Air Corona Research Articles

Using an optical fibre anemometer to measure the speed of the electric wind in a negative polarity, atmospheric corona discharge

Coronas are partial discharges that occur in regions of non-uniform electric fields adjacent to conductors stressed to high voltages. Negative, Trichel-pulse coronas in air occur when a dc, negative-polarity, high voltage is applied to a conductor. Trichel pulses in atmospheric air generate significant amounts of ozone as well as electrical and acoustic noise. Under the right conditions these coronas can be a precursor to complete electrical breakdown of the air gap due to a reduction in the density of neutral molecules resulting from a combination of localised heating and convective air flow generated by the movement of negative ions. An optical fibre anemometer, based on a Mach-Zehnder interferometer has been constructed to measure the speed of the wind generated in the point-plane gap of a negative, Trichel-pulse corona discharge in atmospheric air. The sensing arm of the fibre interferometer is subjected to controlled, repetitive bursts of infrared radiation from a CO2 laser and the combination of localised heating and convective cooling by the corona wind results in fringe shifts which are directly calibrated to the speed of the wind. This paper reports on the nature of the calibration process and presents some radial profiles of wind speed in the corona gap.

Recognition of partial discharge waveshape patterns for gas insulated switchgear

Early warning system based on the detection of UHF SF6 partial discharge (PD) signals is a necessary means for the protection of Gas-Insulated Switchgear (GIS) in service as well as the power system to which it is connected. In order to ensure the safe and reliable operation of GIS, it is important to adopt an effective diagnosing method, which is able to identify signals of harmful defects promptly. Unlike approaches such as Fast Fourier Transform (FFT) and Discrete Wavelet Transform (DWT) based techniques, a novel approach to extract discriminative features directly from time-domain UHF signals is introduced in this paper. With the proposed approach, UHF signal waveforms of different sources are partitioned and quantified so that the harmful (SF6 PD) and not-so-harmful (air corona) UHF signals are classified promptly. The investigation is based on the experiment data measured from a 300 KV GIS with encouraging results, which proves the usefulness of this proposed approach.

Spatiotemporal evolution of the current and the integral and spectral emission characteristics of a negative corona in nitrogen during its transformation into a spark

Results are presented from experimental studies of the conversion of a steady-state negative corona into a spark. It is found that a spark in a negative corona in nitrogen and air is formed in the absence of fast primary streamers. It is shown that, in atmospheric-pressure nitrogen, the conversion of a corona into a spark begins with the propagation of a plasma channel (secondary streamer) from the point electrode (cathode) to the plane electrode (anode). In contrast, the plasma channel in air originates near the plane electrode and then propagates towards the point electrode. The propagation velocity of the secondary streamer is very low, V=103–104 cm/s. Two possible scenarios of the formation of the spark channel in a negative corona in nitrogen are described on the basis of the concept of a contracted volume glow discharge. Results are presented from time-resolved spectral measurements of plasma emission from different regions of the corona during its transformation into a spark.

Surface degradation and hydrophobic recovery of polyolefins treated by air corona and nitrogen atmospheric pressure glow discharge

AbstractThe surface degradation and production of low molecular weight oxidized materials (LMWOM) on biaxially oriented polypropylene (BOPP) and low‐density polyethylene (LDPE) films was investigated and compared for two different dielectric barrier discharge (DBD) treatment types, namely air corona and nitrogen atmospheric pressure glow discharge (N2 APGD). Contact angle measurements, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) analyses were performed in conjunction with rinsing the treated films in water. It is shown that N2 APGD treatments of both polyolefins result in much less surface degradation, therefore, allowing for a significantly higher degree of functionalization and better wettability. Hydrophobic recovery of the treated films has also been studied by monitoring their surface energy (γs) over a period of time extending up to several months after treatment. Following both surface modification techniques, the treated polyolefin films were both found to undergo hydrophobic recovery; however, for N2 APGD modified surfaces, γs ceases to decrease after a few days and attains a higher stable value than in the case of air corona treated films. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1291–1303, 2004

Chemistry on the low field anodes and cathodes of air corona gaps

Surface effects of electrical coronas have been studied on the low field electrodes of point-to-plane air gaps with discharges burning in negative and positive glow regimes, i.e., mainly producing chemical effects without significant sputtering or thermal effects. Most of these chemical effects are due to oxidation and reduction processes taking place in aqueous microdroplets formed by the discharge where oxides, nitrates, and cyanides, are mainly found. Generally oxidation prevails providing very high oxidation states, but using preoxidized gold samples as cathodes enabled the separate observation of reduction phenomena.

