Air Gap Discharge Research Articles

Observation of Removal Process by single discharge in Air Gap

Observation of Removal Process by single discharge in Air Gap

Study on Spatial-Temporal Distribution Characteristics of the Discharge Process in a 1 m Rod-Plate Gap Under Different Polarity Lightning Impulses

The observation of the air gap discharge process is the basis of revealing the physical mechanism of gas discharge and establishing the accurate numerical simulation model. In order to obtain the variation characteristics at different development stages during the air gap discharge processes under positive and negative lightning impulses in this paper, repetitive discharge tests of 1 m rod-plate air gap under standard lightning impulse were carried out in the National Engineering Laboratory (Kunming) for Ultrahigh-voltage Engineering Technology. An improved discharge observation method, using electron-multiplying intensified charge-coupled device (EMICCD) camera, was adopted in the test. A series of discharge spatial-temporal distribution images with nanosecond exposure time and interval time were captured by changing the shooting time delay of the EMICCD in repetitive discharges. The complete discharge development process was reproduced by the image stitching. The development process and the characteristics of the air gap discharge under lightning impulse were analyzed qualitatively. The characteristics of the measured discharge current under the positive lightning impulse were discussed. The testing results and the analysis show that the shape and the development process of the ionization region of the streamer are quite different under different lightning impulses. There are spherical and fan-shaped streamer regions under the positive and negative lightning impulse, respectively. After the streamer runs through the entire gap, the leader develops from the rod electrode to the plate electrode (under the positive impulse) or from both the rod and the plate electrode to the middle area (under the negative impulse). Then the final jump occurs and the gap is broken down. If the streamer still cannot penetrate the entire gap when the applied impulse voltage exceeds the peak, the gap will not be broken down finally.

A Method of Monitoring Partial Discharge in Switchgear Based on Ozone Concentration

We accurately established an air discharge simulation experiment system to obtain the variation of ozone concentration in air discharge process, which includes pin-to-plate discharge, air-gap discharge, creeping discharge as well as metal particle discharge model, consisting of voltage console, transformer, and discharge device, gas absorption cell, spectrometer, and computer. Through online monitoring, the ozone concentration in a relatively closed container to simulate internal partial discharges for an hour with the voltage increases from 3 to 0.2 kV increment, we obtained four basic models with discharge inception voltage grade of 4.0, 4.5, 4.7, and 4.1. Besides, we applied the method of Fourier transform-differential absorption spectroscopy to process and analyze the experimental data, and obtained the ozone concentration curve. The ozone concentration increases with the increasing voltage loading time, and it tends to be stable and reaches an equilibrium state under a certain voltage.

绝缘气隙放电对高压电缆的电学和热学性能的影响研究

绝缘气隙放电对高压电缆的电学和热学性能的影响研究

Number of stems around the H.V. electrode in a 0.74-m air gap under positive impulse

Stem is a special short bright zone for the streamer to leader transition in long air gap discharges, whose characteristics play an important role in leader initiation and development. For the purpose of observing the number of stems around the H.V. electrode at the initial corona discharge, experiments were conducted under positive switching and lightning impulses with different crest values, and the schlieren photographs of stems and the discharge current were recorded synchronously. The associated analysis of the experimental results shows that the number of stems around the electrode at the initial corona discharge is always more than one in most cases. In addition, the average number of stems increases with the radius of the electrode and the crest value of applied impulse. If we focus on the characteristic of only one stem for special study, the conditions with a lower crest value under switching impulse and the electrode with a radius as small as possible are preferable.

