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Sökning: WFRF:(Cooray Vernon) > (2015-2019)

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1.
  • Cooray, Vernon, et al. (författare)
  • On the possible mechanism of keraunographic markings on lightning victims
  • 2015
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 136:Part A, s. 119-123
  • Tidskriftsartikel (refereegranskat)abstract
    • During a lightning strike to a human the high electric field that exists at the point of contact of the lightning flash can generate electrical discharges known as streamer discharges along the skin. Previous research work has shown that the electric field at the head of these streamer discharges are large enough to accelerate electrons to relativistic speeds. In this paper it is shown that the streamers propagating along the skin will bombard the skin with energetic electrons. In this paper an estimation of the energy dissipated by these energetic electrons on the skin is estimated. Since beta radiation generated by radioactive substances consists of energetic electrons the effects of the energetic electrons generated by streamer discharges would be similar to the effects caused by low level beta radiation. It is suggested that the feather like marks, called keraunographical marks, that is sometimes observed on the skin of lightning victims is a result of superficial radiation injury with following inflammation in the epidermis and superficial layers of the dermis caused by energetic electrons.
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2.
  • Cooray, Vernon, 1952-, et al. (författare)
  • A Novel Interpretation of the Electromagnetic Fields of Lightning Return Strokes
  • 2019
  • Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 10:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Electric and/or magnetic fields are generated by stationary charges, uniformly moving charges and accelerating charges. These field components are described in the literature as static fields, velocity fields (or generalized Coulomb field) and radiation fields (or acceleration fields), respectively. In the literature, the electromagnetic fields generated by lightning return strokes are presented using the field components associated with short dipoles, and in this description the one-to-one association of the electromagnetic field terms with the physical process that gives rise to them is lost. In this paper, we have derived expressions for the electromagnetic fields using field equations associated with accelerating (and moving) charges and separated the resulting fields into static, velocity and radiation fields. The results illustrate how the radiation fields emanating from the lightning channel give rise to field terms varying as 1/r and 1/r2, the velocity fields generating field terms varying as 1/r2, and the static fields generating field components varying as 1/r2 and 1/r3. These field components depend explicitly on the speed of propagation of the current pulse. However, the total field does not depend explicitly on the speed of propagation of the current pulse. It is shown that these field components can be combined to generate the field components pertinent to the dipole technique. However, in this conversion process the connection of the field components to the physical processes taking place at the source that generate these fields (i.e., static charges, uniformly moving charges and accelerating charges) is lost.
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3.
  • Cooray, Vernon, 1952-, et al. (författare)
  • Electromagnetic fields of accelerating charges : Applications in lightning protection
  • 2017
  • Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 145, s. 234-247
  • Tidskriftsartikel (refereegranskat)abstract
    • Electromagnetic fields generated by accelerating charges can be utilized to evaluate the electromagnetic fields generated by systems where moving charges and/or propagating currents are present. The technique can be used easily to evaluate the electromagnetic fields generated by systems in which propagating currents are present. This is illustrated by utilizing the equations to derive expressions for the electromagnetic fields generated by systems in which current pulses injected by lightning flashes are propagating.
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4.
  • Cooray, Vernon, 1952-, et al. (författare)
  • Generalized Electric Field Equations of a Time-Varying Current Distribution Based on the Electromagnetic Fields of Moving and Accelerating Charges
  • 2019
  • Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 10:7
  • Tidskriftsartikel (refereegranskat)abstract
    • In several studies conducted recently, it was shown that equations pertinent to the electric and magnetic fields produced by electrical charges in motion can be used to calculate the electromagnetic fields produced by current pulses propagating along linearly restricted paths. An example includes the case of current pulses propagating along conductors and conducting channels such as lightning. In this paper, it is shown how the technique can be applied to estimate the electromagnetic fields generated by current and charge distributions moving in arbitrary directions in space. The analysis shows that, depending on the way the problem is formulated using the field equations pertinent to accelerating charges, one procedure leads to the generalized dipole equations, which are independent of the velocity of propagation of the current, and the other procedure leads to a set of equations that depend on the velocity. Using the well-tested transmission line model of lightning return strokes as an example, it is shown that both sets of field equations give rise to the same total electromagnetic field.
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5.
  • Cooray, Vernon, 1952-, et al. (författare)
  • On the Action of the Radiation Field Generated by a Traveling-Wave Element and Its Connection to the Time Energy Uncertainty Principle, Elementary Charge and the Fine Structure Constant
  • 2017
  • Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 8:3
  • Tidskriftsartikel (refereegranskat)abstract
    • Recently, we published two papers in this journal. One of the papers dealt with the action of the radiation fields generated by a traveling-wave element and the other dealt with the momentum transferred by the same radiation fields and their connection to the time energy uncertainty principle. The traveling-wave element is defined as a conductor through which a current pulse propagates with the speed of light in free space from one end of the conductor to the other without attenuation. The goal of this letter is to combine the information provided in these two papers together and make conclusive statements concerning the connection between the energy dissipated by the radiation fields, the time energy uncertainty principle and the elementary charge. As we will show here, the results presented in these two papers, when combined together, show that the time energy uncertainty principle can be applied to the classical radiation emitted by a traveling-wave element and it results in the prediction that the smallest charge associated with the current that can be detected using radiated energy as a vehicle is on the order of the elementary charge. Based on the results, an expression for the fine structure constant is obtained. This is the first time that an order of magnitude estimation of the elementary charge based on electromagnetic radiation fields is obtained. Even though the results obtained in this paper have to be considered as order of magnitude estimations, a strict interpretation of the derived equations shows that the fine structure constant or the elementary charge may change as the size or the age of the universe increases.
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6.
  • Cooray, Vernon, et al. (författare)
  • On the Momentum Transported by the Radiation Field of a Long Transient Dipole and Time Energy Uncertainty Principle
  • 2016
  • Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 7:11
  • Tidskriftsartikel (refereegranskat)abstract
    • The paper describes the net momentum transported by the transient electromagnetic radiation field of a long transient dipole in free space. In the dipole a current is initiated at one end and propagates towards the other end where it is absorbed. The results show that the net momentum transported by the radiation is directed along the axis of the dipole where the currents are propagating. In general, the net momentum P transported by the electromagnetic radiation of the dipole is less than the quantity U/c, where U is the total energy radiated by the dipole and c is the speed of light in free space. In the case of a Hertzian dipole, the net momentum transported by the radiation field is zero because of the spatial symmetry of the radiation field. As the effective wavelength of the current decreases with respect to the length of the dipole (or the duration of the current decreases with respect to the travel time of the current along the dipole), the net momentum transported by the radiation field becomes closer and closer to U/c, and for effective wavelengths which are much shorter than the length of the dipole, P approximate to U/c. The results show that when the condition P approximate to U/c is satisfied, the radiated fields satisfy the condition Delta t Delta U >= h/4 pi where Delta t is the duration of the radiation, Delta U is the uncertainty in the dissipated energy and h is the Plank constant.
