Positive Breakdown Research Articles

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.

Insulation characteristics of fluoronitriles / CO2 gas mixture under DC electric field

Environmentally friendly DC gas insulated transmission line (GIL) is currently one of the hot topics in advanced electric power equipment. Recently, fluoronitriles have been widely studied and considered to replace SF6 in GILs. In this paper, the breakdown voltage, insulator flashover voltage, and the partial discharge inception voltage of fluoronitriles/CO 2 gas mixtures are reported in a DC electric field. The mole ratio of the mixed gases are 4 : 96 % and 8 : 92 %, and the gas pressure is in the range 304.7 to 717.83 kPa. Based on the results, the breakdown field strength of fluoronitriles/CO 2 gas mixture shows a growth tendency with increasing gas pressure in a uniform electric field. At 717.83 kPa, the positive breakdown field strength is higher than the negative and reached a maximum. In a non-uniform electric field, the negative breakdown voltage of fluoronitriles/CO 2 gas mixture increases with increasing gas pressure. In the contrast, the positive breakdown voltage decreases with the rise of gas pressure, which exhibits a significant polarity effect. The flashover voltage of fluoronitriles/CO 2 gas mixture increases with increasing gas pressure. At the same gas pressure, the negative flashover voltage is slightly higher than the positive. The negative flashover voltage of fluoronitriles/CO 2 gas mixture at 717.83 kPa is equal to 96.06 % of SF6 gas at 0.5 MPa abs. The negative partial discharge inception voltage (PDIV) of the fluoronitriles/CO 2 gas mixture is greater than the positive, and both 4 : 96 % and 8 : 92 % gas mixtures showed higher PDIV than the SF6. Considering the insulation characteristics, the fluoronitriles/CO 2 mixture has the potential to replace SF6 in DC gas insulated transmission lines.

Effect of nanoparticles on streamer propagation and breakdown of vegetable oil-pressboard interface in non-uniform electric field

Electrical equipment is always subjected to various of operating conduction and voltage waves which cause insulation failure on the surface of pressboard (PB). In this paper, pre-breakdown streamers of surface discharge on the interface of PB in vegetable oil based Fe3O4 nanofluids (NFs) were observed by the shadowgraph method under lightning impulse voltage. The images indicate that streamers of NFs impregnated PB show more branches than that of pure oil impregnated PB. The stopping length of streamers propagation is calculated by shadowgraph images for NFs and pure oil. Results suggest that the stopping length of NFs impregnated PB is shorter under the same extra voltage. Secondary reverse streamer is generated at the dissipation process by the reverse electric field, which is caused by residual space charge imparted by primary streamer. Results indicate that the Fe3O4 nanoparticles have reduced the length of secondary reverse streamer. The positive and negative lightning breakdown voltage of Fe3O4 NFs impregnated PB is increased by 24% and 12% at 50mm gap, respectively. In addition, nanoparticles have effectively changed the electric field distribution resulting in the alleviation of streamer concentrated along the parallel direction of PB, and increased the lightning impulse breakdown voltage.

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.

Thermal Stability Study of Commercial Lithium-Ion Batteries As a Function of Cathode Chemistry and State-of-Charge

The safety of lithium-ion batteries is mainly quantified by thermal runaway characteristics such as onset temperature, peak heating rate (HR), and maximum pressure release. This talk evaluates the materials aspects of thermal runaway of commercial lithium-ion batteries by correlating the contributions from individual cell components to thermal runaway trends. An accelerating rate calorimeter was used to define the thermal runaway behavior of three commercial 18650 lithium-ion batteries (LiCoO2, LiFePO4, and LiNixCoyAl1-x-y­O2) at various states of charge (SOC). Cells were also disassembled and the thermal properties of their component materials was evaluated by thermogravimetric analysis, differential scanning calorimetry, and temperature-resolved X-ray diffraction. Broadly, lithium-ion battery thermal runaway process begins with solid-electrolyte interface and negative active material decomposition followed by the positive active material breakdown. The experimental results show that the onset temperature is generally independent of SOC and peak HR increases with SOC as a result of cathode destabilization. Overall, LiNixCoyAl1-x-yO2 decomposes readily, and charged LiCoO2 is also metastable, although it decomposes more gradually than LiNixCoyAl1-x-y. Contrary to the layered transition metal oxides, LiFePO4 is stable to > 500oC, even when charged. The results are aimed at informing future engineering and fire protection of commercial lithium-ion batteries. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2018-3360 A

Effect of Nanoparticle Morphology on Pre-Breakdown and Breakdown Properties of Insulating Oil-Based Nanofluids.

