Change search
Refine search result
12 1 - 50 of 88
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Aljure, M.
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, M.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Pallon, L. K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration2016In: 2016 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (IEEE CEIDP), IEEE conference proceedings, 2016, p. 687-690Conference paper (Refereed)
    Abstract [en]

    We present experiments and simulations on the electrical conduction currents of purified transformer oil with and without surface-modified MgO nanoparticles. Results show that on the injection regime of the voltage-current characteristics, nanoparticles increase the charge production in the fluid. It is also found that the conduction currents in the space-charge-limited regime increased at a lower rate as a function of the voltage in the presence of nanoparticles. The numerical simulations suggest electron attachment is increased due to the nanoparticles, leading to larger accumulation of negative ionic space charge close to the needle in the space-charge-limited regime. It is concluded that electron attachment may be significantly increased with nanoparticles, becoming an important process of electrical conduction in nanofluids.

  • 2.
    Aljure, Mauricio
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Karlsson, Mattias
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Polymeric Materials.
    Erratum to: Aljure, M.; Becerra, M.; Karlsson, E.M. Streamer inception from ultra-sharp needles in mineral oil based nanofluids2018In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 11, no 11, article id 2900Article in journal (Refereed)
    Abstract [en]

    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.

  • 3.
    Aljure, Mauricio
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering. ABB Corp Res, Vasteras, Sweden..
    Karlsson, Mattias E.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).
    On the injection and generation of charge carriers in mineral oil under high electric fields2019In: JOURNAL OF PHYSICS COMMUNICATIONS, ISSN 2399-6528, Vol. 3, no 3, article id UNSP 035019Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Aljure, Mauricio
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Karlsson, Mattias E.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Polymeric Materials.
    Streamer Inception from Ultra-Sharp Needles in Mineral Oil Based Nanofluids2018In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 11, no 8, article id 2064Article in journal (Refereed)
    Abstract [en]

    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.

  • 5.
    Aljure, Mauricio
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Pourrahimi, Amir Masoud
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
    Electric Conduction in Mineral Oil based ZnONanofluids under Intense Electric FieldsManuscript (preprint) (Other academic)
    Abstract [en]

    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.

  • 6.
    Aljure, Mauricio
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corporate Research, Sweden.
    Jonsson, B. Lars G.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Simulation of the electrical conduction of cyclohexane with TiO2 nanoparticles2014In: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014, IEEE , 2014, p. 6893119-Conference paper (Refereed)
    Abstract [en]

    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.

  • 7.
    Aminlashgari, Nina
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. ABB, Sweden.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB, Sweden.
    Hakkarainen, Minna
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Characterization of degradation fragments released by arc-induced ablation of polymers in air2016In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 49, no 5, article id 055502Article in journal (Refereed)
    Abstract [en]

    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.

  • 8.
    Arevalo, L
    et al.
    Division for Electricity, Uppsala University.
    Becerra Garcia, Marley
    Division for Electricity, Uppsala University.
    Roman, F
    Division for Electricity, Uppsala University.
    Understanding the point discharge DC current produced by corona needles2006In: Proceedings of the 28th Internat Conference on Lightning Protection, Japan, 2006, p. 1328-1333Conference paper (Refereed)
  • 9. Arevalo, L
    et al.
    Becerra, Marley
    Division for Electricity, Uppsala University.
    Roman, Francisco
    Numerical Simulations of the corona current development2005In: International Symposium on Lightning Protection, 2005Conference paper (Refereed)
  • 10.
    Ariza, David
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hollertz, R.
    Pitois, Claire
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Measurements of the charge of streamers propagating along transformer oil-solid interfaces2014In: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014, IEEE , 2014, p. 6893165-Conference paper (Refereed)
    Abstract [en]

    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.

  • 11.
    Ariza, David
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corp Res, Stockholm, Sweden.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pitois, Claire
    On the initiation of negative streamers at mineral oil-solid interfaces2015In: IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE conference proceedings, 2015, p. 563-565Conference paper (Refereed)
    Abstract [en]

    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.

