| 1. |
- Aljure, Mauricio, et al.
(författare)
-
Electric Conduction in Mineral Oil based ZnONanofluids under Intense Electric Fields
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The electric conduction processes in mineral oil based ZnO–C18 nanofluids under intense electric fields are investigated. For this, conduction currents are measured usinga needle-plane electrode configuration. Furthermore, an electrohydrodynamic (EHD) model is used here to discuss the charge generation mechanisms and the electronic properties of the ZnO–C18 nanofluids. The analysis of the conduction currents shows that ZnO–C18 nanoparticles increase the generation of charge carriers, and at the same time they augment the scavenging of quasi-free electrons compared with the measurements with mineral oil only. It is found that the existing nanoparticle electron scavenging model reported in the literature grossly underestimates the electron scavenging process here reported. A new analytical formulation for the nanoparticle electron scavenging process is proposed. The EHD model is also used to simulate the electric conduction processes just before negative streamer inception in mineral oil and ZnO–C18 nanofluids. It is shown that ZnO–C18 nanoparticles hinder the streamer initiation process by reducing the effective electric field at the tip of the needle. This electric field reduction is caused by the combined effect of the generation of charge carriers and the electron scavenging of ZnO–C18 nanoparticles.
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| 2. |
- Aljure, Mauricio, et al.
(författare)
-
Erratum to : Aljure, M.; Becerra, M.; Karlsson, E.M. Streamer inception from ultra-sharp needles in mineral oil based nanofluids
- 2018
-
Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 11:11
-
Tidskriftsartikel (refereegranskat)abstract
- The authors wish to make the following corrections to their paper [1]: i. On pages 13 and 14, the numbering of references from 17 to 30 is incorrect. References 17 to 30 should be renumbered from the original order below: 17. Liu, Z.; Liu, Q.; Wang, Z.D.; Jarman, P.; Krause, C.; Smith, P.W.R.; Gyore, A. Partial discharge behaviour of transformer liquids and the influence of moisture content. In Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids (ICDL), Bled, Slovenia, 29 June–3 July 2014. 18. Yamashita, H.; Yamazawa, K.; Wang, Y.S. The effect of tip curvature on the prebreakdown streamer structure in cyclohexane. IEEE Trans. Dielectr. Electr. Insul. 1998, 5, 396–401. 19. Dumitrescu, L.; Lesaint, O.; Bonifaci, N.; Denat, A.; Notingher, P. Study of streamer inception in cyclohexane with a sensitive charge measurement technique under impulse voltage. J. Electrostat. 2001, 53, 135–146. 20. Pourrahimi, A.M.; Hoang, T.A.; Liu, D.; Pallon, L.K.H.; Gubanski, S.; Olsson, R.T.; Gedde, U.W.; Hedenqvist, M.S. Highly efficient interfaces in nanocomposites based on polyethylene and ZnO nano/hierarchical particles: A novel approach toward ultralow electrical conductivity insulations. Adv. Mater. 2016, 28, 8651–8657. 21. Li, J.; Du, B.; Wang, F.; Yao, W.; Yao, S. The effect of nanoparticle surfactant polarization on trapping depth of vegetable insulating oil-based nanofluids. Phys. Lett. A 2016, 380, 604–608. 22. Aljure, M.; Becerra, M.; Pallon, L.K.H. Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration. In Proceedings of the 2016 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Toronto, ON, Canada, 16–19 October 2016; pp. 687–690. 23. Primo, V.A.; Garcia, B.; Albarracin, R. Improvement of transformer liquid insulation using nanodielectric fluids: A review. IEEE Electr. Insul. Mag. 2018, 34, 13–26. 24. Jin, H.; Andritsch, T.; Morshuis, P.H.F.; Smit, J.J. AC breakdown voltage and viscosity of mineral oil based SiO2 nanofluids. In Proceedings of the 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, Montreal, QC, Canada, 14–17 October 2012; pp. 902–905. 25. Jin, H.; Morshuis, P.; Mor, A.R.; Smit, J.J.; Andritsch, T. Partial discharge behavior of mineral oil based nanofluids. IEEE Trans. Dielectr. Electr. Insul. 2015, 22, 2747–2753. 26. Du, Y.; Lv, Y.; Li, C.; Chen, M.; Zhong, Y.; Zhou, J.; Li, X.; Zhou, Y. Effect of semiconductive nanoparticles on insulating performances of transformer oil. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 770–776. 27. Dung, N.V.; Høidalen, H.K.; Linhjell, D.; Lundgaard, L.E.; Unge, M. Effects of reduced pressure and additives on streamers in white oil in long point-plane gap. J. Phys. D Appl. Phys. 2013, 46, 255501. 28. McCool, J.I. Using the Weibull Distribution; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2012. 29. Lesaint, O.L.; Top, T.V. Streamer initiation in mineral oil. part I: Electrode surface effect under impulse voltage. IEEE Trans. Dielectr. Electr. Insul. 2002, 9, 84–91. 30. Becerra, M.; Frid, H.; Vázquez, P.A. Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane. Phys. Fluids 2017, 29, 123605. to the following, corrected numbering: 17. Dumitrescu, L.; Lesaint, O.; Bonifaci, N.; Denat, A.; Notingher, P. Study of streamer inception in cyclohexane with a sensitive charge measurement technique under impulse voltage. J. Electrostat. 2001, 53, 135–146. 18. Liu, Z.; Liu, Q.; Wang, Z.D.; Jarman, P.; Krause, C.; Smith, P.W.R.; Gyore, A. Partial discharge behaviour of transformer liquids and the influence of moisture content. In Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids (ICDL), Bled, Slovenia, 29 June–3 July 2014. 19. Yamashita, H.; Yamazawa, K.; Wang, Y.S. The effect of tip curvature on the prebreakdown streamer structure in cyclohexane. IEEE Trans. Dielectr. Electr. Insul. 1998, 5, 396–401. 20. Becerra, M.; Frid, H.; Vázquez, P.A. Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane. Phys. Fluids 2017, 29, 123605. 21. Pourrahimi, A.M.; Hoang, T.A.; Liu, D.; Pallon, L.K.H.; Gubanski, S.; Olsson, R.T.; Gedde, U.W.; Hedenqvist, M.S. Highly efficient interfaces in nanocomposites based on polyethylene and ZnO nano/hierarchical particles: A novel approach toward ultralow electrical conductivity insulations. Adv. Mater. 2016, 28, 8651–8657. 22. Li, J.; Du, B.; Wang, F.; Yao, W.; Yao, S. The effect of nanoparticle surfactant polarization on trapping depth of vegetable insulating oil-based nanofluids. Phys. Lett. A 2016, 380, 604–608. 23. Aljure, M.; Becerra, M.; Pallon, L.K.H. Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration. In Proceedings of the 2016 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Toronto, ON, Canada, 16–19 October 2016; pp. 687–690. 24. Primo, V.A.; Garcia, B.; Albarracin, R. Improvement of transformer liquid insulation using nanodielectric fluids: A review. IEEE Electr. Insul. Mag. 2018, 34, 13–26. 25. Jin, H.; Andritsch, T.; Morshuis, P.H.F.; Smit, J.J. AC breakdown voltage and viscosity of mineral oil based SiO2 nanofluids. In Proceedings of the 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, Montreal, QC, Canada, 14–17 October 2012; pp. 902–905. 26. Jin, H.; Morshuis, P.; Mor, A.R.; Smit, J.J.; Andritsch, T. Partial discharge behavior of mineral oil based nanofluids. IEEE Trans. Dielectr. Electr. Insul. 2015, 22, 2747–2753. 27. Du, Y.; Lv, Y.; Li, C.; Chen, M.; Zhong, Y.; Zhou, J.; Li, X.; Zhou, Y. Effect of semiconductive nanoparticles on insulating performances of transformer oil. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 770–776. 28. Dung, N.V.; Høidalen, H.K.; Linhjell, D.; Lundgaard, L.E.; Unge, M. Effects of reduced pressure and additives on streamers in white oil in long point-plane gap. J. Phys. D Appl. Phys. 2013, 46, 255501. 29. McCool, J.I. Using the Weibull Distribution; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2012. 30. Lesaint, O.L.; Top, T.V. Streamer initiation in mineral oil. part I: Electrode surface effect under impulse voltage. IEEE Trans. Dielectr. Electr. Insul. 2002, 9, 84–91. ii. On the last paragraph of page 9, the last sentence should be changed from: However, the results in [11] also show the consistent increase in the initiation voltage of prebreakdown phenomena in both polarities, as reported in Figure 11. to the following, corrected version: However, the results in [26] also show the consistent increase in the initiation voltage of prebreakdown phenomena in both polarities, as reported in Figure 11. iii. On the last paragraph of page 10, the third sentence should be changed from: Even though the existing hypotheses of the dielectric effect of NPs [8–10] were proposed for blunter electrodes (where charge generation before streamer initiation is less important [30]), they should still apply under the experimental conditions here reported. to the following, corrected version: Even though the existing hypotheses of the dielectric effect of NPs [5,6,16] were proposed for blunter electrodes (where charge generation before streamer initiation is less important [30]), they should still apply under the experimental conditions here reported. The authors would like to apologize for any inconvenience caused to the readers by these changes. The changes do not affect the scientific results. The manuscript will be updated and the original will remain online on the article webpage, with a reference to this Correction.