Nanoparticles currently in use are challenged in further improving the dielectric strength of insulating oil. There is a great need for a new type of nanoparticle to promote the application of insulating oil-based nanofluids in electric industries. This paper experimentally investigates the effect of nanoparticle morphology on pre-breakdown and breakdown properties of insulating oil-based nanofluids. The positive impulse breakdown voltage of insulating oil can be significantly increased by up to 55.5% by the presence of TiO2 nanorods, up to 1.23 times that of TiO2 nanospheres. Pre-breakdown streamer propagation characteristics reveal that streamer discharge channels turn into a bush-like shape with much denser and shorter branches in the nanofluid with TiO2 nanorods. Moreover, the propagation velocity of streamers is dramatically decreased to 34.7% of that in the insulating oil. The greater improvement of nanorods on the breakdown property can be attributed to the lower distortion of the electric field. Thus, when compared with nanospheres, pre-breakdown streamer propagation of nanofluid is much more suppressed with the addition of nanorods, resulting in a greater breakdown voltage.

Positive Breakdown Streamers and Acceleration in a Small Point-Plane Liquid Gap and Their Variation with Liquid Properties.

Positive breakdown streamers have been investigated in a short (5 mm) point-plane gap in cyclohexane and n-tridecane with and without the additives N,N-dimethylamine (DMA) with low ionization potential and azobenzene with a not quite as low ionization potential but a very low lowest excitation level. The effects off additives are fairly similar to what is known from longer gaps with DMA or pyrene added: the additives caused reduced breakdown voltage, significantly increased acceleration voltage and increased streamer velocity around breakdown. Molecule for molecule, azobenzene has less effect than DMA. The ratio between the acceleration and the breakdown voltage is significantly higher than in long gaps, though. Nonetheless, tests in short gaps are sufficiently good for screening (a large number of) liquid combinations.The two base liquids without additives, and also neat trichloroethene and DMA have been tested with and without a tube causing spatial restriction of the streamer. Restriction causes increased breakdown voltage and promotes the fastest streamer modes.

Клінічна ефективність у віддалений період відновлювального лікування хворих з ішемічною хворобою серця після хірургічної реваскуляризації хворих

Мета роботи: вивчити клінічну ефективність у віддалений період санаторного етапу реабілітації хворих з ІХС після ХРМ.Матеріали і методи. Обстежено 151 пацієнта з ІХС після хірургічної реваскуляризації міокарда (ХРМ). 111 пацієнтів основної групи пройшли санаторне лікування. Курс лікування склав 21 день, після якого були надані рекомендації щодо модифікації способу життя. Пацієнти групи порівняння (40 осіб) не проходили санаторного лікування. Віддалені результати вивчали за даними амбулаторних карт пацієнтів (наявність інфаркту міокарда протягом 12 місяців після ХРМ, кількість госпіталізацій з приводу ускладнень ІХС, кількість нападів стенокардії), всім пацієнтам проводили неінвазивні дослідження, які включали ЕКГ, ЕХоКС. З метою діагностики уражень коронарного русла пацієнтам із позитивною пробою на велоергометрі (ВЕМ) проводили коронарографію.Результати досліджень та їх обговорення. За даними ВЕМ, у 7,5 % пацієнтів, які не проходили санаторний етап лікування, виявлено ознаки ішемії міокарда проти 1,5 % пацієнтів в основній групі. Кількість нападів стенокардії, в результаті якої була проведена повторна процедура реваскуляризації (чрезшкірне коронарне втручання), в основній групі було виявлено у 3 (2,7%) пацієнтів, тоді як у групі порівняння вірогідно більшим – у 5 (12,5 %) пацієнтів (р<0,05).

