Effect Of Ground Conductivity Research Articles

3-D FDTD Computation of Electromagnetic Fields Associated With Lightning Strikes to a Tower Climbed on a Trapezoidal Mountain

In this paper, we are interested in the study of electromagnetic (EM) field waveforms generated by a lightning strike to a tower located on a trapezoidal mountain. The latter has a height of 500 m and a steepness angle of slope of 45°. The EM field components are calculated at six points and in three directions from the tower. Among these six points, four of them are placed on a flat ground and located at close distances from the mountain bottom while the other two points are located on the mountain at close distances from the tower. Also, because of the three-dimensional (3-D) geometry of the problem being treated, the computation is performed in three dimensions by using the finite-difference time-domain (FDTD) method. In this computation, the lightning channel is represented by the transmission line model extended to include a tall strike object. To evaluate the influence of the presence of mountain on the EM field waveforms, a comparison between the results obtained in the presence and absence of the mountain has been done. On the other hand, the ground conductivity effect on the EM field components has been also studied. At the end of this study, we show that the trapezoidal mountain shadowing effect causes a reduction of the vertical electric field calculated on a flat ground at close distances from the mountain bottom, while the field is enhanced on the top of the mountain. The same conclusion is drawn for the azimuthal magnetic field. Furthermore, the initial peaks of the vertical electric field and the azimuthal magnetic field, above the ground, have been attenuated because of the finite conductivity of ground. The horizontal electric fields above the ground are characterized by oscillations due to the presence of tower and mountain. The underground horizontal electric field and azimuthal magnetic field have the same waveforms as those obtained above the ground but with smaller magnitudes. Thus, the underground vertical electric is initially bipolar and then essentially zero. Finally, the 3-D-FDTD developed program was validated (see the Appendix).

High Frequency Ground Wave Propagation

Electromagnetic ground wave propagation has been a topic of practical interest for broadcasting and underground prospection in the medium and lower radio frequencies for more than a century. A brief review of ground wave propagation and its development since the work of Zenneck and Sommerfeld a century ago is presented. Recently interest has been revived in using ground waves for wireless communication with remote under-populated areas using the high frequency (HF) range which extends from 3-30 MHz. We present numerical results for the fields excited by a vertical electric dipole (VED) on the earth' surface in this range of frequencies and study the effect of ground conductivity on the received fields. In addition, we explore the possibility of using horizontal electric dipoles (HED) as transmitting and receiving dipoles. In this case, it is necessary to elevate both dipoles above the earth's surface by a distance comparable to a wavelength in order to enhance the received signal level. This is practically feasible in the HF regime where the wavelengths are sufficiently short We conclude that elevated HED's can perform as well as the VED, or even better, when the operating frequency lies in the upper end of the HF range.

Effects of Lightning Current and Ground Conductivity on the Values of Vertical Electric Fields

In this paper, the relationship between current front time and front time of vertical electric field due to lightning channel at non perfect ground is considered. Results showed that the peak of simulated vertical electric fields under non perfect ground conductivity condition is decreased compared to the corresponding field at perfect ground while the front time of field is increased at non perfect case compared to the perfect one. Likewise, the effect of ground conductivity on the peak and front time of simulated vertical electric field is considered and the results are discussed accordingly.

Engineering Lightning Return Stroke Models Incorporating Current Reflection From Ground and Finitely Conducting Ground Effects

A return stroke model that incorporates a reflected wave from ground without introducing any current discontinuities at the return stroke front is introduced. The incident current is treated using current generation concepts and the reflected current using current dissipation concepts. It is shown that the effect of the reflected current wave is to cause flattening of close electric field waveforms within about 10 μs. Additionally, it is shown how a return stroke model could be utilized to study the effect of ground conductivity on the return stroke current. The results show that the peak time derivative of current in lightning strokes terminating on poorly conducting ground is significantly lower than in the case of highly conducting ground. The model is also used to predict the spatial variation of return stroke velocity. The results show that the return stroke velocity increases initially, reaches a peak, and then decays with increasing height.