The development characteristics of the discontinuous leader under the positive switching impulse with low rate of voltage rising

Discontinuous leader development is the most important discharge process under the application of the switching impulse voltage with the low rate of voltage rising, which is of great significance to study the external insulation characteristics of ultra-high voltage (UHV) large scale air gap. Based on the CMOS high-speed camera, a discharge test with different operating impulse voltage is carried out by constructing a comprehensive observation platform of rod-plate air gap discharge, and a clear discontinuous leader development process picture is captured. Moreover, the leader current, injection charge and leader channel unit length charge, and their characteristics of the change trend are also obtained. Further analysis based on the experimental results shows that the discontinuous leader development under the action of the impulse voltage with low rate of voltage rising has two different laws. Finally, this paper uses the thermodynamic equation, combined with the test results, the channel temperature changes in the discontinuous leader development stagnation stage were calculated. The results show that the leader channel temperature is still greater than 1500 K in the hundreds of microsecond time scales in the leader stagnation stage, and the subsequent leader can continue to develop on the original leader channel.

Factors Affecting the Discharge Characteristics of Rod-Rod Long Air Gaps at Different Altitudes

The impulsive discharge characteristics of the air gap is one of the main bases for the design of the external insulation of high-voltage electrical equipment; meanwhile, air gap discharge characteristics at different altitudes have been of concern for a long time. In this paper, the impact discharge tests of rod-rod gaps at different altitudes were carried out to study the influences of elements such as electrode shape, voltage polarity, altitude and temperature. It can be found that, with the increase of the gap distance, the 50% impulse voltage of the switching impulse tends to be saturated, and the discharge characteristic curve can be fitted by exponential function. The 50% impulse voltage of the lightning impact and the gap distance shows a good linear relationship which can be fitted by the linear function. This is no direct relationship with temperature and altitude. The saturation tendency of switching impulse’s 50% discharge voltage decreased with the decrease of temperature. With the increase of the altitude and the decrease of the 50% impulse discharge voltage, the polar effect of the air gap between the rod-rod and the rod-plate decreases. The results are of great reference value for Insulation configuration of electrical equipment in high altitude.

A three-dimensional downward leader model incorporating geometric and physical characteristics

In this paper, a three-dimensional, stochastic model of a lightning downward leader is proposed. The background electric field in which the downward leader propagates is used to determine the direction of propagation. The critical background electric field for the development of a downward leader was obtained by extrapolating the results of negative long air gap discharge experiments. The distribution of charge in the downward leader was modelled with the charge simulation method. The calculation took into account branching of the leader channel. By choosing a suitable value of the probability exponent, the simulation results of fractal dimension and charge in the downward leader are shown to closely match observational data of lightning. Finally, the model was used to reproduce a negative ground flash. The ground electric field calculated by the model was found to agree well with measurement results from the field.

Study on the lightning strike discharge characteristics of switchgear air gap at low air pressure condition

The usage of medium-voltage switchgear is more and more common in high altitude areas. Few relative standards about the selection of switchgear air gap in low air pressure conditions are currently available for referring. In this study, four different types of switchgear air gaps, as well as a typical rod-plane gap, are taken as the samples, and their positive and negative lightning strike discharge characteristics under low air pressure are tested in the artificial climate chamber. The effects of the electrode structure, polarity, air pressure, humidity, and gap distance on the discharge performances are analysed as well. Research results indicate that the four switchgear air gaps and typical rod-plane gap differs greatly in lightning impulse discharge performances and polarity effects. In low air pressure conditions, the lightning impulse breakdown voltage of either positive or negative polarity can be corrected by U 50 = U 50,0 × (P/P 0) n . The discharge voltage under positive lightning impulse is more easily affected by air pressure than that under negative lightning impulse. Atmosphere humidity will cause dispersiveness of air-gap lightning impulse discharge voltage, which performs to be non-linear with a gap distance lower under 400 mm. Research results are referable for the design, selection and installation of medium-voltage switchgear in high altitude areas.