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7.
  • Cooray, Vernon, et al. (författare)
  • On the Remarkable Features of the Lower Limits of Charge and the Radiated Energy of Antennas as Predicted by Classical Electrodynamics
  • 2016
  • Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 7:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Electromagnetic energy radiated by antennas working in both the frequency domain and time domain is studied as a function of the charge associated with the current in the antenna. The frequency domain results, obtained under the assumption of sinusoidal current distribution, show that, for a given charge, the energy radiated within a period of oscillation increases initially with L/lambda and then starts to oscillate around a steady value when L/lambda>1. The results show that for the energy radiated by the antenna to be equal to or larger than the energy of one photon, the oscillating charge in the antenna has to be equal to or larger than the electronic charge. That is, U >= hv or UT >= h double right arrow q >= e, where U is the energy dissipated over a period, v is the frequency of oscillation, T is the period, h is Planck's constant, q is the rms value of the oscillating charge, and e is the electronic charge. In the case of antennas working in the time domain, it is observed that U Delta t >= h/4 pi double right arrow q >= e, where U is the total energy radiated, Delta t is the time over which the energy is radiated, and q is the charge transported by the current. It is shown that one can recover the time-energy uncertainty principle of quantum mechanics from this time domain result. The results presented in this paper show that when quantum mechanical constraints are applied to the electromagnetic energy radiated by a finite antenna as estimated using the equations of classical electrodynamics, the electronic charge emerges as the smallest unit of free charge in nature.
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8.
  • Cooray, Vernon, et al. (författare)
  • The Deep Physics Hidden within the Field Expressions of the Radiation Fields of Lightning Return Strokes
  • 2016
  • Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 7:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Based on the electromagnetic fields generated by a current pulse propagating from one point in space to another, a scenario that is frequently used to simulate return strokes in lightning flashes, it is shown that there is a deep physical connection between the electromagnetic energy dissipated by the system, the time over which this energy is dissipated and the charge associated with the current. For a given current pulse, the product of the energy dissipated and the time over which this energy is dissipated, defined as action in this paper, depends on the length of the channel, or the path, through which the current pulse is propagating. As the length of the channel varies, the action plotted against the length of the channel exhibits a maximum value. The location of the maximum value depends on the ratio of the length of the channel to the characteristic length of the current pulse. The latter is defined as the product of the duration of the current pulse and the speed of propagation of the current pulse. The magnitude of this maximum depends on the charge associated with the current pulse. The results show that when the charge associated with the current pulse approaches the electronic charge, the value of this maximum reaches a value close to h/8 where h is the Plank constant. From this result, one can deduce that the time-energy uncertainty principle is the reason for the fact that the smallest charge that can be detected from the electromagnetic radiation is equal to the electronic charge. Since any system that generates electromagnetic radiation can be represented by a current pulse propagating from one point in space to another, the result is deemed valid for electromagnetic radiation fields in general.
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9.
  • Abegunawardana, Sidath, et al. (författare)
  • Audible Frequency Analysis of Ground Flashes
  • 2018
  • Konferensbidrag (refereegranskat)abstract
    • Thunder signatures categorized into three types based on peak pressure and variation in fundamental frequency, have been studied by using acoustic spectrum of thunder. S-transformation has been used to estimate the dominant frequency variation around the peak pressure. The mean fundamental frequencies of type 3 ground and cloud flashes are 160 Hz and 98 Hz respectively. The mean frequencies of type 2 ground and cloud flashes are 108 Hz and 82 Hz respectively.
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10.
  • Abegunawardana, Sidath, et al. (författare)
  • Sound Source Localization of Lightning Discharges
  • 2018
  • Konferensbidrag (refereegranskat)abstract
    • The lightning channel can be retraced by thunder signature recorded with an array of microphones. Four microphones were arranged to capture thunder sound, and acoustic cross-correlation of captured pressure signals estimated the time lag of each pair of microphones. A wave segment with 0.25 s or 0.5 s of duration was compared with the acoustic signal recorded by other microphones to estimate time lapse. The direction of channel propagation has been estimated by the time difference of arrival of each pair of microphones. Thunder source locations estimated by this method can be extended to investigate the channel propagation inside clouds.
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11.
  • Abewardhana, Ruwan, et al. (författare)
  • Lightning Localization Based on VHF Broadband Interferometer Developed in Sri Lanka
  • 2018
  • Ingår i: 2018 34th international conference on lightning protection (ICLP 2018). - New York : IEEE. - 9781538666357
  • Konferensbidrag (refereegranskat)abstract
    • A basic broadband digital interferometer was developed, which is capable of locating Very High Frequency (VHF) radiation sources in two spatial dimensions and time. Three antennas sensed the time series of broadband electromagnetic (EM) signals and digitized with 4 ns sampling interval for a duration of several milliseconds. A technique based on cross-correlations has been implemented for mapping lightning source locations. A map of the first return stroke (RS) and the preceding stepped leader was mapped successfully, using the system with a time resolution of few milliseconds. The result was compared with the visible events of the ground flash to validate the system.
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12.
  • Ahmad, Mohd Riduan, et al. (författare)
  • Emission Heights of Narrow Bipolar Events in a Tropical Storm over the Malacca Strait
  • 2017
  • Ingår i: 2017 International Conference On Electrical Engineering And Computer Science (Icecos). - : IEEE. - 9781479976751 - 9781479976768 - 9781479976744 ; , s. 305-309
  • Konferensbidrag (refereegranskat)abstract
    • Emission heights for narrow bipolar events (NBEs) have been reported mostly from observations at mid latitudes but none have been reported from tropical regions. In this paper, we are reporting for the first time the heights of NBE emissions from a tropical storm over the Malacca Strait, a narrow water passage between the Malay Peninsula and Sumatra Island. A total of 49 positive NBEs (+NBEs) were detected from the storm. The NBE activity can be divided into two stages according to the emission heights and radar reflectivity data. The first stage (or S1) lasted for only 6 minutes, started with the first detected NBE, and produced 20 NBEs (41%). The emission heights ranged between 12.0 and 16.7 km. Radar reflectivity data showed that the storm reached maximum values at 55 dBZ within the period S1. In contrast, the second stage (S2) lasted longer (32 minutes) and produced 29 NBEs (59%). The emission heights were lower and ranged from 8.5 to 13.7 km. Radar reflectivity data showed that the storm reached maximum values at 50 dBZ within the period S2.
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13.
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14.
