for HV / EHV substations situated at such locations where soil of the substation
site ... methodology for design of substation grounding system is discussed. Next
...
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012)
Optimal Design of Grounding System for HV/ EHV Substations in Two Layered Soil Kaustubh A. Vyas1, J. G. Jamnani2 1 2
Department of Electrical Engineering, Institute of Technology, Nirma University, Ahmedabad, Gujarat, India Department of Electrical Engineering, Institute of Technology, Nirma University, Ahmedabad, Gujarat, India 1 2
[email protected] [email protected]
Abstract— Successful operation of entire power system depends to a considerable extent on efficient and satisfactory performance of substations. Hence substations in general can be considered as heart of overall power system. In any substation, a well designed grounding plays an important role. Since absence of safe and effective grounding system can result in mal-operation or non-operation of control and protective devices, grounding system design deserves considerable attention for all the substations. Grounding system has to be safe as it is directly concerned with safety of persons working within the substation. Main purpose of this work is designing safe and cost effective grounding systems for HV / EHV substations situated at such locations where soil of the substation site is not uniform. Here soil is considered to be horizontally stratified into two layers. Initially significance of two layered soil model is introduced and design methodology for design of substation grounding system is discussed. Next, two design problems have been solved by using newly developed software and results obtained here are verified with the help of Ground Grid Systems module of ETAP – professional software. Comparison of the results obtained by both the software shows nice agreement of the results given by ESGSD with those obtained from ETAP. Hence this software is found to be quite satisfactory tool in the field of design of grounding system for HV / EHV substations.
Grounding system discharges hazardous fault current and lightning strokes to the earth. It also keeps step and touch voltages within permissible limits [1]. Consequently properly designed and installed grounding system guarantees reliable performance of substations thereby improving integrity of overall power system. A well designed grounding system ensures following [2].
Keywords— Earthing, Earth electrodes, Ground grid, HV/EHV substations, Power systems, Programming, Safety
This paper describes significance of appropriate earth model to be considered while designing any grounding system in order to take into account the variation in characteristics of soil. Uniform resistivity soil is hardly encountered in actual practice. Usually soils have two or more horizontally and / or vertically stratified layers of different resistivity, former being more common. While the most accurate representation of a grounding system should certainly be based on the actual variations of soil resistivity present at the substation site, it will rarely be economically justifiable or technically feasible to model all these variations. However, in most cases, representation of a ground electrode based on an equivalent two - layered earth model is sufficient for design of safe grounding system [2].
1.
It provides means of dissipating electrical current into earth without exceeding operating limits of equipment.
2.
It provides safe environment to protect personnel in the vicinity of the grounded facilities from the dangers of electric shock under fault conditions.
Grounding system comprises of all of the interconnected grounding facilities in the substation area including ground grid, overhead ground wires, neutral conductors underground cables etc. ground grid being the main component. Ground grid consists of horizontal interconnected conductors often supplemented by vertical ground rods. Being chief component of overall grounding system, design of grounding grid should be such that total grounding system is safe and all together it is cost – effective [2].
I. INTRODUCTION Beginning from generating station to the final distribution station, electrical power passes through number of different kind of substations. Role of substations as a whole is crucial in overall functionality of complete power system. Grounding system plays vital role in satisfactory operation of a substation. It provides place for connecting system neutral points, equipment body and support structures to the earth. Earthing system ensures safety of persons working within the substations. It also facilitates earth fault detection and assists control and protection systems. 383
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) Two layered soil can be represented by an upper layer soil having finite depth above lower layer earth of different resistivity and infinite depth. The abrupt change in resistivity at the boundaries of each soil layer can be described by means of a reflection factor K defined by
K
Where r = probe spacing in meters
a =
apparent resistivity measured form Earth Resistivity Testing (ERT) in m
2 1 2 1 ……………….……. (1)
If uniform soil model is used, in this case equivalent apparent resistivity comes out to be
= 205 m
Where 1= Resistivity of top layer
Whereas if two layered soil model is used, values of resistivities of two layers and depth of upper layer (h 1) are as follows. 1 = 383.54 m h1 = 2.56 m 2 = 147.68 m
2= Resistivity of bottom layer Here software named ―Economical Substation Grounding System Designer (ESGSD)‖ has been developed using MATLAB as computing as well as programming tool. This software follows the safety criteria given in [2]. ESGSD is capable of simply analyzing the grounding system in given conditions of soil and system parameters. Additionally it can suggest most appropriate, safe and cost-effective i.e. optimal design of grounding system.
