Analysis on Influence of Long Vertical Grounding Electrodes on Grounding System for Substation Rong ZENG, Jinliang HE, Zanji Wang, Yanqing GAO Dept of Electrical Engineering, Tsinghua University Beijing 100084, China Abstract: Three dimensional grounding system is introduced to decrease the grounding resistance, step and touch voltages of grounding system in areas with high soil resistivity or with limited areas for grounding system The relationship between the number of vertical grounding rods and the grounding resistance is discussed The new means to add grounding rods to the grounding system with explosion grounding technique, have efficiently decreased the grounding resistance in the actual grounding engineering Reasons why vertical grounding rods can efficiently decrease the maximum touch and step voltages of the earth surface above the grounding system are analyzed Calculated results shows the vertical grounding rods can also effectively decrease the influence of seasonal factor on safety of grounding system This paper provides the rule to choose the vertical grounding rods in multi-layer soil based on the relationship analysis between the number and the length of the vertical grounding rods Key words: Grounding system, vertical grounding electrode, step voltage, touch voltage, seasonal factor I INTRODUCITION With the rapid increase of electrical load in recent years in China, in order to promise the safety of the power system, it is urgent to decrease grounding resistance of the grounding system, especially when power generating stations or substations are built in the soil with high resistivity The limitations of the land requisition or the landform make researchers turn their sight from the horizontal grid to the vertical rod But so far most of the discussions on the vertical rods are only samples of application or conclusion of project experiences[ 13 Several researchers analyzed the role and the design rule of rods, but they focused on the uniform soil The detailed discussion of three-dimensional grounding system design in the different soil structures almost cannot be found[2,3] This paper will discuss how to design threedimension grounding system scientifically and economically Weimin SUN Shandong Electric Power Bureau Jinan 250001, shandong, China Qi SU Dept of Electr and Computer Sys Eng Monash University Clayton, Victoria 3168, Australia 11 NUMERICAL ALGORITHM RESEARCH OF GROUNDING SYSTEM Since 1972, scholars have introduced various numerical algorithm into the calculation of grounding parameters In 90’s liner filter method and complex image method were used Numerical algorithm solves many problems emerged with the calculation in experiential formula Comprehensively considering the actual grounding system structure and the noneven dispersion of the fault current in the different parts of the grounding system, numerical algorithm helps to get exact result of any complex grounding system and makes it possible to compare the economic efficiency of the projects scheme on the basis of technical feasibility Dawalibi has improved the theoretical model and optimized the numerical algorithm over 20 years His multi-layer soil model numerical algorithm based on the base image method has come into mature CDEGS (grounding system parameters calculating software developed by him and his collaborators) has been widely used in the world This software can not only get comparatively accurate parameters of various grounding system and also get grounding system parameters in the layered soil This paper makes simulative calculation by the means of CDEGS 111 DESIGNING RULE ANALYSIS OF VERTICAL GROUNDING RODS The discussion is based on a llOkV substation as shown in Fig The horizontal grounding grid is 150x150m2in area The even grounding conductor span of the grounding grid is 15m and horizontal conductor radius is 0.01 Im (r,=0.01lm) The uniform soil resistivity is 200R.m A Number of Vertical Grounding Rods Vertical rods are added to the existed horizontal grounding grid The length of the rods is 50m The relationship between the number N of the rods and the grounding resistance R is illustrated in Fig.2 Here the rod radiuses are r2=3.5m and r2=0.025m used in calculation, Radius of 3.5m is the result considering the effect of explosion technique From Fig.2, we observed that while other conditions are fixed, grounding 0-7803-6338-8/00/$10.00(~)2000IEEE 1475 resistance decreases with the increase of the number of rods However, the decrease of R reaches saturation when N reaches a certain number because the shielding effects increase with the decrease of the interval among rods Besides rods can restrain the dispersed current on the grid, i e the total grounding resistance is not the simple parallel connection of grounding resistances of the grounding grid and vertical grounding rods There is a shielding coefficient of horizontal grounding grid to the vertical grounding rods The shielding coefficient increases with the number of the grounding rods * I* same as the upper Now a rod is arranged in the center of the grid and another in the comer, to check which rod has better resistance decreasing effects The grounding resistance decreasing rate is defined as: here, R, is the grounding resistance after rods are added Ro is the grounding resistance of the horizontal grounding grid The equivalent radius of the horizontal grounding grid and is defined as: req= J s/~ here, S is the area of the horizontal grounding grid The relationship between the grounding resistance decreasing rate and the length of vertical grounding rods under different rod locations are shown in Fig.3 Obviously, the resistance decreasing effect of the rod arranged on the periphery of the grid is better than that of the rod arranged in the center due to the shielding effect between the horizontal conductor and the vertical rods increases when the rod is arranged in the center of thegrid ’ Fig The arrangement of grounding grid - 1.5 I 10 I 15 I 20 I 25 I 30 2.0 2.5 35 Grounding Rod Number Fig.2 The relationshipbetween the number N of the rods and the grounding resistance R When the rod radius is 3Sm, their effects to decrease grounding resistance reach 35%, which is better than that when the rod radius is 0.025m B Location of Vertical Grounding Rods Usually rods are well distributed in the grid and the rod density in the center of the grounding grid is often higher than that on the periphery of the grounding grid Contrast analysis illustrates that such a design is improper Horizontal grounding rid is the 1476 Fig.3 The relationship