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  • 1.
    Akyuz, Mose
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Larsson, Anders
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Franke, Axel
    Characteristics of Laser-triggered Electric Discharges in Air2005In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 12, no 5, p. 1060-1070Article in journal (Refereed)
  • 2. Amarasinghe, Dulan
    et al.
    Sonnadara, Upul
    Berg, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Correlation between brightness and channel currents of electrical discharges2007In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, no 5, p. 1154-1160Article in journal (Refereed)
    Abstract [en]

    Channel brightness of 500 mm long electrical discharges were measured by analyzing a set of digitized images taken by 3 cameras placed symmetrically around a discharge gap at a radial distance of 200 cm from the axis of the spark. The sparks were generated between a steel rod and a plane electrode. The distribution of the brightness across the channel represented a Gaussian distribution. A linear correlation was seen between the channel brightness measured by different cameras looking at the same spark channel. No correlation was seen between the channel brightness and the channel depth (direction perpendicular to the camera plane). The measured peak current and the brightness of the main spark channel show a high degree of correlation (R-2=0.97). The sum of brightness of branches was equal to the brightness of the parent channel. One can use this result to calculate the relative distribution of branch currents in complex electrical discharges including natural lightning flashes. If the current in the parent channel is known, branch currents can be calculated by measuring the optical intensities using photographic techniques.

  • 3.
    Bolund, Björn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Poynting theorem applied to cable wound generators2008In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 15, no 2, p. 600-605Article in journal (Refereed)
    Abstract [en]

    The use of cable windings in generators and transformers has a physical background which is hard to neglect. The work done by Maxwell, Poynting and Slepian combined with powerful finite element solver of today allows for visualization of electric and magnetic fields in different geometries. The electromagnetic fields and power flows for generator stator cables are in this article associated with Poynting's theorem. Geometrical design and insulation material properties are then linked to Poynting's theory showing that circular stator cables enable higher voltages while maintaining a high power flow. Today several high voltage generators and two transformers have been built and are currently in operation. This paper discusses the application of the Poynting Theorem to cable wound generators.

  • 4.
    Diaz, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Vayanganie, S P A
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Experimental study of leader tortuosity and velocity in long rod-plane air discharges2016In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, no 2, p. 806-812Article in journal (Refereed)
    Abstract [en]

    Long air gap electrical discharges are of particular interest among scientists and engineers working on high voltage techniques and lightning research. In the present work we report experimental results obtained while testing a long rod-plane air gap with positive switching-like voltage impulses to study the velocity and tortuous progression of the leader discharge. Voltage and current waveforms were recorded. Two still digital cameras were used to track the leader tortuous path. By using a fast digital camera, the leader temporal evolution was recorded and its propagation velocity was estimated. Three angles were used to describe the leader tortuous progression.

  • 5.
    Greijer, Helena
    et al.
    ABB Corporate Research.
    Olsson, Jörgen
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Mobility and charge transport mechanisms of a field grading material2017In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Article in journal (Refereed)
  • 6.
    Jonsson, AnnaKarin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
    Frenning, Göran
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
    Nilsson, M
    Strømme Mattsson, Maria
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
    Niklasson, Gunnar A
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
    Dielectric study of thin films of Ta2O5 and ZrO22001In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 8, no 4, p. 648-651Article in journal (Refereed)
    Abstract [en]

    Electronic conduction in sputtered Ta2O5 and ZrO2 thin films have been studied using impedance spectroscopy, isothermal transient ionic current, and current-voltage measurements. The dielectric properties of Ta2O5 were shown to be sensitively dependent on deposition parameters with two different frequency responses: a flat loss behavior with very low DC conductivity, or a relaxation peak together with a somewhat higher DC conductivity. ZrO2 has different dielectric properties when fresh, i.e. newly deposited, or aged. A fresh sample arbitrarily can show two different behaviors, consisting of a DC conductivity with a relaxation peak superimposed on it. The DC conductivity shows either of two different values. The aged sample has a lower permittivity and DC conductivity, and the relaxation peak is found at much lower frequencies. Fresh samples of ZrO2 also show switching behavior

  • 7. Oria, Carmela
    et al.
    Carrascal, I.
    Fernández, I.
    Ferreño, D.
    Afshar, Reza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Gamstedt, K.
    Experimental and Numerical Analysis of Cellulosic Insulation Failures of Continuously Transposed Conductors under Short Circuits and Thermal Ageing in Power TransformersIn: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Article in journal (Refereed)
    Abstract [en]

    The integrity of the cellulosic insulation in power transformers is considered one of the most relevant parameters that affects their performance and reliability. Electric faults, such as short circuits, have thermal and mechanical effects that degrade the paper and can eventually produce the end-of-life of the transformer. The evolution of the properties of the paper insulation of a commercial continuously transposed conductor due to thermal ageing was characterised through the degree of polymerisation and tensile testing. Failure initiation and propagation in the paper was analysed macroscopically and microscopically using scanning electron microscope. A finite element numerical mechanical model of the conductor was implemented to reproduce the experiments and to obtain the load level and strain state that produce failure at each ageing state, aiming at developing a failure model for the insulation. This model may contribute to an improvement in manufacturing processes and management of the electrical system.

