Mesh Voltage Research Articles

Numerical analysis and safety design of grounding systems in underground compact substations

This paper presents a mathematical and numerical formulation to design and analyze grounding systems in underground electrical substations. The developed approach is based on the well known Maxwell’s Equations. The proposed problem is solved by means of the Boundary Element Method (BEM). The utilization of BEM allows to introduce the geometry and electrical characteristics of the enclosure in a uniform soil. Therefore, a more realistic approximation of the soil structure is achieved. This formulation allows to obtain the main parameters of these protection systems (the grid resistance, the Ground Potential Rise, and the step and mesh voltage). It also allows to compute the surface and step voltage distributions. In addition, as a secondary result, the voltage and current density distributions over the enclosure are obtained. Finally, two grounding system analyses of real underground electrical substations are presented to demonstrate the industrial application and the modelization capabilities of the proposed formulation.

Mechanism analysis and experimental verification of electromagnetic sail, a new solar propulsion system without propellent

The electromagnetic sail, a new space propulsion system without propellant, which propels a spacecraft by deflecting solar wind with an electromagnetic field, is proposed. The thrust mechanism and the performance variations of the electromagnetic sail are analyzed with both particles in cell and magnetohydrodynamic models, and the interaction between the incoming plasma fluid and the electromagnetic field is investigated by tests on the ground experimental system. The simulation results demonstrate that a magnetosphere is created in front of the sail and its area increases with magnetic field intensity; with appropriate dipole moment, the thrust of the electromagnetic sail is higher than a pure magnetic sail or a pure electric sail. In test experiments, the plume structure of a helicon plasma source changes obviously with coil current and metal mesh voltage, which verifies the repulsion of the magnetic field and electric field to the incoming plasma. These results indicate that the combination of the magnetic sail and electric sail can increase the utilization efficiency of solar wind, which means the electromagnetic sail is a feasible propulsion system.

Analysis and Mitigation of Ground Grid Lightning Potential Rise

Grid design depends mainly on bringing grounding resistance, mesh voltage, and step voltage below tolerable limits when subjected to any ground fault. But, it should be noticed that the grid performance has to be tested when subjected to lightning stroke. The performance of the grid under impulse current differs than that of under power–frequency fault. In this paper ATP-EMTP software is used to analyze the performance of the ground grid when subjected to lightning stroke. Different methods are suggested to mitigate the lightning overvoltage, all of these methods depend mainly on modifying the design by adding more conductors or rods at specific places. The case study under consideration is Al-Mostakbal substation 220/22 kV which is located in Al-Mostakbal city, Cairo, Egypt.

Design of a Substation Grounding System for Fast Front Overvoltage

Grounding studies allow an optimized design of grounding system to provide protection for electrical components and personnel at substation. The design of grounding system needs to comply with IEEE Standard 80:2013. Field work is conducted with Fall-of-Potential method and Wenner’s method to perform measurement soil test. Data from measurement test is used to develop two-layer soil model by using Sunde’s graphical method. Calculation for tolerable step voltage and touch voltage are under 50kg body weight condition. Based on the findings, calculated values for tolerable step voltage and touch voltage are higher than calculated values for mesh voltage and touch voltage. The percentage of error is 7% when computed values and calculated values for tolerable voltages are compared. Higher tolerable voltages minimise the risk of getting electrical shock to any individual presents at substation during fault or lightning strike occurs. However, there is limitation in this paper related to software licensing where full simulation with grounding software required full access.

Evaluations of the apparent soil resistivity and the reflection factor effects on the grounding grid performance in three‐layer soils

In this study, the apparent soil resistivity evaluation is estimated for three-layer soils by four strategies and the obtained values are compared with experimental measurements done by using the four-electrode method. Different methods are suggested with the aid of IEEE 80 to estimate the apparent soil resistivity of three-layer soils, in which a grounding system consisting of a grid with rods is constructed. The effect of the reflection factor on the mesh and the step voltages, the grounding system resistance and the mesh voltage is investigated. It is concluded that the first three methods are close together, and also with the experimental findings done by the authors and others the fourth method gives some tolerance with the first three, but it is in agreement with the experimental measurements. It is concluded also that the reflection factors of non-uniform soil have significant effects on the apparent soil resistivity, the step and the mesh voltages and the ground resistance. For the verification of the calculated results, experimental model is used. It is noted that the measured values are slightly high compared with the calculated values; the reason may be due to the boundary effects of the used model.

