Ground Conductivity Research Articles

Modeling Geoelectric Fields and Geomagnetically Induced Currents Around New Zealand to Explore GIC in the South Island's Electrical Transmission Network

AbstractTransformers in New Zealand's South Island electrical transmission network have been impacted by geomagnetically induced currents (GIC) during geomagnetic storms. We explore the impact of GIC on this network by developing a thin‐sheet conductance (TSC) model for the region, a geoelectric field model, and a GIC network model. (The TSC is composed of a thin‐sheet conductance map with underlying layered resistivity structure.) Using modeling approaches that have been successfully used in the United Kingdom and Ireland, we applied a thin‐sheet model to calculate the electric field as a function of magnetic field and ground conductance. We developed a TSC model based on magnetotelluric surveys, geology, and bathymetry, modified to account for offshore sediments. Using this representation, the thin sheet model gave good agreement with measured impedance vectors. Driven by a spatially uniform magnetic field variation, the thin‐sheet model results in electric fields dominated by the ocean‐land boundary with effects due to the deep ocean and steep terrain. There is a strong tendency for the electric field to align northwest‐southeast, irrespective of the direction of the magnetic field. Applying this electric field to a GIC network model, we show that modeled GIC are dominated by northwest‐southeast transmission lines rather than east‐west lines usually assumed to dominate.

Calibration of the logarithmic-periodic dipole antenna (LPDA) radio stations at the Pierre Auger Observatory using an octocopter

An in-situ calibration of a logarithmic periodic dipole antenna with a frequency coverage of 30 MHz to 80 MHz is performed. Such antennas are part of a radio station system used for detection of cosmic ray induced air showers at the Engineering Radio Array of the Pierre Auger Observatory, the so-called Auger Engineering Radio Array (AERA) . The directional and frequency characteristics of the broadband antenna are investigated using a remotely piloted aircraft carrying a small transmitting antenna. The antenna sensitivity is described by the vector effective length relating the measured voltage with the electric-field components perpendicular to the incoming signal direction. The horizontal and meridional components are determined with an overall uncertainty of 7.4+0.9−0.3% and 10.3+2.8−1.7% respectively. The measurement is used to correct a simulated response of the frequency and directional response of the antenna. In addition, the influence of the ground conductivity and permittivity on the antenna response is simulated. Both have a negligible influence given the ground conditions measured at the detector site. The overall uncertainties of the vector effective length components result in an uncertainty of 8.8+2.1−1.3% in the square root of the energy fluence for incoming signal directions with zenith angles smaller than 60°.

On-Chip Meander Line N-Well Resistor with Shielded Ground Conductor for Q-Factor Improvement

ABSTRACTThe on-chip meander line n-well resistor with a shielded ground conductor inserted parallel between the meander line on the silicon substrate is studied. The new design configuration concept with the shielded ground conductor improved the Q factor compared to those without the shielded ground conductor operating at high frequency. It was confirmed that the design configuration for the shielded ground conductor with a larger coverage area and increasing the number of parallel with the meander signal line was significant in improving the Q-factor performance operating at high frequency. Using this improvement method, the Q-factor value for proposed design meander line n-well structure with shielded ground conductor at 2 µm width in two T shapes has improved around 8% to 9% at 8, 9, and 10 GHz and reduced the parasitic mutual capacitance by 49.35 fF at 10 GHz when compared to the conventional structure of meander line n-well resistors. Shielded ground conductor technique is effective due to reduction of parasitic mutual capacitance. The resulting simulation of the return loss was improved using the shielded ground conductor method, but has poor of insertion loss, due to the skin effect. Overall, it has better performance compared to other designs because it has had significant improvement for Q factor to beyond 4 GHz and better return loss for all the range of frequency.

