Ground Grid Design

Ad A d v anc an c ed Sub Su b s t ati at i o n Gro Gr o u n d i n g Gri Gr i d Desi Des i g n

Potential (Volt) 960 920 880 840 800 760 720 680 641 601 561 521 481 441 401 361 321 281 242 202 162 122 82 42 2

Touch Potential 3D Graph for Substation ABC

EDSA MICRO CORPORATION

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Version 4.80.00

October 2008

EDSA MICRO CORPORATION

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i

Ad van ced Sub st ati on Gro un di ng Grid Gri d

Table Table of Contents 1.

ADVANCED SUBSTATION GROUNDING GRID DESIGN CAPABILITIES, FEATURES AND FUNCTIONS................ FUNCTIONS.. ............................ ............................ ............................ ............................ ............................ ............................ ............................ ........................... ........................... ....................... ......... 1

2.

FOREWORD................ FOREWORD.. ............................ ............................ ............................ ............................ ............................ ........................... ........................... ............................ ............................ ....................... ......... 2

3.

INTRODUCTION, SUBSTATION GROUNDING.. ........................... ........................................ ........................... ............................ ............................ ..................... ....... 2

4.

SAFETY IN GROUNDING GROUNDING ................ .. ............................ ............................ ........................... ........................... ............................ ............................ ............................. ............................ ............. 2

5.

GROUNDING OF AC SUBSTATIONS.. ............................ .......................................... ............................ ............................ ............................ ............................ ...................... ........ 3

6.

DESIGN PARAMETERS PARAMETERS .............. .. .......................... ............................ ........................... ........................... ............................ ............................ ........................... .......................... .................... ....... 3

7.

GROUNDING GROUNDING SYSTEM ANALYSIS ANALYSIS ............... .. ........................... ........................... ........................... ............................ ............................ ........................... ........................... .............. 3

8.

THEORY AND COMPUTATIONAL PROCEDURES; GENERAL CONCEPT.. ......................... ...................................... .................. ..... 4

9.

GROUND POTENTIAL RISE (GPR), STEP, TOUCH, MESH.. ........................... ......................................... ........................... ........................... ................ .. 4

10.

COMPUTATION OF BODY CURRENT.. ........................... ........................................ ........................... ........................... ........................... ........................... ....................... .......... 5

11.

USE OF CRUSHED ROCK LAYER ( rs ) ............... ............................. ............................ ............................ ............................ ............................ ............................ .................. .... 5

12.

RESISTANCE RESISTANCE OF THE HUMAN BODY ............... .. ........................... ........................... ........................... ............................ ............................ ........................... .................... ....... 6

13.

BASIC SHOCK SITUATIONS; SITUATIONS; RECTANGULAR GRID .. ........................... ........................................ ........................... ........................... ...................... ......... 6

14.

RESISTANCE OF THE GROUND BENEATH THE TWO FEET.. ......................... ....................................... ........................... ......................... ............ 7

15.

COMPUTATION OF ALLOWABLE STEP AND TOUCH POTENTIALS.. .......................... ....................................... ....................... .......... 7

16.

DETERMINATION DETERMINATION OF MAXIMUM GRID CURRENT CURRENT .. .......................... ........................................ ........................... ........................... ......................... ........... 8

17.

SOIL MODEL ............... ............................. ............................ ........................... ........................... ............................ ........................... ........................... ............................. ............................. ..................... ....... 8

18.

SUMMARY OF THE PROGRAM PROGRAM CAPABILITIES CAPABILITIES ................ ... ........................... ........................... ........................... ........................... ........................... ................ 11

19.

INPUT INPUT DATA ................ .............................. ............................ ............................ ............................ ............................ ............................ ............................ ............................. ............................. ................ .. 11

20.

MENUS OF SUBSTATION GROUNDING GRID DESIGN PROGRAM... .......................... ....................................... ....................... .......... 12

21.

HOW TO SETUP A STUDY CASE... ........................... ......................................... ........................... ........................... ............................ ............................ .......................... ............ 15

22.

EXAMPLE OF SUBSTATION SUBSTATION GROUNDING GRID ASSEMBLY HAVING SLANTED SLANTED RODS ... .......... 29

23.

TUTORIAL EXAMPLE ................ ... .......................... ........................... ............................ ............................ ........................... ........................... ............................. ............................. ................ .. 33

24.

A FEW IMPORTANT NOTES... .......................... ........................................ ........................... ........................... ............................ ............................ ............................ ..................... ....... 53

25.

SUPPLIED EXAMPLES WITH THE SUBSTATION GROUNDING GRID DESIGN PROGRAM... ........56 ........56

26.