Consider that a ground grid along with ground rods is installed at a substation having soil structure described in table - I. Data related to grid geometry and general system parameters are as shown in table - II. While analyzing performance of this grid two approaches can be adopted. In first approach, soil can be assumed as uniform one and in the other approach soil can be modeled as two layered one.
II. SIGNIFICANCE OF TWO LAYER MODELING OF SOIL IEEE Guide 80 – 2000 gives equations related to uniform soils only. However as stated in [2], uniform soils are rarely encountered in actual practice. Hence grounding system design according to IEEE guide only is not sufficient and one has to go for two layered modeling of soil [3]. In some cases where soils have multiple layers of different resistivities, even two layered model is not quite satisfactory and modeling of soil as multilayered structure is necessary. In such conditions equivalent two layer soil model can be derived which serves better than the uniform soil model. Uniform soil model should be used only when there is moderate variation in apparent resistivity [4]. If variation in measured apparent resistivity is large, uniform soil model is likely to give erroneous results. This will be clear from following example. Data for soil modeling is taken from [5].
TABLE II INPUT DATA FOR GROUNDING SYSTEM DESIGN
Ground grid parameters and system data Grid shape Length in X direction (Lx) Length in Y direction (Ly) Length of each ground rod (Lr) No. of conductors in X direction (Nx) No. of conductors in Y direction (Ny) No. of vertical ground rods(Nr) Cost of grounding grid conductors Cost of grounding rods Maximum symmetrical rms Fault current (I) Depth of burial (h) Fault duration (tf) Shock duration (ts) Ambient temperature (Ta) Surface layer resistivity ( s )
TABLE I SOIL RESISTIVITY MEASUREMENT DATA
r (m)
a ( m )
2.5 5.0 7.5 10 12.5 15
320 245 183 162 168 152
Thickness of surface layer (hs)
Rectangular 100 m 80 m 3m 26 21 80 2000 `/m 2000 `/m 15 kA 0.5 m 1 sec 0.5sec 50 oC
3000 m 0.1 m
Performance parameters for grounding grid and supplementing ground rods having specifications given in table – II are calculated considering if they are installed in both of the abovementioned soils. These results are shown in table III. 384
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) TABLE III PERFORMANCE EVALUATION OF GROUNDING SYSTEM
Parameter Ground resistance ( R g ) GPR (Volts) EStep (Volts) ETouch (Volts) ES actual (Volts) Em actual (Volts) Safety
Uniform soil 1.06
9680 2263.1 688.88 1076.2 585.1 Safe
These tolerable safety criteria have been established based on fibrillation discharge limit of body current. In order to make grounding system safe, equivalent grounding system impedance should be low enough to assure that fault current dissipates mainly through grounding grid into earth. While designing grounding system, main consideration is to be given to the fact that under any circumstances actual mesh and step voltages must not exceed the tolerable values given by following equations.
