Air Corona Research Articles

Corona discharge from electrospinning jet of poly(ethylene oxide) solution

Corona discharges from electrospinning jets were observed and photographed at the tip of the Taylor cone, and in a cylindrical region around the jet, a few millimeters below the tip. The corona discharge was also faintly visible to a dark adapted eye. At the position at which the cylindrical corona discharge became apparent, typical conditions were a jet diameter of 30μm, an applied potential of 12kV, and a calculated radial electric field of 400kV/cm, The calculated electric field required to create a corona in air around a metal wire of the same diameter, calculated from Peek's empirical formula, was only about 200kV/cm. The cross sectional shape of some segments of the electrospun fibers had two or three lobes. The lobes often separated, and formed smaller fibers.

Specific features of radiation from a negative air corona operating in the Trichel-pulse mode

Experimental studies of spatiotemporal characteristics of radiation from a negative corona operating in the Trichel-pulse mode in the point-to-sphere electrode geometry have revealed two emission zones. In addition to the well-known glow near the point electrode, there is also an anode glow, whose intensity depends substantially on the shape of the anode. It is found that the anode glow is delayed with respect to the beginning of the Trichel pulse by a time that depends on the gap length and gap voltage. The emission spectrum of the anode glow in the wavelength range 300–400 nm is identified as the spectrum of the second positive system of nitrogen (the C 3Π u -B 3Π g transition).

Peculiarities of the corona discharge in air

The results of a comprehensive experimental study of the positive and negative corona discharges and their accompanying processes are reported. It is shown that specific features of the corona in air under atmospheric pressure are related to the electric wind.

DC Corona Electric Discharges for Air Pollution Control, 2—Ionic Intermediates and Mechanisms of Hydrocarbon Processing

AbstractA mechanistic study is reported on i‐octane and hexane processing with +DC and −DC corona in air at room temperature and pressure. Current/voltage profiles are matched with the ion analysis data obtained by APCI mass spectrometry. With a −DC corona, the hydrocarbons do not modify the negative ion population with respect to uncontaminated air. In contrast, with a +DC corona many hydrocarbon‐derived positive ions form. O(3P) and •OH were also investigated using chemical probes (ozone formation and CO oxidation, respectively). The results, combined with efficiency and product data, suggest that with −DC corona radical initiation steps occur, whereas with +DC corona ionic reactions prevail.magnified image

Plasma Process. Polym. 2/2008

Cover: The picture shows current/voltage profiles for DC coronas in pure air (green) and in air doped with i‐octane (blue) or hexane (red). Hydrocarbons have no effect on –DC but strongly reduce +DC corona current. Ion analysis provides a rationale for these macroscopic observations: Numerous hydrocarbon‐derived positive ions but no negative ions are detected in the plasma. Further details can be found in the article by E. Marotta, A. Callea, X. Ren, M. Rea, and C. Paradisi* on page 146.

Erosion of a copper cathode in a negative corona discharge

The pulsed-periodic regime of a negative corona (Trichel pulses) in atmospheric-pressure air, which leads to explosion emission mechanisms (ecton generation) of pointed cathode erosion, is investigated. The jet erosion process at the copper cathode is discovered, and micrometer dendritelike structures formed by erosion products returning to the cathode are detected.

Pulse Analysis Algorithms for Distinguishing Discharge Sources using Different Types of Sensors

It is very important to discriminate noises such as air corona in measuring on-site partial discharges (PD). Therefore, in order to investigate the possibility of separating PD and noises through the pulse wave shape analysis (PWA), pulse wave shapes measured by a resistive sensor and a high frequency current transformer (HFCT) have been analyzed and PWA were performed. For the purpose, the high frequency partial discharge (HFPD) detection and PWA system has been developed. Also void discharges and air corona were adopted as the artificial defect and noises, respectively. For the PWA algorithm, two kinds of methods such as TF-KK (Kurtosis of time domain and frequency domain) and MMR-S (the ratio of max/min value and the summation of the captured data) were compared. As a result, the developed system in this study showed good ability of distinguishing small void discharges from very large noises such as big air corona and ground floating discharges, which implies that the developed HFPD diagnostic system with the PWA algorithm has been proved to have very good performance of eliminating noises.

Effect of oxidation on the wettability of poly(dimethylsiloxane) surfaces

The wetting of amorphous poly(dimethylsiloxane) (PDMS) surfaces by water has been studied using molecular dynamics simulations. PDMS surfaces were generated by compressing a long PDMS chain between two elastic boundaries at atmospheric pressure. Oxidation of the PDMS surface, achieved in real systems by exposure to air plasma or corona discharge, was modeled by replacing methyl groups on the PDMS chain with hydroxyl groups. Three surfaces of varying degrees of oxidation were characterized by measuring the water contact angle and the roughness. The dependence of the microscopic contact angle on drop size was measured from time averaged density profiles. The macroscopic contact angle was measured directly using a cylindrical drop of infinite length with zero contact line curvature. The measured macroscopic contact angle ranged from approximately 125 degrees on the untreated surface to 75 degrees on the most oxidized surface studied. The line tension was found to increase with increasing degree of oxidation, from a negligible value on the untreated surface to approximately 5x10(-11) J m(-1) on the most heavily oxidized surface.