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| 3. |
- Aljure, Mauricio, et al.
(författare)
-
On the injection and generation of charge carriers in mineral oil under high electric fields
- 2019
-
Ingår i: JOURNAL OF PHYSICS COMMUNICATIONS. - : IOP PUBLISHING LTD. - 2399-6528. ; 3:3
-
Tidskriftsartikel (refereegranskat)abstract
- Charge injection and generation mechanisms under intense electric fields (up to 10(9)Vm(-1)) in mineral oil are assessed experimentally and numerically. For this, current-voltage characteristics under positive and negative polarities are measured in a needle-plane configuration using sharp needles (with tip radius R-tip <= 1.1 mu m). In addition, a state of the art electro-hydrodynamic (EHD) model is implemented to calculate the contribution of the different mechanisms on the high-field conduction currents in the liquid. In order to evaluate exclusively the contribution of field emission, experiments are also performed in vacuum. It is found that neither field emission nor field ionisation can explain the conduction currents measured in mineral oil. It is proposed that field molecular ionisation, as described by Zener tunnelling model for solids, and electron impact ionisation are the processes dominating the generation of excess electron-ion pairs in mineral oil under positive and negative polarity, respectively. It is also shown that Zener molecular ionisation alone grossly overestimates the measured currents when parameters previously suggested in the literature for mineral oil are used. Preliminary model parameters for these mechanisms that best fit the conduction currents measured in mineral oil are presented and discussed.
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| 4. |
- Aljure, Mauricio
(författare)
-
Pre-breakdown Phenomena in Mineral Oil Based Nanofluids
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mineral oil is a dielectric liquid commonly used in high voltage equipment such as power transformers. Interestingly, it has been experimentally observed that the dielectric strength of the mineral oil is improved when nanoparticles are added. However, the mechanisms behind these improvements are not well understood, hindering the further innovation process of these so-called nanofluids. This thesis aims to contribute to the understanding of the mechanisms explaining the dielectric strength improvement of the base oil when nanoparticles are added.For this, several experiments and numerical simulations are performed in this thesis. The initiation voltage of electric discharges infive different kind of nanofluids was measured. The large data set obtained allowed to cast experimental evidence on the existing hypotheses that are used to explain the effect of nanoparticles. It is found that hydrophilic nanoparticles hinder the electric discharge initiation from anode electrodes. On the other hand, electric discharge initiation from cathode electrodes was hindered by nanoparticles with low charge relaxation time.The electric currents in mineral oil and nanofluids were also measured under intense electric fields (up to 2GV/m). It is found that the addition of certain nanoparticles increases the measured currents. The possible physical mechanisms explaining the measured currents inmineral oil with and without nanoparticles were thoroughly discussed based on results of numerical simulations. Preliminary parameters used in this thesis to model these mechanisms led to a good agreement between the measured and simulated electric currents.
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| 5. |
- Aljure, Mauricio, et al.
(författare)
-
Simulation of the electrical conduction of cyclohexane with TiO2 nanoparticles
- 2014
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Ingår i: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014. - : IEEE. - 9781479920631 ; , s. 6893119-
-
Konferensbidrag (refereegranskat)abstract
- Nanoparticles mixed with transformer oil can potentially increase the breakdown strength of the base liquid. Unfortunately, the basic physical mechanisms leading to such improvement are still not clear. This paper implements two existing theories to model the electrical conduction of cyclohexane with TiO2 nanoparticles in a needle to plane configuration. The generation and drift of carriers in the liquid are simulated by coupling the continuity equations for electrons, positive ions, negative ions, and nanoparticles with Poisson's equation for the electric field. The current-voltage characteristics are simulated and compared with the case of pure cyclohexane. The nanoparticles are modeled as either absorbers of electrons or as source of shallow traps in the fluid, according to the existing theories. The simulations show that the considered theories predict no significant effect of nanoparticles added to cyclohexane on the conduction current from a negative point electrode in steady state or under transient conditions.
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| 6. |
- Aljure, Mauricio, et al.
(författare)
-
Streamer Inception from Ultra-Sharp Needles in Mineral Oil Based Nanofluids
- 2018
-
Ingår i: Energies. - : MDPI. - 1996-1073. ; 11:8
-
Tidskriftsartikel (refereegranskat)abstract
- Positive and negative streamer inception voltages from ultra-sharp needle tips (with tip radii below 0.5 m) are measured in TiO2, SiO2, Al2O3, ZnO and C-60 nanofluids. The experiments are performed at several concentrations of nanoparticles dispersed in mineral oil. It is found that nanoparticles influence positive and negative streamers in different ways. TiO2, SiO2 and Al2O3 nanoparticles increase the positive streamer inception voltage only, whilst ZnO and C-60 nanoparticles augment the streamer inception voltages in both polarities. Using these results, the main hypotheses explaining the improvement in the dielectric strength of the host oil due to the presence of nanoparticles are analyzed. It is found that the water adsorption hypothesis of nanoparticles is consistent with the increments in the reported positive streamer inception voltages. It is also shown that the hypothesis of nanoparticles reducing the electron velocity by hopping transport mechanisms fails to explain the results obtained for negative streamers. Finally, the hypothesis of nanoparticles attaching electrons according to their charging characteristics is found to be consistent with the results hereby presented on negative streamers.
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| 7. |
- Aminlashgari, Nina, et al.
