses in the radial direction. SEM and energy spectrum scanning of different spacers show that there are more exposed alumina particles on the surface of the polished area. The microdischarge induced by alumina particles is the source of heteropolar charges. When the polished area of the spacers is repaired with epoxy paint, heteropolar charges disappear on the surface of the repaired spacer. It is confirmed that charge accumulation and flashover along the spacer surface are mainly affected by the change of surface morphology on the polished area. [ABSTRACT FROM AUTHOR]

In this study, a downsized direct current (dc)-gas-insulated transmission line (GIL) model is applied to investigate the surface charging and discharging characteristics of the basin-type spacer in 0.1-MPa SF6 and air. Under a low electric field, the surface charge accumulation is dominated by the bulk conduction of the spacer. As the charging voltage increases, gas ionization occurs in the localized strong electric field, taking over the dominant position. In 0.1-MPa SF6 gas, flashovers typically develop along the convex surface of the spacer under both positive and negative voltages because the homo-charge accumulation induced by brush discharges on the concave surface aggravates the electric field distortion at the convex side. In air, the dc flashovers have the polarity effect. Under the negative voltage, the flashover development is similar to that in SF6 gas. But for positive polarity, the space charge region of positive polarity intensifies the electric field ahead of the corona region, thus promoting the air-gap breakdown at the concave side of the spacer. [ABSTRACT FROM AUTHOR]

Space plasmas, though tenuous, can lead to the development of large magnitude potentials on spacecraft surfaces relative to the ambient plasma in a process known as spacecraft charging. This effect poses a threat to spacecraft when surfaces charge differentially as it increases the risk of electrical arcing. Spacecraft engineers have primarily focused on the mitigation of charging and not its exploitation. The potentials and current collected by the spacecraft present an opportunity to harvest power from the space environment. Although the current density and floating potential due to incident charged species in the orbit-motion-limited model are identical for all spacecraft surfaces with similar geometries, material property differences between surfaces or charge control devices break the symmetry and allow for net current to flow. Differences in the secondary electron yield, in particular, produce strong differences in floating potential even when the spacecraft is eclipsed or far from the sun. This article presents a theory for computing the power available to harvest from spacecraft charging given the parameters of ambient space plasmas and surface material properties. Given a load connecting two differentially charged surfaces, optimal power is harvested by matching the load resistance to the input plasma impedance. While this holds for any model of the ambient plasma, the orbit-motion-limited theory yields closed-form solutions to the charging equations for spherical and planar surfaces. These solutions provide an upper bound on generated power that scales with the density and temperature of the cold electron population. An environment-specific optimal anode-to-cathode area ratio exists that minimizes the input impedance to the harvesting circuit. Deployable anodes are recommended to satisfy this condition and increase the amount of power harvested. Energy harvesting from spacecraft charging can be used in the Jovian magnetosphere to generate on the order of 10 mW/m2 of power. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The surface charge accumulation is very likely to trigger the surface flashover, which limits the large-scale application of DC GIL/GIS. This article comprehensively reviews the effect of six factors, including insulator-electrode shape, surface roughness of the insulator and conductor, metal particles, temperature, humidity, and gas type, on the insulator surface charging property. Furthermore, three models i.e. 'analogous ineffective region' expansion model, charge cluster triggered surface flashover model, and synergistic model of adsorbed gas, revealing the mechanism of charge triggered surface flashover phenomenon are reviewed and discussed. Future work from the perspective of theoretical analysis and engineering application are suggested in this field. [ABSTRACT FROM AUTHOR]

