Mechanism Of Surface Flashover Research Articles

Customizing superior surface insulation properties of polymeric dielectric via in-plane molecular chain orientation

Surface flashover is a common breakdown phenomenon on material surfaces for which surface charge migration property, determined by surface composition and molecular chain structure, is crucial. Precise modulation of charge migration property by simple and efficient methods to improve surface flashover voltage is the goal in industry. Here, in-plane molecular chain orientation (MCO) modulation by uniaxial stretching was proposed to achieve this goal and investigate the intrinsic mechanism of charge migration on flashover. Flashover voltage and accompanying leakage current, performed with electrodes oriented at varying angles to the MCO direction, show a consistent trend, which skillfully reveals that the facilitated charge migration is favorable for improving flashover voltage. When the stretching ratio is 3.5, the flashover voltage along the stretching direction (SD) increases by up to 48.7%, while the in-plane minimum flashover voltage remains essentially unchanged with the change in stretching ratio. Molecular chain segment motion properties along different directions further elucidate that the surprising improvement of flashover voltage along SD is primarily due to the MCO that promotes intra-chain charge migration. This work provides a new perspective on anti-flashover modification of polymeric dielectric and will promote the development of surface flashover mechanisms.

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.

Inversion Algorithm for Surface Charge Distribution on Insulator in Shift-Invariant System Based on Constrained Least Square Filter

Surface flashover on an insulator caused by the charge accumulation at the gas–solid interface is one of the main reasons limiting the development of gas insulated equipment in high voltage direct current (HVDC) condition. Therefore, it is helpful to clarify the mechanism of surface flashover and surface charge behavior by measuring charge distribution with high accuracy. In this article, a surface charge inversion algorithm based on constrained least square filter (CLSF) is proposed for surface charge measurement in a shift-invariant system. By taking the image restoration technology as an analogy, the algorithm transfers matrix equation into frequency domain by 2-D Fourier transform, which significantly simplifies the computational process. Then the CLSF is used to suppress the noise signal and actualize the conduction from surface potential distribution to surface charge density distribution. The calculation process and accuracy of the algorithm are discussed in detail with the simulated examples, and the inversion performance of CLSF is compared with Wiener filter. At last, the effectiveness of the algorithm is verified by performing surface charge measurement and dust figure. This methodology gives an innovative idea of potential-charge transformation process with higher accuracy and stability, which benefits the development of surface charge measurement technology.

Research on the Characteristics of a Triggered Vacuum Switch with a Surface Flashover Trigger Device

A triggered vacuum switch (TVS) has been applied in high pulsed power systems. It is operated by using the plasma sequentially generated at a trigger device and a main electrode. Firstly, the initial plasma is ignited at the trigger device by surface flashover mechanism. Then, the main plasma, which is induced by the initial plasma, is generated at the main electrode. In general, the research on the characteristics of trigger jitter, main jitter, and delay is essential to develop a highperformance TVS. These characteristics are crucially affected by the trigger device. Accordingly, we have fabricated a trigger device with a ring electrode and measured the trigger jitter, main jitter and delay at voltage up to 20 kV. The measured values of delay are 4.3–9.6 µs, of the trigger jitter are 0.56–2.1 µs, and of the main jitter are 3.1–5.0 µs. As a next step, we plan to continue research of characteristics with a new trigger pulse generator having a rise time of hundreds of nanoseconds.

The mechanism of gas pressure and temperature dependent surface flashover in compressed gas involving gas adsorption

In surface science, when a high voltage is applied, a penetrating discharge occurs along the gas-solid interface, which is called surface flashover. Although the electron stimulated gas desorption is a crucial process for surface flashover in vacuum, the underlying gas-solid interaction physics process for surface flashover is still ambiguous in compressed gas. In this study, the effect of gas adsorption on surface flashover is taken into consideration, surface flashover voltage, gas breakdown voltage, gas adsorption quantity are tested at various gas pressures and temperatures in N2, and the surface gas density is calculated. It is found that the difference between surface flashover voltage and gas breakdown voltage (Vf/Vb) has a positive correlation with the relative gas adsorption ability (Va×T/p or Nsur/Nav) in a wide gas pressure, temperature and density ranges. The results indicate surface flashover occurs in the gas adsorption layer, which density is higher than the average gas density in the gas phase. The gas adsorption improves the surface flashover voltage by impeding electrons collision ionization process in compressed gas. The findings in this research provide a new perspective for studying surface flashover, and offer a route to improve surface flashover performances by tailoring surface adsorption characteristics.

