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- Akyuz, Mose, 1972-
(författare)
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Positive streamer discharges in air and along insulating surfaces: experiment and simulation
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The power quality of modern society relies on the electrical properties of the dielectric insulators used in the power industry. Much research work has been conducted with an aim to understand and predict the insulating behaviour of such materials under different kinds of atmospheric conditions, but still there are many unsolved problems. In particular, there is a lack of knowledge concerning the electrohydrodynamic and electrophysical processes at the insulator surface and the surrounding medium. No detailed knowledge exists at present of the processes governing the development of electrical discharges along the surface of insulators.With an aim to enhance the knowledge in this field in general and on the electrical performance of outdoor insulators in particular a detailed study of the positive streamer discharges in air and along dielectric surfaces was conducted. The study was also extended to gain more knowledge on the water drop initiated electrical discharges in air and the attachment of natural lightning flashes to a Franklin conductor.In the first phase, the study was focused on positive streamer discharges propagating in air. The spatial distribution of the charge of a branched streamer discharge was obtained and the charge contained in a single streamer branch was quantified. In the second phase measurements and simulations of streamer discharges propagating along insulating surfaces were conducted with an aim to understand how the insulating surfaces interact with streamer discharges. In addition to quantifying the parameters of streamer discharges propagating along insulating surfaces, the results of these studies made it possible to separate and quantify the effects of the dielectric constant and the surface properties on the streamer discharges. In the third phase a comprehensive computer algorithm was developed to simulate 3-dimensional propagation of positive streamer discharges in air and along dielectric surfaces taking into account the branching effect. The conditions necessary for the initiation of streamer discharges were applied to obtain the minimum strength of the background electric field required to initiate electrical discharges in the presence of water drops. In particular the study provided an explanation of how lightning flashes are initiated in thunderclouds in background electric fields as low as 200 kV/m. Finally, the study was extended to understand the performance of lightning conductors paying special attention to the influence of conductor radius and the streamer inception criterion.
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| 2. |
- Flache, Denis, et al.
(författare)
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Different modes of charge transfer to ground in upward lightning observed at the Peissenberg tower
- 2009
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We analyzed high-speed video images andcorresponding current records for eight upward lightningflashes initiated by the Peissenberg tower (160 m) inGermany. These flashes contained a total of 33 measurableinitial stage (IS) current pulses, which are superimposed onsteady IS currents. Seven IS pulses had relatively short (< 8μs) 10-to-90% risetimes and 26 IS pulses had relatively long(> 8 μs) risetimes. Six (86%) of seven IS current pulses withshorter risetimes each developed in a newly-illuminatedbranch, and 25 (96%) of 26 IS pulses with longer risetimesoccurred in already luminous (current-carrying) channels.These results support the hypothesis that longer risetimesare indicative of the M-component mode of charge transferto ground, while shorter risetimes are associated with theleader/return stroke mode. Similar results were obtained forM-component pulses that are superimposed on continuingcurrents following return-stroke pulses.
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| 3. |
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| 4. |
- Johari, Dalina, 1976-
(författare)
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Features of the Electric Fields Generated by Lightning with Special Attention to Positive Ground Flashes
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the main processes in positive ground flashes and the distant lightning environment for both positive and negative ground flashes. It presents the characteristics of the preliminary breakdown pulses (PBPs), the characteristics of the electric field pulses observed during leader propagation, and the characteristics of the electric fields produced by the first and the subsequent return strokes. It also features the observations of distant positive and negative ground flashes at distances up to 1000 km. The results were based on electric field measurements conducted remotely during summer thunderstorms in Sweden in 2014.We found that the majority of the positive ground flashes were preceded by PBPs. Some were preceded by more than one PBP train, and the parameter values for the subsequent PBP trains were found to be smaller than the values for the first PBP train. Three types of PBPs were also identified. The results suggest that the PBPs in positive ground flashes during summer thunderstorms in Sweden are weak, and the inverted dipole charge cloud configuration is consistent with our observation.A small percentage of these positive ground flashes were observed to be preceded by pronounced leader pulses. The presence of these pulses suggests that the leaders propagate in a stepped manner. We inferred that these pulses were due to the upward-connecting negative leader since their characteristics were similar to those of a negative stepped leader. On the basis of the leader pulses’ time of initiation and the average speed of the leader, we estimated the distance travelled by the leader. One case of positive return stroke preceded by negative leader pulses was also observed, and the occurrence of these pulses was the first in positive ground flashes to be reported.The majority of these positive ground flashes were found to be single-stroke. Comparison between the first and the subsequent return strokes showed that the average durations of the subsequent stroke parameters were smaller than that of the first strokes. The distances reported by the lightning location system suggest that the subsequent strokes probably created new terminations to ground. Two possible reasons were given to explain the reason for the shorter duration of the subsequent return strokes parameters compared to the first strokes.Finally, observations of distant positive and negative ground flashes showed that the electric field waveforms have a typical shape, like a distorted ‘W’ (or distorted ‘M’ for negative ground flashes) followed by small oscillations. These small oscillations were more pronounced in negative ground flashes, especially at a greater distance. The heights of the ionospheric reflections estimated for both ground flashes were found to likely correspond to a D-layer of the ionosphere. Two possible reasons were suggested for the small oscillations observed in the waveforms.
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| 6. |
- Miki, Megumu, et al.
(författare)
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Initial stage in lightning initiated from tall objects and in rocket-triggered lightning
- 2005
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Ingår i: Journal of Geophysical Research: Atmospheres. ; 110:D2, s. D02109-
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Tidskriftsartikel (refereegranskat)abstract
- We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.
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