Air Gap Breakdown Research Articles

Theoretical and experimental studies of air gap breakdown triggered by free spherical conducting particles in DC uniform field

In this study, mechanical analysis of the air-gap breakdown with free spherical metal particle is conducted in a DC uniform field to address the problem of metal-particle pollution in DC gas insulation. The criterion and spacing of micro-discharge between the particle and electrode is calculated based on the analysis of electric-field distortion with a lifting particle by using the streamer theory. The gas-breakdown equation that considers the effects of free particle is modified with the new concept of equivalent radius, and the different breakdowns triggered by various sizes of aluminum or copper particles are analyzed in ramp voltages. Different states of particle lifting, direct breakdown, or moving breakdown are also considered. The concept of risky radius is also proposed. The breakdown process is observed using a high-speed camera, and the model is verified through experiments. The breakdown voltage of the moving particle is lower than the still particle's. Three breakdown regimes in the DC uniform field triggered by free metal particles were defined, among which the lifting-voltage breakdown occurs only when the particle is close to the upper electrode for the first time. The breakdown voltage showed a minimum value when the particle moved nearer to the risky radius. The proposed research provides fundamental basis in evaluating the dangerous level of free conducting particles within DC GIL as to improve the insulation design.

Characteristics of long air gap discharge current subjected to switching impulse

Measuring the pre-breakdown current of long sparks in air is important for investigating the discharge mechanism. Since the breakdown of long air gaps is conducted by a series of streamer-leader processes, the corresponding current signals cover a bandwidth of 0 to more than 20 MHz. Measurement accuracy of the current from the high voltage side is affected by the displacement current and impulse electromagnetic interference. In this paper, a coaxial current sensor with a DC bandwidth of 74.45 MHz is developed. A displacement current-restrained electrode structure is proposed to reduce the equivalent capacitance between the current sensor and the ground over 30 times. Combined with the digital optical fiber synchronous acquisition unit, a current measurement system for long air gap discharge is established. For the purpose of the UHV system's external insulation optimization design, the discharge current waveform of a 6 m rod-plane air gap under positive switching impulse voltage with 250 μs and 1000 μs time to crest is obtained. Discharge images and stressed voltage are combined to analyze the continuous feature of a current waveform under critical time to crest impulse and discontinuous feature under long front duration impulse. For the purposes of a lightning protection study, the current waveform of a 10 m rod-plane air gap is subjected to negative switching impulse. Finally, the pulse characteristics of the current corresponding to the single channel and branching stepped negative leader are discussed.

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.

Formation of extended directional breakdown channels produced by a copper wire exploding in the atmosphere

Experimental data for switching initiated by the electrical breakdown of air gaps up to 1.9 m long with an arbitrary geometry that are produced by an exploding copper wire 90 μm in diameter are presented. At an initial voltage of 11 kV, the stored energy equals 100–2100 J. Two channel formation conditions are possible: explosion of a wire without electrical breakdown and electrical breakdown in a channel produced by an exploding wire with a delay (current pause) no longer than 250 μs. Current and voltage waveforms across the discharge gap, as well as the resistivity values, under the electrical breakdown conditions are shown. Mechanisms and conditions for streamer initiation at a mean electric field strength in the discharge gap of 5.3–17.0 kV/m are discussed. The geometrical dimensions of plasma objects in the forming channel, the run of the electrical current under breakdown, and the formation mechanism of wire explosion products are found from color microphotographs. The formation mechanism of large aerosols in the form of tiny spherical copper and copper oxide (CuO, Cu2O) particles under wire explosion conditions is discussed.

