Presence Of Corona Discharge Research Articles

Power Transformer Fault Diagnosis using DGA based on Three Gas Ratio and Fuzzy Logic

For power system equipment with oil as insulating medium such as power transformer, Dissolved Gas Analysis (DGA) of oil is very helpful method in order to detect faults below oil level. Early detection of fault conditions in a transformer is possible if analysis of gases is done which gets evolved in it. Analysis of the specific value of every gas helps in diagnosing faults. Faults which can be identified by this method include disturbances like presence of corona discharge, partial discharges, arcing and increase in temperature. If correct preventive actions are initiated early for the diagnosis of gases produced, failure to equipment will get prevented. Even though many methods are researched for fault identification and analysis in power transformers, DGA is much superior in comparison with other techniques as it gives more helpful data about the condition of the transformers in running condition. Different techniques, like key gases and their ratio, and their analyzing them graphically are mainly used to understand DGA samples. For a transformer having multiple faults, above methods fail to diagnose. IEC standards are in use for DGA from many years and valuable experience gained over theses years around the world is in use to diagnose internal faults on transformers. IEC three gas ratio technique suggested by IEC is mainly preferred, but in some conditions it can’t correctly identify conditions like no suitable codes for diagnosis and multiple faults. The limitations of the traditional three gas ratio method are: with gas ratio is on the verge of crossing the coding boundary, there is a sharp change in the codes, but actually fuzzied boundary should be used. In this paper, codes "zero", "one", "two" are represented by fuzzy membership functions, then "AND" and "OR" conditions of three gas ratio method are coded into fuzzy logic based statements. MATLAB based scripts prove that the presented technique surely overcome the limitations of the traditional three gas ratio method, hence, it largely reduces the errors in diagnosis. In this paper, a method on the basis of fuzzy logic is explained which is able to identify many faults in oil insulated equipment. The presented diagnosis technique uses values of the ratios C2H4 /C2H6 , C2H2 /C2H4 and CH4 /H2 and the concentration of specific gases namely methane (CH4 ), hydrogen (H2 ), acetylene (C2H2 ), carbon monoxide (CO), ethylene (C2H4 ), carbon dioxide (CO2 ) and / ethane (C2H6 ). Values of these three ratios reflect various patterns of faults inside the transformer. Fuzzy three ratio technique can also quantitatively indicate the likelihood of identified fault with more accuracy as compared to conventional three ratio method. This tool will prove to be very useful to the engineers in DGA result interpretation.

Modified field‐to‐line coupling model for simulating the corona effect on the lightning induced voltages of multi‐conductor transmission lines over a lossy ground

A modified field-to-line coupling model for simulating the corona effect on the lightning induced voltages of the overhead multi-conductor transmission lines is proposed in this study. Since the model is non-linear, it is solved in an iterative way combining with the time-domain finite-element method and is validated by the measurements of Inoue's experiments. Furthermore, the unconditional stability of the numerical method is theoretically proved. The proposed model is a simplified method according to the method recently proposed by Thang et al., in which finite difference time domain (FDTD) is used to calculate the induced voltages. Compared with FDTD, the proposed model can achieve good efficiency because the transmission line theory is adopted instead of the electromagnetic solution. Both the effects of the corona and the lossy ground on the induced voltage are studied, furthermore, the relationship between the corona effect and the soil conductivity is discussed. The simulation results demonstrate that the corona effect will cause a rise time increase and waveform distortion of the induced voltage, and the magnitudes of induced voltages in the presence of corona discharge are larger than those computed without considering corona.

