Discharge In SF Research Articles

Comprehensive Simulation Research on Microprocess and Macroscopic Electromagnetic Signals of Surface Discharge in SF6

The surface discharge on the dielectric is a common type of partial discharge in GIS, but the microscopic development process of surface discharge in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> is unclear. To find out the micro-process of surface discharge and its correlation mechanism with the partial discharge detection signals of GIS, the fluid-chemical simulation model of the streamer is used to simulate the micro-process of surface discharge in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> . Then the current signal obtained by the micro-process simulation is used as the excitation to simulate the electromagnetic signals based on the FITD method. A close connection between the microscopic discharge and the macroscopic signals is established. The results show that the electron concentration at the streamer head and the streamer velocity decrease first and then increase during the discharge, with an average velocity of about 9.84×105m/s. The discharge current reached a peak value of about 0.22A when the surface streamer reaches the cathode. In terms of waveform correlation, the absolute value of the average correlation coefficient between the electric field signals simulated in this paper and the actual measured signals is above 0.7, which is much higher than the simulated signal excited by the Gaussian source. By changing the applied voltage and comparing with the experiment results, it is found that the simulation method in this paper can reflect the change of signals under the influence of external conditions. As the applied voltage rises, the amplitude of the electric field signals increases, and the main frequency component on the frequency spectrum grows, with the main frequency value at about 1.2 GHz. The research in this paper is an effective supplement to the existing study on the microscopic mechanism of partial discharge signals.

Numerical Study on the Effect of Electric Field Non-uniformity on the Pulse Characteristics of Positive Corona Discharge in SF6/N2 Gas Mixtures

This paper establishes a two-dimensional axisymmetric needle-to-plane geometry model in SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> gas mixtures to simulate the effect of the electric field non-uniformity on the pulse characteristics of positive corona discharge. By changing the needle tip radii from 25 µm to 300 µm, the corona discharge process under different electric field non-uniformity is simulated. Meanwhile, the simulated results are verified by the experiments on a corona discharge experimental setup. The results show that the rising and falling edges of positive corona current pulses correspond to the development and termination of streamers, respectively. The sub-pulse will be generated during the formation of the first current pulse, and this phenomenon is closely related to the applied voltage. The generation of sub-pulse is expected to be a feature of positive corona discharge in SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> gas mixtures entering the brush-pulse stage. With increasing the electric field non-uniformity, the onset voltages of two characteristic pulse stages, including the streamer-pulse stage and brush-pulse stage, decrease. Moreover, the first current pulse amplitude increases, and the pulse-repetition rate presents an approximate exponential increasing trend.

Effect of Gas-Mixture Ratio on the Characteristics of Positive DC Corona Discharge in SF6/N2 Gas Mixtures

The characteristics of positive DC corona discharge in SF6/N2 gas mixtures under different gas-mixture ratios are investigated with a needle-to-plane electrode configuration. With increasing applied voltage, a discharge-stage transition process including the Townsend stage, streamer-pulse stage, brush-pulse stage and breakdown stage is observed. The variation in current-pulse characteristics, including pulse-time parameters, mean pulse amplitude and pulse-repetition rate, is investigated under different gas-mixture ratios. The pulse-time parameters are not affected by gas-mixture ratio. The pulse-repetition rate increases with the increase of SF6 concentration and mean current, ranging from hundreds of kilohertz to thousands of kilohertz. The streamer-pulse amplitude increases with increasing SF6 concentration. With increasing mean current, the small-amplitude brush pulses become dominant, thus the mean pulse amplitude decreases. In order to further explore the influence mechanism of gas-mixture ratio on positive corona discharge, a fluid model of positive corona discharge is developed and relevant simulations show the formation process of current pulse, and verify the validity of the pulse characteristics changing with gas-mixture ratio.

