Grounding Resistance Measurement Analysis of Grounding System in Vertical-Layered Soil

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<ul><li><p>IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 19, NO. 4, OCTOBER 2004 1553</p><p>Grounding Resistance Measurement Analysis ofGrounding System in Vertical-Layered Soil</p><p>Rong Zeng, Member, IEEE, Jinliang He, Senior Member, IEEE, Yanqing Gao, Student Member, IEEE, Jun Zou,and Zhicheng Guan</p><p>AbstractHow to precisely measure the grounding resistance ofsubstations is the fundamental factor to guarantee the safe oper-ation of power systems. The placement rules of test electrodes forthe grounding resistance measurement of grounding system in ver-tical-layered soil were analyzed by the numerical method in thispaper. The actual demanded location of the test potential electroderespective to the true grounding resistance of grounding system indifferent measuring directions changed (in a wide range) when thegrounding system was arranged in different soil models. The testerror of 0.618 method to measure grounding resistance sometimesis very high. The test route that the test circuit is in parallel with thevertical boundary is recommended for measuring the groundingresistance of grounding system in the vertical-layered soil area. Thescientific and suitable measuring method of grounding resistance isbased on analyzing the actual soil model and the grounding systemstructure to obtain the suitable test route and choose the correctcompensated point location of the potential electrode.</p><p>Index TermsFall of potential method, grounding resistance,grounding system, substation, vertical-layered soil.</p><p>I. INTRODUCTION</p><p>THE GROUNDING system of a substation is a fundamentalcountermeasure to guarantee the safe operation of powersystems [1]. The grounding resistance is an important index ofgrounding systems for substations and power plants, and is alsoa parameter to measure the efficiency, safety of grounding sys-tems, and to check whether the grounding systems meet thedemand of design. Due to the nonuniformity of soil and mea-suring error of soil resistivity data and some other factors, whichcannot be considered in simulating analysis, the designed valueof grounding resistance must be checked by the field test afterthe grounding system is constructed [2], [3]. On the other hand,in order to examine the actual condition of the grounding systemof an operating substation, the grounding resistance must bemeasured periodically. The measurement of the grounding re-sistance is routine work to ensure the safe operation of powersystems in China [4].</p><p>Measuring the grounding resistance simply and precisely is abig problem in power systems [5], [6]. And correctly arrangingthe measuring circuit according to the actual soil structure isthe key to precisely measure the grounding resistance. Presentlyin China, the grounding resistance is still measured by 0.618</p><p>Manuscript received January 8, 2003; revised May 27, 2003. Paper no.TPWRD-00011-2003.</p><p>The authors are with the Department of Electrical Engineering, Tsinghua Uni-versity, Beijing 100084, China (e-mail: zengrong@tsinghua.edu.cn; hejl@ts-inghua.edu.cn; gaoyq@emc.eea.tsinghua.edu.cn; zoujun@tsinghuaedu.cn;guanzc@tsinghua.edu.cn).</p><p>Digital Object Identifier 10.1109/TPWRD.2004.835283</p><p>method (the test potential electrode is placed at the location of0.618 , where is the distance between the groundingsystem and the test current electrode); therefore, we often findthat the grounding resistance of a grounding system measuredin one direction is very different from the result in another direc-tion. After analyzing the geological structure of soil, we foundthis condition usually exists in substations where the soil isnonuniform in the horizontal direction. For example, a 220-kVsubstation with a lake or a river in the front and a mountain be-hind, the measured results of grounding resistance in differentdirections are different. So it is very important to discuss thearrangement rule of measuring electrodes for grounding resis-tance test circuit of a grounding system in vertical-layered soil.</p><p>The actual vertical-layered soil structures vary in differentareas. In order to acquire some useful conclusions for groundingresistance measurement of substations in vertical-layered soils,several typical models of vertical-layered soil were assumed andanalyzed in this paper, and all results illustrated were simulatedones.</p><p>A professional software package was applied in the analysisof this paper, which is the CDEGS software package developedby Safe Engineering Services and Technologies Ltd., Canada(SES) [7]. Extensive scientific validation of the software usingfield tests and comparisons with analytical or published researchresults has been conducted [8].