Two layered soil 1.65 15048 2317.7 702.53 2013.3 1094.5 Not safe
Estep _ 50 1000 6 s C s
Here EStep and ETouch are tolerable step and touch voltages ES actual and Em (touch) voltages
actual
Estep _ 70 1000 6 s C s
are actual step and mesh
0.116 ts
…….... (2)
0.157 ts
…...…. (3)
Etouch_ 50 1000 1.5 s C s
GPR stands for Ground Potential Rise As seen from above example, analysis results drastically change while using uniform soil model as compared to the results obtained by two layered soil model. As said before if soil conditions are not uniform actually and it is modeled as uniform for the purpose of calculations in grounding system, erroneous results are likely to be obtained. The same problem when solved by different software shows that the two layer soil model gives more realistic results as compared to uniform soil assumption. Hence it is essential to use equivalent two layered soil model rather than uniform soil model. Merely using uniform soil model for design of grounding system is not sufficient for majority of cases. Therefore equations given in IEEE – 80 – 2000 need to be modified in order to be used with two layered soil model. Software ESGSD takes this fact into account and allows selection of soil model as uniform soil or two layered soil.
Etouch_ 70 1000 1.5 s C s
0.116 ts
….…. (4)
0.157 ts
…….. (5)
Where Estep_50 and Estep_70 are tolerable step voltages for person weighing 50 Kg and 70 Kg respectively Etouch_50 and Etouch_70 are tolerable touch voltages for person weighing 50 Kg and 70 Kg respectively IV. DOMINANT FACTORS CONTRIBUTING TO GROUNDING SYSTEM PERFORMANCE
Out of many parameters affecting grounding system performance, the main factors having substantial effect on grounding system design and performance are as follows
III. SAFETY CRITERIA FOR GROUNDING SYSTEM DESIGN A reliable grounding system should be able to maintain the actual mesh and step voltages within a substation well below the tolerable level. Before 1960s the design criterion for substation earthing system was low earthing resistance alone. However during 1960s the new criterion for design and evaluation of substation earthing system was evolved which included following three conditions Low step voltage, Low touch voltage and, Low earth resistance [6]
Spacing between adjacent conductors (D),
Depth of burial of grid (h),
Number of vertical ground rods (Nr)
Area of grounding system (A),
Following is brief description of the effects of these parameters along with graphs
385
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) A. Spacing between adjacent conductors (D) Spacing between the adjacent conductors and No. of conductors are certainly dependent on each other for fixed area of grid. The more conductors are installed, the smaller the distance between the conductors. With reduced separation and increased No. of conductors, Mesh voltage (Em) decreases but at the same time Step voltage (ES) increases. However effect of reduction in E m is more than that of increase in ES as seen from fig. 1. So designer has to carefully decide no. of conductors and separation between them to keep both voltages below tolerable limits. Also physically, there is a limit on how close conductors can be installed and should be design consideration [7]. Additionally, as specified in [2] there are certain geometrical constraints that must be satisfied in order to use equation given in IEEE guide for designing of the grounding system for any substation.
portion more current tends to flow toward lower resistivity earth surface thereby increasing potential at the earth surface [8].
Fig. 2 Variation of actual mesh and step voltages with change in depth of burial of grounding grid
C. Number of vertical ground rods (Nr) Vertical ground rods discharge the grid current in the soil at sufficient depth. Thus they effectively reduce grounding system resistance and GPR. Also with more number of ground rods, total length of conductors buried in the earth increases thereby decreasing step and mesh voltages. This is also shown in fig. 4. In actual practice ground rods are considered to be an effective means of reducing resistance of combined grounding system and also actual mesh and step voltages whenever design modifications are necessary. For same total length of conductor to be installed vertical rods are more cost – effective than horizontal grid conductors because they penetrate into lower layers of soil in the deep earth which generally have lower resistivity [9]. Multiple driven electrodes are, everything being equal, more effective than equivalent ground grids made of horizontal conductors. This is true even when soil is uniform. However, when lower layer resistivity is high, the horizontal conductors are more effective because they reduce significantly the touch voltages [10]
Fig. 1 Variation of actual mesh and step voltages with change in No. of conductors and conductor spacing
B. Depth of burial of grid (h) Depth of burial of grid does not have great effect on mesh voltage but has drastic effect on step voltage as seen from fig. 2. With increased depth of grid step voltage decreases significantly because as current flows up toward surface, most of the voltage is dropped in the soil itself and at the surface of earth less step voltage is experienced. Also GPR reduces with increased depth of burial until and unless lower layer earth has higher resistivity than upper layer. This is because with higher resistivity layers in the lower
D. Area of grounding system (A) Area occupied by the grounding grid has major effect on GPR, step voltage as well as on mesh voltage. With increased area all the three types of potentials reduce 386
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) significantly as observed form fig. 4. Area contributes to reduction in grid resistance [11] and thus GPR directly as is apparent from the relevant equations given in [2], also with increased area the length of buried conductors increases and thus actual step and touch voltages reduce.
