Point-plane Gap In Air Research Articles

Impact of barriers on dielectric strength and electric field distribution in point-plane air gap configurations

This study presents a numerical investigation into the dielectric strength of point-plane air gaps, specifically focusing on the influence of dielectric barriers on the potential and electric field distributions under alternating current (AC) voltage. By utilizing two-dimensional (2D) electrostatic simulations through the finite element method (FEM), both configurations—with and without dielectric barriers—are analyzed to assess their impact on vertically arranged point-plane gaps. The study demonstrates that the inclusion of a dielectric barrier plays a crucial role in redistributing the electric field, leading to a more uniform distribution and reducing localized peaks. This results in enhanced dielectric strength, allowing the system to withstand higher voltages without breakdown. The findings are significant for the design and optimization of high-voltage insulators, as incorporating dielectric barriers can improve overall insulation performance and reliability. By improving the electric field distribution, these barriers contribute to reducing the risk of insulation failure, thereby increasing the efficiency and longevity of high-voltage components, making them more reliable for applications in power transmission systems.

Energetic electrons and their contribution to the breakdown of a point-plane air gap with a positive nanosecond pulse

It is found that the polarity reversal phenomenon occurs in the nanosecond pulse breakdown experiment. Two-dimensional axisymmetric particle-in-cell/Monte Carlo collisions’ calculation is used to observe energetic electrons at positive nanosecond pulse voltage in atmospheric air and compared with previous calculation results under negative polarity in an attempt to explain the polarity reversal phenomenon. During the evolution of the positive polarity discharge, the difference in spatial net charge distribution leads to distortion of the electric field, which makes the electric field in the area of the ionization channel head very high, exceeding the threshold at which electrons can enter the high-energy state. The simulation results show that although electrons travel in the opposite direction to the ionization channel, energetic electrons can also be generated during the positive polarity discharge’s evolution, which may differ from what some researchers expected. However, it is also found that the maximum energy of energetic electrons under positive nanosecond pulse voltage is lower than that under negative nanosecond pulse voltage (only about 1/4). This may be mainly because in the case of positive polarity, the energetic electrons in the head of the ionization channel will move to the low-field intensity region inside the ionization channel and cannot be accelerated continuously. However, it must be pointed out that in the case of positive polarity, energetic electrons still contribute significantly to the rapid breakdown of the air gap. This study provides a deeper understanding of the physics of nanosecond pulse discharge.

Polluted Barrier Effect on the Electric Field Distribution in Point-Plane Air Gaps under AC Applied Voltage: Based on Experimental Model

The aim of this paper is to study the effect of the surface condition of an insulating barrier on the electric field distribution in point-plane air gaps with the presence of a space charge, under AC voltage. The pollution was modelled as a uniform conductive layer on the barrier surface. Electric field analysis was carried out by changing the conductivity, permittivity, and thickness of the pollution layer. Using the Finite Element Method (FEM), the geometric model has been implemented in COMSOL Multiphysics software. This method is used to solve the partial differential equations that describe the field with the presence of space charge. The electric field increases when the conductivity and thickness of the polluted layer increases. Uniform pollution on the side of the high voltage point greatly reduces the insulation quality of the barrier. In addition, a limit level of pollution, from which its minimal electric strength is equivalent to that of a conductive barrier, has been determined. This model has been validated by comparing with the experimental results of a point-barrier-plane configuration with a distance between electrodes equal to 5cm. The distribution of the electric field predicted by the numerical model is in accordance with the experimental results. The latter indicate that this model has a great contribution in the physics of discharges in the air under various polluted environments.

Numerical simulation of the electric field distribution in point-barrier-plane air gaps

The main aim of this paper is to study the distribution of the electric field in the point-plane air gaps with an insulating barrier in presence of a space charge under AC voltage. The introduction of an insulating barrier in a point-plane gap increases the dielectric strength of the system by changing the electric field distribution in the air gap in general and on the plane in particular. The influence of the barrier is mainly related to the accumulation of charges on its surface on the side of the active electrode. In addition, the variation of the electric field is influenced by parameters concerning the barrier itself, and the configuration of the interval. The simulation was performed with the COMSOL Multiphysics® simulation software based on finite element method. Our hydrodynamic model that we elaborate including a set of nonlinear partial differential equations represents the motion and the diffusion of charge carriers associated with the reactions between them. The influence of several parameters is investigated in detail based on the model. This model has been validated by comparing with the experimental results obtained in the case of a point-barrier-plane system with a distance between electrodes equal to 5cm. The distribution of the electric field predicted by the numerical model is in accordance with the experimental results.