Computation of inception voltage and inception time of positive impulse corona in rod–plane gaps

A method is proposed for computing the inception voltage and time of the corona in air in rod-plane gaps under a positive switching impulse and ramp-shaped voltages. The method is based on the rate of natural production of free electrons in the atmosphere as a result of cosmic ray activity, local radioactivity or ultraviolet radiation from the sun. The computed inception voltages agree reasonably with those measured experimentally for different values of the steepness of the applied switching impulse and ramp-shaped voltages and also with those measured experimentally for a rod gap stressed by a switching impulse voltage. The method is applied to different rod-plane gaps with varying rod radius and gap spacing.

Comparison of Known PD Signals With the Developed and Commercial HFCT Sensors

Increasing need of cost effective identical sensors for partial discharge (PD) location prompted us to take up this research study. Four high-frequency current transformers (HFCT) were assembled and its performances with a used commercial sensor were compared with the known type of PD signals. In this paper, the responses due to surface, oil corona and air corona are analyzed. PD occurrence in 20 ms period and the shape of single PD with more sampled data points are characterized using seven techniques. The study indicates that HFCT with biflar winding is more sensitive in identifying different types of known PD. The number of turns and gauge of the used wire in HFCT play a role in increasing the sensitivity of PD detection.

Pulse-Sequence Analysis of Discharges in Air, Liquid and Solid Insulating Materials

Electrical discharges may occur in gas, liquid as well as solid insulating materials. This paper describes the investigation results on the discharges in air, silicone oil and low density polyethylene (LDPE) using needle plane electrode system under AC voltage of 50 Hz. The experimental results showed that for discharge in air (corona), discharge pulses were concentrated around the peak of applied voltage at negative half cycle. For silicone oil positive as well as negative discharges were observed which concentrated around the peak of applied voltage. The positive pulse number was smaller but the magnitude was higher than that of negative discharge. Discharges in void took place at wider range of phase of applied voltage. The unbalance in pulse number and magnitude similar to that of oil discharges were observed. For electrical treeing in LDPE, the discharges were spread before the zero cross of the applied voltage up to the peak at both positive and negative half cycles. The discharge pulse sequence analysis indicated that the PD occurrence in air, oil and void were strongly affected by the magnitude of applied voltage. However, for electrical treeing it was observed that the discharge occurrence was strongly affected by the time derivative of the applied voltage (dv/dt).

Online source recognition of partial discharge for gas insulated substations using independent component analysis

To perform reliable insulation diagnosis for gas-insulated substation (GIS), detectable partial discharge (PD) should be identified quickly and effectively. With the increasing application of high voltage DC transmission, PD identification in such systems becomes more and more important. Therefore, a novel technique based on the analysis of ultra high frequency (UHF) resonance waveforms is proposed in this paper to meet the requirement. With the help of independent component analysis, the most dominating features are identified directly from UHF resonance signals without phase information. Using the identified features as the input, a neural network is implemented for recognizing sources of PD in SF6 and separating from the corona in air within a very short time

LIF imaging of OH radicals in DC positive streamer coronas

In this study, the LIF detection of the hydroxyl (OH) radicals was performed in a nozzle-to-plane electrode system having an electrode gap of 30 mm during the steady-state positive streamer corona discharge at atmospheric pressure. For monitoring the ground-state OH radicals, OH transition [ A 2 Σ +( v′ = 1) ← X 2Π( v″ = 0)] at 282 nm was used. The time relationship between the regular streamer coronas, laser pulse, OH fluorescence and laser-induced streamer was measured. The time dependence of OH radicals between the successive streamers was measured for the evaluation of OH dynamics when the discharge was in a steady-state condition. The two-dimensional OH distribution in the DC streamer corona discharge was observed. The obtained results showed that the ground-state OH radicals were generated mainly in the filamentary part of the streamers. It was found that LIF detectable amount of ground-state OH radicals stayed in the region where streamers propagate during the steady-state DC positive streamer corona in open air.

Positive corona inception in air at elevated temperatures

The conditions for the inception of non-uniform field corona in air are discussed. A model, that requires simple assumptions, is used to define the characteristics of the critical avalanches needed to initiate streamer corona as the temperature increases and the air density decreases. An iteration procedure that combines measured values of the inception voltages with simple criteria and published data on basic processes is used to define such avalanche characteristics as length, avalanche number and the positive ion space-charge field at criticality. The Meek criterion is invoked to equate the space-charge field to the local applied field, at which replication of the avalanche and streamer initiation is assumed to occur. The procedure is repeated using trial inception voltages to simulate the range measured experimentally at temperatures up to /spl sim/530 K, which corresponds to a relative air density of /spl sim/0.55. It is shown that for relative air densities </spl sim/0.8, the formation of critical avalanches follows a different law from that operating in the range between 1.0 and 0.8. The assumptions made in the calculations result in a good agreement between the calculated and experimental properties.

