CHAPTER 02 LITERATURE SURVEY

Transcription

1 CHAPTER 02 LITERATURE SURVEY The electrical properties of a substation grounding grid such as grid resistance, ground potential rise, touch and step voltage are the functions of soil resistivity. During freezing season, soil resistivity increases multiple times. Therefore, resistance of substation grounding grid and tower footing devices increases which may change the maximum fault current division factor. The fault current division factor is the ratio of grid current to the fault current. The grid resistance, GPR, touch and step voltages are directly proportional to the maximum grid current. A method to determine fault current division factor, influence of seasonal frozen soil on it, effect of thickness of frozen soil, number of transmission lines and overhead ground wires connected to substation is presented [17]. It is at most important to consider the effect of frozen soil on fault current division factor while designing the substation grounding grid. The analysis result shows that use of tower footing devices eliminates the seasonal influence. It is necessary to extend the work for two layer and multilayer soil with ground rods. Effect of potential difference within large grounding grid on fault current division factor analyzed for 1000 kv Ultra high voltage substation using multiport circuit model for grounding grids with multiple grounding points is addressed [18]. The simulation is carried out using Simulik module of MATLAB software. According to authors, the multiport circuit model can be easily implemented for finding fault current distribution and fault current division factor. The factors which governs the potential difference within the grounding grid and the fault current division factor are the area of grid, soil resistivity, material of the grid conductor, and relative position of the grounding points on the fault current distribution are thoroughly analyzed. The simulation result revels that, the large amount of grid current discharged into earth ; when area of grounding grid is large, or low soil resistivity, or the high resistivity and permeability of the over head line conductors, or large distance between the grounding points. The fault current distribution in power system depends on type of fault, location of fault and conductor configuration. Fault current is composed of two components. 34

2 First one is supplied by local sources such as substation transformer, inductive reactors etc and other feed by terminal substations via phase conductors. The fault current supplied by phase conductors is three times zero sequence current of the line 3I 0. When fault occurs, part of the fault current is diverted by overhead ground wires or neutral conductor connected to the substation grid and remaining current flows through the earth and returns to terminal substation which feed the fault. This current is known as grid current which is responsible for safety touch and step voltages. The current diverted by the over head ground wires discharge to earth through towers and terminal substation grid. The complete analysis of fault current division factor and ground resistance measuring current distribution factor and their correlation is available [19]. The fault current division factor is the ratio of sum of the fault current diverted by ground wires to the sum of fault current supplied by all the phase conductor lines. The current supplied for ground resistance measurement by battery operated meter is also distributed in the same manner except that there is no current distribution in the phase conductors. Thus, the measuring current division factor is the ratio of sum of the currents diverted by ground wires to the total current supplied for measurement. The fault current division factor is greater than the measuring current division factor. This is because the fault current discharged by ground wires are due to two phenomenons like conduction and mutual induction between ground wires and phase conductors. However, in case of measuring current, there is no induction effect. Selection of material for earthing system in substation shall be governed by both technical and economical considerations. Substation earthing systems shall be designed to produce the desired technical requirements at optimum cost. But in many cases this consideration has not always formed the basis of selection of material for earthing grid of a substation and preference is given to copper material. Considering the scarcity, high cost and pilferages it is necessary to use other cost effective materials for earthing grids. Generalized formula for calculation of the sizes of earthing conductors of steel, copper and other materials, basic considerations for determining corrosion allowance to be provided while calculating the size of steel conductor for reliable performance is presented [20]. Corrosion of steel depends on resistivity of the soil hence study should be extended to evaluate the effect of seasonal changes on resistivity of the soil to obtain the changes in corrosion factor and the effect of DC offset current should also be taken into account. 35

3 Optimum design of substation grounding in a two layer earth structure is presented. The computer program had been developed which calculate the surface potential, ground resistance and potential probe location of soil resistivity measurement in a two layer soil model with complex grid. The complete optimization procedure has been divided into three parts. In first part, theoretical aspects of the computer program and two analytical methods of potential calculations, titled the summation and the integration methods are described. The second part explains the comparison of experimental results with the analytical results for different configuration of grounding grids. The comparisons of grid parameters such as ground resistance, ground potential rise, mesh and step voltage in two layer soil structure is presented. Authors had also developed the scale down model in two layer earth structure. In third part, detailed study of influence of top layer height, bottom layer and number of meshes on performance of grounding grid is explained. It is concluded that the results shows good agreement with Schwartz formula whereas the Laurent s formula and IEEE STD 80 formulae reflects the pessimistic results [21]. In equally spaced grounding conductors, the grid becomes over designed in central region of the grid and under designed at perimeter and corner of the grid; as a result, it makes surface potential more non uniform. To overcome this difficulty, the unequal conductor spacing technique was first time proposed by G J Sverak [22]. Author had presented the computer program for optimization of grounding grid with considering the effect of spacing geometry. The technique is based on the existing method of grid design documented in IEEE Guide No. 80 for uniform soil model. No ground rods are used in the program. Further, the procedure for finding grid conductor spacing is not presented. Many of the researchers had presented the equations for the computation of surface potential near and above the buried ground electrode are based on the assumption that leakage current density is the same for all the grid conductors near the periphery and conductors in central region of grid. However, this assumption is wrong and gives pessimistic results. In actual practice, for uniformly distributed grid conductors, the leakage current density, dissipated by the conductors near the periphery is much more than the conductors in middle part of the grid due to shielding effect. The equations for leakage current density along with surface potential for uniform and non uniform distribution of conductors are presented [23]. The solution 36

4 to this problem is to calculate the values of leakage currents in segments of the conductors, and using these leakage current values to compute the voltage at any desired point on the surface. The method presented here permits the effects of variation of leakage current density caused due to the proximity effect of parallel conductors, cross conductors, angled conductors, and end effects. This method can be used to find the surface potential in the vicinity of grounding grid made up of inter connected conductors of any shape and size such as square, rectangular, stars, polygons and conductors oriented at any angle in uniform soil. The program requires very less memory and speed of calculation is very high. It is necessary to extend the research for multilayer soil model. Substation grounding grid design plays an important role in high voltage substations systems. Usually, a substation ground grid is composed of horizontally buried parallel conductors at equal distance designed by utilizing standard procedures. The purpose of grounding grid design is to retain step and touch voltages within the safety tolerance limits and to keep ground resistance small. Various researchers have discussed the influence of many factors on grounding resistance R, step and touch voltages for obtaining the optimum design of equally spaced grounding grid, are the number of grid conductors, buried depth, conductor diameter, conductor spacing, ground rods and grounding grid configuration. However, equally spaced grounding grids have several disadvantages. Because of shielding and fringing effect, more current emanates from peripheral conductors, of a grid resulting in touch voltages on corner of the grid much higher than those in center. Unequal spacing technique has been proposed in published research literature, to overcome the drawbacks of equally spaced grid. The method of determining the conductor spacing and designing suitable unequally spaced grid is discussed [24]. All calculations were done by using computer programming and scaled down model measurements have been conducted on many grids and field test have been performed on several installed grounding grids. However, the effect of soil model and number of vertical rods are not considered while determining the spacing of the grid conductors. The safety of the grounding system for equally or unequally spaced grounding conductors, with or without vertical ground rods is presented by numerical analysis [25]. The effect of number of grounding conductors on grounding resistance, touch and step voltage is presented. The relationship between ground resistance and touch 37

5 and step voltage is developed in uniform soil. The effect of conductor spacing on performance of grounding grid is analyzed. Use of vertical rods decreases the grid resistance, touch and step voltages. At the same time it also decreases the number of grid conductors required for the same level of safety. The comparison of touch voltage for equally and unequally spaced grid is presented graphically. For cost effective grounding design, unequally spaced grid is recommended. A new method of optimization of grounding grid keeping safety touch and step voltage below the maximum permissible limit set by IEEE std 80 is proposed [26]. The proposed methods are based on evolutionary computation (EC) and using genetic algorithm for rectangular grids. However, according to authors, it can be used for any shape of grounding grid. The grid conductor spacings obtained with the genetic algorithm shows little expensive than the optimal spacing of grid conductors using EC. It is due to fine adjustment of conductors and superiority of evolutionary method. The work should be extended for the use of ground rods. Quantitative arrangement of the grid conductors is a hot topic of discussion even today. Many researchers are working on this issue. Numbers of reviewed publications are available in the literature. For a given amount the conductors, optimization can be achieved by arranging the conductors in such fashion that the maximum touch voltage generated would be minimum. For unequal spacing of conductors, it is well known that more conductors are concentrated along periphery than the central region of the grid. An algorithm had been proposed for finding optimal unequal spacing of grid conductors for square grounding grids for given values of fault current, cross-section of the cylindrical conductors, burying depth and resistivity of the soil [27].The outcome of algorithm is based on finding the optimal compression ratio OCR which results minimum touch voltage. The work is limited to square grid which should be extend to rectangular grid. An algorithm had been developed using MATLAB software for the optimization of grounding grid cost. It is based on charge simulation method. The predefined data is provided as an input to algorithm. The data like number of conductors length in X and Y directions, number of meshes per side, conductor diameter, number of rods with their diameter and length. Algorithms select the input data one by one and calculate the safety parameters and stops at convergence [28]. 38

6 Two mathematical techniques; one by using multiple coefficients and other by power coefficients was suggested to get different grid configurations [29]. Optimal value of coefficient which gives the minimum value of touch voltage and also optimizes the length of conductors is presented. Number of grids are analyzed with different values multiple coefficient and power coefficients with and without ground rods. The effect of number of conductors on grounding resistance, ground potential rise, touch and step voltages at different values of power and multiple coefficients are discussed. The results obtained are validated using ETAP software. Earth surface potentials and GPR of substation grounding grid in a uniform or non-uniform soil is presented by using image theory [30]. GPR and surface potential are evaluated for various values of grid compression ratios. For CR=1, grid conductors are uniformly distributed. Lesser the value of CR more is the non uniformity of grounding grid. Higher the value of CR, grid approaches to uniform spacing of conductors and increases the touch voltage reducing the safety level. At a certain compression ratio, touch voltage, step voltage and grid resistance becomes minimum is known as optimal compression ratio. The grounding system earth surface potential under transient condition has also analyzed. Further, the grounding impedance is obtained under transient condition of lightening. It is necessary to further study the effect of lightening transients on touch and step voltage and percentage of utilisation of grounding grid. Different methods used by various researches to obtain grid conductor spacings which can give the optimum number of conductors and reduce the cost are available in literatures. Optimization of grounding grid in uniform soil is depicted [31].It uses the rational number of grounding conductors, which can distribute the leakage current uniformly by minimizing the shielding effect and markedly decreases touch voltage and step voltage on the grounding grid. The methodology is also applicable to equally spaced as well as unequally spaced grounding grids. But the methodology does not use the ground rods. Determination of earth surface potential for calculation of touch and step voltage become a topic of interest among the researchers. Two analytical methods are proposed for finding grid resistance and earth surface potential. The first one is the charge simulation method (CSM) and the other is the boundary element method 39

7 (BEM). Both the methods are based on numerical calculations [32]. These methods are used for the analysis of grounding grid parameters like grid resistance, touch voltage, step voltage and ground potential rise in uniform soil model. Moreover, the effect of number and location of vertical ground rods, on grounding parameters is analysed. It is necessary to extend the work for two layer and multilayer soil model. The cost optimization of grounding system by keeping safety touch and step voltage less than the maximum permissible limit in accordance with IEEE std has become a most important topic all over the world among the researchers. Many publications are available in reviewed literature on these issues. A new mathematical model has been proposed for the minimization of cost function. It calculate number of conductors required, conductor diameter, distance between parallel conductors, depth of grid burial, number of rods, length of rods, total area of excavation and installation in a iterative way. A novel hybrid particle swarm genetic algorithm optimization (HPSGAO) method is proposed for designing optimal grounding grid of HV substation [33]. Simultaneously, two other techniques based on genetic algorithm optimization (GAO) and particle swarm one (PSO) have been also elaborated. The cost optimization problem is achieved by minimizing the cost function of the grounding grid. Results revels that HPSGAO technique is superior to GAO and PSO methods as far as cost and time of operation is concern. A simplified approach to the design of substation grounding grids in nonuniform soil implemented at Florida Power Corporation is based on interpretation of the application of the IEEE Standard 80 equations and data obtained from actual field test is available [34]. According to authors while design the substation grounding grid ; the parameters like ground grid resistance, grid current, overhead ground wire (OHGW) details, mesh voltage, ground potential rise (GPR), and neutral conductor influence, etc. are to be consider interdependent on each other. The complete interpretation of the grounding grid design based on short circuit current, fault current distribution, fault clearing time, resistivity of rock layer, allowable touch and step voltages, area available for substation grid, conductor spacing and irregularity factor, grid resistance and use of ground rods duly taking into account the resistance of overhead ground wires, neutral connections is presented. Author had selected multilayer soil model having top and bottom layer resistivity along with their depth randomly, based on measured resistivity field data and correlated with the IEEE 40

8 std.80 equations for the analysis of substation grid. However, such analysis / methodology may be true for one site but could not be generalized unless tested and verified at several sites. An extensive parametric study of grounding grid and performance in multilayer soil structure is discussed first time [35]. Various practical cases has been examined, and the grounding grid resistances, current distributions, earth surface potentials and touch potentials have been presented and compared for different soil structures. Effect of frozen/ partially frozen, soil conditions on the multilayer structure of soil must be achieved. Effect of soil freezing and thawing is discussed [36].However, the multilayer analysis is restricted to vertical layers and not for horizontal layers. In some practical situation, the soil resistivity changes one vertical layer to another as well as horizontally. A method to determine equivalent resistivity of heterogeneous soil to be used in the available expressions for uniform soil, employed to calculate ground resistance, mesh and step voltages are discussed [37]. The results obtained with proposed equivalent resistivity are compared with results obtained from two layer and multilayer model of the soil and with the results from the computer programming developed by authors. The proposed method does not required to model the heterogeneity of the earth as it is based directly on potential measurement made at site. The effect of frozen and thawing soil are not considered while calculation of mesh and step voltages. A detailed analysis of effects of grid configuration on grounding performance has been presented for uniform and horizontally stratified soils with multiple layers [38]. The result analysis reveals that the most efficient and cost effective design is highly dependent on soil structure types and characteristics. In the absence of ground rods, grounding grids with uniform mesh size are quite efficient if soils having a thin high resistivity top soil. While grids with small mesh size, at the periphery of the grid provide optimum performance in uniform soil and soil with low resistivity top soil. According to authors ground rods were found to be effective only when significant portion of their length is lying in low resistivity soil. The study is extended to multiple soil structure considering variation of soil resistivity up to vertical three layers only and horizontal layer variation is not considered. The comparison between uniformly 41

9 and non-uniformly spaced grounding grid conductors is not carried out to differentiate which method of grounding grid design is efficient. The grounding system analysis involves the determination of Green s function generated by a point current source in the multilayer soil. However, when the number of layers become more than three, the analytical expression of Green s function becomes too complicated. Further, it needs to calculate much number of infinite integrals including Bessel functions. J. Zou et al developed the technique which do not requires calculation of analytical expressions of Greens function [39]. It provides sampling technology of Green s function in an iterative way and computing the Greens function using the vector matrix pencil technology which enhances the computing efficiency by large amount. The software developed can be applied to any grounding system having an arbitrarily layered soil without deriving the analytical expression of Green s function. According to authors, the technique can be extended to resolve a high-frequency grounding problem. An efficient algorithm for determination of horizontal multilayer soil model by using the test data of soil resistivity measured by Wenner four probe method is available [40].It employs the complex image method.it has been very effectively used to calculate the apparent resistivities of multilayer soil, which had increased the efficiency and the accuracy of the computation. The partial derivatives of the apparent resistivities with respect to the soil parameters can be directly calculated from complex images that had been obtained from complex image method. The method requires less number of iterations and has very high speed of calculation of multilayer soil parameters. According to authors, any number of soil layers can be analyzed by this method. It can also be used to investigate the mineral deposits at different layers. An adequate grounding grid is a fundamental requirement to maintain reliable power system operation. The earth surface potential distribution in a substation should meet the IEEE Std 80 requirements, for touch and step voltages, by maintaining low ground resistance. But in the area of high soil resistivity decreasing ground resistance of a grounding system may constitute a formidable task. Various methods are proposed by various researchers to decrease the ground resistance. A new method is proposed [41] to decrease ground resistance. The proposed method requires three steps a) Drilling deep holes in ground. b) Filling the holes with low resistivity 42

10 material under pressure and c) Creating cracks in the soil by means of explosions in the holes. An application of this method to power system grounding is presented together with measurement results. The major impedance of the grounding system consists of four parts, the impedance of bonding leads, the impedance of ground conductor, contact resistance between ground conductor and soil and the distributed resistance to the remote earth. While calculation of ground resistance, first three parts contributing to resistance are neglected. Computed resistance values are correspond to grid and vertical rod only, without considering low resistivity material field cracks. Simplified equations for mesh and step voltages in an ac substation having grounding grid of any arbitrary shape are presented. The formulae available in reviewed published literature and recommend by IEEE Std are only for square and rectangular shape grids. However, in actual practice, one has to deal with any arbitrary shape while designing the grounding grid. The shape of grounding grid depends on area available. Authors have proposed the simplified equations for finding mesh and step voltages of any shape of grid that include square, rectangular, triangular, T-shape, L-shape or any practical arbitrary shape. The authors have developed a software program; RESIS, to determine the ground resistance of a grounding system has been extended for calculation of mesh and step voltages. The program was based on finite element method. The proposed method results for touch and step voltage, shows good agreement with results published in literature. These equations would be very useful for grounding grid design engineers [42]. The calculation of ground resistance of the feet has paramount importance as it controls the flow of shock current through the human body. The magnitude of flow of current that makes ventricular fibrillation is the criterion for calculation of limiting value of touch and step voltages. The method for determination ground resistance of human feet in high voltage switchyard is available [43]. It is modified by the proximity effect to find the mutual resistance between the two feet. The principle of Thevenin s theorem was applied to find the feet resistance. The resistance to the flow of shock current path consists of body resistance, resistance of hand gloves, shoes and socks and resistance offered by two feet in parallel. The resistance of gloves, shoes and socks is negligible where as body resistance of 1000 Ω is recommended by IEEE std 80. The foot is modeled as circular plate or rectangular plate to find the ground resistance. The proposed method is verified by using analog model study. 43

11 The various objectives of substation groundings are the personal safety and reliable operation of substation equipments during flow of high fault current to substation grounding which may be due to lightning or line to ground fault (L-G). To assure personal safety, substation grounding should have low grounding resistance, touch and step voltages within tolerable human limit. According to IEEE std 80, in substation area there are some points which are considered as a point of special danger. Various techniques are proposed in various research literatures to control the touch voltage by the way of metal plate above the grid to equalize the potential distribution. There are two approaches to control body current. a) Uniform surface potential which demands closer grounding grid, increasing the cost of substation grounding. b) use of the high resistivity surface layer material. To make the substation grounding cost effective, a new technique of design substation grounding system by adding the plastic sheets above grounding grid, at some hazardous area such as equipments installations, at lightning arresters etc. and how to increase the foot grounding resistance [44]. According to authors, new technique uses 10 to 30 % less grounding grid material for safe and economic design. The proposed computer program used for design of grounding system modifies automatically the design parameters such as, space between parallel conductors, number of meshes, length of conductors, depth of burial grounding grid, thickness of crushed rock, and the depth of plastic sheets. While use of plastic sheets, shall be subjected to life and mechanical damage over a period of time, which may impose a serious safety concerns. Substation grounding grid made of interconnected conductor bars, buried under earth has to meet safety and system requirements. While simulation of grounding grid most of the methods proposed in published literatures were based on equal potential model. That is ignoring the resistance of the conductor. When the radius of the conductor is very small, the resistance may cause obvious potential difference of the grid. Hence, it is always required to investigate the proper radius of the conductor of the grid of certain sizes, buried in the soil of certain value of conductivity. The equal potential model is not adequate to calculate proper radius of the grid conductor. The method forming the equation system for simulating substation grounding grid with unequal potential is presented based on theory of combination of field and circuit [45]. The equation system with potential at discrete point of the grid had obtained by the method of node analysis circuit theory and the determination of mutual resistance 44

12 between conductors located in conducting media concerns with electric field theory. According to authors, the presented method is capable to calculate the grid with arbitrary structure of floating electrodes in multiple layer earth models. Comparison of calculation results of grid of equal and unequal position is presented. Adequacy and effectiveness of substation grounding is governed by the value of ground resistance for reliable operation and to provide adequate protection for personal and apparatus during fault conditions. The maintenance of low value of ground resistance is essential throughout the year. The soil resistivity is governed by various factors such as soil type, nature of soil, compactness and dissolved substances, temperature, moisture content etc. Out of various factors, concentration and composition of dissolved substances, temperature and moisture content of the soil will vary from season to season resulting into increasing ground resistance above the required value leading to series of failures, loss of revenue and imposing serious threat to personal safety. Hence, maintenance of grounding system resistance is vital important from time to time. Best approach is automatic monitoring and maintenance of the grounding system which is proposed [46]. This paper describes the system that can improve the grounding system reliability and efficiency for a substation based on computer aided technique. The proposed technique consists of PC, Terminal Measurement Unit, Data Acquisition Unit, and Relay Board. It measures the ground resistance and has ability to actuate fluid sprinkler system at substation grounding. For better maintenance of the resistivity of the soil, the moisture content shall be 20 % and concentration on dissolved substances. For cost effective maintenance of soil resistivity, it is necessary to evaluate which parameter has resulted in to increase in resistivity of soil. It may happen that moisture content may be more than 20 % and resistivity of soil has increased due to less concentration dissolved substances. The proposed system may start sprinkler to increase moisture content which will not help to decrease ground resistance. For automatic maintenance of ground resistance, it is necessary to decide whether moisture or dissolved substances are required to be supplemented. The sprinkler system may impose certain safety issues in a H.V. / E.H.V. switch yard. Hence, to avoid the same, it is necessary to provide a drip type system. The grounding system of H.V. installation in general fulfills its function only in the moment of unbalanced fault, when increased potential appears at the places where 45

13 normally they do not exist. In order to ensure protection against undesired consequences, such as loss of human lives, burning of grounding wires and damage to telecommunication equipments entering the station, it is necessary to evaluate the values of these potentials as accurately as possible. Therefore, when designing substation grounding it is of prime importance to evaluate correctly the ground fault current distribution. To limit the potentials, it is necessary to take measures by which the ground faults current distribution to be changed a dangerous voltages appearing in the stations and its vicinity. The solution can be achieved by special custom constructed the supply furnishing sufficiently low value of reduction factor. However, if the feeding line is relatively long the solution may prove to be very expensive. The use of bare copper wire of a certain length in the same trench has the cable feeding line for changing the ground fault current distribution helps to decrease dangerous voltages. An analytical procedure for the determination of the part of ground fault current enaminating the substation grounding grid in the conductor when the particular measure has been taken to reduce grid current is proposed [47]. According to authors, the presented method enables a quick and accurate evaluation of copper wire size providing the maximum possible reduction of substation grid current. IEEE Std , A Guide for safety in A.C. substation grounding, was developed to provide guidance pertains to safe grounding of AC substations. Three editions of Std. IEEE -80 are published during 1961, 1976, 1986 to address the various issues related to AC substation grounding. The revisions in substation grounding Std. 80 are carried out for substation grounding design and analysis. The revised IEEE Std. -80 took place in The major changes with respective 1986 revision which affects the grounding design and analysis presented [48]. Comparisons are made for portions of the two versions of the guide where the major changes took place. The proposed changes in 2000 are, calculation of surface layer de rating factor, application of the decrement factor for the DC offset which affects the permissible touch and step voltages. Uniform soil model is considered during analysis, however uniform soil seldom exists. To overcome this drawback, in IEEE Std , Multilayer soil model is added. The grounding practice of substation fence is also discussed. The substation grounding system always insure that the Ground Potential Rise due to ground fault would not lead to destroy power apparatus and the mean time 46

14 should insure that the step voltage and touch voltage would not harm the operator or other peoples. When a grounding system is designed, the fundamental method to ensure the safety of human beings and power apparatus is to control step voltage and touch voltages in their respective safe region, which are governed by ground resistance variation. The resistivity of surface layer would be changed, in different seasons, which would affect the safety of grounding system and grounding resistance. Hence, the seasonal variation in ground resistance and safety of grounding system needs to be analyzed visually. Previously, the measured soil resistivity was multiplied by seasonal factor to consider the influence of winter or dry season on soil. But this method does not satisfy the actual conditions since the surface soil resistivity is only affected in different seasons. Hence, analysis of seasonal influences on safety of substation grounding systems is very much essential. Analysis of seasonal influences on the safety of grounding system by numerical analysis is presented [49].The authors have carried out the analysis of influences of rainy seasons and freezing seasons on the grounding resistance, step and touch voltages, considering the granite layer. While doing analysis the thickness of affected soil layer during freezing seasons is considered up to 1.6 meters. The effect of depth of freeze penetration above the grid and below the grid is discussed. The analyses of seasonal influences are required to study for multilayer soil model where the resistivity of soil layer changes insignificantly in vertical directions. The ground resistance of the substation grounding for a given fault current determines hazardous voltage exists inside or around the substation or generating stations. Hence, ground resistance is important parameter which governs the safety of people and equipments. When the grounding resistance is too high and earth faults occur, the person may be killed or injured and equipment may be damaged. Hence, determination of resistance of grounding grid precisely is one of the important aspects. The method for calculation of resistance of grounding grid using Finite Element Method is proposed [50]. The proposed method enable the user to calculate the resistance of grounding grid of any shapes, e.g. square, rectangular, triangular, T shape, L shape etc. in uniform, two layer and multilayer soil and soil with finite heterogeneities or horizontally non stratified multilayer soil structure viz. three consecutive quickly performed tests. The proposed method is tested in practice to analyze symmetrical and non symmetrical soil model of any shape. It is necessary to 47

15 extent the work for multilayer soil with equally as well unequally spaced grid conductors. The effect of freezing and thawing of soil layer is not considered while calculating grid resistance. With increase in fault level in today s interconnected power system; there is major emphasis on safety. In other words, it is necessary to determine maximum value of tolerable electric current flowing in the human body, in order to provide protection against electrical shock. Considering the same, the assessment of substation grounding design is necessary. A practical case study for the safety assessment of substation grounding grid design of 169/69 KV indoor and outdoor type substation in the system of Taiwan Power company is presented [51].Safety assessment of ground grid design of Indoor and Outdoor type substation in TPC system is carried out by computation of mesh and step voltages compared to minimum step and touch voltages, for humans of weight 50 kg and 70 kg are quantified. According to authors the factor S f, depth of grounding grid, grid conductors spacing, governs the safety design of substation grounding. The proposed work had carried out on customer s 169/69 KV substation to ascertain the adequacy of safety of peoples. Detailed analysis of influence of ground rods on performance of substation grounding grid buried in two layer soil model is discussed. To meet safety criterion, determination of rod length, number and location is evaluated.the study discusses the inadequacy of uniform soil when actual two layer structure exists. It compares the ground resistances and touch voltage for uniform soil model and two layer soil models with different top layer height and different resistivity reflection factors. When the upper layer height is more than the rod length, then the effect of two layer soil model vanishes. The resistance of rod becomes equivalent to uniform soil with top layer resistivity. As soon as top layer height becomes smaller than the rod length, there will be abrupt increase/ decrease of resistance; depending on resistivity reflection factor ( k) whether positive or negative. The resistance increases for k > 0 and decreases with k < 0.Thus, ground rods are effective only when its substantial length penetrates into low resistivity subsoil [52]. The measurement of apparent soil resistivity by driven rod method and by Wenner s four probe method is compared and presented [53]. A simple formula for determination of apparent soil resistivity is derived for driven rod method. It has been 48

16 successfully validated using test results of Wenner s four probe method. Further, the limitations of both the methods are discussed. It is concluded that, driven rod method is more tedious and time consuming while dealing with extensive soil resistivity measurements. Calculation of earth resistance for a deep-driven rod in a multi-layer horizontal earth structure using numerical analysis is presented. The authors had extended their work for two layer soil model to multilayer horizontal soil model. The analysis is based on determination of potential in each layer which led to find the equations for ground resistance in multilayer driven rods. It also presents the influence of various soil layers on the resistance. The proposed equations of resistance calculation would aid the grounding grid designer, to calculate optimum length of bore to meet the targeted ground resistance [54]. The rapid economic development in developing countries imposes an increasing demand for electric power. The capacity and size of electric power stations continues to increase to meet the increasing power demand. Nowadays very advanced and sensitive electronic equipments are installed. Therefore, better grounding practices are essential to adequately protect substation equipments and personals. According to authors, a complete grounding analysis consists of four major steps: measurements of soil resistivity, ground impedance, calculations of fault current distribution and grounding system performance analysis. When dealing with extensive grounding systems in large electric power stations, it is necessary to pay special attention on grounding analysis. A complete grounding analysis of an extensive grounding system of an large electrical power station is presented [55].The proposed work addresses, how to construct an adequate soil structure based on measurement of soil resistivity with short and long probe spacings. How to measure and interpret the ground impedance of large grounding system. How to determine if, worst fault occurs inside or outside the station due to presence of local sources such as converters, synchronous condensers, filter banks and transformers. How to selects appropriate fault location to cover the worst scenarios in the system of grounding system performance. The proposed study was carried out for 400 KV substations and necessary to extend for bigger UHV substations. 49

17 The estimation of ground resistance of grounding grid is an important step in determining the size and basic layout of grounding system for an A.C. substation. The various methods proposed in published research literature uses the following simplifying hypotheses such as soil is an infinite medium, which is flat, isotropic, and stratified in layers of uniform thickness, the laws of electromagnetism may be applied to calculate ground resistance and potential distribution in soil, ground grid rods are assumed to be linear, interconnected, and buried close to the soil surface, grounding grid behavior at power frequency (50 Hz) can be determined using electromagnetic field analysis techniques for stationary fields (propagation time is rejected). A practical approach for determination of ground resistance of a grounding grid is proposed [56]. A new method of calculation of grounding grid resistance using FEM is proposed considering ground resistance is independent of earth fault current. The results obtained using the proposed method is compared with results measured experimentally and published by other researcher. The result obtained in proposed method is used as a basis for formulating and easy to use equations for calculating the ground resistance of grounding grid in uniform soil. However, in many practical cases, the soil is non-uniform and the resistivity of the soil during freezing conditions varies drastically hence it is necessary to extend the proposed work for determination of grounding resistance for non-uniform soils with unequally spaced grounding grid. Ideally the earthing system should be designed with zero impedance to provide an effective discharge of fault currents and to avoid the ground potential rise at an around the substation. In practice, however zero impedance could never be achieved. In order to obtain adequate grounding system performance, earthing system should be designed with low resistance, and optimum design of earthing system can be achieved by considering two parameters Earth Electrode Geometry / configuration and soil properties. Investigation on characteristics of low resistivity material under high magnitude fast impulses is reported [57]. The characteristics of Sodium Chloride (NaCl) were investigated under both low and high magnitude impulse current in order to assess the effect on electrical properties of sand Salt mixture wet clay and sand mixed with a controlled amount of NaCl and water content were used as a test media in the proposed work. It was shown that for lower conductivity of the test media (high percentage of NaCl and moisture content) the 50

18 resistance becomes less current dependent. It is necessary to extend the proposed work for other dissolved substances than NaCl. The Ground Potential Rise is governed by ground resistance and current during worst fault condition. The depth of grid layer influences the grounding resistance and hence the GPR. A substation grounding grid analysis with variation of soil layer, depth of a grounding grid system of a practical 22 KV substation is discussed [58]. The soil resistivity was interpreted as a two soil layer structure with low resistivity top layer and high resistivity bottom layer. The study of grounding grid system installed with constant three meter ground rods, when the ground layer depth of grid increases it directly affects the decrease in value of GPR. According to author s higher depth of high resistivity bottom layer, result more value of GPR. The grid with different ground rod lengths also give the different results, GPR continuously decreases when the rod lengthen, even though the bottom layer resistivity is greater than top layer. It is necessary to study the effect of ground layer depth on GPR for multilayer soil and non stratified horizontal soil layers. The same authors extended their work for substation grounding grid analysis with variation of soil layer depth with required number of ground rods to be incorporated in the grounding grid. The design and construction of the grounding grid in the area with top layer resistivity less than bottom layer resistivity can lessen the number of ground rods used in the grid because the value of GPR is insignificantly different [59]. According to authors, deeper the grid buried, less the value of GPR. The study had conducted for different cases such as ground grid with constant ground rods, increasing length of rods. It is necessary to extend the study for different geometry / shape of grids in multilayer soil. The performance of grounding system subject to high impedance; current plays an important role in safe and reliable operation of the power system. The lightning protection effects of substation grounding are related to impulse characteristics of grounding devices, for transmission line towers and grounding grids for the substations. In order to obtain correct design of an electrical system with respect to the protection of installations against anomalous events, it is fundamental to predict the impulse performance of the grounding system. The influence of different parameters of transient characteristics of grounding grid for substation considering 51

19 soil ionization subjected to lightning impulse current are analyzed and effect of area of grounding grid under lightning is discussed [60]. The authors proposed the effective method, transient performance analysis of the grounding grid with is a numerical calculation approach based on circuit model of distributed time variable parameters. It accurately takes into consideration of non linear effects of breakdown in the soil surrounding the grounding conductors. The soil was assumed to be homogenous and isotropic. It is necessary to evaluate lightning impulse performance of grounding grid for non homogenous soil. Lightning is the external transient phenomenon which results outages of transmission-lines, damages to equipments, catastrophic- induced over voltages, as well as electromagnetic-compatibility (EMC) problems. Grounding grid has to dissipate large amount of current within a very small duration, when lightning strikes on electric substation. Thus, lightning performance of a grounding system has paramount importance for reliable operation of power system. This high magnitude of current may damage the equipments and threaten the safety of the personnel in substation. The transient nature of the lightning impulse current causes a significant amount of potential difference between any two points of the grounding grid resulting in flow of shock current through the human body. It can cause the ventricular fibrillation; in which death may occur. The lightening performance of grounding grid had been evaluated using Monte Carlo simulation of the grounding grid response to the probabilistic distribution of the injected current is reported [61]. The frequencydomain analysis has been obtained by the use of the fast Fourier transform (FFT) and inverse FFT techniques. The proposed method used for evaluation of grounding grids during lightening can be used to determine the worst case condition of human safety. A method for finding resistance to earth of earthing grids buried in multi-layer soil using finite element method is proposed. Equations that are proposed for the resistance of grounding grid buried in homogeneous soil, two layer and three-layer soils are based upon the examination of a large set of grids and soil structures using finite element method. Resistance of grounding grid in multilayer soil has been obtained by making corrections in formula used for uniform soil. Empirical correction factors are introduced to modify the earth resistance formulae for non uniform soil. The results obtained are compared with other published methods and shows good agreements [62]. 52

20 The complete analysis of ground electrode resistance measurements in non uniform soil is presented [63]. Measurement of ground resistance is usually carried out by Fall of Potential Methods (FOP). The 61.2 % rule is suitable for the homogeneous soil only. However, for non homogeneous soil, true grounding grid resistance is given by zero slope region of fall of potential curve. For two layer soil model, the location of potential probe (PP) may vary from 0.5 to 0.9 times the distance between auxiliary current probe and centre of grounding system(d EC ), depending on values of soil resistivity reflection factor K. For negative values of K, the variation may be from 0.5 to and to 0.90 for positive values of K. The location of PP increases as magnitude of K increases. It also depends on upper layer height of two layer soil model. If the ratio of upper layer height h to D EC is small enough 0.01 or less it becomes uniform soil model with resistivity of bottom layer with probe position D EC. Conversely, when h/d EC is more than 5, it becomes a very thick layer. It may be treated as uniform soil model with upper layer resistivity. Three or four layer soil model may be converted to equivalent two layer model for location of PP, else as stated earlier, zero slopes (flat portion) of fall of potential curve can be considered for analysis. Influence of overhead transmission line on ground impedance measurement of large scale substation is presented. How to simply and precisely measure the ground impedance of large substations is the problem of power systems. If the impedance measurement is carried out with over head ground wires connected to the grounding grid, the measured grounding impedance would be smaller than the actual impedance of the grounding system. This is because of parallel connection of impedance of over head ground wires with substation grid impedance. In other way this can be interpreted as; the part of the current injected in the grounding system for the measurement purpose get diverted to ground wires, and hence, reading shown by the meter for ground impedance would be less than the actual one. Analysis has been carried out on an actual 500-kV grounding system. Further, it discusses how to analyze the influence of the overhead ground wire on the measured grounding impedance and how to obtain the real grounding impedance of the grounding [64]. The effectiveness of grounding systems can be verified by the measurements of touch and step voltages and ground resistance. However, measurement suffers from some difficulties like placing of auxiliary current electrode out of influence of 53