INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET)

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1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME ISSN (Print) ISSN (Online) Volume 3, Issue 2, July September (212), pp IAEME: Journal Impact Factor (212): (Calculated by GISI) IJEET I A E M E MEASUREMENTS THE EARTH SURFACE POTENTIAL FOR DIFFERENT GROUNDING SYSTEM CONFIGURATIONS USING SCALE MODEL Mosleh Maiet Al-Harthi Electrical department-faculty of Engineering Taif University Taif KSA (Taif-AlHaweih P.O.Box 888-Zipcode 21974) E. Mail: mosleh2k@hotmail.com Sherif Salama Mohamed Ghoneim 1,2 1 Electrical Department- Faculty of Engineering- Taif University, 2 Electrical Department-Faculty of Industrial Education- Suez Canal University 1 Taif-KSA, 2 Suez- Egypt s.ghoneim@tu.edu.sa ABSTRACT Based on the similarity theory one can replace the investigations of a phenomena in nature by the investigation of an analogous phenomena in a model of smaller or larger scale under specified Laboratory conditions. This paper presents an experimental work using scale model for different grounding system configuration such as vertical rods with different shapes and different grounding grids to measure the Earth Surface Potential (ESP) in one and two layer model soil. The scale factor of the system is taken as 3 for grounding vertical rod and 1 for grounding grids. The AC voltage is applied on the scaled models with different configurations as well as the DC voltage. The two layer model soil is simulated using two glass tanks that have a conducting connection between different water resistivities via copper rods. The effect of the depth, length and radius of the model as well as the water resistivities on the ESP is studied. The effect of the temperature rise on ESP can be investigated. The results obtained from the scale model are compared with those given by computer simulation, which is based on Current and charge Simulation Method. Keywords--Grounding system, two-layer model soil, current simulation method I. INTRODUCTION Great Damages may occur to the electrical equipments and individuals during lightning strike if the grounding system is not properly designed. A safe grounding design has two main objectives, one of them is carrying the electric currents into earth under normal and fault conditions without exceeding operating and equipment limits or adversely affecting continuity of service. The second is ensuring that the person in the vicinity of grounded facilities is not exposed to the danger of electric shock. The need of accurate design procedures for grounding systems becomes highly important and should consider safety and cost issues, since the number and complexity of AC substations 45

2 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME increases. Analytical techniques in some cases are so complex and therefore, studying the phenomenon using scale model is greatly convenient. The goal of the experimental work of the grounding system design is based on scale model to analyze the grounding system behavior during faults with different grounding system structures. Grounding grids are considered an effective solution for grounding systems for all sites which must be protected from lightning strokes such as telecommunication towers, petroleum fields, substations and power plants. Grounding grids produce an equi-potential surface and should provide very small impedance. However, the grounding grids have complex arrangements and many research efforts have been made to explain the performance of the grounding impedance under lightning and fault conditions. Tracking the potential distribution of complicated ground electrode arrangements cannot be easily performed by analytical based methods, since the difficulty degree increases with the increase of grounding system combinations and configurations. For mesh type electrode arrangements with irregular depth of burial, which is the way used with potential control purposes for other complicated structures, it is common to use models [1, 2]. For this purpose such model measurements using an electrolytic tank were undertaken [3-5]. Some attempts to measure the Earth Surface Potential (ESP) with the scale model for some different grounding system configurations such as grounding grids were implemented [3-11]. Measurement of Earth Surface Potential is performed by injecting DC and AC current into a scale model of different grounding system configurations for one and two layer model soil. Some parameters affect the surface potential on the water surface, such as burial depth, length and radius of the model electrodes as well as the resistivity of the water in the tank. The effect of temperature in top and bottom layers on the surface potential is studied. The ESP for real case is calculated using Current and Charge Simulation Method. The results from simulation are compared with the scale model results. II. EXPERIMENTAL SETUP FOR ONE LAYER MODEL SOIL The purpose of the scale model experiments is to investigate the effect of the grounding configurations on grounding resistance of the water as well as the Earth Surface Potential (ESP), when the discharging current is injected into it. Figure 1 depicts the elements of the experimental setup which are, the electrolytic tank that simulates the homogenous earth with dimensions 75cm long, 75cm wide, and 5cm height, Powers supply (AC or DC), some voltmeters and ammeter devices. Different grounding system configurations are presented in Figure 2. The scale factor between the reality and scale model is taken as 3:1 in case of rods and 1:1 in case of grounding grids. Distilled water with salted tap water is used as an electrolyte, which serves as an adequately conducting medium, representing the homogeneous earth. Change in the salinity causes a change in the liquid resistivity. Under the basis of ohm's law, the value of the water resistance (R) can be got. On the other hand the resistivity of the water, as the soil resistivity ρ can be calculated by using the test tube as in Figure 3 from the following equation 1, RA ρ = (1) where, ρ = Water resistivity (Ω.m) L=Length of the tube (m) A= Cross-section of the tube in (m 2 ) L 46

