Flashover Process Research Articles

Research on the prediction method of porcelain insulator pollution degree based on electric field monitoring

The online monitoring of insulator surface pollution states is crucial for predicting/preventing pollution flashover accidents. This paper conducts artificial pollution tests on porcelain insulators at five pollution levels to obtain the electric field signal during the flashover process. Subsequently, the time-frequency performance of the electric field signal waveform at various discharge stages is discussed in detail. Finally, a method for predicting pollution degrees by combining back propagation (BP) neural networks and non-contact electric field signal monitoring is proposed and validated. Research results indicate that during the pollution flashover process of the porcelain insulator, the peak value of the electric field signal shows an upward trend in sections. As the equivalent salt deposit density (ESDD) increases, the curves of electric field values rise. The development of dry-band arcing clearly distorts the electric field signal waveform at its crest and trough parts, increasing the total harmonic distortion (THD) rate. The THD can reach 100–130 % when closing to flashover, with the third harmonic acting as the main component. By taking kmax (the rising rate of the E-field signal amplitude), g (the ratio of Emax to Erms), K3 (the third harmonic coefficient), and THD as inputs to the BP neural network model, an ESDD prediction with a relative error lower than 10.63 % is realized. This research can offer novel technical support for predicting pollution degrees.

Investigation of sheath structure in surface flashover induced by high-power microwave

Flashover is a major limiting factor for the transmission and miniaturization of high-power microwave (HPM) devices. We conducted a study to investigate the developmental process of surface flashover on HPM dielectric windows through particle-in-cell-Monte Carlo collision simulations. A one-dimensional spatial distribution and three-dimensional velocity distribution model is established, encompassing the entire process of surface flashover, which includes electrode field emission, single-surface multipactor, outgassing, and gas breakdown. The nonuniform mesh generation method is employed to enhance the simulation accuracy. The growth rates of electron and ion densities increase as gas pressure rises. Additionally, the discharge transitions gradually from multipactor to gas ionization dominance. Notably, a space-charge-limited (SCL)-like sheath occasionally forms during an rf cycle near the surface under intermediate background pressure (∼0.05 Torr). The SCL-like sheath cannot exist stably. Instead, it periodically disappears and appears as the rf electric field changes. The underlying physics are explained by the variations of the rf electric field, which lead to the variations in the surface charge density, thereby affecting the normal electric field. The normal electric field interacts with the spatial distribution of charged particles, ultimately leading to the formation of the SCL-like sheath. This work may facilitate a comprehensive understanding of the developmental processes of surface flashover.

Research on Leakage Current Waveform Spectrum Characteristics of Artificial Pollution Composite Insulator

In order to know about the change features of leakage current during the pollution flashover process of composite insulators, the study of the pollution flashover test on composite insulator under the pollutional condition is presented in the artificial fog cabinet. The characteristics of leakage current waveforms in time-domain and frequency-domain are measured and analyzed during the test. The analysis results show that there is a strong correlation among the fundamental harmonic, the third harmonic and fifth harmonic. The characteristics of leakage current spectrum are represented by the maximum value of leakage current (Im), total harmonic distortion (THD), and phase difference of fundamental leakage current (θ). During a near-flash event, the leakage current exhibits high amplitude, low harmonic content, and small phase difference between the fundamental leakage current and voltage. The research findings provide a basis for establishing a pollution flash warning system based on leakage current characteristic quantities.

