| 1. |
- Anatory, Justinian, et al.
(författare)
-
The effects of load impedance, line length, and branches in the BPLC transmission-lines analysis for-medium-voltage channel
- 2007
-
Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 22:4, s. 2156-2162
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents the effects of load impedance, line length and branches on the performance of medium-voltage power-line communication (PLC) network. The power-line network topology adopted here is similar to that of the system in Tanzania. Different investigation with regard to network load impedances, direct line length (from transmitter to receiver), branched line length and number of branches has been investigated. From the frequency response of the transfer function (ratio of the received and transmitted signal), it is seen that position of notches and peaks in the magnitude and phase responses are largely affected in terms of attenuation and dispersion by the above said network parameters/configuration. These are observed in the time domain responses too. The observations presented in the paper could be helpful in suitable design of the PLC systems for a better data transfer and system performance.
|
|
| 2. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
On the estimation of the lightning incidence to offshore wind farms
- 2018
-
Ingår i: Electric power systems research. - : Elsevier Ltd. - 0378-7796 .- 1873-2046. ; 157, s. 211-226
-
Tidskriftsartikel (refereegranskat)abstract
- Field observations have shown that the frequency of dangerous lightning events to wind turbines, calculated according to the IEC standard 61400-24:2010, is grossly underestimated. This paper intends to critically revisit the evaluation of the incidence of downward lightning as well as self-initiated and other-triggered upward flashes to offshore wind power plants. Three different farms are used as case studies. The conditions for interception of stepped leaders in downward lightning and the initiation of upward lightning is evaluated with the Self-consistent Leader Inception and Propagation Model (SLIM). The analysis shows that only a small fraction of damages observed in the analysed farms can be attributed to downward lightning. It is also estimated that only a small fraction (less than 19%) of all active thunderstorms in the area of the analysed farms can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can be triggered even under low thundercloud fields once a sufficiently high electric field change is generated by a nearby lightning event. Despite of the uncertainties in the incidence evaluation, it is shown that upward flashes triggered by nearby positive cloud-to-ground flashes produce most of the dangerous lightning events to the case studies.
|
|
| 3. |
- Berg, Marcus, et al.
(författare)
-
Hydrophobicity estimation of HV polymeric insulating materials. Development of a digital image processing method.
- 2001
-
Ingår i: IEEE transactions on dielectrics and electrical insulation. - : Institute of Electrical and Electronics Engineers (IEEE). - 1070-9878 .- 1558-4135. ; 8:6, s. 1098-1107
-
Tidskriftsartikel (refereegranskat)abstract
- Image analysis of water drop patterns on an inclined flat polymeric insulator surface has been performed in order to find a simple mathematical function that indicates the level of hydrophobicity of the insulator surface. A simple function, given the acronym average of normalized entropies (ANE), seems to correlate well with hydrophobicity as defined by the Swedish Transmission Research Institute (STRI) hydrophobicity classification. It is a composition of three other functions, viz. the standard deviation, the Shannon information entropy and the 'fraction of small differences'. All these are in turn based on the histogram of horizontal nearest-neighbor pixel differences for a given digital greyscale image of a water drop pattern. ANE is fairly independent of illumination intensity (exposure) as well as total gain and offset in a camera system (linear sensor). The experimental results also indicate that ANE is fairly independent of limited changes in the surface inclination, although this needs further investigation. Some of the various pitfalls associated with the photography of water drop patterns and processing of images are identified, together with possible solutions for avoiding them.
|
|
| 4. |
- Flache, Denis, et al.
(författare)
-
Different modes of charge transfer to ground in upward lightning observed at the Peissenberg tower
- 2009
-
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.
|
|
| 5. |
- Flache, D., et al.
(författare)
-
Initial-stage pulses in upward lightning : Leader/return stroke versus M-component mode of charge transfer to ground
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:13, s. L13812-
-
Tidskriftsartikel (refereegranskat)abstract
- We analyzed high-speed video images and corresponding current records for eight upward lightning flashes initiated by the Peissenberg tower (160 m) in Germany. These flashes contained a total of 33 measurable initial stage (IS) current pulses, which are superimposed on steady IS currents. Seven IS pulses had relatively short (< 8 mu s) 10-to-90% risetimes and 26 IS pulses had relatively long (> 8 mu s) risetimes. Six (86%) of seven IS current pulses with shorter risetimes each developed in a newly-illuminated branch, and 25 (96%) of 26 IS pulses with longer risetimes occurred in already luminous (current-carrying) channels. These results support the hypothesis that longer risetimes are indicative of the M-component mode of charge transfer to ground, while shorter risetimes are associated with the leader/return stroke mode. Similar results were obtained for M-component pulses that are superimposed on continuing currents following return-stroke pulses.
|
|
| 6. |
|
|
| 7. |
- Jansson, Carl-Gustaf, et al.
(författare)
-
Experiments in creating online course content for signal processing education
- 2020
-
Ingår i: Proceedings 2020 IEEE International Conference on Acoustics, Speech, and Signal Processing. - : Institute of Electrical and Electronics Engineers (IEEE). ; , s. 9220-9223
-
Konferensbidrag (refereegranskat)abstract
- The creation of the NPTEL platform in India has led to a vast population of engineering students getting access to quality online content for Signal Processing. These courses are globally accessible, free of cost, and also provide a means of obtaining certificates of proficiency by taking a proctored examination. Recently, a European Union funded project, MIELES, has supported the activity of creating online courses in the fields related to Signal Processing. This paper presents the details and experiences of creating course content and presents guidelines for prospective content creators.
|
|
| 8. |
- Kande, Mallikarjun, et al.
(författare)
-
Rotating Electrical Machine Condition Monitoring Automation-A Review
- 2017
-
Ingår i: Machines. - : MDPI AG. - 2075-1702. ; 5:4
-
Forskningsöversikt (refereegranskat)abstract
- We review existing machine condition monitoring techniques and industrial automation for plant-wide condition monitoring of rotating electrical machines. Cost and complexity of a condition monitoring system increase with the number of measurements, so extensive condition monitoring is currently mainly restricted to the situations where the consequences of poor availability, yield or quality are so severe that they clearly justify the investment in monitoring. There are challenges to obtaining plant-wide monitoring that includes even small machines and non-critical applications. One of the major inhibiting factors is the ratio of condition monitoring cost to equipment cost, which is crucial to the acceptance of using monitoring to guide maintenance for a large fleet of electrical machinery. Ongoing developments in sensing, communication and computation for industrial automation may greatly extend the set of machines for which extensive monitoring is viable.
|
|
| 9. |
- Liu, Yaqing, et al.
(författare)
-
An Improved Transmission-Line Model of Grounding System
- 2001
-
Ingår i: IEEE transactions on electromagnetic compatibility (Print). - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9375 .- 1558-187X. ; 43:3, s. 348-355
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a time-domain transmission line model of grounding system, which includes the mutual electromagnetic coupling between the parts of the grounding structure and the influence of air-earth interface. The model can be used to simulate the transient behavior of the grounding system under lightning strike. The simulation results are in good agreement with that of the model based on the solution of full Maxwell's equations. The influence of different parameters, such as the soil relative permittivity /spl epsi//sub /spl tau//, the soil resistivity /spl rho/, and the conductivity and diameter of the conductor, on the transient voltage distribution of the grounding system is investigated. It shows that, among the parameters investigated here, the soil resistivity is the most important parameter that affects the transient response of bare buried conductors. The soil permittivity has very little influence on the transient response of the grounding system when the grounding system is buried in the soil with low resistivity, but have moderate influence in the soil with extremely high resistivity. The conductivity of the conductor and skin effect have practically no influence on the peak transient voltage of the grounding system. Increase in conductor diameter tends to decrease the peak transient voltage. The model presented in this paper is simple, but sufficiently accurate and can be used easily in engineering practice. Since the model is in the time domain, it could be easily coupled to the other time-domain models of nonlinear surge-protection components.
|
|
| 10. |
- Liu, Yaqing, et al.
