Negative Lightning Impulse Research Articles

Comparative Analysis of Selected Insulating Liquids Including Bio-Based Hydrocarbon and GTL in Terms of Breakdown and Acceleration Voltage at Negative Lightning Impulse

Comparative Analysis of Selected Insulating Liquids Including Bio-Based Hydrocarbon and GTL in Terms of Breakdown and Acceleration Voltage at Negative Lightning Impulse

Combined effect of lightning impulse voltage and temperature stress on the propagation of creeping discharge of oil-impregnated paper

This article presents the results of an experiment designed to study the impact of temperature on the characteristic parameters of creeping discharges. The insulating interfaces consist of a thermally enhanced cellulose surface immersed in mineral oil, palm kernel oil methyl ester (PKOME) and castor oil methyl ester (COME). The study was carried out under a standard negative lightning impulse voltage (1.2/50 μs). The article also presents the complete algorithms for calculating the maximum extension of the discharges, the ionisation rate and the charge produced by them. The results of the study show that temperature favors the propagation of discharges and the ionisation rate. It was observed that liquids with a higher dielectric constant and high electrical conductivity were more exposed to the impact of temperature. The results show ionisation increments of 0.973%/°C, 1.093%/°C and 1.076%/°C in mineral oil (MO), COME and PKOME respectively. The maximum extension of the discharges shows a linear evolution with the applied voltage and temperature but a non-linear increment with the temperature. As for the charge produced, it shows a constant increment with temperature and voltage in each liquid. These values are (5.839%/°C, 1.977%/kV), (6.047%/°C, 2.082%/kV) and (6.177%/°C, 2.113%/kV) respectively in MO, COME and PKOME.

Atomic scale microparameter analysis of modified natural ester molecules related to impulse discharge characteristics under electric field

Modified natural ester (MNE) has significant potential for application in transformers as an environmentally friendly and renewable energy source. In comparison to natural esters, MNE offers notable advantages in terms of viscosity and power frequency breakdown voltage. However, the lightning impulse breakdown characteristics of MNE and the atomic-scale microscopic mechanisms related to its discharge characteristics remain unclear. Understanding the lightning impulse breakdown characteristics of MNE is crucial for ensuring the safe operation of transformers. In this paper, the density functional theory is used to calculate the change law of the microscopic parameters related to the discharge characteristics of MNE under the electric field under the action of electric field, so as to theoretically evaluate the lightning impulse discharge behavior of MNE. The results reveal that the electronic and geometric structures of MNE molecules are influenced by electric fields, which is the primary factor contributing to the variations in microscopic parameters under different electric field strengths. As the electric field intensity increases, the dipole moment of MNE tends to increase, while the ionization potential and excitation energy of MNE tend to decrease. The decrease of ionization potential, excitation energy and energy gap of MNE molecules is about 6.61%–68.01%, 4.34%–47.66% and 2.01%–69.23%. By comparing the calculated results of mineral oil, natural ester, and MNE microparameters, it can be inferred that the lightning impulse breakdown behavior of MNE is similar to that of natural ester and lightning impulse breakdown voltage of MNE is higher than that of natural ester but still lower than that of mineral oil. The average negative polarity lightning impulse voltages of MNE, natural ester and mineral oil in the 85 mm oil gap are about 245 kV, 199 kV and 304 kV, respectively. The test results confirmed the simulation conclusions. This study provides robust theoretical support for understanding the lightning impulse breakdown characteristics of MNE and ensuring its safe operation in transformers.

Statistical analysis and interpretation on lightning impulse breakdown voltages of ester liquids under varying field uniformity

AbstractEster liquids (ELs), as the alternative transformer liquids to the conventional mineral oils (MOs), have been increasingly used in power transformers with high voltage ratings. Among various properties, the lightning impulse breakdown voltages (LIBDVs) have been studied elaborately in the past two decades under different electric field uniformities. The electric field uniformities do have an effect on the breakdown characteristics, for example, breakdown mechanisms, polarity effects, and differences of LIBDV between ELs and MOs, which is quantitatively investigated based on the LIBDVs collected from the literatures. The breakdown mechanism transition phenomenon between streamer initiation dominated breakdown and streamer propagation dominated breakdown has been reviewed firstly. Then, the transition field factor boundaries to observe the polarity effect on LIBDVs are taken at around 20 for ELs and 10 for MOs. Besides, the transition field factor boundaries to observe the LIBDV distinctions between ELs and MOs are about 109 and 14 under positive and negative lightning impulse, respectively. The systematic analysis and interpretation on the transition field factor boundaries among different phenomena can provide insights on insulation design of ELs filled power transformers.