  • Ahmad, Noor Azlinda, et al. (författare)
  • Radiation field spectra of long-duration cloud flashes
  • 2015
  • Ingår i: Atmospheric Science Letters. - : Wiley. - 1530-261X. ; 16:2, s. 91-95
  • Tidskriftsartikel (refereegranskat)abstract
    • The preliminary results of radiation electric fields produced by long-duration cloud flashes have been Fourier analyzed to determine the frequency spectrum in the range of 10kHz-10MHz. The flashes were recorded within a distance of less than 20km. The spectrum was normalized to 50km distance and it shows a f(-1) dependence within the entire frequency range.
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15.
  • Amarasinghe, Dulan, et al. (författare)
  • Fractal dimension of long electrical discharges
  • 2015
  • Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 73, s. 33-37
  • Tidskriftsartikel (refereegranskat)abstract
    • The fractal dimension of 500 mm long electrical discharges is presented by analyzing a set of photographic images. Three popular fractal dimension estimation techniques, box counting, sandbox and correlation function methods were used to estimate the fractal dimension of the discharge channels. To remove the apparent thickness due to varying magnitudes of current in the discharge channels, edge detection algorithms were utilized. The estimated fractal dimensions for box counting, sandbox and correlation function for long laboratory sparks were 1.20 +/- 0.06,1.66 +/- 0.05 and 1.52 +/- 0.12 respectively. Within statistical uncertainties, the estimated fractal dimensions of positive and negative polarities agreed very well. (C) 2014 Elsevier B.V. All rights reserved.
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16.
  • Arevalo, Liliana, et al. (författare)
  • The leader propagation velocity in long air gaps
  • 2018
  • Ingår i: 2018 34th international conference on lightning protection (ICLP 2018). - : IEEE. - 9781538666357
  • Konferensbidrag (refereegranskat)abstract
    • Experimental measurements of long gap discharges and its interpretation are the base of engineering equations and complex models to design clearance distances, lightning protection systems, among others. Parameters like leader propagation velocity, average electric field of the leader channel, stability electric field of the streamer region, etc. are derived from experimental measurements for rod-plane arrangements. However, in high voltage engineering geometries are not only rod-plane arrangements but also rounded electrode geometries. Experimental measurements of sphere - plane arrangements are presented in this paper. Attention is given to the velocity of propagation of the leader. The velocity of propagation of the leader is compared for two different applied voltage conditions, such as overvoltage and non-overvoltage. Experimental observations indicate that the velocity of the leader does not have a linear relationship with the increment of the applied voltage, as described by other authors for rod-plane arrangements. By means of a model based on the physics of the discharge, it is observed that the velocity of propagation of the leader depends on the injected charge to the leader channel. The injected charge depends on the background electric field, the potential at the leader tip, the steepness of the applied voltage waveform, among other parameters.
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17.
  • Arevalo, L., et al. (författare)
  • The mesh method in lightning protection analyzed from a lightning attachment model
  • 2016
  • Ingår i: 2016 33RD INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP). - 9781509058433
  • Konferensbidrag (refereegranskat)abstract
    • Based on the well-known rolling sphere method, international standards recommend the location of the external lightning protection system of structures. The design of the external lightning protection system of structures of height of less than 60 m can be done by installing a mesh on top of a roof of the structure or by creating a mesh with wires at certain distance from the roof of the structure. The prospective downward leader current that the mesh can incept depends on the size of the mesh and the current magnitude is given as recommendation in the international standards. This paper analyses the relation prospective negative downward leader current vs. mesh size from a lightning attachment model. The model is applied to a perfectly grounded structure with maximum height of 50 m protected by two different external lightning protection systems recommended by the international standards. The results showed difference on magnitude of the prospective downward leader current the standards recommend and the ones obtained using the lightning attachment model for meshes of shorter size. Discrepancies concerning the minimum downward leader current that can be incepted by a mesh made by wires located at certain distance from the ground structure and a mesh located on top of the building are obtained.
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18.
  • Arevalo, Liliana, 1978-, et al. (författare)
  • Unstable Leader Inception Criteria of Atmospheric Discharges
  • 2017
  • Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 8:9
  • Tidskriftsartikel (refereegranskat)abstract
    • In the literature, there are different criteria to represent the formation of a leader channel in short and long gap discharges. Due to the complexity of the physics of the heating phenomena, and the limitations of the computational resources, a simplified criterion for the minimum amount of electrical charge required to incept an unstable leader has recently been used for modeling long gap discharges and lightning attachments. The criterion is based on the assumption that the total energy of the streamer is used to heat up the gas, among other principles. However, from a physics point of view, energy can also be transferred to other molecular processes, such as rotation, translation, and vibrational excitation. In this paper, the leader inception mechanism was studied based on fundamental particle physics and the energy balance of the gas media. The heating process of the plasma is evaluated with a detailed two-dimensional self-consistent model. The model is able to represent the streamer propagation, dark period, and unsuccessful leaders that may occur prior to the heating of the channel. The main processes that participate in heating the gas are identified within the model, indicating that impact ionization and detachment are the leading sources of energy injection, and that recombination is responsible for loss of electrons and limiting the energy. The model was applied to a well-known experiment for long air gaps under positive switching impulses reported in the literature, and used to validate models for lightning attachments and long gap discharges. Results indicate that the streamer-leader transition depends on the amount of energy transferred to the heating process. The minimum electric charge required for leader inception varies with the gap geometry, the background electric field, the reduction of electric field due to the space charge, the energy expended on the vibrational relation, and the environmental conditions, among others.
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19.
  • Baharin, S. A. S., et al. (författare)
  • Wavelet Analysis of the Onset of VHF and Microwave Radiation Emitted by Lightning
  • 2018
  • Ingår i: 2018 INTERNATIONAL CONFERENCE ON ELECTRICAL ENGINEERING AND COMPUTER SCIENCE (ICECOS). - : IEEE. - 9781538657218 ; , s. 297-300
  • Konferensbidrag (refereegranskat)abstract
    • Lightning flash is an electrical discharge in air (dielectric breakdown) which emits electromagnetic (FM) fields across very wide spectra from a few Hertz up to visible wavelength. Electrical breakdown process is an important event that initiates lightning. For electrical breakdown process to occur, it must fulfill two conditions which are at least has one free electron and the electric field region is more than 3 MV/m. This process starts with electron avalanche in millimeter scale then grows into streamer in centimeter scale. Lastly, from streamer it will grow into leader in meter scale. It has already established that streamer emits intensely at Very High Frequency (VHF) band as it's already proven both theoretically and experimentally. A study by Cooray, theoretically proved that emission of electron avalanche peaks at microwave band. Air-gap parallel plate antenna which could operate at 1 GHz with remote sensing is designed and simulated to measure the microwave radiation emitted by lightning. Both temporal and wavelet analyses are used to compare the onset of microwave radiation and VHF radiation in both time and frequency domains to determine electron avalanche appears at which electromagnetic band.