(författare)
-
Investigating the validity of existing definitions and empirical equations of effective length/area of grounding wire/grid for transient studies
- 2007
-
Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 65:5-6, s. 329-335
-
Tidskriftsartikel (refereegranskat)abstract
- There are various definitions for effective length/area of grounding wire/grid for lightning transients [A.S. Farag, T.C. Cheng, D. Penn, Grounding terminations of lightning protective systems, IEEE Trans. Dielectics, Elect. Insul 5(6) (1998) 869-877; B.R. Gupta, B. Thapar, Impulse impedance of grounding grid, IEEE Trans. Power Apparatus Syayem PAS-99(6) (1980) 2357-2362; Y. Liu, N. Theethayi, R. Thottappillil, An engineering model for transient analysis of grounding system under lightning strikes: non-uniform transmission line approach. IEEE Trans. Power Delivery 20 (2) (2005) 722-730; M.I. Lorentzou, N.D. Hatziargriou, Modelling of long grounding conductors using EMTP, in: IPST'99, International Conference on Power System Transients, Budapest, 20-24 June, 1999; L.D. Grcev, M. Heimbach, Frequency dependent and transient characteristics of substation grounding system, IEEE Trans. Power Delivery 12 (1997) 172-178.]. The present work investigates and discusses the validity of those existing definitions. Further, practical methods for estimating the effective length/area of different grounding structures are proposed for engineering applications. The calculations for effective length/area based on non-uniform transmission line approach (Liu et al., 2005) show that, for a single grounding wire, the empirical equation for effective length in Farag et al. (1998) is not valid when the injection current has very fast rise time. Also, the empirical equation for effective length of grid edge in Gupta and Thapar (1980) is not applicable for grids with large inner mesh size.
|
|
| 11. |
- Liu, Y. Q., et al.
(författare)
-
An engineering model for transient analysis of grounding system under lightning strikes : Nonuniform transmission-line approach
- 2005
-
Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 20:2, s. 722-730
-
Tidskriftsartikel (refereegranskat)abstract
- A nonuniform transmission line approach is adopted in this paper for modeling the transient behavior of different types of grounding systems under lightning strikes in time domain by solving Telegrapher's equations based on finite-difference time-domain (FDTD) technique. Electromagnetic couplings between different parts of the grounding wires are included using effective per-unit length parameters (l, c, and g), which are space and time dependent. The present model can predict both the effective length and the transient voltage of grounding electrodes accurately, while, an uniform transmission line approach with electrode length dependent per-unit length parameters [19]-[22] fails to predict the same. Unlike the circuit theory approach [1]-[4], the present model is capable of predicting accurately the surge propagation delay in the large grounding system. The simulation results for buried horizontal wires and grounding grids based on the present model are in good agreement with that of the circuit and electromagnetic field approaches [3], [9]. From an engineering point of view, the model presented in this paper is sufficiently accurate, time efficient, and easy to apply.
|
|
| 12. |
- Liu, Y. Q., et al.
(författare)
-
An improved model for soil ionization around grounding system and its application to stratified soil
- 2004
-
Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 60:2-4, s. 203-209
-
Tidskriftsartikel (refereegranskat)abstract
- An improved model for taking into account the effect of the soil ionization around grounding system under lightning strike is proposed in this paper. In this model, the soil ionization region is assumed to retain 7% of its pre-ionization resistivity, which is consistent with the experimental results on soil ionization found in literature (Trans. SA Inst. Electr. Eng. (1988) 63; AIEE Trans. 61 (1942) 349; Proc. IEE 121(2) (1974) 123) and our own laboratory experiments (Time domain modelling of the response of grounding systems subjected to lightning currents, Licenciate Thesis, Uppsala University, 2003). Compared with modelling the soil ionization as an increase in the size of the ground conductor, the model presented here will not overestimate the beneficial influence of the soil ionization in reducing the ground potential rise, especially in high resistivity soil. The model is also applied to study the transient behaviour of grounding conductors in stratified soil under lightning strike including soil ionization. It shows that making the grounding conductor to penetrate the lower resistivity soil layer could help to decrease the ground potential rise at the injection point several times.
|
|
| 13. |
- Liu, Yaqing, et al.
(författare)
-
The Residual Resistivity in Soil Ionization Region Around Grounding System for Different Experimental Results
- 2003
-
Ingår i: IEEE Symposium on Electromagnetic Compatibility. Symposium Record. - 0780378350 ; , s. 794-799
-
Konferensbidrag (refereegranskat)abstract
- The residual soil resistivity at peak of injection current with lightning current wave shape in soil ionization region around the grounding system is analyzed based on different experimental results in the literature by E.E. Oettle (1988), P.L. Bellaschi et al. (1942) and A.C. Liew and M. Darveniza (1974) and the recent experimental results in the high voltage laboratory at Uppsala University. The results show that the residual resistivity in soil ionization region is changing largely versus different soil resistivity ranging from 50 /spl Omega/m to 827 /spl Omega/m. Combining all the data, the geometric mean of the residual soil resistivity in ionization region is approximately 7% of the original soil resistivity. This value of 7% can be used in modeling of the transient behavior of grounding system under the lightning strike including non-linear soil ionization phenomenon.
|
|
| 14. |
- Long, Mengni, et al.
(författare)
-
On the attachment of dart lightning leaders to wind turbines
- 2017
-
Ingår i: Electric power systems research. - : Elsevier Ltd. - 0378-7796 .- 1873-2046. ; 151, s. 432-439
-
Tidskriftsartikel (refereegranskat)abstract
- Wind turbines are prone to damages due to lightning strikes and the blades are one of the most vulnerable components. Even though the blade tip is usually protected in standard designs, lightning damages several meters away from it have also been observed in some field studies. However, these damages inboard from the tip cannot be explained by the attachment of downward stepped leaders or the initiation of upward lightning alone. In this paper, the attachment of dart leaders in an upward lightning flash is investigated as a mechanism of strikes to inboard sections of the blade and the nacelle of large wind turbines. Dart leaders in an upward lightning flash use the channel previously ionized by the preceding stroke or the continuous current. The analysis is performed with the self-consistent leader inception and propagation model (SLIM). A commercial large wind turbine with 45 m long blades and hub height of 80 m is analysed as a case study. The impact of the prospective return stroke peak current, the rotation angle of the blade and the wind on the location of lightning strikes on this mechanism is analysed. The probability of lightning attachment of dart leaders along the blade for the case study is also calculated. It is shown that this damage mechanism could create a new strike point only when the blade of a wind turbine rotates sufficiently from its initial position (at the inception of the initial upward leader) until the start of the dart leader approach. Thus, dart leader attachment is a mechanism that can explain lightning strikes to the nacelle and to the inboard region several meters away from the blade tip in large wind turbines. However, dart leader attachment cannot explain the lightning strikes observed in the close vicinity of the blade tip (in the region between 1.5 and 6 m from it).
|
|
| 15. |
- Manyahi, M. J., et al.