Streamer and Breakdown Characteristics of Transformer Liquids in Moderately Uniform Fields Under Negative Lightning Impulse

The identification of transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown is important for understanding the breakdown mechanisms of transformer liquids and for the transformer insulation design in practice. Most previous studies focused on either very divergent field or nearly uniform field. This paper reports on breakdown voltage tests with three transformer liquids in moderately-uniform fields. The results show that the transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown under negative lightning impulse is liquid dependent. It is 9.6 for a mineral oil, 7.3 for a GTL oil, and 42.0 for a synthetic ester. The difference of the transition field factor boundary helps explain the observed breakdown voltage distinction among the three transformer liquids. In addition, the characteristics of negative streamers leading to breakdown were also investigated in the moderately-uniform fields. Streamer mode transitions with gap distance were observed and clarified, which is from 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> mode streamer to 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> mode streamer in both the mineral oil and GTL oil, while it is the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> mode streamer to fast streamer in the synthetic ester.

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.

Experimental Analysis of Streamer Currents Under Lightning Impulse Voltage Using Fast Fourier and Discrete Wavelet Transform Methods

This paper presents an experimental comparative study of streamer currents propagating in mineral oil, natural esters (olive oil and rapeseed oil) and synthetic ester (tetra-ester) under both positive and negative lightning impulse voltages. Because of the similarity in the shapes of streamer currents in investigated liquids, it is difficult to differentiate between the current signals corresponding to each liquid. For that purpose, two techniques are used for the analysis of these signals. The first one is the Fast Fourier Transform (FFT) technique, which is based on the frequency spectrum analysis. The second one is the Discrete Wavelet Transform (DWT) method, which has demonstrated its ability for the analysis of the non-stationary signals. The obtained experimental results show spectral signature specific to each liquid, thus making it possible to differentiate the various streamer currents. Overall, the two techniques show good performance. However, the accuracy and effectiveness of the DWT method is better than those of the FFT technique.

Breakdown Characteristics of Double Vacuum Gaps in Series Under Lightning Impulse Voltage: Optimal Gap Distance Distribution

An optimal distribution of gap distance for the double vacuum gaps (VGs) in series can improve its dielectric strength. The objective of this article is to quantitatively determine the optimal distribution of gap distance for the double VGs in series. The double VGs were built up by vertically connecting two pairs of copper sphere-plane electrodes in series. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> was the percentage of the upper VG distance in the total gap distance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{T}$ </tex-math></inline-formula> of 5 mm. A negative standard lightning impulse voltage (LIV) was applied in the dielectric test. The experimental results reveal that, under <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> of 70%, the breakdown (BD) voltage of double VGs reached the maximum 192.7 kV, which was 13.2% higher than that of a single VG with the same <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{T}$ </tex-math></inline-formula> . Therefore, the optimal gap distance distribution of double VGs was determined to be 70%/30% in this experiment. Moreover, when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> was less than 40% or greater than 90%, the dielectric strength of double VGs was mainly determined by the VG with a longer distance. When <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> was in the range of 40%–90%, the dielectric strength of double VGs can be supported by both VGs and significantly improved. The dielectric strength should reach the maximum on the condition that the maximum electric field strengths at the cathode of two VGs were balanced.

Study on the Electron Density Measurement of 1 m Rod-plate Gap Discharge Process under the Lightning Impulse

Observation and measurement of the microscopic parameters in the long air gap discharge process play an important role in studying its physical mechanism and the discharge simulation model. In this paper, we combined an electron-multiplying intensified charge-coupled device (EMICCD) camera and a spectrometer, and built a lightning discharge spectrum observation platform of spatiotemporal distribution for the 1 m rod-plane air gap with nanosecond exposure time. The spatiotemporal distributions of the spectral information of the 1 m air gap discharge under positive and negative lightning impulses were obtained. Based on the atomic emission spectroscopy diagnostic method, N and O atomic spectral lines were used to calculate the corresponding electron density at different positions and time points during the breakdown discharges. The testing results were compared with other literature and the differences also were discussed.

