Efficient ground grid designs in layered soils

A detailed analysis of the effects of ground grid configuration on grounding performance has been carried out for uniform and horizontally stratified soils with multiple layers. The results of the analysis reveal that the most efficient and cost-effective design is highly dependent on soil structure type and characteristics. In the absence of ground rods, grounding grids with uniform mesh size are quite efficient in soils having a thin (relative to grid size) high resistivity (relative to the lower layers) top soil, while grids with small mesh size at the periphery of the grid provide optimum performance in uniform soils and soils with low resistivity top soils. Ground rods were found to be effective only when a significant portion of their length is in contact with a low resistivity soil, as expected     

Resistance to earth of earthing grids buried in multi-layer soil

Expressions for the resistance to earth of 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 introduced to modify the earth resistance formulae for uniform soil to account for 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.     

不同结构土壤中接地网冲击特性的测量与分析

变电站接地网的冲击特性研究对于接地网的优化设计具有重要意义,为能系统真实反映接地装置在不同土壤结构中的冲击特性,基于量纲相似性原理,利用冲击电流发生器进行了接地网在均匀及双层土壤中的冲击接地模拟试验。在均匀土壤中测量了5种埋深、3种冲击电流注入点位置时对应的接地网冲击接地电阻值,并测量了典型情况下接地网的地表电位分布情况;在双层土壤中测量了3种土壤厚度、3种冲击电流注入点位置时对应的接地网冲击接地电阻值。通过分析不同试验条件下接地网模型的冲击接地电阻及电位分布得到了冲击电流注入点位置、地网埋深及土壤结构等因素对接地网的冲击接地特性的影响规律。     

Nonuniformity correction factors for maximum mesh-voltages ofcombined grid-multiple rods electrodes

Correction factors for evaluating maximum mesh-voltages associated with combined grid-multiple rods ground electrodes in two-layer soils are calculated for a wide set of ground electrodes to expand the application of available uniform soil expressions to nonuniform soil cases. Square and rectangular grids are examined reinforced by rods of fixed length distributed along the grid perimeter only and along the grid perimeter and grid diagonals. Results of the calculations of 2200 cases generated by varying electrodes and soil parameters are displayed as graphs appropriate for practical application. Approximate analytical expressions for correction factors are also derived matching reasonably well with the calculated data. These expressions offer an insight in the most effective ground electrodes parameters which might serve in the design process     

A Two-Layer Soil Model for the Calculation of Electrical Parameters of Grounding Systems under Lightning Strikes

This article aims to overcome the drawback of the consideration a uniform soil model in approaches based on the transmission line model by considering a two-layer soil structure. In this regard, the electrical parameters equations for the two-layer soil model based on the method of images in the literature which are suitable for power frequency waves are developed to be applicable to the lightning studies. Also, the developed equations are then used in the multiconductor transmission line approach to study the lightning behaviors of grounding systems. Furthermore, in order to check the validity of the procedure, the results of the multiconductor transmission line approach are compared with those obtained by the electromagnetic field approach through the simulation of horizontal, vertical and mesh grounding grids. The results show the accuracy of the two-layer soil model for the lightning analysis of grounding systems.     

Efficient numerical simulation of GaAs MESFET's based on energy model and using interpolating wavelet

A fast wavelet based non‐uniform grid generation method is presented for time domain simulation of active semiconductor devices. The proposed approach solves the active part model of the semiconductor device. This approach is used to solve the non‐linear equations of semiconductors on the self‐adaptive grids, obtained by applying wavelet. Non‐uniform mesh size is implemented and controlled by the wavelet coefficients. A fine mesh is used where the unknowns are varying rapidly and a coarser mesh where the unknowns are varying slowly. The energy model is considered for active layer of transistor and its results are compared with those of the classical drift diffusion model. Performance of this method is compared with the conventional finite difference on a uniform mesh. A reduction over 80 percent of unknowns in grid together with good accuracy in simulation is obtained using this non‐uniform mesh while accurate physical behaviour of the device can be predicted. This represents an ongoing effort toward a numerical technique that uses wavelet to solve physical modelling problem of semiconductor devices. Copyright © 2006 John Wiley & Sons, Ltd.     