Experimental study on thermal characteristics of positive leader discharges using Mach-Zehnder interferometry

Leader discharge is one of the main phases in long air gap breakdown, which is characterized by high temperature and high conductivity. It is of great importance to determine thermal characteristics of leader discharges. In this paper, a long-optical-path Mach-Zehnder interferometer was set up to measure the thermal parameters (thermal diameter, gas density, and gas temperature) of positive leader discharges in atmospheric air. IEC standard positive switching impulse voltages were applied to a near-one-meter point-plane air gap. Filamentary channels with high gas temperature and low density corresponding to leader discharges were observed as significant distortions in the interference fringe images. Typical diameters of the entire heated channel range from 1.5 mm to 3.5 mm with an average expansion velocity of 6.7 m/s. In contrast, typical diameters of the intensely heated region with a sharp gas density reduction range from 0.4 mm to 1.1 mm, about one third of the entire heated channel. The radial distribution of the gas density is calculated from the fringe displacements by performing an Abel inverse transform. The typical calculated gas density reduction in the center of a propagating leader channel is 80% to 90%, corresponding to a gas temperature of 1500 K to 3000 K based on the ideal gas law. Leaders tend to terminate if the central temperature is below 1500 K.

Prediction the AC Breakdown Voltage in Point/Plane Air Gaps with Barrier Using Design of Experiments

Breakdown voltage characteristics of air gaps considered a random process we have investigated how to find relevant statistical different leading to breakdown or with standing during an impulse voltage test under the same condition for a plane high voltage electrode and a grounded electrode configuration.In the present paper methods of the modelling and prediction of the AC breakdown voltage in point-plane air gaps are investigated. An analysis based on experiments design method has been developed with indicates that.Measurements to contain some relevant information test at early stages in reduced time frame investigation were done on a point-plane with barrier varying in position and size. The barrier used was mounted vertically between the electrodes.The use of methodology of experiments design is one of this methods and present an original idea in high voltage prediction problems several factors were considered,namely the distance between electrodes and different parameters of the barrier such us is dimension and his holes.The experimental results are compared with results from numerical simulations.We firstly present the principals of this model then we apply it to the study of barrier effect. A good agreement has been found between the computed and experimental results.

Investigation of positive streamers by double-pulse experiments, effects of repetition rate and gas mixture

Streamer discharges are often operated in a repetitively pulsed mode and are therefore influenced by species left over from the previous discharge, especially free electrons and ions. We have investigated these effects by applying two consecutive positive high voltage pulses of 200–700 ns duration to a point-plane gap in artificial air, pure nitrogen, other nitrogen–oxygen mixtures and pure argon at pressures between 67 and 533 mbar. The pulses had pulse-to-pulse intervals (Δt) between 200 ns and 40 ms. We imaged both discharges with two ICCD cameras and combined this to a compound image. We observe for values of Δt below 0.5–15 µs (at 133 mbar, with Δt depending on gas mixture) that during the second pulse the streamers continue the paths of the first-pulse streamers. We call the maximal time for which this continuation still occurs the continuation time. For N2–O2 mixtures, this time has a maximum at an oxygen concentration of about 0.2%. According to our plasma-chemical modelling this maximum is determined by the electron loss rate which has a minimum around this oxygen concentration. Depending on oxygen concentration the dominant recombining positive ion is , or where dominates around 0.2% O2 and recombines slowest.For increasing values of Δt we observe that after the continuation phase first no new streamers occur at all, then new streamers show up that avoid the entire pre-ionized region. Next we see new thin streamers that follow the edges of the old channels. For larger Δt (10–200 µs) the new streamers start to increase in size and move to the centre of the old channels. Finally, around millisecond timescales the new channels are completely independent of the old channels.Together this points to the combination of two mechanisms: streamers search the proximity of regions with increased electron density, but cannot penetrate regions with very high electron density.

Defective Barrier on Voltage Optimization for Small Airgap

Aim: To investigate the effect of defective barrier on the optimum breakdown voltage, using positive and negative needle electrodes in an air medium of 10cm gap distance. Methodology: The barriers for the tests were placed at 2.5cm from the point electrode for each test. The defective barriers were created by having holes of 6mm, 8mm, 12mm and 20mm diameter at the centre of the barrier. For each barrier position the breakdown test for positive and negative polarity for needle electrodes were carried out. Also, tests were carried out with non defective barrier and with point-plane airgap (without barrier). Result: From the test without barrier the negative point electrode offered higher breakdown voltage (1.8 times), than the positive point. When with plain barrier the positive point was optimized to 1.6 times, while the negative point was lowered. The optimum breakdown voltage decreased gradually as the hole diameter increased and at 20mm hole diameter the effect was like the plain barrier. Conclusion: From the results, optimization is only effective with positive point’s electrode and it endures even with small opening within the ionization zone. It is necessary to check this in practical situations because the specified optimum voltage of an equipment may be lowered.

