Surface Flashover Research Articles

Tuning Nanofillers in Sprayed Coating Toward High Flashover Strength

Various surface modification methods have been developed to improve flashover strength in gas–solid composite insulation, among which composite coating is recognized as a flexible and promising approach. However, there is still limited knowledge about the effect of different nanofillers on the surface insulation properties of nanocoating. To this end, spraying coating of high concentration of nanofillers is developed as a universal strategy to improve surface flashover strength and hydrophobicity, in which the nanofillers are tuned to achieve high flashover strength. Five kinds of nanofillers including Al2O3, SiO2, TiO2, SiC, and ZnO are introduced to fabricate the coatings. The effect of nanofillers physical properties on surface trap characteristics and micro-topography is explored to guide the development of coatings and understand the underlying mechanism. Micro-nanoscale hierarchical morphology or porous structure are created in the spraying process, which hinders the development of collision ionization and enhances the flashover voltage of all kinds of coatings. Shallow traps are introduced in the coatings with semi-conductive fillers such as ZnO, SiC, and TiO2, which is beneficial to charge dissipation. Owing to the combined effect of accelerated charge dissipation and hindrance of collision ionization by nanosized pore structures, the SiC coating presents largest enhancement of flashover strength among all coatings, with an increment of 50.3%. The spraying coating of high centration of nanofillers provides a universal strategy for simultaneously improving the flashover strength and hydrophobicity.

Improved surface flashover voltage of epoxy following polythiourea-assisted coating with high gas adsorption ability

Surface flashover as a well-known discharge phenomenon on the gas–solid interface can lead to the failure of power equipment. Although intensive studies have been devoted to improving the flashover voltage in gases, the perspective on exploiting gas characteristics has so far been ignored. We here directly utilize a polythiourea-assisted coating on epoxy to achieve beneficial gas adsorption and enhance the flashover voltage. The results show that the flashover voltage of the polythiourea-assisted epoxy is significantly improved in both N2 and SF6 compared with the blank epoxy. Combining the experimental and computational results, it is proved that the striking improvement in the flashover performance is attributed to the increased gas adsorption quantity resulting from the unique thiourea units in polythiourea. Although both esters in epoxy and thioureas in polythiourea act as active function sites for guest gases, polar thioureas not only lead to pronounced charge displacements but also own the larger density, which further enhances the adsorption energy and promotes the gas adsorption. The resulting increased gas adsorption quantity prevents the collision ionization effectively, thus improving the flashover voltage. Our research innovatively proposes a functional polythiourea-assisted coating that can enhance the flashover voltage by improving the gas adsorption ability.

Unraveling Triggering Factors of Surface Flashover of Epoxy/Glass Composites under AC/DC Superimposed Voltage

Unraveling Triggering Factors of Surface Flashover of Epoxy/Glass Composites under AC/DC Superimposed Voltage

Study on explosive emission characteristics of the silicon carbide modified graphite cathodes with varied morphologies

Explosive emission cathodes are extensively used in high power microwave sources. Growing requirements are highlighted in the performance of the explosive emission cathode with the development of the high power microwave technology. In order to improve the emission properties of the graphite cathode, modifications using SiC particles or whiskers are carried out by the in situ chemical vapor reaction method. The experiments demonstrate that SiC whiskers accelerate the explosive emission turn-on speed of the graphite cathode for large field enhancement factors and improve the emission uniformity due to the surface flashover plasma generation mechanism. SiC particles increase the emission capability of the graphite cathode, possibly corresponding to the dielectric properties of SiC particles. These results suggest that the SiC whiskers modified graphite cathode is suitable for applications under low magnetic field and large emission area conditions, while the SiC particles modified graphite cathode has a brighter application prospect in the long-time stable operation.

Improvement of breakdown and flashover properties against thermal aging of oil paper insulation through addition of TiO 2 nanoparticles

TiO2 nanoparticles can improve the electrical performance of oil paper insulation. In this study, 10 nm TiO2 nanoparticles were prepared and dispersed into transformer oil, and then immerse pressboards in it to obtain nanofluid impregnated pressboards (NP). Its thermal aging was accelerated together with the reference sample at 130°C for 36 days. Analyzing the acid value in the oil, polymerization degree, surface topography, AC breakdown strength, surface flashover characteristics and surface potential decay characteristics. It was observed that TiO2 nanoparticles can reduce the acid value and improve AC breakdown strength of oil. However, the degree of polymerization and surface appearance of NP and oil impregnated pressboards (OP) during thermal aging did not differ significantly. During the aging process, the energy level of pressboard surface traps shifted first towards the shallow and then gradually towards the deep. During all the aging stages, the surface flashover voltage of NP was always higher or close to that of OP. The mechanism of TiO2 nanoparticles improving the breakdown and surface flashover characteristics of oil–paper insulation during thermal aging is discussed from the perspective of water molecules and aging by-products adsorption and introduction of shallow traps by nanoparticles.