The demand for DC gas-insulated switchgear/gas-insulated transmission line (GIS/GIL) becomes more and more urgent in recent years owing to the rapid development of high voltage direct current (HVDC) transmission systems. Insulators in DC GIS/GIL could easily accumulate a large number of charges on its surface during long-term operation, which might lead to overstressing of polymeric insulation or surface flashover. Therefore, it is of great significance to propose an effective method for dissipating extra surface charges. However, achieving charge dissipation without opening the tank is still a great challenge to SFC6-insulated equipment for a long time. In this paper, a novel method, which is named X-ray based non-contact charge dissipation method (XNCDM) for insulators in SF6-insulated equipment, is proposed. The effects of X-ray irradiation on the charges dissipation and the influences of irradiation dose were experimentally investigated in SF6. The mechanisms underlying are discussed in detail. Results show that surface charges on the insulator can be almost completely dissipated after short-time X-ray irradiation with enough dose, owing to the significant enhancement of electric conduction along the insulator surface and charge neutralization by gas ions near the gas-solid interface. This paper provides a novel technique for realizing rapid surface charge dissipation on the insulator, which is of great significance for a variety of engineering applications of DC GIS/GIL. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on epoxy insulator installed in gas insulated switchgear (GIS) or gas insulated transmission line (GIL) has been recognized as an important factor for electric field distortion that results in reduction of surface flashover voltage. This paper reports on surface charge accumulation behavior on a actual epoxy insulator for 100 kV GIL prototype with multi-arc surface profile. A coaxial electrode system is used to investigate the charging behavior under DC voltages with metal particles positioned at various positions of the surface so as to estimate its influence on the charge accumulation behavior using a Kelvin type probe. The results show that positive charges are accumulated mainly on the nonplanar region around the high voltage electrode in the shape of speckles, and the average charge distribution along the radial direction presented a double-peak feature. Moreover, the charge accumulation is enhanced by metal particles contacting to the grounded electrode. Using a numerical simulation method, it is suggested that the charge accumulation is mainly determined by gaseous ionization which is enhanced by the metal particle induced electric field distortion. [ABSTRACT FROM AUTHOR]

[Display omitted] • An ultraviolet irradiation in vacuum technique enhancing flashover voltage of SIR. • Illustrate the influence of UVIV on the chain structure of SIR. • Establish a relation between crosslinking of SIR and surface trap parameters. • Explain the effects of surface traps on surface charging and flashover. • Clarify the UVIV improved flashover mechanisms from microscopic to macroscopic. In surface science, surface flashover, which poses a severe danger to the safe operation of modern silicone rubber (SIR)-based electrical equipment, is a continuous discharge event that happens when an intense electric field is produced along the gas-solid interface. To improve the flashover performance, the ultraviolet irradiation in vacuum (UVIV) technique is proposed to modify the SIR surface and the DC flashover threshold increases approximately 14.8 % after 6 h UVIV treatment. Then, the influence mechanisms of UVIV are investigated from microscopic molecule modification to macroscopic flashover performance: The UVIV generates high-energy photons to break C H and Si C bonds in SIR chains, and atoms, group, side chains, and free radicals in chains recombine through the thermally dynamic, causing SIR chains to crosslink. The crosslink chain structure enhances the deep trap energy around the Si O Si bond and introduces large amounts of deep traps. The increment in surface deep trap energy and density captures electrons. It establishes a high electrostatic near the cathode barrier that impedes subsequent electron injection, causing surface charge density decreases far away from the cathode. The reduced surface charges restrict the collision ionization in gas, finally increasing the DC surface flashover voltage through UVIV treatment. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on polymer insulating material has been recognized as an important factor for electric field distortion that results in reduction of surface flashover voltage. This paper reports on the charge accumulation behavior between a pair of finger-shaped electrodes on epoxy/Al2O3 nanocomposites under dc voltage superimposed by repetitive pulses. The charge distribution was measured by means of an electrostatic voltmeter, from which the average charge density could be calculated. Effects of dc voltage, nanofiller content, and pulse voltage on charge accumulation behavior were investigated. Obtained results showed that with the increase in the filler content from 0 wt% to 5 wt%, the average charge density increased initially then appeared to decrease. The superposition of repetitive pulses on dc voltage led to a remarkable growth in charge accumulation as compared with that when only dc voltage was applied. It is suggested that the charges are originated from local gaseous ionization and/or electrode injection, which is determined by the combined voltage. The charge density is dependent upon the surface trap, which is affected by nanofiller content. The superposition of repetitive pulse is considered as a possible reason to encourage local gaseous ionization that results in enhanced accumulation of surface charge. [ABSTRACT FROM AUTHOR]

Superconducting magnet is one of the core components of international thermonuclear experimental reactor (ITER). Due to the harsh operating environment, for the sake of ensuring the stable operation of the superconducting magnet, the superconducting magnet insulation is required to have excellent electrical and irradiation resistance. However, pure epoxy resin is difficult to adapt to the harsh environment in the ITER, so it needs to be modified to improve its insulation performance. In this paper, the epoxy/AlN nanocomposites with 1 wt% filler content of AlN nanoparticles were prepared, and the dispersion of nanoparticles was observed. High-energy electron beam was used to treat nanocomposites with the dose of 300, 500, and 700 kGy. The relative permittivity and conductivity of the nanocomposites after irradiation were tested. In addition, the data of surface potential were measured after surface charging for analyzing the dynamic behavior of surface charge and calculating the trap distribution of the nanocomposites with various irradiation doses. The flashover voltage of the nanocomposites influenced by irradiation in LN2 was also analyzed. The results show that irradiation can cause the change of trap distribution and flashover voltage, and the change trend is not monotonous. [ABSTRACT FROM AUTHOR]