Improving the surface properties of cellophane by air plasma treatment

Air plasma treatment at low pressure was applied to modify the surface of a cellulose film with the aim to improve its wettability, dyeability and adhesion properties. The contact angles of different polar liquids on the treated films show an exponential decay with treatment time at a given power; the power–time reciprocity is followed. The calculated surface tension values exponentially rise to the same maximum value with a decrease of the polar fraction. ATR-FTIR analyses suggest that a cellulose dehydration takes place rather than a surface oxidation. The plasma treatment improves also the cellophane dyeability with typical dyes for cellulose fibers: the results of dye uptake follow the same trend as the surface energy. The bond strength of lap joints of cellophane with LLDPE film shows a strong improvement of the adhesion depending on the duration and the power of treatment. The whole results are consistent with ablation effects like those observed with air corona treatment rather than oxygen plasma.

Negative corona, glow and spark discharges in ambient air and transitions between them

Obtaining new information about different forms of self-sustained dc discharges that can be realized in pin-to-plane electrode geometry in ambient air is the goal of this paper. Experimental and numerical calculation data uncovering the physics of the temporal and spatial evolution of the negative corona and glow discharge (GD), with increase in current up to the transition to the spark, are presented. Special attention is paid to the properties of diffusive GD at atmospheric pressure, which is a necessary stage (steady-state or transient) preceding the spark and determining the threshold conditions of sparking.

Effect of relative humidity on electron distribution and ozone production by DC coronas in air

The effect of relative humidity on the electron distribution and the ozone production in the direct current (dc) corona discharge from a thin wire is evaluated with a numerical model. The model is based on the prior models of ozone production by dc coronas in dry air, with modifications to incorporate the effect of water vapor on the corona plasma and the plasma chemistry for ozone production. The distribution of electron number density is obtained by solving coupled continuity equations for charge carriers and the Maxwell's equation. The electron kinetic energy distribution is solved from the Boltzmann equation. Both the electron number density distribution and the distribution of electron kinetic energy are insensitive to the variation in the relative humidity due to the relatively low water content. The electron distribution data are then combined with the chemical mechanisms of ozone production in humid air and the two-dimensional (2-D) transport processes to obtain the distribution and the production of ozone. In qualitative agreement with prior experimental findings, the ozone production rate decreases with increasing relative humidity in both positive and negative coronas due to the removal of atomic oxygen by water molecules, and the ozone production rate in the negative corona is one order of magnitude higher than that in the positive corona.

Separation of Corona Using Wavelet Packet Transform and Neural Network for Detection of Partial Discharge in Gas-Insulated Substations

It is essential to detect partial discharge (PD) as a symptom of insulation breakdown in gas-insulated substations (GIS). However, the accuracy of such measurement is often degraded due to the existence of noise in the signal. In this paper, a method using wavelet packet transform and neural network is proposed to separate the PD pulses from corona in air, which enables more accurate detection of insulation breakdown of GIS.

Source classification of partial discharge for gas insulated substation using waveshape pattern recognition

Frequency-domain analysis of ultra-high frequency (UHF) signals for source identification of partial discharge (PD) occurring in SF/sub 6/ inside gas-insulated substation (GIS) has been widely covered in literature. In this, Fast Fourier Transform and Discrete Wavelet Transform based techniques have been extensively applied to derive classifying features from transformed patterns. On the other hand, it appears feasible to develop a time-domain classifier, which derives features directly from the original waveshape. The time-domain classifier is conceptually simple, and requires potentially less computing resources and simpler algorithmic interface with other intelligent techniques due to elimination of frequency-domain transformation. A novel classifier to extract features directly from time-domain waveforms is proposed for classifying SF/sub 6/ PD from air corona and among the three types of SF/sub 6/ PD, regardless of changes in PD locations and measurement conditions. Three sets of classifying features are proposed. Encouraging results have been achieved with comprehensive experimental data, which verifies and proves the usefulness and feasibility of the time-domain classifier.

Diagnosis of OH Radical by Optical Emission Spectroscopy in Atmospheric Pressure Unsaturated Humid Air Corona Discharge and its Implication to Desulphurization of Flue Gas

OH radical in the corona discharge with pipe–nozzle–plate electrode has been diagnosed by optical emission spectroscopy. Spatial variations of OH radical emission in discharge gap have been measured. Relative intensity of OH radical emission spectroscopy increases with increasing water vapor flux injected into the reactor or intensity of electric field supported. In positive pulsed corona discharge, relative intensity is higher than that in positive DC corona discharge and lower than that in negative DC corona discharge. Strongest intensity of OH radical spectrum appears within the range of 5 mm near the discharge nozzle- electrode. In addition, it is proved that the efficiency of desulphurization from flue gas by pulsed corona discharge plasma processes can be improved when OH radical is produced in the reactor.