Current Injection Point Research Articles

Grounding grid effective area and impulsive impedance considering statistical–probabilistic characteristics of lightning currents

The grounding grid transient behavior is an important substation design criterion. This behavior is determined through interaction with impulsive currents typical of lightning. Based on the technical literature, the behavior in question is determined considering fixed current waves, with median parameters (peak values and front and tail times etc.). Therefore, important statistical–probabilistic characteristics of lightning currents, such as correlations and statistical regressions, are not included. From this perspective, this paper includes research into the sensitivity of impulsive impedance and effective area values of substation grounding grids in relation to the waveform of currents typical of the first return strokes. The results (original) illustrate significant sensitivity of the grids transient behavior in relation to those mentioned statistical–probabilistic characteristics, considering a wide range of resistivities in both current injection points considered in this study.

3D pole-pole responses of buried anisotropic targets and various anisotropic backgrounds in direct-current resistivity survey

Abstract Many rocks often have the electrical anisotropy property that produces quite different responses from the electrical isotropic rocks. We applied an advanced modeling method to calculate the apparent resistivity responses over an anisotropic target in a homogenous isotropic backgound and an isotropic target embedded in different anisotropic host rocks. We demonstrated comprehensive modeling experiments to characterize the responses of surface pole-pole surveys to the buried targets in different geological backgrounds, which vary in the principal resistivities and Euler angles of the targets or the host rocks. Our modeling results show that: (i) the location of current injection controls the apparent resistivity images in value and shape; (ii) almost circle-contours or more or less elongated contours appear in the apparent resistivity images for the anisotropic targets; (iii) the isotropic target embedded in the anisotropic host rock generates some radial patterns from the current injection point; (iv) the anisotropy strike angles can be obtained from the symmetry axis of the apparent resistivity contours; and (v) multiple locations of current injections throughout the detection region are necessary for the surface pole-pole survey to avoid misleading data interpretation.

Test Method for Transient Characteristics of Grounding Grid With Current Return Point Located in the Same Grid

Lightning strikes in substations may result in extremely uneven ground potential distribution, which will threaten the safety and stability of equipment in substation. However, the main parameters for evaluating the performance of grounding grids are grounding resistance and impulse impedance, and little attention is paid to the distribution of the transient ground potential differences. In addition, when measuring the impulse impedance of a grounding grid, the length of the current leads should be very large, which requires a high-capacity impulse generator to suppress the influence of lead inductance on current waveform. In this paper, a test method is proposed for the transient characteristics of grounding grid with the current-return point located in the same grid. The feasibility of the method is illustrated by analyzing the ground impedance between the current injection point and the return point, longitudinal current of the grounding conductor, transient ground potential rise and transient ground potential difference. The effects of parameters such as soil resistivity, lightning waveform, size of the grounding grid, spacing and materials of the grid conductor are discussed to demonstrate that the method can be applied to most substations. Finally, the practicality of the method is verified by experiments in a 220 kVsubstation.

Bare versus insulated conductors for improving the lightning response of interconnected wind turbine grounding systems

This paper investigates the lightning response of a pair of interconnected wind turbine grounding systems assuming their connection to be performed by a bare or insulated underground conductor. Typical first and subsequent stroke current waveforms are injected at one of the grounding systems and the transient response is studied for different soil resistivities considering frequency-dependent soil parameters. The ground potential rise (GPR) at the current injection point and the voltages transferred to the adjacent grounding system are calculated. GPR peak reductions are obtained using either a bare or insulated conductor, but the former is more effective. It is shown that when the wind turbine grounding systems are interconnected by a bare conductor, the GPR peak reduction is essentially due to the interconnecting wire. On the other hand, when the interconnection is made through an insulated conductor, the GPR reduction is related to the current that is partly diverted to the adjacent tower, especially for high resistivity soils. For an insulated interconnecting conductor and first lightning return-stroke currents, the adjacent grounding system also contributes to the GPR peak decrease.