Surface Flashover Over a Microprofiled Cylinder in Air

Increasing the lightning impulse (LI) flashover voltage of insulators can require significant changes to the geometry and material properties of the insulator or electrodes. In some cases, such changes can be difficult to implement as they may require more volume or may compromise e.g. mechanical properties of the insulating parts. In this work, a new micro-scale surface profile which inhibits streamer discharge propagation without introducing major changes to the insulation system geometry or materials is demonstrated. Polyoxymethylene (POM) cylinders with 25 % glass fibre in air were machined with 0.5x0.4 mm rectangular grooves spaced 0.6 mm apart and stressed with LIs. A non-profiled cylinder and a cylinder with a larger, semi-circular grooves were tested for comparison. The cylinders were tested in a 48.5 mm atmospheric air gap with an exposed triple junction as the positive electrode. The rectangular surface profiles increased the LI flashover voltage (U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> %) from 70.0 to 96.5 kV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> . High-speed imaging showed positive surface streamer discharges stopping on the surface profiles. Discharge inception calculations were used to explain the influence of the surface profile on the flashover process. The results are relevant for technological applications with electrically stressed solid insulators in gases.

Arc feature extraction and flashover warning of direct current composite insulator based on image processing using visible discharge images

Insulator flashover seriously affects the safe operation of power grid. To improve the arc development mechanism and achieve flashover warning, this paper researches the arc evolution characteristics during direct current (DC) flashover process of composite insulator using visible discharge images. Firstly, an arc segmentation algorithm based on edge brightness and local threshold is proposed. It aims to solve the problems of uneven arc brightness, insulator reflection and arc halos in visible discharge images. Then, to quantify segmented arcs, an arc feature extraction method with four feature quantities of arc width, projected length, offset distance and centroid ordinate is proposed. Finally, this paper also extracts and studies the variation characteristics of the four feature quantities in two types of DC flashover processes respectively. The result shows that the proposed feature quantities effectively describe the arc state. Additionally, the arc width change rate k exceeding 1.3kmax, the insulation failure percentage reaching 75%, and the arc offset distance reaching the set threshold can serve as the critical criteria for arc development stages, dividing the two types of DC flashover processes into three stages. Building upon the critical criteria, it is expected to achieve flashover warning.

Switching Impulse Flashover Performance of Nonuniform Pollution Insulators in Natural Environment

Under natural conditions, insulators show a nonuniform pollution state, which has a great influence on the switching impulse flashover voltage of insulators. Meanwhile, the insulation level of transmission lines is selected according to the switching impulse flashover voltage of nonuniform pollution insulators. For this purpose, in this article, the wet pollution method is used to simulate nonuniform pollution conditions. Then, the switching impulse flashover tests and the switching impulse flashover tests with preloading ac voltage are conducted on two types of insulators, XP-70 and LXY-70, in natural environment. The flashover voltage of insulators is obtained, flashover performances are analyzed, and the flashover processes of insulators are photographed. Moreover, the equations of flashover voltage gradient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{{50}}$ </tex-math></inline-formula> under nonuniform pollution conditions are established. The results indicate that there is a conspicuous polarity effect and a polarity reversal phenomenon under different conditions. Besides, as the increase of the ratio of bottom surface pollution to top surface pollution, the flashover voltage changes in the opposite case of preloading and non-preloading ac voltage. Finally, the switching impulse flashover voltage of ordinary porcelain and glass suspension insulators under nonuniform pollution conditions can be calculated based on the established equations of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{{50}}$ </tex-math></inline-formula> . The results can provide theoretical support for the reasonable selection of the insulation level of transmission lines, which in turn can greatly reduce the cost of the transmission lines.

Research on the trigger process of surface flashover triggered vacuum switch

The key to promoting the development of triggered vacuum switch is an in-depth research on the physical process of its trigger phase. This paper mainly divides the trigger process into metal electrode melt evaporation, neutral metal vapor layer expansion, electron collision ionization multiplication, and charged particle migration. An approximate analytical formulation of the trigger delay time theory is presented based on a one-dimensional heat conduction model and an electron collision ionization multiplication process. The trigger delay time experimental platform is built with a removable vacuum chamber. The influence factors involved in the mathematical description proposed are studied experimentally. Theoretical analysis and experimental results show that both trigger current rise rate and main electrode operating voltage have an approximate inverse relationship with trigger delay time. The main electrode gap distance is directly proportional to the trigger delay time, but the effect will be weaker as the main electrode gap distance increases. Overall, the consistency of the experimental results and theoretical analysis on influence trends well support the theoretical model.

