Positive Lightning Impulse Research Articles

Lightning impulse breakdown path and characteristics with dielectric BMC barrier inserted sphere‐plane electrode

This study deals with the electrical breakdown characteristics of the sphere-plane electrode system with the dielectric barrier of bulk moulding compound (BMC) material under the positive lightning impulse voltage. The sphere electrode has a spherical shape of 50 mm in diameter and suspended with a metal rod of 10 mm in diameter, while the plane electrode has a circular with a diameter of 350 mm. The dielectric barrier has a square plate shape of BMC material, and the dimension is changed from 80 × 80 to 240 × 240 mm2. Experimental results show that electrical breakdown paths could be identified into three categories, depending on the dielectric barrier position and the dimension. The larger and farther away from the sphere electrode the dimension of the dielectric barrier is, the occurrence probability of electrical breakdown through the penetration of dielectric barrier becomes. It was confirmed that the breakdown voltage considerably depends on the dimension of the dielectric barrier and the dielectric barrier position.

Differences in Characteristics of Long-Gap Lightning Impulse Breakdown between Vegetable and Mineral Insulating Oil

Vegetable insulating oils, with higher fire point, lower biological hazard and better availability compared to traditional mineral insulating oils, have enormous potential to become the substitute of the latter, given the trend of green development of the power equipment, for example, the transformers. However, the obviously different constitution of the vegetable oils makes the circumstance hard when considering their application on the insulation for actual transformers. One of the significant deficiencies of the vegetable oils in working condition is their insufficient lightning impulse endurance under partial high electric fields and large discharge gaps. In this article, three kinds of transformer insulating oils, rapeseed oil, FR3 and 25# mineral oil were chosen to investigate the performances of these insulation materials under long-gap lightning impulses with positive and negative polarity respectively. The variation rules of breakdown voltage and velocity with the gap length were obtained. And the difference of the lightning impulse voltage endurance between the vegetable and mineral insulating oils as well as its causes based on theories of streamer and ionization potential (IP) were analyzed, which led to the conclusion that the gap increase causes more threat to the positive lightning impulse breakdown of vegetable insulating oils, compared to its counterpart.

Time-Frequency Analysis of Partial Discharge Current Pulses in Different Gas Environment under Lightning Impulse

Abstract This paper deals with a time-frequency analysis of the measured partial discharge (PD) currents in different insulation gases. These gas environments consist of a pure SF6 (sulphur-hexafluoride) and sulphur-hexafluoride and nitrogen (1 % SF6 + 99 % N2) mixture, under both positive and negative lightning impulse (LI) voltage stresses. In this study, the short time Fourier transform was used to extract the time-frequency information of PDs for different gases at different pressures, and these results were compared to each other. Thus, the relationship between the time, amplitude, and frequency of PD currents was studied. Moreover, some statistical formulas, such as mean, standard deviation, kurtosis, and skewness were applied to the time-dependent PD current data. As a result, a correlation between obtained statistical results and PD frequencies was examined. In most cases, the frequency of partial discharge decreased when the pressure increased. The amplitude of the partial discharges for negative polarity was more than that for positive polarity gas insulations. The partial discharge amplitudes of the pressure of 2 bar were mostly high compared to other pressures. This case demonstrated that SF6 had a maximum minimum character in terms of breakdown. The partial discharge frequency of a 1 % SF6 mixture was higher than that of pure SF6. It is thought that SF6 suppresses the discharge frequency, and statistical evaluations support the experimental results.

Experimental study on the final jump phase in long air gap discharges

The final jump is an essential process of long air gap discharge. This paper will focus on the experimental study on the characteristics of the final jump stage under positive switching and lightning impulses in the long air gaps. To demonstrate the propagation of the leader, a high-speed camera (FASTCAM SAX2) was used to record. The lightning impulse (1.9/50 μs) that was produced by a 7.2 MV Marx generator applied to the 3 m or 5 m air gaps. By looking at the results for the gaps as mentioned above; the observation results showed that the average final jump length under lightning impulse is 1.06 m and 1.91 m, respectively. The average velocity during the final jump phase under positive switching and lightning impulses is obtaining from the observation images by fitting the measurements. According to the discharge images, the method to calculate the length and velocity during the final jump stage. Finally, the current under the lightning impulse voltage is thousands of amperes as compared to switching impulse.