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20.
  • Baharudin, Zikri Abadi, et al. (författare)
  • On the characteristics of positive lightning ground flashes in Sweden
  • 2016
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 138, s. 106-111
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study the stroke characteristics of positive cloud-to-ground flashes in Sweden were obtained from the electric field records measured from 14 thunderstorms. The electric fields were measured with nanosecond resolution. Together with the fast and the slow electric field records, the narrowband radiation field at 3 and 30 MHz signals were also measured simultaneously. Out of a total of 107 flashes, 30 flashes had two strokes, 7 had three strokes and 3 flashes had four strokes. The arithmetic and geometric means of the interstroke intervals were found to be 116 and 70 ms, respectively. The arithmetic and geometric mean ratio between the peak electric field of the Subsequent Return Stroke (SRS) and the first Return Stroke (RS) were 0.48 and 0.36, respectively. Of the 40 positive multiple-stroke ground flashes, 5% have at least one SRS with field peak higher than the first RS. The percentage of SRS with field peaks greater than the first RS was 6%. In our best of our knowledge, this is the first time a large sample of positive return strokes in Sweden was analysed. It was found to be statistically more significant than the previous studies.
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21.
  • Bodhika, J. A. P., et al. (författare)
  • Characteristics of thunder pertinent to tropical lightning
  • 2018
  • Ingår i: 34th International Conference on Lightning Protection (ICLP) (ICLP 2018). - : IEEE. - 9781538666357
  • Konferensbidrag (refereegranskat)abstract
    • Acoustic effects of lightning flashes are not well studied. Thunder also destructive as electric fields do. Thunder flashes of tropical lightning occurred in Hambantota (81(0) 0'E, 6(0) 10'N) close to the southern coast of Sri Lanka were studied. About 90 thunder flashes recorded with the sampling rate of 100 kS/s in 45 s windows were analyzed to identify their characteristics and damaging effects. Results of the analysis on duration of thunder flashes, peak sound pressure levels, occurrence of thunder claps and frequency distribution were presented. The shortest and longest duration of thunder recorded were 2.2 sec and 33.7 sec respectively. About 75% of thunder flashes were remained for about 5-20 sec. The frequency spectrum of a thunder signal could spread up to 1500 Hz. The peak sound pressure level of 30% of recorded thunder flashes at least 3 km away from the lightning were higher than 2 Nm(-2) (100 dB). One to five claps were identified in thunder flashes with 62% having 1 or 2 claps. Sound pressure level, frequency range and its duration are required quantities to estimate the harmful effects of thunder.
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22.
  • Chandimal, A. P. L., et al. (författare)
  • Impedance Behavior of Earth Enhancing Compound Under Lightning Transient Conditions
  • 2018
  • Ingår i: 2018 34th international conference on lightning protection (ICLP 2018). - New York : IEEE. - 9781538666357
  • Konferensbidrag (refereegranskat)abstract
    • This study was aimed at finding the performance of commercially available earth enhancing compound (EEC) under lightning conditions. Further to that, this is an extension of a previous study conducted by the same authors referring to the same EEC. The selected material seems to perform impressively under lightning conditions passing considerably large current values. The prepared EEC mixture and the setup was tested under dry conditions and exhibited low dynamic impedance characteristics for lightning impulse voltages. Ten-stage Marx-generator was used for the experiment, and due to high conductive nature of the material, HV supply between the sample was unable to increase more than few kV. Thus the 12 cm gap inside the material was complemented with an external air gap of 90 cm. The study was a comparison between "air-gap breakdown" and "air-gap plus material-gap breakdown". The results confirmed a low impedance characteristic for the selected EEC and suggested a very low electric field inside the material gap. An approximate value for the impedance of the test setup was calculated from the collected data. A time window was selected during the steady current discharge appeared just after the point of current peak and the impedance curves for both experiments were compared at a previously defined point at the time axis. The impedance value for EEC setup was estimated to be in the region of 1 - 2 Omega.
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23.
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24.
  • Cooray, Vernon (författare)
  • A return stroke model based purely on the current dissipation concept
  • 2015
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 136:Part A, s. 61-65
  • Tidskriftsartikel (refereegranskat)abstract
    • A return stroke model based purely on the current dissipation concept is introduced. With three model parameters the model is capable of generating electric and magnetic fields that are in reasonable agreement with experimentally observed electromagnetic fields.
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25.
  • Cooray, Vernon, 1952-, et al. (författare)
  • Advances in lightning research
  • 2017
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1364-6826 .- 1879-1824. ; 154, s. 181-181
  • Tidskriftsartikel (refereegranskat)
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26.
  • Cooray, Vernon, 1952-, et al. (författare)
  • Effects of Propagation of Narrow Bipolar Pulses, Generated by Compact Cloud Discharges, over Finitely Conducting Ground
  • 2018
  • Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 9:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Propagation effects on the narrow bipolar pulses (NBPs) or the radiation fields generated by compact cloud discharges as they propagate over finitely conducting ground are presented. The results were obtained using a sample of NBPs recorded with high time resolution from close thunderstorms in Sri Lanka. The results show that the peak amplitude and the temporal features such as the full width at half maximum (FWHM), zero-crossing time, and the time derivative of NBPs can be significantly distorted by propagation effects. For this reason, the study of peak amplitudes and temporal features of NBPs and the remote sensing of current parameters of compact cloud discharges should be conducted using NBPs recorded under conditions where the propagation effects are minimal.
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27.
  • Cooray, Vernon, 1952-, et al. (författare)
  • Formulation of the Field-to-Transmission Line Coupling Equations in Terms of Scalar and Vector Potentials
  • 2017
  • Ingår i: IEEE transactions on electromagnetic compatibility (Print). - 0018-9375 .- 1558-187X. ; 59:5, s. 1586-1591
  • Tidskriftsartikel (refereegranskat)abstract
    • More than 60 years ago, Prof. S. Rusck introduced a coupling model to take into account the interaction of lightning-generated electromagnetic fields with overhead power transmission and distribution lines. The model which assumes that the ground is perfectly conducting has served the power system research community for many decades. Recently, it was found that this model is not complete because some of the forcing terms in the relevant transmission line equations, which are based purely on the scalar and vector potentials, were missing. In the present paper, the correct transmission line equations pertinent to the interaction of external electromagnetic fields with overhead lines, described in terms of scalar and vector potentials, are derived by incorporating the effects of a finitely conducting ground. The model presented in this paper can be considered as a correction and an extension of the Rusck model to account for a finitely conducting ground.
