Point-plane Geometry Research Articles

Numerical approaches in simulating Trichel pulse characteristics in point-plane configuration

In this work, a detailed comparison is made of a few different approaches to numerical modeling of non-equilibrium gas discharge plasmas in dry ambient air at atmospheric conditions, leading to Trichel pulse discharge. Simulation models are based on a two-dimensional axisymmetric finite element discretization of point-plane geometry. The negative corona discharge and the hydrodynamic approximation for generic ionic species (electrons, positive and negative ions) are used. The models account for the drift, diffusion, and reactions of the species. They comprise continuity equations coupled to Poisson’s equation for the electric field. Three different formulations were used to specify the ionic reaction rate coefficients. In the first one, the reaction coefficients are approximated by the analytical expressions as a function of the electric field intensity. Two others extract the reaction coefficients from the solution of the Boltzmann equation as a function of the reduced electric field or the electron energy. The effect of gas flow and heating on the pulse characteristics is also investigated. The accuracy of the models has been validated by comparing them with the experimental data.

Influence of Molecule Structure on Lightning Impulse Breakdown of Ester Liquids

Ester liquids are environmentally friendly insulating oils, and they can be used as an alternative to mineral oil in transformers, even though in most countries spills of ester oils must be treated like spills of mineral oil. Furthermore, the breakdown characteristics of ester liquids are worse than those of mineral oils in heterogeneous electric fields. In this paper, we present a comprehensive experimental research on both positive and negative lightning impulse breakdown properties in point-plane geometries with gaps varying from 1 mm to 50 mm. The breakdown voltages and streamer velocities of five kinds of ester liquids, including natural ester, synthetic ester, and three kinds of single component esters have been measured. The results show that the double bonds have no effect on the breakdown voltage of ester liquids. The average streamer velocities of mono-esters are faster than that of other esters under positive polarity, and the breakdown voltages of all esters are close.

Dynamics of the Spatial Structure of a Microsecond Pulsed Barrier Discharge Initiated in Atmospheric Pressure Air in the Point–Plane Geometry at Different Polarities of Feed Voltage

The evolution of a microsecond pulsed barrier discharge initiated in atmospheric pressure air in the point–plane electrode geometry has been investigated using streak camera photorecording and shadow photography with micrometer resolution and nanosecond exposure. It has been shown that the discharge develops in series and its structure evolves both within a series and from series to series. It has been found that, irrespective of the polarity of a feed voltage pulse, the discharge develops from the point electrode, having the form of a set of many micrometer-diameter channels. At the late stage, the discharge consists of a number of glowing microstructured channels, which occupy the space between the point and barrier and deviate from the point axis, and many faint channels radially propagating over the surface of the barrier. Numerical parameters of the structured have been obtained.

Plasma and Gas-Dynamic Near-Electrode Processes in the Initial Phase of a Microstructured Spark Discharge in Air

The results of studies of the dynamics of near-electrode processes in the initial phase of a spark discharge in atmospheric-pressure air in the point-plane geometry are presented. After breakdown, regions of about 50 μm or more in size with an increased electron concentration of (3–8) × 1019 cm–3 near the surface of the flat cathode have been observed. With allowance for the experimental data on erosion effect of the discharge on the surface of a flat electrode and the associated gas-dynamic effects, a physical model of near-electrode processes has been proposed, based on which the gas-dynamic parameters are estimated.

Comparison of dielectric properties of olive oil, mineral oil, and other natural and synthetic ester liquids under AC and lightning impulse stresses

This paper presents experimental results of a comparative study of the characteristics of streamers propagating in olive oil and breakdown voltage with mineral oil, rapeseed oil (natural ester) and tetra-ester and methyl oleate (synthetic ester liquids). The comparison focuses more particularly on the pattern, stopping length, associated current and electrical charge of streamers propagating in a point-plane electrodes geometry under (1.2/50 μs) lightning impulse (LI) voltage as well as breakdown voltages under AC and LI voltages. A statistical analysis of breakdown voltages is also achieved. It is shown that the streamer characteristics (streamers shape, stopping length, associated current and electrical charge) of olive oil in small gaps are comparable to those of mineral oil. Breakdown voltages under AC and LI voltages are found to be higher in olive oil than in mineral oil. Consequently, olive oil can constitute a potential substitute for mineral oil in in high voltage power transformers.