Research on injection charge characteristics of long streamer under positive lightning impulse voltage

Streamer is the main process of air gap discharge, and the injection charge characteristics of a long streamer under positive impulse voltage are of great significance for revealing the discharge mechanism of long air gap, selecting reasonable air gap distance, optimizing insulation configuration, and solving the key problems faced by current transmission engineering. Taking the 1 m rod–plate gap as the research object, the long streamer with a length of 20–35 cm was obtained at the rod electrode head by applying positive lightning impulse voltage, and the total injection space charge with a different radius and length was obtained by measuring the streamer discharge current at the high voltage end and integrating it. The empirical formulas between the total injection charge and the initial voltage and the length of the streamer are deduced theoretically and verified by experimental data. The total injection charge has a linear relationship with the square of the initial voltage and the square of the streamer length. The key physical parameters such as the initial physical process of the streamer under impulse voltage, the minimum initial field strength of the streamer, and the internal electric field in the streamer region are also discussed. Research and discussions [Ebert and Sentman, J. Phys. D: Appl. Phys. 41, 230301 (2008)] were carried out.

Pre-Breakdown Streamer Propagation and Positive Lightning Breakdown Characteristics of Palm Oil Impregnated Aged Pressboard

This paper presents the investigation on the breakdown characteristics and pre-breakdown streamer propagation of Palm Oil (PO) impregnated aged pressboard under positive lightning impulse voltages. The experimental work was carried out under a non-uniform field with needle-plane electrodes configuration. The streamer stopping length and breakdown voltage of 2 types of refined, bleached, and deodorized palm oil were examined in the presence of new and aged pressboards. The pressboard was placed in parallel to the needle-plane electrode at a gap distance of 50 mm. The lightning breakdown voltage was applied to the samples based on 1 shot per step rising voltage method under positive polarity as per IEC 60897. The presence of impregnated pressboard in both PO slightly increases the 50% positive lightning breakdown voltages. PO impregnated pressboards have lower 50% positive lightning breakdown voltages than MO. After subjected to ageing, the positive lightning breakdown voltages for PO and MO impregnated pressboards decrease. In the presence of aged pressboard, the streamers in PO generally propagate further than MO at the same voltage level.

Effect of electric field uniformity on positive streamer and breakdown characteristics of transformer liquids

Streamer development in liquid has different dominant mechanisms under uniform and divergent electric fields, respectively initiation and propagation. This paper presents a study on the influence of electric field uniformity on the pre-breakdown and breakdown characteristics of transformer insulating liquids. Streamers were investigated experimentally in different needle-plane non-uniform field and plane-needle-plane semi-uniform field configurations under standard positive lightning impulses. For the same gap distance, streamers under non-uniform field propagate with a more side-branched structure whereas streamer propagation under semi-uniform field is more constrained to the axial direction. Field enhancement factor was used to indicate the degree of inhomogeneity of the field configuration. Streamers in fields with higher field enhancement factors occupy larger areas. As field enhancement factor reduces, the streamer withstanding probability distribution gets closer to the breakdown probability distribution and hence the breakdown happens at a lower normalized voltage level. Furthermore, a transition phase from a propagation-dominated to an initiation-dominated process of streamer development was evident as the field enhancement factor decreases. The field enhancement factor at which this transition happens was quantitatively derived. It is 54.1 for the investigated mineral oil under positive lightning impulses.

On the stem diameter under positive impulses in long air gaps

The diameter of a stem, a common origin point for streamers where the transition from the streamer to the leader takes place, has a significant effect on the simulation of streamer-to-leader transition. To measure the diameter of the stem, a schlieren system was established with the temporal and spatial resolutions of 5 μs and 70 μm, respectively. Experiments were conducted under positive lightning and switching impulses; the applied voltage, discharge current, and schlieren images were recorded synchronously. A method was proposed to measure the diameter of the stem based on the schlieren images, with which the stem diameter as a function of time was obtained. Moreover, the initial diameters of the stem under different conditions were obtained through the linear fitting. At last, a relation between the initial cross-sectional area of the stem and the discharge current was given within a certain range.

A Promising Nano-Insulating-Oil for Industrial Application: Electrical Properties and Modification Mechanism.

Despite being discovered more than 20 years ago, nanofluids still cannot be used in the power industry. The fundamental reason is that nano-insulating oil has poor stability, and its electrical performance decreases under negative impulse voltage. We found that C60 nanoparticles can maintain long-term stability in insulating oil without surface modification. C60 has strong electronegativity and photon absorption ability, which can comprehensively improve the electrical performance of insulating oil. This finding has great significance for the industrial application of nano-insulating oil. In this study, six concentrations of nano-C60 modified insulating oil (CMIO) were prepared, and their breakdown strength and dielectric properties were tested. The thermally stimulated current (TSC) curves of fresh oil (FO) and CMIO were experimentally determined. The test results indicate that C60 nanoparticles can simultaneously improve the positive and negative lightning impulse and power frequency breakdown voltage of insulating oil, while hardly increasing dielectric loss. At 150 mg/L, the positive and negative lightning impulse breakdown voltages of CMIO increased by 7.51% and 8.33%, respectively, compared with those of FO. The AC average breakdown voltage reached its peak (18.0% higher compared with FO) at a CMIO concentration of 200 mg/L. Based on the test results and the special properties of C60, we believe that changes in the trap parameters, the strong electron capture ability of C60, and the absorption capacity of C60 for photons enhanced the breakdown performance of insulating oil by C60 nanoparticles.