</p><p>II. PLACEMENT RULE OF POTENTIAL ELECTRODE INVERTICAL TWO-LAYER SOIL</p><p>In our simulation, a 100 100 square grounding grid witha grounding conductor span of 10 m in vertical-layered soil isassumed as the grounding system model. Different measuringroutes of the grounding resistance of a grounding system in ver-tical two-layer soil model are shown in Fig. 1, the resistivity</p><p>of the soil region in the left side of the vertical boundary islow, equal to 100 m, and the resistivity of the soil re-gion in the right side of the vertical boundary is high, equal to500 m. The measuring current is injected into the center ofthe grounding grid, and we assume equal to 100 m, whichis the side length of the grid. Ordinarily, the fall-of-potentialmethod is applied to measure the grounding resistance of a sub-station recommended by IEEE Std. 81-1983 [9], the apparentgrounding resistance curves in different measuring routes by thefall-of-potential method were analyzed. In our analysis, the truegrounding resistance of the grounding system is calculated bythe definition of grounding resistance, which is the ratio of thegenerated potential of the grounding system and the injected</p><p>0885-8977/04$20.00 2004 IEEE</p></li><li><p>1554 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 19, NO. 4, OCTOBER 2004</p><p>Fig. 1. Measuring routes of grounding resistance of a grounding system builtin a vertical two-layer soil model.</p><p>measuring current between the grounding system and the re-mote point where the potential is zero.</p><p>As we know, when the fall-of-potential method is applied tomeasure the grounding resistance, if the soil is uniform, andthe grounding system is a hemisphere, then the location ofpotential electrode to obtain the true grounding resistance ofthe grounding system is at the point of 0.618 betweenthe grounding system and the current electrode, this point iscalled as the compensated point of the potential electrode.If the soil is nonuniform, and the grounding system is not ahemisphere, the compensated point location of the potentialelectrode would deviate 0.618 , sometimes moving to theside of the grounding system, and sometimes moving to the sideof the current electrode. The compensated point locations ofthe potential electrode in different vertical-layered soil modelswere analyzed in this paper.</p><p>A. Grounding System in Low Resistivity Soil SideAs illustrated in Fig. 1, the grounding system is arranged</p><p>in the low-resistivity (100 m) soil side and the distancebetween the center of the grounding system and the verticalboundary is . The diagram in Fig. 1 is a sectional drawing, soonly half of the grounding grid is illustrated; it is the same inFigs. 4 and 5. Five different measuring routes were consideredin our analysis. When the current electrode of the measuringcircuit is arranged in high-resistivity (500 m) soil region,three measuring routes were assumed in our analysis, thedistance between the center of the grounding system and thetest current electrode is 2 , 4 , or 8 , respectively, for thelocation 1, 2, or 3 in Fig. 1, the current measuring lead isperpendicular to the vertical boundary. And their respectiveapparent grounding resistance curves in Fig. 2 are presentedas INLOW2D, INLOW4D, and INLOW8D. When the testcurrent electrode is arranged in the soil side with low resis-tivity, the test current electrode is located in the left side of thegrounding system and is perpendicular to the vertical boundary,the distance between the center of the grounding system andthe current electrode is 8 , this measuring route is illustratedas location 4 in Fig. 1, the respective apparent groundingresistance curve in Fig. 2 is presented by INLOWLEFT. An-other discussed case is that the measuring route parallel to thevertical boundary, respective for the location 5 in Fig. 1, and</p><p>Fig. 2. Apparent grounding resistance curves when the grounding system isbuilt in the low-resistivity region of a vertical two-layer soil model.</p><p>the distance between the center of the grounding system andthe current electrode is 8 , the respective apparent groundingresistance curve in Fig. 2 is presented by PARALLEL. All ofthe apparent grounding resistance curves except curve 4 inFig. 2 have three regions: the left region is obtained when thepotential electrode P is placed at the opposite side with respectto current electrode C. The middle region is obtained whenthe potential electrode is placed between the grounding systemand the current electrode, and the right region is obtained whenthe potential electrode P is located on the same side as currentelectrode C but away from it. In fact, the middle and the rightregions of the apparent grounding resistance curve should becontinuous at the location of the current electrode, but thetest current electrode is small. When the potential probe isnear the current electrode, the respective apparent groundingresistance is very large, so the curve region with high apparentgrounding resistance near the current electrode is cut off, andthe illustrated curves are interrupted.</p><p>For the above five different measuring routes, in order to ob-tain the true grounding resistance in the field test, the requiredcompensated point locations for the potential electrode, wherethe true grounding resistance would be measured, were analyzedand shown in Table I. On the other hand, the 0.618 method isstill widely used for the measurement of grounding resistance inChina; therefore, the difference between the true grounding re-sistance and the measured one by 0.618 method was compared,and the error of the 0.618 method is the relative error ofto</p><p>(1)</p><p>where is the measured grounding resistance by 0.618method, and is the true grounding resistance of thegrounding system.