SOIL MODEL IN EDSA’s ADVANCED GROUNDING GRID DESIGN PROGRAM... ............................57 ............................57

27.

REFERENCES REFERENCES ................ ... ........................... ........................... ........................... ............................ ............................ ........................... ........................... ........................... ........................... ................... ..... 58

ii

Ad van ced Sub st ati on Gro un di ng Grid

Lis t Of Tables

Table 1: Sample Soil Resistivity Measurements Using Wenner Method ... ................................... 10 Table 2: List of Supplied Examples (Jobfiles) ... ...........................................................................56 List Of Figures

Figure 1: Basic Shock Situations.....................................................................................................6 Figure 2: Example of a Rectangular Grid ......................................................................................6 Figure 3: “Wenner” Soil Resistivity Test Method..........................................................................9 Figure 4: Starting Substation Grounding Program from EDSA Technical 2005 Main Menu... .. 15 Figure 5: Main Menu of The Substation Grounding Program... .................................................. 15 Figure 6: Opening A New Job file ................................................................................................16 Figure 7: General Data Dialog of Substation Grounding... .........................................................16 Figure 8: General Substation Data Dialog ..................................................................................17 Figure 9: Soil Resistivity Model Data Dialog ..............................................................................18 Figure 10: Defining the Substation Grid Land Size ... ..................................................................19 Figure 11: Regular Ground Grid Data Dialog ............................................................................19 Figure 12: Entering Regular Grid Data... ....................................................................................20 Figure 13: Ground Grid Rod Data Dialog... ................................................................................20 Figure 14: Viewing Substation Ground Grid Layout ... ................................................................21 Figure 15: Selection EquiPotential Calculation Option Icon ... ................................................... 24 Figure 16: Defining Area for EquiPotential Calculation (setting the start point) ... .................... 25 Figure 17: Defining Area for EquiPotential Calculation (setting the end point)... ...................... 25 Figure 18: Data Dialog for Defining Area for EquiPotential Calculation... ............................... 26 Figure 19: Calculated EquiPotential Lines Plot ... .......................................................................26 Figure 20: Copying EquiPotential Plots to the Clipboard... ........................................................27 Figure 21: Printing Text Result Report for EquiPotential Calculation ... .................................... 27 Figure 22: Sample Text Report... ..................................................................................................29 Figure 23: Sample Jobfile using Grounding Grid Assembly Having Slanted Rod... .................... 29 Figure 24: Icons for Defining Multiple Slanted Rods and/or Single Slanted Rod ... .................... 29 Figure 25: Data Dialog for Defining Single Slanted Rod ... .........................................................30 Figure 26: Data Dialog for Defining Multiple Slanted Rods... .................................................... 31 Figure 27: Multiple Slanted Rods Added... ...................................................................................31 Figure 28: EquiPotential Lines Plot for Sample Jobfile With Slanted Rods... ............................. 32 Figure 29: Substation Layout of a L-Shaped Grid with Ground Rods, IEEE80-2000... .............. 33 Figure 30: Opening a New Job File, IEEE80-2000 -- example b-4... .......................................... 34 Figure 31: General Data Dialog, IEEE80-2000 –example-b4 ... ................................................. 34 Figure 32: General Substation Data Dialog, IEEE80-2000-example-b4 ... ................................. 35 Figure 33: Soil Resistivity Data Dialog, IEEE80-2000-example-b4... ......................................... 35 Figure 34: Defining the substation Grid Land Size, IEEE80-2000-example-b4... ....................... 36 Figure 35: Entering Ground Grid Data – Step 1, IEEE80-2000-example-b4 ... .......................... 36 Figure 36: Entering Regular Grid Data – Screen 1, IEEE80-2000-example-b4 ... ...................... 37 Figure 37: Entering Rectangular Grid– Screen 2, IEEE80-2000-example-b4 ... ......................... 38 Figure 38: Entering Rectangular Ground Grid Data – Screen 3, IEEE80-2000-example-b4... .. 38