between the grounding resistance decreasingrate and the length of vertical grounding rods under different rod locations Analyzing the results in Fig.3, the dispersed current by vertical grounding rods on the periphery of the grids are more than twice of the current dispersed by the rod of the same length in the center of the grid As a result rods should be arranged on the periphery of the grid if rods are added to decrease the grounding resistance In this way the shielding effects between the grid and rods can reach the minimum With the number of the rods increase, the advantage of this arrangement becomes apparent To increase the resistance decreasing effect and to decrease the shielding coefficient of horizontal grounding grid to the vertical grounding rods, the best way (if possible) is that rods are arranged far away from the grid and connected with the grid through the horizontal grounding conductors This way can benefit to fault current dispersing, efficiently decrease the grounding resistance and can make the touch voltage distribution rational The calculation conditions of two-layer soil are the same as that of uniform soil The soil resistivity of the upper layer, p , equals 200R.m, and the soil resistivity of the lower layer, f i equals 600Q-m., the upper soil layer depth is 20m The analysis results is shown in Fig.4 (b) C Length of Vertical Grounding RODS We observed in Fig.4 that the two curves are almost in superposition, i.e grounding resistance decreasing rates of horizontal grounding grids with different areas are almost the same when the ratio of equivalent radius and the rod-length is the same Another analysis result from Fig.4 is that the two curves coincide with each other when they are far from the interface of the two layers; while they differ greatly when they are near the interface That is also the rule of current intensity pass through the interface (discussed in Part 111) Since parameters that influence the grounding resistance are too much, all the discussion of this part is based on the horizontal grounding grid with a fixed area Analysis indicates that these conclusions are still in effect when the area is changed Take a square horizontal grounding grid as an example, the ratio of the grounding resistance decreasing rate to the vertical grounding rod length, and another ratio of grounding resistance decreasing rate to grounding grid radius are obtained In the analysis, the grounding resistivity is 200R-m One used horizontal grounding grid area is isox isom', while the other is lOOx 1OOm' The span between the horizontal conductors of the grounding grid is 10m and the buried depth of the horizontal grounding grid is 0.6m The number of vertical grounding rods are The influence of the vertical grounding rod length on the grounding resistance decreasing rate with different grid areas is illustrated in Fig.4 IV ARRANGEMENT OF VERTICAL GROUNDING RODS IN THE NONUNIFORM SOIL The arrangement of long rods in the nonuniform soil from the point of dispersed current distribution on the rods are discussed here A Current Distribution on A Single Rod 40 3.0 The calculation illustrates that in the similar soil conditions and the same number of rods, the grounding resistance decreasing rates not change if the ratios of grounding-grid radius to rod-length is fixed 0.5 The reflective coefficient of two-layer soil is defined as k =(p, A ,p, are soil resistivities Fig.5 is the distribution curves of the current intensity J dispersed into earth along rod length x under different reflective coefficient Here, the length L equals 20m; radius r, equals 0.02m; upper-soil resistivity p, equals 100SZ.m; the depth h is 7.5m 1.o 1.5 2.0 s I (a) uniform soil 225 S:150m*150m S:lOOm*l OOm $150 v ? 75 O ' " ' 10 " 15 " 20 ' x (m) Fig3 Current distribution on the rods in the hvo-layer soil (b) two-layer soil From Fig.5, we observed that the current is well distributed on the rods except that current intensity increases quickly at the Fig4 Influences of rod length on grounding resistance decreasing rate 1477 bottom of the rods in the uniform soil It also shows that current intensity increases a little with the increase of depth However, the high current intensity area occupies a little percentage As a result, the current distribution is considered as well-distributed, this would not cause apparent error The dispersed current distributions differ with each other when rods are in the two-layer soil The current intensity in the lowresistivity soil layer is higher than that in the high-resistivity soil layer In each layer current distribution almost has no change but there is a sharp shift along the interface The difference of current distributions in different layers rises when reflective coefficient increases For example, when k equals 0.8, the current intensity is 300" in the upper layer, but it is only 25A/m in the lower layer Hence the proper choice of rod length can not only decrease grounding resistance efficiently but also achieve better economic effect B Arrangement of Rods in Two-layer Soil Here is the discussion how vertical grounding rods affect the electrical behaviors of the three-dimensional grounding grid in the two-layer soil The relationship between the rod length and the grounding resistance decreasing rate with different reflective coefficient is shown in Fig.6 Here the horizontal grounding grid area is lOOx loom*, the horizontal conductor span is 10m; the upper layer soil resistivity is 200R.m Four steel rods are arranged on the comers of the grid, the upperlayer soil thickness h is 40m "0 23- -0.9 4- -0.5 Therefore, the method of long rods does not fit the situation that the soil resistivity of bottom layer is very high On the other hand, when the soil resistivity of the bottom layer is low, adding of long rod can achieve good results From current distribution curves in Fig.5, it is understandable that a tuming point emerges on the grounding resistance decreasing rate curve in Fig.6 when k ... Fig.4 IV ARRANGEMENT OF VERTICAL GROUNDING RODS IN THE NONUNIFORM SOIL The arrangement of long rods in the nonuniform soil from the point of dispersed current distribution on the rods are discussed... 10m and the buried depth of the horizontal grounding grid is 0.6m The number of vertical grounding rods are The influence of the vertical grounding rod length on the grounding resistance decreasing... arranged on periphery of the grid instead of in the center The discussion of rod length in various nonuniform soil shows it is wrong that long rods can always reach better results The choice of rod