Onset corona fields and electrical breakdown criteria

Predictions are made of the electric field necessary for the onset of corona in air for spheres and cylinders from the general breakdown criterion, ; α′ is the net ionization coefficient, r is the radius and Q is a constant. It is found that Q = 104 gives a fair fit to experimental results for both wires and points, and for positive and negative corona, for radii varying from 0.01 to 20 cm. By assuming a linear variation of α′ with the difference of the electric field from the critical field where ionization equals attachment, an analytic formula can be derived for the breakdown field for the onset of corona as a function of the radius of the sphere, which is similar to, but different from, Peek's formula for the onset of corona for cylinders or wires. The formulae also give the breakdown field as a function of temperature and pressure. For Townsend and Streamer breakdown mechanisms, values of Q are typically 3 × 104 and 108, respectively. Thus the value of Q derived from experimental values of the onset field for corona for both wires and points is significantly lower than would be expected from either the Townsend breakdown criterion or the Streamer breakdown criterion. It is suggested that the most likely collision process to explain the observed low value of Q is indirect ionization, for example by collisions between excited molecules.

Shape of the current tube of a negative point-to-plane corona in air

Results are presented from the experimental studies of a negative point-to-plane air corona discharge with controlled geometry. It is shown that, in a certain range of parameters, the steady-state current-voltage characteristic of the corona can be fitted by a parabola. The results obtained are used to determine the time-averaged shape of the current tube of a negative corona.

About the significance of pd measurements in liquids

This paper presents investigations concerning the variations and significance of PD measurements in mineral oil, when detectors with different frequency response are used. Typical durations of discharge phenomena in liquids are much longer than in other media, and the measurement technique must be adapted accordingly. However, depending on the application concerned, this is not always possible. In this study, experiments and calculations are carried out to determine typical errors produced when classical RLC measurement impedances with various frequency responses are used to measure PD's in liquids. Experiments are done with corona and creeping discharges in mineral oil, and compared with corona and spark discharges in air. These examples allow us to conclude that when a calibrated high frequency PD measurement system is used, the ratio of measured to actual charge is much lower for discharges in liquids than in gases. If measurements are carried out on a practical device that may include various types of PD sources, the high frequency measurement system will strongly underestimate discharges in liquids compared to other discharge types. This discrepancy versus discharge type increases when the frequency response of the measurement system is increased.

A phase-related investigation of ac corona in air

A computer-based partial discharge detector has been used to measure AC corona pulses in air in a point-cup electrode system. The pulse height distribution was measured as a function of phase angle and pulse height for a range of voltages. The onset voltages for both positive and negative half-cycles are the same, and significantly lower than for the DC case, due to space charge effects. In all other respects the patterns for the two half-cycles are different: pulses only occur at the onset voltage (rising) for the positive half-cycle, but continuously for the negative half-cycle, except for the higher applied voltages which exhibit a glow discharge transition near the (negative) peak. Differences between the distributions for avalanches and streamers increase as the breakdown voltage is approached. Qualitative explanations of the patterns and behavior are given, together with calculations showing that space charge from one cycle can linger in the gap between half-cycles.

Positive corona in air using a point/cup electrode system

The authors have previously published some measurements of negative corona currents in air using a spherically-symmetric point/cup electrode system, and also the related simulations. In this paper the corresponding current/voltage measurements and simulations for positive-point corona, again using the highly symmetric electrode system, are described. Although the corona onset voltages are again predicted with reasonable accuracy for different point radii and air pressures, the currents are very much lower, presumably due to the effect of positive space charge accumulating close to the point electrode (since the simulations assume the absence of space charges). In agreement with this assumption, the streamer pulses are only seen at or just above the onset voltage. Only avalanches occur from about 1 kV above the onset voltage.