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28.
  • Cooray, Vernon, 1952- (författare)
  • Latitude dependence of peak lightning return stroke current : A theoretical explanation
  • 2018
  • Ingår i: 34th International Conference on Lightning Protection (ICLP 2018). - : IEEE. - 9781538666357
  • Konferensbidrag (refereegranskat)abstract
    • The experimental data show that the peak currents of first and subsequent lightning return strokes increase with decreasing latitude. In this paper the reason for this dependence of current amplitude is explained using the fact that the height of the charge centers increases with decreasing latitude. The results obtained show that in tropical regions peak first and subsequent return stroke currents are located in the vicinity of 42 kA and 15 kA, respectively. This theoretical prediction is in agreement with available experimental data.
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29.
  • Cooray, Vernon, 1952-, et al. (författare)
  • Merging of current generation and current dissipation lightning return stroke models
  • 2017
  • Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 153:SI, s. 10-18
  • Tidskriftsartikel (refereegranskat)abstract
    • Current generation and current dissipation return stroke models are engineering models based on the theory associated with the propagation of current pulses along transmission lines undergoing corona. However, neither of these models incorporates the complete theory associated with the phenomenon. One can make the physical scenario complete by combining the current generation concept with the current dissipation concept. In this paper how this can be done is demonstrated by creating a return stroke model which is a combination of these two model types. The new model encompasses the full theory associated with the pulse propagation along transmission lines under corona. The paper provides a full description of the model together with a description of the spatial and temporal variation of the return stroke current and the electric and magnetic fields generated at different distances as predicted by the model.
  •  
30.
  • Cooray, Vernon, 1952-, et al. (författare)
  • Modeling the Stepping Process of Negative Lightning Stepped Leaders
  • 2017
  • Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 8:12
  • Tidskriftsartikel (refereegranskat)abstract
    • A physical model based on the mechanism observed in experimental investigations is introduced to describe the formation of negative leader steps. Starting with a small length of a space leader located at the periphery of the negative streamer system of the stepped leader, the model simulates the growth and the subsequent formation of the leader step. Based on the model, the step length, the step forming time, and the propagation speed of stepped leaders as a function of the prospective return stroke peak current are estimated. The results show that the step length and the leader speed increase with increasing prospective return stroke current. The results also show that the speed of the stepped leader increases as it approaches the ground. For prospective return stroke currents in the range of 15 kA–60 kA, the step lengths lie within the range 5 m–100 m, the step forming times lie within the range 10 μs–250 μs, and the leader speed lies within the range 105 m/s −1.5 × 106 m/s. The results obtained are in reasonable agreement with the experimental observations.
  •  
31.
  • Cooray, Vernon (författare)
  • On the mimimum length of leader channel and the minimum volume of space charge concentration necessary to initiate lightning flashes in thunderclouds
  • 2015
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 136:Part A, s. 39-45
  • Tidskriftsartikel (refereegranskat)abstract
    • Minimum length to which a leader channel has to grow before it can propagate continuously as a stable leader as a function of the background electric field inside a thundercloud is estimated. For electric field magnitudes comparable to the values measured inside thunderclouds, the minimum length of the leader channel that is required for it to propagate continuously is about 3-5 m. In other words, a leader discharge that originated inside a thundercloud has to grow to a length of 3-5 m before it can culminate in a stable and continuously propagating leader discharge that can give rise to a lightning flash. The minimum size of charge concentrations that can make this event possible have radii in the range of 2-4 m and should carry about 300-900 mu C of electric charge, respectively. This in turn shows that the high field regions inside the cloud where electrical discharges that can culminate in stable leader discharges, and hence in lightning discharges, may be confined to volumes which are no larger than a few meters in radius.
  •  
32.
  • Cooray, Vernon, 1952- (författare)
  • Origin of the Fine Scale Tortuosity in Sparks and Lightning Channels
  • 2018
  • Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 9:6
  • Tidskriftsartikel (refereegranskat)abstract
    • The physical reason for the small-scale tortuosity observed in sparks and lightning channels is unknown at present. In this paper, it is suggested that the small-scale tortuosity of the discharge channels is caused by the natural tendency for subsequent leader streamer bursts to avoid each other but at the same time to align as much as possible along the direction of the background electric field. This process will give rise to a discharge channel that re-orients in space during each streamer burst creating the small scale tortuosity.
  •  
33.
  • Cooray, Vernon, 1952- (författare)
  • The influence of lightning conductor radii on the attachment of lightning flashes
  • 2017
  • Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 153:SI, s. 138-143
  • Tidskriftsartikel (refereegranskat)abstract
    • The influence of the tip radius of lightning conductors on their lightning attractive distance as predicted by the self-consistent leader inception and propagation model (SLIM) is presented. The results show that in the absence of any glow corona from the tip of the conductor a smaller tip radius gives rise to a larger attractive radius than a larger radius. It is suggested that the reason for the experimental observations which show that blunt conductors are more efficient lightning receptors than sharp ones is the presence of glow corona at the tip of the sharp ones during the time of lightning strikes. Moreover, in a given background electric field, the probability of the inception of glow corona at the conductor tip increases with increasing conductor height and decreasing conductor radius. Thus, in a given electric field, as the conductor height increases its radius has to be increased to avoid the inception of glow corona at the tip. For this reason, the conductor radius that performs best as a lightning interceptor depends on the height of the conductor and the best performance shift from smaller radii to larger ones with increasing height of the conductor.
  •  
34.
  • Cooray, Vernon, 1952- (författare)
  • The power, energy, momentum and the action of the electromagnetic radiation fields of lightning return strokes
  • 2018
  • Ingår i: 2018 34th international conference on lightning protection (ICLP 2018). - : IEEE. - 9781538666357
  • Konferensbidrag (refereegranskat)abstract
    • The energy, momentum and the action associated with the radiation fields generated by a typical subsequent lightning return stroke is estimated. The action is defined as the product of the total radiation energy emitted and the duration of emission, and the momentum refers to the vertical component. The results obtained show that a typical subsequent return stroke generates 2 x 10(6) J, 2 x 10(6) Kgm/s and 2 x 10(6) Js respectively of energy, momentum and action.
  •  
35.
