An Effective Design of Grounding Rods in a Complicated Soil Layer

Transcription

1 An Effective Design of Grounding Rods in a Complicated Soil Layer Ju-Hong Eom, Sung-Chul Cho, Jae-Jun Kim * 1 and Bok-Hee Lee * 2 KESRI, 130 Dong, Seoul National University, 56-1, Shilim-dong, Kwanak-gu, Seoul, Korea * 1 KEPCO, 167, Samseong-dong, Gangnam-gu, Seoul, Korea * 2 HIERC, 253, Yonghyun-dong, Nam-gu, Inchon, Korea Abstract Soil resistivity measurement must be made at the construction site of a grounding system in order to analyze the soil structure because the performance of grounding system is influenced mainly by the resistivity of the soil surrounding the grounding electrode. Wenner 4-probe arrangement is used most widely by the method to measure soil resistivity and the measured data with the Wenner method are apparent resistivities of the soil. Therefore, the soil structure can be analyzed easily from the measured apparent resistivity, but the real soil resistivity is difficult to know correctly at a particular depth or at a specific location on earth surface, and it is very puzzling problem to decide the scale or the location of an electrode being provided vertically in the subsurface layer. This paper introduces a method that can be used to decide the suitable burial depth and the electrode scale of a grounding rod effectively using the soil structure analysis equipment based on the new dipole-dipole method. Keywords: Soil resistivity, Grounding system, Grounding system impedance, dipole-dipole method, Apparent resistivity 1 INTRODUCTION It is necessary to have knowledge of how various parameters affect the performance of the grounding system in order to efficiently design a safe grounding system. Some of these parameters include conductor arrangement, number of grounding rods, location and length, and soil resistivity. Among the parameters of grounding system, soil resistivity is very important for the estimation of grounding system performance in multi-layered soil structure.[1] Because of the wide variations in the structure and properties of soil materials, there are numerous methods and technicques for determining true resistivity of soil. Most resitivity measurement techniques are variations of the four equally-spaced probe arrangement originally described by Frank Wenner. The Wenner arrangement is shown in Fig. 1. Four probes are driven in the earth along a straight line. The spaces between probes are uniform and the burial depth of the probe is usually less than 10 percent of the space between probes. The ratio of V/I with dimension of resistance is propotional to the apparent resistivity at spacing a. However, the measured resitivity value is not the true resistivity of the subsurface, and the relationship between the apparent resistivity and the true resistivity is very complicated except homogeneous soil structure. This complexity interacts by errors to determine the ground impedance in design of grounding system. I V C1 P1 P2 C2 a a a Figure 1. Wenner 4-probe method Therefore, real resistivity was measured using dipole-dipole method widely used in geology field to analyze the soil structure, and a new method to decide

2 the installation location of grounding rod was proposed efficiently. 2 MEASUREMENT METHOD 2.1 Impedance measurement method The fall-of-potential method is one of the most used method for measuring the grounding system impedance. In classical fall-of-potential method, the direction of the potential probe is the same with the direction of the grounding electrodes under test-the return current electrode, the potential probe is located between the grounding electrode under test and the return current electrode. Therefore, significant measurement errors can be caused by the inductive coupling between the potential probe and return current probe leads at high frequency. The test circuit for measuring the grounding system impedance using the revised fall-of-potential method employed in this work was shown in Fig. 2. The potential probe is installed at the opposite side of the return current electrode to avoid any inductive coupling between the two leads of voltage and current probes system at high frequency.[2] PC V EP Ground Impedance Calculation Program Isolator Digital Filter A/D Converter V-F F Conversion Controller Inverter displayed on a monitor of the computer. 2.2 Soil resistivity measurement The measurement methods of soil resistivity used in this investigation are Wenner method and dipole-dipole method. The Wenner method has been used widely for quantitative interpretation of soil structure. The measured apparent resistivity values are normally plotted on a log-log scale graph, and the soil structure consisting of horizontal layers can be derived from the result. The center point between the inner probes remains fixed in this method, but the spacing between the outer probes is increased to obtain resistivity values at the deeper soil. Therefore, the surveying line becomes overlong for the investigation of deeper subsurface and the increase of surveying line acts on a limitation in detailed earth structure analysis. Another difficult procedure of the Wenner method is to convert the true resistivity values from the apparent resistivity. To obtain the true resistivity, it is assumed that the subsurface consist of homogeneous horizontal layers, and the resistivity of the soil changes only with the depth. However, the horizontal changes in the soil resistivity are commonly found at a construction site, and the effect of such horizontal changes can results in error during the resistivity conversion procedure and the layer thickness determination. The lateral changes also must be taken into account in the analysis of soil structure.[3] I T P 100 m E 100 m C Figure 2. Grounding system impedance measurement Frequency of the test current was linearly controlled during the established time in the range of khz. The waveforms of the test current and grounding electrode potential rise were directly recorded by a data acquisition system controlled by a personal computer. The magnitude, phase, resistive and reactive components of grounding impedance are analyzed by the specially installed program and the graphs of the measured results as a function of frequency are Figure 3. Dipole-dipole method Figure 3 shows the field surveying method of soil resistivity using the dipole-dipole method and a possible sequence of measurement for the dipole-dipole method. First time, measured values from 40 electrodes are stored to a measuring instrument with coordinates