Fast imaging of intermittent electrospraying of water with positive corona discharge

The effect of the electrospraying of water in combination with a positive direct current (dc) streamer corona discharge generated in air was investigated in this paper. We employed high-speed camera visualizations and oscilloscopic discharge current measurements in combination with an intensified charge-coupled device camera for fast time-resolved imaging. The repetitive process of Taylor cone formation and droplet formation from the mass fragments of water during the electrospray was visualized. Depending on the applied voltage, the following intermittent modes of electrospraying typical for water were observed: dripping mode, spindle mode, and oscillating-spindle mode. The observed electrospraying modes were repetitive with a frequency of a few hundreds of Hz, as measured from the fast image sequences. This frequency agreed well with the frequency of the measured streamer current pulses. The presence of filamentary streamer discharges at relatively low voltages probably prevented the establishment of a continuous electrospray in the cone–jet mode. After each streamer, a positive glow corona discharge was established on the water filament tip, and it propagated from the stressed electrode along with the water filament elongation. The results show a reciprocal character of intermittent electrospraying of water, and the presence of corona discharge, where both the electrospray and the discharge affect each other. The generation of a corona discharge from the water cone depended on the repetitive process of the cone formation. Also, the propagation and curvature of the water filament were influenced by the discharge and its resultant space charge. Furthermore, these phenomena were partially influenced by the water conductivity.

FDTD Simulations of Corona Effect on Lightning-Induced Voltages

In this paper, a simplified model of corona discharge for the finite-difference time-domain (FDTD) computations has been applied to analysis of lightning-induced voltages at different points along a 5-mm radius, 1-km long single overhead wire taking into account corona space charge around the wire. Both perfectly conducting and lossy ground cases were considered. FDTD computations were performed using a 3-D nonuniform grid. The progression of corona streamers from the wire is represented as the radial expansion of cylindrical weakly conducting (40 μS/m) region around the wire. The magnitudes of FDTD-computed lightning-induced voltages in the presence of corona discharge are larger than those computed without considering corona. The observed trend is in agreement with that reported by Nucci et al. and by Dragan et al ., although the increase predicted by our full-wave model (up to 5% and 9% for negative and positive lightning return strokes, respectively) is less significant than in their studies (up to a factor of 2) based on the distributed-circuit model with sources specified using the electromagnetic field theory. The disparity is likely to be related to the use of different charge-voltage diagrams, explicitly assumed by Nucci et al. and Dragan et al . and resulting from our FDTD model with corona in the present study. When corona is considered, there is a tendency for induced-voltage rise time to increase. It appears that the distributed impedance discontinuity, associated with the corona development on the wire, is the primary reason for higher induced-voltage peaks and longer voltage rise times, compared to the case without corona.

Increasing the efficiency of deep photocatalytic oxidation of organic vapors under the action of negative atmospheric corona discharge

The influence of atmospheric corona discharge on the photocatalytic oxidation of acetone (83 ppm), acetaldehyde (108 ppm), and toluene (12–46 ppm) vapors on TiO2 under the action of soft UV light in a closed chamber has been studied. Corona discharge was created around the wire of the charging unit of an electric filter at the inlet of a photocatalytic reactor. The rate of the removal and deep oxidation of substances to CO2 increased considerably and reached maximum for acetaldehyde. The concentrations of inter-mediates in the course of experiments did not exceed 1 ppm. In the presence of corona discharge, the rate of deep oxidation per unit of consumed electric power increased by a factor of 1.2–3.6. The results show that combined photocatalytic and plasmochemical oxidation is promising for the purification of released gases.

Charge-to-mass ratio and dendrite structure of diesel particulate matter charged by corona discharge

The electric charge of diesel particulate matter (DPM) from a corona discharge device is measured for understanding the electrical characteristics of the DPM; an electrostatic precipitator (ESP) is employed for trapping DPM. Exhaust gas from a diesel engine is sampled by isokinetic sampling, and DPM contained in the gas is charged by negative corona discharge. The charged DPM is collected by a Faraday cup, and its electric charge is measured by a Coulomb meter. In addition, the appearance of the collected DPM is observed using a scanning electron microscope. It is found that when the gas velocity is 0.28 m/s, the maximum charge-to-mass ratio of the DPM is −126 μC/g. The DPM appears to agglomerate into a large particle in the absence of corona discharge; however, the agglomerated DPM forms a dendrite structure in the presence of corona discharge. The dendrite structure causes an increase in the surface area of the agglomerated DPM. Therefore, the oxidation removal of the DPM trapped by the ESP by using ozone and nitrogen dioxide can be enhanced because an oxidation reaction occurs on the surface of the DPM.

