Anik Chatterjee Novartis, Hyderabad, India Clopper-Pearson Confidence Interval in SAS Using Proc Freq Disclaimer: The views expressed within this paper/presentation ...
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where Cs is the surface layer derating factor which is a thickness of 10 cm on top of a native soil with a soil. function of the surface layer thickness and the reflection resistivity of 100 m K 0 9 CS1986 0 555 and. coefficient between the surface layer resistivity and the CS2000 0 705 giving a 27 increase Using Equations. native soil resistivity defined as K s s 1 and 2 we can see that the increase in the Cs factor. results in a 20 increase in tolerable touch voltage and. In Standard 80 1986 the expression for Cs is 25 increase in tolerable step voltage assuming a body. resistance of 1000, OP The lower tolerable touch and step voltages based. on Standard 80 1986 mean that the design of a,n 1 1 2 nh b 2. s grounding system is a conservative one possibly an. overdesign The higher tolerable touch and step, where hs is the surface layer thickness voltages based on Standard 80 2000 make the design of. grounding system easier Note that the differences in the. In Standard 80 2000 the expression for Cs is tolerable touch and step voltages based on the two. versions of the Standard generally increase as the. magnitude of the reflection coefficient increases and the. Cs 1 K r sin 1,b 2 n 1 0 R1 R2, H K 6 surface layer thickness decreases as evidenced by Fig. where R1 and R2 are give by,IV THE DECREMENT FACTOR Df. R1 r b 2 2 nhs 2 j 1 2,7 Because the design of a grounding system must. e r b 2 nh j, 2 consider the asymmetrical current a decrement factor is. introduced to take into account the effect of DC current. offset during a fault Careful readers may have noticed. Fig 1 compares the Cs curves based on the formulae that the expressions for the decrement factor in the two. from both versions of the Standard for K 0 1 K versions of the Standard are different However there is. 0 5 and K 0 9 no intended change in the expression of the decrement. factor Df The difference arises from a typographical. error in the 1986 version The 2000 version has simply. corrected this In the application of the decrement. factor however the two versions have slightly different. interpretations A table showing typical values of the. decrement factor in the 2000 version includes time. durations of the fault up to 1 s while the 1986 version. recommends a value of 1 0 for Df for fault durations of. 0 5 s or more and considers such a value to be, conservative The 2000 version states that a value of 1 0. for Df for fault duration of 0 5 s or more can be used. without further comment It is obvious that if the, computed value for Df is used for longer fault durations. then the grounding design will always be conservative. For example for a fault duration of 0 5 s and a typical. X R ratio ratio of the system reactance to resistance of. 20 the calculated value of Df is 1 052 If Df 1 052 is. used in the safety calculation of a grounding design. quantities such as grid GPR Ground Potential Rise,touch voltage and step voltage will be 5 2 larger. compared to the use of Df 1 0, Fig 1 Cs curves based on Standard 80 1986 and Standard 80 2000. It can be seen that the Cs factor based on Standard V THE UNIFORM SOIL ASSUMPTION. 80 2000 is generally higher than that based on Standard There are major changes in the chapters dealing with. 80 1986 The difference is larger when the reflection soil structure and selection of a soil model Chapter 13. coefficient is closer to 1 The implication of higher Cs in Standard 80 2000 and Chapter 11 in Standard 80. values is that the corresponding foot resistances based 1986 These changes include a new table showing. on Standard 80 2000 are higher As a consequence the typical surface material resistivities a description of the. tolerable touch and step voltages are higher as can be uniform soil assumption procedure addition of a two. seen from 1 and 2 For example for a surface layer model and an associated graphical method and. crushed rock layer with a resistivity of 2000 m and a the introduction of a brief discussion of multilayer soils. All the changes provide useful information except for. Copyright Safe Engineering Services technologies ltd. the change dealing with the uniform soil assumption 300 m 100 m. which may lead to incorrect grounding designs if not Computer simulated soil resistivity measurements in. applied carefully The misleading effect is further the above two soil types are made and the results are. enhanced