Scale Model Studies of Grounding Grids in Non-Uniform Soils

A new improved scale model to study the behaviour of grounding systems in non-uniform (two layer) soils has been developed. Constructional details and instrumentation has been discussed. To verify the accuracy of the results obtained from the tests conducted on the model, they have been compared with the data available in the literature. The results giving the potential profile, grounding resistance, mesh and step potentials for a number of grids in two layer soils are presented.     

Measurements and computations of the performance of groundingsystems buried in multilayer soils

The resistances and earth potentials of two Florida Power Corporation electric substation grounding systems buried in soils with multiple horizontal layers were computed and measured at various phases of their installation, i.e., starting from a one mesh grid to the final design consisting of many regular meshes. Soil resistivity at each site was measured and interpreted to obtain equivalent two-layer and multilayer earth structures. It is shown that good agreement is obtained between the measured and computed values if multilayer earth structure models are used in the computations. Poor agreement results when uniform soil models are considered     

一类垂直双层土壤中地网接地电阻的简易计算公式

针对特高压输电系统变电站地网常跨越垂直分层土壤,研究垂直双层土壤中地网接地电阻的工程计算方法。用待求接地电阻与参考电阻的比值以及双层土壤两侧电阻率比值减少参数数量,并通过数据拟合得到这两个比值的函数关系,分析自变量等于1和趋向无穷大对应的地网接地电阻和土壤结构获得函数系数的表达式,将表达式代入函数之后进行化简,得到垂直双层土壤中地网接地电阻的计算式。计算式中包含的参考电阻和辅助电阻都是均匀土壤中地网接地电阻,这两个接地电阻可用IEEE标准或中国电力行业标准的推荐公式代替。与仿真软件相比,所得式不需建立土壤和地网模型,不需要仿真计算电流分布,能简单快捷地计算出接地电阻。通过几个算例分析可知,所得计算式的精度受推荐公式的影响,其精度能满足工程设计的需要。     

Method for calculating the ground resistance of grounding grids using FEM

Taking as given that ground resistance is independent of earth fault current, this paper presents a new way of calculating ground resistance of grounding grids using the finite-element method (FEM). Ground resistance is calculated by means of three consecutive tests, with an acceptable number of elements, and fast resolution. The results of the proposed method are compared with those measured experimentally or determined by other authors. Once ground resistance and earth fault current are known, grid potential and touch-and-step voltages are calculated. The main advantage of the method proposed here is that it permits us to analyze symmetrical and nonsymmetrical grids of any shape in uniform, two-layer, and multilayer soils. The method developed in this paper constitutes a fast-resolution industrial application, with acceptable results, for calculating the ground resistance of grounding grids of any shape.     

Maximum step voltages of combined grid-multiple rods groundelectrodes

Expressions are suggested for assessing the maximum step-voltages and associated parameters of combined grid-multiple rods ground electrodes buried in uniform soil. The expressions proposed are successfully verified by comparison to exact computer calculations performed for a wide set of square and rectangular ground grids reinforced by ground rods distributed around the grid perimeter and within the grid     

多层土壤接地网设计的机辅分析方法

提出了基于计算机辅助分析的变电站接地网设计方法。主要内容包括：从现场实测在接地电阻率出发,得到多层土壤结构模型;计算出多层不均匀土壤条件下不同地风结构,包括立体地网的接地电阻;分析并设计得到更经济合理的地网;同时给出地网的电位分布。     

Simplified equations for mesh and step voltages in an AC substation

A large number of grounding grids in substations have shapes other than a square or a rectangle, whereas the simplified formulas for mesh and step voltages available in the literature are derived for square or rectangular grids. Improvements in the simplified equations for determining mesh and step voltages are presented. With the modified simple equations, mesh and step voltages at a substation of any practical shape can be estimated with reasonable accuracy. The results obtained with the improved equations have been compared with accurate results obtained via computer. The error in the values obtained with the derived equations is within 16% for mesh voltage and less than 30% for step voltage     

引外接地对降低接地网接地阻抗的作用分析

工程上常采用引外接地的措施,一般根据经验在离主接地网1~2km范围内另敷设一辅助接地网,并采用扁钢将此辅助接地网与主接地网相连,但国内现行接地设计方法及故障电流下地网等电位计算模型均不能计算引外接地电阻。为此采用接地网工频接地参数分析软件,计算了均匀土壤中引外接地网对降低主接地网接地阻抗的作用。结果表明,辅助接地网与主接地网间的距离超过一定数值后,辅助接地网的作用可忽略,且在土壤电阻率较高的地区,引外接地具有较好的降阻作用。     