An improved method for expressing the transient ground resistance of overhead transmission lines by exponential polynomial

In this study, a method is proposed for expressing the transient ground resistance of the overhead transmission lines (TLs) by the exponential polynomial. The relation of the poles due to the rational approximation of the ground resistance between the ground conductivity, permittivity and the line geometry is fully discussed. A concept of ‘normalise poles’ is introduced, which makes it very easy to evaluate the poles according to any ground conductivity, permittivity and line geometrical geometry. A new and easily programmed method is achieved which can overcome the uncertain error caused by the conventional vector fitting method when approximating the ground resistance. Based on this method, a recursive convolution can be achieved for analysing the multi-conductor TLs system, so good efficiency can be obtained. Several examples are provided to demonstrate the validation of the proposed method for evaluating the transient ground resistance with different ground parameters and line geometry, and an application on calculating the lightning induced voltages of the overhead TLs system is also provided.

Configuration Detection of Substation Grounding Grid Using Transient Electromagnetic Method

Optimizing the grounding grid configuration is an effective approach to improve the performance of grounding grid. On the other hand, configuration of grounding grid is necessary for some defect diagnosis method to detect the faults existing in grounding conductors. This paper employs the transient electromagnetic (TEM) method to draw the configuration of a substation grounding grid. The surface magnetic field is recorded by receiver coils above the grounding grid. Based on the regular distribution characteristics of a recorded magnetic field, the grounding conductor is accurately located from the curve of magnetic intensity. The complete process of configuration detection of the grounding grid is described in detail. After fast inversion calculation of the recorded magnetic field, the equivalent resistivity is obtained. The configuration of the grounding grid will be clearly and accurately shown in the contour map and three-dimensional (3-D) surface map of equivalent resistivity. Model study is performed, and a field test is also conducted. The results are in good agreement to demonstrate the effectiveness of TEM method for configuration detection of the substation grounding grid.

Long‐term geomagnetically induced current observations in New Zealand: Earth return corrections and geomagnetic field driver

AbstractTranspower New Zealand Limited has measured DC currents in transformer neutrals in the New Zealand electrical network at multiple South Island locations. Near‐continuous archived DC current data exist since 2001, starting with 12 different substations and expanding from 2009 to include 17 substations. From 2001 to 2015 up to 58 individual transformers were simultaneously monitored. Primarily, the measurements were intended to monitor the impact of the high‐voltage DC system linking the North and South Islands when it is operating in “Earth return” mode. However, after correcting for Earth return operation, as described here, the New Zealand measurements provide an unusually long and spatially detailed set of geomagnetically induced current (GIC) measurements. We examine the peak GIC magnitudes observed from these observations during two large geomagnetic storms on 6 November 2001 and 2 October 2013. Currents of ~30–50 A are observed, depending on the measurement location. There are large spatial variations in the GIC observations over comparatively small distances, which likely depend upon network layout and ground conductivity. We then go on to examine the GIC in transformers throughout the South Island during more than 151 h of geomagnetic storm conditions. We compare the GIC to the various magnitude and rate of change components of the magnetic field. Our results show that there is a strong correlation between the magnitude of the GIC and the rate of change of the horizontal magnetic field (H′). This correlation is particularly clear for transformers that show large GIC current during magnetic storms.

Modeling of UHV and Double-Circuit EHV Transmission-Line Exposure to Direct Lightning Strikes

This paper comprises the first systematic investigation into the factors that render ultra-high voltage and double-circuit extremely high voltage transmission lines more vulnerable to direct lightning strikes by shielding failure. Such lines are typically characterized by one or more of the following features: massive conductor bundles, extreme conductor heights, and exceptionally high operating voltage. The model addresses the effects of the upward connecting leader onset of conductor proximity, ground field due to cloud charges prior to negative leader descent, positive space-charge generation at ground wires, as well as the operating voltage during the positive half cycle. It has been found that such factors have significant effects on the lateral attractive distance of both conductor bundles and ground wires. An agreement between model findings and field experience is found to be generally satisfactory.