New Features of the Generation of Runaway Electrons in Nanosecond Discharges in Different Gases

This paper presents new experimental data on the generation of runaway electron beams and X-rays in repetitively pulsed discharges in helium and air with an inhomogeneous electric field. Both negative and positive voltage pulses of duration 1-5 ns and amplitude in the incident wave up to 18 kV were applied to a “tube-to-plane” gap at a pulse repetition frequency of up to 1 kHz. It is shown that at a helium pressure of several and tens of torrs, whatever the polarity of the main voltage pulse, an electron beam is generated on arrival of reflected negative voltage pulses in the gap. X-rays were detected over a wide pressure range of helium and air, including atmospheric pressure. The breakdown of atmospheric-pressure air gaps with a special cathode design at a rate of voltage rise of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> V/s was studied with subnanosecond and picosecond time resolution. At a voltage amplitude higher than 100 kV, a fast electron beam in atmospheric-pressure air was detected in the space downstream of the cathode, which was made of thin wires arranged parallel to a thin flat foil. The current of the fast electron beam downstream of the cathode depended strongly on the anode material.

Application of dynamic displacement current for diagnostics of subnanosecond breakdowns in an inhomogeneous electric field

The breakdown of different air gaps at high overvoltages in an inhomogeneous electric field was investigated with a time resolution of up to 100 ps. Dynamic displacement current was used for diagnostics of ionization processes between the ionization wave front and a plane anode. It is demonstrated that during the generation of a supershort avalanche electron beam (SAEB) with amplitudes of ~10 A and more, conductivity in the air gaps at the breakdown stage is ensured by the ionization wave, whose front propagates from the electrode of small curvature radius, and by the dynamic displacement current between the ionization wave front and the plane electrode. The amplitude of the dynamic displacement current measured by a current shunt is 100 times greater than the SAEB. It is shown that with small gaps and with a large cathode diameter, the amplitude of the dynamic displacement current during a subnanosecond rise time of applied pulse voltage can be higher than 4 kA.

Using Subgridding FDTD Method to Incorporate Air Gap Breakdown in the Analysis of Lightning Induced Voltage

A subgridding formulation of Agrawal coupling equations of transmission line with branch and distributed electromagnetic excitation is proposed. This algorithm enables us to solve the transmission line system using different temporal and spatial step on different branches. The method is used to calculate the lightning induced over-voltage on the transmission line installed with air gap component branch. The air gap is modeled by disruptive effect method with small time step, while the transmission line is simulated using large time step. With the help of this method, the simulation of transmission line with air gap component can avoid long time computation. As an example, the protection effectiveness of surge arrester with series air gap is studied using the proposed method.

Soft X-ray generation and its role in breakdown of air gap at elevated pressures

Fast runaway electrons, which appear during the electric breakdown in an air-filled discharge gap under the action of a short-front voltage pulse, generate 525-eV photons from the K-shell of oxygen. This soft X-ray emission effectively generates new electrons in the region of enhanced field. The proposed mechanism may explain the fast streamer propagation and the formation of a diffuse discharge at atmospheric pressure.

Impulse breakdown of small air gap in electric field Part I: Influence of barrier position

The influence of barrier position on breakdown voltage in air dielectric has been investigated. Needle and Cone positive point electrodes were used and the effects of electrode curvature on barrier position for maximum breakdown voltage were compared, with air gap for the point to plane electrode fixed at 10 cm for all the tests. The breakdown probability was found for barrier position, and at every barrier distance, the breakdown voltage differed. The position for maximum breakdown for needle electrode was in the range of 28 - 30 % from the poi-nt electrode. The position for optimum breakdown for cone electrode was 50 % from the point electrode, and the deviation originated from the electrode configuration. The effect of defective barrier was tested by introducing holes of different diameter at the barrier centre, and the maximum breakdown voltage was affected depending upon the diameter of the hole on the barrier.

Physical mechanisms for reduction of the breakdown voltage in the circuit of a rod lightning protector with an opening microswitch

The effect of air gap breakdown voltage reduction in the circuit with an opening microswitch is substantiated from the physical point of view. This effect can be used to increase the efficiency of lightning protection system with a rod lightning protector. The processes which take place in the electric circuit of a lightning protector with a microswitch during a voltage breakdown are investigated. Openings of the microswitch are shown to lead to resonance overvoltages in the dc circuit and, as a result, efficient reduction in the breakdown voltage in a lightning protector-thundercloud air gap.