Surface charge accumulation of polymer insulating materials is harmful to the safe operation of electrical equipment, especially under DC electric field excitation. In order to ensure the safety and stable operation of HVDC power system such as Gas-insulated transmission line, it is necessary to understand the influence of the surface charge on the flashover and improve the surface charge dynamic characteristics. In this paper, Epoxy resin (EP) is deposited by SiOx thin films with different plasma treatment conditions. The flashover voltages are measured with and without surface charges. The results show that the flashover occurs at lower voltage due to the accumulated charges on the EP surface. In particular, the surface charge with the same polarity of flashover discharge can reduce the surface flashover voltage significantly. Moreover, the charge decay rate is increased up to 90% by plasma treatment of 3 min, corresponding flashover voltage is increased. In addition, the flashover voltages of samples during the surface charge dissipation are obtained. Both static impact index I and dynamic impact index S are introduced to estimate effect of surface charge on dielectric surface flashover. [ABSTRACT FROM AUTHOR]

Laser-ablated template and fluoroalkyl silane modified composite coatings are used to prepare a micro- and nano-structure on silicone rubber surfaces. By testing the surface potential accumulation and dissipation process, we assess the suppression of surface charge on superhydrophobic silicone rubber. The surface physicochemical properties and resistivity are examined. The electron and hole trap distribution are analysed using isothermal current decay theory. Peak density of traps associated with the superhydrophobic surfaces are found to be promoted to lower energy levels relative to unmodified silicone. Surface micro-structure increases trap density and reduce trap depth; nano coatings are shown to further reduce surface trap depth. The improvements shown to suppress surface charge accumulation is beneficial to a range of electrical industries and helps mitigate surface flashover on insulation. [ABSTRACT FROM AUTHOR]

The surface charges that accumulate on polypropylene (PP) film can induce electrical field distortion and surface flashover, which accelerate the electrical aging of a capacitor. This paper investigated the effect of combined voltage on the surface charge behaviors and flashover voltage and a modification method to inhibit the accumulation of surface charge. The carrier mobility in different conditions was calculated using a surface potential decay process to analyze the transportation of surface charges. Samples that were fluorinated for 15, 30, 45 and 60 min and unfluorinated samples were prepared prior to experiments. The samples were corona charged with a pair of needle-plate electrodes by DC voltage and a combination of DC voltage and pulse voltage. The flashover experiments were performed under DC voltage, pulse voltage and superimposed voltage in atmospheric air. The results indicate that the maximum carrier mobility and maximum flashover voltage can be obtained when the samples are surface modified for 45 min. The surface charge density decreases with an increase in the carrier mobility under DC voltage. Under a combination of DC voltage and pulse voltage, the initial surface charge density is influenced by both the carrier mobility and the pulse voltage. The results suggest that surface modification for a certain time can significantly inhibit the surface charge accumulation, improve the carrier mobility and increase the flashover voltage. [ABSTRACT FROM AUTHOR]

Surface charge accumulated on insulators plays an important role in flashover process in vacuum, which has not yet been fully understood. In this paper, charging behaviors on insulator surface were experimentally examined under negative DC and impulse voltage. For the case of DC voltage, two charging stages were observed, i.e., the area near cathode was at first negatively charged under lower voltage (stage I) and then followed by a positively charged cathode-anode surface (stage II). Surface charging at stage I is considered to be caused by Schottky injection of homo-charges from electrode into sample surface layer. While at stage II secondary electron emission (SEE) process is responsible to the positively charged anode-cathode surface. However, charging at stage I is quite sensitive to pre-stress process, as well as the electrode-insulator interface conditions. The ongoing injection of electrons from cathode can lead to gradually decreasing in surface charge accumulation as the voltage duration increases. For the case of impulse voltage, Schottky injection of homo-charges has little influence on surface charging as the time is too short for charge injection and migration in surface layer. The SEE process will govern surface charging due to its extremely fast establishment process. A good linear relationship is obtained between saturated surface charge density and applied voltage, which is generally consistent with the numerical calculation result based on secondary electron emission avalanche theory. At last, it is confirmed that the ionization of desorbed gas from insulator surface and subsequent neutralization of electrons with positive surface charge is the main cause to eliminate surface charge in flashover. [ABSTRACT FROM AUTHOR]