Dust particles motion in back discharge

The back discharge is a type of discharge that takes place in the electrostatics precipitation process, especially in coal-fired power plants for coal of low content of sulfur. This type of discharge takes place in the presence of corona discharge and occurs on an electrode covered with a high resistivity dust layer. The breakdown of dielectric layer causes an increase in the discharge current and re-entrainment of dust particle from collection electrode into the flowing gas. The investigations of the motion of dust particle emitted from the collection electrode are presented in this paper. Additionally, acrylic powder layer placed between a mica plate with a small pinhole and the plate electrode were also used in the experiments in order to generate repeatable back discharge. An effect of back discharge on the collection efficiency in the electrodes system was also presented.

Development of short pulsed corona on two-wire transmission line

Propagation of short high-voltage pulses along a 6-m-long twin line was studied in the presence of corona discharge. The experimental setup included a twin-line arrangement, a 100-kV, 5-ns pulse generator, voltage and current probes, and photomultipliers. The dependencies on line geometry and wire conductivity and on pulse voltage and polarity have been studied. PSpice and electrostatic simulations are presented; the latter are compared to still photos of corona discharge. Of most interest was the finding that the discharge was very much asymmetric when one of the pulser's terminals was grounded, even when the line height was much greater than the distance between the wires. This effect will be pronounced to heights of hundreds of meters. Pulsed corona study on NiCr resistive wires revealed intense pulse decay: only 1% of the launched energy reaches the end.

Laboratory studies of back-discharge in fly ash

The back-discharge is a type of discharge that takes place in the presence of corona discharge and occurs at an electrode covered with a dielectric layer of resistivity higher than about 10 8 Ω m. Back-discharge can be observed in electrostatic precipitators when dust covering the collection electrode has low conductivity. In this paper, the studies of back-discharge generated in ambient air, in point-to-plane geometry with the plate electrode covered with fly ash are presented. The discharge is characterised in terms of its visual forms, current–voltage characteristics, and light emission spectra. Three forms of back-discharge were investigated: glow discharge, streamers, and low-current back-arc discharge. The current of the back-arc discharge was only a few milliamps. The discharge was stabilised by a high series resistance. It was noted that the voltage of ignition of the back-discharge for negative polarity is lower than for a positive one. Spectroscopic measurements of emission spectra provided information on elements present in the discharge column. The elements present in the fly ash, including toxic metals, can be re-entrained into the gas as particles or can be emitted as ions or neutrals during the discharge, and can decrease the collection efficiency of electrostatic precipitators. These elements were detected in the emission spectra. The effect of the discharge on the fly ash layer was also discussed. It was observed that sinter-like leftovers remain in the dust layer after a back-arc discharge.

Dielectric behaviour of particle-contaminated air-gaps in the presence of corona

Abstract The operating conditions of corona-electrostatic separators, of various electroprecipitators, and of other similar high-voltage devices are influenced by the size and shape of the particles which pass through the inter-electrode air-gap. The presence of corona discharge affects the movements and the charge of the particles, and hence modifies the breakdown threshold. The numerical analysis of the electric field, using a charge simulation program, enabled the evaluation of the effects. Two other computer programs were written in order to analyse the conditions in which micro-discharges can occur between charged particles and electrodes of opposite polarities, and to estimate the consequences of such an event on the global dielectric strength of an ionized air-gap. Nevertheless, a systematical experimental study was necessary to fully-understand the phenomena. The basic electrode arrangement consisted in a matrix of corona-emitting points situated above a plate electrode. In some experiments, a grounded metallic grid was interposed between the two electrodes connected to high-voltage supplies of opposite polarities, so that the ionic current density and the intensity of the electric field be independently controlled. Both the computational results and the experimental data demonstrated that the presence of particles in air-gaps affected by corona discharges has a more important influence on the dielectric strength than in the case of a charge-free electric field.

A novel high-voltage peak voltmeter

The paper describes a new type of high-voltage peak voltmeter which is free from the errors normally encountered in such instruments. The instrument, which includes rectifiers in a feedback loop, is sensibly independent of rectifier characteristics, and is particularly suited to the measurement of high voltage in the presence of corona discharge.