Performance Analysis of Actual Step and Mesh Voltage of Substation Grounding Systemwith the variation of Length and Number of Ground Rod

The performance of Earthing grid system is very important to ensure the human and protective devices in safe environment. Actual Step and Mesh voltage of a substation must keep under the maximum allowable limits under fault condition. Ground potential rise, GPR is greatly influence on actual step and mesh voltage of substation grounding system. Ground potential rise also mainly depends on the length and numbers of ground rods and grid spacing. This paper presents performance analysis of actual step and mesh voltages of a substation grounding system under the variance of length and number of ground rods. The performance result is also carried out by using current injection method and with the help of MULTISIM simulation software.

Comparison of the safety criteria used for ground grid design at 161/23.9-kV indoor-type substation

This paper compares seven different standards, i.e., IEEE Std. 80, 2000 edition, IEC 60479-1, BS 7354, EA-TS 41-24, CENELEC HD 637 S1, DL/T 621 and ITU-T K.33 used for ground grid design at 161/23.9-kV indoor-type substation in the system of Taiwan Power Company. The calculated results from equations proposed by Sullivan and the Current Distribution Electromagnetic Grounding and Soil structure analysis (CDEGS) software package are also included. The results show IEC 60479-1 gives the lowest limits of tolerable touch and step voltages whereas BS 7354 gives the highest when no chippings on the ground surface are considered. The designed maximum predicted touch voltage obtained from Sullivan’s equation is higher than those obtained from the formulas of BS 7354, EA-TS 41-24, DL/T 621 and CDEGS, the mesh voltage is higher than those from IEEE Std. 80, 2000 edition and CDEGS.

GROUNDING GRID PERFORMANCE OF SUBSTATION IN TWO LAYER SOIL - A PARAMETRIC ANALYSIS

During ground faults, fault current flows in the ground which results in potential gradient and ground potential rise (GPR). The potential gradient affects the safety of operating person due to step and touch voltage and GPR affects the insulation of protective devices and control cables. The potential gradient and GPR are the functions of ground electrode geometry. In this paper, performance assessment and analysis of grounding grid is carried out for different types of grid electrodes with two layer soil model for both positive and negative reflection factor K. The simulation is carried out to evaluate grounding grid performance indices such as Ground resistance, Mesh voltage, Step voltage, Ground Potential Rise. The various parameters used for the simulation are, depth of ground grid, Number of meshes and length of ground rods. The results are tabulated, and discussed. The SES Autogrid Pro grounding software, Canada is used for the analysis.

봉상접지극과 Mesh접지극의 매설깊이 변화에 따른 위험전압 분석에 관한 연구

Grounding electrodes with higher burial depths are evaluated to have better performance, due to the domestic practice that puts the grounding resistance as the standard of performance evaluation, while grounding resistance decreases as the burial depth increases. However, The dangerous voltage is necessary for the analysis. Because the performance evaluation of grounding electrodes should include not only grounding resistance but also the dangerous voltage(mesh voltage and step voltage). So in this paper, The dangerous voltages of mesh grounding and rod grounding were analyzed for using computer simulation and miniature grounding model.

메쉬접지극의 내부도체 배치에 따른 개선된 메쉬접지극 모델

국내?외 메쉬접지극은 격자형태의 등간격 접지Grid로 설계하고 있다. 격자형 접지Grid의 경우 구석의 접촉전압이 중심 부분에 비해 높아지는 문제점이 있다. 이러한 문제점을 극복하기 위해 사선형태의 접지 Grid를 사용하면 메쉬전압이 발생하는 구석부분의 면적이 감소함으로써 메쉬전압이 감소한다. 따라서 본 논문에서는 기존의 격자형태의 접지Grid의 형태와 다르게 사선형태의 접지Grid를 제안하며, 격자형태의 접지Grid와 사선형태의 접지Grid로 설계된 메쉬접지극에 동일한 접지설계제원을 적용하고 메쉬전압, GPR, 접지저항, 접지도체의 총길이를 비교 검토하여 사선형태 접지Grid의 우수성을 검증하였다.