iii


Figure 39: Entering Rectangular Ground Grid Data – Screen 4, IEEE80-2000-example-b4... .. 39 Figure 40: Entering Rectangular Ground Grid Data –Screen 5, IEEE80-2000-example-b4... ... 39 Figure 41: Adding Multiple Ground Rods- Screen 1... .................................................................40 Figure 42: Adding Multiple Ground Rods- Screen 2... .................................................................40 Figure 43: Adding Multiple Ground Rods- Screen 3... .................................................................41 Figure 44: Adding Multiple Ground Rods- Screen 4... .................................................................41 Figure 45: Adding Multiple Ground Rods- Screen 5... .................................................................42 Figure 46: Adding Multiple Ground Rods- Screen 6... .................................................................42 Figure 47: Adding Multiple Ground Rods- Screen 7... .................................................................43 Figure 48: Adding Multiple Ground Rods-Screen 8... ..................................................................43 Figure 49: Adding Multiple Ground Rods - Screen 9... ................................................................44 Figure 50: Adding Multiple Ground Rods - Screen 10... ..............................................................44 Figure 51: Adding Multiple Ground Rods - Screen 11... ..............................................................45 Figure 52: Adding Multiple Ground Rods - Screen 12... ..............................................................45 Figure 53: Adding Multiple Ground Rods - Screen 13... ..............................................................46 Figure 54: Adding Multiple Ground Rods - Screen 14... ..............................................................46 Figure 55: Adding Multiple Ground Rods - Screen 15... ..............................................................47 Figure 56: Plotting 3-D Potential and Equipotential Couture lines – Screen 1 ... ....................... 47 Figure 57: Plotting 3-D Potential and Equipotential Couture lines – Screen 2 ... ....................... 48 Figure 58: Touch Potential and Equipotential lines ... .................................................................48 Figure 59: Touch Potential and 3D Graph ... ...............................................................................49 Figure 60: Allowable Touch and Step Voltages ... ........................................................................49 Figure 61: Calculate Allowable Touch and Step Voltages... ........................................................50 Figure 62: Potential Along the Axis Calculation – Icon ... ...........................................................50 Figure 63: Potential Along the Axis Calculation – Mouse Shape ... ............................................. 51 Figure 64: Potential Along the Axis Calculation – Define an Axis ... ........................................... 51 Figure 65: Potential Along the Axis Calculation – Define an Axis Continuation... ..................... 52 Figure 66: Potential Along the Axis Calculation – Step Potential Plot ... .................................... 52 Figure 67: Multiple grounding systems... .....................................................................................54 Figure 68: Report for Multiple Grounding... ................................................................................55

Note: 

You can view this manual on your CD as an Adobe Acrobat PDF file. The file name is: Advanced Substation Ground Grid

Ground_Grid_Design.pdf

You will find the Test/Job files used in this tutorial in the following location: = Advanced Substation Grounding C:\DesignBase\Samples\GG3D  Test Files:

IEEE80-2000-EXAMPLE-B1, IEEE80-2000-EXAMPLE-B2 IEEE80-2000-EXAMPLE-B3, IEEE80-2000-EXAMPLE-B4 IEEE80-2000-EXAMPLE-B2-SLANTED

ALL RIGHTS RESERVED COPYRIGHT 2008

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1.

R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R

ADVANCED SUBSTATION GROUNDING GRID DESIGN CAPAB ILITIES, FEATURES AND FUNCTIONS IEEE 80 (2000 & 1986), IEEE 665 Modeling, and IEEE 80/IEC 490 for Safety Analysis English or Metric Units Ease of use and visual design of grounding system Fast and Accurate Analysis of complex an d large grounding grid assembly Grid/Rod/Profile Wizards to Set Up Initial System Rectangular, Square, or Custom Grid Shapes and 3D Plots Optimize Number of Conductors and/or Rods Equal or Irregular Conductor Spacing Parallel or Arbitrary Oriented Run of Conductors Rods & Conductors of any shape, size and in any 3-D Direction (Vertically driven or Slanted) Built-In Library of Rods and Grids Multi Layer Soil Model Development of the Two Layer Soil Resistivity Model Based on the Wenner Measurement Techniques Using Advanced Optimization Method Passive and Return Grids and Pipe Model User Specified Fault Current Safety Analysis Including Surface Materials Based on Body and Exposure Time Parallel Resistance of Tower Footing and Substation Transformer Grounding Computation of Ground Grid Assembly Resistance, Ground Potential Rise, Step, Touch and Mesh Voltages Touch and Surface Potential Analysis Data Entry for Earth Model, Rods and Grids in Spreadsheet Format Current Decrement/Division Factor Move the ground grid assembly upward/downward/left/right with a click of mouse 3-D, Cross Sectional, and Top Graphical Interface Views Ability to Analyze the Potential Rise for Each Ground System Including Neighboring Passive Grids or Rods Calculation of Tolerable Step and Touch Voltages on the Basis of the Person Weighing, fault duration and soil resistivity Touch and Potentials within user defined irregular shapes Identification of the location of the worst touch potential (i.e. maximum touch) Identification of the location of the worst absolute potential (i.e. maximum absolute potential) Export absolute/touch/step potentials in 2-D and 3-D to Excel 2-D and 3-D touch, step, & absolute potential graphs 2-D and 3-D touch, step, & absolute potential graphs in an irregular user defined area Graphical and Color-coded display of Danger Areas Step and Touch Voltage Profile on user-defined area or along a specified path User-Defined Thresholds for Danger Area Evaluation User-Defined Color Coding for Graphical Safety Analysis Powerful Zoom Feature Zoom in/out to visualize details or global views Modeling of ground grid & rod conductors assembly & automatic setup of self & transfer impedance matrix Grid Conductor Current Displacement Using Matrix Analysis Ground Resistance Calculated Using Conductance Matrix Comprehensive Report for Grid and Rod Configuration