Study of electric field distorted by space charges under positive lightning impulse voltage

Abstract Actually, many insulation problems are related to electric fields. And measuring electric fields is an important research topic of high-voltage engineering. In particular, the electric field distortion caused by space charge is the basis of streamer theory, and thus quantitatively measuring the Poisson electric field caused by space charge is significant to researching the mechanism of air gap discharge. In this paper, we used our photoelectric integrated sensor to measure the electric field distribution in a 1-m rod-plane gap under positive lightning impulse voltage. To verify the reliability of this quantitative measurement, we compared the measured results with calculated results from a numerical simulation. The electric-field time domain waveforms on the axis of the 1-m rod-plane out of the space charge zone were measured with various electrodes. The Poisson electric fields generated by space charge were separated from the Laplace electric field generated by applied voltages, and the amplitudes and variations were measured for various applied voltages and at various locations. This work also supplies the feasible basis for directly measuring strong electric field under high voltage.

Observation of Material Removal Process by Single Discharge in Air Gap

Electrical discharge machining (EDM) can be used to perform high-precision machining. However, the material removal rate is low. One of the reasons for the low material removal rate is that only a small part of the melted material is removed even when the material is melted by electrical discharge. From this viewpoint, understanding the process by which the material is removed in EDM is extremely important. In this study, the state of a workpiece being processed by a single discharge was observed using a high-speed camera equipped with a high-magnification lens. A machine condition was selected, in which a long pulse duration by the needle electrode can be found in air gap, to enable an easy observation of the material behavior. An observation system that employs a bandpass filter, which allowed only a specific wavelength to pass through, was constructed using a laser beam as illumination. The workpiece behavior during discharge could be observed without the influence of plasma emission. Several explosions intermittently occurred during a single discharge duration. Consequently, several small craters superposed on the surface were observed.

Study on the streamer inception characteristics under positive lightning impulse voltage

The streamer is the main process in an air gap discharge, and the inception characteristics of streamers have been widely applied in engineering. Streamer inception characteristics under DC voltage have been studied by many researchers, but the inception characteristics under impulse voltage, and particularly under lightning impulse voltage with a high voltage rise rate have rarely been studied. A measurement system based on integrated optoelectronic technology has been proposed in this paper, and the streamer inception characteristics in a 1-m-long rod-plane air gap that was energized by a positive lightning impulse voltage have been researched. We have also measured the streamer inception electric field using electrodes with different radii of curvature and different voltage rise rates. As a result, a modified empirical criterion for the streamer inception electric field that considers the voltage rise rate has been proposed, and the wide applicability of this criterion has been proved. Based on the streamer inception time-lag obtained, we determined that the field distribution obeys a Rayleigh distribution, which explains the change law of the streamer inception time-lag. The characteristic parameter of the Rayleigh distribution lies in the range from 0.6 to 2.5 when the radius of curvature of the electrode head is in the range from 0.5 cm to 2.5 cm and the voltage rise rate ranges from 80 kV/μs to 240kV/μs under positive lightning impulse voltage.

Characteristics of long-gap AC streamer discharges under low pressure conditions

The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of long-gap discharge in a high-altitude area. This paper describes laboratory investigations of streamer discharge under alternating current (AC) voltage in a low pressure test platform for a 60 cm rod–plane gap at 30 kPa, and analyzes the characteristics of streamer generation and propagation. The results show that the partial streamer and breakdown streamer all occur in the positive half-cycle of AC voltage near the peak voltage at 30 kPa. The partial streamer could cause the distortion of current and voltage waveform, and it appears as the branching characteristic at the initial stage. With the extension of the streamer, the branching and tortuosity phenomena become gradually obvious, but the branching is suppressed when the streamer crosses the gap. The low-pressure condition has little influence on the tortuosity length and the tortuosity number of the streamer, but affect the diameter of streamer obviously.