(författare)
-
Simplified model for estimation of lightning induced transient transfer through distribution transformer
- 2005
-
Ingår i: International Journal of Electrical Power & Energy Systems. - : Elsevier BV. - 0142-0615 .- 1879-3517. ; 27:4, s. 241-253
-
Tidskriftsartikel (refereegranskat)abstract
- In this work a simplified procedure for the formulation of distribution transformer model for studying its response to lightning caused transients is presented. Simplification is achieved by the way with which the model formulation is realised. That is, by consolidating various steps for model formulation that is based on terminal measurements of driving point and transfer short circuit admittance parameters. Sequence of steps in the model formulation procedure begins with the determination of nodal admittance matrix of the transformer by network analyser measurements at the transformer terminals. Thereafter, the elements of nodal admittance matrix are simultaneously approximated in the form of rational functions consisting of real as well as complex conjugate poles and zeros, for realisation of admittance functions in the form of RLCG networks. Finally, the equivalent terminal model of the transformer is created as a pi-network consisting of the above RLCG networks for each of its branches. The model can be used in electromagnetic transient or circuit simulation programs in either time or frequency domain for estimating the transfer of common mode transients, such as that caused by lightning, across distribution class transformer. The validity of the model is verified by comparing the model predictions with experimentally measured outputs for different types of common-mode surge waveform as inputs, including a chopped waveform that simulate the operation of surge arresters. Besides it has been verified that the directly measured admittance functions by the network analyser closely matches the derived admittance functions from the time domain impulse measurements up to 3 MHz, higher than achieved in previous models, which improves the resulting model capability of simulating fast transients. The model can be used in power quality studies, to estimate the transient voltages appearing at the low voltage customer installation due to the induced lightning surges on the high voltage side of the transformer. The procedure is general enough to be adapted for any two-port devices that behaves linearly in the frequency range of interest.
|
|
| 16. |
- Manyahi, M. J., et al.
(författare)
-
Transient response of transformer with XLPE insulation cable winding design
- 2005
-
Ingår i: International Journal of Electrical Power & Energy Systems. - : Elsevier BV. - 0142-0615 .- 1879-3517. ; 27:1, s. 69-80
-
Tidskriftsartikel (refereegranskat)abstract
- Significant advances in XLPE insulation cables that have higher electric field strength withstand capability have made it possible to apply these high voltage (HV) cables as windings in generators and transformers. Therefore, the recent advent of HV generator (Powerformer) that can be connected directly to the power transmission line has motivated the design of HV transformer (Dryformer) that performs one step transformation from transmission to distribution voltage levels. Since the dryformer will be connected directly to transmission lines, they will be subjected to transients resulting from direct and indirect lightning strikes as well as fast switching surges from Gas insulated circuit breakers. This paper presents the results of experimental studies on the cable winding power transformer (Dryformer) to study its response to various transients. Experimental investigations have been carried to obtain the transformer model parameters based on terminal measurement of admittance functions using Network Analyser, and hence for comparing the model predictions with experimentally obtained responses. The model has been successfully used in estimating the dryformer transient responses at its terminals due to surge application of various front times and peak amplitudes that are representative of lightning and switching caused transients. Experiment and simulation results show that there are considerable differences in the transient response characteristics of dryformer windings as compared to that of transformers with traditional winding design. These differences on transient responses are discussed in perspective of their basic difference in winding design features.
|
|
| 17. |
- Rachidi, Farhad, et al.
(författare)
-
Determination of lightning currents from far electromagnetic fields
- 1993
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 98:D10, s. 18315-18321
-
Tidskriftsartikel (refereegranskat)abstract
- Expressions relating far electromagnetic fields and return stroke channel base currents have been derived in the literature for various lightning return stroke models. The use of such relations permits the estimation of channel base currents of return strokes, testing of the individual models using sets of simultaneous current and field measurements, and also the determination of nondirectly measurable parameters of the models. In this paper we derive a relation between the far electromagnetic field and the channelbase current for the Diendorfer-Uman (DU) and the modified Diendorfer-Uman (MDU) models. Additionally, the current field relations for all models for which such an analytical relation can be derived are summarized. Finally, a numerical solution technique is proposed to solve the equations relating the far field and channel base current of the DU and the traveling current source (TCS) models.
|
|
| 18. |
|
|
| 19. |
- Rakov, Vladimir A., et al.
(författare)
-
Electric field pulses in K and M changes of lightning ground flashes
- 1992
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 97:D9, s. 9935-9950
-
Tidskriftsartikel (refereegranskat)abstract
- From electric field records of 27 ground flashes near Tampa, Florida, and 19 ground flashes at the NASA Kennedy Space Center (KSC) the occurrence and waveshape of microsecond-scale electric field pulses associated with both millisecond-scale steplike K changes and millisecond-scale hook-shaped M changes are examined to test and disprove the following two hypotheses: (1) that K changes contain a microsecond-scale pulse component which can be described by the characteristic pulse waveform proposed by Arnold and Pierce (1964) and (2) that there is essentially no difference between K and M processes, as argued by Kitagawa et al. (1962). Microsecond-scale electric field variations exceeding by at least 50% the system noise level were observed in 23% of 135 K changes from Tampa and in 25% of 128 K changes from KSC, while such field variations were found in 44% of 88 M changes from Tampa and in 77% of 30 M changes from KSC. In the majority of the K changes having microsecond-scale pulse activity, that activity did not occur at the beginning of the K step, while in most cases the pulses associated with M changes occurred at the initial portion of the M hook. These results can be interpreted to imply that K changes and M changes are associated with dissimilar physical processes, in refutation of hypothesis (2) above. The microsecond-scale pulse activity during K changes and M changes was highly variable and sometimes irregular in waveshape. Not all the pulses had the same polarity as the K step or the initial portion of the M hook on which they were superimposed. The relation of the microsecond-scale variations to the overall K changes in ground flashes as regards the frequency of occurrence, the position of pulses within the slower field change, and the shape of the pulses is similar to that reported by Bils et al. (1988) for K changes in cloud flashes. The observed microsecond-scale field variations associated with K changes are not consistent with the characteristic electric field pulse waveform attributed by Arnold and Pierce (1964) and some other investigators to K changes, in refutation of hypothesis (1) above. No relation was observed between the magnitude of a K change and the presence or absence of corresponding microsecond-scale field variations. M changes during continuing-current field changes of relatively short duration (less than 20 ms or so) are more likely to have pulses than M changes during continuing-current field changes of longer duration.
|
|
| 20. |
- Rakov, Vladimir A., et al.
