THREE-DIMENSIONAL DESIGN OF GROUNDING NETWORKS Fikri Barış UZUNLAR baris.uzunlar@tr.schneider-electric.com Özcan KALENDERLİ ozcan@elk.itu.edu.tr Istanbul Technical University, Faculty of Electrical and Electronics Engineering, Department of Electrical Engineering, Istanbul ABSTRACT This study presents a computer model for the design and analysis of grounding systems compliant with IEEE Std 80-2000, IEEE Std 81-1983, and IEEE standards, using an advanced methodology and computer program. The methodology and computer program have been validated with real system measurements. The accuracy of the computer algorithm depends on how well the grounding model and physical arrangement are reflected in real field conditions. Allowable voltage limits and maximum estimated voltage values were calculated using the experimental formula given in the standard. Step, touch, network voltages, and high-voltage current-carrying regions were calculated according to the recommendations given in the standard, and the differences between them were examined and explained. Simulations were performed to realize the possibility of introducing design criteria for engineering applications. The facility grounding network design and analysis module is specifically designed to assist engineers in strengthening existing networks and optimizing the design of new networks of all kinds, with its ease of use and built-in hazard point assessment capability. It also offers some useful practical keywords: Grounding network, Touch Voltage, Step Voltage, Network Voltage. 1. INTRODUCTION In transformer substations, grounding networks are a type of ground electrode consisting of conductors embedded in the ground, placed parallel to the ground surface, and connected to each other to form a network, covering a large area. In energy and systems, they are used in large structures and locations where small potential differences are desired, to keep grounding resistance, step and contact voltages small and to ensure uniform potential distribution. The economical design and analysis of grounding networks suitable for these purposes constitutes an engineering problem. Design calculations are generally based on experience and empirical formulas. These calculations are insufficient in examining three-dimensional ground electrode conditions, in many definitions of the ground, and in obtaining realistic solutions, making them difficult and time-consuming. The design of a grounding network requires the evaluation of parameters such as network dimensions and shape, number of meshes, burial depth, and soil properties. Performing this through experiments on a real grounding network is both difficult, time-consuming, and expensive. A simpler, shorter, and more economical method is to experiment on scaled-down models or to perform calculations on realistic models using numerical methods on a computer. Therefore, computer-aided design and analysis of grounding networks has become popular in recent years. This requires computer algorithms that provide more accurate and realistic results in the design of grounding systems. For this purpose, method and software development studies are being conducted, and software packages are being produced. In this study, a three-dimensional network design, based on the finite element method, is presented with explanations and applications using a grounding network design program. 2. OBJECTIVES AND GOALS The main objectives of this study are: - To provide a suitable reference containing the necessary principles for grounding system designers to focus on the most efficient design, - To keep touch and step voltages within the allowed safety limits and reduce grounding resistance, - To design a grounding installation in accordance with IEEE Std 80-2000 [1], IEEE Std 81-1983 [2] and IEEE Std 837-2002 [3] standards, - To ensure the safety of personnel who may be exposed to electric shock in case of faults, - To determine the characteristics of the network conductors that should be used in the grounding system, - To observe the effect of factors such as the installation area, the number of grounding stakes, the cross-section of the network conductor and the burial depth of the conductor on the calculation of resistance, - To calculate the ground voltage rise (GPR), - To calculate the maximum allowable touch and step voltages, - To visualize the surface voltage distribution along any direction as a ratio of the ground voltage rise, - To present a report containing all design details. 