Prebreakdown phenomena in liquids: propagation ‘modes’ and basic physical properties

Prebreakdown phenomena in liquids (usually called ‘streamers’) were characterized experimentally in a wide range of experimental conditions, liquid nature and additives. The description of these phenomena leads to a complex and frequently confused situation, since their properties widely vary when parameters such as liquid nature or voltage are changed. In this review paper, we try to obtain a more comprehensive presentation of pre-breakdown phenomena in liquids by considering different propagation ‘modes’, which can be recognized in a wide range of liquid nature and experimental conditions. On the basis of experimental results either published previously or original, we will review the main parameters and features able to better characterize these modes, and show their general interest. ‘Positive’ streamers observed in point-plane geometry in non-polar liquids are mainly considered here. Some elements allowing a comparison with water, characterized by a much higher permittivity and conductivity are given. The text is concluded by considerations concerning the different physical processes involved in modes.

Influence of fast rise voltage and pressure on partial discharges in liquid embedded power electronics

Initiation of partial discharges at the highly stressed regions of an Insulated Gate Bipolar Transistor (IGBT) can lead to degradation of the insulation and eventual total breakdown of the system. In this work, an experimental setup has been designed for the study of partial discharges (PDs) under different voltage waveforms. PD behavior of IGBT insulation was investigated using conventional and optical techniques. Influence of pressure and voltage wave shape is documented. The test environment was first characterized with point-plane geometry under sinusoidal and slow rise square voltage of up to 20 kVpeak and fast rise square voltage of up to +50 kV. The measured electrical and optical PDs showed good correlation, revealing that optical PDs can be relied on for the characterization of PD phenomena. High slew rate of the square voltage reduced the inception voltage and increased magnitude. The PD pattern from the trench shows the existence of space charges. The PDs which occurred within the triple point region are most likely attracted along the board interface and become surface discharges. Pressure suppresses the initiation and propagation of the discharge.

Light emission spectra of molecules in negative and positive back discharges in nitrogen with carbon dioxide mixture at atmospheric pressure

Results of spectroscopic investigations and current-voltage characteristics of back discharge generated in point-plane electrode geometry with plate covered fly ash layer in a mixture of N2 + CO2 at atmospheric pressure, for positive and negative polarity of the discharge electrode are presented in this paper. Point-plane electrode configuration was chosen in these studies in order to simulate the physical processes occurring in electrostatic precipitator. Three forms of back discharge for both polarities were investigated: glow, streamers and low-current back-arc. Diatomic reactions and dissociation products of N2 and CO2 (OH, NO, CN), atoms from fly ash layer (N, Ti, Na), free radicals, molecules or ions, which have unpaired valence electrons, and other active species, e.g., N2 (in C,B,A-state), N 2 + (B) were identified in the discharges by the method of optical emission spectroscopy (OES). The measurements shown that atomic and molecular optical emission spectral lines from back discharge depend on the forms of discharge and the discharge current. In normal electrical discharges, the emission spectra are dominated by gaseous components, but in the case of back discharge, atomic lines belonging to chemical compounds of fly ash were also recorded and identified.

Quasi-stationary numerical model of the dielectric barrier discharge

Abstract The aim of this paper is to present a numerical algorithm for investigating the dynamic characteristics of a simple model of the dielectric barrier discharge in air. The proposed algorithm is based on the finite element method and is capable of determining the distribution of electric field intensity and space charge density in the point-plane geometry, when the plane is covered with a thin dielectric layer and the needle is energized with a sinusoidal voltage. The simulation results show the behavior of corona current and space charge density for a few values of the input voltage and frequency. The aim of this parametric study is to investigate the effect of voltage, and frequency on the time variation of the discharge.

Fractal analysis of lightning impulse surface discharges propagating over pressboard immersed in mineral and vegetable oils

This paper is aimed at the experimental study of discharges propagating over pressboard immersed in mineral and vegetable oils and the analysis and comparison of the fractal dimension of these discharges. The shape, stopping length (i.e., the maximum extension of creeping discharge) L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> , current and electrical charge of discharges are investigated versus the thickness (e) of pressboard and the type of oil in a point-plane electrode geometry stressed by a standard lightning impulse voltage. It is shown that L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> decreases when e increases and it is longer when the point is positive than with a negative point. The smaller e, the more branched the discharge structures are, indicating the important role of the electric field and capacitive effect in the propagation mechanism. L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> is generally greater of 60% in vegetable oils than in mineral oil when the point is positive and it is of 40% greater when the point is negative. By using the box counting method, we show that the observed discharge patterns present a fractal dimension D which depends on e and the kind of oil. D decreases when e increases. The value of the density of branches increases when the e decreases. D is generally higher in mineral oil than in vegetable oil. This dependency on the thickness of pressboard and the nature of oil reveals the existence of a relation between the fractal dimension and the physicochemical parameters of interface.