(författare)
-
M-component mode of charge transfer to ground in lightning discharges
- 2001
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 106:D19, s. 22817-22831
-
Tidskriftsartikel (refereegranskat)abstract
- The M-component mode of charge transfer to ground is examined using (1) multiple-station measurements of electric and magnetic fields at distances ranging from 5 to ∌ 500 m from triggered-lightning channels and (2) measured currents at the channel base. Data have been obtained in 1997, 1999, and 2000 at the International Center for Lightning Research and Testing at Camp Blanding, Florida, for (1) “classical†M-components that occur during the continuing currents following return strokes and (2) impulsive processes that occur during the initial stage of rocket-triggered lightning and are similar to the “classical†M components. All lightning events considered here effectively transported negative charge to ground. For one triggered-lightning event the electric field 45 km from the lightning channel was measured together with the current and close fields. The shapes and magnitudes of the measured close electric and magnetic fields are generally consistent with the guided-wave mechanism of the lightning M component. Specifically, the M-component electric field peak exhibits logarithmic distance dependence, ln(kr−1), which is indicative of a line charge density that is zero at ground and increases with height. Such a distribution of charge is distinctly different from the more or less uniform charge density that is characteristic of the dart leaders in triggered lightning, as inferred from close electric field measurements. The M-component magnetic field peak decreases as the inverse distance (i.e., r−1), which is generally consistent with a uniform current within the lowest kilometer or so of channel. The M-component electric field at 45 km appeared as a bipolar, microsecond-scale pulse that started prior to the onset of the M-component current at the channel base. M-component-type processes can produce acoustic signals with peak pressure values of the same order of magnitude as those from the leader/return stroke sequences in triggered lightning.
|
|
| 21. |
- Rakov, Vladimir A., et al.
(författare)
-
Mechanism of the lightning M component
- 1995
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 100:D12, s. 25701-25710
-
Tidskriftsartikel (refereegranskat)abstract
- Analysis of simultaneous measurements of the channel-base current and the vertical electric field 30 m from triggered lightning reveals that the fields associated with M components, although essentially electrostatic, appear to be proportional to the time derivatives of the associated M currents. Based on this finding, coupled with other observations and modeling, a mechanism for the lightning M component is proposed. According to this mechanism an M component involves a downward progressing incident wave (the analog of a leader) followed by an upward progressing reflected wave (the analog of a return stroke). However, as opposed to a leader-return stroke sequence in which the latter removes the charge deposited by the former, both the upward and the downward processes contribute about equally to the total charge flowing from the bottom of the channel at any instant of time. Such a mode of charge transfer to ground, distinctly different from a leader-return stroke sequence, is possible because of the presence of a path capable of supporting the propagation of a traveling wave (facilitated by a continuing current flowing to ground) and the fact that the ground is essentially a short circuit for the downward incident wave, so that the magnitude of the current reflection coefficient at ground is virtually equal to unity. We show that some observed properties of M components can be explained if the lightning channel traversed by an M-current wave is represented as a linear R-C transmission line. In this view, the preferential attenuation of the higher-frequency components on an R-C line is responsible for the lack of frequencies above several kilohertz in both the M-current pulses measured at the channel base and the M-light pulses observed in the bottom 1 km or so of the channel. Further, the relatively high characteristic impedance of the channel, of the order of tens to hundreds of kilohms for frequencies below some kilohertz, inferred from the linear R-C line approximation, is consistent with the observation that even a relatively poor ground is sensed by an incident M wave as essentially a short circuit. However, on a linear R-C transmission line the higher-frequency components travel faster than lower-frequency components (this velocity dispersion implying that the original pulse would spread while propagating along the line), whereas the shape of the M-light pulses does not change much within the bottom 1 km or so, as if the channel were a distortionless transmission line. We speculate, on physical grounds, that the front of the traveling M-current pulse heats the channel so that the pulse tail encounters a lowered resistance and, as a result, accelerates. By virtue of these two opposing effects, velocity dispersion and channel nonlinearity, an M pulse is formed whose more-or-less symmetrical shape is preserved over a relatively large distance, as in the case of a soliton.
|
|
| 22. |
- Rakov, V. A., et al.
(författare)
-
New insights into lightning processes gained from triggered-lightning experiments in Florida and Alabama
- 1998
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 103:D12, s. 14117-14130
-
Tidskriftsartikel (refereegranskat)abstract
- Analyses of electric and magnetic fields measured at distances from tens to hundreds of meters from the ground strike point of triggered lightning at Camp Blanding, Florida, and at 10 and 20 m at Fort McClellan, Alabama, in conjunction with currents measured at the lightning channel base and with optical observations, allow us to make new inferences on several aspects of the lightning discharge and additionally to verify the recently published “two-wave†mechanism of the lightning M component. At very close ranges (a few tens of meters or less) the time rate of change of the final portion of the dart leader electric field can be comparable to that of the return stroke. The variation of the close dart leader electric field change with distance is somewhat slower than the inverse proportionality predicted by the uniformly charged leader model, perhaps because of a decrease of leader charge density with decreasing height associated with an incomplete development of the corona sheath at the bottom of the channel. There is a positive linear correlation between the leader electric field change at close range and the succeeding return stroke current peak at the channel base. The formation of each step of a dart-stepped leader is associated with a charge of a few millicoulombs and a current of a few kiloamperes. In an altitude-triggered lightning the downward negative leader of the bidirectional leader system and the resulting return stroke serve to provide a relatively low-impedance connection between the upward moving positive leader tip and the ground, the processes that follow likely being similar to those in classical triggered lightning. Lightning appears to be able to reduce, via breakdown processes in the soil and on the ground surface, the grounding impedance which it initially encounters at the strike point, so at the time of channel-base current peak the reduced grounding impedance is always much lower than the equivalent impedance of the channel. At close ranges the measured M-component magnetic fields have waveshapes that are similar to those of the channel-base currents, whereas the measured M-component electric fields have waveforms that appear to be the time derivatives of the channel-base current waveforms, in further confirmation of the “two-wave†M-component mechanism.
|
|
| 23. |
- Rakov, Vladimir A., et al.
(författare)
-
On the empirical formula of Willett et al. relating lightning return-stroke peak current and peak electric field
- 1992
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 97:D11, s. 11527-11533
-
Tidskriftsartikel (refereegranskat)abstract
- The empirical formula proposed by Willett et al. (1989) for the estimation of lightning return-stroke peak current, I, from measured peak electric field, E, at a range D, is analyzed and discussed. The formula of Willett et al. (1989), obtained from the regression of E on I, is not the least squares fit and hence is not the best expression for predicting the peak current from the measured peak electric field. Based on the same data, the least squares fit and, hence, the best expression is obtained from the regression of I on E, given by I = 1.5 − 0.037DE, where I is in kA and taken as negative, E is positive and in V/m, and D is in km. The Willett et al. (1989) formula results in an error with respect to the best peak current estimating expression that varies from −15% to +2.6% over the range of peak field values of 1.9 to 11 V/m (normalized to 100 km) used to derive the two relations. When the data of Willett et al. (1989) are separated into a high returnstroke speed group (1.5 × 108 to 1.9 × 108 m/s) and a low return-stroke speed group (1.2 × 108 to 1.4 × 108 m/s), the I-E regression lines differ for the two groups, with the difference in the regression line slopes being statistically significant at the 0.01 significance level. If the difference between intercepts of these two regression lines, found to be statistically insignificant, is neglected, the observed difference in slopes suggests that the group with the higher measured return-stroke speed is associated with a lower peak electric field for the same peak current. Finally, the practical applications of the Willett et al. (1989) formula presently found in the literature are reviewed, and the several cases of improper use, mostly related to misinterpretation of Willett et al. ’s (1989) sign convention, are corrected.
|
|
| 24. |
- Rakov, Vladimir A., et al.