3. GROUNDING NETWORK DESIGN SOFTWARE The following characteristics are sought in computer algorithms used in the modeling of grounding systems: - Ability to model grounding conductors and grounding stakes as separate elements, - Ability to define each separate element as a set of equations, - Calculation of the ground fault current flowing to the ground, - Calculation of the surface potential at any desired point. The first step in implementing grounding design is to determine the ground model to be used. Secondly, the ground model to be used must be decided upon. At this stage, an electrical calculation is performed, including the maximum allowable step and touch voltages for a specific surface and condition, as defined in IEEE Std 80-2000. In the third stage, the dimensions of conductors and electrodes such as stakes are determined, taking into account the parameters that may occur in the worst-case scenario. In the next stage, the geometric dimensions and shape information of the site, such as the burial depth and physical dimensions of the electrical conductor, are entered. Finally, it is verified whether the plant design meets the desired safety criteria. A potential distribution plot, touch and step voltages must be generated. If the safety criteria are not met, the network design needs to be reviewed. The method is repeated from the third stage until acceptable results are obtained. The software used in this study consists of the following three main modules: A. Soil Analysis Module: This module contains the electrical characteristics of the site and the properties of the network conductors to be used (Figure 2). The following information is entered into this module: 1) Operating voltage of the facility (V), 2) Ground fault current (A), 3) Number and length of grounding stakes and conductors, 4) Ground voltage rise (GPR) (V), 5) Calculated ground resistance (ohm), 6) Equivalent impedance (ohm). A. Ground Analysis Module This module contains the following parameters necessary for the grounding design of the area to be simulated (Figure 1): Figure 2. Grounding network analysis module C. Three-Dimensional Potential Distribution Module In this module, the potential distribution found as a result of the simulation using the finite element method can be observed in three dimensions (Figure 3). 308 If a >> b, then (2) is written from equation (1). The maximum allowable touch and step voltages are calculated in accordance with the IEEE Std 80-2000 standard. The purpose of the calculation is to use a high-resistance surface layer and thus keep the allowable touch voltage high. The surface layer reduction coefficient (Cs) used in the calculations is calculated with the following formula: (3) Touch and step stresses, respectively, for a body weight of 50 kg: (4) (5) Touch and step stresses for a body weight of 70 kg: (6) (7) The parameters used in the above equations are: • ts represents the shock duration in seconds, • s represents the resistivity of the material on the surface of the soil in ohm-meters, • Cs represents the reduction coefficient depending on the use of high-resistance surface material and the reflection factor (K) and the thickness of the top layer (h), • hs represents the thickness of the high-resistance surface material, • represents the resistivity of the soil under the high-resistance surface material. 5. SIMULATIONS To verify the results obtained in this study, the examples given in Appendix B of the IEEE Std 80-2000 standard were referenced and comparative tables and related graphs were used. Design information is given in Table 1. Table 1: Design information used in network design Feature Value Figure 3. Three-dimensional potential distribution modulus 4. GROUNDING NETWORK DESIGN METHOD Many graphical and analytical approaches have been proposed for years to obtain practical soil models [4-8, 10]. Due to the prevalence of multi-layered soils consisting of layers with different soil resistivities, grounding resistance measurement techniques are used according to the multi-layered soil model. For years, a two-layered model consisting of a lower layer of infinite depth and different resistance and an upper layer of finite depth has been used in the practical approach of plant grounding. The software [9] supports the Wenner four-stake soil resistivity measurement technique where the distance (a) between each pair of stakes is equal (Figure 4). According to this method, while a current I, whose value is measured by an ammeter, is applied from the outer electrodes, the voltage V created by the applied current in the soil resistance between the inner electrodes is measured by a voltmeter, and Ohm's law is applied to find the resistance (R = V / I). Figure 4. Wenner four-stake method. The soil resistivity ( ) is found according to the measured V and I values, the length of the grounding stake (b) and the distance between stakes (a) using the following relationship: (1) 309 2222421)/(4baabaaIVa)/(2IVa0,09(1/)120,09sssCh50(10001,5)0,116/dokunmasssECt50(10006,0)0,116/sadımssECt70(10001,5)0,157/dokunmasssECt70(10006,0)0,157/sadımssECt Body weight Resistivity of gravel Thickness of gravel layer Fault opening time Soil resistivity Highest fault current IG, X/R ratio Fault current IG Separation Coefficient Sf Conductor material Ambient temperature Network conductor diameter Network burial depth 70 kg 2500 Ω.m 0.102 m 0.50 s 400 Ω.m 6814 A, 16.2 3180 A 0.6 Hard-drawn copper 40 C 0.01 m 0.5 m D. Square network without grounding stakes Figure 5 shows a three-dimensional drawing of a square network with dimensions of 70 m x 70 m and 9 x 9 meshes without grounding stakes. Table 2 presents the results obtained with both IEEE Std 80-2000 and the software used in this study for comparison