A New Method for Fast Evaluation of the Cooray–Rubinstein Formula in Time Domain

The Cooray–Rubinstein formula is most widely used to evaluate the horizontal electric field generated by a lightning return stroke over a lossy ground. In this paper, a new method for fast evaluation of the Cooray–Rubinstein formula in time domain is proposed, by means of approximating the integral form of the modified Bessel function of the first kind using Gauss numerical integral. In the proposed method, the integral form of the modified Bessel function of the first kind is expressed by a summation of exponential functions, and a recursive convolution is achieved, which makes the calculation of the Cooray–Rubinstein formula in time domain much more efficient. Besides, the residues and poles due to the expression of the kernel function K ( s ) can be obtained directly for any ground conductivity and permittivity which makes the method proposed very simple and easy to program. The numerical examples demonstrate the validity and efficiency of the proposed method.

Arsenic in Shallow Aquifers Linked to the Electrical Ground Conductivity: the Mekong Delta Source Example

Arsenic in Shallow Aquifers Linked to the Electrical Ground Conductivity: the Mekong Delta Source Example

CFD prediction of the daytime climate evolution inside a greenhouse taking account of the crop interaction, sun path and ground conduction

The management of the local climate inside a greenhouse is essential to control the plant growth and reach the expected quality level for commercialization. In recent years, Computational Fluid Dynamics (CFD) proved to be an efficient tool to predict the greenhouse climate and therefore could serve as decision-making tool in the next future. Nevertheless, up until now, very few studies included together the sun path, plant interaction with local environment, and ground inertia effects. In addition, validation was generally based on only one or two parameters. The present study aims at analysing the evolution of the climate inside a greenhouse using CFD and taking account of the main heat and mass transfer processes at a daily time scale. A particular attention was paid to the validation of the model, considering air temperature and humidity inside and above the crop, as well as leaf temperature, crop transpiration and ground temperature. The CFD model solved the Reynolds Averaged Navier-Stokes equations, using a k-e closure for turbulence. Specific submodels were established to include the sun path as well as the crop interaction with the local climate. The ground was also meshed to assess its inertia effects. 2D unsteady simulations were launched, considering only cases for which the wind blew perpendicular to the ridge. The validation of the model was undertaken on the basis of experiments conducted for a 100 m2 Venlo-greenhouse with Impatiens pot plants grown on shelves. A typical sunny day of spring in Angers, France, was retained. The model showed its ability to correctly predict the temperature and humidity dynamics inside the greenhouse as well as the leaf temperature and crop transpiration evolutions. Although slight discrepancies were observed for the air temperature and humidity inside the crop, the main trends were correctly predicted.

The comparative analysis of calculating methods for ac impedance of R65 type rails and track 1520 mm gauge in the audio frequency range

Knowledge of the exact AC rail impedance in a wide frequency range is necessary for de-signing and modeling audiofrequency track circuits, determining of the traction return current distribution in rails of high-speed railways, testing new types of vehicles on electromagnetic compatibility with track circuits, etc. The aim of the work is to perform a comparative analysis of the methods for calculating of the AC impedance of the R65 type rails of a track 1520 mm gauge in the audiofrequency range. Results of AC rail impedance measurements and theoretical description of frequency dependence of impedance for wires above lossy ground are briefly reviewed. Mathematical formulation of Carson's method and the complex image method proposed by Deri with co-workers for impedance of transmission lines were represented. The impedance of running rails of R65 type and track of 1520 mm gauge have been calculated by using Carson's method and the complex image method. Due to the insufficient amount of literature data concerning AC impedance measurements for rails of type R65 of 1520 mm gauge, the results of calculations for these rails were compared with the measured values for both rails of type R65, 1520 mm gauge, as well as for rails of type UIC 60 with 1435 mm gauge. Calculated frequency dependencies of the AC impedance of R65 type rails are in good qualitative agreement with literature data for rails UIC60 in frequency range Hz. The results for rails of R65 type and 1520 mm gauge calculated according to Carson's method and complex image method differ from reference data presented in literature, and these differences increase with increasing of frequency. Such behavior may be due to error of calculation methods caused by small height of rails above lossy ground and high electrical conductivity between rails and a ground.

