Prebreakdown Phenomena Research Articles

Effect of electric field configuration on streamer and partial discharge phenomena in a hydrocarbon insulating liquid under AC stress

This paper concerns pre-breakdown phenomena, including streamer characteristics from a fundamental perspective and partial discharge (PD) measurements from an industrial perspective, in a hydrocarbon insulating liquid. The aim was to investigate the possible changes of the liquid’s streamer and PD characteristics and their correlations when the uniformity of the AC electric field varies. In the experiments, a plane-to-plane electrode system incorporating a needle protrusion was used in addition to a needle-to-plane electrode system. When the applied electric field became more uniform, fewer radial branches occurred and streamer propagation towards the ground electrode was enhanced. The transition from streamer propagation dominated breakdown in divergent fields to streamer initiation dominated breakdown in uniform fields was evidenced. Relationships between streamer and PD characteristics were established, which were found to be electric field dependent. PD of the same apparent charge would indicate longer streamers if the electric field is more uniform.

Prebreakdown phenomena in liquids: propagation ‘modes’ and basic physical properties

Prebreakdown phenomena in liquids (usually called ‘streamers’) were characterized experimentally in a wide range of experimental conditions, liquid nature and additives. The description of these phenomena leads to a complex and frequently confused situation, since their properties widely vary when parameters such as liquid nature or voltage are changed. In this review paper, we try to obtain a more comprehensive presentation of pre-breakdown phenomena in liquids by considering different propagation ‘modes’, which can be recognized in a wide range of liquid nature and experimental conditions. On the basis of experimental results either published previously or original, we will review the main parameters and features able to better characterize these modes, and show their general interest. ‘Positive’ streamers observed in point-plane geometry in non-polar liquids are mainly considered here. Some elements allowing a comparison with water, characterized by a much higher permittivity and conductivity are given. The text is concluded by considerations concerning the different physical processes involved in modes.

Pre-breakdown phenomena in new vegetable oil - based jatropha curcas seeds as substitute of mineral oil in high voltage equipment

Due to their high biodegradability, non-toxicity and higher fire safety guarantee, vegetablebased oils are considered today as a potential substitute for mineral and synthetic liquids for electrical insulation and especially in high voltage power transformers. However most of known vegetable oils are derived from food materials (rape-seeds, sunflower, soybeans, palms, corns and others). And one has to be vigilant to the fact that the replacement of mineral oil by natural ester fluids based on “renewably sourced” vegetable oils, does not create new problems as this could cause global food crisis due to diversion of food into insulating material oils or others (fuel). An interesting product can be jatropha curcas oil (JCO) extracted from the fruit seeds of jatropha curcas plants (“miracle tree”) which is non-food crops. These plants can be grown on marginal or degraded soils and thus enables to avoid the need to utilize those more fertile soils currently being used by smallholders to grow their staple crops; and they readily grow in areas where annual rainfall levels are significantly lower than those required by other species such as rape-seeds, sunflower, palm, soybeans, corns and others. This paper presents an experimental study of streamers phenomena in jatropha curcas methyl ester oil (JMEO) and mineral oil (MO) under positive and negative lightning impulse voltages (1.2/50 μs); JMEO is obtained by alkali base catalyzed trans-esterification process of JCO to reduce the viscosity and acidity. It is shown that basing on the streamer characteristics (shape, stopping length, velocity, current and electrical charge), JMEO could constitute a potential substitute for mineral for electrical insulation and especially in high voltage power transformers.

Bipolar model for electrical breakdown in polyethylene materials under dc high electrical fields

Abstract In this paper, we have applied our bipolar transport model for studying the charge dynamics in the insulating polyethylene materials, from nano to micro -scales and under dc high applied field. We have also applied the model for studying the electrical breakdown phenomenon for these mentioned scales. The principal results are dedicated to the evolution of the external current in low density polyethylene samples. Under notable dc high applied field, the electrical pre-breakdown phenomenon is indicated by an abrupt increase that occurs during the steady state of the external current.

Stationary prebreakdown volt-ampere characteristics of weakly conductive liquid dielectrics and slightly ionized gases

A theoretical model of electrohydrodynamic prebreakdown phenomena in slightly ionized (weakly conductive) media is proposed. The electric high voltage conduction of weakly conductive liquids and slightly ionized gases in intense electric fields using this model is considered. The formula for the calculations of volt-ampere characteristics under high voltage spherical capacitor field is analytically obtained. The experimental corona discharge volt-ampere characteristics of slightly ionized gas are presented. Theoretical and experimental results are compared. These results may be applied to the analysis of electrohydrodynamic and electrogasdynamic media flow from thin high voltage axis symmetrical electrodes.