Study of Creeping Discharges Propagating Along a Solid Insulating Surface Immersed in Mineral Oil and Palm Kernel Oil Methyl Ester Under Negative Lightning Impulse Voltage

This article presents the results of an experiment designed to study creeping discharges on a solid insulating surface immersed in palm kernel oil methyl ester (PKOME) compared with those produced from mineral oil (MO). The investigation takes place under standard negative lightning impulse voltage application, and for the three following solid insulating surfaces: hardened glass (HG), porcelain (PO), and simple glass (SG). The distributions of the radial discharge occupancy densities as a function of the thickness of the solid material and the insulating liquid are evaluated. A statistical study of the total and final length of the branches produced by the discharges is presented, as well as the current amplitudes and the charge produced by this phenomenon. A fractal analysis of the phenomenon is also presented. The experimental results show that the distribution of the average radial density of the discharge, the total and final lengths of the branches, and fractal dimension depend strongly on the physicochemical characteristics of the insulation system.

Breakdown characteristics of triple vacuum gaps in series under lightning impulse voltage: Effect of floating potential gap

Inside a high-voltage vacuum interrupter, multiple floating shields form the multiple vacuum gaps (VGs) in series. When the number of series VGs exceeds three, there will exist floating potential VGs in which both electrodes have floating potentials. The floating potential VG is important for the dielectric strength of multiple VGs in series. The objective of this paper is to quantitatively determine the effect of the floating potential VG distance on the breakdown (BD) characteristics of triple VGs in series. In the experiments, triple VGs were built up by series-connecting three pairs of sphere-plane electrodes vertically. A negative standard lightning impulse voltage (1.2/50 μs) was applied. The experimental results revealed that a critical gap distance of the floating potential VG was required to support the dielectric strength of triple VGs in series. The distance was approximately 0.7 mm in this experiment. The critical distance of the floating potential VG can be determined by an intersection of the maximum electric field strength at cathode and the 100% BD electric field strength of the floating potential VG. This method can be used to estimate the critical distance of the floating potential VG in a high-voltage vacuum interrupter.

Breakdown voltage characteristics of combined air gaps under negative lightning impulse

Long air gaps are the main external insulation medium of overhead transmission lines. When there are floating conductors in the air gap, the gap’s configuration changes, and the combined air gaps is formed. We built an experimental platform to obtain the breakdown characteristics of a 2 m rod–plane air gap with a rod floating conductor in different positions under negative lightning impulse. The results show that as the distance between the floating conductor and the high voltage conductor changes, the breakdown voltage (BV) and timing change. Moreover, the breakdown voltage increases with the length of sub-gap 1, showing a trend of first increasing, then decreasing, and then increasing, which is different from the trend under positive lightning impulse. The reason for this phenomenon is inferred from the analysis to be related to the polarity effect.

Lightning Impulse Breakdown Voltage RBDPO with the Presence of Metallic Particles Under a Uniform Electric Field

The performance of liquid insulation in power transformer is vulnerable to particles, especially the metallic particles. This paper presents the experimental study on the effect of metallic particles on the palm oil (PO) as dielectric insulating fluid under lightning impulse breakdown voltage. The type of PO used in this study is refined, bleached and deodorized Palm Oil (RBDPO) Olein. Two different types of metallic particles (copper and iron) with different concentrations (clean, low, medium, and high) were studied. The lightning impulse test has been carried out according to IEC 60897 standard and under the influence of a uniform electric field. For the comparative purpose, similar test has been carried out with mineral oil (MO). The presence of metallic particles reduces the average lightning impulse breakdown voltage of MO and PO but shows less significant effect to MO. This is because the streamers in the PO propagate faster and further than in the MO at the same voltage level. Hence, causing the breakdown voltage of PO lower than MO. Under negative lightning impulse, the breakdown voltage of MO is slightly higher than RBDPO. From the study, the increment of the number of particle level contamination will reduce the lightning impulse breakdown voltages of the PO.