Portable audio magnetotellurics - experimental measurements and joint inversion with radiomagnetotelluric data from Gotland, Sweden

Field setup of an audio magnetotelluric (AMT) station is a very time consuming and heavy work load. In contrast, radio magnetotelluric (RMT) equipment is more portable and faster to deploy but has shallower investigation depth owing to its higher signal frequencies. To increase the efficiency in the acquisition of AMT data from 10 to 300 Hz, we introduce a modification of the AMT method, called portable audio magnetotellurics (PAMT), that uses a lighter AMT field system and (owing to the disregard of signals at frequencies of less than 10 Hz) shortened data acquisition time. PAMT uses three magnetometers pre-mounted on a rigid frame to measure magnetic fields and steel electrodes to measure electric fields. Field tests proved that the system is stable enough to measure AMT fields in the given frequency range. A PAMT test measurement was carried out on Gotland, Sweden along a 3.5 km profile to study the ground conductivity and to map shallow Silurian marlstone and limestone formations, deeper Silurian, Ordovician and Cambrian sedimentary structures and crystalline basement. RMT data collected along a coincident profile and regional airborne very low frequency (VLF) data support the interpretation of our PAMT data. While only the RMT and VLF data constrain a shallow (~20-50 m deep) transition between Silurian conductive (<30 Ωm resistivity) marlstone and resistive (>1000 Ωm resistivity) limestone, the single-method inversion models of both the PAMT and the RMT data show a transition into a conductive layer of 3 to 30 Ωm resistivity at ~80 m depth suggesting the compatibility of the two data sets. This conductive layer is interpreted as saltwater saturated succession of Silurian, Ordovician and Cambrian sedimentary units. Towards the lower boundary of this succession (at ~ 600 m depth according to boreholes), only the PAMT data constrain the structure. As supported by modelling tests and sensitivity analysis, the PAMT data only contain a vague indication of the underlying crystalline basement. A PAMT and RMT joint inversion model reveals all the aforementioned units including the less than 80 m deep limestone and marlstone formations and the conductive sedimentary succession of Silurian, Ordovician and Cambrian units. Our test measurements have proven the PAMT modification to be time saving and easy to set up. However, PAMT data suffer from the same noise disturbances as regular AMT data. Since man-made EM noise can propagate over great distances through resistive underground, PAMT measurements are recommended to be carried out in areas with low resistivity. The PAMT method is proven to be applicable in shallow depth studies, especially in areas where normal AMT measurements are inconvenient and/or too expensive to carry out.

Impact and Improvement Method of the Biological Effects during the Transmission Tower Being Lightning Struck

When lightning strikes the transmission line towers, the lightning current flow through the overhead line shunted along the grounding conductor dispersed in the earth, the lightning current amplitude of tens of thousands of amperes or even hundreds of thousands of amperes, making the local current density is very large, seriously affecting the surrounding biological safety. This paper analyzes that different fish fatalities in fish ponds after lightning strikes the transmission line towers, the typical accident towers were simulated and the correctness of the model was verified by CDEGS. Meanwhile the effects of lightning current on fish were simulated, according to the simulation results that the fish current density is related to body length and the current angle in the electric field. By comparing the simulated results with the fish tolerance limits, we can quantitatively evaluate the fish bioelectromagnetism of the relationship between soil resistivity and fishpond distance. The modification of the external grounding network based on the insulated lead wire can be used to guide the related grounding devices.

Spectral scaling technique to determine extreme Carrington‐level geomagnetically induced currents effects

AbstractSpace weather events produce variations in the electric current in the Earth's magnetosphere and ionosphere. From these high‐altitude atmospheric regions, resulting geomagnetically induced currents (GICs) can lead to fluctuations in ground currents that affect the electric power grid and potentially overload transformers during extreme storms. The most extreme geomagnetic storm on record, known as the 1859 Carrington event, was so intense that ground‐based magnetometers were saturated at high magnetic latitudes. The most reliable, unsaturated observation is the hour resolution data from the Colaba Magnetic Observatory in India. However, higher‐frequency components—fluctuations at second through minute time cadence—to the magnetic field can play a significant role in GIC‐related effects. We present a new method for scaling higher‐frequency observations to create a realistic Carrington‐like event magnetic field model, using modern magnetometer observations. Using the magnetic field model and ground conductivity models, we produce an electric field model. This method can be applied to create similar magnetic and electric field models for studies of GIC effects on power grids.