Diverse effects of gases on surface flashover in a wide pressure range from atmospheric to near vacuum pressure

Investigations into surface flashover characteristics and underlying mechanisms in the range from atmospheric to near vacuum pressure are significant for developing high-power equipment based on the application of gases used for flashover prevention. In this work, polyetherimide (PEI) films with controllable outgassing properties were prepared. Then, the gas-related factors, namely, gas pressure, the type of gases, and outgassing were clarified by investigating the DC surface flashover of films in a wide pressure range (10−4–105 Pa) in different gas atmospheres. In all conditions, the variation of flashover voltage was found to be divided into three regions with pressure increasing, while its deeper reason is that the number density of molecules of the gas layer is affected by outgassing and ambient gas pressure, which affects the collision of electrons. In region I, the surface flashover voltage is closely related to outgassing and shows the highest value. In region II, the surface flashover is jointly affected by outgassing and ambient gas, along with a rapid decrease of flashover voltage with increasing pressure. In region III, the surface flashover is dominated by ambient gas, and the surface flashover voltage shows a U-shaped curve with increasing pressure. Transition points of the three regions shift significantly with the change of outgassing properties. In region I, the desorption of adsorbed gas has little effect on the surface flashover voltage, while the escape of dissolved gas dominates. Finally, a model of the diverse effects of gases on surface flashover in a wide pressure range was established.

Charging Mechanism of Surface Flashover on Polyimide Under Electron Beam Irradiation

The surface flashover along the interface of solid dielectric and vacuum seriously restricts the improvement of the voltage level of vacuum electrical equipment. Surface charging is an important factor in the process of surface flashover, but its mechanism is still unclear. In this article, in situ surface potential measurement is used to measure the surface potential of polyimide radiated by electron beams with different energies, and the charging curves of polyimide at different temperatures are obtained. The experimental results show that the surface potential of polyimide is close to zero after the low-energy electron beam radiation. When the electron beam energy exceeds the critical value, the surface potential is approximately proportional to the electron beam energy. The critical values of different materials are different, such as 1.18 keV for polyimide and 1.86 keV for Teflon. The surface charging characteristic is related to sample thickness, secondary electron yield (SEY), conductivity, permittivity, and so on.

Surface trap characteristics of polyimide and effect of surface charge accumulation on surface flashover

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.

Effects of Ion Beam Treatment on DC Flashover Characteristics of Epoxy

The effect of ion beam treatment on surface flashover characteristics of epoxy is evaluated and explored. Epoxy samples are prepared and ion beam treatment is carried out for 10-, 15-, 20-, 25-, and 30-min durations using an end-Hall ion source. Flashover test of both pristine and treated samples is conducted at negative dc voltage in air. The results indicate the effectiveness of ion beam treatment to enhance the flashover properties of epoxy. The flashover voltage of the epoxy resin increased to 21.35% after ion beam treatment for a duration of 20 min. Optical microscopic images, scanning electron microscopy (SEM) images, and results from Fourier transform infrared spectroscopy reveal the change in physicochemical characteristics of epoxy resin after treatment. The obtained surface potential from the surface potential decay (SPD) test shows a faster decay rate of surface charges for treated samples than the pristine epoxy. Moreover, from the trap distribution characteristics, it is found that ion beam treatment reduces the surface trap depth, which increases the surface conductivity of the epoxy insulator and helps to promote the charge migration and transport along the insulator surface.