V. ECONOMICAL SUBSTATION GROUNDING SYSTEM DESIGNER (ESGSD) As mentioned previously the software ESGSD has been developed using MATLAB as mathematical computing tool and programming environment as well [12, 13]. ESGSD follows guidelines given in IEEE guide and also uses modified equations for two layered modeling of soil which are not given in [2].
Fig. 3 Variation in Actual mesh and step voltages with change in No. of vertical ground rods Fig. 5 Main page of the software ESGSD
Fig. 5 shows main page of the software ESGSD. It takes various basic data from the user and also allows access to all other input and processing modules of the software. Results can be obtained and reports can be generated from this page itself. This software is designed using MATLAB GUI Development Environment as the platform for programming as well as mathematical computing tool. Additionally it has been deployed as windows standalone application using MATLAB compiler addon. Hence it can be used on any system where MATLAB is not installed. Here performance of grounding system can be analyzed in two ways Fig. 4 Variation in actual mesh and step voltages and GPR with change in area occupied by the grounding system
387
Simple performance analysis of grounding system by taking data related to grid geometry, system parameters and soil parameters. Optimal design of grounding system for all the five grid shapes in uniform as well as two layered soil by taking the same data as for first option.
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) A. Salient features of ESGSD [14] GUI developed with the help of MATLAB makes the software quite user friendly and any novice user can also work with it without going for detailed literature study. Only thing the user should be aware of is basic design methodology as per IEEE guidelines and related terminology. A list of commonly used conductor materials is provided and program automatically takes the standard values of material constants for the selected conductor material. This software calculates required conductor size and automatically chooses the most appropriate standard conductor size available in the market. It is capable of handling various grid geometries like square, rectangular, triangular, L shaped and T shaped grounding grids supplemented by vertical ground rods. It can analyze technical performance of any proposed grounding system in terms of grounding system resistance, tolerable and actual values of touch and step voltages and safety as per standard guidelines. In addition to analyzing grounding system as per IEEE guide lines, ESGSD can apply optimization to the proposed grounding system design in various ways and suggest the most appropriate safe and economical preliminary design of grounding system by keeping necessary safety margin. Results obtained here are found to be quite matching with those given by ‗Ground Grid Systems‘ module of ETAP professional software used for solving problems related to power system by many utilities. In certain cases its performance is found to be quite better than that of ETAP GGS. Where the latter gives unrealistic results, ESGSD successfully tackles all possible cases and gives realistic results. Option for derivation of soil model from earth resistivity testing (ERT) data is available when mathematical model of soil is not known in advance. This is an additive feature compared to ETAP. By implementing the standard procedure given in appendix of IEEE standard 81 and using steepest descent algorithm soil model has been derived for uniform or two layered soil. It gives well formatted output in the form of Microsoft Word file.
additionally two layered soil modeling and optimization are implemented in it.