Special issue of the IEEE Transactions on Dielectrics and Electrical Insulation on Vacuum Insulation, Fundamentals and Applications

We are pleased to announce that the February 2016 issue of the IEEE Transactions on Dielectrics and Electrical Insulation (TDEI) will be a special issue on Vacuum Insulation, Fundamentals and Applications. This issue is open to all authors including presenters of papers at the 26th International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV 2014) held on 28 September – 3 October 2014 in Mumbai India who are especially encouraged to submit their work to this special issue. Topics include, but are not restricted to: A: Breakdown and Flashover 1. Vacuum breakdown and pre-breakdown phenomena 2. Surface discharges and flashover phenomena 3. RF breakdown and multipactoring phenomena 4. High field effects in microelectromechanical systems and nano-structures B: Vacuum arcs 1. Switching in vacuum and related phenomena 2. Interaction of vacuum arcs with magnetic field 3. Vacuum arc physics 4. Computer modeling and computer aided design 5. Pulse power physics and technology C: Applications 1. Vacuum interrupters and their applications 2. Deposition of coatings by vacuum arc plasmas and related technologies 3. Electron, ion, neutron, X-ray and other beam and light sources 4. Accelerators and fusion reactor related issues 5. Space related technologies 6. Vacuum arc melting and degassing

Photoionization produced by low-current discharges in O2, air, N2 and CO2

An analysis of gas ionization by the radiation produced in a low-current gas discharge is performed here on the basis of the comparison of measurements and the results of direct simulation of the excitation of identified extreme ultraviolet transitions, radiation transport and ionizing absorption of radiation.In unitary gases and in their mixtures with low admixtures of other gases, the ionizing radiation is generated in one electron impact by direct dissociative excitation and by direct ionizing excitation followed by a radiative transition of an excited neutral atom or ion, respectively. In N2–O2 mixtures, photoionization predominantly occurs via ionization of O2 molecules due to the absorption of the emission produced by excited N2 and, possibly, some N I, both produced by direct electron impact.The results obtained in this work provide the missing data required for performing direct numerical simulation of pre-breakdown phenomena, including the inception and propagation of coronas and creeping sparks, in the above-mentioned gases and mixtures.

Electrical breakdown in air: new experiments and statistical and numerical models

The electrical breakdown and discharges in air are of great significance for science and technology due to wide applications. Besides a breakdown itself, the pre-breakdown phenomena and relaxation after the breakdown have a significant role. In this report the analysis of DC glow discharge and its relaxation processes is presented. Statistical analysis of time delay distributions is presented and the statistical model based on the mixture distribution is applied to data measured with different electrodes from which electron yield is calculated. The occurrence of mixture distributions is physically explained based on analysis of the cathode surface. Also, by analyzing the memory curve measured in synthetic air the early and late relaxation of DC discharge are discussed. The processes responsible for the memory effect are identified and corresponding rate coefficients are determined. In order to confirm that the correct particles are identified, the 2D numerical model for relaxation is developed.

Experimental study on hard x-rays emitted from metre-scale negative discharges in air

We investigate the development of metre long negative discharges and focus on their x-ray emissions. We describe appearance, timing and spatial distribution of the x-rays. They appear in bursts of nanosecond duration mostly in the cathode area. The spectrum can be characterized by an exponential function with 200 keV characteristic photon energy. With nanosecond-fast photography we took detailed images of the pre-breakdown phenomena during the time when x-rays were registered. We found bipolar discharge structures, also called ‘pilot systems’, in the vicinity of the cathode. As in our previous study of x-rays from positive discharges, we correlate the x-ray emission with encounters between positive and negative streamers. We suggest that a similar process is responsible for x-rays generated by lightning leaders.

Photo-ionization and residual electron effects in guided streamers

Complementary experiments and numerical modeling reveal the important role of photo-ionization in the guided streamer propagation in helium-air gas mixtures. It is shown that the minimum electron concentration ∼108 cm−3 is required for the regular, repeated propagation of the plasma bullets, while the streamers propagate in the stochastic mode below this threshold. The stochastic-to-regular mode transition is related to the higher background electron density in front of the propagating streamers. These findings help improving control of guided streamer propagation in applications from health care to nanotechnology and improve understanding of generic pre-breakdown phenomena.