Leader Transition Properties of Propagating Partial Discharges in SF&lt;sub&gt;6&lt;/sub&gt; Gas and the Scaling Law under the Negative Lightning Impulse Voltage Application

Leader Transition Properties of Propagating Partial Discharges in SF&lt;sub&gt;6&lt;/sub&gt; Gas and the Scaling Law under the Negative Lightning Impulse Voltage Application

Diffuse discharges in SF 6 and mixtures of SF 6 with H 2 , formed by nanosecond voltage pulses in non‐uniform electric field

Results of experimental study of nanosecond diffuse discharge in SF6 and gas mixtures of SF6 with H2, D2 and C2H6 are presented. The aim of this work is to study parameters of discharge between two extended electrodes with a small radius of curvature in SF6 and SF6 with additives. It was shown that diffuse discharge can be formed in SF6 at elevated pressure between blade electrodes with length of 30 cm. It was also confirmed that in a sharply non-uniform electric field a beam of runaway electrons is generated and that the gap breakdown occurs due to ionisation waves which begin on electrodes with small radius of curvature. Laser action in the infrared spectral region was obtained in SF6–H2 (D2, C2H6) mixtures. The laser output up to 110 mJ was easily achieved which corresponds to ultimate intrinsic efficiency (with respect to deposited energy) of 10%.

High resistance of surface fluorinated epoxy insulators to surface discharge in SF&lt;inf&gt;6&lt;/inf&gt; gas

Our recent studies have indicated that direct fluorination can suppress surface charge accumulation of epoxy resin insulators and thus enhance the DC flashover performance. It was also confirmed that the fluorinated epoxy surface layer has a significantly improved resistance to corona discharge. Further, in this work, the degradation or tracking resistance to surface discharge and the AC flashover performance of the surface fluorinated epoxy insulator were investigated. A sheet sample/finger electrode arrangement was used in the AC flashover test in SF6 gas. The sample/electrode arrangement can keep the discharge along the same surface path for successive flashover tests, and thus can cause a lasting damage of the sample surface. Test results show that the fluorinated epoxy surface layer has a high degradation or tracking resistance, although direct fluorination only leads to a marginal increase in AC flashover voltage for the sample/electrode arrangement. The resistance improvement is mainly attributed to the formation of C-F bonds and the disappearance of conjugated double bonds in the surface layer by substitution and addition of fluorine atoms, as indicated by ATR-FTIR analysis. The marginal effect of the fluorination on AC flashover voltage is due to limited surface charge accumulation for the symmetrical sample/electrode arrangement during the short period between the partial discharge initiation and the occurrence of flashover.

Resistance of surface fluorinated epoxy resin to corona discharge in SF&lt;inf&gt;6&lt;/inf&gt; gas

Our previous results have shown that direct fluorination can modulate surface electrical properties and suppress surface charge accumulation of epoxy insulators, and thus has potential applications for improving the flashover performance of epoxy spacers in gas-insulated equipment. In this study, to investigate the resistance of the fluorinated epoxy surface layer to partial discharge in SF 6 gas, surface fluorinated epoxy samples together unfluorinated (virgin) ones were corona treated using a multi-needle-to-plate electrode system. ATR-IR analyses and SEM surface and cross section images show that the corona discharge caused the chemical modification and chain scission at some weak points in the fluorinated layer and the crosslinking reaction, and thus led to the formation of nodules on the corona treated fluorinated surface. However, the corona treatment did not change the fluorinated layer thickness. In contrast with this, severe degradation, most likely in the form of etching, occurred for the virgin surface layer during the same treatment. Measurements of surface potential decay and water contact angle indicate that the corona discharge obviously reduced surface conduction of the fluorinated sample, and rendered the surface conduction insensitive to humidity although the corona treatment almost did not change surface wettability of the fluorinated sample. On the contrary, the measurements show that surface conduction of the virgin sample increased and became sensitive to humidity after the corona treatment.

Ultraviolet Images of 50-Hz AC Discharges in SF6 Atmosphere

Detection of ultraviolet images is proved to be both effective and convenient in discharge monitoring nowadays. In a self-designed gas chamber with observation windows installed, characteristics of 50-Hz ac discharges in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> atmosphere were observed in the needle-to-plane model and the floating potential model. Ultraviolet images of the discharges in different periods of the voltage process are presented, and correspondence is found with the variation of the discharge capacity.