B. Algorithm for working of ESGSD [1] Flow chart in fig. 6 shows algorithm for working of the software ESGSD. It is as per IEEE guide lines and
Fig. 6 Flow chart depicting algorithm of development of the software
388
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) As seen from the flow chart the software initially takes required input data from user and derives the mathematical model of soil if not known. Next, a safety criterion as per equations (2) - (5) is established as per recommendations given by IEEE and maximum fault current is calculated considering various factors. Then the software iteratively calculates safety parameters for grounding system for various combinations of ground grid conductors and ground rods. After checking all the safety and geometrical constraints the optimization algorithm finds the most economical and safe design among all the designs that are found feasible to be used for grounding system design. Thus the software gives the design that is safe and cost effective i.e. optimal design. Following are details about optimization problem and its attributes.
D1 = Spacing between conductors in X direction (m) D2 = Spacing between conductors in Y direction (m) D = Average separation between any two consecutive conductors in the ground grid (m) X = Specified percentage of the tolerable value. These safety criteria state that actual mesh and step voltages must be less than X percent of the corresponding tolerable values, X is dependent on user requirement, spacing between the conductors in horizontal grid should be uniform and almost equal in both directions and spacing between two adjacent conductors should be between 2.5 m to 12.5 m. By satisfying geometrical constraints the design assures that equations given in [2] are applicable to the present grounding system with a fair degree of accuracy and by satisfying safety constraints electrical safety of persons working in the substation site is ensured. Minimization of cost objective function gives economical and yet safe i.e. cost effective design of grounding system. Thus design given by solving above optimization problem is optimal because it reveals minimal cost among all the designs giving similar performance. Optimization works in both ways i.e. if the system is already safe it suggest the most cost effective design for economic savings without compromising the technical aspects and if the original system is not safe then optimization gives safe design at lowest possible prize. This fact becomes clear from following design problems.
In order to get optimal design, following cost objective function has to be minimal subjected to safety criteria listed below it [5]. Optimization problem: Minimize the cost of grounding system given by f ( N x , N y , N r , Lr ) ( N r Lr Cri ) ( N r Cr ) (Cci Cc ) ( N x Ly Lx N y )
………….. (6) Subjected to safety constraints given as follows
Em _ act X Em _ tol ..........(7) Es _ act X Es _ tol ..........(8)
VI. PERFORMANCE EVALUATION OF ESGSD AS COMPARED WITH ETAP – GGS
0.95 D1 D2 1.05 D1..........(9)
2.5 D 12.5..........(10)
Here sample systems have been taken to analyze grounding system performance and evaluate working of ESGSD by comparing with GGS module of ETAP. Input data required for design of grounding system having rectangular shaped ground grid, supplemented by ground rods and located in two layered soils is shown in table IV.
Where Nr = No. of vertical ground rods Lr = Length of each vertical ground rod (m) Cri = Installation cost of vertical ground rods (` / m)
Performance analysis of the abovementioned grounding system shows that it is already safe. Application of optimization by using ESGSD gives optimal design of the grounding system which contributes in terms of cost benefits. Next, the same system is considered by taking different value of the fault current and as the result the system becomes unsafe as observed form simple performance analysis. Now application of optimization makes the system safe and at the same time it is economical as well. Results given by the software are compared with those given by GGS module of ETAP [15, 16] for both cases as shown in following subsections.
Cr = Material cost for ground rods (` / rod) LC = Total length of conductors in ground grid (m) Ci = Installation cost for grid conductors (` / m) CC = Material cost of conductors (` / m) Em_act = Actual mesh voltage (Volts) Em_tol = Tolerable mesh voltage (Volts) ES_act = Actual step voltage (Volts) ES_tol = Tolerable step voltage (Volts) 389
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) layered soil which shows that this design is safe. However actual mesh voltage, 182 volts, is much lower (about 39%) than the tolerable value i.e. 467 volts. Hence this grounding system seems to be over designed and may not be the optimal design. Now, application of optimization by using ESGSD and ETAP – GGS gives design of the grounding system which is the optimal one for given conditions.
