Point-plane Electrode Arrangement Research Articles

Prebreakdown and Breakdown Behavior of Low Pour Point Dielectric Liquids Under Negative Lightning Impulse Voltage

In this paper, some investigations on the prebreakdown and breakdown phenomena of low pour point insulating liquids under negative lightning impulse voltage are reported. The tested liquids include mineral oil, a typical synthetic ester, and two low pour point synthetic esters. These liquids underwent accelerated thermal aging. The non-aged and aged samples were subjected to lightning impulses using a point-plane electrode arrangement. The discussions are focused on the initiation of partial discharges, propagation of streamers, and breakdown behaviour in the non-aged and aged liquids. The investigated parameters include inception voltage, lightning impulse breakdown voltage, streamer acceleration voltage, and streamer velocity. The results are supported by the oscillographs of the light activity that is recorded during the discharge process. The prebreakdown phenomenon noticed in the typical synthetic ester vis-à-vis mineral insulating oil is in line with the existing literature. Importantly, it is noticed that the inception and breakdown voltages of the non-aged low pour point synthetic esters are similar to non-aged mineral oil. In addition, the inception and breakdown voltages of the aged low pour point synthetic esters are noticed to be higher than that of the aged mineral oil. These results add to the arguments in favor of replacing mineral oils in power transformers.

Positive streamers: inception and propagation along mineral-oil/solid interfaces

This paper presents an experimental characterization of the prebreakdown phenomena in liquid/solid interfaces. The characterization is devoted to the 2nd mode positive streamers initiated and propagated along interfaces of mineral-oil and solids with different chemical composition and physical properties. Polymers of low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF) and papers made of kraft paper and a kraft fibril paper (made from cellulosic micro and nano fibrils), lignin-free paper and paper with high lignin content (referred to as k107 kraft paper) are used as the solid to study their influence on the streamer inception and propagation. The streamers are initiated at the interface by applying steps of voltage to a point-plane electrode arrangement with a solid (dielectric barrier) into the gap. The solid is placed diagonal to the oil gap and near to the point electrode. Shadowgraphs, charge and light intensity recordings are obtained during the inception and propagation of the streamers. Thus, estimations of the streamer length, velocity, current and average charge, are also presented. A time delay has been observed before the initiation of the streamer. This delay is probably correlated to the initiation process and formation of the gaseous phase of the streamer near to the interface. The threshold propagation voltage of the 2nd mode streamers at mineral-oil/solid interfaces is shown to be independent of the interface. However, the inception voltage is highly influenced by the interface. Additionally, the observed characteristics of streamers propagation (e.g. current, length, velocity, etc) along the tested interfaces cannot be fully explained by a capacitive coupling effect (permittivity mismatch). This open a discussion for the possibility that properties of the solid such as chemical composition, wettability and surface roughness can influence the streamer propagation.

Electrical Detection of Creeping Discharges over Insulator Surfaces in Atmospheric Gases under AC Voltage Application

Creeping discharges over insulator surfaces have been related to the presence of triple junctions in compressed gas insulated systems. The performance of dielectric materials frequently utilised in gaseous insulating high voltage applications, stressed under triple junction conditions, has been an interesting topic approached through many different physical perspectives. Presented research outcomes have contributed to the understanding of the mechanisms behind the related phenomena, macroscopically and microscopically. This paper deals with the electrical detection of creeping discharges over disc-shaped insulator samples of different dielectric materials (polytetrafluoroethylene (PTFE), epoxy resin and silicone rubber) using atmospheric gases (dry air, N2 and CO2) as insulation medium in a point-plane electrode arrangement and under AC voltage application. The entire approach implementation is described in detail, from the initial numerical field simulations of the electrode configuration to the sensing and recording devices specifications and applications. The obtained results demonstrate the dependence of the generated discharge activity on the geometrical and material properties of the dielectric and the solid/atmospheric gas interface. The current work will be further extended as part of a future extensive research programme.