Luminescence reveals micro discharge as a potential triggering factor for surface flashover

The undetermined surface flashover of insulators limits the development of ultra-high voltage AC gas insulated equipment. In this paper, we report a micro discharge potentially triggered surface flashover phenomenon, which is revealed directly by luminescence activities of samples with different micro morphologies. The results show that sandblasted samples, which are prone to a lower surface flashover voltage, present luminescence pulses corresponding to micro discharge activities over the surface. Polished samples have the mold release agent layer removed and inhibit local micro ionization, which eliminates the pulse luminescence and improves the surface flashover voltage. This paper brings novel ideas to insulation surface flashover mechanism under AC voltage and lays a foundation for further developing high reliable gas insulted equipment.

Surface charge transport behavior and flashover mechanism on alumina/epoxy spacers coated by SiC/epoxy composites with varied SiC particle size

Charge accumulation phenomenon on gas–solid interface greatly restricts the development of HVDC energy transmission system. In this study, the surface charge transport behavior and flashover performance on alumina/epoxy spacer coated by SiC/epoxy composites are experimentally investigated under DC stress. SiC/epoxy composites with varied SiC particle size are fabricated and deposited on spacer surface. Nearly a charge free surface is achieved especially at smaller SiC particle size, even when metallic wires are adhered on spacer surface and connected to high voltage electrode. The DC flashover voltage increases with the decrease of SiC particle size. On the one hand, smaller SiC particle size exhibits more outstanding nonlinear conductivity characteristics, contributing to accelerating charge dissipation, whereas on the other hand, it would introduce plenty of shallow traps due to the increased interfacial regions between SiC particle and epoxy matrix, resulting in aggravating charge accumulation. A theoretical model is proposed to reveal the control mechanism of SiC/epoxy coating with different SiC particle size on flashover performance. The validity of this model can be confirmed based on the surface trap distribution, carrier mobility and our previous investigations on gas–solid interface flashover development process.

Investigation on surface electric field distribution features related to insulator flashover in SF6 gas

The gas-solid interface is generally considered to be the weakest part in gas-insulated systems. The flashover voltage of a gas-solid interface is difficult to predict because it involves an extremely complex physical process affected by multiple factors. In this paper, we employ the support vector regression (SVR) method based on mass data on the features of the electric field that are indicators of the flashover voltage. We cast 48 kinds of epoxy resin post insulators as samples, and 61 features are extracted to characterize the surface electric field distribution and the flashover voltage under standard lightning impulse voltage. Four indicators that may be useful in predicting the flashover voltage were obtained; the maximum electric field along the surface of the insulator (E max ), the normal electric field components of E max (E n_Emax ), the maximum gradient of the surface tangential electric field component (E t_gm ), and the integral of the area in which the E value exceeds x% of E max along the insulator surface (V t60 ). We establish an SVR flashover voltage prediction model with these four features, whose average deviation is 7% in 10,000 predictions. It is expected that this study may provide a reference for the continued study of the gas-solid surface flashover mechanism.

Mechanism of Saline Deposition and Surface Flashover on High-Voltage Insulators near Shoreline: Mathematical Models and Experimental Validations

This paper deals with sea salt transportation and deposition mechanisms and discusses the serious issue of degradation of outdoor insulators resulting from various environmental stresses and severe saline contaminant accumulation near the shoreline. The deterioration rate of outdoor insulators near the shoreline depends on the concentration of saline in the atmosphere, the influence of wind speed on the production of saline water droplets, moisture diffusion and saline penetration on the insulator surface. This paper consists of three parts: first a model of saline transportation and deposition, as well as saline penetration and moisture diffusion on outdoor insulators, is presented; second, dry-band initiation and formation modelling and characterization under various types of contamination distribution are proposed; finally, modelling of dry-band arcing validated by experimental investigation was carried out. The tests were performed on a rectangular surface of silicone rubber specimens (12 cm × 4 cm × 8 cm). The visualization of the dry-band formation and arcing was performed by an infrared camera. The experimental results show that the surface strength and arc length mainly depend upon the leakage distance and contamination distribution. Therefore, the model can be used to investigate insulator flashover near coastal areas and for mitigating saline flashover incidents.