  • Cooray, Vernon, 1952- (författare)
  • The Similarity of the Action of Franklin and ESE Lightning Rods under Natural Conditions
  • 2018
  • Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 9:6
  • Tidskriftsartikel (refereegranskat)abstract
    • In the lightning rods categorized as Early Streamer Emission (ESE) types, an intermittent voltage impulse is applied to the lightning rod to modulate the electric field at its tip in an attempt to speed up the initiation of a connecting leader from the lightning rod when it is under the influence of a stepped leader moving down from the cloud. In this paper, it is shown that, due to the stepping nature of the stepped leader, there is a natural modulation of the electric field at the tip of any lightning rod exposed to the lightning stepped leaders and this modulation is much more intense than any artificial modulation that is possible under practical conditions. Based on the results, it is concluded that artificial modulation of the electric field at the tip of lightning rods by applying voltage pulses is an unnecessary endeavor because the nature itself has endowed the tip of the lightning rod with a modulating electric field. Therefore, as far as the effectiveness of artificial modulation of the tip electric field is concerned, there could be no difference in the lightning attachment efficiency between ESE and Franklin lightning rods.
  •  
36.
  • Diaz, Oscar, 1980-, et al. (författare)
  • Experimental study of leader tortuosity and velocity in long rod-plane air discharges
  • 2016
  • Ingår i: IEEE transactions on dielectrics and electrical insulation. - 1070-9878 .- 1558-4135. ; 23:2, s. 806-812
  • Tidskriftsartikel (refereegranskat)abstract
    • 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.
  •  
37.
  • Diaz, Oscar, et al. (författare)
  • Leader channel models for long air positive electrical discharges
  • 2015
  • Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 76, s. 208-215
  • Tidskriftsartikel (refereegranskat)abstract
    • The models proposed for the positive long air gap electrical discharge can be considered to be either engineering or physical in their approach. In this work, we make a general review of the available models and use two of them for a comparison with experimental data. Common underlying assumptions were found in most of the models analyzed. The comparison with the experimental data revealed that the results obtained from the models were a good representation of the physical situation when the leader potential distribution and the leader-corona region evolution were described with certain physical assumptions.
  •  
38.
  • Diaz, Oscar, 1980-, et al. (författare)
  • Methodologies for the charge estimation in the leader corona region used in modeling long air gaps underpositive voltage impulses
  • 2016
  • Konferensbidrag (refereegranskat)abstract
    • Different methodologies have been proposed to represent the physical phenomena taking place in a laboratory electrical breakdown event. The implementation of these methodologies in numerical routines is based in several physical assumptions and a proper calculation of the electrostatic potential distribution. The whole electrical breakdown in air tested with switching-like voltage impulses can be subdivided into three main stages: first, the streamer inception (first corona), then the streamer to leader transition (second corona, leader inception) and the leader propagation. An important element in the last stage is the representation of the leader corona region (streamer region) in front of the leader tip channel as it propagates towards ground. In this paper, with the aid of a finite element method solver to determine the electric potential distribution, two new methodologies to quantify the amount of charge produced in the leader corona region were presented and compared with other ones available in the literature.
  •  
39.
  • Diaz, Oscar, 1980-, et al. (författare)
  • Numerical Modeling of Electrical Discharges in Long Air Gaps Tested With Positive Switching Impulses
  • 2018
  • Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 46:3, s. 611-621
  • Tidskriftsartikel (refereegranskat)abstract
    • The numerical modeling of electrical discharges occurring in atmospheric air has been in continuous development during the past decades in different fields, such as high-voltage techniques and lightning protection. Different methodologies have been proposed to represent the physical phenomena taking place at a single full discharge event, departing both from experimental and theoretical approaches. The implementation of these methodologies in numerical routines combined with the use of numerical methods to determine the electric potential distribution permits the creation of models whose predictions closely agree with the real case situations, where electrode arrangements might have nonsymmetric geometries. In this paper, we present an improved version of a simulation methodology for representing electrical discharges in long air gaps. This simulation methodology includes new elements like: 1) the 3-D leader channel tortuosity based on laboratory experimental measurements and 2) two new methods for the estimation of the electric charge contained in the so-called leader-corona region based on the electrostatic potential of fictitious potential rings representing the active region in front of the leader tip. Results from the simulation were compared with experimental records and a reasonably good agreement is found between them.
  •  
40.
  • Diaz, Oscar, 1980- (författare)
  • Numerical modelling of positive electrical discharges in long air gaps
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This dissertation deals with research on the numerical modelling of electrical discharges in laboratory long air gaps excited with positive switching impulses. It begins with the preliminary work of several scientists during the last decades, making a detailed analysis of different approaches for modelling all the stages in a full discharge. The relations between these models are identified as well as the effect on the outcome when modifying some important input parameters.The general concept describing the discharge phenomenon usually includes three main elements: the streamer inception, the streamer-to-leader transition and the stable leader propagation. These elements are present in many of the analysed models and the main differences between them are the assumptions and simplifications made by each author at a specific point in their methodologies. The models are usually simplified by assigning experimentally determined values to physical constants pertinent to different stages of the full discharge. These constants are the potential gradient in the leader-corona region to sustain the leader propagation, the charge per unit length along the leader channel which depends on the atmospheric conditions and the voltage impulse wave shape; and the leader propagation velocity, which is closely related to the discharge current. The dissertation includes the results of laboratory work related the study of leaders in long gap discharges, electrical parameters and optical records. By reconstructing the three-dimensional leader propagation for the rod-to-plane configuration, it was possible to study the random tortuous path followed by the leader as it propagates.One important element included in the discharge modelling is the representation of the leader-corona region in front of the leader tip as it propagates towards the grounded electrode. For the calculation of the net charge available in the leader-corona region, two new methodologies were pro-posed based on the electrostatic potential distribution obtained from a finite element method solver. This allowed the inclusion of more elements representing different parts of the discharge in the simulation domain.In the final part, all the analysed elements and the new proposed ones were included in a new methodology for the modelling of electrical dis-charges in long air laboratory gaps. The results obtained from this methodology were compared to experimental data. A good agreement was found between the simulation results and the experimental data.
  •  
41.
  • Diaz, Oscar, 1980-, et al. (författare)
  • Parameter variation in leader channel models used in long air gap discharge simulation
  • 2016
  • Ingår i: Electric power systems research. - : Elsevier. - 0378-7796 .- 1873-2046. ; 139, s. 32-36
  • Tidskriftsartikel (refereegranskat)abstract
    • Theoretical models have been developed to predict the 50% breakdown voltage of long air gaps arrangements, based on the physics of the discharge. These models are capable of estimating electric fields, leader and streamer region propagation, among others. An important parameter within this calculation is the leader model and its electric potential distribution along the discharge channel. In the present work, we compared engineering and physical leader models against experimental data recorded for a rod-to-plane electrode arrangement tested with switching-like voltage impulses. The analysis showed that the leader channel evolution depends strongly on the potential gradient assumed to sustain streamers.
  •  
42.