Mesh voltages at earthing grids buried in multi-layer soil

Expressions for the mesh voltages caused by earth fault currents leaking from earthing grids buried in uniform, two- and three-layer soils are proposed based upon the examination of a large set of grids and soil structures using the finite element approach. Simple empirical correction factors are developed to modify the mesh voltage formulae for uniform soil to take into account the multi-layer soil structure. A comparative analysis of available expressions for uniform and two-layer soils is performed in order to check their applicability in various cases.

Study on the count plateau, gas gain and energy resolution of the Micromegas detectors

In this paper, we describe the structure of Micromegas (micro mesh gaseous structure) detector which has been developed recently by our group. And its count plateau, gas gain and energy resolution in the condition of a 55Fe X-ray source and various Ar-CO2 mixtured gases are plotted as functions of mesh voltage. Based on the experimental results, we confirm that we have mastered the technique of fabricating this kind of detector with high energy resolution and excellent performance.

Acquisition of X-ray images by using a CNT cold emitter

Carbon nanotubes (i.e., CNTs) are tubular carbon molecules with properties that make them potentially useful in extremely small scale electronic and mechanical applications. Because of this, CNTs are widely used in many fields such as field emission display (FED), nanoscale sensors, vacuum electronic devices, and so on. In this study, CNTs were applied for an X-ray source. CNTs were grown on the Si-wafer substrate by thermal CVD method and the length of the grown CNTs was about 30–50 μm. The electrical properties of the grown CNT emitter were tested in an X-ray tube, which has triode structure (i.e., a cathode as a CNT emitter, a grid, and an anode). Electron beam focusing characteristics as well as correlations between emission currents and grid mesh structures (or grid mesh voltage) were also studied by using OPERA 3D simulation. The detailed descriptions of the manufactured X-ray triode were reported and some preliminary X-ray images were presented.

Comparison of 161/69-kV Grounding Grid Design Between Indoor-Type and Outdoor-Type Substations

This paper presents a practical case for the safety assessment of ground grid design at 161/69-kV indoor-type and outdoor-type substations in the system of Taiwan Power Company (TPC). For a substation grounding system to be safe, the touch and step voltages should be limited below the minimum tolerable voltages. Since the mesh voltage is usually the worst possible touch voltage inside an ac substation (excluding transferred potentials), the mesh voltage is used as the criteria for a safe ground grid design. In other words, safety assessment of a ground grid design at 161/69-kV indoor-type and outdoor-type substations in the TPC system is referred to as a procedure by which the mesh and step voltages at substations are computed and compared to the minimum touch and step voltages. Computation results of safety assessment for humans with 50- and 70-kg body weight are quantified in this paper.

Safety-limit curves for earthing system designs: appraisal of standard recommendations

The tolerable voltage limits and the maximum predicted voltage values calculated using empirical formulae given in five currently applicable standards, namely IEEE 80 (2000), EA-TS 41-24 (1992), BS 7354 (1990), ITU-T K.33 (1996) and CENELEC HD 637 S1 (1999) are compared. The calculated results from CDEGS-HIFREQ are also included. The step, touch and mesh voltages, and hot zones are calculated according to the recommendations given in the five standards, and the differences between them are investigated and clarified. The results show that IEEE 80 with 50 kg body weight touch voltage case gives the lowest tolerable limits whereas BS 7354 gives the highest in most cases. The maximum predicted touch voltage obtained from CDEGS-HIFREQ is higher than that obtained from the empirical formulae of EA-TS 41-24 and BS 7354, and the mesh voltage is lower than that from IEEE 80. The derivation of 'safety limit-curves' is proposed related to the magnitude and duration of fault current for a given earthing grid design based on the assumptions used in each standard. Following this new proposed procedure, it is established that, if the fault current parameters, i.e. grid current magnitude and duration, are within the safety zone of IEEE 80, they also satisfy the criteria of EA-TS 41-24 and BS 7354.