The interactions between soil–biosphere–atmosphere (ISBA) land surface model multi-energy balance (MEB) option in SURFEXv8 – Part 2: Introduction of a litter formulation and model evaluation for local-scale forest sites

Abstract. Land surface models (LSMs) need to balance a complicated trade-off between computational cost and complexity in order to adequately represent the exchanges of energy, water and matter with the atmosphere and the ocean. Some current generation LSMs use a simplified or composite canopy approach that generates recurrent errors in simulated soil temperature and turbulent fluxes. In response to these issues, a new version of the interactions between soil–biosphere–atmosphere (ISBA) land surface model has recently been developed that explicitly solves the transfer of energy and water from the upper canopy and the forest floor, which is characterized as a litter layer. The multi-energy balance (MEB) version of ISBA is first evaluated for three well-instrumented contrasting local-scale sites, and sensitivity tests are performed to explore the behavior of new model parameters. Second, ISBA-MEB is benchmarked against observations from 42 forested sites from the global micro-meteorological network (FLUXNET) for multiple annual cycles.It is shown that ISBA-MEB outperforms the composite version of ISBA in improving the representation of soil temperature, ground, sensible and, to a lesser extent, latent heat fluxes. Both versions of ISBA give comparable results in terms of simulated latent heat flux because of the similar formulations of the water uptake and the stomatal resistance. However, MEB produces a better agreement with the observations of sensible heat flux than the previous version of ISBA for 87.5 % of the simulated years across the 42 forested FLUXNET sites. Most of this improvement arises owing to the improved simulation of the ground conduction flux, which is greatly improved using MEB, especially owing to the forest litter parameterization. It is also shown that certain processes are also modeled more realistically (such as the partitioning of evapotranspiration into transpiration and ground evaporation), even if certain statistical performances are neutral. The analyses demonstrate that the shading effect of the vegetation, the explicit treatment of turbulent transfer for the canopy and ground, and the insulating thermal and hydrological effects of the forest floor litter turn out to be essential for simulating the exchange of energy, water and matter across a large range of forest types and climates.

Soil Resistivity: A Limiting Determinant to Zero-Sequence Currents for Grounded Conductors in South African Low Voltage Networks

Soil Resistivity: A Limiting Determinant to Zero-Sequence Currents for Grounded Conductors in South African Low Voltage Networks

Karoo Array Telescope Berm Shielding: Efficient Computational Modeling and Multicopter Measurement

The square kilometer array is planned to be the most sensitive radio telescope of the 21st Century. Electromagnetic site quietness is a critical element in achieving the full science goals. We evaluate the shielding effectiveness of a soil berm at the South African Karoo site. The berm is electrically large and computationally expensive to model. A hybrid GO/MoM model using FEKO is proposed. Simulated data are compared to full-scale multicopter measurements. Broadband antennas are built into the landing structure and their patterns are deembedded during postprocessing. Simulations and measurements over 100–900 MHz (vertically polarized) show that the berm becomes effective from the point where its height reaches ten wavelengths. Shielding effectiveness values in the range of 25 dB are predicted by an approximate perfect electrical conductor ground model; the validity of this approximation is investigated. A simple knife-edge model is shown to provide a useful estimate of likely shielding. The results show that 3-D berm optimization using computational techniques is possible.