Partial discharges in SF&lt;sub&gt;6&lt;/sub&gt; gas filled void under standard oscillating lightning and switching impulses in uniform and non-uniform background fields

Behaviors and characteristics of partial discharges in SF6 gas filled void are investigated under oscillating lightning impulse voltage (OLI) and switching impulse voltage (OSI). The negative and positive PD sequences under impulses are described and analyzed under the applications of uniform and non-uniform background fields. Four types of discharge pulses in sequences are distinguished by their magnitudes, waveforms and time interval and well interpreted by a charge memory physic model. The distribution of PD points under OLI and OSI in normalized phase range (φN, Im) are proposed to describe the occurrence of PD under periodic oscillation impulses statistically. It is indicated that the discharges with low magnitudes are distributed in the relative large ranges of phase and the discharges with high magnitudes are mainly distributed in the relative narrow ranges of phase. By experimental results, the influences of the gas pressure and the void dimension ratio are studied and it is concluded that both the increases of these two factors could bring the obvious restrictions on PD activity under oscillating impulses.

Partial discharges in SF6 gas filled void under standard oscillating lightning and switching impulses in uniform and non-uniform background fields

Behaviors and characteristics of partial discharges in SF6 gas filled void are investigated under oscillating lightning impulse voltage (OLI) and switching impulse voltage (OSI). The negative and positive PD sequences under impulses are described and analyzed under the applications of uniform and non-uniform background fields. Four types of discharge pulses in sequences are distinguished by their magnitudes, waveforms and time interval and well interpreted by a charge memory physic model. The distribution of PD points under OLI and OSI in normalized phase range (φN, Im) are proposed to describe the occurrence of PD under periodic oscillation impulses statistically. It is indicated that the discharges with low magnitudes are distributed in the relative large ranges of phase and the discharges with high magnitudes are mainly distributed in the relative narrow ranges of phase. By experimental results, the influences of the gas pressure and the void dimension ratio are studied and it is concluded that both the increases of these two factors could bring the obvious restrictions on PD activity under oscillating impulses.

Flashover and Puncture Characteristics of Epoxy Molded Electrodes under Repetitive Discharges in SF&lt;sub&gt;6&lt;/sub&gt; Gas

This paper introduces excellent insulating features of dielectric molded electrodes, studying the flashover and the puncture characteristics under repetitive discharges in SF6 gas. The goal of the study is to clarify the essential breakdown characteristics of thick dielectric-layered electrodes when applied as substitutes for metallic electrodes in high voltage power equipment. The flashover characteristics are investigated by optical measurements in view of the partial discharge onset properties. The puncture characteristics are studied based on the observation of the punctured trace and the utilization factor in the epoxy at the moment of the flashover in SF6. Considering both of the flashover and the puncture characteristics, the remarks on the selection of the appropriate filler in the epoxy is also presented.

The Changes in Electrical and Interfacial Properties of Polyimide Exposed to Dielectric Barrier Discharge in SF6Medium

The formation mechanism of space charges in polyimide (PI) which was exposed to dielectric barrier discharge (DBD) in SF6 medium and the effects of the space charges on interfacial and electrical properties of PI were investigated. The variation of normalized surface charge density on PI sample was calculated and illustrated for different DBD exposure times. The surface potential was measured to determine the effect of the space charges on the sample. Then, the contact angle values were measured to obtain the relation between the surface energy and the surface charge density. The expressions for the total charge and the concentration of trapped electrons were derived by using Poisson and continuity equations at stationary state. The space charges were determined experimentally by using thermally stimulated depolarization current (TSDC) method. Also, SEM image and FTIR spectrum of virgin and treated samples were presented to observe the structural variations. It was seen that the approach for the formation mechanism of the space charges agreed with the experimental data. However, it was concluded particularly for the short-time DBD treatments that the space charges accumulated in the sample should be considered besides the effects of surface functionalization in the determination of the surface energy.