(författare)
-
Characterization of degradation fragments released by arc-induced ablation of polymers in air
- 2016
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Ingår i: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 49:5
-
Tidskriftsartikel (refereegranskat)abstract
- Polymers exposed to high intensity arc plasmas release material in a process called arc-induced ablation. In order to investigate the degradation fragments released due to this process, two different polymeric materials, poly(oxymethylene) copolymer (POM-C) and poly(methyl methacrylate) (PMMA), were exposed to a transient, high-power arc plasma in air. A small fraction of the ablated material drifting away from the arcing volume was deposited on a fixed glass substrate during the total duration of a 2 kA ac current semicycle. In addition, another fraction of the released material was deposited on a second moving substrate to obtain a time-resolved streak 'image' of the arc-induced ablation process. For the first time, mass spectra of degradation fragments produced by arc-induced ablation were obtained from the material deposited on the substrates by using laser desorption ionization time-of-flight mass spectrometry (LDI-ToF-MS). It was found that oligomers with mean molecular weight ranging between 400 and 600 Da were released from the surface of the studied polymers. The obtained spectra suggest that the detected degradation fragments of POM could be released by random chain scission of the polymer backbone. In turn, random chain scission and splitting-off the side groups are suggested as the main chemical mechanism leading to the release of PMMA fragments under arc-induced ablation.
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| 8. |
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| 9. |
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| 10. |
- Ariza, David, et al.
(författare)
-
First Mode Negative Streamers along Mineral Oil-solid Interfaces
- 2017
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Ingår i: IEEE transactions on dielectrics and electrical insulation. - : IEEE. - 1070-9878 .- 1558-4135. ; 24:4
-
Tidskriftsartikel (refereegranskat)abstract
- This document presents an experimental study on the propagation of first mode negative streamers along mineral oil-solid interfaces. Samples made of an oil impregnated kraft paper and a low-porosity paper made from cellulosic micro and nano fibrils, as well as different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)) are used as the solid. A comparison of the length, charge and velocity of streamers for all different mineral oil-solid interfaces is reported. It is shown that streamers propagate longer and faster along mineral oil-solid interfaces with low surface roughness, low porosity and higher electrical permittivity than mineral oil. Those streamers show a quasi-continuous injection of charge in the early stage of their propagation. This quasi-continuous charge injection consists of a sequence of small charge steps separated by few tens of nanoseconds in between. In comparison, the streamers that propagate along surfaces with similar permittivity to the mineral oil have lower injection of charge and higher stopping voltage conditions than streamers propagating free in the liquid without any solid barrier.
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| 11. |
- Ariza, David, et al.
(författare)
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Inception of first mode negative streamers at liquid/solid interfaces
-
Ingår i: Journal of Physics D: Applied Physics.
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents an experimental study on the inception of first mode negative streamers at different mineral-oil/solid interfaces. This study is performed with a point-plane configuration immersed in mineral oil (point cathode). The mineral-oil/solid interface is done by assembling a solid in an inclined position into the point-plane gap. The solid is in contact (or in the proximity) with the point electrode tip. The tested solids are a kraft paper, a paper made from cellulosic micro and nano fibrils and different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)). These solids have different relative permittivity and structural composition. It is found that the streamer inception voltage is statistically similar for the cases with solids with higher permittivity than mineral oil. The streamer inception voltage for the case without any solid barrier is also statistically similar to the cases with solids with higher permittivity than mineral oil. It is also found that the inception voltage is higher for streamers initiated at permittivity-matched interfaces (cases with LDPE and PTFE). Additionally, the influence of the spatial limitation with the solid surface to the volume where the streamer initiation process takes is performed with PTFE by varying the distance between the point electrode and the solid surface. It is shown that the streamer inception voltage depends on the distance between the point electrode and the solid surface. Furthermore, It is observed that the streamer inception voltage is also influenced when the distance between the point electrode and the surface of the PTFE is several micrometers (twenty times longer than the penetration depth of the avalanche responsible of streamer initiation). Additionally, it is also shown that a recently-proposed streamer inception criterion is unsuitable to predict the streamer inception conditions close to permittivity matched and mismatched liquid/solid interfaces.
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| 12. |
- Ariza, David, et al.
(författare)
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Influence of Paper Properties on Streamers Creeping in Mineral Oil
- 2017
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Ingår i: Proceedings of IEEE International Conference on Dielectric Liquids. - : IEEE. - 2153-3725 .- 2153-3733.
-
Tidskriftsartikel (refereegranskat)abstract
- This work presents an experimental study ofsecond mode positive streamers propagating along mineral oilpaperinterfaces. A point-plane arrangement immersed inmineral oil with the paper inclined 60 degrees to the planeelectrode is used to create the liquid-solid interface. Kraft paperand a kraft fibril paper, made from cellulosic micro and nanofibrils, with higher density and lower surface roughness are usedas the solid materials. High speed shadowgraphy and chargerecordings are used to compare the propagation of second modepositive streamers along the mineral oil-kraft paper and mineraloil-kraft fibril paper. Streamers creeping along the mineral oilkraftpaper interface propagate mainly into the liquid, with oneor two main filaments. In comparison, the streamers propagatingalong the kraft fibril paper show a strong reduction of thebranching; these streamers consist of a single filament thatpropagates exactly on the solid surface. Streamers along the kraftfibril paper also have longer propagation time than for the casewith kraft paper. Mutual electrostatic shielding betweenfilaments is observed for the streamers creeping on the kraftpaper. An electrostatic analysis of the influence of permittivity,density and surface roughness of the solid in the electricalproperties of the streamer filaments is also performed.
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| 13. |
- Ariza, David, et al.
(författare)
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Measurements of the charge of streamers propagating along transformer oil-solid interfaces
- 2014
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Ingår i: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014. - : IEEE. - 9781479920631 ; , s. 6893165-
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Konferensbidrag (refereegranskat)abstract
- This paper presents an experimental study of the positive streamer charge in transformer oil and oil-solid interfaces in a point-plane gap (5mm) under impulse voltage 50ns\1800μs (maximum peak voltage 24 kV). The experiment is intended to compare the charge of streamers propagating in transformer oil with and without a solid barrier along their path. The solid barrier constricting the streamer volume consists of two parallel strips of either impregnated pressboard or a polymeric film (PA6) installed at both sides of the point electrode. The maximum cumulative charge of the streamer is found for the case when the impregnated pressboard is used and the minimum cumulative charge for the case of the polymer as a barrier.
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| 14. |
- Ariza, David, 1985-
(författare)
-
On the inception and propagation of streamers along mineral-oil/solid interfaces
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents an experimental study of positive and negativestreamers propagating along mineral-oil/solid interfaces under square highvoltage pulses. The thesis includes the design and construction of anexperimental setup to studying the streamers in a point-plane configurationcapable to detect streamer parameters such as velocity, length, propagationtime, emitted light and charge.The first part of this experimental study is devoted to the analysis of thestreamer inception at mineral-oil/solid interfaces under negative polarity. Thestreamer inception voltage and charge recordings are reported for eachmineral-oil/solid interface. It is found that only solids with a permittivitysimilar to that of mineral oil can influence the streamer inception voltage.Solids with matched permittivity such as LDPE and PTFE increase the inceptionvoltage. The cases with solids with higher permittivity than mineral oil havesimilar inception voltage as the streamer incepted in the liquid bulk withoutsolid. The second part is devoted to studying the propagation of first mode negativestreamers along different mineral-oil/solid interfaces. A comparison of theelectrical and physical properties of the streamers (e.g. charge, length,velocity, etc.) is presented. Solid samples with different chemical compositionand different physical properties are used. The solid samples are anoil-impregnated kraft paper and an low-porosity paper referred to as kraftfibril paper made from cellulosic micro and nano fibrils. Polymeric films madeof low density polyethylene (LDPE), polyethylene terephthalate (PET),polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) are also usedas the solid. Streamers propagating along the liquid/solid interface arecompared with streamers developing in mineral oil without solid. Streamerspropagate longer and faster along solids with low surface roughness, lowporosity and higher electrical permittivity than mineral oil showing aquasi-continuous injection of charge in the early stage of propagation.The third part of the experimental study deals with second mode positivestreamers propagating along mineral-oil/solid interfaces. The inception andpropagation of the streamer are investigated using different mineral-oil/solidinterfaces. Measurements of the streamer velocity, charge, stopping length,propagation time, together with light recordings and shadowgraphs are reported.It is found that the interface can influence the streamer inception voltage,branching, stopping length, velocity, charge and current. Properties of thesolid and of the interface (i.e. surface roughness, permittivity, wettability)are parameters that influence the streamer propagation.