Estimating the energy of electric breakdown in air gap between electrolyte and metal electrode

The influence of induced electric charge (localized on the surface of a suspended copper rod) on the formation of a protrusion (Taylor cone) on the inducing liquid (aqueous solution) surface is considered. At an applied voltage of U ≤ 12 kV, the protrusion height in the interval of pre-breakdown voltages (U < U P) is limited by the electric field strength. At U > U P, the growth of protrusion is terminated by an electric discharge, which drives the liquid to oscillate in a broad range of applied voltages U at almost constant multiple frequencies f = f 0 n, which are resonantly switched at certain fixed U values. By measuring the amount of evaporated liquid, the energy (27.8 × 10−3 J) and current (64.9 A) of single discharge were evaluated and the electric capacitance (7.6 × 10−10 F) of a system comprising the water surface and suspended copper electrode was estimated. Serial connection of an additional capacitor (100 μF) to the copper electrode with induced electric charge leads to a threefold increase in these parameters.

Analysis of Space Charge Generating Devices for Lightning Protection: Performance in Slow Varying Fields

This paper starts with a review of anode corona modes with particularly streamer-free glow known as ultra-corona, with the view of exploring its application in lightning protection. Due to the inherent intensification of the ambient electric field atop a tall structure, strict conditions have to be imposed on the streamer-free space charge generation and the stability of that mode of corona when exposed to rapid field variations due to remote lightning. A novel ultra-corona electrode satisfying the above conditions is introduced. High voltage test results on such electrodes are presented including corona current, charge and laboratory air gap breakdown voltage. By applying dimensional analysis, a generalized formula for corona currents from such a device atop a tall structure is presented which includes the effects of the ambient ground field, structure height, ambient electric field at the top of the structure, electrode dimensions, and wind speed.

Power-Frequency Voltage Withstand Characteristics of Insulations of Substation Secondary Systems

The insulation of secondary system in substation, including secondary cables and devices, have to endure the high ac power-frequency voltage during the short-circuit fault. The ac withstand characteristics of the secondary cables and devices are experimentally investigated and discussed in this paper. The insulation breakdown mechanism of relays can be classified into three modes: air gap breakdown, creeping discharge, and solid insulation breakdown, that of the microcomputer protection device is solid state component damage. The ac power-frequency breakdown voltages of the microcomputer protection device, the small-size electromagnetic and solid-state relays all are as low as about 2 kV. The statistic method to obtain the withstand voltage of long communication cable from experimental data of short cables is proposed. The minimum 3-s withstand voltages of secondary system are 6.5 kV and 2 kV, respectively, this should be considered in substation design. What's more, the volt-second characteristics of the power-frequency withstand voltage of the secondary cables and devices are analyzed and compared in detail, which would be important in the selection of secondary cables and devices. At last, the method how to guarantee high ac breakdown voltage of electromagnetic and solid-state relays and microcomputer protection device is suggested.

The influence of the sand-dust environment on air-gap breakdown discharge characteristics of the plate-to-plate electrode

The experiments of plane-plane gap discharge was carried out in an environment of artificial sandstorm. By comparing and analyzing the differences in gap breakdown voltage between the sand & dust environment and clean air, some problems were investigated, such as effects of wind speed and particle concentration on the breakdown voltage, differences of gap discharge characteristics between the dust & sand medium and the clean air medium. The results showed that compared with the clean air environment, the dust & sand environment had a decreased gap breakdown voltage. The longer the gap distance, the greater the voltage drop; the breakdown voltage decreased with the increase of particle concentration in flow. With the increase of wind speed, the breakdown voltage decreased at the beginning and rose afterwards. The results of the paper may helpful for further research regarding the unidentified flashover and external insulation characteristics of the HV power grid in the dust & sand environment.

A Numerical Model of Streamer Breakdown in Air Gap at Atmospheric Pressure

This article presents a numerical model of streamer breakdown of sphere gap in air at atmospheric pressure. The streamer propagation is described by a system of equations, including charge continuity equation and Poisson equation. In addition, the photoionization effect is simplified and integrated into the model. The numerical results was performed and compared with the measurement of breakdown current. From the results, the increase of discharge current before the gap breakdown could be divided into three stages according to the development of each type of streamer.We propose that the sphere gap will breakdown when positive streamer reaches the cathode and the rate of increase of discharge current is about 1.4A/ns.

