| 1. |
- Berg, Marcus, et al.
(author)
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Hydrophobicity estimation of HV polymeric insulating materials. Development of a digital image processing method.
- 2001
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In: IEEE transactions on dielectrics and electrical insulation. - : Institute of Electrical and Electronics Engineers (IEEE). - 1070-9878 .- 1558-4135. ; 8:6, s. 1098-1107
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Journal article (peer-reviewed)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.
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| 2. |
- Liu, Yaqing, et al.
(author)
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An Improved Transmission-Line Model of Grounding System
- 2001
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In: IEEE transactions on electromagnetic compatibility (Print). - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9375 .- 1558-187X. ; 43:3, s. 348-355
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Journal article (peer-reviewed)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.
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| 3. |
- Liu, Y. Q., et al.
(author)
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An improved model for soil ionization around grounding system and its application to stratified soil
- 2004
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In: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 60:2-4, s. 203-209
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Journal article (peer-reviewed)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.
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| 4. |
- Liu, Yaqing, et al.
(author)
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The Residual Resistivity in Soil Ionization Region Around Grounding System for Different Experimental Results
- 2003
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In: IEEE Symposium on Electromagnetic Compatibility. Symposium Record. - 0780378350 ; , s. 794-799
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Conference paper (peer-reviewed)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.
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| 5. |
- Rakov, Vladimir A., et al.
(author)
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M-component mode of charge transfer to ground in lightning discharges
- 2001
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In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 106:D19, s. 22817-22831
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Journal article (peer-reviewed)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.
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| 6. |
- Theethayi, Nelson, et al.
(author)
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On the influence of conductor heights and lossy ground in multi-conductor transmission lines for lightning interaction studies in railway overhead traction systems
- 2004
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In: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 71:2, s. 186-193
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Journal article (peer-reviewed)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.
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| 7. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
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Electric and magnetic fields from a semi-infinite antenna above a conducting plane
- 2004
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In: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 61:3-4, s. 209-221
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Journal article (peer-reviewed)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.
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| 8. |
- Thottappillil, Rajeev, 1958-
(author)
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Electromagnetic pulse environment of cloud-to-ground lightning for EMC studies
- 2002
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In: IEEE transactions on electromagnetic compatibility (Print). - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9375 .- 1558-187X. ; 44:1, s. 203-213
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Journal article (peer-reviewed)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.
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| 9. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
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On different approaches to calculating lightning electric fields
- 2001
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In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 106:D13, s. 14191-14205
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Journal article (peer-reviewed)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.
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