by the introduction of the erroneous Annex E shown in Table II If 10 is used to compute the. entitled Equivalent Uniform Soil Model for average resistivity then the two average resistivities. Nonuniform Soils corresponding to the two soil types are 1 195 3 m. and 2 199 7 m respectively Actually if the, Two formulae are presented for the calculation of measurement spacing is large enough we will always. the resistivity of a uniform soil model based on,have 1 2 200 m Let us now consider a 64 mesh. measured resistivity in Chapter 13 of Standard 80,5m 5m grounding grid and a 64 mesh 200m 200m. grounding grid buried at a depth of 0 5 m in Soil Type. 1 and Soil Type 2 The ground resistances computed, a 1 a 2 a 3 a n with the two layer soils and with the average soil. n resistivity of 200 m are shown in Table III,a min a max. av 2 10 TABLE II,2 COMPUTER SIMULATED SOIL RESISTIVITY. where a 1 a 2 a 3 a n are the measured MEASUREMENTS IN SOIL TYPES 1 AND 2. apparent resistivity data at different spacings using the. four pin method n the number of measurements and Electrode Separation Soil Type 1 Soil Type 2. a min and a max the minimum and maximum values Apparent Apparent. ft m Resistivity Resistivity,of the measured apparent soil resistivity m m. The flaws in the uniform soil assumption using 9 1 0 305 99 7 299 0. and 10 are as follows 3 0 915 100 1 299 5,5 1 524 100 6 298 4. a Uniform soils seldom exist in practice 15 4 573 111 3 271 6. b Both formulae only relate the average soil 20 6 098 121 0 248 7. resistivity to the measured apparent 30 9 146 143 1 202 6. resistivities not to the electrode spacings at,50 15 24 181 7 144 2. which they were measured It is well known,70 21 34 208 8 120 3. that the shallow soil resistivities exert greater, influence on smaller grounding grids and deep 90 27 44 227 2 110 7. soil resistivities have a greater effect on larger 110 33 54 241 5 106 4. grounding grids Measurements made at short 130 39 63 251 8 104 2. electrode spacings reflect surface soil,150 45 73 259 6 103 1. resistivities and measurements made at large, electrode spacings reflect deeper soil 200 60 96 272 7 101 7. resistivities 400 121 92 290 9 100 4,c Depending on the distribution of electrode. spacings chosen the average soil resistivity TABLE III. using 9 will vary significantly For example GROUND RESISTANCES COMPUTED WITH. if more measurement points are taken for short TWO LAYER SOILS COMPARED WITH. spacings and fewer measurement points for THOSE COMPUTED WITH AVERAGE UNIFORM SOIL. large spacings then the top soil resistivity will, have more weight in the calculation of the Grid Soil Soil Uniform. Dimension Type 1 Type 2 200 m,average resistivity using 9 See 6 for details. on how limited electrode spacings can 5m 5m 9 22 19 4 15 0. introduce error into grounding analysis 200m,0 58 0 33 0 48. predictions 200m, Let us consider two two layer soil types as shown in It can be seen from Table III that in Soil Type 2 the. Table I ground resistance of the small grid is larger than that in. TABLE I Soil Type 1 while for the large grid the reverse is true. TWO LAYER SOIL TYPES, These results show that for the small grid the top layer. resistivity has a larger influence and for the large grid. Soil Type 1 Soil Type 2 the bottom layer has a larger influence Use of the. Soil Layer Soil Layer average soil resistivity results in an error of 63 for the. Resistivity Thickness Resistivity Thickness small grid in Soil Type 1 and 23 in Soil Type 2 In. 100 m 6 1 m 300 m 6 1 m fact to obtain the same ground resistance of the small. Copyright Safe Engineering Services technologies ltd. grid in Soil Type 1 the average soil resistivity should the grid current i e the current discharged into the. be 123 m instead of 200 m as proposed by the use earth by the grounding grid of the faulted substation In. of 10 Similarly to obtain the same ground resistance Standard 80 2000 a new annex Annex C has been. of the large grid in Soil Type 2 the average soil added to analyze the current division using a graphical. resistivity should be 138 m instead of 200 m approximate method Examples have also been. provided to illustrate the use of the graphical analysis. The newly added Annex E in Standard 80 2000 The information provided in Annex C is useful. gives an example of equivalent uniform soil for However it is preferable to use computer programs to. nonuniform soils In the above example the two soil compute fault current distribution because accurate. types are the same as those in Annex E of Standard 80 results can easily be obtained While using the graphical. 