Enhanced Electrical Insulation and Heat Transfer Performance of Vegetable Oil Based Nanofluids

Nanoparticles enhance the electrical insulation and thermal properties of vegetable oil, and such improvements are desirable for its application as an alternative to traditional insulating oil for power transformers. However, the traditional method of insulating nanofluids typically achieves high electrical insulation but low thermal conductivity. This work reports an environmentally friendly vegetable oil using exfoliated hexagonal boron nitride (h-BN) showing high thermal conductivity and high electrical insulation. Stable nanofluids were prepared by liquid exfoliation of h-BN in isopropyl alcohol. With 0.1 vol.% of the nano-oil, the AC breakdown voltage increased by 18% at 25°C and 15% at 90°C. Both the positive and negative lightning impulse breakdown voltages of the nano-oil were also enhanced compared with those of the pure oil. Moreover, the thermal conductivity of the nano-oil increased by 11.9% at 25°C and 14% at 90°C. Given its high thermal conductivity, the nano-oil exhibited faster heating and cooling effects than the pure oil. Nano-oils with an electric field (either DC or AC) displayed a faster thermal response than that without an electric field. The reason is that h-BN is oriented under the electric field and formed a thermal network to increase the heat transfer.

Discharge characteristics in inhomogeneous fields under air flow

This research investigates the impact of high velocity air flow on Partial Discharge (PD) patterns generated in strongly inhomogeneous fields. In the laboratory, a needle plane electrode configuration was exposed to a high electrical DC-field and a laminar air flow up to 22 m/s. The needle was connected to a variable DC potential of up to 100 kV over a grounded plate in order to trigger different corona modes. The impact of the air flow on the space charges created in the vicinity of the electrode is evaluated by means of PD measurements in time domain. The results indicate that the wind increases the frequency and magnitude of partial discharges in the vicinity of the electrode due to an increased rate of space charge removal around the tip of the needle and in the gap. The positive polarity shows higher dependency on air flow compared to the negative polarity. It is shown that positive breakdown streamer corona can be extinguished if wind speeds of more than 14.3 m/s are applied to the electrode.

Fast positive breakdown in lightning

AbstractVHF radiation sources produced by positive breakdown during lightning discharges are generally considered to be both weak and slowly propagating. However, as VHF lightning mapping systems have become more sensitive, even this weak radiation can be mapped. In addition to being a faint process, positive breakdown often produces bursts of energetic activity. During the bursts, the VHF emission is extremely bright, and the breakdown propagates at much higher speeds. Here we present VHF interferometric and time‐of‐arrival measurements of such fast positive breakdown events produced during three example flashes. Electric field change measurements show that the fast breakdown process carries positive charge. The extent and velocity of the breakdown is estimated by converting the angular source locations provided by the interferometer into Cartesian coordinates using three‐dimensional lightning mapping observations of the flash as a guide. Fast positive breakdown events are found to extend 100–2400 m into virgin air beyond the tip of the preceding positive leader, at speeds of 0.9–9 ×107 m s−1. The observations expand upon earlier observations of such breakdown and are similar to recently reported results that fast positive breakdown is the cause of high‐power narrow bipolar events.

Abnormal Polarity Effects of Streamer Discharge in Propylene Carbonate under Microsecond Pulses**Supported by the National Natural Science Foundation of China under Grant No 51677190, and the Hunan Provincial Natural Science Foundation of China under Grant No 2017JJ1005.

Propylene carbonate (PC) has a great potential to be used as an energy storage medium in the compact pulsed power sources due to its high dielectric constant and large resistivity. We investigate both the positive and negative breakdown characteristics of PC. The streamer patterns are obtained by ultra-high-speed cameras. The experimental results show that the positive breakdown voltage of PC is about 135% higher than the negative one, which is abnormal compared with the common liquid. The shape of the positive streamer is filamentary and branchy, while the negative streamer is tree-like and less branched. According to these experimental results, a charge layer structure model at the interface between the metal electrode and liquid is presented. It is suggested that the abnormal polarity effect basically arises from the electric field strength difference in the interface between both electrodes and PC. What is more, the recombination radiation and photoionization also play an important role in the whole discharge process.

Observations of the initial stage of a rocket-and-wire-triggered lightning discharge

Observations have been obtained of the initial stage of a rocket-and-wire triggered lightning flash with a high resolution broadband VHF interferometer. The discharge produced 54 precursor current pulses (PCPs) over 883 ms during the rocket's ascent. The interferometer observations show that the PCPs were produced by breakdown at the ascending tip of the rocket, and that individual PCPs were produced by weak upward positive breakdown over meters-scale distances, followed by more energetic, fast downward negative breakdown over several tens of meters distance. The average propagation speeds were ~5 x 106 m s-1 and ~3 x 107 m s-1, respectively. The sustained upward positive leader (UPL) was initiated by a rapid, repetitive burst of 14 precursor pulses. Upon initiation, the VHF radiation abruptly became continuous with time. Significantly, breakdown during the UPL appeared to extend the discharge in a similar manner to that of the precursor pulses.