Streamer propagation in a non-uniform electric field under lightning impulse in short gaps insulated with natural ester and mineral oil

Abstract This article describes the comparative experimental studies on streamer propagation in natural ester and mineral oil under a high voltage lightning impulse. These studies were concentrated around the small electrode gaps and the point-plane electrode arrangement. The spatial shapes of the developing streamers, light emission and propagation velocity were analyzed and compared between the two different dielectric liquids. In both of them two streamer propagation modes were registered during the studies performed. Propagation of slow 2nd mode streamers took place below the so-called acceleration voltage while fast 3rd mode streamers developed at acceleration voltage and above. Comparing the streamer shapes corresponding to a given voltage polarity, no visible differences were observed between the liquids tested. Concerning the light emission, higher frequencies of light pulses were registered however in the case of natural ester. The significant differences between both liquids were noticed in the value of the acceleration voltage estimated. In the case of positive polarity streamers started to develop in natural ester as a 3rd mode at lower value of testing voltage than in the case of mineral oil. For negative polarity, within the applied testing voltages, 3rd mode streamers appeared only in natural ester. On this basis, the fundamental conclusion is that natural ester may have a lower ability of preventing the development of fast and energetic 3rd mode streamers, even at small electrode gaps.

Streamer propagation in small gaps of synthetic ester and mineral oil under lightning impulse

This paper describes experimental studies on streamer propagation in synthetic ester and mineral oil under lightning impulse voltage. These studies are concentrated around the small electrode gaps (15 and 20 mm) of the point-plane electrode arrangement. The spatial shapes of the developing streamers, oscillograms of light emitted and propagation velocities were analyzed and compared. Two streamer propagation modes were observed during the studies performed. Slow 2nd mode streamers developed below the so-called acceleration voltage while fast 3rd mode streamers developed at acceleration voltage and above. Independently of the liquid used the registered streamers shapes corresponding to a given voltage polarity were identical. Concerning the light oscillograms, higher frequencies of light pulses were observed in the case of synthetic ester. A significant difference between both the considered liquids was however noticed in the threshold value of the testing voltage at which the 3rd mode streamers appeared. Fast positive streamers started to develop in synthetic ester at lower testing voltage (of about 26 kV for 20 mm gap and 20 kV for 15 mm gap) than in the case of mineral oil. For negative polarity of lightning impulse, within the applied testing voltages, 3rd mode streamers were observed only in synthetic ester. For both electrode gaps acceleration voltage for negative polarity was 2.2 of estimated inception voltage. This allowed forming the conclusion that synthetic esters have a lower ability of preventing the development of fast and energetic streamers, even at small (15 and 20 mm) electrode gaps.

Streamers Optical and Electrical Characteristics Correlations in Liquid Dielectrics Under Alternating Current Stress

This paper is devoted to the analysis of optical and electrical characteristics of positive streamers (namely the emitted light and current signals, electric field, velocity and electrical charge) propagating in mineral oil, tetra-ester and toluene, in a point-plane electrode arrangement, under alternating current voltage. Correlations are established between the different characteristics of streamers especially between the velocity and the current derivative indicating that the streamers present also a certain inductive nature, contrarily to the generally used assumptions where streamers propagating are considered of only resistive and capacitive nature. The frequency spectra of the current pulses are also investigated and enlighten all similarities and differences between the studied oils. Each type of discharge can be well identified in relation with the nature of the liquid by the analysis of the frequency behaviour of the corresponding currents or emitted lights. The behaviour of each current streamer is found to be highly influenced by the liquid temperature of evaporation and its electronic affinity.

Investigation on creeping discharges propagating over pressboard immersed in mineral and vegetable oils under AC, DC and lightning impulse voltages

This paper deals with the experimental characterization of discharges (namely the shape and stopping length) propagating over pressboard of different thicknesses immersed in mineral and vegetable oils, under AC, DC and lightning impulse voltages (1.2/50 μs) using a pointplane electrode arrangement; the point being perpendicular to pressboard. It's shown that the thickness of pressboard and the type of oil as well as the magnitude and polarity of voltage significantly influence the characteristics of discharges. For a given thickness, the stopping length of discharges Lf increases quasi-linearly with the voltage (U); and it is longer with a positive point than with a negative point whatever the type of voltage. Lf decreases when the thickness increases. The thinner the pressboard, the more branched are the discharge structures. The density of discharge branches increases when the pressboard thickness is reduced. On the other hand, for a given voltage and thickness of pressboard, Lf is longer under AC than under DC whatever the type of oil. Also, the characteristics Lf(U) in presence of processed vegetable oil and mineral oil are very close except for thin pressboard. The influence of electric field and capacitive effects on the propagation mechanism is also evidenced.