  • 12.
    Ariza, David
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corporate Research, Sweden.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pitois, Claire
    ABB Corporate research .
    Propagation of negative streamers along mineral oil-solid interfaces2015In: Proceedings of the 2015 CEIDP Conference on Electrical insulation and Dielectric phenomena, CEIDP 2015, IEEE conference proceedings, 2015, p. 566-569Conference paper (Refereed)
    Abstract [en]

    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.  

  • 13.
    Ariza, David
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Inception of first mode negative streamers at liquid/solid interfacesIn: Journal of Physics D: Applied PhysicsArticle in journal (Refereed)
    Abstract [en]

    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.

  • 14.
    Ariza, David
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Second Mode Positive Streamers Propagating Along Mineral-oil/solid InterfacesManuscript (preprint) (Other academic)
    Abstract [en]

    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.

  • 15.
    Ariza, David
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pitois, Claire
    ABB AB Corporate Research.
    First Mode Negative Streamers along Mineral Oil-solid Interfaces2017In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, no 4Article in journal (Refereed)
    Abstract [en]

    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.

  • 16.
    Ariza, David
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Methling, Ralf
    INP - Leibniz Institute for Plasma Science and Technology.
    Gortchakow, Sergey
    INP Leibniz Institute for Plasma Science and Technology.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Influence of Paper Properties on Streamers Creepingin Mineral Oil2017In: Proceedings of IEEE International Conference on Dielectric Liquids, ISSN 2153-3725, E-ISSN 2153-3733Article in journal (Refereed)
    Abstract [en]

    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.

  • 17.
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Glow corona discharges and their effect on lightning attachment: Revisited2012In: Lightning Protection (ICLP), 2012 International Conference on, IEEE , 2012, p. 6344402-Conference paper (Refereed)
    Abstract [en]

    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.

  • 18.
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Self-consistent leader inception and propagation model —SLIM—: Response to some criticism2012In: Proceedings of the International Conference on Lightning Protection (ICLP), 2012, IEEE conference proceedings, 2012, p. 1-7Conference paper (Refereed)
    Abstract [en]

    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.

  • 19.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Aljure, Mauricio
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Nilsson, Janne
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Assessing the production and loss of electrons from conduction currents in mineral oil2019In: 2019 IEEE 20th International Conference on Dielectric Liquids (ICDL), IEEE Computer Society, 2019Conference paper (Refereed)
    Abstract [en]

    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.

  • 20.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Laboratory experiments cannot be utilized to justify the action of early streamer emission terminals2008In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 41, no 8Article in journal (Refereed)
    Abstract [en]

    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.

  • 21.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Lightning attachment to power transmission lines: on the validity of the electrogeometric model2008In: Proceedings of the 29th International Conference on Lightning Protection ICLP, 2008Conference paper (Refereed)
  • 22.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    On the velocity of lightning upward connecting positive leaders2007In: IX International Symposium onLightning Protection, 2007Conference paper (Refereed)
    Abstract [en]

    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.

  • 23.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    On the velocity of positive connecting leaders associated with negative downward lightning leaders2008In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, no 2Article in journal (Refereed)
    Abstract [en]

    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.

  • 24.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    The early streamer emission principle does not work under natural lightning!2007In: IX International Symposium on Lightning Protection, 2007Conference paper (Refereed)
    Abstract [en]

    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.