Research on Creeping Flashover Characteristics of Nanofluid-Impregnated Pressboard Modified Based on Fe₃O₄ Nanoparticles under Lightning Impulse Voltages.

Creeping flashover of mineral-oil-impregnated pressboard under impulse stress is a common insulating failure in oil-immersed transformers, arousing increasing attention. Recent studies have shown that the breakdown strength of transformer oil under positive lightning impulse voltage can be significantly improved through nanoparticles-based modification, and Fe3O4 has shown the best improvement in breakdown strength compared to other nanoparticles that have been used. This paper presents the creeping flashover characteristics of pure oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP) based on Fe3O4 nanoparticles under positive and negative lightning impulse voltages, respectively. It was found that NIP possessed higher resistance to creeping flashover than OIP. The relative permittivities of oil and oil-impregnated pressboard before and after nanoparticles-based modification were measured, and the results revealed that the addition of nanoparticles led to a better match in relative permittivity between oil and oil-impregnated pressboard, and a more uniform electric field distribution. Furthermore, the shallow trap density in NIP was obviously increased compared to that of OIP through the thermally stimulated depolarization current (TSDC), which promoted the dissipation of surface charges and weakened the distortion of the electric field. Therefore, the creeping flashover characteristics of oil-impregnated pressboard were greatly improved with Fe3O4 nanoparticles.

Effect of Shape, Surface Modification and Concentration of Al2O3 Nanoparticles on Breakdown Performance of Transformer Oil

The Al2O3 nanoparticles (NPs) with favorable features were prepared in our lab to develop transformer oil/paper insulation system with better insulation performance. The nanofluids (NFs) were prepared with different shapes (nanorods, nanoplates), surface modifications (oleic acid, C2, C18) and concentrations (0.6 g/L, 0.8 g/L, 1.2 g/L) of Al2O3 NPs. The positive lightning impulse (LI) breakdown voltages (VBDs) of mineral oil were improved by 1.11 times with nanorod shape, oleic acid surface modification and 0.8 g/L concentration of NPs. The effects of Al2O3 NPs with different gap distances on insulating properties of NFs under LI voltage were examined. It was noted that after Al2O3 NPs suspension, the positive LI VBDs of transformer oil (TO/MO) were enhanced but the negative LI VBDs of were decreased. The significant improvement in insulating performance of NFs is interpreted in light of electron traps theory. It was found that suspended NPs can enhance the shallow trap desity. The generated electrons would lose their kinetic energy through repeatedly trapping and de-trapping process, leading to decline in elcetron mobility. The higher trap density of NPs will lower average energy, and lower mobility of electrons are obtained. The electrons with lower energy are easy to be captured by those molecules with high electron affinity, and be converted into negative space charges. These negative space charge decreases the electrical field strength at the front of positive streamer and therefore reduce the propagating velocity of positive streamer.

Study on Spatial-Temporal Distribution Characteristics of the Discharge Process in a 1 m Rod-Plate Gap Under Different Polarity Lightning Impulses

The observation of the air gap discharge process is the basis of revealing the physical mechanism of gas discharge and establishing the accurate numerical simulation model. In order to obtain the variation characteristics at different development stages during the air gap discharge processes under positive and negative lightning impulses in this paper, repetitive discharge tests of 1 m rod-plate air gap under standard lightning impulse were carried out in the National Engineering Laboratory (Kunming) for Ultrahigh-voltage Engineering Technology. An improved discharge observation method, using electron-multiplying intensified charge-coupled device (EMICCD) camera, was adopted in the test. A series of discharge spatial-temporal distribution images with nanosecond exposure time and interval time were captured by changing the shooting time delay of the EMICCD in repetitive discharges. The complete discharge development process was reproduced by the image stitching. The development process and the characteristics of the air gap discharge under lightning impulse were analyzed qualitatively. The characteristics of the measured discharge current under the positive lightning impulse were discussed. The testing results and the analysis show that the shape and the development process of the ionization region of the streamer are quite different under different lightning impulses. There are spherical and fan-shaped streamer regions under the positive and negative lightning impulse, respectively. After the streamer runs through the entire gap, the leader develops from the rod electrode to the plate electrode (under the positive impulse) or from both the rod and the plate electrode to the middle area (under the negative impulse). Then the final jump occurs and the gap is broken down. If the streamer still cannot penetrate the entire gap when the applied impulse voltage exceeds the peak, the gap will not be broken down finally.