3 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME Figure 1. Experimental Setup for homogenous soil Figure 2. Scale model of grounding system configurations Figure 3. Measurement of the water Resistivity 47

4 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME III. EXPERIMENTAL RESULTS FOR ONE LAYER MODEL SOIL The effect of the depth, length and radius of the vertical rods and other rod configurations, burial depth of the rods as well as the water resistivities on the ESP are investigated. A. Effect of rod length Based on experimental measurements conducted with vertical rods shown in Figure 4, the variation of the ESP with the distance from the vertical rods is presented in Figures 5, and Table I. The ESP and the Ground Potential Rise (GPR) decrease with an increase of the rod length. The touch voltage (V t ) from Eq 2 and step voltage (V S ) from Eq 3 decreases since the reduction rate of the GPR is greater than the reduction rate of the ESP. GPR V 1 V touch % = 1 (2) V1 wher,v 1 = Surface potential at distance 1 cm from the rod. V1 V 2 V step % = 1 (3) V1 where V 1,V 2 = the surface potential at points 1,2 on the surface potential profile with 1 cm apart. Figure 4. Configuration of vertical rods the used in the scale model (different length and diameter) 2 L=3 cm L=8 cm Figure 5. Effect of rod length (L) on the ESP (Diameter of Rod(D)= 3mm, Depth of Rod(H) =, Resistivity of water (ρ) =7.63Ω.m and DC Voltage) TABLE I. R G, V T AND V S FOR DIFFERENT VERTICAL ROD LENGTH Test L D H ρ RL V N t V s cm mm cm Ω.m cm Ω % % Another configuration to study the effect of vertical rod length is presented as in Figure 6. The radial rod length is constant with the variation of vertical rod length. As shown in Figure 7 and Table II, an increase in vertical rod length is accompanied with a reduction in ESP and R g. 48 R g

5 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME Figure 6. Configuration of rods used as scale model for the effect of rod length with radial length constant (RL)=5cm L=1 cm L=3 cm L=8 cm Figure 7. Effect of rod length on the ESP(D= 5mm,H =, ρ =7.63Ω.m, Radial Length (RL)=5cm, Number of Radial(N)=1 and DC Voltage) TABLE II. RG, VT AND VS FOR DIFFERENT VERTICAL ROD LENGTH Test L D H ρ RL V N t V s cm mm cm Ω.m cm Ω % % B. Effect of rod diameter As shown in Figures 8 and Table III, the effect of rod diameter on the ESP is not significant, since the variation of the vertical rods diameter causes a small change on ESP as well as GPR, therefore, the effect of vertical rod diameter can be neglected. 2 R g D=3 mm D=5 mm Figure 8. Effect of rod diameter (D) on the ESP (L= 3cm,H =,ρ =7.63Ω.m and DC Voltage) 49

6 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME TABLE III. RG, VT AND VS FOR DIFFERENT VERTICAL ROD DIAMETER L D H ρ RL V Test N t V s cm mm cm Ω.m cm Ω % % C. Effect of water resistivity Figure 9 illustrates the variation of water resistivity with ESP. A significant change on ESP is noticed with the variation of water resistivity. The ESP and GPR decrease with a decrease in water resistivity (ρ) as a reduction of the grounding resistance (R g ). A decrease in water resistivity will cause a decrease in step voltage (V S ), as well as touch voltage (V t ), this fact is presented in Table IV. D. Effect of number of radial of rod Figure 1 shows some of rod configurations with different number of radials. As seen in Figures 11, An increase of the redial number that connected to the vertical rod plays an important part in a reduction of ESP and R g. Further, when the radial number is increased, the Step and touch voltages will be decrease, as seen in Table V. E. Effect of applied voltage type As seen in Figure 12, the variation of power supply from DC supply into AC supply causes a reduction of ESP and R g. Further, the Step and touch voltages will be decrease, as seen in Table VI ρ =7.63 Ω.m ρ =1 Ω.m Figure 9. Effect of water resistivity (ρ) on the ESP ( L= 3cm, D = 3mm, H =3 and DC Voltage) TABLE IV: RG, VT AND VS FOR DIFFERENT WATER RESISTIVITY Test L D H ρ RL V N t V s cm mm cm Ω.m Cm Ω % % R g R g Figure 1. Configuration of rods used as scale model for the effect of Number of Rod Radial (N) and Radial Length (RL) 41

