| 1. |
- Cooray, Vernon, 1952-, et al.
(författare)
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Electromagnetic fields of accelerating charges : Applications in lightning protection
- 2017
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Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 145, s. 234-247
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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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| 2. |
- Cooray, Vernon, 1952-, et al.
(författare)
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Formulation of the Field-to-Transmission Line Coupling Equations in Terms of Scalar and Vector Potentials
- 2017
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Ingår i: IEEE transactions on electromagnetic compatibility (Print). - 0018-9375 .- 1558-187X. ; 59:5, s. 1586-1591
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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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| 3. |
- Frost, Anna E., 1988-, et al.
(författare)
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Do pre-recorded lessons in combination with formative assessments fulfil the learning necessity of engineering students?
- 2022
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Ingår i: Bidrag från 8:e Utvecklingskonferensen för Sveriges ingenjörsutbildningar. - : Karlstads universitet. - 9789178672813 ; , s. 21-25
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- As an effort to increase digitization of education during the ongoing pandemic in Spring 2021, the course ”Electromagnetism II with electric circuits” at Uppsala University has been conducted in a blended form with both pre-recorded sessions and live online sessions. About half of the lectures were live sessions, and the rest were pre-recorded. Among the lessons, more than 75 % were pre-recorded. When the teaching is pre-recorded, the teachers do not get reactions directly from the students, either verbally or from facial expressions. Therefore,self-correcting quizzes were implemented for every lecture, and short formative evaluations were implemented for every lesson. The main objective was to increase the number of students participating in the lessons, which hopefully would lead to more students passing the course. More students watched the pre-recorded lessons than attended the lessons on campus previous years on the course. More students passed the exam on the course compared to previous years, but it cannot be excluded that this is partially or completely due to the exam being different due to the pandemic. The short formative evaluations had at least seven respondents during the first four lessons, but from lesson five and onward, the number of respondents was two or fewer. The quizzes which most students attempted were right after or right before the pre-exams (”duggor” in Swedish), but otherwise relatively few students answered the quizzes. When asked in the course evaluation, it varied a lot whether the students liked, disliked, or were neutral to have pre-recorded lessons, with slightly more students saying they preferred live sessions.
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| 4. |
- Ahmad, Mohd Riduan, et al.
(författare)
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Interference from cloud-to-ground and cloud flashes in wireless communication system
- 2014
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Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 113, s. 237-246
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Tidskriftsartikel (refereegranskat)abstract
- In this study, cloud-to-ground (CG) flash and intra-cloud (IC) flash events that interfere with the transmission of bits in wireless communication system operating at 2.4 GHz were analyzed. Bit error rate (BER) and consecutive lost datagram (CLD) measurement methods were used to evaluate BER and burst error from 3 tropical thunderstorms on November 27, 28, and 29 during 2012 northeastern monsoon in Malaysia. A total of 850 waveforms from the electric field change recording system were recorded and examined. Out of these, 94 waveforms of very fine structure were selected which matched perfectly with the timing information of the recorded BER. We found that both CG and IC flashes interfered significantly with the transmission of bits in wireless communication system. The severity of the interference depends mainly on two factors namely the number of pulses and the amplitude intensity of the flash. The interference level becomes worst when the number of pulses in a flash increases and the amplitude intensity of pulses in a flash intensifies. During thunderstorms, wireless communication system has experienced mostly intermittent interference due to burst error. Occasionally, in the presence of very intense NBP event, wireless communication system could experience total communication lost. In CG flash, it can be concluded that PBP is the major. source of interference that interfered with the bits transmission and caused the largest burst error. In IC flash, we found that the typical IC pulses interfered the bits transmission in the same way as PBP and mixed events in CG flash and produced comparable and in some cases higher amount of burst error. NBP has been observed to interfere the bits transmission more severely than typical IC and CG flashes and caused the most severe burst error to wireless communication system.
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| 5. |
- Amarasinghe, Dulan, et al.
(författare)
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Fractal dimension of long electrical discharges
- 2015
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Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 73, s. 33-37
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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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| 6. |
- Arevalo, Liliana, 1978-, et al.
