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- Liu, Yaqing, et al.
(författare)
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An Improved Transmission-Line Model of Grounding System
- 2001
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Ingår i: 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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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.
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- Liu, Yaqing, et al.
(författare)
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Investigating the validity of existing definitions and empirical equations of effective length/area of grounding wire/grid for transient studies
- 2007
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Ingår i: Journal of Electrostatics. - : Elsevier BV. - 0304-3886 .- 1873-5738. ; 65:5-6, s. 329-335
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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.
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- Liu, Yaqing, et al.
(författare)
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The Residual Resistivity in Soil Ionization Region Around Grounding System for Different Experimental Results
- 2003
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Ingår i: IEEE Symposium on Electromagnetic Compatibility. Symposium Record. - 0780378350 ; , s. 794-799
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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.
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- Liu, Yaqing, 1969-
(författare)
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Transient Response of Grounding Systems Caused by Lightning: Modelling and Experiments
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In order to achieve better lightning protection and electromagnetic compatibility (EMC) requirements, the needs for a proper grounding system and the knowledge of its transient behaviour become crucial. The present work is focused towards developing engineering models for transient analysis of grounding system with sufficient accuracy and simplicity for lightning studies. Firstly, the conventional uniform transmission line approach for a single grounding conductor is modified and extended to grounding grids. Secondly, in order to overcome the drawbacks of all the existing transmission line approaches, for the first time, a non-uniform transmission line approach is developed for modelling the transient behaviour of different types of grounding systems. The important feature of such an approach is in its capability to include the electromagnetic couplings between different parts of the grounding system using space and time dependent per-unit length parameters. High voltages and currents induced in the grounding systems due to lightning always produce ionization in the soil. This phenomenon should be included during the transient analysis of grounding systems. In the present work, an improved soil ionization model including residual resistivity in ionization region is developed. The fact that there exists residual resistivity in ionization region (7 % of the original soil resistivity) can be proved by the experiments reported in the literature and the experiments carried out at the high voltage lab of Uppsala University. The advantage of including residual resistivity is that the beneficial influence of soil ionization in reducing the potential rise of grounding system will not be overestimated, especially in high resistivity soil. Finally, the transmission line approaches are adopted for studying the response of grounding systems due to lightning for different applications. These are, influence of soil parameters on the transient behaviour of grounding systems, transient analysis of grounding structures in stratified soils, investigation of the validity of existing definitions for effective length/area of different grounding structures, current distribution in the shields of under ground cables associated with communication tower, and influence of insulator flashover and soil ionization around the pole footing on surge propagation in Swedish railway system.
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| 14. |
- Ren, Luyao, et al.
(författare)
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Quartet DNA reference materials and datasets for comprehensively evaluating germline variant calling performance
- 2023
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Ingår i: Genome Biology. - : BioMed Central (BMC). - 1465-6906 .- 1474-760X. ; 24:1
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: Genomic DNA reference materials are widely recognized as essential for ensuring data quality in omics research. However, relying solely on reference datasets to evaluate the accuracy of variant calling results is incomplete, as they are limited to benchmark regions. Therefore, it is important to develop DNA reference materials that enable the assessment of variant detection performance across the entire genome.RESULTS: We established a DNA reference material suite from four immortalized cell lines derived from a family of parents and monozygotic twins. Comprehensive reference datasets of 4.2 million small variants and 15,000 structural variants were integrated and certified for evaluating the reliability of germline variant calls inside the benchmark regions. Importantly, the genetic built-in-truth of the Quartet family design enables estimation of the precision of variant calls outside the benchmark regions. Using the Quartet reference materials along with study samples, batch effects are objectively monitored and alleviated by training a machine learning model with the Quartet reference datasets to remove potential artifact calls. Moreover, the matched RNA and protein reference materials and datasets from the Quartet project enables cross-omics validation of variant calls from multiomics data.CONCLUSIONS: The Quartet DNA reference materials and reference datasets provide a unique resource for objectively assessing the quality of germline variant calls throughout the whole-genome regions and improving the reliability of large-scale genomic profiling.
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| 15. |
- Theethayi, Nelson, et al.
(författare)
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A theoretical study on the consequence of a direct lightning strike to electrified railway system in Sweden
- 2005
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Ingår i: Electric Power System Research. - : Elsevier BV. ; 74:2, s. 267-280
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Tidskriftsartikel (refereegranskat)abstract
- Direct lightning strike to a single-track electrified railway system in Sweden is modeled in this paper. Using this model, the induced voltages in each of the nine conductors at heights varying from 0.5 m (tracks) to 10 m above the ground are estimated. The effect of the finitely conducting ground is included using a time domain expression for the transient ground impedance that has better early time and late time behavior. The main interconnection between the conductors and the flashover strength of the supporting insulators is included in the simulations. A simple model for the arc channel during flashover of the insulators and the ionization of the soil around the pole foundations is also included in the model to assess the possible realistic surge voltage distribution in the system. It is shown in the paper that finite ground conductivity, interconnections between the conductors, arcing phenomena of insulation flashover and grounding of the poles decide the voltage/current distribution in the conductors. Simulations have been also carried out to determine the voltages on the lines and across the rails as function of distance from the point of strike as it could be a necessary data for deciding the possible future protection schemes. It was found that for a lightning stroke of 31 kA peak, large common mode and differential mode surges exist on the lines which could create excessive voltages between the line and neutral of the transformer and might pose a threat to the various low voltage equipments used for telecommunication, signaling and control.
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| 16. |
- Theethayi, Nelson, et al.