Two modes of vacuum flashover process under pulsed voltage

Vacuum flashover process can be influenced by various factors, such as gap length, applied voltage form, electric and magnetic field structures, and so on, leading to different flashover voltages and flashover times under a pulsed voltage. Particle-in-cell (PIC) simulation results in this paper show two typical vacuum flashover modes: (a) the area of high space charge density appears after electrons run across the gap and extends from the anode to cathode and (b) the area of high space charge density appears before electrons run across the gap and extends from the middle to both ends of the gap. In order to verify the PIC simulation results, a probe that measures the change in the electric field between the gap is embedded in the anode, the experimental waveform of which shows different features when the shape of insulator is different. Simulation results and experimental waveforms indicate that the flashover process of certain cylindrical insulators corresponds to the first mode, while the flashover process of certain conical insulators with an acute angle between the cathode and dielectric corresponds to the second mode.

Study on Initial Discharge Process of Insulator Pollution Flashover Considering Quantum Transition

Insulator pollution flashover has a high probability in the faults of transmission and distribution system. Due to its complex formation process and various influencing factors, the relevant studies are mostly based on experimental tests and empirical formulas, which do not fully consider the impact of random factors on the flashover process. The quantifiable algorithms are lacking, and it is difficult to effectively predict pollution flashover. In this article, the quantum transition theory is introduced in the streamer theory to calculate the energy-level transition probability of the particle on the surface of polluted insulator. The randomness of particle collision cross section is considered, and the initial discharge voltage is calculated by probability theory. The effects of active particle content, ambient temperature, and relative humidity on the initial charge number of insulator pollution flashover are analyzed. On this basis, the initial discharge process of insulator pollution flashover is studied. The experimental results show that the calculated results of insulator pollution flashover initial discharge voltage based on quantum energy-level transition probability are consistent with the experimental values, which has engineering application value for insulator pollution flashover prediction.

Effect of Different DC Prestressed Time on Flashover Characteristics of Epoxy Resin Under Polarity Reversal Voltage

DC wall bushing is one of the key equipment in high-voltage direct current (HVDC) transmission project. At present, post insulators based on epoxy composites are widely used for supporting insulation in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> -insulated apparatus. However, the flashover faults caused by surface charge accumulation can be a severe threat to the long-term operation of the post insulators under dc field. Therefore, the issue of charge accumulation on epoxy post insulators in HVDC wall bushing and its effect on flashover has been widely concerned. Aiming at this problem, the flashover characteristics of epoxy samples in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas under polarity reversal conditions with dc voltage prestressed for 10–600 min were measured in this article. The flashover process is analyzed through surface charge distribution measurement and electric field computation. The results show that with the increase of prestressed time, the accumulated negative charge density could reach <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-14.8\,\,\mu \text{C}/\text{m}^{{2}}$ </tex-math></inline-formula> , which led to a 12% increase in maximum field strength. As a result, the polarity reversal flashover voltage could decrease by 12% and the flashover time delay could decrease by 84.6%. This article can offer as a reference for the effect of surface charge accumulation on flashover in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas-insulated equipment.

The surface flashover process under positive lightning impulse voltage: initial stage and evolution