(författare)
-
Review of lightning properties from electric field and TV observations
- 1994
-
Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996 .- 2169-8996. ; 99:D5, s. 10745-10750
-
Tidskriftsartikel (refereegranskat)abstract
- From analysis of simultaneous electric field and TV records of 76 negative cloud-to-ground lightning flashes in Florida, various lightning properties have been determined and several new facets of lightning behavior inferred. Only 17 % of the flashes were single-stroke flashes, less than half the commonly claimed percentage (e.g., Anderson and Eriksson, 1980). The initial electric field peak (and, by inference, current peak) for the only strokes in single-stroke flashes was smaller than for first strokes in multiple-stroke flashes. Half of all flashes, single and multiple stroke, struck ground at more than one point, with the spatial separation between the channel terminations being up to many kilometers. One third of multiple-stroke flashes had at least one subsequent stroke whose distance-normalized initial electric field peak exceeded that of the first stroke in the flash. Thus such flashes are not unusual, contrary to the implication of most lightning protection and lightning test standards. Subsequent strokes of the order of 2 through 4 were more likely to create a new channel termination on ground than strokes of the order of 5 and higher. Further, leaders of lower-order subsequent strokes following previously formed and not-too-aged (100 ms or less) channels were more likely to show stepping, as opposed to continuous propagation (i.e., to be dart-stepped leaders rather than dart leaders), than were leaders of higher-order strokes. Finally, lower-order subsequent return strokes exhibited a larger initial electric field peak than did higher-order strokes. The second leader of the flash (the first subsequent leader) encounters the least favorable propagation conditions of all subsequent strokes: more than half of the second leaders either deflected from the previously formed path to ground or propagated in a stepped, as opposed to a continuous, fashion along the lowest part of that path. It is important to note that interstroke intervals preceding second strokes are similar to or shorter than those preceding higher-order strokes. These observations indicate that channel conditions for the propagation of a subsequent leader are determined not just by the immediately preceding channel heating and cooling processes but rather by the entire channel history. In particular, the status of the channel apparently depends on the number of strokes that have participated in its cumulative conditioning. The overwhelming majority of long continuing currents, those with a duration longer than 40 ms, were initiated by subsequent strokes of multiple-stroke flashes as opposed to either the first stroke in a multiple-stroke flash or the only stroke in a single-stroke flash. Strokes that initiate such long continuing currents were (1) relatively small (in terms of both return-stroke field peak and, as determined from an independent study in New Mexico, stroke charge), (2) followed relatively short interstroke intervals, and (3) showed a tendency to be preceded by a relatively large stroke. Millisecond-scale K and M electric field changes appeared different in terms of both microsecond-scale pulse content and interevent time intervals. Often no microsecond-scale K and M field pulses were detected. When they were present, such pulses were highly variable and sometimes irregular in waveshape, as opposed to the alleged characteristic K-pulse waveform described by Arnold and Pierce (1964), which has been extensively used in atmospheric radio-noise studies. There is a remarkable similarity between many lightning characteristics in Florida and in New Mexico.
|
|
| 25. |
- Raysaha, Rosy Balaram, et al.
(författare)
-
SPECIAL CASE OF LIGHTNING STRIKE TO TALL OBJECTS ON GROUND
- 2010
-
Ingår i: Proceedings of the 30TH International Conference on Lightning Protection.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Analysis of lightning strike to Tall Grounded Objects (TGOs) is of practical importance. Actual measurements on TGOs have shown the presence of reflected current waves originating from either ends of TGO. With regard to the channel dynamics for these waves, there seems to be some uncertainties like: (i) will there be any reflection when these waves approach the wavefront, (ii) what would be their final status on the channel etc. The present work makes an attempt to address these issues using a macroscopic physical model for the lightning return stroke. Results show that waves transmitted from TGO on to the channel approaches the wavefront with higher velocity except near the wavefront. In fact, it gets attenuated and merges with the wavefront without any sign of reflection whatsoever. Both, the non-linear channel dynamics at the wavefront and corona sheath are responsible for this phenomenon. Incidentally, the simulation results also indicate a double initial peaking in the far field, significant reflection at the ground end of TGO and some noticeable induction to TGO well before the arrival of wavefront.
|
|
| 26. |
- Silfverskiöld, Stefan, 1964-, et al.
(författare)
-
Induced Voltages in a Low-Voltage Power Installation Network Due to Lightning Electronmagnetic Fields : An Experimental Study
- 1999
-
Ingår i: IEEE transactions on electromagnetic compatibility (Print). - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9375 .- 1558-187X. ; 41:3, s. 265-271
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents the experimental results on the induced common-mode (CM) voltages in a residential low-voltage power installation (LVPI) network exposed to electromagnetic fields from lightning. The objective of this investigation was to find out the severity of inducedvoltages in a low voltage network due to direct coupling of the lightning electromagnetic fields with the network in the absence of transient overvoltages coming through the distribution lines. Therefore, during the experiment the LVPI network was disconnected from the distribution network to exclude conducted transients in the measured signals entering through the mains. The induced CM voltages in a power outlet of the network were measured simultaneously with the vertical component of the electric field near the installation. The sequence of induced voltage events during the complete duration of one negative cloud-to-ground (CG) flash, one positive CG flash, and one cloud flash (CC), thought to be typical, is presented simultaneously with the electric field. It is shown that in addition to the return strokes in ground flashes, other discharge events taking place in the cloud; notably, the bipolar pulse trains associated with the initiation of both CG lightning and cloud lightning, are also important in determining the transient environment of electronic devices connected to a LVPI network. Flashes at a distance of about 25 km produced many induced-voltage pulses as large as 100 V, six such pulses in a time period of 0.4 ms in a negative CG flash, and 11 such pulses in a time period of 3 ms in a CC that immediately followed the ground flash.
|
|
| 27. |
- Theethayi, Nelson, et al.
(författare)
-
On the influence of conductor heights and lossy ground in multi-conductor transmission lines for lightning interaction studies in railway overhead traction systems
- 2004
-
Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 71:2, s. 186-193
-
Tidskriftsartikel (refereegranskat)abstract
- Railway overhead traction system is a classic example of scattered conductor configuration, where tracks and other wires form multi-conductor transmission lines (MTLs) with large variation in conductor heights above ground and they are spread across regions having different soil conditions. Lightning transient analysis in such systems has not received much attention earlier. Here we analyze the influence of conductor heights and lossy ground on the induced voltages in a two conductor MTLs for the case of a direct lightning strike. For transient analysis, modified time domain transient ground impedance expressions having better early and late time behavior was used. The dependence of transient ground impedance on conductor heights and ground resistivity are presented and discussed. The early time transient ground impedances are unaffected by ground resistivity but their decaying nature is highly dominated by ground resistivity. It is found, if one of the conductors is close to ground (a rail) and if it is at large vertical distance from struck conductor (an auxiliary power line), then with increasing ground resistivity the peak induced voltages in the conductor close to ground initially increase, then decrease and finally tend to remain constant (within 100-10,000Ωm). This phenomenon is opposite to that compared to conductors that are close to each other with minimum vertical separation (two auxiliary power lines), where the peak induced voltages increase with increasing ground resistivity. The study focuses mainly to access when a mutual coupling due to system geometry or due to ground losses becomes dominant in determining induced effects from lightning in MTLs, which could be an important contribution to the lightning interaction studies for electrified railway systems.