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| 15. |
- Ariza, David, et al.
(författare)
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On the initiation of negative streamers at mineral oil-solid interfaces
- 2015
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Ingår i: IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP). - : IEEE conference proceedings. - 9781467374989 ; , s. 563-565
-
Konferensbidrag (refereegranskat)abstract
- This paper reports the probability distribution of negative streamers initiated in mineral oil with and without a solid interface. In addition, the charge injected by conduction currents prior to the streamer inception is presented. Impregnated paper and polymeric films made of PET, PTFE and PVDF are tested as solid materials. Comparison of the conduction charge for the different oil-solid interfaces is presented. It is found that the permittivity of the material used at the solid interface does not influence significantly the condition for streamer initiation in mineral oil. A nonlinear increasing of the charge readings for the impregnated paper, PET and PVDF cases are observed.
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| 16. |
- Ariza, David, 1985-, et al.
(författare)
-
Propagation of negative streamers along mineral oil-solid interfaces
- 2015
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Ingår i: Proceedings of the 2015 CEIDP Conference on Electrical insulation and Dielectric phenomena, CEIDP 2015. - : IEEE conference proceedings. ; , s. 566-569
-
Konferensbidrag (refereegranskat)abstract
- This paper introduces an experimental study on the propagation of negative streamers along mineral oil-solid interfaces. A standard type of impregnated paper and different polymeric films (made of PET, PTFE and PVDF) are selected as solid materials immersed in mineral oil. The effect of the solid material on the streamer propagation along the interface formed with transformer oil is studied. Streamer velocities classified as first mode propagation point cathode are reported. Voltage gradient of the streamer channel and its stopping voltage are calculated for all the cases. Comparison of streamer charge and stopping length propagation are reported.
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| 17. |
- Ariza, David, et al.
(författare)
-
Second Mode Positive Streamers Propagating Along Mineral-oil/solid Interfaces
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents an experimental study on second mode positive streamers propagating along mineral-oil/solid interfaces. The inception and propagation of these streamers is investigated with different impregnated solids (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF), two papers referred to as kraft paper and a kraft fibril paper, made from cellulosic micro and nano fibrils, a lignin-free paper and a paper with high lignin content referred to as k107 kraft paper). Streamers are initiated in a point-plane configuration under step voltages with 35 ns rise time. The radius of the tip is 2.9 μm and the solid is installed in an inclined position in close contact to the point electrode. Shadowgraphs, charge and light recording of the streamers are reported for each case. Furthermore, estimations of the streamer stopping length, velocity, current and average charge are reported. It is found that the streamer inception is influenced by the solid interface indicating that the inception process is not only conditioned by the field at the tip but also by the interface. A time delay is observed before the initiation of the streamer and probably correlated with the initiation process and formation of the gaseous phase. Additionally, the threshold propagation voltage of the second mode streamers at mineral-oil/solid interfaces is shown to be independent of the interface. It is also shown that the different characteristics of streamers propagating along the tested interfaces cannot be fully explained by the capacitive coupling effect due to permittivity mismatch. Thus, it is suggested that the characteristics of streamers propagating near interfaces is affected by other properties of the solid such as chemical composition, wettability and surface roughness.
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| 18. |
- Augustin, Tim, et al.
(författare)
-
Advanced Test Circuit for DC Circuit Breakers
- 2018
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Ingår i: 20th European Conference on Power Electronics and Applications (EPE'18 ECCE EUROPE).
-
Konferensbidrag (refereegranskat)abstract
- In future HVDC systems, many DC circuit breakers (DCCBs) will be required. In this paper, an advanced test circuit for DCCBs is described. A DC source is combined with a capacitor bank. In contrast to other test circuits, the proposed test circuit allows to replicate constant DC and temporary faults. In addition to conventional faults, this enables testing of auto-reclosing, proactive commutation, and complex test sequences combining all of these modes. The test circuit is easy to setup and also suitable for smaller research facilities. Experimental results from a down-scaled mock-up are included to demonstrate the capabilities of the test circuit.
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| 19. |
- Augustin, Tim, et al.
(författare)
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Enhanced Active Resonant DC Circuit Breakers Based on Discharge Closing Switches
- 2021
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Ingår i: IEEE Transactions on Power Delivery. - : IEEE. - 0885-8977 .- 1937-4208. ; 36:3, s. 1735-1743
-
Tidskriftsartikel (refereegranskat)abstract
- Direct current circuit breakers (DCCBs) have become a large research topic and are considered one of the critical components for future DC grids. Proposed DCCB concepts may be grouped into hybrid DCCBs and active resonant DCCBs. In this work, the enhanced active resonant (EAR) DCCB family is introduced. EAR DCCBs combine elements of hybrid and active resonant DCCBs. The EAR DCCB family consists of one unidirectional and six bidirectional concepts. All concepts feature proactive commutation. The main characteristic of the EAR DCCBs is that discharge closing switches are used instead of semiconductors with turn-off capability. Relevant discharge closing switch technology is reviewed, a laboratory prototype is explained, and experimental results are presented to demonstrate the feasibility of the proposed DCCB concepts.
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| 20. |
- Augustin, Tim, 1990-
(författare)
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Enhanced Active Resonant DC Circuit Breakers for HVDC Grids
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-voltage DC (HVDC) grids are considered promising for the electricity grid expansion required to integrate renewable energy sources into the existing infrastructure. DC fault currents increase rapidly and lack a current zero crossing. Therefore, HVDC grids require complex DC circuit breakers (DCCBs) capable of interrupting faster than AC circuit breakers to protect against DC faults. Being complex, DCCBs can offer functionality in addition to interruption. Most DCCBs can be categorized as current-injection DCCBs or hybrid DCCBs. Hybrid DCCBs feature more functionality than current-injection DCCBs. Nevertheless, the power semiconductors used in hybrid DCCBs are expensive. The enhanced active resonant (EAR) DCCBs studied in this work are an intermediate solution with the functionality of hybrid DCCBs and the interruption mechanism of current-injection DCCBs. The core of EAR DCCBs are discharge closing switches, which are simple, robust and available for high current and high voltage.Like all HVDC DCCBs, EAR DCCBs need a fast mechanical switch. A Thomson-coil actuator with active damping is used to open and close the mechanical switch fast. A novel Thomson-coil driver recycling energy during actuation simplifies the Thomson-coil actuator system. Experimental results demonstrate the open-close and open-close-open operation of the Thomson-coil actuator. Extensive experimental studies investigate the DC interruption capability and functionality of a prototype EAR DCCB in a specialized DCCB test circuit. The tests results show that the prototype EAR DCCB can interrupt up to 1.2 kA, abort proactive commutation, and auto-reclose. The studies of the discharge closing switch used find that its minimum voltage is not a serious limitation and that the discharge can become unstable after commutationat low currents. An alternative commutation technique allows EAR DCCBs with less components to operate reliably at all currents.