Research on a practical de-noising method and the characterization of partial discharge UHF signals

Experimental studies on partial discharges (PD) occurred in oil-insulated devices using ultra-high frequency (UHF) sensors to detect PD signals and using wavelet analysis to process the acquired UHF signals are presented, and a threshold selecting method adapting to various noise circumstances when de-noising UHF signals using wavelet transform is proposed in this paper. The threshold method was based on numerical fitting of standard deviations and manual setting thresholds for brush-fire samples from a large scale. Horizontal comparisons between different geometries and vertical comparisons between different intensities of partial discharges in oil were also discussed and the mathematical tool was wavelet analysis. Multi-resolution analysis (MRA) results show the significant differences that can be used to recognize the discharge pattern and to judge the intensity. Particularly attention was given to needle-plate discharge geometry, which was often used by researchers focusing on the mechanism of pre-breakdown phenomena in liquid dielectrics. The results of needle-plate model show good agreement with electronic theory of liquid pre-breakdown mechanism.

Degradation analysis and current collapse imaging of AlGaN/GaN HEMTs by measurement of electric field-induced optical second-harmonic generation

On the basis of electric field-induced optical second-harmonic generation (EFISHG) measurements, the degradation of AlGaN/GaN high electron mobility transistor (HEMT) devices was analyzed and the current collapse phenomenon was imaged. Strong second-harmonic (SH) signals were observed at the gate edge at the site of a small defect during the application of a high drain stress voltage (300V), thus indicating that this technique can identify degradation of HEMTs. The EFISHG technique also generated images of the trapped carriers that lead to current collapse in these devices during the on-state. Immediately following the completion of drain-stress voltage application in the off-state (that is, 1μs), SH signals were observed at the gate edge on the drain electrode. This signal location is in good agreement with the well-known virtual gate model of current collapse. The EFISHG measurement technique is therefore capable of detecting pre-breakdown phenomena and producing images of current collapse in AlGaN/GaN HEMT devices.

An Overview of Electrical Tree Growth in Solid Insulating Material with Emphasis of Influencing Factors, Mathematical Models and Tree Suppression

Nowadays, the most widely used insulating materials in high voltage equipment such as cables are polymeric insulations due to the numerous merits they possess with regards to electrical performance compared to paper insulations. However, electrical treeing, one of the dielectric pre-breakdown phenomena, has been considered as a major contribution to the failure of insulating polymeric materials. Thus, this paper provides an overview on the factors affecting the initiation and propagation of electrical tree. Discussions on parameters that affect the growth of electrical treeing such as applied voltage, electric field enhancement, partial discharge, frequency of applied voltage and temperature are given. Some discussions on the various models are also highlighted. The tree related models being discussed herein include Weibull, Lognormal, Johnson SB, Dielectric Breakdown Model, Discharge-Avalanche Model, and Field-Driven Tree Growth. In addition, discussions on the use of nano-sized fillers in polymeric insulating material to inhibit electric treeing are highlighted.

Characterization of streamers in liquid nitrogen under AC voltages using UHF techniques

Pre-breakdown phenomena in liquid nitrogen (LN2), often referred as streamers, are the precursor of total breakdown due to the more conductive nature of density-change channels compared to the liquid. The paper characterizes streamer behavior in LN2 under AC voltages using ultra high frequency (UHF) and radio frequency current transformer (RFCT) measurements. The aim is to further understand the role of LN2 as insulation as well as the application of non-conventional partial discharge (PD) detection techniques in condition monitoring of future HTS devices. An experiment has been developed to measure PD events in LN2 using narrowband and wideband methods simultaneously. PD detection following IEC 60270 standards allows measurement of the amount of charge transferred at the terminals while the use of a UHF senor with a bandwidth up to 3 GHz can provide better resolution in the time domain. Results from the UHF sensor show that PD in LN2 appears in the form of multiple pulses which are suggested to be due to the expansion and collapse of the gaseous channels. The pulse burst phenomenon was not observed by the narrowband detection. Instead, a slowly damped signal was measured, which may result from the integration of multiple individual events. The experiment also allows correlation of PD measurements with images of pre-breakdown streamers using shadow photography. The characteristics of streamers in LN2 are discussed based on the observation of the density-change channels together with the corresponding discharge signature.

Phase resolved analysis of partial discharges in liquid nitrogen under AC voltages

Liquid Nitrogen (LN2) has been used as a coolant as well as a part of the insulation system in High Temperature Superconducting power apparatus. In recent decades, there has been scientific research into the pre-breakdown phenomena in this liquid. The focus of previous studies is on streamer behavior and their governing mechanisms. However, research into the characterization of partial discharge patterns in a composite insulation of liquid nitrogen and solid dielectrics has not been widely reported. This paper presents a characterization of partial discharges in LN2 under AC voltages by the means of phase resolved analysis. The results suggest that discharges in a LN2/solid insulation system occur primarily in the first and third quadrant of the applied voltage waveform due to charge storage effects. Partial Discharge (PD) pulses were also observed before the zero crossing of the waveform under higher electric stress. The conditions under which density change streamers cross a LN2 gap and reach the solid insulation barrier are also discussed. Different PD patterns were observed for different solid dielectric barriers indicating the effects of the solid/liquid interface on the behavior of discharges in point-plane composite systems at cryogenic temperature.