Fabrication, characterization, and insulating property of Fe3O4 nanofluids

ABSTRACTMonodisperse Fe3O4 nanoparticles modified by oleic acid were synthesized by a solvothermal method and dispersed into mineral transformer oil to prepare Fe3O4 nanofluids. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis indicated that the obtained nanoparticles are single crystals with an average diameter of 15 nm. The test results indicate that the nanoparticles exhibited good dispersibility in the nanofluids at a wide range of concentrations. Moreover, the positive impulse breakdown strength of nanofluids was greatly improved by 36.6% at the optimum concentration of nanoparticles.

Comparative analysis of lightning breakdown voltage of natural ester liquids of different viscosities supported by light emission measurement

This article presents the results of the studies on lightning impulse breakdown voltage measurements performed for two commercial natural ester liquids of different viscosities. These viscosities measured at 100°C were 8.0 mm2/s and 4.6 mm2/s respectively. The dielectric test results presented in this paper concern the measurements performed using the recommendations of the IEC 897 standard where point-to-sphere electrode system is dictated. In addition to measuring the breakdown voltage other experimental techniques were applied such as time to breakdown estimation, light emission registration using the photomultiplier and static photograph collection. The results showed that the differences between the liquids tested concern only positive polarity of lightning impulse voltage. In the case of negative polarity from the standard based calculations and statistical analysis using Weibull distribution the breakdown voltages may be treated as a very similar. In turn the other discharge characteristics such as light waveforms, static photographs and propagation velocities did not showed any differences. In the case of positive lightning breakdown voltage the results were assessed only on the basis of the average values calculated because a deep statistical analysis was not possible due to the small scattering of data. These results indicated on slightly better lightning properties of the natural ester of higher viscosity. Confirmation of this conclusion shall constitute the light waveforms registered where the discharges developing in the low viscosity natural ester are characterized by higher intensity than corresponding discharges developing in the “classical” natural ester. However the static photos did not indicate any differences between the esters tested.

Transformer oils-based graphene quantum dots nanofluid as a new generation of highly conductive and stable coolant

Transformer oil-based graphene quantum dots (GQD) nanofluid with superior colloidal stability has great potentials as a new generation of high performance transformer oil. To this end, graphene quantum dots were initially synthesized with a novel and cost-effective exfoliation approach. To eliminate the acidity, a covalently-functionalization process was employed to change GQD to amine-treated graphene quantum dots (AGQD). The morphological analysis confirmed that the diameter and average height of the mono-layered AGQD were mostly in the range of 5–17nm and <1nm, respectively. Transformer oil-based AGQD nanofluid at very low weight fraction has been shown experimentally to have substantially higher positive voltage breakdown, thermal conductivity, natural and forced heat transfer rate, and flash point levels compared to that of pure transformer oil. A comprehensive rheological and electrical analysis of the transformer oil-based AGQD nanofluid showed no significant enhancement in its viscosity compared to pure transformer oil, which is a great advantage of this new generation of transformer oil. Case studies showed that the transformer oil-based AGQD nanofluid has a superior colloidal stability, offering improved high voltage equipment performance and reliability.

Enhanced breakdown performance of Anatase and Rutile titania based nano-oils

Nano-oils synthesized by dispersing dielectric nanostructures counter common intuition as such nano-oils possess substantially higher positive dielectric breakdown voltage with reduced streamer velocities than the base oils. Nano-oils comprising stable and dilute homogeneous dispersions of two forms of titanium (IV) oxide (TiO 2 ) nanoparticles (Anatase and Rutile) have been experimentally examined and observed to exhibit highly enhanced dielectric breakdown strength compared to the conventional transformer oils. The present study involves titania dispersed in two different grades of transformer oils, both with varied levels of thermal treatment, to obtain consistent and high degrees of enhancement in the breakdown strength, as well as high degrees of increment in the survival of the oils at elevated electrical stressing compared to the base oils, as obtained via detailed twin parameter Weibull distribution analysis of the experimental observations. The experimental results demonstrate higher augmented breakdown strength for Anatase compared to the Rutile phase of titania. In-depth survey of literature indicates that mostly Rutile based oils are used. However, the present study shows that they exhibit relatively less breakdown strength enhancement compared to the Anatase based oils. It is also observed that heat treatment of the nano-oils further enhances the dielectric breakdown performance. The differences in the performance of Anatase and Rutile has been explained based on the electronic structure of the two and the affinity towards electron scavenging and the theory has been found to validate the experimental observations.