Surface flashover is a crucial issue in the field of electrical insulation, and it involves many complex physical processes. In this paper, the development process of the surface flashover was studied from different methods. The initial process of positive surface streamers in different gas environments (air, N2, CO2) was studied by photoelectric observation. The evolution of positive surface streamers in the air was described based on a 2D fluid model. The influence of the surface trap energy level on flashover development and the relationship between gas adsorption and surface trap energy level was discussed by density functional theory calculation preliminarily. The results showed that the initial ionization process of surface flashover is considered as the collision ionization between the initial electrons and gas molecules and photoionization of high-energy photons. Some of the high-energy photons can not only ionize some gas molecules but also cause the surface of the insulator to emit electrons (photoemission process), which could promote the development of the streamer. Both the ionization of the gas molecules and the photoelectric emission of the insulator surface may determine the initial development of surface flashover. During the process of the flashover, the electron density of the surface streamer (∼1021 m−3) is high and the main streamer tends to develop towards the insulator surface. The attraction of surface streamers changes with the position of the initial electron, and the positive surface charge brings the stronger ionization process, and the negative surface charge has the opposite effect. The band gap of the insulator surface is affected by the adsorption of gas molecules, which is considered as introducing shallow traps on the surface. For impulse voltage, the charge accumulation and internal charge migration may be not evident, the initial photoionization process and initial surface charge distribution affect the flashover process primarily.

Effects of Static Icing on Flashover Characteristics of High-Speed Train Roof Insulators

To study the icing flashover characteristics displayed by insulators on the roof of a high-speed train at a low-speed or static state, an environment with a low temperature and high humidity is established in the artificial climate chamber to carry out the icing flashover test of composite insulator under a static state. The results of the study indicate the presence of ice ridges bridging the sheath at the edge of the sheath when the ambient wind speed is less than 2m/s. The arc appears firstly at the end of the ice ridge, and it is likely that the arc at the adjacent tip converges in the air and then further extends to the edge of the next sheath. The arc melts the ice ridges at the end and enables it to stick to the tip of the ice ridge in the form of water droplets, and therefore the existence of droplets results in the distortion of the electric field near them. Moreover, the elongation of droplets and the bridging of ice edges accelerate the process of flashover. The research results can offer a reference to the structural optimization design of anti-icing insulators for high-speed trains in the future.

Surface Flashover Characteristics of Epoxy Resin Composites in SF6/CF4 Gas Mixture with DC Voltage

SF6/CF4 mixture is considered a potential substitution for SF6 in gas-insulated equipment owing to the considerable insulation and low temperature characteristics of CF4. This paper concerns the substitutability of the mixture on the surface flashover property. Epoxy resin composites for solid insulators are applied since flashover at the insulator interface often occurs and results in severe damage to the safe operation of the equipment. Two kinds of electrode systems are designed to study DC flashover properties with different electric field uniformities. The obtained result shows that the flashover voltage in the 20% SF6/80% CF4 mixture achieved more than 70% of the voltage in SF6 at the same pressure. The mixture shows considerable synergy in the surface flashover process and performed different tendencies with varying gas pressures and contents under different electrode systems due to the influence of surface accumulated charge. Moreover, it is considered that surface charge due to ionization may result in a reversal phenomenon for voltage polarity with increasing pressure. The results indicate the possibility of SF6/CF4 functioning as an efficient substitution of SF6 with a considerable insulation property and improved environmental protection characteristics.

Excellent Vacuum Pulsed Flashover Characteristics Achieved in Dielectric Insulators Functionalized by Electronegative Halogen–Phenyl and Naphthyl Groups

Designing electrical insulation materials with excellent surface flashover strength in a vacuum environment is crucial for high-power equipment and aerospace devices. In the present paper, the effect of two types of electronegative groups, the halogen-phenyl groups and the aromatic π-conjugated naphthyl groups, is used to greatly improve the vacuum flashover characteristics of polystyrene (PS), a commonly used polymer dielectric material in high-power devices. By polymerization of the monomers containing these electronegative groups, the bulk insulation material as a whole is modified expediently. In comparison to the base polymer PS, the electron affinity of the structures containing strong electronegative groups is studied with first-principles calculations based on the density functional theory. The nanosecond pulsed vacuum flashover testing results show that the vacuum flashover strength is increased by 10% after replacing the PS pendant phenyl groups with fluorophenyl groups and increased by 44% when replaced with the naphthyl groups. Furthermore, the thermally stimulated current and secondary electron emission yield spectroscopies are measured, to study the influence of strong electronegative groups on the trapping characteristics and further the electron-emitting features of the polymer dielectrics, which are closely related to the charged particle multiplication process during the vacuum flashover. The results prove that introducing strong electronegative groups can inhibit the triggering of vacuum flashover, suppress the electron emission, delay the flashover process, and thus greatly increase the vacuum flashover voltage. The study of this paper not only puts forward two groups of easily processable polymers with excellent vacuum flashover strength but also paves ways for the future material design of special insulation polymers.