  • 8.
    Saxén, Carl
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Gamstedt, E. Kristofer
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Afshar, Reza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Paulsson, Goran
    ABB Corp Res, SE-72178 Vasteras, Sweden.
    Sahlen, Fredrik
    ABB Corp Res, SE-72178 Vasteras, Sweden.
    A Micro-Computed Tomography Investigation of the Breakdown Paths in Mica/Epoxy Machine Insulation2018In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 25, no 4, p. 1553-1559Article in journal (Refereed)
    Abstract [en]

    X-ray computed tomography is a technology capable of visualizing a wide variety of materials in 3D. In this investigation, insulation samples were electrically aged and computed tomography imaging was used for analysis of the microstructure and breakdown paths. Samples consisting of mica tape wrapped around aluminum tubes and impregnated with epoxy resin were produced. Two samples were electrically aged at 40 kV(rms) AC and another two at 30 kV(rms) AC until electrical breakdown. The samples were then scanned and reconstructed using x-ray micro computed tomography. The reconstructed images were analyzed and used for rendering volumetric 3D-models of the breakdown channels in the insulation. The results of mu-CT investigation show the location of vaporized insulation as well as the directions of the breakdown paths. When inspecting the 3D models, it is possible to follow the breakdown from electrode to electrode. The results also show that the mica tape layers will act as barriers forcing the breakdown path to follow the epoxy resin layers. It is noted how the breakdown path takes a tortuous course around the mica tape layers and how resin pockets inside the insulation weaken the insulation. Conversely, an improved continuous coverage of the mica tape without wrinkles would improve the insulation properties.

  • 9.
    Theethayi, Nelson
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Thottappillil, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Diendorfer, Gerhard
    Mair, Martin
    Pichler, Hannes
    Currents in buried grounding strips connected to communication tower legs during lightning strikes2008In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 15, no 4, p. 1153-1161Article in journal (Refereed)
    Abstract [en]

    During a lightning strike to communication tower stroke currents are shared by the tower and by the shields of the cables along the tower. The currents in the tower proceed towards the grounding system (possibly a combination of counterpoises or ring conductors or ground rods or grounding grids) connected to tower legs' foundation. In this paper, lightning strike to communication tower on mount Gaisberg in Austria is considered and measured currents at the tower top and those shared by an instrumented grounding strip connected to one of the tower leg's are presented. The measured currents at different locations on the 70-m long ground strip are compared with the predictions of a frequency dependant lossy transmission line (TL) model and reasonably good agreement was found. From this validation it is claimed that the TL models are appropriate for lightning transient analysis of grounding systems.

  • 10.
    Theethayi, Nelson
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Thottappillil, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Paolone, Mario
    Department of Electrical Engineering University of Bologna.
    Nucci, Carlo Alberto
    Department of Electrical Engineering University of Bologna.
    Rachidi, Farhad
    Swiss Federal Institute of Technology.
    External Impdeance and Admittance of Buried Horizontal Wires for Transient Studies Using Transmission Line Analysis2007In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, no 3, p. 751-761Article in journal (Refereed)
    Abstract [en]

    The paper investigates the applicability of some closed form expressions for the ground impedance and ground admittance of buried horizontal wires (bare and insulated) for lightning or switching transient analyses based on transmission line (TL) theory. In view of the frequency contents that typically characterize such transients, the behavior of the ground impedance and admittance is studied for a wide frequency range up to 10 MHz. Low frequency approximation of the ground impedance is not always appropriate for transient analysis. Sensitivity analyses show that, unlike overhead wires, the ground impedance for buried wires is little sensitive to the ground conductivity. On the other hand, the ground admittance varies strongly with the ground conductivity. The paper also discusses the results of transient analysis of buried cables performed by means of electromagnetic transient programs (EMTP) that neglect the ground admittance. The limits of such an approximation are discussed in order to evaluate the applicability of EMTP-like programs to the transient analysis of buried conductors. Transient pulse propagation in time domain based on finite difference time domain (FDTD) method of solution of TL equations is also discussed for a future inclusion of non-linear phenomena, like soil ionization and arcing/breakdown mechanisms, in the soil. The analysis presented could be useful in estimating surge propagation characteristics of buried wires for appropriate insulation coordination and transient protection.

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