TABLE IV INPUT DATA FOR GROUNDING SYSTEM DESIGN IN TWO LAYERED SOIL
Particular Grid shape Length in X direction (Lx) Length in Y direction (Ly) Length of each ground rod (Lr) No. of conductors in X direction (Nx) No. of conductors in Y direction (Ny) No. of vertical ground rods (Nr) Cost of grounding grid conductors
Value Rectangular 80 m 41 m 2.4 m 19 15 50
Cost of grounding rods
2000 `/m 0.6 m 1.5 m 15 kA 1 sec 0.5sec 50 oC 1500 m
Thickness of surface layer (hs) Resistivity of first layer ( 1 )
Resistivity of second layer (2) Fault current split factor (Sf)
TABLE VI POST OPTIMIZATION RESULTS FOR GROUNDING SYSTEM DESIGN
2000 `/m
Depth of burial (h) Depth of first layer (h1) Maximum symmetrical rms Fault current (I) Fault duration (tf) Shock duration (ts) Ambient temperature (Ta) Surface layer resistivity ( s )
Results for optimization for rectangular grid described in above case in two layered soil structure are shown in table – VI.
Results by ESGSD 0.103 929
Results by ETAP – GGS 0.12 1061.6
EStep (Volts) ETouch (Volts)
1375.5 466.9
1375.5 466.9
ES actual (Volts) Em actual (Volts)
237.6 440.8
267.7 409.4
8
10
Particular Ground resistance (Ω) GPR (Volts)
0.2 m 34.15 m
(NX Optimal) (NY Optimal)
6.64 m 0.6
Pre optimization safety
A. Cost savings obtained with application of Optimization Grounding system having specifications shown in tableIV is analyzed with the help of both aforementioned software and the results are shown in following table- V.
5
5
Safe
Safe
Post optimization safety
Safe
Safe
Pre optimization cost (`)
2,15,900
2,15,900
Post optimization cost (`)
90,800
99,000
1,25,100
1,16,900
Saving (`)
Results by ESGSD
Results by ETAP
0.105
0.12
950.1
1045.6
EStep (Volts)
1375.48
1375.5
ETouch (Volts)
466.9
466.9
ES actual (Volts)
258.8
258.5
Observation of the results given in above table shows that, optimization gives safe design which is also cost effective, thereby giving considerable financial benefit which in this case is 58% of original installation cost. When there are numerous substations being installed every year this factor may be of greater interest to utility. This comparison of results before and after optimization reveals excellent performance of ESGSD as verified from GGS module of ETAP. This proves technical soundness of the software developed herein. It is quite useful for getting preliminary optimized design of grounding system for any substation.
Em actual (Volts)
181.9
181.7
B. Application of Optimization for making system safe
Safety
Safe
Safe
Same data as the previous case is used with little modification in fault current only. In this case fault current of 45 kA is assumed instead of 15 kA as taken in previous subsection. Simple performance analysis of this grounding system shows that it becomes unsafe with current values of
TABLE V PRE OPTIMIZATION RESULTS FOR GROUNDING SYSTEM ANALYSIS
Particular Grounding system resistance (Ω) GPR (Volts)
As seen from above table, results obtained from ESGSD are quite comparable to those given by ETAP – GGS. This is simple performance analysis of grounding system in two 390
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) soil and system parameters. Result for performance analysis of the above grounding system as obtained from both the software are shown in table VII
optimization has served its purpose of making the design safe. Also observed here is that the results given by ESGSD are quite comparable to those obtained from GGS module of ETAP. Thus technical performance of the novel software is again validated for this purpose as well. Hence now ESGSD can be considered as an effective solution in the field of grounding system design at least at preliminary stage. Based on site specific requirements detailed design can be prepared and analyzed by this software.