Hollow Cathode Glow Discharge Initiation in a Fore-Vacuum Plasma–Cathode Electron Source

In this article, we present the results of a study on the initiation of a hollow cathode discharge in the electrode system of a fore-vacuum plasma–cathode electron source. It is shown that the development of the hollow cathode effect is facilitated by the back-streaming ion flux of the high-voltage glow discharge (HGD) that arises in the accelerating gap of the electron source. The effect of the type of operating gas and the cathode hollow geometry on the value of the initiating accelerating voltage, which ensures the HGD back-streaming ion flux, have been studied. It is proposed to use an auxiliary pulsed surface flashover discharge to trigger the hollow cathode discharge. The two methods used jointly provide a stable discharge initiation in the entire operating pressure range of the source, including the ultimately low-pressure range where the operation of the hollow cathode discharge in its low-voltage form is still stable.

Analysis of electron emission characteristics of triggered vacuum switch based on cold cathode materials

A trigger vacuum switch that works for a long time is subject to the stable emission of initial electrons. Cold cathode materials such as velvet and carbon fiber have the characteristics of large emission electron area, uniformity, and stability. In this paper, two cold cathode materials, namely T4 rayon velvet and T300 carbon fiber board, are attached. They were used on the surface of the vacuum switch cathode, the trigger electrode was located in the middle of the cathode, and the trigger vacuum switch was triggered by a positive polarity high-voltage pulse along the surface flashover. The breakdown voltage and dispersion were studied in the form of capacitive discharge, and the electron emission characteristics of the two materials were explored from a microscopic perspective, such as through scanning electron microscope and energy dispersive spectrometer element analysis. The results show that the triggering vacuum switch using this cold cathode material has stable triggering, wide operating voltage, and low breakdown voltage dispersion. The electron emission of the velvet has both fiber tip emission and lateral flashover mechanism, and carbon fiber is prone to surface damage; the anode metal surface is partially carbonized due to electron sputtering.

Understanding Effects of Deep Traps on DC Surface Flashover Characteristics of Epoxy/MWCNTs-TiO2 Nanocomposites in a Vacuum

Surface flashover is likely to be triggered when a high electric field is applied due to its voltage being much lower than the breakdown voltage of either gas or solid, which restricts the development of advanced power electrical and electronic devices. To improve dc surface flashover performances in a vacuum and further explore the effects of deep traps on surface flashover, conductive multiwall carbon nanotubes (MWCNTs) and nonconductive nano-titania (TiO2) particles were incorporated into the epoxy (EP) matrix to tailor the surface deep trap level. Surface trap parameters, surface conductivity, surface charge density, and surface flashover voltage were calculated and measured. It indicates that surface flashover voltage improves by 23.07% and 14.45% compared to pristine EP when solely introducing MWCNTs or nano-TiO2 particles into the EP matrix, while it increases by 36.04% for C0.1T2 as simultaneously introducing both particles. When adding small amounts of particles, the surface charge transport is suppressed due to the introduction of bonded layers in the interfacial region, and the surface deep trap level is improved. As the surface deep trap level increases, charge transport and electron emission processes in the solid surface layer are hindered, which reduces the surface positive charge density and the distortion of the electric field and further results in the reduction of surface conductivity and the improvement of dc surface flashover voltage in a vacuum. The explanation for the effects of surface deep traps on surface flashover is essential for further improvements of surface flashover.

Exploring the Basic Physical Mechanisms of Cathode- and Anode-Initiated High-Voltage Surface Flashover

Surface flashover in vacuum imposes a substantial physical limit on modern, large-scale pulsed power. One of the ramifications is a minimum size requirement for new machines, which in itself becomes a hard barrier to the modernization and improvement of existing infrastructure. Pulsed power topologies require the physical mechanisms of both anode- and cathode-initiated flashover to be considered. Originally, the geometrical implications of field emission at the cathode triple junction (CTJ) motivated the usage of configurations that avoid electrons impinging on the insulating material. This will largely suppress the cathode-initiated flashover, which is best described by the secondary electron avalanche mechanism, gas desorption, and final breakdown in the desorbed gas. It depends on the cascade growth of a conducting plasma along the length of the insulator from the cathode. Mitigating the cathode-initiated flashover typically comes at the cost of a significant field enhancement at the anode triple junction (ATJ). In a typical implementation, the anode field may be three times higher than the cathode field for a given voltage, making the corresponding anode-initiated flashover much more common than otherwise. In the case of pulsed, anode-initiated flashover, experimental evidence suggests that charge is directly extracted from the insulator resulting in the insulator taking on a net positive charge advancing the anode potential. Along with accompanying gas desorption from the surface, the potential will then propagate from the anode toward the cathode until the effective length of the gap is sufficiently reduced to support flashover. The underlying physical mechanisms of cathode- and anode-directed flashover are discussed in light of previously gathered experimental data and recent experiments with pulsed, high-gradient, anode-initiated flashover.