大型水电站接地网接地电阻的初步计算

现有的适合于一般发变电站地网设计的均匀和垂直或水平分层土壤结构的计算模型，并不适用于大型水电站接地系统。文中首次提出了在水平和垂直方向都分层的复合分层土壤结构的计算模型，并介绍了基于此模型的边界元算法，编制了通用的计算程序。所介绍的计算方法对于我国大中型水电站的接地设计具有较大的实用价值。     

Analysis of grounding systems in soils with cylindrical soilvolumes

A theoretical model for the analysis of grounding systems located in soils with cylindrical soil volumes is presented for the first time. Exact closed-form analytical expressions for earth potentials due to current sources in different regions of such soil structures have been obtained. More precisely, the soil models considered contain horizontal semi-cylindrical soil volumes and vertical cylindrical soil volumes. Numerical results are presented for different grounding systems and for different types of cylindrical soil volumes. The results clearly show that these soil structures have a significant influence on the performance of grounding systems. The results obtained are in agreement with well-known simple case results and converge asymptotically to the uniform soil case     

Simplified two layer model substation ground grid designmethodology

A simplified approach to the design of substation ground grids in nonuniform soil conditions is presented. The procedure is based on the interpretation of principles enunciated in the IEEE Standard 80-1986. The supporting data were obtained from actual field tests on substation ground grids. The mesh voltage, ground potential rise (GPR), ground grid resistance, grid current, overhead ground wire (OHG), and neutral conductor influence, etc. need to be treated as an interdependent system. How each item is used in the computer model is described. Use of each item in the computer model is based on the results of field tests and subsequent analysis (using substation grounding theory) of field data. The individual terms used in the document are defined. the procedure used to determine the effect of the various parameters which influence the grid design is described     

Influence of inductive coupling between leads on resistivitymeasurements in multilayer soils

A detailed study of the influence of inductive coupling between current and potential leads on soil resistivity measurements has been carried out. Two commonly used measurement arrangements, Wenner and Schlumberger, are modeled and analyzed at different frequencies and for different soil structures. The study quantifies the coupling between current and potential leads for typical separation distances between the leads for uniform and multilayer soils. The results show that high operating frequencies or small separation distances between leads result in high inductive coupling levels, as expected. The study also shows that when the resistivity of a uniform soil or the bottom layer resistivity of a multilayer soil is low, the measurement error caused by the inductive coupling is large. Coupling levels presented in this paper can be used as a reference to estimate possible errors in soil resistivity measurements     

Production of large‐diameter uniform plasmas and control of electron temperature

In order to produce large‐diameter uniform plasmas, we have proposed two plasma sources: a plane electron‐cyclotron‐resonance (ECR) slotted antenna (PESA) source for ECR plasma production and a modified magnetron‐type (MMT) radio frequency (RF) source for RF plasma production. The PESA has magnet rings behind the slotted antenna to provide the magnetic field for the ECR. The MMT RF source provides high‐density uniform plasmas with the help of the LC resonance of auxiliary electrode placed parallel to the substrate. Both plasma sources produce large‐diameter uniform plasmas exceeding 30 cm in diameter. We have proposed a grid method for electron temperature control. By applying a negative dc voltage to the grid which separates a plasma‐processing region from a plasma production region, the electron temperature is decreased by one order of magnitude in the processing region. The same effect is obtained by changing the mesh size of a floating grid. The change of a slit between two grids also causes variation of the electron temperature. These techniques are quite effective for chemical reaction control in processing plasmas. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 1–7, 2000     

铅布作为铅酸电池板栅的研究

通过物理方法和电化学方法对铅布作为铅酸电池板栅的可行性,在性能方面(强度、导电性、耐蚀性等)进行了研究,并对铅布板栅采用表面涂覆一层铅锡合金进行改进。研究结果表明:纯铅纤维复合板栅材料的抗拉强度、导电性、耐腐蚀性能较常用的铸造铅合金板栅材料都有显著提高;改进后的铅布板栅的正、负极活性物质利用率均有明显提高。