A Comparison of Corona-Treated and Flame-Treated Polypropylene Films

The comparison of corona-treated and flame-treated polypropylene (PP) films provides insight into the mechanism of these surface-oxidation processes. Atomic force microscopy (AFM), contact-angle measurements, and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize surface-treated biaxially oriented PP. While both processes oxidize the PP surface, corona treatment leads to the formation of water-soluble low-molecular-weight oxidized materials (LMWOM), while flame treatment does not. Computational modeling of the gas-phase chemistry in an air corona was performed using a zero-dimensional plasma-chemistry model. The modeling results indicate that the ratio of O to OH is much higher in a corona discharge than in a flame. Chain scission and the formation of LMWOM are associated with reactions involving O atoms. The higher ratios of O to OH in a corona are more conducive to LMWOM production. Surface-oxidized PP exhibits considerable thermodynamic contact-angle hysteresis that is primarily caused by microscopic chemical heterogeneity.

Numerical simulations of Trichel pulses in a negative corona in air

Numerical simulations of a negative corona in air demonstrate that the experimentally observed regime of self-oscillations, known as Trichel pulses, is well described by a three-dimensional axisymmetric model that is based on the standard transport equations and in which only the ion-induced secondary electron emission at the cathode is taken into account. The quantitative difference between the measured and calculated values of the mean current and the pulse repetition rate most likely stems from the insufficiently large dimensions of the computation region and from the fact that the point shape adopted in simulations somewhat inexactly conforms to that used in experiments. It was found that the transverse discharge structure near the cathode radically changes during the pulse. Specifically, as the current grows, a cathode sheath forms at the discharge axis and expands over the cathode surface. When the current falls off, the cathode sheath is rapidly destroyed; as a result, the characteristic field structure is well defined only near the discharge axis and becomes virtually indistinguishable as the current decreases by an order of magnitude.

Negative corona in air at atmospheric pressure due to a voltage impulse

This article presents a numerical modeling of a negative corona caused by an overvoltage pulse (∼250%), in a point-to-plane electrode system. The field value applied to the cathode is high, thus, the numerical simulation described in this article attempts to show that the field-effect emission can be responsible for the ignition of the corona and for its first stage of development, while the positive-ion bombardment is responsible for sustaining the discharge during the current decay phase. A time-dependent enhancement factor is introduced in the classical Fowler–Nordheim relationship in order to model the switch on of some emissive site at the cathode surface. Preliminary experiments were carried out in order to better understand the ignition and development of the corona. The role of the field-effect emission is underlined and the dimension of the corona is optically determined. The negative corona current pulse and its associated emitted UV light are computed and compared to experimental results.

Chemical kinetics with electrical and gas dynamics modelization forNOx removal in an air corona discharge

A non-stationary reactive gas dynamics model in a mono-dimensional geometry, including radial mass diffusion, gas temperature variation and chemical kinetics, is developed in this paper. The aim is to analyse the spatio-temporal evolution of the main neutral species involved in a corona discharge used for NO pollution control in polluted air at atmospheric pressure and ambient temperature. The present reactive gas dynamics model takes into account 16 neutral chemical species (including certain metastable species) reacting following 110 selected chemical reactions. The initial concentration of each neutral species is obtained from a 1.5D electrical discharge model. The gas temperature variations are due to direct Joule heating during the discharge phase, and also result from the delayed heating due to the relaxation of the vibrational energy into a random thermal energy during the post-discharge phase. The simulation conditions are those of an existing experimental setup (anode voltage of 10 kV in the case of a point to plane geometry with an interelectrode distance of 10 mm). The obtained results show that the diffusion phenomena and the gas temperature rise affect quite well the gas reactivity and the neutral species evolution. This allows us to better understand the different reaction processes and transport phenomena affecting the NO concentration magnitude inside the discharge channel.