  • 25.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Hartono, Z.A
    Lightning Research Pte. Ltd., Kuala Lumpur, Malaysia.
    Identification of lightning vulnerability points on complex grounded structures2007In: Journal of Electrostatics, ISSN 0304-3886, E-ISSN 1873-5738, Vol. 65, no 9, p. 562-570Article in journal (Refereed)
    Abstract [en]

    The identification of the most vulnerable points on a given structure to be struck by lightning is an important issue on the design of areliable lightning protection system. Traditionally, these lightning strike points are identified using the rolling sphere method, through anempirical correlation with the prospective peak return stroke current. However, field observations in Kuala Lumpur and Singapore haveshown that the points where lightning flashes strike buildings also depend on the height and geometry of the structure. Since a lightningstrike point is believed to be the place on a grounded structure where a propagating upward leader is first initiated, a physical leaderinception model is used here to estimate the background electric field required to initiate a stable upward leader from the corners of somecomplex buildings. The computed location of the points from where leaders are incepted are compared with the damaged points onbuildings struck by lightning. The observed lightning strike points coincide rather well with the corners of the buildings which arecharacterized by lower leader inception electric fields. Furthermore, it is found that the geometry of the buildings significantly influencesthe conditions necessary to initiate upward leaders and, therefore, the location of the most likely strike points.

  • 26.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Roman, F
    National University of Colombia, Ciudad Universitaria, Departamento de Ingeniería Eléctrica.
    Lightning striking distance of complex structures2008In: IET Generation, Transmission & Distribution, ISSN 1751-8687, E-ISSN 1751-8695, Vol. 2, no 1, p. 131-138Article in journal (Refereed)
    Abstract [en]

    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.

  • 27.
    Becerra Garcia, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Soula, S
    Laboratoire d’Ae´rologie, UMR CNRS, Universite´ Paul Sabatier, Toulouse, France.
    Chauzy, S
    Laboratoire d’Ae´rologie, UMR CNRS, Universite´ Paul Sabatier, Toulouse, France.
    Effect of the space charge layer created by corona at ground level on the inception of upward lightning leaders from tall towers2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no 12Article in journal (Refereed)
    Abstract [en]

    Electric field measurements above ground have shown that the space charge layercreated by corona at ground level shields the background electric field produced by thethundercloud. Therefore it is expected that this space charge layer can also influence theconditions required to initiate upward lightning from tall objects. For this reason, anumerical model that describes the evolution of the main electrical parameters below athunderstorm is used to compute the space charge layer development. The time variationof the electric field measured at 600 m above ground during the 1989 rockettriggered lightning experiment at the Kennedy Space Center (Florida) is used to drive themodel. The obtained space charge density profiles are used to compute the conditionsrequired to initiate stable upward lightning positive leaders from tall towers. Corona at thetip of the tower is neglected. It is found that the space charge layer significantly affectsthe critical thundercloud electric fields required to initiate upward lightning leadersfrom tall objects. The neutral aerosol particle concentration is observed to have asignificant influence on the space charge density profiles and the critical thundercloudelectric fields, whereas the corona current density does not considerably affect the resultsfor the cases considered in the analysis. It is found that a lower thundercloud electric fieldis required to trigger a lightning flash from a tall tower or other tall slender groundedstructure in the case of sites with a high neutral aerosol particle concentration, like polluted areas or coastal regions.

  • 28.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Vazquez, Pedro A.
    Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 12, article id 123605Article in journal (Refereed)
    Abstract [en]

    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.

  • 29.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Long, Mengni
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Schulz, W.
    Thottappillil, Rajeev
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    On the estimation of the lightning incidence to offshore wind farms2018In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 157, p. 211-226Article in journal (Refereed)
    Abstract [en]

    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.

  • 30.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Pettersson, Jonas
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Optical radiative properties of ablating polymers exposed to high-power arc plasmas2018In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 12, article id 125202Article in journal (Refereed)
    Abstract [en]

    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.

  • 31.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering. KTH Royal Inst Technol, Sch Elect Engn & Comp Sci, S-10044 Stockholm, Sweden.;ABB Corp Res, S-72226 Vasteras, Sweden..
    Pettersson, Jonas
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Franke, Steffen
    INP Leibniz Inst Plasma Sci & Technol, D-17489 Greifswald, Germany..
    Gortschakow, Sergey
    INP Leibniz Inst Plasma Sci & Technol, D-17489 Greifswald, Germany..
    Temperature and pressure profiles of an ablation-controlled arc plasma in air2019In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 52, no 43, article id 434003Article in journal (Refereed)
    Abstract [en]

    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.