Safety assessment of AC earthing systems in a DC traction-supply substation

The paper assesses safety of AC earthing systems in a DC traction-supply substation (TSS) by comparing the designed mesh and step voltages to the maximum tolerable voltages. Generally, IEEE 80 criteria are based on the maximum tolerable body current to prevent fibrillation of the human heart, and this current is then translated into tolerable touch and step voltages. As a result, safety assessment of AC earthing systems in a DC TSS is referred to a procedure by which the designed mesh and step voltages are computed and compared with the tolerable touch and step voltages. Computation results of AC earthing systems for the orange line at O19 TSS in Taipei rapid-transit systems are presented as an example.

Computation of current-division factors and assessment of earth-grid safety at 161∕69 kV indoor-type and outdoor-type substations

The paper computes current-division factors and assesses earth-grid safety at 161/69 kV indoor-type and outdoor-type substations in the system of Taiwan Power Co. For computation of current-division factors, a method based on the grid current is proposed and the results obtained by the proposed method are compared with the Endrenyi method presented in IEEE 80:2000 as well as the current-distribution electromagnetic-grounding and soil-structure analysis (CDEGS) program. On the other hand, mesh and step voltages are computed and compared with minimum touch and step voltages for assessing earth-grid safety, respectively. The values of current-division factor obtained by the proposed method and the CDEGS program are smaller than those from the Endrenyi method, since coupling effects between overhead earth wires and phase conductors are taken into account. Nevertheless, the design of earthing systems at both types of 161/69 kV substations is safe for humans with 50 kg and 70 kg body weight.

Safety Assessment of Bulk and Traction Supply Substations in Taipei Rail Transit Systems

This paper presents a practical case for safety assessment of bulk supply substations (BSS) and traction supply substations (TSS) in Taipei Rail Transit Systems (TRTS). For a substation grounding system to be safe, the maximum touch-and-step voltages should not exceed postulated safety criteria. The safety criterion in IEEE Standard 80 is defined in terms of permissible body current. The permissible body current is then translated into the permissible touch-and-step voltages. Since the mesh voltage is usually the worst possible touch voltage inside the substation (excluding transferred potentials), the mesh voltage is used as the safety criteria. In other words, safety assessment of an ac substation grounding system in TRTS is referred to a procedure by which the mesh and step voltages are computed and compared to the maximum permissible (safe) touch-and-step voltages. Computation results for the orange line at O19 station are presented.

Evaluation of mesh and touch voltage at substations

Site tests, to measure the voltages on the surface of the ground above a grid for comparison with calculated values, are only of general interest if the ground has uniform resistivity at and below the grid. However, such conditions rarely if ever exist so the only practical alternative way of obtaining general data is to make tests on model grids in a tank where the electrolyte can be maintained at an homogeneous resistivity. A comprehensive series of such tests has been made in which voltages calculated using empirical formulas given in four of the most commonly used standards are compared with model test results and, as the correspondence is unsatisfactory, new empirical formulas have been developed. That identified the need to account for the shape of the grid and the meshes, and the distortion of the voltage contours close to the grid due to the heterogeneous nature of the local ground. New equations more closely agree with test results. A method of taking into account that the grid is not at the ground surface in the calculation of the HOT distance is given.

Beam characteristics of a high-gain avalanche rushing amorphous photoconductor field-emitter image sensor

A new type of image sensor that uses a field emitter array (FEA) and a high-gain avalanche rushing amorphous photoconductor (HARP) target, with a mesh electrode inserted between them, has been designed and tested. The effects of the voltage and position of the electrode were investigated in a vacuum chamber. The resolution and dynamic range were improved by applying an adequate mesh voltage. The voltage needed for the mesh electrode was decreased by reducing the distance between the FEA and the electrode. Most of the electrons emitted from the FEA are captured by the mesh electrode and therefore contribute nothing to the read-out of the signal on the target. The mesh electrode structure thus needs further study. In addition, the decelerating electric field formed between the mesh and target needs to be considered because it spreads the electron beam, thus degrading the resolution and dynamic range of the device.