Biaxially Oriented Polypropylene (BOPP) Surface Modification by Nitrogen Atmospheric Pressure Glow Discharge (APGD) and by Air Corona

We compare two surface treatments of biaxially-oriented polypropylene (BOPP), which are carried out in the same dielectric barrier discharge (DBD) apparatus, namely air corona, and N2 atmospheric pressure glow discharge (APGD). Changes in the surface energy and chemistry are investigated by contact angle measurements, by X-ray photoelectron spectroscopy (XPS) and by attenuated total reflectance infrared spectroscopy (ATR-FTIR). It is shown that N2 APGD treatment leads to a higher surface energy than air corona treatment, and to the formation of mostly amine, amide, and hydroxyl functional groups at the polypropylene surface. Finally, hydrophobic recovery of the treated film is studied; for both treatment types, the increased surface energy is found to decay in a similar manner with increasing storage time after treatment.

Surface chemistry induced by air corona discharges in a negativeglow regime

This paper deals with oxidation phenomena observed on surfacessubmitted to the action of corona-activated ambient air. The study wasmainly performed with gold and silicon surfaces chosen for their greatmaterial stability. During the discharge, the surfaces become covered withmicronic water droplets which look like granules. They grow in number withthe duration of the sample exposure to the discharge, their mean sizedepending on the material, and they resist the vacuum of the electronscanning microscope, but not its electron beam, especially when it isfocused on them. From a chemical point of view, the surfaces are nowoxidized in a way comparable with what can be obtained in liquidelectrochemical solutions and, apart from the electrode material oxides orhydroxides observed, one can also find nitrogen oxides, but in clearlysmaller quantities.

Fractal structure of sprites

A large scale model of sprites based on a phenomenological probabilistic approach to modeling of streamer corona discharges is developed. The model utilizes the experimentally documented macroscopic properties of positive and negative streamer corona in air and allows a realistic determination of the propagation of multiple breakdown branches in a self‐consistent electric field. The model results reproduce the large scale volumetric shapes of sprites, agree with the experimentally documented thundercloud charge moment changes in sprite producing cloud to ground lightning discharges (CGs), and demonstrate fundamental asymmetries between sprites generated by CGs of positive and negative polarity.

Negative corona in air using a point/cup electrode system

Calculations and simulations of potential and charged-particle distributions have been made by various workers for single pulses along the axis of point/plane electrode systems but not for the overall current. The difficult geometry makes this inevitable. In this paper the authors describe measurements of corona currents in air using a high-symmetry 'point/cup' electrode system comprising a hemispherical-tipped point which is concentric with a concave hemispherical electrode; and simulations based on a spherically symmetric geometry. The simulations assumed avalanches to originate from the random release of electrons in the inter-electrode gap. The predicted onset voltages and the shape of the experimental curves for corona current vs. Voltage were in good agreement with the experimental results over a range of point-tip radii (0.5 to 3 mm) and pressures (0.02 to 0.10 MPa). Space charge effects were not included in the simulation so it follows that the accumulation of space charge is negligible at the onset voltage in air, although it is likely that it will have an increasing effect as the voltage increases.

Characterization of point-plane corona in air at radio frequency using a FE-FCT method

The development of corona under RF conditions in point-plane configuration has been investigated in this paper, using the recently developed finite-element flux-corrected transport (FE-FCT) method. The study was made in air for a 1 cm gap with 50 µm tip radius needle at 40 MHz and the effects of different parameters, such as the peak applied voltage (in the range 3-4.5 kV) on the coronal development and particularly on the current and light output, are examined. Polarity and hysteresis effects are observed, as well as a plateau in the current waveform, which are confirmed by previously published experimental results. Finally, a Fourier analysis on the steady-state current waveform is performed.

Current and electric field measurements in coaxial system during the positive DC corona in humid air

A stable positive corona discharge in coaxial electrode system with varying humidity is analysed using a model which separates the corona in two distinct regions. A careful choice of boundary conditions in the drift region, particularly at the cathode surface, is needed. A new method of current and electric field measurements with a linear biased probe, incorporated in the outer cylinder, is used. The corona current, at a given voltage, decreases with humidity but the electric field at the outer cylinder is independent of humidity. The fields at the surfaces of the cathode and the anode vary linearly with the applied voltage between electrodes. The exact value of the average mobility of positive ions in the drift region is deduced from current and field measurements. This mobility increases when the potential difference across the drift region increases and it decreases slightly as the air is made more humid. Other parameters of the positive corona are successively determined by the current and field measurements, in particular, the total field distribution between electrodes.