Analysis of the performance of grounding grids buried in heterogeneous soil under impulse current

The present paper is devoted to analyzing the transient behaviour of simple grounding grids subjected to impulse lightning current. The transmission line method (TLM) involving mutual coupling between conductors will be used. The transient behaviour of grounding grids buried in homogeneous and in heterogeneous soil is going to be evaluated into a complete time domain solution. Different simulations carried out altering, the influence of the grid dimensions, the kind of the ground and the current injection point on the grid voltage and impedance will be presented. Simulation results will be shown for two extreme cases: in the first case the current is in the center of the grid and in the second one it is injected in one corner of the two configurations of grids. The obtained results show that the grid 1x1 gives the lowest transient potential when the injection point is in lower resistivity side, and the grid 2x2 presents better behavior when the current is injected at center point. It is obvious that the suggested simulations are in a good agreement, with corresponding results of other researchers.

Impulse Impedance and Effective Area of Grounding Grids

High values and uneven distribution of the transient ground potential rise (TGPR) at grounding grid conductors in cases of lightning can disrupt the system safety and reliability. To assess the transient performance of grounding grids under lightning currents, a parametric analysis was performed with a computer simulation of 13,978 cases of time-domain responses with a broad range of parameters. From this study, graphs of impulse impedance and a new formula for the effective area have been developed. The graphs are general, describe grids of any shape and size and depend on three parameters: grid conductor spacing, the resistivity of earth, and the zero-to-peak time of the lightning current pulse. Although the parametric analysis was based on a uniform soil model, the graphs can also be used for the two-layer soil model. In this paper, graphs and formula are used in a simple procedure to estimate the effectiveness of a mitigation measure to reduce the TGPR by increasing the grounding grid mesh density in a limited area around the current injection points. The results from the procedure are compared with experimental results, and the application and limitations of the procedure are illustrated by considering large grounding grids as examples.

Efficiency and Catastrophic Failure of High-Power Blue GaN LEDs During Extremely High Temperature and Current Stress

This paper reports an extensive analysis of the short-term effects of extremely high driving current on the performance and reliability of commercial high-power blue LEDs. To this aim, five different groups of devices, having different structures and chip areas, were submitted to increasing levels of bias current, until catastrophic failure was reached. For the first time we provide information (i) on the reliability limits of LEDs outside the safe-operating area, (ii) on the related failure processes, and (iii) on the impact of current and temperature in determining the degradation of the devices. In addition, the results of failure analysis indicate that for almost all devices, failure occurs in correspondence of a major current injection point, often determined by the presence of crowding phenomena. Failure is ascribed to the high localized power dissipation and temperature. Device layout and intrinsic defects may favour the observed degradation processes.

A new optimization design for grounding grid

In this paper, a new approach for designing groudging grid is proposed. The design is based on constracting an auxiliary annular grounding grid and transforming the lingtning rod’s grounding poing from main grounding grid to auxiliary grounding grid. Modal analysis and softwave simulataes is employed to obtain the transformed lightning current (and the voltage) and the reduced ressitance. The dual port model is used to calculate the impedance value of the grounding grid. The calculation results of the model are compared with the standard grounding body model EMF. At the same time, the relationship between the induced voltage of each point of the grounding grid and the distance of the lightning current injection point is studied by using the dual port model. The reduced resistance is simulated by building annular grounging grid model and square grounging grid model and making a comparison in performance betwen them. Farther, when there is a lingtning event, instead of providing a low impedance path by the main grounding grid, the design provide anther path by the auxilary grounding grid, which can transfer the lightning current into the auxilary grounding grid. All the model analysis and simulation results are presented and discussed in the paper.

Investigation on Impulse Characteristic of Full-Scale Grounding Grid in Substation

Impulse performance of grounding grid is needed for lightning protection design of power system. This paper describes an impulse test facility established for the full-scale grounding grid (50 m long and 52.5 m wide) in a 110 kV substation. Test method is presented, including configuration of current injection and return point, measurement of potential rise and longitudinal current. Impulse test is carried out with 2.6/50 and 8/20 μ s current waveform, and the maximum current amplitude is 4 kA. A Current Distribution, Electromagnetic fields, Grounding and Soil structure simulation for the tested grounding grid is also adopted. Impulse performance of full-scale grounding grid is affected by soil ionization and conductor inductance. There is steeper peak on the potential rise waveform near the injection point with shorter wave front current, which leads to more uneven potential rise distribution. Partial soil ionization could increase the current dispersing into the soil at partial area of the grounding grid, and the longitudinal current flowing to the next area decreases. These two factors pose a high potential difference hazard to the secondary equipment via the secondary cable.