Effect of Ground Conductivity on Radiation Pattern of a Dipole Antenna

This paper aims firstly to develop a theoretical formulation of electromagnet ic fields above the earth surface taking into account the finite and infinite conductivity of the ground- which has the advantages of being reasonably fast to compute, secondly to investigate the effect of ground properties on the electric field intensity. For this purpose we have analyzed radiation pattern for finite and infinite ground conductivity, frequencies and different antenna lengths. The monopole is fed to its base. The current distribution along the portion below ground is the image of the current distribution along the original monopole.The image source for calculating electric field, above the infinitely conducting ground surface is just opposite to that of the original source. The image source for calculating electric field above the finitely conducting ground surface is not just opposite to that of the original source. It is affected by the ground properties. For same frequency and identical antenna length but at different ground conductivity, there exists variation in radiation field. It is observed that at small value of ground conductivity the radiation field is small also the direction of radiation field changes. Furthermore it is obtained that rise of frequency increases the radiation field sharply in a particular direction. Finally we can say that at a fixed antenna length, there exists a minimum frequency for measurable radiation field.

Lightning induced voltages on multiconductor power distribution line

The magnitude and waveshape of the induced voltages on 33 kV, three-phase overhead power distribution lines of both single-circuit and double-circuit configurations due to a nearby lightning stroke have been computed. The effect of ground conductivity and the presence of multiple conductors on the magnitude and waveshape of the induced voltages are also studied. A comparison is made on the induced voltages for a single-conductor overhead line as well as for single- and double-circuit power distribution line configurations. It has been observed that the ground impedance as well as the presence of multiple conductors has a significant influence on the induced voltages. The first peak of the induced voltage at the line termination is found to be less when only ground conductivity is taken into account in the horizontal electric field computation. It is reduced further when ground conductivity is included in the calculation of both the horizontal electric field as well as in the coupling of field to overhead conductor. At the midpoint of the line, the influence of ground conductivity on the induced voltage is such that it increases the first peak. The first peak of induced voltage near the line termination gets reduced in the presence of other conductors in a multiconductor line, whereas it remains almost constant at the midpoint of the line. However, the second peak increases with number of conductors in both cases. It is also observed that the induced voltage on a conductor is reduced if the number of conductors in a power distribution line circuit are increased. In the time-domain coupling equation used for the computation of induced voltage, the ground impedance is usually represented by a convolution integral. The low-frequency approximation used for the transient ground resistance presents singularity as t→0. However, for the solution of the convolution integral a new approximate expression has been used, which circumvents the singularity.

On Two Methods Using Magnetometer-array Data for Studying Magnetic Pulsations

We present some recent progress in interpreting the results from the two powerful methods that analyze the magnetic pulsations observed by ground magnetometer arrays. The first method involves an inversion technique that requires the observations from a chain of magnetometer stations located on the same latitude. This method can estimate several factors that affect the pulsation amplitude on the ground, namely the magnitude of the wave ”event”, the local time distribution, and the ground conductivity effect. By analyzing several months of pulsation data, we present an alternative approach to estimate the statistical values of the ground conductivities. We confirm the conductivity structure obtained by one day of data presented by Chi et al. (1996). We also demonstrated the local time dependence of wave amplitude for different frequency bands using this method. The second technique is the ”the gradient method” that calculates the difference in phase or amplitude of the signals measured at two closely separated stations on the same meridian. This technique has been successful in ”observing” the eigenfrequencies of magnetospheric field lines. Although most studies to date analyze the phase difference in the H -component, we find that the phase difference was in fact largest in the Z-component. In order to interpret such difference quantitatively, we apply the conventional field line resonance theory to the condition in which the ground signals are induced by the Hall currents in the ionosphere. We also demonstrated that the same formulation can be used to understand the characteristics of field line resonance from the observations of phase differences.

Comments on "Calculation of fault rates of overhead power distribution lines due to lightning-induced voltages including the effect of ground conductivity"

The author comments on the original paper, "Calculation of fault rates of overhead power distribution lines due to lightning-induced voltages including the effect of ground conductivity" (H. Hermosillo et al., see ibid., vol. 37, p. 392-99, 1995) and in particular examines the use of the concept of surface impedance to extend the calculation of the electric field E/sub /spl rho// tangential to the plane of a lossy ground, to the near-field zone.

Calculation of fault rates of overhead power distribution lines due to lightning-induced voltages including the effect of ground conductivity

Fault rates in power distribution lines are estimated from probability distributions for induced voltages, obtained through a Monte Carlo simulation of lightning, striking in the proximity of a line over a finitely conductive ground. Waveshapes are presented for various locations relative to the point of interest along the line. The effects of line height, ground conductivity, and return stroke current risetime are analyzed. Results show that the expected number of faults increases for lower ground conductivities.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of ground conductivity on the secondary field of a horizontal current-carrying loop

The radiation field of a circular loop carrying an electric current and located horizontally above a lossy ground is formulated using the dyadic Green's functions. The resulting field vectors are then split into the primary and secondary components. The primary components represent those that would exist in the absence of the ground, leaving the secondary components to represent the contribution of the Earth. For different frequencies and various ground conductivities, the secondary components are determined and compared with the primary ones. The results indicate that in the vicinity of the loop, both within and outside the ground, the secondary fields are small at low frequencies and may be neglected. This provides a simplifying approximation to handle the return response of buried objects, excited by a circular loop located in the air.