7 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME Figure 11. Effect of Number of Rod Radial (N) on the ESP ( L= 5cm, D = 5mm, ρ=7.63ω.m, H =, Radial Lenght(RL)= 2cm and DC Voltage) TABLE V: RG, VT AND VS FOR DIFFERENT NUMBER OF RADIALS Test L D H ρ RL V N t V s cm mm cm Ω.m Cm Ω % % N=4 N= R g DC AC Figure 12. Effect of Supply Voltage on the ESP (L= 3cm,D = 3mm,ρ =1Ω.m and H =3) TABLE VI: RG, VT AND VS FOR DIFFERENT SUPPLY TYPE Test L D H ρ RL V N t V s cm mm cm Ω.m Cm Ω % % F. ESP for grounding grid Figure 13 shows the earth surface potential (ESP) for scaled grid with scale factor 1:1. The distribution of ESP versus the distance from the center of grid is presented. Figure 14 explains the effect of grid depth on the value of ESP. an increase of the grid depth will lead to a reduction of the ESP due to the grounding resistance decreases. Figure 15 shows the effect of number of meshes on ESP. when the number of meshes increases the ESP will decrease because of the reduction of Grounding resistance. R g 411

8 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME Grid 3cm*3cm No of meshes=16 Resistivity=7.63 W.m H=.5m ESP(V) Distance from cenetr of grid (cm) Figure 13. The ESP versus the distance from the grid center ESP(V) Grid 3cm*3cm No of meshes=16 Resistivity=7.63 Ω.m D=.5 cm D=1 cm Distance from the center of grid (cm) Figure 14. Effect of depth on the ESP Figure 15. Effect of number of meshes on the ESP IV. EXPERIMENTAL SETUP FOR TWO LAYER MODEL SOIL The experimental setup for two layer model soil is presented in Figure 16. The current is injected to the scaled electrode and digital voltmeter measures the voltage on the water surface. Electrolytic tanks that simulates the bottom layer has dimensions 75cm long, 75cm wide, 5cm height and top layer tank has dimensions 65cm long, 65cm wide, 25cm height. The scale factor between the reality and scale model is taken as 3:1. In this section, the effect of some parameters on ESP, V t, V s and R g will be studied. 412

9 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME Figure 16. Experimental Setup for two layer model soil V. EXPERIMENTAL RESULTS FOR TWO LAYER MODEL SOIL A. Effect of rod length The relationship between the distance from the rod in cm and the ESP in V at different rod length is presented in Figure 17. The ESP and GPR decrease with the increase of the rod length. B. Effect of Bottom Layer Resistivity The effect of the bottom layer resistivity is not significant since the rod is located in the upper layer. The effect is appeared when the rod penetrate the bottom layer or change the resistivity of upper layer. The effect is shown in figure 18. C. Effect of Top Layer Thickness As seen in Figure 19, An increase of the TLT causes a higher of ESP and R g. Further, when the TLT is increased, the Step and touch voltages will be increase. D. Effect of applied voltage type As seen in Figure 2, the variation of power supply from DC supply into AC supply causes a small reduction of ESP and R g. Further, the Step and touch voltages will be decrease. E. Effect of temperature Figure 21 illustrates that the temperature plays a great role in decreasing the resistivity and hence the resistance of the water. It is shown in the Figure 21 that when the top layer temperature is greater than that of bottom layer, the ESP and GPR is decreased since the rod located in the upper layer only and hence the resistivity of the top layer has a great effect on the ESP and GPR L=3cm L=8cm Distance from the electrode (cm) Figure 17. Effect of rod length on the ESP (D= 3mm, H = cm,ρ1=7.63ω.m, ρ2 =3Ω.m, TLT =15cm and DC Voltage) 413