2000 for comparison purposes It should be pointed out method the system under consideration has to be. that Tables E 1 and E 2 in Annex E of Standard 80 reasonably similar to those covered by the graphics. 2000 contain erroneous values Table E 2 in Annex E Principles of such programs are described in 11 12. shows the mathematically derived apparent resistivities Typical computation results can be found in 13. for the two soil types The first three values of the Following is an example showing the computation of. resistivities have significant errors For example for grid current. Soil Type 1 in Table E 2 of Annex E the first derived. soil resistivity value corresponding to a probe spacing. of 0 305 m is 56 94 m Anyone with experience in, soil resistivity measurement and interpretation will. immediately realize that this value theoretically cannot. be lower than 100 m unless overly long electrodes, are used for the measurements and no correction is. made to account for the coupling between them This. appears to be the case in Annex E the wrong data is. therefore used in the calculation of the average soil. resistivity in Annex E Consequently the results shown. in Table E 1 of Annex E cannot be referenced with any. confidence In reality if the electrode spacings include a. very short spacing and a very large spacing the, calculated average resistivity should be 200 m for. both soil types This is why we have used 200 m in, our example Obtaining a good equivalent uniform soil. to represent a multilayer soil is never an easy task if. ever possible As described in 7 in some cases Fig 2 System and configuration data for fault current. different equivalent uniform or two layer soils can be distribution computation. used to establish lower and upper bounds for ground. parameters in a multilayer soil Interested readers may. refer to 7 for a detailed analysis of the equivalence of. uniform and two layer soils to multilayer soils,VI MULTILAYER SOIL MODEL. A subsection regarding multilayer soil models, which is not in Standard 80 1986 is added in Standard. 80 2000 It briefly describes the method of multilayer. analysis and presents some references In practice most. soil structures are multilayered and require appropriate. computer modeling for accurate results Interested, readers may refer to 8 10 for grounding analysis in. multilayer soils,VII GRID CURRENT COMPUTATION, It is known that for a typical grounding analysis the. basic information required concerns the following, grounding system configuration and characteristics soil. structure and fault current Generally the information. provided regarding the fault current is the fault current. flowing into the faulted substation from the faulted Fig 3 Fault current in the phase and neutral conductors. phase conductors This information however cannot be. used directly in a grounding analysis in most cases Fig 2 shows the system and configuration data Fig. without undue conservatism Instead it is used to derive 3 shows the computed current distribution in the phase. Copyright Safe Engineering Services technologies ltd. and overhead ground OHGW conductors of the fence to the right of isolating sections 1 and 2 always. transmission line and in one of the three distribution remain disconnected from the grid when isolating. feeder neutrals It can be seen that there is a significant sections are installed. amount of the fault current returning to the sources. We can see from Fig 5 that without any isolating,through the OHGW of the transmission line. section the touch voltages are very high in the, Furthermore there is a significant amount of the fault. proximity of the fence away from the grid The reason. current flowing in the distribution feeder neutrals. is that high potentials are transferred to this portion of. Obviously the existence of the distribution feeder. the fence while fence posts alone are not adequate to. neutrals will help lower the fault current discharged by. raise the earth potentials significantly along the fence. the grounding grid at the faulted substation In this. and thereby reduce touch voltages at these locations. example the fault current discharged by the grounding. When isolating sections 1 and 2 are installed the touch. grid is 2422 A representing 27 of the total fault,voltages at these locations decrease considerably. current Using the graphical method presented in Annex. except at locations close to the isolating sections To. C of Standard 80 2000 the grid current is 2726 A 13. reduce the high touch voltages at locations close to. more than in reality The discrepancy and