(författare)
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Experimental Investigation of Lightning Transients Entering a Swedish Railway Facility
- 2007
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Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 22:1, s. 354-363
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Tidskriftsartikel (refereegranskat)abstract
- Transients caused by lightning in railway facilities have not received much attention. In this paper, we describe the measurements of lightning transients entering a Swedish railway facility during the summer of 2003. The measurements of the transients were made in a technical house that provides an uninterrupted power supply for telecommunication systems and the signal systems. An analysis of the data has shown that transients in excess of 7 kV (peak to peak) can appear across the line-to-neutral supply system due to an indirect lightning strike. Some typical characteristics of the line-to-neutral transient voltages in terms of stroke locations and stroke amplitudes are presented. Further, from the experimental data, an empirical relation for predicting the line-to-neutral transient voltage in terms of stroke location and stroke current amplitude is obtained. Simple induced voltage calculations are presented to identify the levels of induced voltages appearing at the input of the technical house. The influence of ground conductivity on those induced voltages is also presented. The information presented in the paper is an important electromagnetic-compatibility issue associated with the lightning protection for railway systems.
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| 17. |
- Theethayi, Nelson, et al.
(författare)
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Important Parameters That Influence Crosstalk in Multiconductor Transmission Lines
- 2007
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Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 77:8, s. 896-909
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Tidskriftsartikel (refereegranskat)abstract
- Transient surges in one of the overhead conductors, due to direct lightning strikes, causes crosstalk [C.R. Paul, Analysis of Multiconductor Transmission Lines, John Wiley & Sons, Inc., 1994; C.R. Paul, Introduction to Electromagnetic Compatibility, John Wiley & Sons, Inc., 1992] in other adjacent conductors. It is a common electromagnetic interference (EMI) phenomenon observed in power lines, communication lines and electrified railway lines. In this paper we investigate the crosstalk in multiconductor transmission lines (MTLs) above finitely conducting ground as a function of ground conductivity, heights of the receptor conductor and the terminal loads. For receptor conductor close to the ground, compared to the emitter conductor [C.R. Paul, Analysis of Multiconductor Transmission Lines, John Wiley & Sons, Inc., 1994; C.R. Paul, Introduction to Electromagnetic Compatibility, John Wiley & Sons, Inc., 1992], the decrease in ground conductivity increases the crosstalk peak currents at near end (end near to the source in the emitter conductor) of the receptor conductor, but at the far end it could either increase or decrease depending upon the line height and ground conductivity.It is found that the ground impedance [J.R. Carson, Wave propagation in overhead wires with ground return, Bell. Sys. Tech. J. 5 (1926) 539–554; Y.J. Wang, S.J. Liu, A review of methods for calculation of frequency dependant impedance of overhead power transmission lines, Proc. Natl. Sci. Conc. ROC (A), 25 (6), (2001) 329–338; E.D. Sunde, Earth conduction effects in transmission systems, 1st ed., Dover Publications Inc., New York, 1968; A. Deri, G. Tevan, A. Semlyen, A. Castanheira, The complex ground return plane a simplified model for homogenous & multilayer earth return, IEEE Trans. PAS 100 (8) (1981) 3686–3693; K.C. Chen, K.M. Damrau, Accuracy of approximate transmission line formulas for overhead wires, IEEE Trans. EMC 31 (4) (1989) 396–397; A. Semlyen, Ground return parameters of transmission lines an asymptotic analysis for very high frequencies, IEEE Trans. PAS 100 (3) (1981) 1031–1038; E.F. Vance, Coupling to Cable Shields, Wiley Interscience, New York, 1978; J.R. Wait, Theory of wave propagation along a thin wire parallel to an interface, Radio Sci. 7 (6) (1972) 675–679; R.G. Olsen, J.L. Young, D.C. Chang, Electromagnetic wave propagation on a thin wire above earth, IEEE Trans. Anten. Propag. 48 (9) (2000) 1413–1418; M. D’Amore, M.S. Sarto, Simulation models of a dissipative transmission line above a lossy ground for a wide-frequency range. I. Single conductor configuration, IEEE Trans. EMC 38 (2) (1996) 127–138; M. D’Amore, M.S. Sarto, Simulation models of a dissipative transmission line above a lossy ground for a wide-frequency range. II. Multiconductor configuration, IEEE Trans. EMC 38 (2) (1996) 139–149; F. Rachidi, C.A. Nucci, M. Ianoz, C. Mazzetti, Influence of lossy ground on lightning induced voltages on overhead lines, IEEE Trans. EMC 38 (3) (1996) 250–264; F. Rachidi, C.A. Nucci, M. Ianoz, Transient analysis of multiconductor lines above a lossy ground, IEEE Trans. Power Deliv. 14 (1) (1999) 294–302; F.M. Tesche, M.V. Ianoz, T. Karlsson, EMC Analysis Methods and Computational Models, John Wiley and Sons Inc., 1997; A.K. Agrawal, H.J. Price, S.H. Gurbaxani, Transient response of multiconductor transmission lines excited by a nonuniform electromagnetic field, IEEE Trans. EMC 22 (2) (1980) 119–129] has profound influence in all the crosstalk cases studied here. Hence, a brief review and comparison of different closed form ground impedance expressions under the limits of transmission line approximation [F.M. Tesche, M.V. Ianoz, T. Karlsson, EMC Analysis Methods and Computational Models, John Wiley and Sons Inc., 1997] and its behavior at both high and low frequencies is presented. It is shown that low frequency approximation of ground impedance is not sufficient for lightning transient studies involving ground conductivities lower than 10 mS/m. The observations presented in the paper have important implications in EMI studies of large distributed outdoor systems, such as the railway network, subjected to lightning strikes.
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| 18. |
- Theethayi, Nelson, et al.
(författare)
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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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Ingår i: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 71:2, s. 186-193
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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.
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