Surface morphology and flashover performance of epoxy resin in SF<inf>6</inf> after discharge aging

The study of the surface morphology changes in epoxy resin under direct current power supply in SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> is of great significance to research into the surface flashover mechanism and the improvement of flashover voltage. In this paper, the epoxy resin was subjected to different degrees of aging in an electric field formed by rod-rod and pin-pin electrodes. Taking the pin-pin electrode as an example, the surface morphology of the same position with different number of flashovers were measured by atomic force microscopy and scanning electron microscopy, and the surface chemical characteristics were explored by Fourier transform infrared spectroscopy and energy dispersion spectrum. Meanwhile, the fractal dimension was introduced to characterize the surface morphology in a quantitative manner: the influence of aging on the flashover voltage was then explored. The research results showed that swelling, cracking, and granular bumps, were three typical surface features, and these were successively formed during aging. As the aging time increased, organic groups on the surface of epoxy resin decreased in number, whilst the inorganic filler mass fraction increased. Moreover, with increasing number of flashovers, the surface morphology gradually changed from disorderly to orderly, from complex to simple, from chaotic to self-similar, when the aging region was changed from the electrode position to the middle of the two electrodes. Furthermore, with prolonged aging, the flashover voltage was reduced to a certain extent.

The potentially neglected culprit of DC surface flashover: electron migration under temperature gradients

This report intends to reveal the role of electron migration and its effects in triggering direct current (DC) surface flashover under temperature gradient conditions when using epoxy-based insulating composites. The surface potential and the surface flashover voltage are both measured using insulators that are bridged between two thermo-regulated electrodes. The space charge injection and migration properties under different temperature are detected. The results show that the surface potential rises significantly because of electron migration near the high voltage (HV) electrode under high temperature conditions, thus creating an “analogous ineffective region”. The expansion of this “analogous ineffective region” results in most of the voltage drop occurring near the ground electrode, which serves as an important factor triggering positive streamers across the insulation surface. This work is helpful in understanding of DC surface flashover mechanism from a new perspective and also has important significance in design of a suitable DC insulator to avoid surface flashover problem.

Mechanism of saline deposition and surface flashover on outdoor insulators near coastal areas part II: Impact of various environment stresses

This is the second in a two-part paper series dealing with sea salt transportation and deposition mechanisms, and discussing the serious issue of degradation of outdoor insulators resulting from various environmental stresses and severe saline contaminant accumulation near the shoreline. The deterioration rate of outdoor insulators near the shoreline depends on the concentration of saline in the atmosphere, influence of wind speed on the production of saline water droplets, moisture diffusion and saline penetration on the insulator surface. This paper comprises two parts. The first part, deals with the impact of different environmental stresses on insulator surface degradation, including wind speed and direction, cold fog and rainfall. The second part concerns the flashover process related to saline contamination of the surface under constant and variable cold fog wetting rates and equivalent salt deposit density (ESDD). The experiments were performed on high voltage insulators based on the model presented in Part-I. Based on the proposed model, the influence of wind speed and direction on the pollution accumulation rate and impact of wetting rate on discharge current and surface flashover process were investigated. The equations S=S0e(Vdep0/αh)[e(−αx/v)−1] and D=Doe(Vdep0/αh)[e(−αx/v)−1] are derived from the model for saline concentration and deposition show good reliability and well represent the results obtained. Test results also show that due to the different wetting and contamination deposition rate, surface discharge current characteristics of tested insulator in rain are different with that in cold fog, which lead to different surface flashover voltages. An experimental setup was mounted for artificial saline contamination deposition. The proposed model can be therefore used to investigate insulator flashover near coastal areas and for mitigating saline flashover incidents.

Surface flashover characteristics in polyimide/ZnO nanocomposite under DC voltage in vacuum

It is well known that the solid materials flashover phenomenon is very complicated and influenced by many factors, such as surface conditions, electrodes and traps, etc. In this paper, dc flashover experiments of polyimide (PI) and PI/ZnO nano-composite are studied in vacuum to reveal the surface flashover mechanism. The trap levels and space charge distributions were obtained by thermally stimulated current (TSC) and pulsed electro-acoustic (PEA) methods respectively. Further researches on surface states of flashover tested sample, such as observation by scanning electron microscopy (SEM), and measurement of secondary electron emission (SEE) coefficient, were carried out as well. Experimental results showed that the dc surface flashover voltage of PI/ZnO nano-composite is significantly enhanced compared to that of pure PI. The experiments results can be explained as that the doped ZnO nano-particles introduced deeper traps, which can reduce the level of charge carrier mobility, secondary electron emission and space charge accumulation. Meanwhile, the gas desorption efficiency of nano-composite is much lower than that of pure PI. It suggests that the doped ZnO nano-particles plays a critical role in affecting the surface flashover performance under dc voltage in vacuum.

N2/O2혼합가스 중 적층된 고체유전체에 대한 연면방전의 메커니즘과 특성

This paper presents the surface flashover mechanism of a laminated solid dielectric and describes the surface flashover characteristics with the inherent capacitance of the laminated solid dielectric in a mixture gas (8:2) under an quasi uniform field. It was found that the electron emission at a cathode and the high-local electric field region around an anode were important factors to reasonably describe the surface flashover mechanism. The surface flashover voltage by the mechanism decreased with the inherent capacitance increase of the laminated solid dielectric. In addition to the surface flashover mechanism and its characteristics, the surface flashover voltage equations as a function of the inherent capacitance were derived by considering a gas pressure used in future eco-friendly GIS and the factors influencing the surface flashover.