resulting. isolating sections 1 and 2 isolating sections 3 and 4 are. overdesign is considerably greater when optical fibre. installed As shown in Fig 5 the touch voltages at these. overhead ground wires are involved, locations are much lower now and at other locations. also remain low By using four isolating sections we. VIII GROUNDING OF SUBSTATION FENCE, have reduced the touch voltage everywhere to below. Newly extended Section 17 3 in Standard 80 2000 800 V The permissible touch voltage in this case is 856. deals with grounding of substation fences in much V for a fault clearing time of 0 35 s a surface crushed. greater detail than in Standard 80 1986 The following stone layer with a thickness of 15 cm and a resistivity of. cases have been discussed 1 fence within the 3000 m and a native soil resistivity of 100 m. grounding area 2 fence on grid perimeter 3 fence,outside the grounding area but close by 4 fence. outside the grounding area and far away One case, which is absent but is of significance is when part of. the fence is close to the grounding grid and part of it far. away The information provided in Section 17 3 is, therefore useful but not complete Interested readers. may refer to 14 for a detailed safety analysis of fence. interconnections to substation grounding systems The. following example is intended to demonstrate, appropriate fence grounding for the case when part of. the fence is close to the grounding grid and part far. Fig 4 shows a grounding grid at the left side of a. large fenced area whose dimensions are 350m 100m, The isolation of the fence between the left and right. sides is made by creating 3 m gaps at the junction. points Each 3 m gap represents an isolated fence, section The left side of the fence is always connected to. the grounding grid,Fig 5 Touch voltages along fence. It should be pointed out that the permissible touch. voltage 856 V is calculated based on Standard 80, 2000 It would be 751 V if it were calculated based on. Standard 80 1986 In this case the grid has to be, enhanced at the location close to isolating sections 1. and 2 in order to lower the touch voltages at the, locations that exceed 751 V We see from this example. Fig 4 Grounding grid in a large fenced area that the changes in Standard 80 do affect grounding. Fig 5 shows the touch voltages with respect to the. IX CONCLUSIONS, fence for three cases 1 no isolating section 2 with. isolating sections 1 and 2 as shown in Fig 4 3 with The major changes in the 2000 version of IEEE. all the four isolating sections shown in Fig 4 Note that Guide for Safety in AC Substation Grounding Standard. the portion of the fence left of isolating sections 1 and 2 80 2000 with respect to the 1986 version Standard 80. is always connected to the grid while the portions of the 1986 which affect the grounding design and analysis. Copyright Safe Engineering Services technologies ltd. have been discussed Comparisons are made for the, portions in the two versions of the Standard where 14 J Ma W Ruan R D Southey and F P Dawalibi Safety. changes occur Examples have been presented to show analysis of fence interconnection to substation grounding. the effects of the changes in the grounding design and system Proceedings of the Fourth IASTED International. Conference on Power and Energy System Marbella Spain pp. analysis 223 228 Sept 19 22 2000,X ACKNOWLEDGMENTS. The authors wish to thank Safe Engineering XII BIOGRAPHIES. Services technologies ltd for the financial support Dr Jinxi Ma M 91 SM 00 was born in Shandong P R China in. and facilities provided during this research effort December 1956 He received the B Sc degree from Shandong. University P R China and the M Sc degree from Beijing University. XI REFERENCES of Aeronautics and Astronautics both in electrical engineering in. 1982 and 1984 respectively He received the Ph D degree in. 1 IEEE Guide for Safety in AC Substation Grounding IEEE electrical and computer engineering from the University of Manitoba. Standard 80 2000 Revision of IEEE Standard 80 1986 Winnipeg Canada in 1991. 2 IEEE Guide for Safety in AC Substation Grounding IEEE From 1984 to 1986 he was a faculty member with the. Standard 80 1986 Revision of IEEE Standard 80 1976 Department of Electrical Engineering Beijing University of. Aeronautics and Astronautics He worked on projects involving. 3 B Thapar V Gerez and P Emmanuel Ground resistance of design and analysis of reflector antennas and calculations of radar. the foot in substation yards IEEE Trans Power Delivery vol cross sections of aircraft Since September 1990 he has been with the. 8 no 1 pp 1 6 Jan 1993 R D Dept of Safe Engineering Services technologies in. Montreal where he is presently serving as manager of the Analytical. 