TABLE VII PRE OPTIMIZATION RESULTS FOR GROUND GRID IN TWO LAYERED SOIL
Particular Grounding system resistance (Ω)
Results by ESGSD 0.1
Results by ETAP – GGS 0.11
GPR (Volts)
3277
3687
EStep (Volts)
1375.5
1375.5
ETouch (Volts)
466.9
466.9
ES actual (Volts)
949.1
947.7
Em actual (Volts)
531.1
530.2
Not safe
Not safe
Safety
C. Discussion Above all theoretical concepts, implementation and results can be briefly summarized as follows 1) Significant findings
As seen for the above results here actual mesh voltage that is 531.1 volts is 14% more than the corresponding tolerable value. Hence now application of optimization to this specified problem will result in safe and economical design. Here some additional expense may be necessary to make design safe but that too will be the least possible cost i.e. optimized solution. Following table VIII shows post optimization results of this grounding system.
TABLE VII POST OPTIMIZATION RESULTS FOR GROUNDING SYSTEM DESIGN
Results by ESGSD 0.1 3215.8
Results by ETAP – GGS 0.11 3682.4
EStep (Volts) ETouch (Volts)
1375.5 466.9
1375.5 466.9
ES actual (Volts) Em actual (Volts)
985.7 442.9
992.3 433.5
(NX Optimal)
23
27
(NY Optimal)
16
14
Pre optimization safety
Not safe
Not safe
Post optimization safety
Safe
Safe
Pre optimization cost (`)
2,15,900
2,15,900
Post optimization cost (`)
2,40,300
2,40,700
24400
24800
Particular Ground resistance (Ω) GPR (Volts)
Additional expenses (`)
As observed form these results, application of optimization has made the design safe which was previously unsafe and that too at extra cost of about 10 % of initial cost. Thus 391
Merely using uniform soil model for design of substation grounding systems may not prove satisfactory in all the cases. Hence better choice is use of two layered soil model unless the soil characteristics are sufficiently uniform or soil is chemically treated. Grounding system of the substations if not designed judiciously may become over designed. Hence application of optimization in many cases is appreciable to get safe and economical design of grounding systems especially for HV/EHV substations. Software developed here is found to be effective tool in the field of grounding system design as it gives safe and cost effective design of grounding system for uniform as well as two layered soils and is compliant with IEEE standard 80 – 2000. This work has explored novel feature of MATLAB called GUIDE that helps in development of graphical software using MATLAB. It has proven quite appreciable for people who are not expert in professional programming languages but can manage with MATLAB coding for their usual purpose in terms of software development. Additionally, MATLAB runtime compiler gives standalone application that can be used in absence of MATLAB. Performance of ESGSD is validated by means of software testing i.e. results are verified by comparing with those given by professional software ETAP.
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012) [6] Rao Sunil S., ‗Switchgear Protection and Power Systems‘. 12th
2) Applications
edition, Khanna Publishers, New Delhi, 2007.
ESGSD developed here can be used satisfactorily for educational purpose for students of electrical engineering and also for training purpose for professionals concerned with substation designing It can be used for obtaining optimized preliminary design of grounding system for any actual substation. Based on site specific requirements it can be used to develop detailed design of actual substation grounding system.
[7] Mc Donald John D., ‗Electrical Power Substation Engineering‘, CRC Press, 2006.
[8] Puttarach A., Chakpitak N., Kasirawat T., and Pongsriwat C., 2007, ‗Ground layer depth and the effect of GPR‘, IEEE PES Conference and Exposition, Johannesburg, South Africa, pp. 1 – 5.
[9] Ahdab Elmorshedy, Rabah Amer, Sherif Ghoneim and Holger Hirsch, November 2006, ‗Surface potential calculation for grounding grids‘, First International Power and Energy Conference, pp. 501 – 505.
[10] Dawalibi F. and Mukhedkar D., 1979, ‗Influence of ground rods on grounding grids‘, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-98, (6), pp. 2089 – 2098.