Development process of impulse surface flashover on alumina dielectrics in vacuum

For higher electrical insulation performance of vacuum circuit breakers (VCB), the surface insulation characteristics in vacuum should be improved. In this paper, we investigated the development mechanism of impulse surface flashover on alumina ceramic insulator in vacuum. We measured the still images and ultra fast framing images of fast developing surface flashover in synchronous with applied impulse voltage and current waveforms. The light emission transition in initiation of surface flashover was also clarified. We found the dark area was formed around the cathode and conducting channel was formed from the anode in developing process of surface flashover in vacuum.

Effect of UV illumination on surface flashover characteristics of epoxy resin under pulse

Surface flashover in vacuum is a primary limitation to pulsed-power devices. Recently, there are more studies on the pulsed vacuum surface flashover focused on the flashover mechanism. In this paper, the UV illumination is used to study the mechanism of vacuum surface flashover. The effects of different illumination positions, different illumination durations and different illumination intensities are investigated. The results show that the pulsed vacuum surface flashover characteristic is affected by the UV illumination significantly. When the UV light illuminates the cathode triple-point junction, the flashover voltage is reduced; this reduction is almost unaffected by the illumination duration, but significantly increased with the enhancement of illumination intensity. When the UV light illuminates the sample surface, the flashover voltage reduction is increased both by the increases of illumination duration and illumination intensity. Moreover, UV illumination can also change the surface potential and thus influence the surface flashover characteristic.

Influence of surface charges on impulse flashover characteristics of alumina dielectrics in vacuum

For higher electrical insulator performance of vacuum circuit breakers (VCB), the surface flashover in vacuum should be improved. In particular, it is important to clarify how strongly surface charges influence the surface flashover mechanism. In this paper, we investigated the surface flashover characteristics based on the existence of surface charges on an alumina insulator in vacuum. We changed the location and magnitude of surface charges and measured surface flashover voltage. From our experimental results, an attempt was made to clarify the influence of charges on alumina dielectrics on impulse surface flashover characteristics in vacuum. From this, we concluded that (1) when surface charges were located near the cathode electrode, a positive charge made the surface flashover voltage lower and a negative charge made the surface flashover voltage higher; and, (2) when surface charges were located in the way of the discharge path, both positive and negative charges made the surface flashover voltage lower. We were able to explain these results by considering the influence of surface charges on the electric field at flashover and on the secondary electron emission avalanche.

Flashover Characteristic of Epoxy Composites Filled With Different Micro-Inorganic Oxide Particles Under Nanosecond Pulse in Vacuum

Research on the vacuum surface flashover mechanisms of insulating material under nanosecond pulses is important for the design and manufacture of high-power pulse generators. Here, the vacuum surface flashover characteristic of epoxy composites filled with different microfillers Al2O3 middot3H2O , Al2O3, BaTiO3, and TiO2 is investigated using a unipolar pulse generator (50-ns rise time and 600-ns full-width at half-maximum). In addition, their thermally stimulated depolarization current and secondary electron emission (SEE) coefficients are measured. The results show that the flashover performance does not depend just on the SEE coefficient, but that the different effects of shallow and deep traps must also to be taken into account.

Influence of electric field at cathode triple junction on flashover characteristics in vacuum

Analytical and experimental investigations have been carried out to clarify the characteristic and the mechanism of surface flashover in vacuum. Various insulator profiles made of PMMA having a certain contact angle with the cathode have been investigated. Simulation of surface charging based on a secondary electron emission process is performed, which analyzes the electric field strength near the cathode triple junction considering the surface charge. It has been shown that the flashover voltage of spacers experimentally obtained significantly depends on the electric field strength near the triple junction at the cathode. © 2000 Scripta Technica, Electr Eng Jpn, 131(3): 1-8, 2000

Influence of electric field at cathode triple junction on flashover characteristics in vacuum

Analytical and experimental investigations have been carried out to clarify the characteristic and the mechanism of surface flashover in vacuum. Various insulator profiles made of PMMA having a certain contact angle with the cathode have been investigated. Simulation of surface charging based on a secondary electron emission process is performed, which analyzes the electric field strength near the cathode triple junction considering the surface charge. It has been shown that the flashover voltage of spacers experimentally obtained significantly depends on the electric field strength near the triple junction at the cathode. © 2000 Scripta Technica, Electr Eng Jpn, 131(3): 1-8, 2000