DC corona discharge from floating particle in low pressure SF/sub 6/

The final objective of this research project is clarification of the particle-triggered corona mechanism in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas-insulated electrical equipment as well as development of high precision diagnostic method of foreign metallic particle in GIS insulated by SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas using the electrical signal due to corona discharge. The following research steps are conducting to achieve our goal: 1) clarification of particle-triggered corona phenomena in air which is well-understood corona mechanism, 2) clarification of particle-triggered corona phenomena under dc and ac applied voltage in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas at low pressure in which a stable corona occurred, and 3) clarification of the pressure effect on the particle-triggered corona mechanism. This study is the second step of our research series research, which focuses on characteristics and the mechanism of particle-triggered corona discharge in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas on the basis of measurements of corona current pulses at a pressure range of 30 kPa to 50 kPa under dc applied voltage. The identification of positive and negative corona pulses in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6 </sub> gas from the waveform, amplitude and repetition rate of corona current pulses is difficult; thus, current and light pulses of coronas from the both tips of particle were measured simultaneously to discriminate the polarity of corona current pulses. Experiments showed that near the corona onset voltage, positive corona accompanied pulse current but negative corona tended to be glow-like corona. As the applied voltage was increased, negative corona changed into streamer type accompanying strong current pulse. Positive corona pulses maintained a small amplitude with an instantaneously high repetition rate, even if the field enhancement on the positive tip of particle occurred due to the positive-ion inflow to the particle produced by negative corona. On the basis of current pulse measurements, the mechanism of particle-triggered corona was discussed. Moreover, corona activity and breakdown voltage were investigated by changing the particle position. This study confirmed that the active corona development and fall in breakdown voltage were due to the effect of intermittent short-circuit arc in the minor gap between particle and electrode when the particle was in the vicinity of the electrode

Light emission from corona discharge in SF6and SF6/N2gas mixtures at high pressure

A spectroscopic investigation of corona discharges in SF 6 /N 2 gas mixtures has been undertaken using an optical multichannel analyser (OMA). A point-to-plane geometry has been used with point radii varying from 3 to 10 μ m. Spectra are measured for high pressures ranging from 0.2 MPa up to 1.4 MPa and for different concentrations of SF 6 in the gas mixture. The spectra in the 200–850 nm spectral range are mainly made of molecular bands, which is indicative of a low temperature discharge. It has been noted that SF 6 emits in the region of 420 nm to 510 nm in positive and negative polarities, although in negative polarity the emission is weaker. For SF 6 /N 2 mixtures, the main source of light emission is from N 2 . The resultant spectra are used to evaluate the rotational T r and vibrational T v temperatures of excited N 2 , T r being considered, due to the high pressure, to be equal to the kinetic temperature T kin in the corona discharge. T r and T v are determined by comparing the experimental spectrum of the second positive system ( of N 2 and the simulated one, which is obtained using a convolution method. As expected, the results show that the measured rotational temperature T r increases steadily with the mean discharge current, while its increase with gas pressure is less pronounced. The values of T r are higher for the positive corona discharge than the negative and also for mixtures having higher amounts of SF 6 . In all conditions, we found and T v is less sensitive to the variation of the current.

Physicomathematical analysis of surface modification of polymers by glow discharge in [formula omitted] medium

In this paper, surface modification of dielectric polymers, having different physical and chemical properties (polyimide—PI, polystyrene—PS, Polyethylene terephthalate—PET), treated in contourized glow discharge in SF 6 + N 2 gas medium near cathode was studied. The electret state, formed as a result of modification, caused an increase in the surface energy. Surface charge densities and contact angles of dielectric polymers whose surfaces were activated under identical conditions, were measured. Upon activation, formation of electret state on the dielectrics strained the sessile drop and consequently the contact angle changed. A mathematical model relating the contact angle and the surface charge density was derived. Experimental results agreed well with the theoretical model.