  • 32.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Saba, M. M. F.
    Liu, Lige
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Visacro, S.
    Using low-level currents measured during lightning events to estimate upward leader properties2018In: 34th International Conference on Lightning Protection, ICLP 2018, Institute of Electrical and Electronics Engineers Inc. , 2018Conference paper (Refereed)
    Abstract [en]

    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.

  • 33.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corporate Research, Power Technologies, Electrical Apparatus and Diagnostics group, Sweden .
    Corona discharges and their effect on lightning attachment revisited: Upward leader initiation and downward leader interception2014In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 149, p. 316-323Article in journal (Refereed)
    Abstract [en]

    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.

  • 34.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Glow corona generation and streamer inception at the tip of grounded objects during thunderstorms: revisited2013In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 46, no 13, p. 135205-Article in journal (Refereed)
    Abstract [en]

    The initiation of streamers prior to a lightning strike can be reportedly inhibited by glow corona discharges generated from tall objects. In contrast to previous studies based on a simplified one-dimensional model of glow corona, a two-dimensional evaluation of the corona ion drift from tall objects is used here to analyse this effect quantitatively. Proper estimates for the corona space charge distribution generated during both the charging process of a thundercloud and the descent of the downward stepped leader are thus calculated. It is found that the shielding effect of the corona space charge on the streamer inception is not as severe as previously reported. Estimations of the effective height of the downward leader tip at which streamer inception takes place are presented and discussed for lightning rods and dissipation array systems.

  • 35.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the Estimation of the Charge of Positive Streamers Propagating in Air2014In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, no 2, p. 627-634Article in journal (Refereed)
    Abstract [en]

    Streamer discharges are an important breakdown mechanism in air-based electrical insulation systems. This paper introduces a method to estimate the spatial distribution of the charge density of positive streamers in air, based on the solution of a Poisson inverse problem by optimization. In contrast to other methods, it does not require tuning parameters and can also be used in configurations including dielectric interfaces or preexisting space charge. Three different experimental datasets reported in the literature are used to validate the method. Good agreement between the measurements and the predictions of the method is found.

  • 36.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    A self-consistent upward leader propagation model2006In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 39, no 16, p. 3708-3715Article in journal (Refereed)
    Abstract [en]

    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.

  • 37.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    A simplified model to represent the inception of upward leaders from grounded structures under the influence of lightning stepped leaders2005In: International Symposium on Lightning Protection, 2005Conference paper (Refereed)
  • 38.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    A simplified physical model to determine the lightning upward connecting leader inception2006In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 21, no 2, p. 897-908Article in journal (Refereed)
    Abstract [en]

    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.

  • 39.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    An improved upward leader propagation model2006In: Proceedings of the 28th Internat Conference on Lightning Protection, 2006, p. 581-586Conference paper (Refereed)
  • 40.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Dynamic modeling of the lightning upward connecting leader inception2006In: Proceedings of the 28th International Conference on Lightning Protection, 2006, p. 543-548Conference paper (Refereed)
  • 41.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    On the Interaction of Lightning Upward ConnectingPositive Leaders With Humans2009In: IEEE transactions on electromagnetic compatibility (Print), ISSN 0018-9375, E-ISSN 1558-187X, Vol. 51, no 4, p. 1001-1008Article in journal (Refereed)
    Abstract [en]