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| 21. |
- Augustin, Tim, et al.
(författare)
-
Experimental Study of Enhanced Active Resonant DC Circuit Breakers
- 2022
-
Ingår i: IEEE transactions on power electronics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-8993 .- 1941-0107. ; 37:5, s. 5687-5698
-
Tidskriftsartikel (refereegranskat)abstract
- Enhanced active resonant (EAR) dc circuit breakers (DCCBs) are a novel type of DCCB that use a discharge closing switch as interruption medium. A technical limitation of discharge closing switches is the minimum voltage across the main gap required for successful triggering. A novel commutation process creating the minimum voltage internally is proposed, which allows to simplify the EAR DCCB configuration and to reduce its component count. In the prototype, the discharge closing switch is implemented with a TVG. Experiments show that the TVG can be triggered reliably down to a voltage of 50 V and that the discharge in the TVG is highly oscillatory at low current. The originally proposed EAR DCCB configuration has to be tuned such that the commutation to the TVG succeeds at low current. Conversely, the novel commutation process decouples the minimum voltage from the current level by adjusting the triggering delay. This allows reliable commutation irrespective of the operating conditions. It is shown that the novel commutation process does not adversely affect dc interruption. Proactive commutation operation and auto-reclosing strategies are demonstrated.
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| 22. |
- Augustin, Tim, et al.
(författare)
-
System Design of Fast Actuator for Vacuum Interrupter in DC Applications
- 2018
-
Ingår i: 2018 28th International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV). - : Institute of Electrical and Electronics Engineers (IEEE). - 9781538643730 ; , s. 527-530
-
Konferensbidrag (refereegranskat)abstract
- One of the major challenges of DC circuit breakers is the required fast mechanical actuator. In this paper, a Thomson coil actuator system for a vacuum interrupter is designed. Active damping is used to decelerate the moving contacts. Challenges are discussed, especially concerning the power supply needed for the Thomson coil actuator. The design philosophy is explained and FEM simulation results are presented. The results indicate that a wide range of combinations of drive circuit capacitance and voltage fulfill the requirements for armature acceleration. However, active damping requires a very careful selection of drive circuit voltage and timing of applied damping.
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| 23. |
- Augustin, Tim, et al.
(författare)
-
Thomson-Coil Actuator System for Enhanced Active Resonant DC Circuit Breakers
- 2022
-
Ingår i: IEEE Journal of Emerging and Selected Topics in Power Electronics. - : Institute of Electrical and Electronics Engineers (IEEE). - 2168-6777 .- 2168-6785. ; 10:1, s. 800-810
-
Tidskriftsartikel (refereegranskat)abstract
- Enhanced active resonant (EAR) dc circuit breakers (DCCBs) are a promising set of recently proposed DCCB concepts. As other DCCBs, EAR DCCBs still require a fast mechanical switch. The requirements on the actuator of the mechanical switch depend on the DCCB concept and the dc grid and are derived here for an EAR DCCB. Thomson-coil actuators (TCAs) can open and close mechanical switches sufficiently fast to satisfy the requirements. This work studies experimentally a TCA system with active damping for an off-the-shelf industrial vacuum interrupter used as mechanical switch in an EAR DCCB. The prototype is explained in detail, and extensive measurement results are presented, showing that active damping must be perfectly timed to be effective. A novel Thomson-coil (TC) driver is proposed and studied experimentally, which operates the TCA more efficiently by recycling energy during the actuation. Moreover, the novel TC driver reduces the capacitive storage by 50% and allows for faster recharging with lower charging current. Finally, the autoreclosing and proactive commutation operation of the TCA system and the interruption capability of the prototype EAR DCCB are demonstrated experimentally.
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| 24. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
Assessing the production and loss of electrons from conduction currents in mineral oil
- 2019
-
Ingår i: 2019 IEEE 20th International Conference on Dielectric Liquids (ICDL). - : IEEE Computer Society. - 9781728117188
-
Konferensbidrag (refereegranskat)abstract
- The evaluation of the high-field generation and loss of charged carriers is a key step to simulate any prebreakdown process in a dielectric liquid. Currently, the electron generation in mineral oil has been widely described in terms of 'electric-field-dependent molecular ionization' and the electron loss is estimated using a fixed attachment time constant. This paper reports our next step towards the quantitative characterization of the production and loss of electrons in mineral oil. In this step, the electrical conduction measurements are performed in mineral oil for a needle-plane configuration (tip radius 3 μm) and submicrometric gap distances (ranging between 10 to 100 μm). Conduction currents in negative polarity are reported from 10-12 to 10-7A, from the ohmic to the space-charge limited regimes. In order to check the validity of existing simulation models for mineral oil, computer simulation is used to calculate the VI characteristic in the liquid considering electrohydrodynamic (EHD) motion. It is shown that the active zone where electrons are produced in front of the needle is around 10 μm long. Furthermore, it is found that electrons travel a similar distance before they attach into ions. It is also shown that the currents are grossly misestimated when parameters proposed in the literature to model generation and loss of electrons in mineral oil are used.
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| 25. |
- Becerra Garcia, Marley
(författare)
-
Glow corona discharges and their effect on lightning attachment : Revisited
- 2012
-
Ingår i: Lightning Protection (ICLP), 2012 International Conference on. - : IEEE. - 9781467318969 - 9781467318983 ; , s. 6344402-
-
Konferensbidrag (refereegranskat)abstract
- Previous studies in the literature have suggested that glow corona discharges could be potentially used to control the frequency of lightning flashes to grounded objects. Such studies use simplified one-dimensional corona drift models or basic empirical equations derived from high voltage experiments to assess the effect of glow corona on the initiation of both streamers and upward connecting leaders under the influence of a descending lightning leader. In order to revisit the theoretical basis of these studies, a two-dimensional glow corona drift model has been implemented together with a self-consistent upward leader inception and propagation model -SLIM-. A 60 m tall lightning rod is used as a study case. It is found that the shielding effect of the glow corona space charge has been strongly overestimated in the literature. Furthermore, it is shown that streamers under the influence of a descending leader are initiated significantly earlier from the cylindrical body rather than from the corona-emitting area of the rod. Considering the effective shielding potential of glow corona, it is also shown that the presence of glow corona reduces the downward lightning attractiveness of 60 m tall lightning rods by less than 15%. This result shows that the efficiency of lightning rods is not strongly influenced by the generation of glow corona as opposed to the suggestions of previous studies.
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| 26. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
High field conduction in mineral oil based zno nanofluids prior to negative streamer inception
- 2021
-
Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 5:4
-
Tidskriftsartikel (refereegranskat)abstract
- The electric conduction under intense electric fields (up to ∼ 10 V/m) in nanofluids using surface-modified ZnO–C nanoparticles dispersed in mineral oil as host, is investigated with both experiments and numerical simulations. The measurements are used to estimate unknown parameters necessary to represent the generation and loss of electrons in an electrohydrodynamic model for mineral oil with and without ZnO–C nanoparticles in a needle-plane configuration. The model suggests that ZnO–C nanoparticles induce an enhanced field emission from negative needles, explaining the significantly larger conduction currents measured in the nanofluid compared with those in the host liquid. It is also found that the scavenging of electrons by ZnO–C nanoparticles is a process which is negligible compared with the loss of electrons due to attachment in mineral oil. It is shown that ZnO–C nanoparticles hinder the streamer initiation process by reducing the effective electric field at the tip of the needle. This electric field reduction is caused by the combined effect of enhanced electron injection through ZnO–C nanoparticles and strong electron attachment in mineral oil. Thus, the electric field on the needle tip reaches the same threshold value when the streamer is incepted in the nanofluid as in mineral oil, although at a larger voltage. Solid evidence indicating that the additional electron scavenging and the reduced electron mobility introduced by nanoparticles has no effect in the conduction currents and in the negative streamer inception in the tested ZnO–C nanofluids is shown.