Effects of high frequency modeling & grounding system parameters on transient recovery voltage across vacuum circuit breakers for capacitor switching in wind power plants

This paper investigates the transient recovery voltages across vacuum circuit breakers used for the switching shunt capacitor bank in wind power plants, which are connected to the power system. Consideration has been given to high frequency models of all power system components, such as, the transformer, wind power plant generator, cable, surge arrester, vacuum circuit breaker and grounding system. The initial phase of study reproduced the results of previous work. In the second phase, the influence of different parameters, such as, the grounding system network, current injection point location to the grounding system, soil resistivity, grounding system length segment and soil ionization phenomena in the grounding system on transient recovery voltage, have been investigated via EMTP-RV. The results show that consideration of the high frequency behavior of the grounding system in wind power plants for prediction of transient recovery voltages is very important.

Modelling and controlling of PV connected quasi Z-source cascaded multilevel inverter system: An HACSNN based control approach

In this paper, the modelling and controller design for quasi-Z source cascaded multilevel inverter (QZS-CMI) based three-phase grid-tie photovoltaic (PV) power system is presented. The control scheme of 3-phase QZS-CMI is designed as two phases and regulates the source PV voltage and output grid current. Here, the first phase utilized Adaptive Cuckoo Search (ACS) algorithm for determining the total PV voltage and the second phase utilized CS algorithm with artificial neural network (ANN) to extract the reference current of grid. In first phase, the ACS algorithm engendered the tuning parameters of the PI controller based on the variation of the PV voltage. The second phase ANN is trained offline manner with the exact dataset prepared by the CS technique and it ensures the grid current control. The proposed method is refined for regulating the DC link voltages and extracting the grid currents. The proposed method based QZS-CMI synchronizes the shoot through duty ratio, reduces the modulation burden, regulates the dc link voltage, current and frequency conditions. By using the proposed method, all presented QZS-H-bridge inverter (HBI) modules achieves the grid-tie current injection and independent maximum power point tracking (MPPT) using system level control. The proposed method based QZS-CMI is designed with MATLAB/Simulink platform and their performances are evaluated with traditional techniques.

Analytical derivation of DC SQUID response

We consider voltage and current response formation in DC superconducting quantum interference device (SQUID) with overdamped Josephson junctions in resistive and superconducting state in the context of a resistively shunted junction (RSJ) model. For simplicity we neglect the junction capacitance and the noise effect. Explicit expressions for the responses in resistive state were obtained for a SQUID which is symmetrical with respect to bias current injection point. Normalized SQUID inductance (where Ic is the critical current of Josephson junction, L is the SQUID inductance, e is the electron charge and ℏ is the Planck constant) was assumed to be within the range l ≤ 1, subsequently expanded up to using two fitting parameters. SQUID current response in the superconducting state was considered for arbitrary value of the inductance. The impact of small technological spread of parameters relevant to low-temperature superconductor (LTS) technology was studied, using a generalization of the developed analytical approach, for the case of a small difference of critical currents and shunt resistances of the Josephson junctions, and inequality of SQUID inductive shoulders for both resistive and superconducting states. Comparison with numerical calculation results shows that developed analytical expressions can be used in practical LTS SQUIDs and SQUID-based circuits design, e.g. large serial SQIF, drastically decreasing the time of simulation.

Potential Rise Characteristics of a Grounding Grid Subjected to Lightning Currents

This paper presents the results of an investigation into the factors that affect the impulse response of a grounding grid under the passage of lightning currents. We carried out simulations of potential rise of a 10m×10m square grounding grid comprising 4 meshes buried in the soil of about 140Ω․m resistivity. The potential rise profiles of the grounding grid stressed by lightning currents were computed by EMTP software. The dissipation current and the potential rise for the 0.25/100μs current wave were significantly concentrated around the current injection point. The impulse coefficient for a corner-positioned lightning strike point showed a doubling of the impulse coefficient compared to the corresponding central strike point of the grounding grid.