  • Gunasekara, T. A. L. N., et al. (författare)
  • Characteristics of Narrow Bipolar Pulses observed from lightning in Sri Lanka
  • 2016
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 138, s. 66-73
  • Tidskriftsartikel (refereegranskat)abstract
    • A detailed study on electric field characteristics of Narrow Bipolar Pulses (NBP) observed in Sri Lanka is presented here. NBP5 analyzed in this work were recorded at a coastal location in the Southern part of Sri Lanka (Matara: 5.95 degrees N, 8.53 degrees E), from five highly active consecutive thunderstorm days during the month of May in 2013. The waveforms were recorded with a 10 ns resolution within a 100 ms time window. Both positive and negative NBP5 were observed in this study with the negative type being the most frequent. Parameters presented in this study were the rise time (Tr), zero crossing time (Tz), the duration of slow front (Ts), the full width of half maximum (FWHM), the pulse duration and the ratio of amplitude of overshoot to the corresponding peak amplitude (Os/Pa). The corresponding average values of negative NBP5 for these parameters were found to be 0.58 mu s, 3.01 mu s, 0.20 mu s, 138 mu s, 19.21 mu s and 0.19 respectively. Similarly, for positive events corresponding values were 1.38 mu s, 4.66 mu s, 0.48 mu s, 1.93 mu s, 16.42 mu s and 0.37 respectively. The above values conforms to a much narrower bipolar events when compared to previously reported values which is considered to be caused by the propagation effects of signals captured by the apparatus.
  •  
43.
  • Gunasekara, T. A. L. N., et al. (författare)
  • Horizontal electric fields of lightning return strokes and narrow bipolar pulses observed in Sri Lanka
  • 2018
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 173, s. 57-65
  • Tidskriftsartikel (refereegranskat)abstract
    • Simultaneous measurement of both vertical and horizontal electric field signatures of lightning was carried out in an elevated location in Colombo, Sri Lanka. The experimental setup used in this work was similar to an earlier study carried out by a different group in the late 1980s. To our knowledge, this is the first instance that such a study is conducted in this region. Data were acquired during the active months (April-May) of the southwest monsoon period in 2014. Lightning flashes from the most active thunderstorm was analyzed by selecting 65 Return Strokes (RS), 50 Negative Narrow Bipolar Pulses (NNBP) and 40 Positive Narrow Bipolar Pulses (PNBP). The wave shapes were initially validated against results of a previous study and subsequently via a theoretical method as well. Since the direction and the distance information was not available, rather than the amplitudes, ratios of the peak amplitudes of vertical electric field (Ev) and corresponding horizontal electric field (Eh) were compared. The average ratio for the return stroke was 0.024 +/- 0.008. The same for the NNBP was 0.041 +/- 0.004. The PNBP had a ratio of 0.031 +/- 0.006. The average 10%-90% rise times (Tr) for Ev for RS, NNBP and PNBP was 2.124 +/- 1.088 mu s, 0.734 +/- 0.077 is and 1.141 +/- 0.311 mu s respectively. The Tr values for Eh for RS, NNBP and PNBP were 1.865 +/- 1.200 mu s, 0.538 +/- 0.061 mu s and 1.086 +/- 0.423 mu s.
  •  
44.
  • Gunasekara, T. A. L. N., et al. (författare)
  • Time-Frequency Analysis of Narrow Bipolar Pulses observed in Sri Lanka
  • 2016
  • Ingår i: 2016 33RD INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP). - 9781509058433
  • Konferensbidrag (refereegranskat)abstract
    • Narrow Bipolar Pulses (NBP) of a single thunderstorm from a coastal location in Sri Lanka were subjected to S-transformation in order to study their time frequency information. This study is an extension to a similar study carried out by the authors where properties of NBPs were analyzed using the wavelet transformation. Keeping in line with the previous study, 18 events pertaining to equal number of positive and negative NBPs with the narrowest temporal widths were studied. The data was obtained from the southern coastal area of Sri Lanka (Matara - 5.95 degrees N, 8.53 degrees E) from a highly active thunderstorm, which occurred during the month of May in 2013. The waveforms were recorded with a 10 ns resolution within a 100 ms time window. The spanning (width) and the ratio of peak power amongst the initial and overshoot pulses were measured and compared for each polarity. The negative NBPs had an average spanning of 134 - 371 kHz. The overshoot of the same had an average range of 127 - 255 kHz. The positive NBPs had an average spanning of 103 - 245 kHz. The overshoot of the same had an average range of 102 - 195 kHz. The ratio of peak power of NBPs to overshoot had an average of 1.08 for both positive and negative pulses. The spectrogram reveals the initial and overshoots pulses to be relatively equal in power intensities.
  •  
45.
  • Gunasekara, T.A.L.N., et al. (författare)
  • Time-Frequency Analysis of vertical and horizontal electric field changes of lightning negative return strokes observed in Sri Lanka
  • 2018
  • Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 179, s. 34-39
  • Tidskriftsartikel (refereegranskat)abstract
    • Simultaneously captured vertical and horizontal (total) electric field signatures of lightning negative Return Strokes (RS) were analyzed to obtain Time-Frequency (TF) variation using Stockwell Transformation (ST). In the study, ST was utilized since it is known to possess comparatively better time resolution at high frequency regions compared to other available TF methods. The data were obtained during the monsoon season of April–May 2014. The vertical and horizontal component of fifty negative RSs was utilized in the study. The resultant ST spectrum was analyzed and the regions of interest were demarcated based on the color which represented their relative power output intensities for different frequency components of the signal. The spread area was identified as the region of frequencies which consisted of power intensity equal or above 90th percentile when compared to the maximum possible value. The spectral area was identified as the area of frequencies in the borderline to the natural background noise. The spread region for the vertical E field had a range between 10 kHz and 650 kHz. Its average values were in between 126 kHz and 331 kHz. The spectral region of the vertical electric field change spanned from 1 kHz to 1020 kHz. Its average distribution was 44 kHz–660 kHz. Horizontal electric fields had a range of 20 kHz–1940 kHz in the spectral region. The same for the spread region was 80 kHz–910 kHz. The averages of the horizontal E field's spectral region were 46–1112 kHz and its spread region varied between 227 and 599 kHz. The results display a higher frequency range for all aspects of the horizontal E field changes which implies that its influence on the high frequency radiation is much higher than its vertical counterpart.
  •  
46.
  • Guo, Jun, et al. (författare)
  • On the Modeling of Non-Vertical Risers in the Interaction of Electromagnetic Fields With Overhead Lines
  • 2019
  • Ingår i: IEEE transactions on electromagnetic compatibility (Print). - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0018-9375 .- 1558-187X. ; 61:3, s. 631-636
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper proposes a simple method to take into account non-vertical risers through an equivalent partial inductance. The proposed approach was validated considering several examples and taking as reference full-wave results obtained using a numerical electromagnetics code numerical electromagnetics code (NEC)-4.