Radiation electron trajectory modulated DC surface flashover of polyimide in vacuum

Improving surface flashover voltage on vacuum-dielectric interface irradiated by electrons is a long-standing challenge for developing high-voltage and high-power spacecraft technology. The basic issue is understanding the role of radiation electrons in the process of surface flashover. In this paper, a ‘three-segment’ curve concerning the surface flashover properties under electron irradiation is discovered experimentally. As the gap distance of electrodes increase, the surface flashover voltage of polyimide during electron irradiation presents a trend of firstly increasing, then decreasing, and finally stabilizing. According to the simulation of the trajectory distribution for kinetic electrons, this trend is found to correspond with three typical stages respectively. In stage A, the kinetic electrons are completely deflected and the varying electrode parameters mainly affect the electric field distribution. In stage B, the kinetic electrons can irradiate the part of polyimide. The promoting effect of those electrons on flashover process enhance with the enlargement of the irradiated region. In stage C, trajectories are no longer seriously deflected and the role of kinetic electrons do not vary with electrode parameters. Combining with the results above, a model with combined effects of both kinetic and deposited electrons on surface flashover in vacuum is thus proposed, base on which the guidance for the methods of improving surface flashover voltage during electron irradiation is provided.

Armature Ejecta and Its Effects on Insulator Degradation in Solid-Armature Launchers

Armature ejecta, which erodes the insulator and reduces surface dielectric strength, has been considered as a threat to solid-armature launchers. Two series of experiments were carried out in a small caliber launcher (SCL) to investigate the sources of armature ejecta and degradation mechanisms of the insulator. In first series of experiment, the ejection of armature was imaged by a high-speed camera. The influence of linear current density on ejection was studied in second series of experiment. Mass difference of insulator samples was used to evaluate the output of armature ejecta. Insulation performance was judged from the surface flashover voltage. And the degradation mechanism resulted from armature ejecta was analyzed based on the process of surface flashover. Results show that the local breakdown at armature–rail interfaces is one of the sources of armature ejecta. The amount of ejecta on samples increased with linear current density. Average flashover voltages decreased when ejecta increased in Zone A (0–200 mm) and Zone B (200–600 mm). And there was little difference of average flashover voltages in Zone C (600–900 mm). The discharge channel of the post-shot sample was composed of arcs and ejecta. The distortion of the surface electric field and the shorten of creepage distance are degradation mechanisms of bore insulators in solid-armature launchers.

Plasma Flows From Linear High-Voltage Nanosecond Vacuum Discharges on the Surface of Polymer Materials at 2–6 kA

Particle flow from vacuum flashover on surface of PTFE, PMMA, and PE initiated by 60-kV nanosecond pulses in self-breakdown mode is studied in this work in the range of discharge currents of 2–6 kA. The feature of this work is changing the discharge current at the constant working voltage (~60 kV). The discharge current is varied by changing a ballast resistor. The time lag between voltage application and current rise is ~12 ns for all the values of the discharge current. This lag corresponds to the stage of conductive channel formation (high-voltage stage) and depends only on working voltage of the generator. In contrast to the low-voltage experiments (~1–5 kV) with external ignition, in our case, plasma flow characteristics depend only on intrinsic properties of the breakdown. The novelty of the proposed approach is that all the measurements are carried out at high repetition rate of pulse application (30–100 Hz). We obtained the dependence of particle flow thrust, mass loss, and share of ion component on the value of the discharge current. The results show that there is a share of the ion beam, intensity of which does not depend on the discharge current. Presumably, these ions are generated at the high-voltage stage of the flashover process. The thrust is directly proportional to the charge passed through the discharge channel. It indicates that the key factor in raise of particle flow intensity is the widening of the discharge channel at higher currents. Some of the results are of practical interest for the development of high-voltage pulsed plasma thrusters.