Effect of Concentration of Al2O3 Nanoparticles on Electrical Properties of Mineral Oil

This study aimed to compare the electrical characteristics of mineral oil-based nanoparticles to those of pure mineral oil. Mineral oil used in distribution transformers was mixed with aluminum oxide (Al2O3) nanoparticles, which serve as an insulator, with 0.01, 0.03, and 0.05 % mineral oil volume concentrations. The moisture content in mineral oil was measured beforehand, and the electrical tests were subsequently conducted in accordance with international standards. The moisture content test revealed that the combination of nanoparticles and mineral oil resulted in an increase in moisture content. As regards the test of AC withstanding voltage and negative polarity lightning impulse voltage, it was found that the mineral oil mixed with the nanoparticles had higher dielectric strength than the original mineral oil. In addition, the test results of positive polarity lightning impulse voltage demonstrated that with a mixture of mineral oil and the nanoparticles at a 0.01 % concentration, the positive polarity lightning impulse dielectric strength of the mineral oil increased. However, at concentrations of 0.03 and 0.05 %, it did not rise, regardless of the increasing volume of nanoparticles. The results, thus, showed promising directions for applications of nanoparticles in the development of electrical properties of mineral oil. HIGHLIGHTS The addition of Al2O3 nanoparticles to the mineral oil resulted in an increase in the oil moisture content. However, it was found that the moisture content in the mineral oil tended to decrease with the increase in the amount of nanoparticles The optimum nanofluid combination in terms of increased AC breakdown strength was created by adding Al2O3 nanoparticles to mineral oil at a rate of 0.05 vol% The Al2O3 nanoparticles being added to the mineral oil can give an increase in the impulse breakdown voltage with the negative polarity For the positive polarity of the impulse breakdown voltage, the best nanofluid mixture in terms of increased breakdown strength was produced by the addition of Al2O3 nanoparticles at a rate of 0.01 vol% to the mineral oil GRAPHICAL ABSTRACT

ОЦІНКА ПАДІННЯ НАПРУГИ В ІНДУКТИВНОМУ ОПОРІ З'ЄДНУВАЛЬНИХ ПРОВОДІВ НЕЛІНІЙНИХ ОБМЕЖУВАЧІВ ПЕРЕНАПРУГ

Purpose. The results of studying the voltage drop in the inductive ractance of the connecting wires of surge arresters installed in open switchgear are presented. Methodology. Free software for circuit simulation was used to solve electri- cal circuits with nonlinear parameters. To take into account the differences between the negative and positive lightning current impulses, the oscillograms of real lightning currents were digitized using free specialized software. Results. The duration of the impulse front (as well as the total impulse duration) of the positive lightning significantly exceeds the corresponding parameters of the negative lightning current, so the charge carried by the positive lightning significantly exceeds the charge carried by the negative lightning. For these reasons, positive lightning, although less common than ЕЛЕКТРОЕНЕРГЕТИКА, ЕЛЕКТРОТЕХНІКА ТА ЕЛЕКТРОМЕХАНІКА Вісник КрНУ імені Михайла Остроградського. Випуск 4/2021 (129) 162 negative lightning, is more dangerous. It is found that due to the slower front of the impulse, the voltage drop in induc- tive reactance of connecting wires of the surge arresters in the case of positive lightning in 8-11 times less than in the case of negative lightning. Analytical formulas for calculating the inductance of the connecting wires, which take into account the geometric dimensions of the wires, provide more accurate results than the average inductance value of 1.3 μH/m given in the literature. Originality. The approach takes into account the differences between impulses of light- ning currents of negative and positive polarity, which cannot be achieved with use of the approximation of lightning currents by simplified mathematical expressions. Practical value. It is found that voltage drops in the inductive reac- tance will be more dangerous in case of long wire lengths and negative lightning current impulses with a steep front. Conclusions. Further development of the proposed approach is seen in its use for the analysis of circuits for protection of electrical equipment of high-voltage electrical networks from lightning overvoltages using surge arresters. Refer- ences 15, figures 10.