Effect of micro-fillers in polytetrafluoroethylene insulators on the characteristics of surface discharges in presence of SF 6 , CO 2 and SF 6 -CO 2 mixture

This study is devoted to the optical and electrical characterisation of discharges propagating over insulators made of polytetrafluoroethylene (PTFE) filled with different kinds of micro-mineral fillers immersed in gas or gaseous mixture, under lightning impulse voltage (1.2/50 µs), using a point-plane electrode arrangement. The fillers the authors investigated are MoS2, Al2CoO4, SiO2 and CaF2. The gases and mixture the authors considered are SF6, CO2 and SF6-CO2. It is shown that the stopping length of discharges Lf increases quasi-linearly with the voltage; Lf is shorter in SF6 than in CO2 and it is higher when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one. The discharges do not always present a radial structure as reported in the literature. Also, the type of filler greatly influences the characteristics of creeping discharges. In a given gas or mixture, the shortest Lf is obtained with PTFE filled with MoS2 or Al2CoO4. Thus, insulators with these fillers appear as the best insulators.

Investigation on creeping discharges propagating over epoxy resin and glass insulators in the presence of different gases and mixtures

This paper deals with the experimental characterization of discharges propagating over insulators of epoxy and glass, immersed in a gas or a gaseous mixture, under lightning impulse voltages (1.2/50 μs), using a point-plane electrode arrangement. The gases and mixtures we considered are SF6, N2, CO2, SF6-N2 and SF6-CO2. The morphology of creeping discharges and their final lengths are investigated versus the kind of insulator material, the amplitude and polarity of the voltage, the type of the gas (resp. mixture) and its pressure. It is shown that the shape of discharges and their final (stopping) lengths Lf depend significantly on the solid insulator and the type of gas. For given solid and gas, Lf increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges do not always present a radial structure as reported in the literature. For given voltage and pressure, Lf is longer when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one; and Lf is longer with glass than with epoxy. Lf is shorter in SF6 than in CO2 or N2. On the other hand, the increase of SF6 content in SF6-CO2 mixture leads to a significant decrease of Lf. Therefore, the addition of small concentration of SF6 in a given gas mixture improves the dielectric strength of insulating structure.

Analysis of creeping discharges activity at solid/liquid interfaces subjected to ac voltage

This paper presents the experimental characterisation of discharges propagating over insulators immersed in mineral oil, under ac using a point-plane electrode arrangement. The influence of voltage magnitude and the nature of insulator on the cumulative number of discharge events and their locations during one voltage cycle are also investigated. It is shown that the nature and the thickness of insulator have a significant influence on the properties of discharges and especially their morphology and stopping length Lf . For a given thickness, Lf increases quasi-linearly with voltage; it decreases when the thickness is increased and/or the dielectric constant of insulator decreases, thus indicating the important role of capacitive effect in the propagation mechanism. The total number of discharges recorded during 500 cycles of applied voltage increases significantly with the voltage magnitude. The threshold voltage Ui of discharges depends on the kind of material; and the average number of discharges ( nmoy ) increases with the dielectric constant e r , of insulator. For instance, nmoy is the highest for Bakelite (e r =4.8) and the lowest for Polycarbonate (e r =2.9). nmoy of negative discharges seems to be slightly higher than that for positive ones irrespective of the solid sample and the amplitude of voltage.

Detection and Recording of Partial Discharges Below the Inception Voltage with a Point-Plane Electrode Arrangement in Air: Experimental Data and Definitions

Detection and Recording of Partial Discharges Below the Inception Voltage with a Point-Plane Electrode Arrangement in Air: Experimental Data and DefinitionsThe problem of insulation damage from partial discharges below the inception voltage has not yet attracted much attention. Indications of possible damage exist from previous research. In this paper, the possibility of existence of such phenomena is investigated with small air gaps and point-plane electrode arrangements. It is shown that random discharges below inception voltage may exist. Such discharges are registered and their waveforms are discussed. Experimental evidence is offered that discharges change from the pulse-type to a pulseless-type as the air gap becomes larger. Phenomena affecting in some way the insulating systems below inception may help us to better understand the mechanism of small current flow at relatively low voltages and may contribute to a better formulation of dielectric materials. Furthermore, besides the experimental results, the problem of definitions regarding these phenomena is discussed and commented upon.