Significantly Improved Electrical Breakdown Strength of Natural Ester Liquid Dielectrics by Doping Ultraviolet Absorbing Molecules

In this paper, we experimentally demonstrate that molecules with maximum absorption wavelength corresponding to the first excitation energy of base liquid molecules are able to significantly improve the electrical performances of natural ester oils. The positive lightning impulse breakdown and acceleration characteristics, ac breakdown characteristics, and streamer structure of the natural ester oils, as well as the electronic properties of the molecules, are presented. The molecules with different absorptions in ultraviolet lights were doped into the natural ester oil. The positive lightning impulse breakdown characteristics of natural ester oil with different doped molecules indicated that the doped molecule with great absorption in the medium wavelength ultraviolet irradiation was able to significantly enhance the positive lightning impulse breakdown and acceleration voltages of natural ester oil by 15% and 60%, respectively. In addition, it also enhanced the ac breakdown voltage by 17%. The base liquid molecules exhibited first excitation energy of 4 eV, which corresponds to the maximum absorption wavelength of such molecules, indicating that the improved electric breakdown strength was a result of the reduced the number of excited molecules and weakened ionization of molecules during the streamer propagation.

Influence of the insulator geometry on the streamer propagation characteristics in polymeric insulators under positive polarity lightning impulse voltages

Polymer composite insulators acquire the position of their counterpart ceramic and glass insulators in high voltage overhead transmission systems due to their remarkable pollution performance. They have different potential and electric field distributions compared to that of ceramic insulators and the absence of intermediate pin and cap metal fittings makes them compact in size by reducing the distance between energised and grounded ends. This study presents a simulation-based approach to compute the streamer propagation characteristics in polymer suspension insulators under positive polarity lightning impulse voltages. The streamer inception, propagation, and sustenance criteria are evaluated using finite element method-based electric field computations. The significance of insulator geometries on streamer inception and propagation phenomena is well established. The insulator geometries such as weather sheds, location of the first shed, and shed radius have shown a strong influence on streamer inception and propagation path, which govern the flashover characteristics of the insulator. The lightning impulse flashover voltages of different insulators were determined experimentally to validate the computed results. A good agreement is found between simulation and experimental results.

Number of stems around the H.V. electrode in a 0.74-m air gap under positive impulse

Stem is a special short bright zone for the streamer to leader transition in long air gap discharges, whose characteristics play an important role in leader initiation and development. For the purpose of observing the number of stems around the H.V. electrode at the initial corona discharge, experiments were conducted under positive switching and lightning impulses with different crest values, and the schlieren photographs of stems and the discharge current were recorded synchronously. The associated analysis of the experimental results shows that the number of stems around the electrode at the initial corona discharge is always more than one in most cases. In addition, the average number of stems increases with the radius of the electrode and the crest value of applied impulse. If we focus on the characteristic of only one stem for special study, the conditions with a lower crest value under switching impulse and the electrode with a radius as small as possible are preferable.

Pre-breakdown streamer propagation and breakdown characteristics of refined bleached and deodorized palm oil under lightning impulse voltage

Vegetable oils are considered nowadays as a potential alternative to Mineral Oil (MO) for dielectric insulation fluids in transformers due to its high biodegradability, non-toxicity and high fire resistance. This paper presents an experimental investigation on the streamer propagation and breakdown of Refined, Bleached and Deodorized Palm Oil (RBDPO) under both positive and negative lightning impulse voltages. The streamer shape, stopping length, velocity and breakdown voltage of 2 types of RBDPO were examined. Needle-plane electrodes configuration with 50 mm gap was used in this study and streamer images were captured by a high speed camera. It is observed that the negative streamer shape patterns are thicker than positive streamer for both RBDPO and MO. At the same applied voltage level and polarity, both RBDPO have longer streamer stopping lengths and the streamers propagate faster than MO.