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| 27. |
- Becerra Garcia, Marley, et al.
(författare)
-
Laboratory experiments cannot be utilized to justify the action of early streamer emission terminals
- 2008
-
Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:8
-
Tidskriftsartikel (refereegranskat)abstract
- The early emission of streamers in laboratory long air gaps under switching impulses has beenobserved to reduce the time of initiation of leader positive discharges. This fact has beenarbitrarily extrapolated by the manufacturers of early streamer emission devices to the case ofupward connecting leaders initiated under natural lightning conditions, in support of thosenon-conventional terminals that claim to perform better than Franklin lightning rods. In orderto discuss the physical basis and validity of these claims, a self-consistent model based on thephysics of leader discharges is used to simulate the performance of lightning rods in thelaboratory and under natural lightning conditions. It is theoretically shown that the initiation ofearly streamers can indeed lead to the early initiation of self-propagating positive leaders inlaboratory long air gaps under switching voltages. However, this is not the case for positiveconnecting leaders initiated from the same lightning rod under the influence of the electricfield produced by a downward moving stepped leader. The time evolution of the developmentof positive leaders under natural conditions is different from the case in the laboratory, wherethe leader inception condition is closely dependent upon the initiation of the first streamerburst. Our study shows that the claimed similarity between the performance of lightning rodsunder switching electric fields applied in the laboratory and under the electric field producedby a descending stepped leader is not justified. Thus, the use of existing laboratory results tovalidate the performance of the early streamer lightning rods under natural conditions is not justified.
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| 28. |
- Becerra Garcia, Marley, et al.
(författare)
-
Lightning striking distance of complex structures
- 2008
-
Ingår i: IET Generation, Transmission & Distribution. - : Institution of Engineering and Technology (IET). - 1751-8687 .- 1751-8695. ; 2:1, s. 131-138
-
Tidskriftsartikel (refereegranskat)abstract
- Traditionally, the location of lightning strike points has been determined byusing the rolling sphere method, but recently the collection volumemethod (CVM) has also been proposed for the placement ofair terminals on complex structures. Both these methods are empiricalin nature and a more advanced model based on physicsof discharges is needed to improve the state of affairs.This model is used to evaluate the striking distance fromcorners and air terminals on actual buildings and the resultsare qualitatively compared with the predictions of the rolling spheremethod and the CVM. The results show that the strikingdistance not only depends upon the prospective return stroke currentand the geometry of the building, but also on thelateral position of the downward leader with respect to thestrike point. A further analysis is performed to qualitatively comparethe lightning attraction zones obtained with the CVM and theleader inception zones obtained for a building with and withoutair terminals. The obtained results suggest that the collection volumeconcept overestimates the protection areas of air terminals placed oncomplex structures, bringing serious doubts on the validity of this method.
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| 29. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
On the estimation of the lightning incidence to offshore wind farms
- 2018
-
Ingår i: Electric power systems research. - : Elsevier Ltd. - 0378-7796 .- 1873-2046. ; 157, s. 211-226
-
Tidskriftsartikel (refereegranskat)abstract
- Field observations have shown that the frequency of dangerous lightning events to wind turbines, calculated according to the IEC standard 61400-24:2010, is grossly underestimated. This paper intends to critically revisit the evaluation of the incidence of downward lightning as well as self-initiated and other-triggered upward flashes to offshore wind power plants. Three different farms are used as case studies. The conditions for interception of stepped leaders in downward lightning and the initiation of upward lightning is evaluated with the Self-consistent Leader Inception and Propagation Model (SLIM). The analysis shows that only a small fraction of damages observed in the analysed farms can be attributed to downward lightning. It is also estimated that only a small fraction (less than 19%) of all active thunderstorms in the area of the analysed farms can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can be triggered even under low thundercloud fields once a sufficiently high electric field change is generated by a nearby lightning event. Despite of the uncertainties in the incidence evaluation, it is shown that upward flashes triggered by nearby positive cloud-to-ground flashes produce most of the dangerous lightning events to the case studies.
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| 30. |
- Becerra Garcia, Marley, et al.
(författare)
-
On the velocity of lightning upward connecting positive leaders
- 2007
-
Ingår i: IX International Symposium onLightning Protection.
-
Konferensbidrag (refereegranskat)abstract
- The point of interception of the downward movinglightning leader by a newly created upward connectingleader depends upon the velocity of both leaders. Eventhough measurements and estimates of the velocity ofdownward moving lightning leaders are more common in theliterature, only few values of the velocity of upwardconnecting leaders have been reported. In this paper, a selfconsistentleader propagation model is used to estimate thevelocity of upward connecting positive leaders initiated froma tall tower under the influence of downward negativelightning leaders. It is predicted that an upward connectingleader propagates with low velocity (lower than about 5 x103 m s-1) immediately after the creation of the first leadersegment. It is not until the electric field produced by thedownward leader is high enough for the continuousacceleration of the leader channel, when the upward leadervelocity reaches values close to the ones observed inlaboratory (about 1-2 x 104 m s-1). Then, the connectingleader continues its movement with increasing velocity untilit intercepts the downward moving leader. The propagationof upward connecting leaders has been found to beinfluenced by the prospective return stroke current, theaverage velocity and the lateral position of the downwardleader channel as well as by the ambient field. This resultclearly shows that the velocity and propagation time ofupward connecting positive leaders change from flash toflash and they cannot be generalized by assuming a givenvelocity ratio of both leaders as it is assumed by the existingleader progression models.
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| 31. |
- Becerra Garcia, Marley, et al.
(författare)
-
On the velocity of positive connecting leaders associated with negative downward lightning leaders
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:2
-
Tidskriftsartikel (refereegranskat)abstract
- A self-consistent leader propagation model is used to estimate the velocity of upward connecting positive leaders initiated from a tall tower under the influence of downward negative lightning leaders. The propagation of upward connecting leaders has been found to be influenced not only by the average velocity of the downward leader but also by the prospective return stroke current, the lateral position of the downward leader channel as well as by the ambient electric field. This result show that the velocity and propagation time of upward connecting positive leaders change from flash to flash due to the variations in these parameters.
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| 32. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
Optical radiative properties of ablating polymers exposed to high-power arc plasmas
- 2018
-
Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 51:12
-
Tidskriftsartikel (refereegranskat)abstract
- The radiative properties of polymers exposed to high-intensity radiation are of importance for the numerical simulation of arc-induced ablation. The paper investigates the optical properties of polymethylmethacrylate PMMA and polyamide PA6 films exposed to high-power arc plasmas, which can cause ablation of the material. A four-flux radiative approximation is first used to estimate absorption and scattering coefficients of the tested materials in the ultraviolet (UV) and in the visible (VIS) ranges from spectrophotometric measurements. The temperature-induced variation of the collimated transmissivity of the polymers is also measured from room temperature to the glass temperature of PMMA and the melting temperature of PA6. Furthermore, band-averaged absorption and scattering coefficients of non-ablating and ablating polymers are estimated from the UV to the short-wavelength infrared (SWIR), covering the range of interest for the simulation of arc-induced ablation. These estimates are obtained from collimated transmissivities measured with an additional in situ photometric system that uses a high-power, transient arc plasma to both illuminate the samples and to induce ablation. It is shown that the increase in the bulk temperature of PA6 leads to a strong reversible increase in collimated transmissivity, significantly reducing the absorption and scattering coefficients of the material. A weaker but opposite effect of temperature on the optical properties is found in PMMA. As a consequence, it is suggested that the absorption coefficient of polymers used for arc-induced ablation estimates should not be taken directly from direct collimated transmissivity measurements at room temperature. The band-averaged radiation measurements also show that the layer of products released by ablation of PMMA produces scattering radiation losses mainly in the VIS-SWIR ranges, which are only a small fraction of the total incident arc radiation. In a similar manner, the ablation layer of PA6 leads to weak absorption radiation losses, although mainly in the UV range.