Gas heating dynamics during leader inception in long air gaps at atmospheric pressure

The inception of leader discharges in long air gaps at atmospheric pressure is simulated with a thermo-hydrodynamic model and a detailed kinetic scheme for N2/O2/H2O mixtures. In order to investigate the effect of humidity, the kinetic scheme includes the most important reactions with the H2O molecule and its derivatives, resulting in a scheme with 45 species and 192 chemical reactions. The heating of a thin plasma channel in front of an anode electrode during the streamer to leader transition is evaluated with a detailed 1D radial model. The analysis includes the simulation of the corresponding streamer bursts, dark periods and aborted leaders that may occur prior to the inception of a propagating leader discharge. The simulations are performed using the time-varying discharge current in two laboratory discharge events of positive polarity reported in the literature as input. Excellent agreement between the simulated and the experimental time variation of the thermal radius for a 1 m rod-plate air gap discharge event reported in the literature has been found. The role of different energy transfer and loss mechanisms prior to the inception of a stable leader is also discussed. It is found that although a small percentage of water molecules can accelerate the vibrational-translational relaxation to some extent, this effect leads to a negligible temperature increase during the streamer-to-leader transition. It is also found that the gas temperature should significantly exceed 2000 K for the transition to lead to the inception of a propagating leader. Otherwise, the strong convection loss produced by the gas expansion during the transition causes a drop in the translational temperature below 2000 K, aborting the incepted leader. Furthermore, it is shown that the assumptions used by the widely-used model of Gallimberti do not hold when evaluating the streamer-to-leader transition.

Calculation model for predicting partial-discharge inception voltage in a non-uniform air gap while considering the effect of humidity

A calculation model for predicting the inception of partial discharge (PD) in a non-uniform air gap on the basis of theories of electron avalanche and streamer phenomena is proposed. “Partial discharge inception voltages” (PDIVs), under an assumed absolute humidity of less than 20g/cm3, calculated using the proposed model were compared with experimentally measured ones and found to be almost equivalent. Furthermore, the relationship between negative ions and humidity was incorporated into the calculation model, and it was confirmed that the calculated and measured PDIVs tend to be the roughly the same under a wide humidity range under various gap length between electrodes. The proposed partial-discharge inception model based on theories mentioned above was therefore judged to be effective for predicting changes in PDIV when humidity changes.

Discharge Simulation of Typical Air Gap Considering Dynamic Boundary and Charge Accumulation

The simulation of typical air-gap discharge process is of great significance to the long air-gap discharge mechanism and UHV transmission project design. A positive fractal dynamic streamer discharge model of a rod-plane air gap was established. On the basis of the original fractal streamer model, this new model proposed an electric field calculation method based on a dynamic boundary condition, as well as a new calculation method of the time parameters based on the charge accumulation process. In the model, the electric field and the development time of the discharge step were obtained by solving the electric field distribution and charge accumulation equations. The processes of streamer starting, streamer development, charge accumulation, and discharge extinguishing were all contained in the model. Discharge process of various voltage amplitudes (235 and 595 kV), the waves of which were lighting impulses (2/50 $\mu \text{s}$ ), was simulated by this model in the 1-m rod-plane air gap. As shown in the results, for a gap under 235 kV, the length of the streamer is 200 mm, the streamer development time is $5.02~\mu \text{s}$ , the velocity is $3.98 \times 10^{{\mathrm {4}}}$ m/s, and the total charge accumulation in the streamer channel is 23.2 $\mu \text{C}$ , while for a gap under 595 kV, the gap is broken down, and the streamer development time is $9.92~\mu \text{s}$ , its velocity is $1.01 \times 10^{{\mathrm {4}}}$ m/s, increasing to $1.60 \times 10^{{\mathrm {5}}}$ m/s before discharge, and the total charge accumulation is $258.3~\mu \text{C}$ . The model matched the test results well in the dynamic processes of the length of discharge channels, the electric field waveform, and charge accumulation. This model is meaningful to research into long air-gap streamer discharge mechanisms and leader discharge processes.