Epoxy resin is widely used in disc-type spacers of gas insulated switchgear (GIS). Surface flashover related to the existence of surface charge often occurs on the spacer, which is a great threat to the application of GIS and the safety of entire grid. Moreover, the influences on surface charge under pulse voltage produced by switch overvoltage, accompanying with the AC voltage, are still unclear. In order to research the comprehensive effect of AC and pulse voltage on surface charge, in this paper, epoxy resin sample was charged by a corona discharge system and the charge distribution was measured by an electrostatic voltmeter. The results are obtained from the research of different pulse amplitudes, pulse numbers and pulse polarities. The surface potential is a negative value after AC voltage applied for a period of time. When AC and negative pulse voltage are applied, smaller pulse voltage would push the original negative charges away. As a result, initial negative surface potential decreases. But bigger pulse voltage could increase the charge accumulation. When the pulse polarity becomes positive, surface potential at the sample center will be positive value at the beginning and then it gradually changes from positive to negative potential. In addition, the influence on surface charge decay rate will be enhanced with the increase of pulse number under the same pulse voltage. Trap energy distributions were used to analyze charge decay process in the last section. [ABSTRACT FROM AUTHOR]

Silicone rubber (SiR), as a basic insulating material, is widely used in insulators for HVDC transmission lines. The prominent characteristics of the SiR coating are the hydrophobicity and the hydrophobicity recovery, which can suppress the development of leakage currents and dry-band discharges, and prevent the flashover incidents. Fluorination is a method to change the chemical component of the surface layer of polymers, which could induce the corresponding change in hydrophobicity and some electrical properties, thus influencing the dc surface flashover characteristics. Room temperature vulcanized (RTV) SiR samples were fluorinated in a stainless reaction vessel at about 298K (25°C) using a F2/N2 mixture with 12% F2 by volume and 0.05 MPa (500 mbar) for respectively 5, 10, 15 and 20 minutes. The hydrophobicity of samples with different fluorination time was measured by the static contact angle tests after corona aging. The variation of hydrophobicity with the lapse of time was recorded. Dc flashover tests in different conditions were carried out between two plane electrodes on top of the material sample. The flashover voltage was found to be associated with the surface hydrophobicity status of samples. Obtained results showed the effects of the fluorination on the surface charge, hydrophobicity performance and dc flashover voltage. It is suggested that the appropriate fluorination can improve surface hydrophobicity and suppress surface charge accumulation, thus raising the dc flashover voltage. [ABSTRACT FROM AUTHOR]

Epoxy is widely used in electronic and electrical devices because of its excellent insulation properties. The addition of nanoparticles to epoxy could change the material structure and then charge behaviors are altered as a consequence. The accumulation of surface charge may cause electric field distortion, which would lead to surface flashover. In this paper, the samples were made by dispersing nano-scale boron nitride (BN) particles in Epoxy with the weight ratios of 5, 10, 20, 30, 40 wt%. The samples were charged by a dc corona triode system using needle-plate electrodes. The distribution of charges was measured with an electrostatic voltmeter and the effect of nanoparticles on charge accumulation and decay behavior was studied. A flashover test was carried out under a dc stress between two finger-type electrodes. The effect of nanoparticles on flashover characteristics and the relation between surface charging and surface flashover were also investigated. The results showed that the traps between nanoparticles and the polymer, which were introduced by the added nanoparticles, could cause surface charge accumulation. In addition, the flashover occurred at a lower voltage because of the accumulated charge on the sample surface. [ABSTRACT FROM AUTHOR]

Effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations. The considered insulator consisted of a glass fiber reinforced epoxy core covered with a layer of silicone rubber and terminated by metallic electrodes with rounded smooth edges. In the experiments, the insulator surface was charged by external corona while keeping the electrodes grounded and different charging levels were realized by varying its intensity. A series of disruptive discharge tests were carried out on the charged insulator under negative dc voltages. It was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the flashover voltage level. A theoretical model has been developed and utilized for analyzing the experimental results. In the model, surface charge density profiles deduced from measured surface potential distributions were used as boundary conditions for calculations of electric fields. The measured and calculated flashover voltages were found to be in agreement indicating that the observed variations in the flashover characteristics could be attributed to the modifications of the electric field produced by the surface charges. [ABSTRACT FROM PUBLISHER]