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| 33. |
- Becerra Garcia, Marley
(författare)
-
Self-consistent leader inception and propagation model —SLIM— : Response to some criticism
- 2012
-
Ingår i: Proceedings of the International Conference on Lightning Protection (ICLP), 2012. - : IEEE conference proceedings. ; , s. 1-7
-
Konferensbidrag (refereegranskat)abstract
- The self-consistent leader inception and propagation model -SLIM- has been increasingly used to study the attachment of lightning flashes to grounded structures. However, criticism about some assumptions of SLIM has been raised in a recent publication. This paper intends to reply to this criticism and to openly discuss the validity of the model. Moreover, solid scientific evidence from theory and experiments is presented to support the validity of the assumptions considered by SLIM. It is shown that the streamer corona charge calculation and the leader inception condition used by SLIM give proper estimates which are completely justified. In addition, it is shown that the predictions of this state-of-the-art model based on the latest developments in the physics of leader discharges has been found in agreement with results available from laboratory, rocket triggered lightning experiments and field observations.
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| 34. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane
- 2017
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 29:12
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by Vazquez et al. ["Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model," Phys. Fluids 12, 2809 (2000)] when the correct longitudinal electric field distribution and liquid velocity radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a high-density charged core. In addition, no significant differences in the model estimates of the plume are found when the simulations are obtained either with the finite element method or with a diffusion-free particle method. It is shown that the model also enables the calculation of the current-voltage characteristic of EHD laminar plumes produced by electron field emission, with good agreement with measured values reported in the literature.
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| 35. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
Spectral and electric diagnostics of low-current arc plasmas in CO2 with N2 and H2O admixtures
- 2024
-
Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 57:1
-
Tidskriftsartikel (refereegranskat)abstract
- Plasma diagnostics is a key tool to support the further development of plasma-induced chemical conversion of greenhouse gases (such as CO2) into high-value chemicals. For this reason, spectroscopic and electric measurements of low current (below 1.7 A), stationary arc plasmas in CO2 at atmospheric pressure with addition of N-2 or H2O are reported. High-speed photography, imaging emission spectroscopy and time-resolved electrical measurements are used to obtain time-space resolved gas temperatures as well as the electric-field current characteristics of the discharge. It is found that the lowest average electric field in a CO2 arc plasma at atmospheric pressure is similar to 20 kV mm(-1) at a current between 0.8 and 1 A. If the current decreases below this level, the arc remains in vibrational-translational (VT) equilibrium by increasing the electric field. However, VT equilibrium conditions can be only maintained until a threshold minimum current of 0.33 +/- 0.05 A, at which the arc transitions into a non-equilibrium condition with further increasing electric fields (reaching 68 +/- 15 V mm(-1) at 0.03 A). The addition of N-2 or H2O did not influence the electrical characteristics of the CO2 arc within to the tested mixtures. However, there is only a significant decrease in the electric field of the formed transition arcs and the threshold minimum current in the presence of N-2. The spectra of the low-current CO2 arc is found to be dominated by emission from the C-2 Swan band system and the O I 777 nm triplet peak. However, the CN band dominates the spectra even when small amounts (0.5 wt%) of N-2 is present in the plasma. The gas temperature at the axis of the CO2 arc plasma decreased slightly with decreasing current, from an estimated 7000 K at 1 A down to 6300 K at 0.4 A. The thermal radius of the arc is estimated to be larger than 1.2 mm, more than two times larger than the optical radius obtained from the emitted radiation. The addition of N-2 and H2O (up to 7 and 9 wt% respectively) lead to only to a 500 K decrease in the axial arc temperature.
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| 36. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
Temperature and pressure profiles of an ablation-controlled arc plasma in air
- 2019
-
Ingår i: Journal of Physics D. - : IOP PUBLISHING LTD. - 0022-3727 .- 1361-6463. ; 52:43
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental measurements of the spatial distribution of temperature and composition of ablation-controlled arc plasmas are a key to validate the predictions of metal evaporation and polymer ablation models. Thus, high-speed photography and space-resolved spectroscopic measurements have been performed to characterize a stable air arc plasma jet controlled by ablation of a polymer nozzle made of Polyoxymethylene copolymer (POM-C) or polyamide (PA6). The spectroscopic analysis is performed along a plane perpendicular to the arc jet axis for a current of 1.8 kA, corresponding to an estimated current density of similar to 65 A mm(-2). Temperature and partial pressure profiles of the plasma for copper, hydrogen and carbon in the gas mixture are estimated as an inverse optimization problem by using measured side-on radiance spectra and radiative transfer spectral simulations. It is shown that the generated ablation-controlled arc has a complicated, non-uniform gas composition. Thus, the generated arc jet has a thin metallic core with a lower almost constant hydrogen pressure, surrounded by a thicker hydrogen and carbon mantle at partial pressures slightly lower than atmospheric pressure. The separation of hydrogen and carbon in the core is a consequence of demixing of the polymer vapour in the plasma. It is found that the overall shape of the temperature and pressure profiles obtained for the arc plasmas with the POM-C and PA6 nozzles are similar although differ in peak values and width.
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| 37. |
- Becerra Garcia, Marley, et al.
(författare)
-
The early streamer emission principle does not work under natural lightning!
- 2007
-
Ingår i: IX International Symposium on Lightning Protection.
-
Konferensbidrag (refereegranskat)abstract
- An appropriate evaluation of the efficiency of airterminals is a key factor for the discussion of the claimedadvantages of the Early Streamer Emission devices ESEcompared to the conventional Franklin rods. In order todiscuss the physical basis and validity of the ESE claim, aself-consistent physical model is used to simulate theperformance of an air terminal under laboratory and undernatural lightning conditions. It is theoretically shown thatthe early initiation of streamers can indeed lead to the soonerinitiation of a self-propagating positive leader in a laboratorylong air gap under switching voltages. However, this is notthe case for positive leaders initiated from the same terminalunder the influence of the electric field produced by thedescent of a downward moving lightning leaders. The timeevolution of the leader development under natural conditionsis different to the case in laboratory, where the leaderinception condition is closely dependant upon the streamerinititation. This is mainly because of the differences in thetime variation of the electric field applied in laboratory andthat produced by the approach of the downward leader.Therefore, it is found that the claimed similarity between theswitching electric fields applied in laboratory and the electricfield produced by the descent of a negative downward leader,used in the literature to extrapolate the early streameremission principle to natural lightning, is not true.