Sub-nanosecond time resolved light emission study for diffuse discharges in air under steep high voltage pulses

Pin-to-plane discharges in centimetre air gaps and standard conditions of pressure and temperature are generated under very high positive nanosecond scale voltage pulses. The experimental study is based on recordings of sub-nanosecond time resolved and Abel-processed light emission profiles and their complete correlation to electrical current waveforms. The effects of the voltage pulse features (amplitude between 20 and 90 kV, rise time between 2 and 5.2 ns, and time rate between 4 and 40 kV · ns−1) and the electrode configuration (gap distance between 10 and 30 mm, pin radius between 10 and 200 µm, copper, molybdenum or tungsten pin material) are described. A three time period development can be found: a glow-like structure with monotonic light profiles during the first 1.5 ns whose size depends on time voltage rate, a shell-like structure with bimodal profiles whose duration and extension in space depends on rise time, and either diffuse or multi-channel regime for the connection to the cathode plane according to gap distance. The transition of the light from monotonic to bimodal patterns reveals the relative effects and dynamics of streamer space charge and external laplacian field. A classical 2D-fluid model for streamer propagation has been used and adapted for very high and steep voltage pulses. It shows the formation of a strong space charge (streamer) very close to the pin, but also a continuity of emission between the pin and the streamer, and electric fields higher than the critical ionization field (28 kV · cm−1 in air) almost in the whole gap and very early in the discharge propagation.

Effect of branching on spikes of positive leader current

Positive leader current spikes were observed in long air gap discharge. However, there has been no explanation on this phenomenon so far, and some leader advancement models can only produce the current without spikes. Therefore, it is necessary to figure out this phenomenon in order to deepen the understanding of leader discharge. In this paper, experiments of leader development were carried out in a 10 m rod-plane gap under positive switching impulse. The discharge current and high speed photographs of discharge process were recorded synchronously. Then the high speed photographs during the period when the leader current fluctuated were carefully analyzed. The experimental results showed that the leader current spikes were with leader branching. Finally, the effect of branching on the spikes of positive current was explained qualitatively and a hypothesis on leader branching was proposed.

Influence of surface emission processes on a fast-pulsed dielectric barrier discharge in air at atmospheric pressure

This paper presents simulations of an atmospheric pressure air discharge in a point-to-plane geometry with a dielectric layer parallel to the cathode plane. Experimentally, a discharge reignition in the air gap below the dielectrics has been observed. With a 2D fluid model, it is shown that due to the fast rise of the high voltage applied and the sharp point used, a first positive spherical discharge forms around the point. Then this discharge propagates axially and impacts the dielectrics. As the first discharge starts spreading on the upper dielectric surface, in the second air gap with a low preionization density of , the 2D fluid model predicts a rapid reignition of a positive discharge. As in experiments, the discharge reignition is much slower, a discussion on physical processes to be considered in the model to increase the reignition delay is presented. The limit case with no initial seed charges in the second air gap has been studied. First, we have calculated the time to release an electron from the cathode surface by thermionic and field emission processes for a work function eV and an amplification factor . Then a 3D Monte Carlo model has been used to follow the dynamics of formation of an avalanche starting from a single electron emitted at the cathode. Due to the high electric field in the second air gap, we have shown that in a few nanoseconds, a Gaussian cloud of seed charges is formed at a small distance from the cathode plane. This Gaussian cloud has been used as the initial condition of the 2D fluid model in the second air gap. In this case, the propagation of a double headed discharge in the second air gap has been observed and the reignition delay is in rather good agreement with experiments.

Partition of the development stage of air-gap discharge in oil-paper insulation based on wavelet packet energy entropy

Air-gap discharge in oil-paper insulation is one of the main types of partial discharge (PD) in power transformer. The discharge development stage for monitoring and diagnosis of transformer potential faults is a significant area of study. The method of wavelet packet energy entropy, which is based on different frequency bands energy distribution of PD signals at different insulation states, is provided to explore the variation characteristics of the whole PD process. In this paper, air-gap discharge model is built in the simulative transformer tank that collects PD signals based on constant voltage method. This model also utilizes wavelet packet decomposition method to partition PD signal bands obtaining signal energy distribution in each frequency band, as well as total signal energy tendency along with PD development process. Wavelet packet energy entropy, which is the new PD feature parameter describing the development process, represents the order degree of PD signals which corresponds to dielectric strength. Finally, because of the cyclic change of this method, the step points of wavelet packet entropy are taken as the way to effectively divide the PD development stage.