Linking trap to G10 surface flashover in liquid nitrogen under DC voltage

The insulation failure is a bottleneck to restrict the safe operation and development of direct current (DC) superconducting power devices. The solid–liquid flashover test is an important evaluation index, but the previous researches mostly focus on engineering application, and less involve in the mechanism analysis. As a characterization method for defects existing in the dielectric, the trap has been applied to the study of flashover to occur polymer surface in liquid nitrogen (LN2) in recent years. However, the mechanism of trap action in solid–liquid flashover is still confusing. Therefore, this paper carries out the following research. Firstly, the flashover voltage and current of glass epoxy laminate sheet (G10) in LN2 were obtained through experiments and analysis using the Weibull model in this paper. Secondly, the flashover process of G10 was monitored. The changes of G10 surface morphology and chemical bond before and after flashover were also observed. Finally, the relationship between polymer flashover and trap in LN2 was analyzed based on the surface trap distribution test and theoretical calculation of the charge transport equation. The results are beneficial to guide the selection and performance improvement of dielectric materials and the design of insulating structures in superconducting devices.

Analysis and Experimental Verification of the Peaking Capacitor’s Flashover Process in an EMP Simulator

The flashover process of the peaking capacitor in the electromagnetic pulse (EMP) simulator is studied based on theoretical analyses and experimental verification in this paper. There are deeper and denser ablation spots on the film surface near the inner core of the destroyed peaking capacitor, while the damage of the outer film is relatively slight, which indicates that the flashover current along the inner film is larger. Besides, the circuit simulation analyses show that the earlier the flashover occurs on the film of the peaking capacitor, the smaller the flashover current. The electric field on the non-conical surface of the capacitor is mainly occupied with normal component. Differently, for the conical surface, the electric field on the outer layer is dominated by the normal component and the parallel component is the main part on the inner layer. It is considered that the flashover on the conical surface originates from outer layers and develops gradually to the inner until the flashover penetrates through all layers. Furthermore, the images of flashover show that the flashover firstly occurs on the outermost layers and develops to the inner layers with the increase of the voltage. For such special structure of the peaking capacitor, the parallel component of electric field is more likely to facilitate the flashover under nanosecond pulse. These results may exhibit specific reference implication in the design of insulation for the peaking capacitor used in EMP simulator.

Features Characterizing the Surface State of HV Insulator Glass Model under Desert Pollution

This paper deals with the monitoring of the surface state of HV insulator model under discontinuous layer of pollution. Natural desert sand is used to simulate the pollution influence in Algerian Sahara regions. Experimental tests are carried out on a glass plane model of 1512L cap-pin insulator largely used in the said regions. Leakage current LC signal is recorded for different positions and widths of the polluted-band. First, the obtained results are discussed to describe the behavior of the insulator model. For each configuration, time-frequency decomposition is established using Discrete Wavelet Transform (DWT) for information extraction corresponding to the flashover process. Decomposition results indicate that the detail D3 is highly correlated with the measured LC signal. Recurrence Plot (RP) technique and Recurrence Quantification Analysis (RQA) are applied to quantify the temporal patterning in LC signal, giving information on the position and width of the polluted-band on the insulating surface. It was found that RQA indicators increase with the polluted-band width according to the position of this polluted-band. Based on the plan model, the findings demonstrate the capability of RQA indicators for the glass insulator monitoring, providing information on the width and position of the polluted-band on its surface.