3 of measuring point respectively. About 1000 measurements gathered along the surveying line are stored sequentially at the storage memory. The measured results are transmitted to PC and converted to true resistivity two-dimensional (2-D) sections using the RES2DINV conversion software. 3 RESULTS AND DISCUSSION Wenner method usually involve about several tens of readings and the converted result is apparent resistivity curve according to the electrode space. Figure 4 shows the apparent resistivities and soil type computed using the CDEGS software. between 4 m and 10 m according to the surveying lines. It looks like easy work to design grounding rod using this results, but it is very difficult to calculate ground resistance actually because the interpretation of measured data can vary considerably where soil with nonuniform resistivities are encountered in the soil layer.[1] 30m Rod 10m Rod Figure 5. Inverted 2-D map of soil resistivity The 2-D imaging survey using the dipole-dipole method is shown in figure 5. The dipole-dipole method involves about 100 to 1000 measurements and displays the results by a 2-D map of the soil structure. The result also shows two heterogeneous resistivity areas. The localized low resistivity area of around 20 Ω m in the top layer is located at a distance between 130 m and 170 m from the origin of the surveying line. The soil resistivity also reaches 1000 Ω m at the bottom layer where the 30 m rod was driven. However, as we can see from the result in figure 4, the localized high or low resistivity area cannot be detected by conventional Wenner 4-probe method.[4] Table 1. Interpretation model of subsurface and calculated resistances using simulation software Figure 4. Apparent resistivity The above measurements were carried out using the Wenner method along the two surveying lines. The soil structure was construed by horizontal two-layer from both of two survey lines. Soil resistivity was analyzed about 80 to 100 Ω m in top layer and 15 to 30 Ω m in bottom layer. Thickness of top layer shows difference Table 1 shows the interpretation models of subsurface and calculated resistances using CDEGS software. Two

4 grounding rods for obtaining a low ground impedance were developed at the position of 100 m and 150 m apart from the measurement origin. Apparent resistivity curve from the computation result #1 shows the two-layer soil type. Calculated ground resistance of 10 m rod using this soil model was 1.89 Ω and this value was lower markedly than the calculated ground resistance of 8.71 Ω using the three-layer model in table 1. two-layer soil structure. However, selected soil resistivities correspond to measurement methods at a bottom layer show significant difference each other. Therefore, calculated ground resistances of the 30 m rod were 6.49 Ω and Ω. (a) Ground impedance (a) Ground impedance (b) Phase (b) Phase (c) Resistive component (c) Resistive component (d) Reactive component Figure 7. Ground impedance of 30 m rod (d) Reactive component Figure 6. Ground impedance of 10 m rod The grounding rod of 30 m long was driven at a distance of 100 m, and both of the two interpretation models of subsurface using computation results #2 in figure 4 and inverted 2-D result in figure 5 show Figure 6 and 7 show the measurement results of the two grounding rods. The ground impedance was measured and analyzed with the magnitude of the impedance, phase, resistive component, and reactive component using the variable frequency inverter system.[5] The measured resistive component is the ground resistance of the rod at low frequency. The ground resistances of

5 10 m and 30 m rods are 11.9 Ω and 13.0 Ω respectively. Measured ground resistance values are much higher than calculated values using the soil models originated Wenner 4-probe method. On the other hand, calculated resistance valus using the model of dipole-dipole were approximated slightly to the measured values. The serious error of ground resistance calculation using Wenner method was caused by the exclusion of possibility of lateral changes in subsurface soil. And also, there is a singular result that the ground resistance of 10 m rod is much lower than the value of 30 m rod. Because the 10 m rod was driven in the localized low resistivity area, the especially low resistivity caused the ground resistance to be low for the 10 m rod. From these results, we can design the electrode scale of a grounding rod and decide the suitable burial depth effectively using the soil structure analysis method based on the new dipole-dipole method. 4 CONCLUSION Field measurement of soil resistivity and calculation method of grounding rods was carried out and the results could be summarized as follows. The conventional Wenner method cannot detect the localized lateral changes of soil resistivity and produce an error for calculating the grounding rod resistance. The 2-D survey introduced in this paper is effective to analyze a more accurate model of the subsurface and can give useful results taking into account horizontal changes in the soil. And effective design of the grounding rod and suitable burial depth can be carry out using the soil structure analysis method based on the dipole-dipole method. REFERENCES [1] IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System, IEEE Std , [2] IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System, IEEE Std , [3] M. H. Loke, 2-D and 3-D electrical imaging surveys, Instrument Tutorial, [4] N. Harid, H. Griffiths, A. Haddad and K. Walker, "Soil resistivity mapping of non-homogeneous soils," proc. of ISH 2003, Rotterdam, Netherlands, [5] Bok-Hee Lee and Ju-Hong Eom, "A New Measurement Method of Grounding Impedance Using a Variable Frequency Inverter," Proc. of of ISH 2003, Rotterdam, Netherlands, 2003, pp.390~393. Author : Ju-Hong Eom Senior Researcher High Voltage and Material Lab. of KESRI eommas@snu.ac.kr