EMP Coupling to Power Lines

A transmission line model of coupling to power lines has been developed that shows the effect of ground conductivity, angle of arrival, polarization, and power line geometry on the open-circuit voltage at the end of the line. With this information, and the characteristic impedance of the line, the current and voltage to any termination may be calculated. This theory has been checked with experimental measurements and shows quite good agreement with measured values of current. The measurements were performed on scale models of power lines. The soil conductivity was scaled appropriately. The results show that, although the transmission-line model is not strictly applicable at the highest frequencies of interest, the error introduced by this approximation is less than the experimental error (≈ ± <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> percent). Measurements have also been made of the propagation of pulses down lines showing effects of attenuation, dispersion, junctions, bends, and spurs. Where available, theoretical calculations have been compared to measured results. From these measurements it is concluded that transmission-line models can adequately be used to predict propagation effects.

EMP coupling to power lines

A transmission line model of coupling to power lines has been developed that shows the effect of ground conductivity, angle of arrival, polarization, and power line geometry on the open-circuit voltage at the end of the line. With this information, and the characteristic impedance of the line, the current and voltage to any termination may be calculated. This theory has been checked with experimental measurements and shows quite good agreement with measured values of current. The measurements were performed on scale models of power lines. The soil conductivity was scaled appropriately. The results show that, although the transmission-line model is not strictly applicable at the highest frequencies of interest, the error introduced by this approximation is less than the experimental error ( \approx \pm 10 percent). Measurements have also been made of the propagation of pulses down lines showing effects of attenuation, dispersion, junctions, bends, and spurs. Where available, theoretical calculations have been compared to measured results. From these measurements it is concluded that transmission-line models can adequately be used to predict propagation effects.

Current induced by a plane wave on a thin infinite wire near the earth

The problem of finding the plane-wave induced current on an infinite wire above the earth is discussed. The formal solution to the general case of arbitrary plane-wave incidence angle and arbitrary polarization is presented. An approximate expression for the current is found which is valid over a wider range of parameters encountered in normal applications. This approximate result is compared with a numerical integration of the exact expression. Numerical results are given for the special cases of the plane of incidence containing the wire and the case of the plane of incidence being perpendicular to the wire. The effect of ground conductivity is found to be very significant in some cases when the incident field is near grazing.

ELECTROMAGNETIC FIELDS ABOUT A LOOP SOURCE OF CURRENT

Integral expressions for the electromagnetic field components produced by a horizontal loop, carrying a current [Formula: see text] and placed on or above the surface of an n‐layered half‐space, are deduced in a form such that numerical integration can be performed easily. The expressions are free of approximations and completely general for all frequencies. They are constrained only to the uniformity of current around the transmitting loop. The resulting computed electromagnetic fields are valid for arbitrary values of the electrical parameters σ, μ, and ε. The quasi‐static approximation for the region above the half‐space, wherein the wave equation is replaced by the Laplace equation, can be avoided. Measurements outside the loop constitute induction depth sounding. Induction depth sounding curves of field components and magnetic polarization parameters show good resolution of subsurface layering. In particular, it is suggested that the measurements of tilt angle and/or ellipticity of the magnetic polarization ellipse should be made to determine earth layering because of the rapidity and ease of these measurements in field operation. It is shown that the radius of the loop should, in the general case, be taken into account in theoretical computations. Measurements at the center of the loop constitute central induction sounding. Central induction sounding responses are diagnostic only for layered earth models in which conductivity increases with depth. Measurement of the quadrature part of the vertical magnetic field is particularly promising. Theoretical curves for earth models consisting of one layer overlying a half‐space are given for the quasi‐static case for induction depth sounding, and for the nonquasi‐static (general) case for central induction sounding. In another application, the response from a homogeneous, conductive, magnetic half‐space with the central induction method at low frequencies reveals the feasibility of in‐situ determination of static magnetic permeability. In a final application, it is shown that the effect of ground conductivity should be included in making the normal correction to Turam data whenever the apparent conductivity of the ground is greater than [Formula: see text].