    Upward connecting leaders can be initiated from humansunder the influence of lightning downward stepped leaders,thereby causing severe injuries. In order to improve the scarceknowledge about the interaction of upward connecting leaders withhumans, a self-consistent model based on the physics of leader dischargesis used in this paper. Furthermore, a current-generationtypereturn-stroke model is applied to calculate the current pulseproduced during the neutralization of unsuccessful aborted upwardleaders. It is estimated that an upward connecting leader canbe initiated even when the victim is located several tens of metersaway from the lightning channel. However, the lightning exposureto a direct strike and to an aborted leader is found to be reduced by50% and 70%, respectively, when an individual standing straightadopts the squat position. In the case of an aborted upward leader,it is estimated that a short-duration pulse of opposite polarity in thekiloampere range would be produced by the neutralization of theleader charge. Rough estimates of the total energy dissipated in thevictim’s body by the current of an aborted unsuccessful upwardleader range between hundred and thousand joules.

  • 42.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Time dependent evaluation of the lightning upward connecting leader inception2006In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 39, no 21, p. 4695-4702Article in journal (Refereed)
    Abstract [en]

    The evaluation of the upward connecting leader inception from a grounded structure has generally been performed neglecting the effect of the propagation of the downward stepped leader. Nevertheless, field observations suggest that the space charge produced by streamer corona andaborted upward leaders during the approach of the downward lightning leader can influence significantly the initiation of stable upward positive leaders. Thus, a physical leader inception model is developed, which takes into account the electric field variations produced by the descending leader during the process of inception. Also, it accounts for the shielding effect produced by streamer corona and unstable leaders formed before the stable leader inception takes place. The model is validated by comparing its predictions with the results obtained in long gap experiments and in an altitude triggered lightning experiment. The model is then used to estimate the leader inception conditions for free standing rods as a function of tip radius and height. It is found that the rod radius slightly affects the height of the downward leader tip necessary to initiate upward leaders. Only an improvement of about 10% on the lightning attractiveness can be reached byusing lightning rods with an optimum radius. Based on the obtained results, the field observations of competing lightning rods are explained. Furthermore, the influence of the average stepped leader velocity on theinception of positive upward leaders is evaluated. The results obtained show that the rate of change of the background electric field produced by a downward leader descent largely influences the conditions necessary for upward leader initiation. Estimations of the leader inception conditions for the upper and lower limit of the measured values of the average downward lightning leader velocity differ by more than 80%. In addition, the striking distances calculated taking into account the temporal change of the background field are significantly larger than the ones obtained assuming a static downward leader field. The estimations of the present model are alsocompared with the existing leader inception models and discussed.

  • 43.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Velocity of Laboratory Electrical Discharges at low Pressure2006Conference paper (Refereed)
    Abstract [en]

    One of the most important characteristics of sprite development is the velocity of the downward tendrils, which has been observed to range from 105 to 3x107 m/s (Moudry et al., JGR, 29, 2002). However, there is a lack of laboratory experimental data on the speed of propagation of electrical discharges at pressures similar to those encountered in the sprite environment. In order to gather more information on this topic, the average velocity of propagation of electrical discharges in quasi-uniform electric fields in air at pressures ranging from 1 to 10 mBar has been measured from optical emission. In the laboratory, the discharge is confined in a glass tube of 0.09 m diameter and between the electrodes placed 0.8 m apart. Each electrode has an equivalent radius of 0.25 m and is segmented into two concentric sections, one of them placed inside the tube. The voltage impulses applied to the cathode and the anode have a risetime of about 20 ns and a decay (half-value) time of 2 ms. The light from the discharge is detected with two optical fibers connected to photomultipliers Hamamatsu R1477-06, placed at 0.05 m from the electrodes. For each considered pressure, a set of impulse voltages with increasing peak values are applied. The lowest voltage applied at a particular pressure corresponds to the voltage for which light signals are detected. The average development velocities of the discharges at 1, 4 and 10 mBar are estimated from the measured optical signatures. For the considered pressures, the measured discharge velocity ranged from 1.5x105 to 1.5x107 m/s for reduced electric fields E/N (where E is the average electric field and N is the gas density) ranging from 120 to 1200 Td. This range of measured discharge velocities correspond to observed velocities of downward tendrils in sprites. In addition, a well-defined empirical relationship as given below, is found between the reduced discharge velocity v/N and the reduced electric field E/N in the range of pressures considered: v/N=102.002log(E/N)-5.234 [10-6 m4/s]