10 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME ρ2 =3 Ω.m ρ2 =1 Ω.m Distance from the electrode (cm) Figure 18. Effect of Bottom Layer Resistivity (ρ 2 ) on the ESP (L=3cm, D=3mm, H=, ρ 1 =7.63Ω.m,TLT=25cm and DC Voltage) TLD=15 cm TLD=2 cm Distance from the electrode (cm) Figure 19. Effect of Top Layer Thickness (TLT) on the ESP (L=3cm,D=3mm,H=cm,ρ 1 =7.63Ω.m, ρ 2 =3Ω.m and DC Voltage) dc voltage AC voltage Distance from the electrode (cm) Figure 2. Effect of power supply on the ESP (L=3, D=5mm,H=3cm, ρ 1 =7.63Ω.m, ρ 2 =3Ω.m,N=6, RL=4cm and TLT =15cm) T1=T2 T1<T2 T1>T Figure 21. Effect of soil temperature on the ESP (L=5, D=5mm, H=, ρ 1 =7.63Ω.m, ρ 2 =3Ω.m, N=6, RL=4cm, TLT =15cm and DC voltage) 414

11 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME VI. VALIDATION OF THE SIMULATION A simulation program based on the Current Simulation Method [1] is used to calculate the Earth Surface Potential (ESP) on the earth for real case. To compare between the simulation and the experimental work, the scale factor is taken as 3:1, i.e the 3 cm in experimental is equivalent to 9 m in simulation. Fig. 22 shows the comparison between the results from simulation and that from experimental. The performance of the two curves are the same, therefore, the simulation is valid for using in ESP calculation. Figure 22. ESP/GPR at scaled distance from the rod for one layer soil. (L= 3cm, H =,ρ =7.63Ω.m and DC Voltage) VII. CONCLUSIONS The objective of this paper is to measure the Earth Surface Potential (ESP) resulted from discharging current flow into different grounding system configurations using a scale model immersed in an electrolytic tank. If all dimensions of the scaled models and other parameters such as water resistivity are reduced by the same factor, the results given with such model can be used as guidelines for safe design of grounding systems. The water resistivity and the ESP are decreased with the increase of water temperature. The experimental work results agree with the simulation results given by the CSM. This is a prove for a validation of the simulation. REFERENCES ESP/GPR Simulation Experimental Scaled distance from the vertical rod [1] IEEE Guide for safety in AC substation grounding, IEEE Std.8-2. [2] W. Koch, Erdungsmassnahmen für höchspannungsanlagen mit geerdetem sternpunkt, Elektrotechnische Zeitschrift, Vol. 71, February 195, pp [3] R. Caldecott, D. G. Kasten, Scale model studies of station grounding grids, IEEE Trans. Power Apparatus and Systems, Vol. PAS-12, no. 3, 1983, pp [4] B. Thapar, S. L. Goyal, Scale model studies of grounding grids in non-uniform soils, IEEE Transactions on Power Delivery, Vol. PWRD-2, no. 4, 1987, pp [5] C. S. Choi, H. K. Kim, H. J. Gil, W. K. Han, and K. Y. Lee, The potential gradient of ground surface according to shapes of mesh grid grounding electrode using reduced scale model, IEEJ Trans. On Power and Energy, Vol. 125, no. 12,,25, pp [6] A. Elmorshedy, A. G. Zeitoun, and M. M. Ghourab, Modelling of substation grounding grids, IEE Proceedings, Vol. 133, Pr. C, No. 5, July [7] I. F. Gonos, F. V. Topalis, I. A. Atathopulos Modeling of a Grounding Grid Using an Electrolytic Tank, 12th ISH Symposium, India, 2-24 August, 21. [8] B. Thapar, S. L. Goyal, Scale Model Studies of Grounding Grids in Non-Uniform Soils, IEEE Transactions on Power Delivery, Vol. PWRD-2, No. 4, 1987, pp

12 International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Online) Volume 3, Issue 2, July- September (212), IAEME [9] S. Ghoneim, H. Hirsch, A. Elmorshedy, R. Amer, Measurement of Earth Surface Potential Using Scale Model, UPEC27, Brighton University, England, September 27. [1] S. S. Dessouky, S. Ghoneim, S. Awad," Ground Resistance, Step and Touch Voltages For A Driven Vertical Rod Into Two Layer Model Soil", International Conference Power System Technology, POWERCON21, Hangzhou, China, October 21. [11] Elsayed M. Elrefaie, Sherif Ghoneim, Mohamed Kamal and Ramy Ghaly, " Evolutionary Strategy Technique to Optimize the Grounding Grids Design", 212 IEEE Power & Energy Society General Meeting, San Diego, California, USA, July 22-26,