|
|
| 28. |
|
|
| 29. |
|
|
| 30. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Comment on ‘’Radio frequency radiation beam pattern of lightning return strokes : A revisit to theoretical analysis” by Xuan-Min Shao, Abram R. Jacobson, and T. Joseph Fitzgerald
- 2005
-
Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 110:D24
-
Tidskriftsartikel (refereegranskat)abstract
- In summary, SJF present a new approach to deriving a radiation electric field equation for the TL model when there is no current discontinuity at the return stroke front. This approach (see their equation (11)) is identical, except for the SJF sign error, to that found in the literature (e.g., equation (47) of TUR). However, their electric field equation (10), asserted to be general, is incapable of handling models with current discontinuity (intrinsic in some models) at the return stroke front, and their electric field equation (12) for the TCS model is incomplete. The correct equation for the TCS model is given by TUR (see their equation (49)). A truly general electric field equation that is valid for any model and for any distance to the field point is equation (7) of Thottappillil and Rakov [2001b].
|
|
| 31. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Comparison of lightning return-stroke models
- 1993
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 98:D12, s. 22903-22914
-
Tidskriftsartikel (refereegranskat)abstract
- Five return-stroke models, each allowing the use of measured channel-base current and return-stroke speed as inputs for the computation of channel current distribution and remote electric field, are compared and evaluated using 18 sets of three simultaneously measured triggered lightning features: channel-base current, return-stroke speed, and electric field at a distance of about 5 km from the channel base. The experimental data were acquired during a triggered lightning experiment at the NASA Kennedy Space Center, Florida, in 1987 and were reported in part by Willett et al. (1989) and Leteinturier et al. (1991). The models compared are the transmission line (TL) model, the modified transmission line (MTL) model, the traveling current source (TCS) model, the Diendorfer-Uman (DU) model, and the modified Diendorfer-Uman (MDU) model. The TL, MTL, DU, and MDU models each predict the measured initial electric field peaks with an error whose mean absolute value is about 20%; the TCS model has a mean absolute error about twice that value. For the prediction of overall measured field wave shape, none of the models is clearly preferred, although for the model parameters assumed, the MDU model gave the best wave shape match. Most of the return strokes that exhibited very narrow sharp initial peaks in the measured electric field waveforms had a maximum rate of rise of channel-base current closer to the peak of the measured channel-base current waveform than did return strokes not exhibiting these sharp field peaks. The calculated fields from the TL and the MTL models do not have narrow sharp peaks similar to those found in many of the measured fields, while the fields calculated from the TCS, DU, and MDU models had somewhat similar peaks in most of the cases where those peaks were found in the measured fields. On the basis of the comparison of the five models, we recommend the TL model for calculating the peak channel-base current from the measured initial peak electric field because the TL model provides a similar or better result from a simpler mathematical relation.
|
|
| 32. |
- Thottappillil, Rajeev, 1958-
(författare)
-
Computation of electromagnetic fields from lightning discharge
- 2014. - 2
-
Ingår i: The Lightning Flash. - London : Institution of Engineering and Technology. - 9781849196918 - 9781849196925 ; , s. 351-403
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Electromagnetic fields from lightning can couple to electrical systems and produce transient overvoltages, which can cause power and telecommunication outages and destruction of electronics. Knowledge of electromagnetic fields at high altitudes produced by return strokes in cloud-to-ground (CG) lightning are required in the study of transient luminous events in the mesosphere. Electric and magnetic field pulses from various electrical breakdown events in the lightning are used in detecting and locating lightning flashes. Therefore, calculation of the electric and magnetic fields from different lightning processes has several practical applications. In this chapter, expressions for electric and magnetic fields are derived for charge and current configurations applicable to lightning. In general lightning currents and charges vary with time. First, simple expressions for non-time varying cases are presented. Then, electric and magnetic field expressions from time-varying lightning sources are presented.
|
|
| 33. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Distribution of charge along the lightning channel: Relation to remote electric and magnetic fields and to return stroke models
- 1997
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 102, s. 6987-7006
-
Tidskriftsartikel (refereegranskat)abstract
- We derive exact expressions for remote electric and magnetic fields as a function of the time‐ and height‐varying charge density on the lightning channel for both leader and return‐stroke processes. Further, we determine the charge density distributions for six return‐stroke models. The charge density during the return‐stroke process is expressed as the sum of two components, one component being associated with the return‐stroke charge transferred through a given channel section and the other component with the charge deposited by the return stroke on this channel section. After the return‐stroke process has been completed, the total charge density on the channel is equal to the deposited charge density component. The charge density distribution along the channel corresponding to the original transmission line (TL) model has only a transferred charge density component so that the charge density is everywhere zero after the wave has traversed the channel. For the Bruce‐Golde (BG) model there is no transferred, only a deposited, charge density component. The total charge density distribution for the version of the modified transmission line model that is characterized by an exponential current decay with height (MTLE) is unrealistically skewed toward the bottom of the channel, as evidenced by field calculations using this distribution that yield (1) a large electric field ramp at ranges of the order of some tens of meters not observed in the measured electric fields from triggered‐lightning return strokes and (2) a ratio of leader‐to‐return‐stroke electric field at far distances that is about 3 times larger than typically observed. The BG model, the traveling current source (TCS) model, the version of the modified transmission line model that is characterized by a linear current decay with height (MTLL), and the Diendorfer‐Uman (DU) model appear to be consistent with the available experimental data on very close electric fields from triggered‐lightning return strokes and predict a distant leader‐to‐return‐stroke electric field ratio not far from unity, in keeping with the observations. In the TCS and DU models the distribution of total charge density along the channel during the return‐stroke process is influenced by the inherent assumption that the current reflection coefficient at ground is equal to zero, the latter condition being invalid for the case of a lightning strike to a well‐grounded object where an appreciable reflection is expected from ground.
|
|
| 34. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Electric and magnetic fields from a semi-infinite antenna above a conducting plane
- 2004
-
Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 61:3-4, s. 209-221
-
Tidskriftsartikel (refereegranskat)abstract
- The electric and magnetic field structures around a semi-infinite thin-wire antenna vertically placed above a perfectly conducting ground plane are investigated when the antenna is supporting two different types of sources. It is shown that when the wire is carrying a uniform line charge, the electrostatic potentials are equal on the surfaces of imaginary cones of fixed cone angles with axis along the wire and apex at the conducting plane. The electrostatic field vectors are shown to be perpendicular to the imaginary cones and tangential to the meridian lines of half-spherical shells centered at the base of the line charge. The vertical components of the electrostatic field on the surface of these imaginary half-spherical shells of a given radius are constant, except at the wire itself. The magnetic field structure associated with a constant current in the semi-infinite antenna is that of an infinite wire. The electric and magnetic fields due to a time-varying charge or current pulse propagating with the speed of light along the vertical thin-wire antenna have a spherical transverse electromagnetic (TEM) field structure, identical to that for the case of a uniform line charge and a uniform current. The connection between the static and dynamic solutions is derived mathematically using two different approaches.