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| 38. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
Using low-level currents measured during lightning events to estimate upward leader properties
- 2018
-
Ingår i: 34th International Conference on Lightning Protection, ICLP 2018. - : Institute of Electrical and Electronics Engineers Inc.. - 9781538666357
-
Konferensbidrag (refereegranskat)abstract
- Low-level currents measured prior to return strokes can potentially provide information about the properties of upward leaders during lightning flashes. However, these currents need to be properly analysed and interpreted in order to be useful for evaluating upward connecting leaders. In this paper, low-level currents measured before return strokes in two lightning events to two structures in Brazil are analysed and interpreted as case studies. The discharge current estimated from one of these events is used as input to a detailed thermohydrodynamic model with an extensive kinetic scheme for N2/O2 mixtures. The model allows the evaluation of the physical and chemical properties of upward connecting leaders. Estimates of the temperature, mass density, electric field and radius of the channel are presented for an upward connecting leader propagating in a lightning event. In addition, estimates of the axial density of electrons, ions and neutral particles (including NO and NO2) are also reported.
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| 39. |
- Becerra, Marley, et al.
(författare)
-
A self-consistent upward leader propagation model
- 2006
-
Ingår i: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 39:16, s. 3708-3715
-
Tidskriftsartikel (refereegranskat)abstract
- The knowledge of the initiation and propagation of an upward movingconnecting leader in the presence of a downward moving lightning steppedleader is a must in the determination of the lateral attraction distance of alightning flash by any grounded structure. Even though different models that simulate this phenomenon are available in the literature, they do not take into account the latest developments in the physics of leader discharges. Theleader model proposed here simulates the advancement of positive upward leaders by appealing to the presently understood physics of that process.The model properly simulates the upward continuous progression of thepositive connecting leaders from its inception to the final connection withthe downward stepped leader (final jump). Thus, the main physical properties of upward leaders, namely the charge per unit length, the injected current, the channel gradient and the leader velocity are self-consistentlyobtained. The obtained results are compared with an altitude triggeredlightning experiment and there is good agreement between the modelpredictions and the measured leader current and the experimentally inferredspatial and temporal location of the final jump. It is also found that the usualassumption of constant charge per unit length, based on laboratoryexperiments, is not valid for lightning upward connecting leaders.
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| 40. |
|
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| 41. |
- Becerra, Marley, et al.
(författare)
-
A simplified physical model to determine the lightning upward connecting leader inception
- 2006
-
Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 21:2, s. 897-908
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, a generalized leader inception model isproposed. It is based on an iterative geometrical analysis of thebackground potential distribution of an earthed structure to simulatethe first meters of propagation of an upward connecting leader.By assuming a static field approach, the leader stabilization fieldsand the striking distances were computed for a lightning rod andfor a building. The obtained results were compared with the existingleader inception criteria. Furthermore, in order to validatethe model, the leader inception condition was computed for a triggeredlightning experiment. Excellent agreement with the experimentalresults was obtained. The present model has several advantagesin comparison with the existing leader inception criteria.One of them is related to the fact that the proposed model can beused to analyze the effect of the space charge on the upward leaderinception.
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| 42. |
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| 43. |
|
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| 44. |
- Becerra, Marley, et al.
(författare)
-
Arc jets blown by outgassing polymers in air
- 2014
-
Ingår i: Proceedings of the 20<sup>th</sup> International Conference on Gas Discharges and Their Applications GD2014. - : GD2014. ; , s. 1-4
-
Konferensbidrag (refereegranskat)abstract
- This paper describes experimental results about the behaviour of arc jets transversely blown in the presence of outgassing polymers (POM –CH2O– or PMMA –C5H8O2–). The arc jets are ignited in air between copper electrodes under a 2 kA, 50 Hz AC current. High speed photography and optical emission spectroscopy are used to study the mechanism leading to the increase of the arc voltage when polymers are used instead of non-ablating materials (e.g. quartz). It is found that the transversal blowing flow caused by the injection of ablation vapours have a weak effect on the arc voltage build-up. Instead, the chemical changes in the plasma environment appear to better explain the observed increase in the arc voltage when polymers are used.
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| 45. |
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| 46. |
- Becerra, Marley
(författare)
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Corona discharges and their effect on lightning attachment revisited : Upward leader initiation and downward leader interception
- 2014
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Ingår i: Atmospheric research. - : Elsevier BV. - 0169-8095 .- 1873-2895. ; 149, s. 316-323
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies have suggested the possibility of using glow corona discharges to control the frequency of lightning flashes to grounded objects. In order to revisit the theoretical basis of this proposal, the self-consistent leader inception and propagation model - SLIM - is used together with a two-dimensional glow corona drift model. The analysis is performed to quantify the effect of glow corona generated at the tip of ground-based objects on the initiation and propagation of upward positive connecting leaders under the influence of downward lightning leaders. It is found that the presence of glow corona does not influence the performance of Franklin lightning rods shorter than 15 m, while it slightly reduces the lateral distance of rods up to 60 m tall by a maximum of 10%. Furthermore, the results indicate that it is not possible to suppress the initiation of upward connecting leaders by means of glow corona. It is found instead that unconventional lightning protection systems based on the generation of glow corona attract downward lightning flashes in a similar way as a standard lightning rod with the same height.
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| 47. |
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| 48. |
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| 49. |
- Becerra, Marley, et al.
(författare)
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Effect of the space charge layer created by corona at ground level on the inception of upward lightning leaders from tall towers
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:D12, s. D12205-
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Tidskriftsartikel (refereegranskat)abstract
- Electric field measurements above ground have shown that the space charge layer created by corona at ground level shields the background electric field produced by the thundercloud. Therefore it is expected that this space charge layer can also influence the conditions required to initiate upward lightning from tall objects. For this reason, a numerical model that describes the evolution of the main electrical parameters below a thunderstorm is used to compute the space charge layer development. The time variation of the electric field measured at 600 m above ground during the 1989 rocket triggered lightning experiment at the Kennedy Space Center (Florida) is used to drive the model. The obtained space charge density profiles are used to compute the conditions required to initiate stable upward lightning positive leaders from tall towers. Corona at the tip of the tower is neglected. It is found that the space charge layer significantly affects the critical thundercloud electric fields required to initiate upward lightning leaders from tall objects. The neutral aerosol particle concentration is observed to have a significant influence on the space charge density profiles and the critical thundercloud electric fields, whereas the corona current density does not considerably affect the results for the cases considered in the analysis. It is found that a lower thundercloud electric field is required to trigger a lightning flash from a tall tower or other tall slender grounded structure in the case of sites with a high neutral aerosol particle concentration, like polluted areas or coastal regions.
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| 50. |
- Becerra, Marley, et al.
(författare)
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Electrohydrodynamic motion due to space-charge limited injection of charges in cyclohexane
- 2014
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Ingår i: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014. - 9781479920631
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Konferensbidrag (refereegranskat)abstract
- A self-consistent numerical model is presented in order to study the electrohydrodynamic (EHD) motion generated by a stationary, space-charge limited injection of charge in the point-plane geometry. In this multiphysics model, the continuity equations for charge carriers and Poisson's equation are coupled with Navier-Stokes equations and the heat equation. This model is used to study the EHD motion of cyclohexane in the negative point-plane geometry for sharp points with tip radius of 0.2 μm. It is shown that the injection of charges from a very sharp point electrode results in the formation of a thin plume with high liquid velocity. The results show large differences in the liquid velocity close to the point electrode compared to the average velocity estimated by the well-known electrohydrodynamic mobility. The difference between the width of the charged core and the hydrodynamic plume is analyzed and presented. It is shown that the local heating of the liquid is strongly reduced by the convective losses generated by EHD motion. Finally, it is found that the liquid temperature in cyclohexane in the vicinitiy of sharp points under space charge limited injection can reach temperatures slightly above boiling temperature, without generating bubbles.
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