Detection of partial discharge in SF 6 gas using a carbon nanotube-based gas sensor

For electrical insulation diagnosis of gas-insulated switchgear (GIS), detection of partial discharge (PD) generated in sulfur hexafluoride (SF 6) gas is important. This paper describes a new detection method of PD generated in SF 6 gas using a gas sensor composed of multiwall carbon nanotubes (MWCNTs). The gas sensor was fabricated by electrokinetic manipulation of semiconducting MWCNTs using positive dielectrophoresis. The MWCNT gas sensor and a point-to-plane electrode system were placed in a steel tank filled with SF 6 gas at atmospheric pressure. AC high voltage was applied to the point electrode in order to generate PD while the electrical impedance of the MWCNT sensor was monitored. When the PD was generated, the electrical conductance of the MWCNT sensor gradually increased. The sensor response was reversible and was influenced by the PD intensity as well as by the relative position of the sensor to the point electrode. It was suggested that the sensor conductance increase was caused by an electronic interaction between MWCNTs and non-identified oxidative decomposition products. The faint PD, which could not be sensed by gas detecting tubes, was successfully detected by the MWCNT sensor on a real time basis.

Characteristics of 1 MHz Discharge in SF 6/O 2 Mixture in a Parallel-Plate System

The influence that mixing SF6 with O2 has on the discharge characteristics varies with the pressure. At the total pressure of 0.5 Torr, the discharge structure changes remarkably and becomes almost the same as that of SF6 discharge by mixing only 5% SF6 with O2. At the total pressure of 0.1 Torr, the discharge-sustaining voltage, the discharge current and the spatiotemporal profile of optical emission do not vary noticeably by mixing only 5% SF6, though the radial distribution of plasma density changes clearly. In this case, the electronegativity of SF6 influences obviously on the discharge characteristics of SF6/O2 mixture at SF6 ratios greater than 50%.

Characteristics of 1-MHz Discharges in SF 6/Ar and CF 4/Ar Mixtures in a Parallel-Plate System

In SF6/Ar and CF4/Ar mixtures at a total pressure of 0.5 Torr, the discharge characteristics at 1-MHz have been investigated as a function of the mixing ratio. In the case of the SF6/Ar mixture, discharge parameters, such as discharge-sustaining voltage and discharge current, suddenly decrease by mixing only 5% SF6 with Ar, and the plasma exists uniformly only inside the gap between electrodes. By increasing the SF6 ratio, the electron temperature rises, and tends to saturate at large SF6 ratios. In the case of the CF4/Ar mixture, the plasma density near the discharge axis is lower than that around the edge of the electrode at small CF4 ratios. At a CF4 ratio of 20%, in particular, the plasma density in the interelectrode gap becomes very low, and the spatiotemporal emission profile across the gap around the edge of the electrode is different from that at the axis. At CF4 ratios greater than 60%, the discharge parameters suddenly change due to the electronegativity of CF4. These characteristics are synergistic.

Research on the Degradation of Insulating Material by Feeble Discharge in SF&lt;sub&gt;6&lt;/sub&gt; Gas due to the Quantitative Measurement of Discharge Light

Research on the Degradation of Insulating Material by Feeble Discharge in SF&lt;sub&gt;6&lt;/sub&gt; Gas due to the Quantitative Measurement of Discharge Light

Voltage-current characteristics of high-current pulsed discharges in SF

A high-current pulsed discharge in SF6 has been used as a closing switch for a pulsed power generator. The characteristics of a high-current pulsed discharge are not clear, since it is difficult to measure the time-dependent arc voltage accurately. In the present paper, the voltage-current characteristics and the time-dependent arc resistance of the high-current pulsed discharge in SF6 are obtained accurately from the arc current and the breakdown voltage by solving the circuit equation. The electrode separation and the pressure are changed from 0.1 to 1 cm and from 200 to 760 torr, respectively. The current rise time is about 380 ns. The minimum arc resistance occurs at the maximum arc current, and is from 0.04 to 0.1 Ω under the experimental conditions. The voltage-current characteristics in SF6, which are characterized by five phases, are similar to the characteristics in air but not in Ar.