The Influence of Nanofilled Aluminum Oxide on Electrical Tree Growth Behavior under High Voltage Non-Uniform Field Stress

This paper proposed a study on solid insulator that can withstands the high electrical field stress. In order to clarify how the solid insulator can withstands the high electrical field stress, an Araldite epoxy resin with specific amount of nanoAluminum Oxide (Al2O3) filler have been chosen for the experiments. The influence of nanoAluminum Oxide (Al2O3) filler on the electrical tree propagation has been investigated. The electrodes of point-plane geometry were impregnated into the sample and tested under high voltage AC. The level of applied voltage was 12 kV (AC), 50 Hz. The results of experiment showed that the specific ratio of nanoAl2O3 (0.1 wt% to 0.3 wt%) filler can inhibit the growth of the electrical tree as well as it can withstands the high electrical field stress.

Partial discharges in ceramic substrates embedded in liquids and gels

In Insulated gate Bipolar Transistor (IGBT) modules, ceramic substrates are used as insulation between semiconductor chips and the ground potential. The substrate, semiconductor chips and connections are covered with silicone gel to prevent partial discharges (PDs) from occurring. In this work, partial discharges are investigated on laboratory samples made of metalized substrates embedded in silicone gel and various liquids. Substrates are made of aluminum nitride (AlN), alumina (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ), or glass/epoxy composite. The objective is to determine the actual origin of PDs, by changing the nature of encapsulating material and substrate. The various liquids used are first characterized using a point-plane geometry. Experiments show that a rather large discrepancy on the PD behavior can be due to the ceramic substrate itself. "Good" and "bad" substrates can be identified on the basis of PD measurements. The influence of the sample preparation (degassing procedure) is also evidenced. The main conclusion obtained in this paper shows that partial discharges do not occur within the encapsulating material, contrary to most previous hypotheses, but within pores existing in sintered ceramic materials. Changing the substrate nature has a large influence on PD behavior, whereas a modification of the encapsulating material has almost no effect.

Studies of corona and back discharges in carbon dioxide

Results of spectroscopic investigations and current-voltage characteristics of corona and back discharges generated in point-plane electrode geometry in CO2 at atmospheric pressure for positive and negative polarity of the discharge electrode are presented in the paper. Three forms of back discharge, for both polarities, were investigated: glow, streamer and low-current back-arc. To generate the back-discharges for the conditions similar to electrostatic precipitator, the plate electrode was covered with fly ash layer. In order to characterize back discharge processes, the emission spectra were measured and compared with those obtained for normal discharge, generated in the same electrode configuration but without the fly ash layer on the plate electrode. The measurements have shown that optical emission spectral lines of atoms and molecules, excited or ionised in back discharge, depend on the forms of the discharge, the discharge current, and are different in the zones close to needle electrode and fly ash layer. From the comparison of spectral lines of back and normal discharges, an effect of fly ash layer on discharge characteristics and morphology has been determined. In normal corona, the emission spectra are mainly predetermined by the working gas components, but in the case of back discharge, the atomic and molecular lines, resulting from chemical composition of fly ash, are also identified. Differences in the spectra of back discharge for positive and negative polarities of the needle electrode have been explained by considering the kind of ions generated in the crater in fly ash layer. For back arc, the emission of spectral lines of atoms and molecules from fly ash layer can be recorded in the crater zone, but in the needle zone, only the emission lines of CO2 and its decomposition products (CO and C2) can be noticed. The studies of back discharge in CO2, as one of the main components of flue gases, were undertaken because this type of discharge, after unwanted inception, decreases the energy and collection efficiencies of electrostatic precipitator. The second reason behind these studies is that CO2 is the main component of flue gas leaving oxyfuel boiler that re-circulates in the combustion-precipitation cycle. It was shown that discharges in CO2 lead to contamination of discharge electrode with carbonaceous products that can cause severe maintenance problems of electrostatic precipitator. The recognition of the characteristics of electrostatic precipitator operating in the oxyfuel system is, therefore, of crucial importance for exhaust gas cleaning in modern combustion systems.

Analytical model of a corona discharge from a conical electrode under saturation

Exact partial solutions are found for the electric field distribution in the outer region of a stationary unipolar corona discharge from an ideal conical needle in the space-charge-limited current mode with allowance for the electric field dependence of the ion mobility. It is assumed that only the very tip of the cone is responsible for the discharge, i.e., that the ionization zone is a point. The solutions are obtained by joining the spherically symmetric potential distribution in the drift space and the self-similar potential distribution in the space-charge-free region. Such solutions are outside the framework of the conventional Deutsch approximation, according to which the space charge insignificantly influences the shape of equipotential surfaces and electric lines of force. The dependence is derived of the corona discharge saturation current on the apex angle of the conical electrode and applied potential difference. A simple analytical model is suggested that describes drift in the point-plane electrode geometry under saturation as a superposition of two exact solutions for the field potential. In terms of this model, the angular distribution of the current density over the massive plane electrode is derived, which agrees well with Warburg’s empirical law.