Frequency Response Characteristics of a Grounding Grid

The dissipation current distribution, potential rises and frequency-dependent impedances of the grounding grid were investigated. The computational simulations are made using the simplified EMTP-based model. Based on experimental and simulation results, the dissipation current at a frequency above ～100㎑ intensively converged near the current injection point of the grounding grid, and decreased far from the current injection point. When the test current was injected at the center of the grounding grid, the high frequency grounding impedances were lower than those measured at the corner or side of the grounding grid. The trend of simulation results for the grounding impedance and dissipation current distribution calculated using a non-uniform distributed parameter circuit model are similar to the data measured at the test site.

Current transfer length in multi-filamentary superconducting NbTi and Nb3Sn strands; experiments and models

The current transfer length of multi-filamentary superconducting NbTi and Nb3Sn strands was measured and analyzed. The aim is to understand and quantify the current distribution process between matrix and superconducting filaments occurring at current injection joints or shunting localized interruptions like originated by transverse cracks or high strain, temperature, or magnetic field in filaments. The current transfer length was investigated on two different multi-filamentary Nb3Sn wires and one NbTi wire. Opposite to earlier clarifications, it was found that the current transfer length cannot be simply represented by a single parameter but depends on the ratio of transport current and critical current and the distance from the current injection point or local interruption of the superconducting path in the filamentary zone. With the aid of our numerical 3D multi-filamentary strand model, simulations were performed showing excellent agreement with the experimental data. For broader use, analytical formulae are proposed to determine the current transfer length for multi-filamentary superconductors with complex cross-sectional layout. The increasing current transfer length with the higher injected current and/or along with the distance away from the current injection point is explained by a progressive current penetration, which is caused by the high resistive matrix layers and complex layout. For the experimental results obtained here, the analytical and numerical simulation results show good agreement against the experimentally measured data from the potential-tips along the strand length.

Technique to increase the effective length of practical earth electrodes: Simulation and field test results

Field experiments and computer simulations of two sectionalized horizontal earth electrodes are presented. The electrodes were energized using various voltage sources (dc, and transients of different shapes), and measurements of current and voltage magnitudes along the length of the electrode were examined. Furthermore, by incrementally increasing the length of the test electrodes, the effective length of the earth electrodes was determined experimentally. The experimental and simulation results show reasonably close agreement which gives confidence in predicting the effective length. A new method is proposed to increase the effective length of the horizontal earth electrode by installing an additional insulated parallel conductor which is bonded to the horizontal electrode at points along its length. Such insulated conductor allows current to be distributed further away from the injection point without high leakage as would occur with standard horizontal electrodes buried in the ground. The results show that the current and voltage distributions are modified such that a greater length of buried conductor is utilized and that this contributes to an additional reduction in the earth impedance, and hence the earth potential rise at the point of current injection. The effect of the proposed technique is simulated on a practical wind turbine earthing design, and shows significant gains.

An analysis on the Frequency-dependent grounding impedance based on the ground current dissipation of counterpoises in the two-layered soils

Abstract Because of the increased usage of power conversion equipment and their susceptibility to lightning strikes, high frequency fault currents flowing through grounding systems are increased. As grounding systems change from individual groundings to common or integrated groundings, the improved performance of grounding systems against high frequency fault currents is urgently required. When regarding the common or integrated grounding systems, most of them are required to satisfy the ground resistance defined by standards or regulations. The grounding system performance against high frequency fault currents is defined by the grounding impedance. A number of studies have reported that grounding system performance is reduced due to the increase of the grounding impedance with increasing of the frequency of fault currents. In this paper, in order to investigate the characteristics of the frequency-dependent grounding impedances of counterpoises installed in the two-layered soils, the ground current dissipation rates in each 10 m section of the counterpoises were measured and analyzed as functions of the length of grounding electrodes and the current injection point. As a result, most of the ground currents above the frequency of 100 kHz are dissipated into the earth near the current injection point and the high frequency grounding impedance of long counterpoises is converged into that of short counterpoises. The high frequency grounding impedance measured at the low soil-resistivity end of the counterpoise buried in the two-layered soils is much lower than that measured at the high soil-resistivity end.