Effect of isolation mode and surface condition of an insulating barrier on the performance of a non-uniform field electrode system under positive DC voltage

The aim of this paper is the study of the effect of the isolation and the surface condition of an insulating barrier on the performance of a non-uniform field electrode system. The study is carried out under positive DC voltage. The dielectric properties of the material were measured using the Schering bridge. The effect of the material isolation mode in the point-plane air gap system on the optimisation of its performance was analyzed. Moreover we present findings of experiments which allow quantifying the effects of the clean or polluted atmosphere and the contamination severity level on the electric strength of the air gap system. Finally, this investigation has been supported by laboratory observations of the discharge phenomena in the air gap from inception to full flashover in all cases using a video camera system. The results from this study show that there is a distance limit of the isolation barrier beyond which its performance is optimal. In addition, a limit level of pollution, from which its minimal electric strength is equivalent to that of a conducting barrier, has been detected.

Determination of ionization conditions characterizing the breakdown threshold of a point-plane air interval using fuzzy logic

The objective of this paper is to study the discharge phenomenon for a point-plane air interval using an original fuzzy logic system. Firstly, a physical model based on streamer theory with consideration of the space charge fields due to electrons and positive ions is proposed. To test this model we have calculated the breakdown threshold voltage for a point-plane air interval. The same model is used to determine the discharge steps for different configurations as an inference data base. Secondly, using results obtained by the numerical simulation of the previous model, we have introduced the fuzzy logic technique to predict the breakdown threshold voltage of the same configurations used in the numerical model and make estimation on the insulating state of the air interval. From the comparison of obtained results, we can conclude that they are in accordance with the experimental ones obtained for breakdown discharges in different point-plane air gaps collected from the literature. The proposed study using fuzzy logic technique shows a good performance in the analysis of different discharge steps of the air interval.

Generalization of the quadratic charge-voltage relation of impulse corona in air

On the one hand, a quadratic relationship between the charge Qi of the first corona and the associated inception voltage Ui has been recently put in evidence for a 1m point-plane air gap submitted to impulse voltages. On the other hand, different models have been proposed to evaluate that relation for coaxial geometries and conductor-plane also submitted to impulse voltages. The aim of this paper is to put in evidence the quadratic charge-voltage relations for all the configurations and to discuss how the assumptions made for one configuration can be exported to the other electrodes geometries.

Study of Discharge in Point-Plane Air Interval Using Fuzzy Logic

The objective of this paper is to study the discharge phenomenon for a point-plane air interval using an original fuzzy logic system. Firstly, a physical model based on streamer theory with consideration of the space charge fields due to electrons and positive ions is proposed. To test this model we have calculated the breakdown threshold voltage for a point-plane air interval. The same model is used to determine the discharge steps for different configurations as an inference data base. Secondly, using results obtained by the numerical simulation of the previous model, we have introduced the fuzzy logic technique to predict the breakdown threshold voltage of the same configurations used in the numerical model and make estimation on the insulating state of the air interval. From the comparison of obtained results, we can conclude that they are in accordance with the experimental ones obtained for breakdown discharges in different point-plane air gaps collected from the literature. The proposed study using fuzzy logic technique shows a good performance in the analysis of different discharge steps of the air interval.

Positive corona progression over profiled insulator surfaces near the breakdown condition

Experimental observations are presented of the development and propagation of positive impulse corona near the breakdown condition in a point plane gap in air at atmospheric pressure in the presence or absence of insulators having simple cylindrical and profiled surfaces. Optical techniques have been used and the effects of the insulator materials have been demonstrated. All the results with corona were obtained using an applied peak impulse voltage of 0.9U 50 in each case, where U 50 is the 50% breakdown voltage of the gap. Comparisons were made between air, polytetrafluoroethylene (PTFE) and porcelain surfaces. It has been demonstrated that corona development in the gap is dependent on the relative permittivity of the insulator material, the profiles and the proximity of the insulator to the high voltage point electrode. Simultaneous photomultiplier records and photographs taken on fast film with Quartz optics show details of corona development using the UV region of the spectrum.

Theoretical investigation on barrier effect on point-plane air gap breakdown voltage based on streamers criterion

A model is developed for the computation of the electric field and potential in the presence of a space charge, which is determined after the knowledge of the efficient ionisation factor. The computation is performed in a point–barrier–plane air gap under a positive voltage using finite elements. As a typical electrostatic application, the electric field distribution is computed in the presence of a space charge and a dielectric barrier in the air gap. The proposed model deals with the phase of electronic avalanches.