Al2O3-Epoxy Flashover Characteristics Under Positive and Negative Nanosecond Impulse in SF6

Surface flashover of dielectric materials often causes extensive damage to pulse power devices. Nowadays, this insulation issue is exacerbated by the demands for ever-higher operating voltages. To solve this issue, the flashover voltage and the streamer development characteristics were investigated under positive and negative nanosecond impulse, respectively, in pressured SF6 gas. First, the surface flashover strength was obtained and the Weibull distribution is introduced to describe the statistical characteristics. The results showed that the negative flashover voltage was much higher than the positive one for Al2O3-epoxy. Within the range of 0.1–0.3 MPa, the negative voltage increased with the gas pressure, while the positive one performed independent with pressure. Then, to investigate the streamer evolving process, a high-speed camera was used to get the discharge picture and the instant velocity of streamers at different conditions. Surface morphology change after the flashover was also analyzed. At last, the possible mechanisms for polarity and pressure effects were given.

A Unified Formula for Five Basic Forms of Discharge in an Electric Field Under Short Pulses

This study proposes a unified formula for five basic forms of discharge—gas, liquid, solid, and vacuum breakdown, and vacuum surface flashover—under short pulses in an electric field. This formula considers the effects of the number of dimensions and pulsewidth on the electric field. It is verified by using the results of experiments reported at the Aldermaston Weapon Research Establishment (AWRE) and the Northwest Institute of Nuclear Technology. The ranges of application of this formula to different discharge forms are also summarized. The proposed formula can be used to transform experimental data at a small scale under a known pulsewidth into those at a large scale under the application pulsewidth and is thus important for designing insulation.

RTV-SiR based composites aged in a multi-stressed environment under AC and bipolar DC voltages

Silicone rubber-based polymers are gradually replacing traditional insulators in outdoor applications due to their numerous advantages, in particular high hydrophobicity that is important for suppressing leakage current and hence surface flashover. However, the main problem with polymeric composites is their aging when exposed to environmental stresses causing loss of their desired properties and expected lifetime. In this work, aging behavior of three types of room temperature vulcanized silicon rubber (RTV-SiR) loaded with nano-silica and micro-ATH is studied. Samples of the materials with different specifications were fabricated and exposed to various environmental stresses in two specially designed weather chambers under both AC and bipolar DC voltages for a time period of 9000 hours. Thereafter, diagnosis was performed for identifying the degree of deterioration using several techniques. These include hydrophobicity classification, measurements of leakage current and mechanical properties, Fourier transform infrared spectroscopy, and Thermogravimetric analysis. Results of the conducted experiments indicated stronger degradation of the desired properties under positive DC stress as compared to the AC and negative DC voltages. Moreover, silicone rubber doped with nano-sized particles of silica filler demonstrated better anti-aging performance as compared to its micro-ATH filled counterpart.

Experimental Study on The Flashover Characteristics of Typical Medium Materials of Spacecraft Along the Surface

Along the surface flashover is one of the main forms of electrostatic discharge on the surface of the aircraft, the construction of low-pressure electrostatic discharge simulation test system, taking the typical dielectric material commonly used in spacecraft as the research object, simulating the typical structure of the spacecraft surface with the metal-medium structure, simulating the charging process of the spacecraft surface in the operating environment by injecting electrons with an electron gun, starting from the formation process of the electrostatic discharge channel and the theory of the discharge, the change law of the characteristics of the dielectric material along the surface flash discharge is studied. Studies have shown that under vacuum conditions, when the dielectric material is charged with an electron gun, the surface potential of the material is gradually increased, and finally reaches a limit value of dynamic equilibrium (surface charging balance potential); And with the increase of the energy of the electron gun, the balance potential of the surface will also increase; The thicker the dielectric material, the lower the discharge frequency, and the higher the surface dynamic balance potential. The change of material electrical parameters and surface roughness caused by chemical changes in the dielectric material during discharge is an important reason for the influence of the flashover voltage along the surface; Increasing the conductivity and surface roughness of the material, introducing transition substances with a small dielectric constant, and increasing the hydrophobicity of the material can effectively reduce the flashover voltage, which is an effective means of flash network protection for dielectric materials.