VII. CONCLUSION
[11] B. Thapar, V. Gerez, A. Balakrishnan and D. A. Blank, April 1991, ‗Evaluation of Grounding Resistance of a Grounding Grid of any Shape‘, IEEE Transactions on Power Delivery, Vol. 6, No. 2, pp. 640-647.
Here effects of various parameters on the performance of grounding system are analyzed and significance of two layered model of soil is discussed with example. Next by using newly developed software ―Economical Substation Grounding System Designer‖, Design problems of grounding system in two layered soil have been solved. The same problems are also solved by using ETAP – GGS module which is professional software widely used by power system engineers around the world. Comparison of results given by ESGSD with those of GGS module of ETAP proves technical soundness of the novel software developed here. Hence it is obvious that the software ESGSD is quite satisfactory for design of grounding system design in uniform as well as two layered soils and it also meets IEEE safety criteria. Hence this can prove to be an effective tool, for at least preliminary design of grounding system in an optimal way, rather than using thumb rules based on experience. Here only results for grid in two layered soil and having rectangular shape have been shown. However ESGSD allows design of grounding system for five grid shapes in two types of soil models described earlier that can be obtained from [17, 18, 19, 20].
[12] Online help for MATLAB available at: http://www.mathworks.in/help/techdoc/creating_guis/bqz6p81.html
[13] Dr. Attia A. El-Fergany, July 2011, ‗Design and Optimize Substation Grounding Grid Based on IEEE Std. 80 - 2000 Using GUI and MATLAB Codes‘. International Journal of Engineering Science and Technology, Vol. 3 No. 7, pp. 6033- 6039.
[14] Help file of the software ESGSD available at: https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVs dGRvbWFpbnxncm91bmRpbmdzeXN0ZW1kZXNpZ258Z3g6Mjg5 ODkzYTE0ZGVhNmE0OQ
[15] ETAP 4.7.0 Documentation: Help on Ground Grid Systems module. [16] N. M. Tabatabaei and S. R. Mortezaeei, March 2010, ‗Design of Grounding Systems In Substations By ETAP Intelligent Software‘. International Journal on Technical and Physical Problems of Engineering, Issue 2, Volume 2, No. 1, pp. 45-49.
[17] Kaustubh A. Vyas and J.G. Jamnani, December 2011, ‗Optimal Design and Development of Software for Design of Substation Grounding System‘. International Conference on Current Trends in Technology – NUiCONE, Ahmedabad, Gujarat, India, pp. 1 – 6.
[18] Kaustubh A. Vyas and J.G. Jamnani, April 2012, ‗MATLAB GUI Development Environment simplifies deployment of standalone software for optimal design of substation grounding system‘. International Conference on Computer Science nad Information Technology – ICCSIT 2012,Pune, Maharashtra, India, pp. 303 -312.
REFERENCES [1] Kaustubh A. Vyas, ‗Optimal Design of Grounding Systems‘, M.
[19] Project report available at:
Tech Major Project Thesis, Institute of Technology, Nirma University, Ahmedabad, Gujarat, India.
https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVs dGRvbWFpbnxncm91bmRpbmdzeXN0ZW1kZXNpZ258Z3g6OTQ 5MWEyNmE4NzA0NmYz
[2] IEEE: 80: 2000, IEEE Guide for safety in AC Substation Grounding.
[20] Project report available at:
[3] W. Sun, J. He, Y. Gao, R. Zeng, W. Wu and Q. Su, December 2000, ‗Optimal Design Analysis of Grounding Grids for Substations Built in Non uniform Soil‘, IEEE International Conference on Power System Technology, Perth, Australia, Vol.3, pp. 1455 - 1460.
https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVs dGRvbWFpbnxncm91bmRpbmdzeXN0ZW1kZXNpZ258Z3g6OTQ 5MWEyNmE4NzA0NmYz
[4] IEEE: 81: 1983, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System.
[5] Gary Gilbert, ‗High Voltage Grounding Systems‘. PhD thesis, University of Waterloo, 2011
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