Secondary emission effects on streamer branching in transient non-uniform short-gap discharges

A new approach is presented for calculation of photoionization rates, in fully three-dimensional grids, that improves the accuracy of the secondary processes calculation without significantly compromising the efficiency of the numerical algorithm. The method is based on generating a coarser secondary grid and interpolating the photoionization values between the two meshes, in order to overcome the enormous effort required for calculation of photoionization in gas discharge problems.A comprehensive study of the effects of photoionization, photoemission and background ionization in a short point-plane gap in air at atmospheric pressure is then presented, by using the above approach for the secondary processes in two dimensions, in conjunction with the two-dimensional axisymmetric finite-element flux-corrected transport algorithm. The secondary processes are modelled individually within a wide range of parametric values to reflect the uncertainty in the experimental data, and their effect on streamer development and propagation is investigated. The significant reduction in time required for the calculations makes numerical modelling an essential tool for better understanding of the very important yet not well understood physical processes central to the propagation and development of streamers.Finally, numerical branching is observed under certain conditions in the absence of an adequate supply of electrons in high field regions.

Evolution of the radial structure of a negative corona during its transformation into a glow discharge and a spark

With the proper stabilization of a negative corona, it is possible to increase the threshold current at which the corona discharge in the point-plane gap in air transforms into a spark. Then, in the current range corresponding to the transition region between the corona discharge and the spark, a new type of discharge arises—an atmospheric-pressure diffuse glow discharge. The transformation of the negative corona into a glow discharge and then into a spark is accompanied by the rearrangement of the discharge structure. The experiments show that, as the corona current increases, the radial current profile at the anode shrinks and the glow diameter near the anode increases. The radial profiles of the current and the corona glow during the transition to a glow discharge are measured. The longitudinal structure of the corona is computed using a 1.5-dimensional model that, unlike the other available models, includes gas ionization in the drift region of the corona. The experimental data are used to determine the effective cross section of the current channel at the anode. The radial glow profile near the anode is calculated using the measured current profile and assuming that the field profile is parabolic.

Simulation of the coronal development in air at radio frequency: the effects of attachment, secondary emission and diffusion

The finite-element flux-corrected transport (FE-FCT) method is used to examine the transient development of corona discharges in a 1 cm hyperboloid (50 /spl mu/m tip radius) point-plane gap in air at a frequency of 40 MHz. The investigation revealed the formation of streamers in the positive part of the voltage cycle during the transient stage. These are suppressed in the steady state due to the formation of negative ions, while a cathode fall region is fully established. The effects of attachment, secondary emission and diffusion are also examined, both in the transient and steady states.

An experimental study of negative discharge in a 1.3 m point-plane air gap: the function of the space stem in the propagation mechanism

This paper reports a study of the origin and propagation of the space stem when a bi-exponential voltage impulse is applied to a 1.3 m point-plane air gap. The tests are carried out by using impulse shapes ranging from 0.25/2000 to 120/2000 mu s. Among these impulse shapes, the optimum with respect to the observation of the space stem formation is the 0.3/2000 mu s one. Some detailed photographs obtained with an image converter allow one to identify the following phenomena: negative streamers, a Trichel zone, a negative corona, a cathodic zone (cathodic stem), positive streamers and space stems. The current measurements allow one to quantify some energetic thresholds. The 5/2000 mu s impulse shape leads to a quasi-continuous space stem propagation in the gap. Thus it is possible to study the role of the positive and negative streamers in the space stem's behaviour, which is defined as their zone of origin. A stem replica mechanism is explained and some associated energetic criteria are discussed.

Thermally stimulated discharge currents in corona-charged aramid paper

Thermally stimulated discharge (TSD) currents were measured in corona-charged aramid paper to study the mechanisms of charge storage and its subsequent release from the bulk of the material. Studies were carried out on paper thicknesses of 76 and 127 μm using a point-plane gap in air at atmospheric pressure. TSD currents were measured over a temperature range of 0–200 °C and the influence of various parameters, such as the poling voltage, rate of heating and effect of electrode materials, were investigated. Corona-charged aramid paper with positive polarity voltage showed that the TSD current is of either polarity depending upon the temperature range. Three distinct peaks were observed, one in the low-temperature range, 20–25 °C, and the other two peaks at higher temperatures. The low-temperature peak was considered to be due to an abnormal TSD current, whereas the currents at higher temperatures were normal TSD currents. Activation energies were determined using the low-temperature tail of the TSD curves and were found to be dependent on extrinsic parameters such as the thickness of the sample. The activation energy for aluminium electrodes was observed to be in the range of 0.5–2.0 eV. The TSD currents for the low-temperature peak was considered to be electronic. It is postulated that the charge carriers are generated within the material by the intense electric field due to corona.

Impulse flashover of air in divergent fields

Impulse voltage flashover of a 4 cm point-plane air gap was investigated. The positive-voltage flashover track was found to wander further away from the axis than that for negative voltage. A space-charge model is proposed to explain the difference between these trajectories.