An analysis of conventional grounding impedance based on the impulsive current distribution of a horizontal electrode

The performance of a grounding system should be evaluated in terms of the grounding impedance for a lightning surge containing high frequency components. The ground resistance is regarded as grounding impedance measured in a low frequency. When designing the grounding system, the grounding impedance and effective length should be considered. In this study, the conventional grounding impedance was measured as functions of the front time of the impulse current and the length of the horizontal electrode used largely as the grounding electrode of the transmission tower. In order to find the relation between the conventional grounding impedance and current distribution, the magnitude of the dispersed impulse current at every 10m interval of the horizontal electrode, which is 50m long, was measured and simulated as a function of the front time of the injected impulse current. The conventional grounding impedance was also measured and simulated as functions of the front time of the impulse current. The current distribution and grounding impedance for the first and second strokes were also examined. As a result, the conventional grounding impedance of a long horizontal electrode was similar to that of a short horizontal electrode for fast front time. As the front time of the injected current became shorter, the current distribution increased near the current injection point. In addition, the simulated results in the multi-layered soil structure were very similar to the measured results. In case of the second stroke containing high frequency components, the simulated conventional grounding impedance of the horizontal electrode was almost the same regardless of the length of the horizontal electrode being longer than 10m since a large portion of the current was dispersed within 0–10m section of the horizontal electrodes.

Induced Voltage Calculation in Electric Traction Systems: Simplified Methods, Screening Factors, and Accuracy

In modern electric traction systems, the calculation of induced voltage on internal and external conductors is always required for both interference and people and equipment safety. The International Telegraph and Telephone Consultative Committee (CCITT) method is based on simplified relationships, which are integrated by suitable screening factors (representing and hiding the traction system complexity). Focus is on the determination of these factors and the accuracy, which are evaluated with simulation and experimental data. The considered systems are rail return, return conductor, and autotransformer (AT) 2 × 25-kV traction systems with victim cables of various lengths; the influence of the most relevant electric parameters (soil resistivity and conductance to earth of the return circuit conductors) is considered. The results confirm the relevance of the conductance to earth and of the position of the current injection point. The induced voltage is approximately proportional to the length, with a slight increase for shorter victim cables. The calculated AT screening factor is always smaller than the values suggested by the standards by a factor of 2 in the worst case. For the influence of separation of the inducing and victim circuits, the adopted methods agree in finding a critical distance around 6-8 m; at larger distances, CCITT values are slightly optimistic. Measured results confirm that the CCITT coefficients are all conservative by a factor of 2-4 (except the AT coefficient for total length exposure), whereas the multiconductor transmission line (MTL) results agree with a worst-case margin of 13%.

Edge effects in four-point direct current potential drop measurements on metal plates

Four-point direct current potential drop (DCPD) measurements are commonly used to measure the conductivity (or resistivity) of semiconductors and ferrous or non-ferrous metals. The measured electrical potential difference is often interpreted in terms of analytic expressions developed for large plates that are either ‘thin’ or ‘thick’ relative to the probe length. It is well known that the presence of the back surface of a plate leads to a solution expressed in terms of an infinite series representing the current source and its images. This approach can be generalized to account for multiple surfaces in order to obtain a solution for a finite plate, but convergence of the series is poor when the plate dimensions are similar to or smaller than the separation of the current injection and extraction points. Here, Fourier series representations of the infinite sums are obtained. It is shown that the Fourier series converge with many fewer terms than the series obtained from image theory, for plates with dimensions similar to or smaller than the separation of the current injection and extraction points. Comparing calculated results for the potential drop obtained by a four-point probe centred on finite plates of varying dimension, with those for a probe in contact with a large (laterally infinite) plate, estimates are given of the uncertainty due to edge effects in measurements on small plates interpreted using analytic formulae developed for large plates. It is also shown that these uncertainties due to edge effects are reduced, for a given plate size, if the probe pick-up points are moved closer to the current injection points, rather than adopting the common arrangement in which the four probe points are equally spaced. Calculated values of DCPD are compared with experimental data taken on aluminium and spring-steel plates of various sizes and excellent agreement is obtained.