purposes. Figure 6. Highest and actual touch voltages for a square network without grounding stakes E. Rectangular network with grounding stakes Figure 7 shows a three-dimensional drawing of a rectangular network with dimensions of 63 m x 84 m and 9 x 12 meshes with grounding stakes. The three-dimensional drawing is shown. The results obtained with both methods for the network in Figure 7 are given in Table 3. Figure 5. Square network without grounding stakes Table 2: Comparative results table for square network without grounding stakes Feature Maximum allowable touch voltage (V) Maximum allowable step voltage (V) Reduction coefficient CS RG ( ) GPR (V) IEEE Std 80-2000 838.20 Software 840.55 2686 2696.10 0.740 2.780 5304.00 0.740 2.675 5105.61 From the potential distribution in Figure 6 obtained for the square network without stakes, it is possible to observe that the maximum value of the touch voltage is exceeded at the corners of the system. Figure 7. Rectangular network with grounding stakes Table 3: Comparative results table for rectangular network with grounding stakes Feature Maximum allowable touch voltage (V) Maximum allowable step voltage (V) Reduction factor CS ) RG ( GPR (V) IEEE Std 80-2000 838.20 Software 840.55 2686 2696.10 0.740 2.620 4998.96 0.740 2.278 4348.00 For the network in Figure 7, the potential distribution obtained with the software used is shown in Figure 8. Again, a potential increase is observed at the corners. 310 Table 5: Comparison table for a two-layer network with equally spaced grounding stakes Feature Maximum allowable touch voltage (V) Maximum allowable step voltage (V) Reduction factor CS RG ( ) GPR (V) IEEE Std 80-2000 838.20 Software 840.55 2686 2696.10 0.740 2.740 4562.49 0.740 2.330 5227.92 As can be observed from this situation, the difference between the results is almost insignificant as the grounding resistance is obtained as low. For the network in Figure 9, the potential distribution obtained with the software used is observed as in Figure 10. 6. CONCLUSION Although computer simulation-based analysis techniques are still expensive, it is clear that their use will lead to inexpensive and safe grounding network arrangements. The parametric analyses performed need to be updated to include multi-layered grounding. Harmonization is needed among the standards regulating the grounding network. Figure 8. Highest and actual touch voltages for a rectangular network with grounding stakes. F. Two-layer network with equally spaced grounding stakes. To exemplify the simulations of two-layer grounding systems used in many applications in practice, in the B.5 example of the IEEE Std 80 standard, with the information in Table 4, the 60.96 m x 60.96 m dimensions shown in Figure 9, A 4 x 4 mesh simulation is performed. The results obtained with both methods for the mesh in Figure 9 are given in Table 5. Table 4: Information used in two-layer mesh design Property Body weight Resistivity of gravel Thickness of gravel layer Resistivity of top layer Thickness of top layer Resistivity of bottom layer Diameter of mesh conductor Network burial depth Length of stakes Stake diameter Fault opening time Soil resistivity Fault current IG Value 70 kg 2500 Ω.m 0.1 m 300 Ω.m 4.572 m 100 Ω.m 0.01 m 0.5 m 9.144 m 0.0127 m 0.50 s 400 Ω.m 1908 A Figure 10. Maximum and actual touch voltages for rectangular mesh with grounding stakes REFERENCES [1] IEEE Std. 80-2000, IEEE Guide for Safety in AC Substation Grounding, IEEE Standard Board, New York, USA, 2000. [2] IEEE Std. 81-1983, IEEE Guide for Measuring Earth Resistivity, Ground Impedance and Earth Surface Potentials of a Ground System, IEEE Standard Board, New York, USA, 1983. [3] IEEE Std. 837, IEEE Standard for Qualifying in Substation Permanent Connections Used 311 Figure 9. Evenly spaced ground pile and two-layer network Grounding, IEEE Standard Board, New York, USA, 2002. [4] CH Lee, APS Meliopoulos, "Comparison of touch and step voltages between IEEE Std. 80 and IEC 479-1", IEEE Proceedings on Generation, Transmission and Distribution, Vol. 146, No. 6, pp. 593–601, 1999. [5] H. Zhao, H. Griffiths, A. Haddad, A. Ainsley, "Safety-limit curves for earthing system designs: appraisal of standard recommendations", IEE Proceedings on Generation, Transmission and Distribution, Vol. 152, No. 6, pp. 871–879, 2005. [6] MH Hocaoğlu, AT Hocaoğlu, "Comparison of high voltage standards for installations", 8th National Congress of Electrical, Electronics, Computer Engineering, Gaziantep, pp. 395-398, 1999. grounding [7] LM Coa, "Comparative study between IEEE Std. 80-2000 and finite elements method application for grounding systems analysis", Transmission & Distribution Conference and Exposition, Latin America, pp. 1-5, 2006. [8] J. Ma, FP Dawalibi, RD Southey, "Effects of the changes in IEEE Std. 80 on the design and analysis of power system grounding", PowerCon International Conference, Vol. 2, 974-979, 2002. [9] CYMGRD, User Guide and Reference Manual, Canada, 2006. [10] S. Meliopoulos, Power System Grounding and Transients, Marcel Dekker, New York, 1998. 312
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