Effect of Electric Field Uniformity on Breakdown Mechanisms of Dielectric Liquids under Lightning Impulse

The electric field uniformity imposes a significant effect on the breakdown mechanisms of dielectric liquids. In general, breakdown is dominated by streamer initiation under uniform field, while it is dominated by streamer propagation under non-uniform field. This paper presents a study on identifying and quantifying the transition field factor boundary, represented by the field enhancement factor f, between the two dominating breakdown mechanisms. The methods for deducing the transition field factor boundary are introduced. The first method from our previous study was developed by measuring and processing both the streamer initiation and breakdown voltages. In this paper, a new method is developed by only processing the breakdown voltage data. Based on the breakdown results reproduced from the literature, the transition field factor boundaries are derived for the mineral oil under both positive and negative lightning impulses. It is found that the negative transition field factor boundary is lower than the positive one. A four-region model is proposed to describe the effect of impulse polarity on breakdown mechanisms of dielectric liquids and to interpret the transition of breakdown mechanisms with respect to the field uniformity.

Numerical Evaluation on the Propagation of Non-breakdown Streamer in Natural Ester under Negative Lightning Impulse Voltage via Shadowgraph Imaging

A modified shadowgraph imaging setup equipped with a high-speed camera is utilized to achieve a numerical evaluation of temperature, number densities of molecules, ions and electrons to the refractive-index of negative non-breakdown streamer in rapeseed insulating oil for revealing the dynamics of streamer propagation. The non-breakdown streamer is found reaching fairly high speed of 3.01-4.66 km/s during 0-1.7 μs. The following 10 μs is dominated by streamer propagation with the grayscale decreasing continuously for the entire streamer. The dissipation is identified starting at ca. 11.7 μs in the stem region while the head of streamer remaining further propagation. This is followed by a dissipation expansion from the stem to the head regions. The double-unsaturated triglycerides are evidenced as the main ionization components in the non-breakdown streamer, which aid to identify the temperature variations, instead of electrons, positive ions, or dissociation of molecules, contribute mainly to the refractive index drop of the streamer. The mapping for the temperature distribution of streamer is resultantly possible and provides the insight to ascertain the inhomogeneous migration of electrons the kinetic superiority for the streamer evolution dynamics.

Negative Lightning Impulse Breakdown Characteristics of Mineral Oil with Different Nitrogen Addition Treatment Methods using Rogowski Electrodes

Negative Lightning Impulse Breakdown Characteristics of Mineral Oil with Different Nitrogen Addition Treatment Methods using Rogowski Electrodes

Influence of Molecule Structure on Lightning Impulse Breakdown of Ester Liquids

Ester liquids are environmentally friendly insulating oils, and they can be used as an alternative to mineral oil in transformers, even though in most countries spills of ester oils must be treated like spills of mineral oil. Furthermore, the breakdown characteristics of ester liquids are worse than those of mineral oils in heterogeneous electric fields. In this paper, we present a comprehensive experimental research on both positive and negative lightning impulse breakdown properties in point-plane geometries with gaps varying from 1 mm to 50 mm. The breakdown voltages and streamer velocities of five kinds of ester liquids, including natural ester, synthetic ester, and three kinds of single component esters have been measured. The results show that the double bonds have no effect on the breakdown voltage of ester liquids. The average streamer velocities of mono-esters are faster than that of other esters under positive polarity, and the breakdown voltages of all esters are close.

Drift-diffusion model of corona discharge in coaxial geometry due to negative lightning impulse voltage

Corona discharge at atmospheric pressure in cylindrical geometry is simulated numerically for negative lightning voltage impulse. The simulation is performed by solving the drift-diffusion-reaction equations for electrons, positive and negative ions. Spatial dependence and time evolution of the concentration of charged particles and created space electric field are used for detailed theoretical analysis of corona discharge based on the values of the transport and reaction gaseous coefficients. This exact drift-diffusion model enables clear physical picture of the corona discharge dynamics. It is used for the determination of the charge-voltage curves necessary for taking into account the corona effects in transient overvoltage calculation on transmission lines. In contrary to the previous engineering corona models, it is shown that the corona charge continues to increase although the input voltage pulse decreases after reaching its maximum value. This behavior is confirmed by measurements in a few experimental studies. Secondly, it has been shown that according to this model there are no positive streamers formation departing from central electrode or wire during decreasing edge of the input voltage pulse. The total charge simply commences to decrease when the negative ions neutralize themselves on the central electrode or wire of positive polarity. A sharp peak of corona current is observed immediately after the beginning of the intense ionization. The drift-diffusion model could also be used to check the validity of other common assumptions and predictions of previous engineering corona models.