|
|
| 35. |
- Thottappillil, Rajeev, 1958-
(författare)
-
Electromagnetic pulse environment of cloud-to-ground lightning for EMC studies
- 2002
-
Ingår i: IEEE transactions on electromagnetic compatibility (Print). - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9375 .- 1558-187X. ; 44:1, s. 203-213
-
Tidskriftsartikel (refereegranskat)abstract
- The characteristics of lightning electromagnetic pulse (LEMP) from individual processes in lightning such as return strokes, preliminary breakdown pulses, pulses associated with the leader process, K-changes and M-changes, the isolated narrow bipolar pulses, and the pulse bursts have been known. However, there is a need for a combined characterization of the total LEMP environment created during the entire duration of lightning. An attempt is made to provide such a description that gives the distribution of the LEMP characteristics, which is important in its ability to interfere with electronic systems, such as peak amplitude, peak time derivative, pulse duration, number of pulses, and time interval between pulses, during the entire duration of a cloud-to-ground lightning. Separate electromagnetic environment characterizations for negative cloud-to-ground lightning and positive cloud-to-ground lightning are proposed, including all the significant LEMP sources. Based on the LEMP characterization, models for negative and positive cloud-to-ground lightning flashes, that could be used in electromagnetic compatibility (EMC) studies are proposed. The electromagnetic environment models for cloud-to-ground lightning can be coupled to the locations of lightning given by the lightning location system to give comprehensive information about the electromagnetic environment at a desired geographical location.
|
|
| 36. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Extension of the Diendorfer-Uman lightning return stroke model to the case of a variable upward return stroke speed and a variable downward discharge current speed
- 1991
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 96:D9, s. 17143-17150
-
Tidskriftsartikel (refereegranskat)abstract
- A new lightning return stroke model has recently been proposed by Diendorfer and Uman (1990). In this model, if one specifies a current at the channel base (ground), a constant return stroke speed, and a channel discharge time constant, one can derive analytically the current and charge as a function of time and height associated with the channel above ground. Here we present a more general and more straightforward derivation of the Diendorfer-Uman model. In the new formulation we allow a variable return stroke speed that can be any arbitrary function of height. The influence of a decrease in speed with height, as occurs in nature, on the channel current and charge distributions and on the radiated electric and magnetic fields is determined and compared with the constant speed case. For a given channel base current, a decreasing speed with height does not change the initial peak electric and magnetic fields appreciably from the values found for a constant speed having the same value as the variable speed at the channel base, but a decreasing speed can cause considerable changes in field waveshapes for both the distant radiation fields and the near electrostatic fields. In addition to allowing an arbitrary return stroke speed, the new formulation of the Diendorfer-Uman model allows the downward propagating current waves released by the return stroke front also to have an arbitrary variable speed, whereas the original model assumed that speed to be constant at the speed of light.
|
|
| 37. |
|
|
| 38. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
K and M changes in close lightning ground flashes in Florida
- 1990
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 95:D11, s. 18631-18640
-
Tidskriftsartikel (refereegranskat)abstract
- Electric field changes produced by K and M processes in Florida lightning ground flashes at distances within 12 km are analyzed and compared. The geometric-mean time durations were similar: 0.7 ms for 135 K changes and 0.9 ms for 80 M changes. The geometric-mean time intervals between events were dissimilar: 12.5 ms for 93 K change intervals and 2.1 ms for 48 M change intervals. Our geometric mean K change interval is about twice the median value observed by Kitagawa et al. (1962) in New Mexico who used a somewhat different measurement technique, while our geometric mean M change interval is about one-third the New Mexico median value although Kitagawa et al. (1962) apparently excluded from their statistical data those M components occurring within 15 ms of the return stroke initial peak field, and we included these. The K electric field changes were not always simple ramps superimposed on the slower interstroke J or final F field changes, but, rather, could be grouped into four general waveshape categories. All M field changes had relatively similar hooked shapes. We argue that the hypothesis proposed by Kitagawa et al. (1962) that K and M processes are essentially the same physical phenomenon is not supported by our data, although it cannot be positively ruled out; and, additionally, that the conclusion of Kitagawa and Brook (1960) that the net interstroke field change (J change) is essentially the sum of the individual K changes is also not supported by our data. Further, our data suggest that there is a significantly higher upper limit for the moment change associated with K processes in ground flashes than the 1 to 2 C-km reported by Brook and Kitagawa (1960) and Ogawa and Brook (1964).
|
|
| 39. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Lightning return stroke model with height-variable discharge time constant
- 1994
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 99:D11, s. 22773-22780
-
Tidskriftsartikel (refereegranskat)abstract
- A new lightning return stroke model is proposed in which the lightning channel, previously charged by the leader, is exponentially discharged with the discharge time constant being a general function of height. The new model is both more general and more physically reasonable than the Diendorfer-Uman model (Diendorfer and Uman, 1990) and its modification by Thottappillil et al. (1991). In the new model a single height-variable discharge time constant is used instead of the two height-invariant discharge time constants used for natural lightning in the Diendorfer-Uman model. As an illustration of the use of the new model, the measured electric fields for three triggered lightning return strokes about 5-km distant are compared with the fields calculated from the model, using simultaneously measured channel base current and return stroke speed as inputs to the model.
|
|
| 40. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Lightning subsequent-stroke electric field peak greater than the first stroke peak and multiple ground terminations
- 1992
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 97:D7, s. 7503-7509
-
Tidskriftsartikel (refereegranskat)abstract
- For 46 multiple-stroke flashes in which each stroke ground termination was located using a TV camera network and thunder ranging, 15 flashes (33%) had one or more subsequent return strokes whose initial electric field peak normalized to 100 km was greater than the first-stroke field peak of the flash. In 9 of these 15 flashes the subsequent strokes with field peaks greater than the first stroke followed the same channel as the first stroke; in five flashes the subsequent strokes with the greater peaks followed a different channel to ground; and in one flash the subsequent strokes with the greater peaks occurred both in the first-stroke channel and in a different channel. The interstroke intervals immediately preceding the 13 larger subsequent strokes that followed the first-stroke channel had a geometric mean (GM) duration of 98 ms, 1.7 times greater than the GM of 57 ms for all 199 interstroke intervals (46 flashes) without any selection. Eight of the 13 larger subsequent strokes for which leader durations were measurable had a GM leader duration of 0.55 ms, 3.3 times smaller than the GM of 1.8 ms for 117 subsequent leaders with measurable duration in a previously formed channel of the 46 multiple-stroke flashes. For the six larger subsequent strokes that created a new channel to ground, the preceding interstroke interval had a GM of 130 ms, and the leader duration had a GM of 15 ms. No subsequent stroke with peak field exceeding the first in any category had a preceding interstroke interval less than 35 ms. Analysis of direct current measurements from Switzerland shows that subsequent-stroke currents exhibit many features similar to those of Florida subsequent-stroke electric fields. In 22 Florida single-stroke and multiple-stroke ground flashes the distances between multiple channel terminations in a given flash (33 measurements) ranged from 0.3 km to 7.3 km, with a GM of 1.7 km.