  •  
47.
  • He, Lixia, et al. (författare)
  • An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Santis Tower
  • 2018
  • Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 123:8, s. 4045-4059
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Santis Tower in Switzerland. The electric field was measured 15km from the tower. Upward flashes always begin with the initial stage composed of the upward-leader phase and the initial-continuous-current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return-stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no-current time intervals; (2) mixed-mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively small junction heights; (3) classical M-component pulses superimposed on the continuing current following some return strokes; and (4) M-component-type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively large junction heights. We consider a data set consisting of 9 return-stroke pulses, 70 mixed-mode ICC pulses, 11 classical M-component pulses, and 19 M-component-type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed-mode and M-component-type pulses, which is based on the current waveform features. The characteristics of M-component-type pulses during the initial stage are found to be similar to those of classical M-component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed-mode ICC pulses were preceded by microsecond-scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond-scale electric field pulses observed for M-component-type ICC pulses and which can be attributed to the junction of an in-cloud leader channel to the current-carrying channel to ground. Classical M-component pulses and M-component-type ICC pulses tend to have larger risetimes ranging from 6.3 to 430s. On the other hand, return-stroke pulses and mixed-mode ICC pulses have current risetimes ranging from 0.5 to 28s. Finally, our data suggest that the 8-s criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M-components. However, mixed-mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28s, as observed in this study.
  •  
48.
  • He, Lixia, et al. (författare)
  • Electromagnetic Fields Associated With the M-Component Mode of Charge Transfer
  • 2019
  • Ingår i: Journal of Geophysical Research - Atmospheres. - : AMER GEOPHYSICAL UNION. - 2169-897X .- 2169-8996. ; 124:13, s. 6791-6809
  • Tidskriftsartikel (refereegranskat)abstract
    • In upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M-component or leader/return-stroke-like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC-carrying channel at relatively short distance from the tower top, resulting in the so-called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M-component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Santis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M-component charge transfer mode (including classical M-component pulses and M-component-type pulses superimposed on the initial continuous current) is analyzed and compared with its effect on the fields associated with the return stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2-Dimentional Finite-Difference Time Domain method, in which the M-component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al., 1995, model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Santis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for classical M-component and M-component-type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M-component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster traveling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M-component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component, which mainly depends on the overall transferred charge. The contribution of the radiation component to the M-component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of the current pulses. However, the results indicate a high variability of the propagation factors as a function of the branch-to-channel junction point height. For junction point heights of about 1 km, the propagation factors reach a value of about 1.6 for the E-field and 1.9 for the H-field. For a junction height greater than 6 km, the E-field factor becomes slightly lower than 1. The obtained results are consistent with the findings of Li, Azadifar, Rachidi, Rubinstein, Paolone, et al. (2016, ) in which an electric field propagation factor of 1. 8 was inferred for return strokes and mixed-mode pulses, considering that junction points lower than 1 km or so would result in a mixed mode of charge transfer, in which a downward leader/return-stroke-like process is believed to take place. It is also found that the field enhancement (propagation factor) for return stroke mode is higher for larger ground conductivities. Furthermore, the enhancement effect tends to decrease with increasing current risetime, except for very short risetimes (less than 2.5 mu s or so) for which the tendency reverses. Finally, model-predicted fields associated with different charge transfer modes, namely, return stroke, mixed-mode, classical M-component, and M-component-type ICC pulse are compared with experimental observations at the Santis Tower. It is found that the vertical electric field waveforms computed considering the mountainous terrain are in very good agreement with the observed data. The adopted parameters of the models that provide the best match with the measured field waveforms were consistent with observations. The values for the current decay height constant adopted in the return stroke and mixed-mode models (1.0 km for the return stroke and 0.8 km for the mixed-mode pulse) are lower than the value of 2.0 km typically used in the literature.
  •  
49.
  • Hettiarachchi, Pasan, et al. (författare)
  • Energy Distribution of X-rays Produced by Meter-long Negative Discharges in Air
  • 2017
  • Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 8:12
  • Tidskriftsartikel (refereegranskat)abstract
    • The energy deposited from X-rays generated by 1 m long laboratory sparks in air created by 950 kV negative lightning impulses on scintillated detectors was measured. Assuming the X-ray energy detected in such sparks results from the accumulation of multiple photons at the detector having a certain energy distribution, an experiment was designed in such a way to characterize their distribution parameters. The detector was screened by a copper shield, and eight series of fifteen impulses were applied by stepwise increasing the copper shield thickness. The average deposited energy was calculated in each series and compared with the results from a model consisting of the attenuation of photons along their path and probable photon distributions. The results show that the energy distribution of X-ray bursts can be approximated by a bremsstrahlung spectrum of photons, having a maximum energy of 200 keV to 250 keV and a mean photon energy around 52 keV to 55 keV.
  •  
50.
  • Hettiarachchi, Pasan, 1977- (författare)
  • Measurements of X-Ray Emission from Laboratory Sparks and Upward Initiated Lightning
  • 2018
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • In 1925 Nobel laureate R. C. Wilson predicted that high electric fields of thunderstorms could accelerate electrons to relativistic energies which are capable of generating high energetic radiation. The first detection of X-rays from lightning was made in 2001 and from long sparks in 2005. Still there are gaps in our knowledge concerning the production of X-rays from lightning and long sparks, and the motivation of this thesis was to rectify this situation by performing new experiments to gather data in this subject.The first problem that we addressed in this thesis was to understand how the electrode geometry influences the generation of X-rays. The results showed that the electrode geometry affects the X-ray generation and this dependency could be explained using a model developed previously by scientists at Uppsala University. The other missing information was the distribution of energy. Using a series of attenuators, we observed how the X-ray photons were attenuated as a function of barrier thickness and using a simple model we obtained the average and the maximum energy of X-rays. All the studies conducted previously was based on the lightning impulses, but in switching impulses, the voltage waveform rises very slowly compared to lightning impulses, and according to some scientists the rate of rise is an important parameter in X-ray development. Our study showed that the switching impulses were as efficient as lightning impulses in generating X-rays even though the rate of rise of voltage in switching impulses was hundreds of times slower.All the observations on X-ray generation from lightning by other scientists were based on either natural downward lightning flashes or triggered lightning in Florida. The first experiments to study the X-ray generation from upward lightning flashes systematically was conducted within this thesis work at Gaisberg Tower in Austria. The results showed that the X-ray emissions from these flashes were much weaker than the ones produced by either natural downward or triggered lightning. An attempt was made to explain this observation by invoking the possible differences in the charge distribution of leaders associated with the triggered lightning flashes in Florida and upward initiated lightning flashes at Gaisberg tower.
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