(författare)
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A new static calculation of the streamer region for long spark gaps
- 2012
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Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 70:1, s. 15-19
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Tidskriftsartikel (refereegranskat)abstract
- Different electrostatic approximations have been proposed to calculate the streamer region without going in deep details of the behavior of density of particles under the effect of high electric fields; this kind of approximations have been used in numerical calculations of long spark gaps and lightning attachment. The simplifications of the streamer region are achieved by considering it to be a geometrical region with a constant geometrical shape. Different geometrical shapes have been used, such as cones or several parallel filaments. Afterward, to simplify the procedures, the streamer region was approximated by two constants, one denoted K-Q, called the geometrical constant and in other cases K named as geometrical factor. However, when a voltage that varies with time is applied to an arrangement of electrodes (high voltage and grounded electrodes), the background electric field will change with time. Thus, if the background electric field is modified, the streamer zone could cover a larger or smaller area. With the aim of reducing the number of assumptions required in the calculation of long gap discharges, a new electrostatic model to calculate the streamer region is presented. This model considers a variable streamer zone that changes with the electric field variations. The three-dimensional region that fulfills the minimum electric field to sustain a streamer is identified for each time step, and the charge accumulated in that region is then calculated. The only parameter that is being used in the calculation is the minimum electric field necessary for the propagation of streamers.
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| 7. |
- Arevalo, L., et al.
(författare)
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The mesh method in lightning protection analyzed from a lightning attachment model
- 2016
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Ingår i: 2016 33RD INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP). - 9781509058433
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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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| 8. |
- Becerra Garcia, Marley, et al.
(författare)
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Laboratory experiments cannot be utilized to justify the action of early streamer emission terminals
- 2008
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:8
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Tidskriftsartikel (refereegranskat)abstract
- The early emission of streamers in laboratory long air gaps under switching impulses has beenobserved to reduce the time of initiation of leader positive discharges. This fact has beenarbitrarily extrapolated by the manufacturers of early streamer emission devices to the case ofupward connecting leaders initiated under natural lightning conditions, in support of thosenon-conventional terminals that claim to perform better than Franklin lightning rods. In orderto discuss the physical basis and validity of these claims, a self-consistent model based on thephysics of leader discharges is used to simulate the performance of lightning rods in thelaboratory and under natural lightning conditions. It is theoretically shown that the initiation ofearly streamers can indeed lead to the early initiation of self-propagating positive leaders inlaboratory long air gaps under switching voltages. However, this is not the case for positiveconnecting leaders initiated from the same lightning rod under the influence of the electricfield produced by a downward moving stepped leader. The time evolution of the developmentof positive leaders under natural conditions is different from the case in the laboratory, wherethe leader inception condition is closely dependent upon the initiation of the first streamerburst. Our study shows that the claimed similarity between the performance of lightning rodsunder switching electric fields applied in the laboratory and under the electric field producedby a descending stepped leader is not justified. Thus, the use of existing laboratory results tovalidate the performance of the early streamer lightning rods under natural conditions is not justified.
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| 9. |
- Becerra Garcia, Marley, et al.
(författare)
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Lightning striking distance of complex structures
- 2008
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Ingår i: IET Generation, Transmission & Distribution. - : Institution of Engineering and Technology (IET). - 1751-8687 .- 1751-8695. ; 2:1, s. 131-138
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Tidskriftsartikel (refereegranskat)abstract
- Traditionally, the location of lightning strike points has been determined byusing the rolling sphere method, but recently the collection volumemethod (CVM) has also been proposed for the placement ofair terminals on complex structures. Both these methods are empiricalin nature and a more advanced model based on physicsof discharges is needed to improve the state of affairs.This model is used to evaluate the striking distance fromcorners and air terminals on actual buildings and the resultsare qualitatively compared with the predictions of the rolling spheremethod and the CVM. The results show that the strikingdistance not only depends upon the prospective return stroke currentand the geometry of the building, but also on thelateral position of the downward leader with respect to thestrike point. A further analysis is performed to qualitatively comparethe lightning attraction zones obtained with the CVM and theleader inception zones obtained for a building with and withoutair terminals. The obtained results suggest that the collection volumeconcept overestimates the protection areas of air terminals placed oncomplex structures, bringing serious doubts on the validity of this method.
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| 10. |
- Becerra Garcia, Marley, et al.
(författare)
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On the velocity of positive connecting leaders associated with negative downward lightning leaders
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:2
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Tidskriftsartikel (refereegranskat)abstract
- A self-consistent leader propagation model is used to estimate the velocity of upward connecting positive leaders initiated from a tall tower under the influence of downward negative lightning leaders. The propagation of upward connecting leaders has been found to be influenced not only by the average velocity of the downward leader but also by the prospective return stroke current, the lateral position of the downward leader channel as well as by the ambient electric field. This result show that the velocity and propagation time of upward connecting positive leaders change from flash to flash due to the variations in these parameters.
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