Cavitation in the vicinity of the high-voltage electrode as a key step of nanosecond breakdown in liquids

Fast shadowgraphy of nanosecond discharge in liquids with different dielectric permittivity, namely in water, ethanol and n-pentane, has been performed. Formation of a gas cavity at a nanosecond time scale was observed as a pre-breakdown phenomenon at amplitudes of the high-voltage pulse close to the breakdown threshold. This phenomenon is considered as a possible key step of high-voltage breakdown in polar liquids.

Streak Observation of Dc Pre-breakdown Phenomenon in Silicone Oil/Low Density Polyethylene (LDPE) with Different Viscosities

We observe streak images of dc pre-breakdown in silicone-oil (10 cSt, 100 cSt and 10000 cSt) / low density polyethylene (30 μm in thickness) composite insulation under needle / sphere electrode by using the 20 meter long image guide scope and a streak camera. In the case of composite insulation with 10 cSt of silicone-oil, breakdown light propagates from the needle side under both voltage polarity. However, in the 10000 cSt case, breakdown light propagates from the film side under the negative voltage polarity. The mechanism of these transient phenomena are discussed in this paper.

Spatiotemporal analysis of propagation mechanism of positive primary streamer in water

Currently, further clarification of pre-breakdown phenomena in water such as propagation mechanisms of primary and secondary streamers are needed because applications of aqueous plasma to environmental and medical treatments are increasing. In this study, a series of primary streamer propagations in ultrapure water was visualized at 100-Mega frames per second (100 Mfps) in the range of 400 μm square using an ultra high-speed camera with a microscope lens when a single-shot pulsed positive voltage was applied to a needle electrode placed in a quartz cell. Every observation was synchronized with the waveforms of the applied voltage and the discharge current. The primary streamer, having many filamentary channels, started to propagate semi-spherically with a velocity of about 2 km/s when the pulsed currents occurred. Although most filamentary channels disappeared 400 ns after the beginning of the primary streamer, a few of them continued propagating with almost the same velocity (about 2 km/s) as long as the repetitive pulsed currents flowed. Shock waves were iteratively generated and streamer channels were formed while the repetitive pulsed currents were flowing. Thus, we concluded that the positive primary streamer in water propagates progressively with each repetitive pulsed current.

Vegetable oil based liquid nanocomposite dielectric

Physically smaller dielectric materials would improve the optimisation of space for power systems. Development of nanotechnology provides an effective way to improve the performances of insulating oils used in power system applications. In this research study, we focused on the development of nanomodified vegetable oils to be used in power transformers. Higher conduction currents were observed in virgin linseed oil than in virgin castor oil. However, for both virgin linseed and virgin castor oil, the DC conduction current increased approximately linearly with the applied DC voltage. In nanomodified linseed oil, the characteristic curve showed two distinct regions: a linear region (at lower applied voltage) and a saturation region (at slightly higher voltage). Conversely, in nanomodified castor oil, the characteristic curve showed three distinct regions: a linear region (at lower applied voltage), a saturation region (at intermediate applied voltage) and an exponential growth region (at higher applied voltage). The nanomodified linseed oil exhibited a better dielectric performance than the nanomodified castor oil. Overall, the addition of nanodielectrics to vegetable oils decreased the dielectric performance of the vegetable oils. The results of this study contribute to the understanding of the pre-breakdown phenomenon in liquid nanocomposite dielectrics.

Measurement and simulation of partial discharge in oil-paper insulation under the combined AC–DC voltage

Depending on unique operation characteristics for voltage applied on valve side winding of the converter transformers, it is extraordinarily significant to study the partial discharge (PD) behavior with oil-paper insulation under combined AC–DC voltage. Therefore, this paper investigated PD inception characteristics by pulse current methods with needle-plate electrode system under combined AC–DC voltage. Furthermore, 3D electric field distributions versus combined AC–DC voltage in different ratios were calculated by Finite Element Analysis (FEA). An experimental conclusion was drew that AC partial discharge inception voltage (PDIV) in pure oil would decrease linearly with the DC component increasing but the inception voltage with oil-paper insulation appeared to be independent of DC voltage and dependent of AC voltage. And 3D electric field distribution deduced from simulation provided a supplementary proof on the experimental results. Moreover, high speed photography was used to capture emitted light produced by discharge, estimate streamer velocity (1.8 km/s) and record streamer initiation and propagation process in oil gap. Previous studies have shown that the prebreakdown phenomena involving the generation and propagation of vapor channels through the oil could be divided into a three-stage process.