Characterization of high performance AIN nanoparticle-based transformer oil nanofluids

Well dispersed aluminium nitride(AlN) nanoparticles were synthesized by two-step ball milling coupled with surface coating. The AlN nanofluids (NFs) were prepared by dispersing as-prepared nanoparticles into transformer oil. The modified NFs is investigated as a new generation of transformer oil. The insulating properties have been investigated firstly. In the case of our AlN NFs with 0.079% < Φ < 0.159% (Φ means the volume concentration of AlN nanoparticles in the transformer oil), the positive lightning impulse breakdown voltage and partial discharge inception voltage (PDIV) increased around 50% and 20% compared to those of the transformer oil while alternating current (AC) breakdown voltage decreased around 20-30%. However, it can still meet the demand of the standard below 330 kV. The results show that AlN NFs have a concentration dependence of AlN nanoparticles. The amount of large particles increases with the increasing concentration of AlN nanoparticles while trap density of NFs increases first and then decreases, both of which have an opposite effect on the dielectric strength. Furthermore, the experimental results showed that the thermal conductivity is increased by 3-7% because of the addition of AlN nanoparticles.

Effect of TiO2nanoparticles on streamer propagation in transformer oil under lightning impulse voltage

Recent experiments have shown that some nanoparticles can influence the breakdown strength of transformer oil under lightning impulse voltage. To reveal the working mechanism, this paper presents an experimental study on the effect of TiO2 nanoparticles on the impulse breakdown strength and prebreakdown streamer propagation process in transformer oil-based nanofluid under both positive and negative lightning impulse voltage. The test results verify that the modification of nanoparticles on breakdown strength of transformer oil has a distinct polar effect: positive breakdown voltage of nanofluid is increased by up to 30.8%, whereas the negative one is decreased by 6.8%. Streamer shape, propagation length and velocity in both pure oil and nanofluid were investigated using the shadowgraph technique. It is revealed that the propagation characteristics of positive and negative streamers in nanofluid are markedly affected by the addition of TiO2 nanoparticles. The positive streamers in nanofluid form a bush-like structure with thicker and denser branches, developing much slower than tree-like streamers in pure oil. While negative streamers in nanofluid have a tree-like shape with much longer branches, propagating faster than the original bush-like streamer in pure oil. These differences in streamer propagation characteristics and breakdown strength in pure oil and nanofluid are closely related to the change of space charge distribution caused by shallow trap in nanofluid. More negative charges are formed through capturing fast electrons into slow electrons in shallow traps induced by the presence of TiO2 nanoparticles, which change the local electric field in front of the streamer tip. Thus, streamer propagation process in nanofluid is dramatically modified, leading to the change in breakdown strength.

Properties of relatively long streamers initiated from an isolated hydrometeor

AbstractThis paper reports a modeling study on the dynamics of relatively long streamers initiated from an isolated hydrometeor in subbreakdown (0.5Ek) and overbreakdown (1.5Ek) electric fields, respectively, where Ek is the conventional breakdown threshold field. The modeling results indicate that at 0.5Ek field, the streamer shows a generally exponential growth feature, but its channel characteristics such as radius, electron density, and current density exhibit colocated fluctuations. On the other hand, at 1.5Ek field, the streamer shows faster exponential growth, and no fluctuations of those parameters are observed. The exponential growth property allows one to use the modeling results to predict the characteristics of longer streamers, as well as to quantify the thunderstorm electrical conditions including the high‐field region size and potential difference in order to accelerate the streamer to the high speed of fast positive breakdown that was observed recently in lightning initiation. At 0.5Ek, a potential difference of ∼0.3–1.8 MV is required to accelerate the streamer to the high speed of 1–5 × 107 m/s, while at 1.5Ek, ∼0.07–0.4 MV is required. The exponentially growing streamer current produces an exponentially increasing magnetic field, leading to electromagnetic (EM) field radiation. It is found that the growth rate of the magnetic field ( ) radiated by a streamer can be obtained by a simple relation , where 1/τ is the growth rate of the streamer. Combining our modeling results with the Fourier analysis reported by Qin et al. (2012), we find that streamers at thundercloud altitudes radiate in the high frequency and very high frequency range of the EM spectrum.