Electric field and charge distribution in back discharge in point-plane geometry

The paper presents experimental and theoretical investigations of electric field and charge distribution on the fly-ash layer covering the plate electrode in the back discharge in point-plane geometry. Two kinds of dielectric layer were used in the experiments: highly resistive fly-ash from an electrostatic precipitator and a dielectric plate (PET) with a small pinhole.

Simulating mode transitions during breakdown in liquids

The primary new information from this investigation is the simulation of a transition from one streamer mode to another during propagation. The transition is characteristic of experimental data, but has not heretofore been described in a model. This demonstrates that the conceptual framework describing prebreakdown streamers in liquids as stochastic growth of a branching fractal tree in point-plane geometry is capable of simulating a wide range of streamer propagation behaviors in insulating liquids. The work also improved the approach in the model for scaling the potential at nearby grid points and explored some of the influences of computational choices, specifically needle length and grid size, on the predicted results.

Electron mobility in liquid and supercritical helium measured using corona discharges: a new semi-empirical model for cavity formation

Electron mobilities in supercritical and liquid helium were investigated as a function of the density. The mobilities were derived from I(V) curves measured in a high-pressure cryogenic cell using a corona discharge in point-plane electrode geometry for charge generation. The presented data spans a wide pressure and temperature range due to the versatility of our experimental set-up. Where data from previous investigations is available for comparison, very good agreement is found. We present a semi-empirical model to calculate electron mobilities both in the liquid and supercritical phase. This model requires the electron-helium scattering length and thermodynamic state equations as the only input and circumvents any need to consider surface tension. Our semi-empirical model reproduces experimental data very well, in particular towards lower densities where transitions from localised to delocalised electron states were observed.

Investigating the impact of harmonics on the breakdown of epoxy resin through electrical tree growth

Many studies have investigated the influence of harmonics on insulation life. A limited number of these studies address electrical tree growth characteristics while even fewer employ harmonics maintaining a constant peak voltage. This experimental study sought to quantitatively identify any distinguishing features of electrical tree growth characteristics as a consequence of harmonic content. The total harmonic distortion (THD) and waveshape (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) indices were varied to a maximum of 40% and 1.6 respectively. Electrical trees were developed in point-plane geometry using 2 μm tip radius hypodermic needles and a 2 mm gap in epoxy resin (LY/HY 5052) samples. Upon electrical tree initiation, 50 Hz composite waveforms incorporating harmonic content were applied as the excitation waveform at a constant voltage of 14.4 kV peak for 2.5 hours. No change in the electrical tree growth characteristics was identified using the width/length ratio. Electrical tree growth resulting from low power quality reduced operational insulation life and decreased the Weibull shape parameter β with the 7 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> harmonic displaying particular characteristics worthy of further investigation.

New experimental system for the study of the effect of temperature and liquid to solid transition on streamers in dielectric liquids: application to cyclohexane

An experimental setup for dielectric testing of insulating liquids in the temperature range from -60 °C to +250 °C has been built. The system uses point-plane geometry, a sensitive differential charge measurement technique, a computer controlled temperature regulator, a photomultiplier and a pulse generator with a fast rise time. This paper presents a detailed overview of the functionality of the system and results obtained in cyclohexane under positive and negative polarity below and above freezing point. Pre-inception current, inception probabilities, streamer charge and emitted light from streamers are reported. The results found in cyclohexane at room temperature compares well to results obtained in other similar setups. A temperature dependence on the injected streamer charge for liquid cyclohexane under positive polarity was observed. No polarity differences were observed for any of the measured quantities in frozen cyclohexane.

Investigating the influence of the lubricant coating on hypodermic needles on electrical tree characteristics in epoxy resin

Many electrical treeing studies have employed the use of hypodermic needles in the point-plane geometry configuration. These hypodermic needles developed for medical usage are coated with a transparent lubricant. Limited literature exists on sample preparation techniques addressing the removal of this lubricant coating, which affects adhesion of the epoxy resin to the hypodermic needle. In this experimental study, the impact of the lubricant coating on electrical tree growth and associated partial discharge activity was investigated. The findings confirm that the lubricant coating inhibits electrical tree growth resulting in limited spread of the electrical tree while yielding the occurrence of 'rabbit-ear' partial discharge patterns. This study also reports pronounced visible gaseous activity upon electrical tree initiation in the presence of the lubricant coating. Electrical trees were developed using point-plane samples of LY/HY 5052 epoxy resin with a gap spacing of 2 ± 0.5 mm. All electrical trees were 50 Hz initiated at either 8.4 or 14.4 kV peak. After electrical tree initiation, 50 Hz composite waveforms incorporating harmonic content were applied as the excitation waveforms.