Contribution of electromagnetic and tomographic technique to the study of the impact of salinity in soils of the experimental station of Al Ain Atti (Errachidia, Morocco)

Soil salinity is widespread particularly in arid areas. Much work has been done to remedy this natural contamination and make them more favorable to receive experimental soil of vegetation adapted to grow in these contrasting environments of scarce water resources. Over the 80 years, Morocco has experienced excessive dryness whose effects have been severely felt in the region of Errachidia. The depletion of the Quaternary alluvial aquifer has required the exploitation of deep reservoirs. In order to optimize the use of salt water in arid, the Institute of Agricultural Research tested soil experimenting three plant species; the Triplex, Acacia and Cypress. This experiment was not only economic objectives; increasing agricultural yields and production, but also reduce the impact of desertification in this region. So these are environmental reasons which have led researchers to realize this experiment whose purpose is to examine the ability of these plants to grow and adapt to irrigation by saline water from the deep aquifer in place. The experimental site is located on the left bank of the national road to Erfoud (Errachidia). In its entirety, the redesigned covering about 10 hectares. The studies that have been conducted mainly concerned pedogenetic analyzes and observations the development height of the plantation tested, regardless of the experimental ground heterogeneity. This is why we undertook a geophysical survey which aims to provide information on the quality of the tested soils and accumulation of salinity at different depths of the experimental ground. To achieve our goal, we used the technique of electrical and electromagnetic investigation in order to identify the main lines of a possible spatial heterogeneity. Recall that the work of OKAY (2010) on an experiment coring by examining the evolution of the newly formed fracturing have proved that the result of resistivity measurement is correlated with that of the chargeability. The electrical resistivity is closely related to the concentration of mineral particles. Characterization studies at the La Ronde tailings mine (Agnico-Eagle Ltd, in Abitibi), showed quite encouraging results (Campos, 2004; Anterrieu, 2006). Since it is a salt-bearing areas (highly conductive) and given that the induced polarization is very sensitive to the ground conductivity; by measurement of residual potential, so we took measures chargeability tomography, the result is compared to the resistivity measured at different soils (control and irrigated), which will better clarify the impact of salinity on the quality of the disturbed soil. Indeed, the chargeability response it possible to characterize the different irrigated soils and its variation is attributed not only to the effect of saline irrigation but also the existence of an heterogeneity of the original ground. Prospecting in electrical tomography has displayed vertically and horizontally anomalies existing within the experimental site of the station of Ain Al Atti, it showed that:- accumulation of salinity becomes wider going from the control ground to that of Acacias.- surveyed the ground at least appear more homogeneous in surface, but their conductivity varies in depth.- salinity origin and the concretion formed on the surface greatly influenced chargeability and resistivity measured on the different experimental soils.

Lightning Surge Propagation on a Single Conductor in Free Space

This paper introduces an analysis of lightning surge propagation on a conductor without any returning current path. The conductor can be either a free-space or grounded conductor as long as the reflected surge from the ground has not arrived. Unlike a TEM transmission line, this conductor is characterized with time/position-variant surge impedance as surge current attenuates during its propagation. In this paper, a simplified formula was derived. Using the unique parameter—attenuation coefficient of current—an iterative method was developed to evaluate actual propagation characteristics. This method was verified numerically, and is much more efficient in calculation and easier in implementation. It is found that the surge impedance is affected by the waveform of an impulse current source, but is independent of the slope of a ramp current source. It increases quickly if the source current has a short rising time or failing time. The simplified formula generates overestimated results, but the difference decreases with increasing distance to the source. The proposed method can be used to address surge voltage on the tower upon the arrival of a reflected surge from the ground.

Extension of the Rusck Model for Calculating Lightning-Induced Voltages on Overhead Lines Considering the Soil Electrical Parameters

The validity of the Rusck coupling model and its equivalence to the model by Agrawal, Price and Gurbaxani have already been demonstrated for the case of a perfectly conducting ground and a vertical stroke channel. However, in spite of its popularity, the restriction of an infinite soil conductivity imposes a strong limitation on the use of the model for the analysis of realistic situations. In this paper, a procedure is proposed to extend the Rusck's model to the case of a finite ground conductivity so as to enable the calculation of lightning-induced transients on overhead lines taking into account the soil electrical parameters. The validity of the model is demonstrated through comparisons of simulation results with measured lightning-induced voltages and currents obtained from scale model and rocket-triggered lightning experiments.