4 F P Dawalibi W Xiong and J Ma Effects of deteriorated. R D Department His research interests are in transient. and contaminated substation surface covering layers on foot. electromagnetic scattering EMI and EMC and analysis of grounding. resistance calculations IEEE Trans Power Delivery vol 8 no. systems in various soil structures,1 pp 104 111 Jan 1993. Dr Ma is the author of more than eighty papers on transient. 5 B Thapar V Gerez and H Kejriwal Reduction factor for the electromagnetic scattering analysis and design of reflector antennas. ground resistance of the foot in substation yards IEEE Trans power system grounding lightning and electromagnetic interference. Power Delivery vol 9 no 1 pp 360 368 Jan 1994 He is a senior member of the IEEE Power Engineering Society a. 6 R D Southey and F P Dawalibi Improving the reliability of member of the IEEE Standards Association and a corresponding. power systems with more accurate grounding system resistance member of the IEEE Substations Committee and is active on Working. estimates IEEE PES CSEE International Conference on Power Groups D7 and D9. System Technology Kunming China Oct 13 17 2002, Dr Farid P Dawalibi M 72 SM 82 was born in Lebanon in. 7 J Ma F P Dawalibi and R D Southey On the equivalence of November 1947 He received a Bachelor of Engineering degree from. uniform and two layer soils to multilayer soils in the analysis of St Joseph s University affiliated with the University of Lyon and the. grounding systems IEE Proceedings Generation M Sc and Ph D degrees from Ecole Polytechnique of the University. Transmission and Distribution vol 143 no 1 pp 49 55 Jan of Montreal. From 1971 to 1976 he worked as a consulting engineer with the. 8 P J Lagace J L Houle Y Gervais and D Mukhedkar Shawinigan Engineering Company in Montreal He worked on. Evaluation of the voltage distribution around torroidal HVDC numerous projects involving power system analysis and design. ground electrodes in n layer soils IEEE Trans Power Delivery railway electrification studies and specialized computer software code. vol 3 no 4 pp 1573 1577 Oct 1988 development In 1976 he joined Montel Sprecher Schuh a. manufacturer of high voltage equipment in Montreal as Manager of. 9 F P Dawalibi and N Barbeito Measurements and, Technical Services and was involved in power system design. computations of the performance of grounding systems buried in. equipment selection and testing for systems ranging from a few to. multilayer soils IEEE Trans Power Delivery vol 6 no 4 pp. several hundred kV In 1979 he founded Safe Engineering Services. 1483 1490 Oct 1991, technologies a company specializing in soil effects on power. 10 F P Dawalibi J Ma and R D Southey Behaviour of networks Since then he has been responsible for the engineering. grounding systems in multilayer soils a parametric analysis activities of the company including the development of computer. IEEE Trans Power Delivery vol 9 no 1 pp 334 342 Jan software related to power system applications. 1994 He is the author of more than one hundred and fifty papers on. 11 F P Dawalibi Ground fault current distribution between soil power system grounding lightning inductive interference and. and neutral conductors IEEE Trans Power Apparatus and electromagnetic field analysis He has written several research reports. Systems vol PAS 99 no 2 pp 452 461 Mar Apr 1980 for CEA and EPRI. Dr Dawalibi is a corresponding member of various IEEE. 12 A P Meliopoulos A D Papelexopoulos and R P Webb Committee Working Groups and a senior member of the IEEE Power. Current division in substation grounding system Proceedings Engineering Society and the Canadian Society for Electrical. of the 1982 Protective Relaying Conference Georgia Institute of Engineering He is a registered Engineer in the Province of Quebec. Technology Atlanta Ga May 1982, 13 G Yu J Ma and F P Dawalibi Computation of return current For the biography of Mr Robert D Southey please see. through neutral wires in grounding system analysis Improving the reliability of power systems with more accurate. Proceedings of the Third IASTED International Conference on grounding system resistance estimates in the proceedings of IEEE. Power and Energy System Las Vegas Nevada pp 455 459 PES CSEE International Conference on Power System Technology. Nov 8 10 1999 Kunming China Oct 13 17 2002, Copyright Safe Engineering Services technologies ltd.
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