</p></li><li><p>ZENG et al.: GROUNDING RESISTANCE MEASUREMENT ANALYSIS OF GROUNDING SYSTEM IN VERTICAL-LAYERED SOIL 1555</p><p>TABLE IDEMANDED COMPENSATED POINT LOCATIONS OF POTENTIAL ELECTRODE</p><p>WHEN THE GROUNDING SYSTEM IS BUILT IN THE LOW-RESISTIVITYSIDE OF A VERTICAL TWO-LAYER SOIL</p><p>Observing the left regions of these curves in Fig. 2, if the po-tential electrode P is placed at the opposite side with respectto the current electrode C in different measuring routes, thatis to say, the current and potential electrodes are arranged in180 angle, even if the potential test lead is longer than ,the measured grounding resistance is still 15% smaller than thetrue one. The soil resistivity is low in the left side, the po-tential falls slowly. And the placed current electrode near thegrounding system leads to the potential of the grounding systembecoming smaller than the respective one when the measuringcurrent is applied between the grounding system and a remotepoint with zero potential. So the potential difference between thegrounding system and the potential electrode is forever smallerthan the potential of the grounding system relative to a remotezero-potential point, which is respective to the true groundingresistance, and is obtained when the measuring current is ap-plied between the grounding system and the remote point withzero potential.</p><p>Since the current densities in two sides of the verticalboundary are equal, and the soil resistivity in the right side ishigh, we can conclude that the potential in the right side fallsquickly from the equation (here, is the electricalfield intensity, is the current density, and is the soil resis-tivity). So, in curves INLOW2D, INLOW4D, and INLOW8D,the apparent grounding resistances in the region between thegrounding system and the current electrode increase morequickly than in uniform soil, so their intersecting points withthe true grounding resistance line move to the left side, the de-manded compensated point locations of the potential electrode(defined as the ratio of potential lead length and currentlead length ) respective to the true grounding resistanceare smaller than 0.618 as shown in Table I.</p><p>When the test current electrode is arranged in the low-re-sistivity region in the left side of the grounding system, weobserved from curve INLOWLEFT in Fig. 2 that its regionbetween the grounding system and the current electrode isvery even, so its intersecting point with the true groundingresistance line moves to the right side, that is to say, the com-pensated point location of the potential electrode respectiveto the true grounding resistance of the grounding system islarger than 0.618 .</p><p>So, for these two cases when the current electrode is arrangedin the right region of the grounding system with high resistivityor in the left region with low resistivity, we can observe thedemanded compensated point locations of the potential elec-trode to obtain the true grounding resistance highly deviate0.618 .</p><p>Fig. 3. Apparent grounding resistance curves when the grounding system isbuilt in the high-resistivity region of a vertical two-layer soil model.</p><p>When the test current lead is arranged in parallel with thevertical boundary, the demanded compensated point location ofthe potential electrode to obtain the true grounding resistance isabout 0.65 as shown by the curve PARALLEL in Fig. 2,and the measuring error of the 0.618 method is very small, it isonly 1.4%.</p><p>Observing the right region of curve INLOW2D, which is inthe right side of the current electrode in Fig. 2, it has anotherintersecting point with the true grounding resistance line, but itis far from the grounding system.</p><p>B. Grounding System in High Resistivity Soil SideAs shown in Fig. 1, when the grounding system is built in</p><p>high-resistivity soil region, and now and are 500 mand 100 m, respectively, the distance between the center ofthe grounding system and the vertical boundary is still . Asshown in Fig. 1, when the test current is placed in location 1,location 2, and location 3, the distance between the center of thegrounding system and the current electrode is , , or ,respectively. Their respective apparent grounding resistancecurves in Fig. 3 are presented by INHIGH2D, INHIGH4D,and INHIGH8D. When the current electrode is placed in thehigh-resistivity region, which is in the left side of the groundingsystem and the current measuring lead is perpendicular tothe vertical boundary, the distance between the center of thegrounding system and the current electrode is ; this caseis illustrated as location 4 in Fig. 1, the respective apparentgrounding resistance curve in Fig. 3 is presented by INHIGH-LEFT. Another case is that the measuring lead parallels t...</p></li></ul>

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