|
|
| 41. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
On different approaches to calculating lightning electric fields
- 2001
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 106:D13, s. 14191-14205
-
Tidskriftsartikel (refereegranskat)abstract
- Three different approaches to the computation of lightning electric fields are compared. These approaches are the traditional dipole (Lorentz condition) technique and two versions of the monopole (continuity equation) technique. The latter two techniques are based on two different formulations of the continuity equation, one used by Thottappillil et al. [1997] and the other by Thomson [1999], the difference between the formulations being related to different treatments of retardation effects. The three approaches involve the same expression for the vector potential but different expressions for the scalar potential. It is analytically shown that the three different expressions for the scalar potential are equivalent and satisfy the Lorentz condition. Further, the three approaches yield the same total fields and the same Poynting vectors. However, expressions in the three approaches for the individual electric field components in the time domain, traditionally identified by their distance dependence as electrostatic, induction, and radiation terms, are different, suggesting that explicit distance dependence is not an adequate identifier. It is shown that the so identified individual field components in the electric field equation in terms of charge density derived by Thottappillil et al. [1997] are equivalent to the corresponding field components in the traditional equation for electric field in terms of current based on the dipole technique. However, the individual field components in the electric field equation based on Thomson’s [1999] approach are not equivalent to their counterparts in the traditional dipole technique equation. Further, in Thottappillil et al.’s [1997] technique and in the traditional dipole technique, the gradient of scalar potential contributes to all three electric field components, while in Thomson’s [1999] technique it contributes only to the electrostatic and induction components. Calculations of electric fields at different distances from the lightning channel show that the differences between the corresponding field components identified by their distance dependence in different techniques are considerable at close ranges but become negligible at far ranges.
|
|
| 42. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Properties of M components from currents measured at triggered lightning channel base
- 1995
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 100:D12, s. 25711-25720
-
Tidskriftsartikel (refereegranskat)abstract
- Channel base currents from triggered lightning were measured at the NASA Kennedy Space Center, Florida, during summer 1990 and at Fort McClellan, Alabama, during summer 1991. An analysis of the return stroke data and overall continuing current data has been published by Fisher et al. [1993]. Here an analysis is given of the impulsive processes, called M components, that occur during the continuing current following return strokes. The 14 flashes analyzed contain 37 leader-return stroke sequences and 158 M components, both processes lowering negative charge from cloud to ground. Statistics are presented for the following M current pulse parameters: magnitude, rise time, duration, half-peak width, preceding continuing current level, M interval, elapsed time since the return stroke, and charge transferred by the M current pulse. A typical M component in triggered lightning is characterized by a more or less symmetrical current pulse having an amplitude of 100–200 A (2 orders of magnitude lower than that for a typical return stroke [Fisher et al., 1993]), a 10–90% rise time of 300–500 Όs (3 orders of magnitude larger than that for a typical return stroke [Fisher et al., 1993]), and a charge transfer to ground of the order of 0.1 to 0.2 C (1 order of magnitude smaller than that for a typical subsequent return stroke pulse [Berger et al., 1975]). About one third of M components transferred charge greater than the minimum charge reported by Berger et al. [1975] for subsequent leader-return stroke sequences. No correlation was found between either the M charge or the magnitude of the M component current (the two are moderately correlated) and any other parameter considered. M current pulses occurring soon after the return stroke tend to have shorter rise times, shorter durations, and shorter M intervals than those which occur later. M current pulses were observed to be superimposed on continuing currents greater than 30 A or so, with one exception out of 140 cases, wherein the continuing current level was measured to be about 20 A. The first M component virtually always (one exception out of 34 cases) occurred within 4 ms of the return stroke. This relatively short separation time between return stroke and the first M component, coupled with the observation of Fisher et al. [1993] that continuing currents lasting longer than 10 ms never occur without M current pulses, implies that the M component is a necessary feature of the continuing current mode of charge transfer to ground.
|
|
| 43. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
-
Protection against lightning surges
- 2009
-
Ingår i: Lightning Protection. - London : Institution of Engineering and Technology. - 9780863417443 - 9781849191067 ; , s. 269-305
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
|
|
| 44. |
|
|
| 45. |
|
|
| 46. |
- Zhou, Helin, et al.
(författare)
-
A New Approach To Estimation Of Effective Height Of Towers On Mountain Tops For Lightning Incidence Studies : Sensitivity Analysis
- 2009
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, we review and evaluate the definitions and methods that could be used to estimate the effective height of a given tower on mountaintop based on the statistical observations . We derive the effective heights based on Rizk’s lightning attachement model, which are less than those predicted by the earlier methods based on statistical observations. Then we perform sensitivity analysis to evaluate the effect of uncertainties in model parameters that influence the effective height. Variations in the effective height as a function of model parameters, including the final quasi-stationary leader gradient, minimum positive streamer gradient, upward connecting positive leader speed, and mountain base radius, are presented, with Gaisberg tower as the example. It’s found that the effective height depends primarily on the structre height, mountain shape and upward positive leader speed. This new approach presented here can be employed to estimate the effective height for towers for which no lightning incidence data needed for the earlier methods are available. And the information could be also used in designing lightning protection of communication/transmission line towers and masts on mountain tops.
|
|
| 47. |
|
|
| 48. |
- Zhou, Helin, et al.
(författare)
-
Continuing Current in Tower-Initiated Lightning
- 2010
-
Ingår i: 2010 30th International Conference on Lightning Protection, ICLP 2010. - : Institute of Electrical and Electronics Engineers (IEEE).
-
Konferensbidrag (refereegranskat)abstract
- We investigate 172 upward-initiated negative lightning flashes containing at least one leader-return stroke sequence observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. A geometric mean value of 8.75 kA of the return stroke peak current is determined, followed by a continuing current of 1.48 ms with a charge transfer of 0.786 C. We find that in case of the continuing current duration larger than 40 ms, usually defined as long continuing current, or when the continuing current charge transfer is greater than 5 C, the initiating peak return stroke current did not exceed 20 kA. Moreover, continuing currents with durations of less than 20 ms or with a charge transfer smaller than 5 C, those continuing currents were initiated by return strokes with peaks of any range.
|
|
| 49. |
- Zhou, Helin, et al.
(författare)
-
Simultaneous current and electric field observations of upward negative leaders initiated from the Gaisberg Tower
- 2010
-
Ingår i: 2010 Asia-Pacific Symposium on Electromagnetic Compatibility, APEMC 2010. - 9781424456215 ; , s. 1174-1177
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, we present simultaneous current and electric field records of two upward negative leaders initiated from the Gaisberg Tower (GBT). It is shown that bipolar current pulses occur at the beginning of the upward negative leaders followed by regular unipolar leader pulse trains, and the corresponding electric field exhibits asymmetrical V-shaped pulses with a hump superimposed at the end of the pulse. The stepped leader characteristics in electric field waveforms at close range are similar to that of downward stepped leader pulses in altitude-triggered lightning flashes. Distinct electric field changes prior to the upward negative leader inception are indicative of nearby lightning discharges.
|
|
| 50. |
- Zhou, Helin, et al.
(författare)
-
Upward Bipolar Lightning Flashes Observed at the Gaisberg Tower
- 2010
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We analyze current records for 21 natural upward-initiated bipolar lightning flashes observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. Thirteen (62%) of 21 bipolar flashes occurred in non-convective season (September- March) and twelve (57%) of them occurred in seasonal transmission periods of March, August, and November in Austria. Thirteen (62%) of them belong to Type 1 associated with a polarity reversal during the initial stage current, based on the classification as suggested by Rakov and Uman [1]. We also find that the initial polarity reversal from negative to positive occurs more often (16 of 21) than that from positive to negative within a bipolar flash, in agreement with observations in other studies. The geometric mean total absolute charge transfer is 99.5 C with a relatively short total duration of 320 ms.
|
|