Influence of the voltage waveform and hydrostatic pressure on morphology and final length of discharges propagating over solid-liquid interfaces

This paper is aimed at the influence of the voltage waveform (lightning impulse, dc and ac) and hydrostatic pressure on the morphology and final length of creeping discharges propagating over solid/liquid insulating surfaces in a point-plane electrode arrangement. Different solid materials immersed in mineral oil are tested. It is shown that under ac and dc, the discharges do not present a radial shape as observed with the negative discharges under lightning impulse voltages. For a given solid sample, the final length L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> (i.e. the maximum extension) of discharges is longer with ac than with lightning impulse voltage and dc. The currents waveforms are similar to those observed in liquids. On the other hand, the length of discharge branches is reduced when the hydrostatic pressure is increased whatever the voltage waveforms and polarity. L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> increases quasi-linearly with the voltage and decreases when the hydrostatic pressure is increased. And for given voltage and pressure, the thinner the solid sample and/or the higher its dielectric constant, the longer the discharges are, indicating the important role of capacitive effects in the propagation mechanisms of discharges.

Optical and electrical characterization of creeping discharges over solid/liquid interfaces under lightning impulse voltage

This paper deals with the experimental characterization of discharges propagating over solid insulators immersed in mineral oil, under impulse voltages using a point-plane electrode arrangement. It's shown that the nature and the thickness of the solid insulator significantly influence the characteristics of creepage discharge and especially the final length Lf and the density of branches rho. For a given thickness, Lf increases quasi-linearly with the voltage; it decreases when the thickness increases. So rho increases with the dielectric constant of the solid insulator; it also increases when the thickness is reduced. This indicates the important role of the capacitive effect in the propagation mechanism. The main discharge is followed by a secondary one of opposite sign to that of the main discharge

Dielectric breakdown of epoxy-based composites: relative influence of physical and chemical aging

The effect of aging on the dielectric strength of epoxy-inorganic particle composites used for insulators in the high voltage industry is reported. A differential scanning calorimetry analysis of an insulator aged twenty years in actual service conditions indicated both a chemical degradation and a structural recovery of the polymer network. This composite exhibited however a breakdown field comparable to that of a fresh sample with the same formulation. An accelerated physical aging was thus performed which lead to a large increase in the high voltage performance of the newly processes composite over time. This improvement was attributed to a densification of the thermoset resin, which impeded tree growth. It was also observed that the choice of the electrode geometry greatly alters the measurements under high electric field. In a quasi-homogeneous field configuration, the breakdown was mainly governed by the major flaws at the sample scale, namely the reinforcing particles. On the contrary, under a divergent field (with a point-plane electrode arrangement), the field was essentially localized at the point electrode tip, and the major flaws might not be reachable by the damage tree. It hence appeared that the measurements performed in a quasi-homogeneous field are not very sensitive to the variations within the polymeric matrix as are the measurements under a divergent field.

Fractal analysis of creeping discharge patterns propagating at solid/liquid interfaces: influence of the nature and geometry of solid insulators

The paper is aimed at the fractal analysis of the creeping discharge patterns propagating over different solid insulators immersed in mineral oil, under negative impulse voltage using a point-plane electrode arrangement. The considered solid insulators are circular samples of different thickness made of polycarbonate, phenoplast resin (Bakelite) and glass. Two methods are used to estimate the fractal dimension, namely the box counting method and the fractal measure relations method. It is shown that the creeping discharges propagate radially; the shape and the length of these discharges depend on the types of solid insulators and their thicknesses. By using the box counting method, we show that the discharge patterns present a fractal dimension D which depend on the thickness of the solid samples (e) and the type of insulator. D decreases when e increases. This suggests the possible implication of capacitive effects on the propagation phenomena of creeping discharges. So D increases with the dielectric constant of the insulator; it is the highest for glass and the lowest for polycarbonate; D is in between for phenoplast resin. This dependency of D on the solid insulators and their thicknesses reveals the existence of a relation between the fractal dimension and the physical parameters. The fractal measure relations method gives a fractal dimension equal to the Euclidian dimension (D = 2) due to the fact that the density of branches ρ(r) remains constant whatever the radius r of the circular domain within which ρ(r) is determined. ρ(r) depends on the thickness and the nature of solid insulating material. The value of the density of branches increases when the thickness of the solid insulating material decreases.

Simulation of electrical tree propagation using cellular automata: the case of conducting particle included in a dielectric in point-plane electrode arrangement

The electrical tree propagation is simulated in this paper using Cellular Automata. The polymethylmethacrylate (PMMA) sample has given dimensions and inside this volume a small spherical conducting particle is considered. Point-plane electrode arrangement is used and a DC voltage is applied. The electric strength is calculated over the insulating volume and the electrical tree propagation is shown as the result of the electrical field and the inhomogeneity factor that incorporates the dielectric constant fluctuations of the insulating material.