  • 44.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Hartono, Z
    Location of the vulnerable points to be struck by lightning in complex structures2005In: International Conference on Lightning and Static Electricity, 2005Conference paper (Refereed)
  • 45.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Cooray, Vernon
    Division for Electricity, Uppsala University.
    Roman, F.
    Division for Electricity, Uppsala University.
    Striking distance of vulnerable points to be struck by lightning on complex structures2006In: Proceedings of the 28th International Conference on Lightning Protection, 2006, p. 608-613Conference paper (Refereed)
  • 46.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Friberg, Andreas
    ABB Corporate Research, Västerås.
    Arc jets blown by outgassing polymers in air2014In: Proceedings of the 20th International Conference on Gas Discharges and Their Applications GD2014, GD2014 , 2014, p. 1-4Conference paper (Refereed)
    Abstract [en]

    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. 

  • 47.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Electrohydrodynamic motion due to space-charge limited injection of charges in cyclohexane2014In: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014, 2014Conference paper (Refereed)
    Abstract [en]

    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.

  • 48.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Division for Electricity, Uppsala University.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the modeling of the production and drift of carriers in cyclohexane2013In: Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP, IEEE , 2013, p. 905-908Conference paper (Refereed)
    Abstract [en]

    The modeling of the mechanisms of generation, loss, multiplication and transport of charge carriers is vital for the simulation of the prebreakdown process in dielectric liquids. Unfortunately, there is a lack of suitable coefficients to describe the electron generation and transport of carriers in liquids, which hinders the development of numerical models with sufficient predictive power. In this paper, the drift-dominated continuity equations for electrons and ions are coupled with Poisson's equation in order to simulate the carrier production and drift in the liquid phase under positive and negative voltages in cyclohexane. The estimations of the model are compared with measurements of current-voltage characteristics and Trichel current pulses reported in the literature for needle-plane configurations. In the analysis, the electron generation mechanisms suggested for dielectric liquids are analyzed and discussed. It is found that estimations based on the Zener equation for field-dependent molecular ionization do not agree with measurements for negative sharp points. It is also shown that the proper estimation of the electric current in the liquid phase should consider a field-dependent attachment term as well as the electrohydrodynamic movement of the liquid.

  • 49.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corporate Research.
    Gati, R.
    Time-resolved ablation of polymers exposed to ac arcs in air2013In: 20th Symposium on Physics of Switching Arc 2013, FSO 2013, Brno University of Technology, Physics , 2013, p. 84-87Conference paper (Refereed)
    Abstract [en]

    Polymers exposed to the radiation emitted by electric arcs generate vapours that are used to improve the interruption performance of electric switching apparatus. However, the basic physics of this process (known as ablation) under the broadband radiation spectra of electric arcs is not well understood. This paper presents measurements of time-resolved, ablation depth profiles of polymers (PA6-C6H11ON- and PMMA-C5H8O2-) exposed to the radiation of a single wall-stabilized arc ignited in a 5 mm air gap under AC currents with a peak of 2.5 kA. The profiles are obtained by exposing the surface of polymers moving with a velocity of 1 m/s, to the arc radiation through a 0.8 mm thick slit on one of the constricting glass walls. The measured profiles are correlated with the electrical and radiated power. It is found that there is a significant shift between the time-variation of the radiated power and the ablation depth, indicating that thermal decomposition is the main mechanism involved in the experiment.

  • 50.
    Becerra, Marley
    et al.
    Division for Electricity, Uppsala University.
    Mejia, A
    Impulse test results independence analysis using runs2003In: International Symposium on High Voltage Engineering, 2003Conference paper (Refereed)
12 1 - 50 of 88
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf