International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 47 Analytical Methods for Earth Surface Potential Calculation for Grounding Grids Sherif S. M. Ghoneim1,3, 2,3Kamel A. Shoush 1 Suez University, Suez, Egypt, 2Al-Azhar University, Cairo, Egypt 3 Taif University, 21974, KSA, ghoneim_sherif2003@yahoo.com, kamel.shoush@yahoo.com 1 Abstract— The grounding resistance (Rg) as well as step and touch voltages determine the performance and quality of grounding grids. The estimation of grounding resistance values and the touch and step voltages is usually carried out by means of formulas and algorithms that take into account the mutual influence between the grid electrodes. Two methods analytical methods are used For Rg and earth surface potential (ESP) calculations. The first one is the charge simulation method (CSM) and the other is the boundary element method (BEM) from commercial software TOTBEM. The validation of two methods is explained by comparing their results and the other results that formulated in IEEE Guide for Safety in AC Substation Grounding (ANSI/IEEE std 80-1986). The soil is assumed to be homogeneous. Vertical rods that added to the grounding grids play an important role in improving the performance of it by reducing not only the grid resistance but also the step and touch voltages to values that safe for human. The paper investigates the effect of vertical rods location on Rg and ESP. A comparison between the addition of horizontal rods and vertical rods to the grounding grids is investigated. Also the paper focuses on the effect of profile location on some parameters on Rg and ESP. Keywords— Grounding grids, Step voltage, Touch voltage, Boundary element approach, and vertical rod location. 1. INTRODUCTION A safe grounding design has two main objectives: 1. To carry the electric currents into earth under normal and fault conditions without exceeding operating and equipment limits or adversely affecting continuity of service. 2. To ensure that the person in the vicinity of grounded facilities is not exposed to the danger of electric shock. To attain these targets, the equivalent electrical resistance (Rg) of the system must be low enough to assure that fault currents dissipate mainly through the grounding grid into the earth, while maximum potential different between close points into the earth’s surface must be kept under certain tolerances (step, touch, and mesh voltages) [1]-[2]. The basic design quantities of the grounding grid are the resistance (Rg) (or ground potential rise (GPR), which is the product of the resistance by the grid current), touch (V t) and step (Vs) voltages. In a uniform soil, the resistance can be calculated with an acceptable accuracy using several simplifying assumptions [3-7]. Touch and step voltages are 1 difficult to calculate by simplified method but it determined by analytical expressions [2], [8-10]. A “Boundary Element Approach” technique for calculating the earth surface potential and then knowing the step and touch voltages, is developed [11]. In the last decades, some intuitive techniques for grounding grid analysis such as the Average Potential Method (APM) have been developed. A new Boundary Element Approach has been recently presented [12-14] that includes the above mentioned intuitive techniques as particular cases. In this kind of formulation the unknown quantity is the leakage current density, while the potential at an arbitrary point and the equivalent resistance for grounding grids must be computed subsequently. Vertical ground rods are considered the most suitable method for earth termination of electrical and lightning protection systems. The addition of the vertical ground rods to the grounding grid achieve a convenient design for grounding system by decreasing the grid resistance, the step and touch voltage to a safe values for human and public. Studying the effect of addition of vertical grounding rods at different locations on earth surface potential (ESP) is investigated in this paper. This study describes how can improve a design of grounding grid with vertical rods that achieve the convenient values of (Rg, Vt and Vs) with the minimal cost of design. Studying the effect of profile locations on earth surface potential (ESP) is also investigated. This study describes the role of profile locations on the values of (R g, Vt and Vs). Studying the volubility to add a horizontal wire or a vertical rod to improve the grounding resistance Rg, the step and touch voltages. This paper will present the comparison between to analytical methods that used for calculating the grounding resistance and earth surface potential, the first method is the Boundary Element Method that have been implemented in a computer aided design (CAD) system for grounding grids of electrical substations called TOTBEM [12], the second one is the Charge Simulation Method which is considered a practical method for calculating the fields and from its simplicity in representing the equipotential surfaces of the electrodes, its application to unbounded arrangements whose boundaries extend to infinity and its direct determination to the electric field [15]. Figure 1 explains the grounding system with grounded electrical equipment and shows the very important parameters in grounding system design. 1310803-7272-IJECS-IJENS © June 2013 IJENS IJENS International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 48 current and Vmin is the minimum surface potential in the grid boundary. Fig. 1: Illustration of the grounding system 2. BOUNDARY ELEMENT METHOD In this paper a computer aided design (CAD) system for grounding grids of electrical substations called TOTBEM [14] is presented to get the grounding resistance and earth surface potential. The effect of the vertical rods location on the earth surface potential and then on Vt and Vs using this technique is presented in this section. The characteristics of the grid are 50m*50m, and 75m*31.25m to achieve the equal area between two cases, the number of meshes is 16, the radius of the grid rods (r) is 0.005 m, the length of vertical rods (Lvr) is 2m, the radius of it (rvr) is (0.005m), the grid depth (h) is 0.5 m, the resistivity of the soil is 2000 ohm.m, and the total ground potential rise (GPR) is defined as 1V. The cases under study are shown in Figure 2. Figure 3 illustrates the 3 dimensions (3D) and contour profile for the case R014P16. It is clear that the ground potential rise (GPR) as well as distribution of the earth surface potential (ESP) during the current flow in the grounding system are important parameters for the protection against electric shock. The distribution of the earth surface potential helps us to determine the step and touch voltages, which are very important for human safe. By definition, the touch voltage is the difference between the ground potential rise (GPR) and the surface potential at the point where a person is standing while at the same time having his hands in contact with a grounded structure, and the mesh voltage is defined as the maximum touch voltage to be found within a mesh of a ground grid. The maximum touch voltage is the difference between the GPR and the lowest potential in the grid boundary [1]. The maximum percentage value of Vtouch is given by Max touch voltage% Vgrid Vt m in GPR 100 (1) Where, Vgrid is the ground potential rise (GPR), which equal the product of the equivalent resistance of grid and the fault Fig. 2: Grounding grids with different meshes and different locations of the vertical rods. (a. without vertical rods, b. with vertical rods at all point, c. the vertical rods at the points across the diagonal and center lines, d. the vertical rods at the perimeter of the grid only) Fig. 3: The distribution of EPS per GPR above the grounding grid (case R04P16 in two cases for rectangle and square grids) Furthermore, the maximum step voltage of a grid will be the highest value of step voltages of the grounding grid. The maximum step voltage can be calculated by using the slope of the secant line. The max step voltage occurs outside the grid boundary, where the slope of the recorded surface potential against distance is a maximum. 1310803-7272-IJECS-IJENS © June 2013 IJENS IJENS International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 Effect of the locations of the vertical rods on the grounding resistance and earth surface potential is presented in Figure 4. 250000 R04_4 R04P_4 R04P16_4 ESP(V) 200000 150000 Table I explains that if the number of meshes increases the grid resistance, step and touch voltages will decrease but we should take into account the total length of the conductors used in the grid (the cost of the grounding system), for example the case R08P gives us the good results but the length of copper used is 1062 m in this case, then the cost is much higher but we can choose the case R04P16 as the optimum case of design because it produces a suitable results and give economical cost. TABLE I GROUND GRID RESISTANCE, STEP AND TOUCH VOLTAGES AT DIFFERENT VERTICAL RODS LOCATIONS WITH GROUNDING GRIDS [ IF= 1000 A] 100000 50000 0 -50 49 0 50 100 150 Distance along the diagonal of the grid from its center (m) Fig. 4: The effect of vertical rod location on the earth surface potential for the case study of square grid It is seen from Figure 4 that the minimum earth surface potential doesn’t vary significantly since the vertical rods are connected to grid in the homogenous soil. The vertical rods that connect to the grid should be used in the multi-soil to reach to the lower resistivity soil. In contrast the grid potential rise decreases considerably when the fault current is considered as 10kA. As a result the touch voltage decreases with increase the number of vertical rods to the grid at the same number of meshes. A small effect in the earth surface potential when changing the vertical rods locations at the same number of meshes then the economical cost plays a great part for choosing the suitable design for the square grids. Figure 5 shows the variation of the touch voltage from the center of grid along the diagonal and it is clear that the maximum touch voltage is at the corner mesh of the grid at different soil resistivity. Case lt (m) Rg (ohm) GPR (kV) R02 R04 R08 R02P R04P R08P R08P57 R02P08 R04P16 R08P32 300 500 900 318 550 1062 1014 316 532 964 23.1 20.3 18.6 22.7 19.9 18.1 18.1 22.7 19.9 18.2 23.1 20.3 18.6 22.7 19.9 18.1 18.1 22.7 19.9 18.2 Vt max % of GPR 41.4 29 20 40.9 28 17.81 17.92 40.92 28 18.08 Vs max % of GPR 17 16 12 20 16 15 15 18 16 15 Figure 6 shows that an increase in the number of meshes makes the curve of earth surface potential much flatter and then a reduction in the grid resistance, touch and step voltages, also it is seen that the max touch voltage moves towards the corner mesh in the grid. ESP/ GPR.1 0.9 1 Me sh 4 Me sh 16 Mesh 36 Mesh 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -20 -10 0 10 Distance from the center of grid (m ) "Diagonal Profile". 20 Fig. 6: Effect of the number of meshes on the earth surface potential Fig. 5: Variation of touch voltage from the center of grid along the diagonal case (R04P16). Table II explains that an addition of ground rods with 2m lengths to the 4 meshes grounding grid for the case of study gives nearest results when add the horizontal conductors to the same grid. The difference in the max touch voltage between the two cases is 26.919 V but the difference in the total length of conductors is 62 m as shown from the table and hence, an increase in the cost of design occurs when add horizontal rods. Therefore the addition of vertical rods plays 1310803-7272-IJECS-IJENS © June 2013 IJENS IJENS International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 50 an important part to get the same results and decreases the cost of design. The effect of profile location of the cases shown in Figure 7 will be studied. Figures 8, 9 and 10 illustrate the effect of profile location, vertical rods and number of meshes for both earth surface potential and ground potential rise. It is noted that the number of meshes and the addition of vertical rods to the grid have a great effect for decreasing grid resistance and then both of ESP and GPR. TABLE II COMPARISON BETWEEN THE ADDITION OF VERTICAL RODS OR HORIZONTAL RODS TO A GROUNDING GRID No of meshes 36 4 No. of vertical rods 0 9 Vertical rods length (m) 0 2 Total grid length (m) 140 78 Resistance (Ω) 4.26 4.29 GPR (V) at 100 A 426 429 Max touch voltage % GPR 25.0 31.1 Max touch voltage (V) 106.5 133.419 Max step voltage % GPR 16 16 Fig. 8: The earth surface potential (ESP) and ground potential rise (GPR) at specified profile location and different number of meshes, with and without vertical rods for square grids It is seen from the results that the minimum earth surface potential doesn’t vary significantly when the number of meshes increases above 16 meshes, but in contrarily the grid potential rise decreases considerably. It is also seen in the pervious figures that the earth surface potential of 16 meshes is above that of 36 meshes because the profile location of the 16 meshes lies above one conductor of the grid which causes the earth surface potential is high in this region. Fig. 9: The earth surface potential (ESP) and ground potential rise (GPR) at specified profile location and different number of meshes, without vertical rods for square grids Fig. 10: The earth surface potential (ESP) and ground potential rise (GPR) at specified profile location and different number of meshes, with vertical rods for square grids. Fig. 7: Grounding grids with different profile locations. (a. square grids (20m*20m) without vertical rods, b. square grids (20m*20m) with vertical rods at all, , c. rectangle grids (10x40m) without vertical rods, d. rectangle grids (10x40m) with vertical rods at all point) Table III shows the values of ground grid resistance, step and touch voltages for the different profile locations of different configurations of grounding grids for fault current (If) of 100 A. It is clear from the table that if the number of meshes increases the grid resistance as well as the step and touch voltages will decrease. The rectangular grid offers good values of the parameters (grid resistance, step and touch voltages) and then represents an improved design for 1310803-7272-IJECS-IJENS © June 2013 IJENS IJENS International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 grounding systems. The profile location also plays a great deal for reducing the values of these parameters, the touch voltage decreases when the profile location comes near to the center line of grid. TABLE III THE EFFECT OF PROFILE LOCATION ON THE STEP AND TOUCH VOLTAGE FOR DIFFERENT RECTANGLE AND SQUARE GRIDS Case R02_1 R02_2 R02P_1 R02P_2 R04_1 R04_2 R04P_1 R04P_2 R06_1 R06_2 R06P_1 R06P_2 S02_2 S02P_2 S04_2 S04P_2 S06_2 S06P_2 R (ohm) 2.273 2.273 2.179 2.179 2.079 2.079 1.965 1.965 2.004 2.004 1.868 1.868 2.626 2.491 2.363 2.20 2.260 2.072 GPR (V) 227.34 227.34 217.97 217.97 207.94 207.94 196.56 196.56 200.47 200.47 186.85 186.85 262.68 249.16 236.30 220.07 226.07 207.28 Vsmax /GPR 0.096 0.096 0.091 0.100 0.091 0.12 0.073 0.093 0.099 0.114 0.082 0.101 0.111 0.1 0.144 0.133 0.110 0.096 Vtmax/ GPR 0.2566 0.2918 0.2567 0.2751 0.1622 0.17 0.1567 0.168 0.1184 0.132 0.116 0.1162 0.3557 0.336 0.1346 0.1143 0.151 0.1214 If at GPR=1 0.4398 0.4398 0.4398 0.4398 0.4808 0.4808 0.5087 0.5087 0.4988 0.4988 0.5351 0.5351 0.3806 0.4013 0.4231 0.4539 0.4423 0.4824 Pij 1 4 51 1 1 d ij d ' ij (3) where, dij is the distance between contour point i and charge point j and d’ij is the distance between the contour point i and image charge point j’ as shown in Figure 11. As in Figure 11, the fictitious charges are taken into account in the simulation as point charges. The position of each point charges and each contour point are determined in X, Y and Z coordinates where the distance between the contour (evaluation) points are calculated as the following ; d ij X j X i Y j Yi Z j Z i 2 2 2 Where, Xj, Yj and Zj are the dimensions of the point charge and Xi, Yi and Zi are the dimensions of the contour point. After solving 2 to determine the magnitude of simulation charges, a number of checked points located on the electrodes where potentials are known, are taken to determine the simulation accuracy. As soon as an adequate charge system has been developed, the potential and field at any points outside the electrodes can be calculated. 3. FIELD COMPUTATION WITH EQUIVALENT CHARGES In the charge simulation method, the actual electric filed is simulated with a field formed by a number of discrete charges which are placed outside the region where the field solution is desired. Values of the discrete charges are determined by satisfying the boundary conditions at a selected number of contour points. Once the values and positions of simulation charges are known, the potential and field distribution anywhere in the region can be computed easily [15]. The basic principle of the charge simulation method is very simple. If several discrete charges of any type (point, line, or ring, for instance) are present in a region, the electrostatic potential at any point C can be found by summation of the potentials resulting from the individual charges as long as the point C does not reside on any one of the charges. Let Qj be a number of n individual charges and Φi be the potential at any point C within the space. According to superposition principle; n i Pij Q j (2) j 1 where Pij are the potential coefficients which can be evaluated analytically for many types of charges by solving Laplace or Poisson’s equations, Φi is the potential at contour (evaluation) points, Qj is the charge at the point charges. Because of the ground surface is flat, the method of images can be used with the charge simulation method and the potential will be characterized for being constant on the grounding grids and its symmetry [16]. The potential coefficients will be as in the following equation; Fig. 11: Illustration of the charge simulation technique The grid is divided into equal segments by the point charges distribution along the axis of grid conductors. Figure 12 shows the distribution of the point charges (dots) for the grounding grid (1 mesh), the number of point charges is distributed on the axis of the grid conductors equally and also the evaluation points distributed on each conductor as shown in Figure 12. The meshes of the grid are always symmetrical. The charge simulation technique is used to get the ground resistance (Rg), ground potential rise (GPR) and then the surface potential on the earth due discharging impulse current into ground grid is known. The touch and step voltages are calculated from surface potential. The duality expression is used to calculate the ground resistance Rg from the next equation; 1310803-7272-IJECS-IJENS © June 2013 IJENS IJENS International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 n C Qj j 1 V Rg C (4) where, V is the GPR that is defined 1 V, Qj is the charge of point charge j that used for the calculation, ρ is the soil resistivity and ε is the soil permittivity. In this section, some figures explain the earth surface potential along diagonal profile for the square grid with different number of meshes. The characteristics of the grid are 50m*50m, the radius of the grid rods (r) is 8 mm, the grid depth (h) is 0.5 m, the resistivity of the soil () is 100 Ω.m, and the total ground potential rise (GPR) is defined as 1 V. Figure 13 shows the Earth surface potential in 3D and the contour map of this case. 52 4. COMPARISON BETWEEN THE BEM AND CSM The following case of study is taken to compare between the results by BEM and CSM, the input data about the grid configuration: Number of meshes (N) = 64, side length of the grid in X direction (X) = 75 m, side length of the grid in Y direction (Y) = 75 m, grid conductor radius = 5 mm, vertical rod length (Z) = 0 (no vertical rod), depth of the grid (h) = 0.5 m, resistivity of the soil (ρ) = 2000 Ω.m and the permittivity of the soil is 9. The following table I explains that the result from the proposed method is close to the other formula in [1] and also the values of resistance that calculated by [11-14]. TABLE IV GROUNDING RESISTANCE BETWEEN THE BEM AND CSM AND THE OTHER FORMULAS THAT USED IN IEEE STANDARDS [1] R (Ω) 11.8 13.29 13.23 12.67 11.11 11.87 Formula Dwight [1] Laurent [1] Sverak [1] BEM [11-14] Schwarz [1] CSM 4320 Points Figure 14 explains that the proposed method satisfies an agreement with the other method that used to calculate surface potential for example Boundary Element Method [11-14]. 14 12 Fig. 12: Distribution of point charges on the grid (1 mesh) 10 ESP(kV) 8 6 4 BEM 2 CSM 0 -150 -100 -50 0 50 100 150 Distance from grid center (m) Fig. 14: Comparison between proposed method and Boundary Element Method for 64 meshes grid Fig. 13a: ESP/GPR for 64 meshes Fig. 13b: Contour map for 64 mesh grid 5. CONCLUSIONS The vertical rods play an important role for reducing the grid resistance, the step and touch voltages. The number of meshes is an effective parameter for reducing the pervious values but it needs more copper then increases the cost. The study explains a small effect in the earth surface potential when changing the vertical rods locations at the same number of meshes hence the economical cost plays a great part for choosing the suitable design for the square grids, then the optimal case of the grids is that consists of 16 meshes with the vertical rods in the perimeter (case R04P16), it gives suitable results for the grid resistance, step and touch voltages and in the same time it presents an economical cost. Additional vertical ground rods gives nearest results to that results when add the horizontal rods to the same grid, 1310803-7272-IJECS-IJENS © June 2013 IJENS IJENS International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 then the addition of vertical rods to the grounding grid gives a good performance and decrease the cost of design. Both vertical rods and number of meshes are considered effective parameters for reducing the grid resistance, the step and touch voltages. The paper demonstrates that another important parameter that effects in the pervious values is the profile location. The man location in case of fault determines the value of step and touch voltage that he will experiences. The figures illustrate that the dangerous point is at the side of the grid and comes in the corner mesh in the grid. The proposed methods (BEM and CSM) that used to calculate the earth surface potential and grounding resistance due to discharging current into grounding grid are efficient. The validation of these methods is satisfying by a comparison between the results from it and the results from the formula in IEEE standard. The proposed methods give a good agreement with the IEEE standard. The two methods give the closest results to each other althought the different techniques that applied in each method. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] IEEE Guide for safety in AC substation grounding, IEEE Std.802000. J. G. Sverak, “Progress in step and touch voltage equations of ANSI/IEEE Std. 80,” IEEE Trans. Power Delivery, vol. 13, no. 3, pp. 762-767, Jul. 1999. Sherif Salama, Salah AbdelSattar and Kamel O. Shoush, "Comparing Charge and Current Simulation Method with Boundary Element Method for Grounding System Calculations in Case of Multi-Layer Soil, International Journal of Electrical & Computer Sciences IJECSIJENS Vol:12 No:04, pp.17-24, August 2012. Thinh Pham Hong ; Quan Do Van ; Thang Vo Viet “Grounding resistance calculation using FEM and reduced scale model” Electrical IEEE Conference on Insulation and Dielectric Phenomena, 2009. CEIDP '09, pp. 278-281, 2009. Salam, M.A. ; Ja'afar, S. ; Ariffin, M. “Measurement of grounding resistance by U-Shape and square grids”, TENCON 2010 - 2010 IEEE Region 10 Conference, pp.102-105, 2010. E. D. Sunde, Earth Conduction Effects in Transmission Systems, D. Van Nostrand Company Inc., New York 194, 1968. G. F. Tagg, Earth Resistances, Pittman Publishing Corporation, London, 1964. J. M. Nahman, V. B. Djordjevic, “Nonuniformity correction factors for maximum mesh and step voltages of ground grids and combined ground electrodes,” IEEE Trans. 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Casteleiro, "A numerical formulation for grounding analysis in stratified soils", IEEE Trans. on Power Delivery, vol. 17, pp 587-595, April 2002. Colominas, I. ; Navarrina, F. ; Casteleiro, M. “Numerical Simulation of Transferred Potentials in Earthing Grids Considering Layered Soil Models”, IEEE Transactions on Power Delivery, Vol. 22, No. 3, pp.1514-1522, 2007. 53 [15] N. H. Malik, “A review of charge simulation method and its application,” IEEE Transaction on Electrical Insulation, vol. 24, No. 1, pp 3-20, February 1989. [16] E. Bendito, A. Carmona, A. M. Encinas and M. J. Jimenez “The extremal charges method in grounding grid design,” IEEE Transaction on power delivery, vol. 19, No. 1, January 2004, pp 118123. BIOGRAPHIES Sherif S. M. Ghoneim Received B.Sc. and M.Sc. degrees from the Faculty of Engineering at Shoubra, Zagazig University, Egypt, in 1994 and 2000, respectively. Starting from 1996 he was a teaching staff at the Faculty of Industrial Education, Suez Canal University, Egypt. Since end of 2005 to end of 2007, he is a guest researcher at the Institute of Energy Transport and Storage (ETS) of the University of Duisburg-EssenGermany. In 2008, he got Ph.D Degree in Electrical power and machines, Faculty of Engineering-Cairo University (2008). He joins now the Taif University as an assistant professor in the Electrical Engineering Department, Faculty of Engineering. His research focuses in the area of high voltage engineering. Dr. Kamel A. Shoush (1961) is associate professor in the Electrical Engineering Department- Faculty Of EngineeringAL-Azhar University- Cairo – Egypt. He received his B. Sc. And M. Sc. Degrees from AL-Azhar University- Cairo – Egypt in 1986 and 1993 respectively. And his Ph. D. degree from AL-Azhar University- Cairo – Egypt in 1998 after having worked for two years in “GerhardMercator-Universität–esamthochschula Duisburg” - Duisburg/Germany. Now, he is an Associate Prof in the Taif University, College of Engineering, Electrical Engineering Department, Saudi Arabia. His Areas of interest Intelligent Systems Applications For Power Systems Optimization And Control. 1310803-7272-IJECS-IJENS © June 2013 IJENS IJENS