Effect of Trap Regulation on Vacuum DC Surface Flashover Characteristics of Nano-ZnO/PI Film.

The operating safety of spacecraft in space environments is closely related to the surface discharging phenomenon of dielectrics such as polyimide (PI) film in solar arrays; moreover, carrier traps in the dielectric can affect its insulation performance. Therefore, to improve the vacuum surface flashover characteristics of PI film by nano modification and reveal the effect of trap distribution on the flashover of PI composite film, first, the original PI and nano-ZnO/PI composite films with different additive amounts (0.5, 1, 2, and 3 wt.%) were prepared by in situ polymerization and their performance was evaluated by the physicochemical properties characterized by methods such as thermogravimetric analysis; second, the surface traps of the original and nanocomposite films were measured and calculated by surface potential decay method, and the carrier mobility was also obtained; finally, the vacuum direct current (DC) surface flashover characteristics and bulk resistivity of all the film samples were measured and analyzed. The experiment results showed that with the increase in the amount of nano-ZnO, both the shallow and deep trap density increased significantly, while the trap energy varied slightly, and the surface flashover voltage also increased obviously. Based on the multi-core model, the increases in the shallow and deep trap density after the introduction of nano-ZnO into the PI matrix was analyzed from the microscopic perspective of the interface. Based on the comparative analysis of the trap distribution and surface flashover voltage characteristics, a bilayer model of vacuum DC surface flashover development was proposed. In the bilayer model, deep traps and shallow traps play a dominant role in the vacuum–solid interface and the inner surface of the dielectric, respectively, and increasing the trap density could effectively inhibit secondary electron multiplication on the surface and accelerate charge dissipation inside the film. Consequently, nano-ZnO can purposefully control the trap distribution, and then improve the flashover characteristics of nano-ZnO/PI composite films, which provides a new approach for improving the spacecraft material safety.

Study on DC Surface Flashover at the Clear Edge of Metallized Polypropylene Films

The surface flashover may occur at the clear edge of metallized films, affecting the insulation performance of metallized film capacitors (MFCs). In this article, an experimental setup was established to investigate the dc surface flashover characteristics of the metallized polypropylene film for MFCs. The effects of voltage setting, parallel capacitance, interlayer pressure, experiment temperature, and clear edge width on the flashover characteristics of metallized films were studied experimentally. The results reveal that, different voltage setting methods affect the surface flashover voltage, with a relatively stable disparity of 14.03%; parallel capacitance has little effect on the flashover characteristics in the range of 0–60 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{F}$ </tex-math></inline-formula> ; the surface flashover voltage and field strength are positively correlated with the interlayer pressure in the range of 50–800 kPa, and negatively correlated with the experiment temperature in the range of 30 °C–90 °C; as the clear edge width of the metallized film increases from 0.5 to 2.5 mm, the flashover field strength substantially decreases by 49.39% and gradually slows down to saturation. In terms of clear edge design, the width should be determined by the surface flashover voltage under the second voltage setting method; the effect of parallel capacitance on surface flashover can be ignored; measures should be taken to increase the interlayer pressure of outer films, such as increasing the turns of wrapped film outside; the clear edge width should be increased for MFCs operating at high temperature and can be selected according to the experimental data in this article.

Peak-type charge barriers enabling high surface-insulating performance of epoxy composites

The rapid increase in electrical and electronic devices integration necessitates the urgent development of epoxy composites with more reliable surface-insulating performance. Charge traps with valley-like features are known to improve the surface-insulating characteristics of insulating materials. Inspired by this strategy, a new approach is reported herein for constructing peak-type charge barriers to enhance the surface-insulating performance of epoxy composites. This paper describes the construction of 0.92 eV peak-type charge barriers on the surface of an epoxy composite by doping with nanodiamonds (NDs), which have a negative electron affinity. The constructed barriers alter the charge migration behavior on the surface of the ND/epoxy composite. Consequently, the charge dissipation rate and surface flashover voltage of the ND/epoxy composite increase by 563% and 17.6%, respectively. Additionally, the NDs improve the arc resistance duration of the epoxy resin by 211%, owing to their electron-emitting effect under strong electrical fields. This strategy, by which the peak-type charge barriers constructed using the NDs exhibit excellent surface-insulating characteristics to the epoxy composites, can be adopted as a new approach to improve the surface-insulating abilities of materials.