Insulation characteristics of triple mixtures of c-C4F8/N2/CO2 under lightning impulse voltage

This paper has researched the insulation characteristics of 10% c-C4F8/N2/CO2 mixtures under lightning impulse voltage by experiment. It is shown that the positive and negative lightning impulse breakdown voltages of 10%c-C4F8/N2/CO2 gas mixtures rise linearly as the electrode gap distance and gas pressure increase and under the same conditions, the positive lightning impulse breakdown voltage of the gas mixtures is always higher than the negative lightning impulse breakdown voltage. As the gas mixtures have a little higher liquefied temperature than SF6 and the comprehensive GWP is about 5% of SF6, and the positive and negative lightning impulse breakdown voltages can both reach 60% of SF6, 10%c-C4F8/N2/CO2 gas mixtures can be applied as insulation gas in electrical equipment such as C-GIS, GIT, GIL and so on.

Effects of additives on dielectric strength of naphthenic transformer oil

In order to investigate the effects of additives on electrical breakdown strength of naphthenic transformer oil, three types of additives including N, N′-dimethyl aniline (DMA), trichloroethylene (TCE) and TiO2 nanoparticles were used to modify the oil with different concentrations. The breakdown strength and prebreakdown streamer propagation process in the pure and modified oils were investigated under positive lightning impulse voltage. The test results indicate that all additives can improve the breakdown strength of naphthenic transformer oil at certain concentrations, and TiO2 nanoparticles exhibit an optimal modification effect with an increase of 39.4%. Moreover, streamer propagation characteristics of modified oils are significantly affected by the type of additives. The presence of DMA increases the number of discharge branches, while TCE shortens the propagation length of branches without any obvious change in the shape. The positive streamers in the oil modified by TiO2 nanoparticles possess both denser branches and much slower velocities. Based on the analysis of molecule ionization and distribution characteristics of space charge, the improvement in breakdown strength and the change of prebreakdown process are mainly attributed to the significant influence of additives on the formation and transportation of charge carriers in the naphthenic transformer oil.

Insulation characteristics of CF3I/c-C4F8/N2 gas mixtures in slightly non-uniform electric field

In this paper, power frequency breakdown tests as well as positive and negative lightning impulse breakdown tests have been performed. The breakdown voltages of CF 3 I/CO 2 /N 2 , c-C 4 F 8 /CO 2 /N 2 , CF 3 I/c-C 4 F 8 /CO 2 gas mixtures have been measured in different gas pressures, mixing ratios and electrodes distances in slightly non-uniform electric fields. In addition, the breakdown voltages of CF 3 I/c-C 4 F 8 /CO 2 gas mixtures have been comparatively analyzed with SF 6 /CO 2 , CF 3 I/CO 2 , c-C 4 F 8 /CO 2 gas mixtures. The results show that CO 2 is excellent buffer gas to CF 3 I and c-C 4 Fρ as it has synergistic effect. The power frequency breakdown voltages of CF 3 I/c-C 4 F 8 /CO 2 gas mixtures are basically the same as that of c-C 4 F 8 /CO 2 gas mixtures, which are higher than that of CF 3 I/CO 2 gas mixtures. Besides, the 50% breakdown voltage of c- CρFρ/CFρI/CO 2 gas mixtures under lightning impulse are higher than that of CF 3 I/CO 2 . Compared with SF 6 /CO 2 gas mixtures, CF 3 I/c-C 4 F 8 /CO 2 gas mixtures have the same insulating performance in short electrode distance, and show more excellent insulating ability at an electrode distance more than 20mm.

Study on Impulse Breakdown Characteristics of Internal-Gap Lightning Protection Device Applied to 35 kV Distribution Line

External environmental factors have no effect on the breakdown performance of the internal gap, leading to the anti-icing and anti-storm features of the internal-gap lightning protection device (ILPD). In this paper, a test platform is created to study the impulse discharge and arc erosion characteristics of the ILPD applied to a 35 kV distribution line. The 50% lightning impulse voltage and discharge stability of the ILPD are experimentally analysed. The results show that the ILPD has good discharge voltage repeatability under multiple impulses. Under a positive lightning impulse, the 50% breakdown voltage of the ILPD is 3.8–11.4% higher than that of the outer-gap lightning protection device (OLPD). A finite element simulation model is created for electric field analysis. The maximum electric field strength of the ILPD is 4.68% lower than that of the OLPD, leading to a higher lightning breakdown voltage. High-speed camera shooting shows that the discharge arc may lead to the erosion of the discharge tube, reducing its insulation performance. A large current impulse test platform is set up for arc energy analysis, which indicates that more than 90% of the energy is absorbed by the varistor during lightning stroke. The quality and leakage current of the discharge tube did not change significantly after testing. Given the current design of varistors, the per unit length energy of arc is less than 4.5 J/mm due to the numerical calculation, which is far less than the experimental arc energy (25.0 J/mm). Therefore, arc erosion will not cause the insulation performance of discharge tube to decrease when using the current varistor design.