Initiation and propagation of the negative leader in transformer oil under impulse voltage

The complex experimental studies of the discharge mechanism in transformer oil under negative polarity (NP) of the lightning impulse were carried out using alternative diagnostic methods. Novel data concerning propagation mechanism and parameters of the predischarge channels were obtained. The results indicate that oil breakdown both under negative and positive polarities are similar in a number of properties to the breakdown in the long air gaps.

Modeling of Proximity Effect on Positive Leader Inception and Breakdown of Long Air Gaps

The paper presents the first effort to mathematically model proximity effects on continuous leader inception and breakdown of long air gaps under critical switching impulses. Three aspects of proximity with considerable practical importance are addressed. These include wall proximity in high voltage switching impulse tests, effects of protrusions and surface roughness on switching impulses performance of large electrodes and finally determination of critical ground fields for upward leader inception of a vertical rod under an energized plane electrode. The model results provide quantitative tools to assess proximity effects as well as means to keep such effects within acceptable limits. Whenever possible the model findings are compared with experimental results and internationally recommended practice and with view of the complexity of the problems, the agreement is quite satisfactory.

Numerical simulation of long laboratory sparks generated by positive switching impulses

A numerical methodology using two different leader channel criteria has been implemented. The methodology is based on Bondiou and Gallimberti's proposition [A. Bondiou, I. Gallimberti, Theoretical modelling of the development of the positive spark in long spark, J. Phys. D: Appl. Phys. 27 (1994) 1252–1266]. The leader channel criteria used are Rizk engineering criterion [Rizk, A model for switching impulse leader inception and breakdown of long air gaps, IEEE Trans. Power Deliv., 4(1) (1989)] and Local thermodynamic – L.T.E. – physical concept [I. Gallimberti, The mechanism of the long spark formation, Colloque C7, J. Phys. (supplement au nro 7, Tome 40) (July 1979) C7–193]. The methodology was tested in three different cases; a deterministic case, a statistical variation and a typical constant level test. Deterministic calculation considered corona inception using stabilization corona electric field criterion of Gallimberti [I. Gallimberti, The mechanism of the long spark formation, Colloque C7, J. Phys. (supplement au nro 7, Tome 40) (July 1979) C7–193] and the leader moving as segments. The statistical simulation has two different statistical delays, one at inception and the other due to the tortuous characteristics of the leader channel. The constant level test consists of 200 positive switching impulses with the same characteristics such as maximum applied voltage, time to crest and time to fall. Time to breakdown and breakdown voltage were found based on the results obtained from the constant level test characteristics. All the numerical results presented are based on experimental conditions reported in [Les Renardières Group, Research on long gap discharges at Les Renardières, Electra N 35 (1973)] from the world class research group namely Les Renardieres Group.

Comparison of air gap breakdown and substrate injection as mechanisms to induce dielectric charging in microelectromechanical switches

Experiments show that air breakdown and substrate carrier conduction can be responsible of the dielectric charging of microelectromechanical devices after electrical stress. Test conducted under vacuum allows us to isolate the two mechanisms. It is also shown that the relative importance of the two mechanisms depends on the dielectric nature and technology.

Modeling of DC Flashover on Ice-Covered HV Insulators Based on Dynamic Electric Field Analysis

In this paper, a model based on dynamic electric field analysis has been developed to predict the flashover voltage of the ice-covered HV insulators, under dc voltage. The potential and electric field calculation models before and after air gap breakdown are built respectively based on finite element method (FEM). The arc initiation process is determined based on the model before air gap breakdown. The critical applied voltage and leakage current to maintain an arc with certain length are obtained based on the electric field calculation model after air gap breakdown and the U-I characteristic of the arc. Moreover, the improved Hampton criterion has been employed to determine the critical flashover of the ice-covered insulator. The results obtained from the dynamic electric field analysis model have been compared with other mathematical and experimental results of other researchers and got a great agreement.