1


R Comprehensive Report for Surface Potential Analysis Featuring Station Data and Currents Diffused R R R R

2.

to Ground by the Grid Elements Danger Point Evaluation Report Viewer Range Checking for Simulation Parameters Advanced numerical method for efficient and accurate conductance matrix calculations

FOREWORD This discussion assumes that the user is a Professional Engineer familiar with the AC Substation Ground Grid Design concepts that affect the electrical distribution systems performance during Ground Fault conditions. Determination of validity of the results, and whether the program is applicable to a system, is the user's responsibility. This program is undergoing continuous development, and EDSA is determined to make this program as comprehensive and easy to use as possible. Additional analyses capabilities will be made available as they are developed. Any comments, suggestions or errors encountered in either the results or documentation should be immediately brought to EDSA's attention. You should read and be familiar with ANSI/IEEE standards and run all the examples in the manual before building your own job file and run the program.

3.

INTRODUCTION, SUBSTATION GROUNDING As required by NEC, an Electrical System must have effective grounding to ensure safety. This can be obtained by the operation of the overcurrent device as soon as a ground fault occurs. If the overcurrent device operates slowly, or not at all, during the occurrence of the ground fault, the following may happen:   

Damage of Equipment, Ignition, Fire, Electrocution of Personnel.

Of all these factors, increased consideration must be given to electrocution of personnel and, therefore, a high degree of grounding protection should be implemented to safeguard human life.

4.

SAFETY IN GROUNDING In principle, the objectives of a Grounding Design are as follows: 



To provide means to carry electric current into the earth under normal and fault conditions without exceeding any operating and equipment limits, or adversely affect continuity of service. To make sure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.

2


The circumstances that make electric shock accidents possible are: 1.

Relatively high fault current to ground in relation to the area of ground system, and its high resistance to remote earth 2. Soil resistivity and distribution of ground current such that high potential gradients may occur at certain points at the earth surface 3. Presence of an individual at such a point in time, and in such a position, that the human body is bridging two points of high potential differences 4. Absence of sufficient contact resistance, or other series resistance, which would limit current through the human body to a safe value under the above circumstances. 5. Duration of fault and body contact, and hence, of the flow of current through a human body for enough time to cause harm at the given current intensity .

5.

GROUNDING OF AC SUBSTATIONS Computation methods based on the book "IEEE Guide for Safety in AC Substation Grounding" ANSI/IEEE Standard 80-2000, has been traditionally the primary tool available to substation engineers for analysis and design of substation grounding systems. The present software uses the conductance matrix approach and IEEE80-2000 is only used for computation of maximum allowable touch and step potentials. IEEE80-2000 technique is only applicable to rather simple ground grid design and it is an approximate method and will be shown in this document that for rather simple grid assembly, IEEE80-2000 can be in error by as much as 15%. The primary objective in the design of a substation grounding is to provide safe conditions for personnel operating in and around the substation. Accidents to personnel result from Grounding Potential Rise (GPR) of the ground system during fault conditions on the connected power system. Therefore, the grounding system must be designed to: 1. 2.

limit the potential rise of the substation ground mat to an acceptable value for any possible fault condition; limit the resulting step, touch, and transfer potentials in and around the substation to an acceptable value.

The surface potentials are approximately proportional to the grid potential rise, and both are determined by the current flow from the grounding grid to earth.

6.

DESIGN PARAMETERS The parameters that primarily determine the performance of the grounding system are:   

7.

Soil resistivities in the vicinity of earth embedded grounding conductors; Grounding grid area and geometry; Elements of connected power system including: transformer connections, overhead ground wires, transmission tower grounding, counterpoise wires, and use of URD cable.

GROUNDING SYSTEM ANALYSIS The analysis of a grounding system design consists of: 

Modeling the grounding system, representing the grounding grid, the connected power system and the earth conductivity.

3


  

8.

Providing procedures to determine grid potential rise for specified fault type and location. Providing procedures for determination of step, touch and transfer potentials. Developing simplified procedures for grid analysis.

THEORY AND COMPUTATIONAL PROCEDURES; GENERAL CONCEPT Considering the user's point of view, this chapter has been prepared to present information based on textbooks, technical papers, and experience in the field. Related definitions of grounding systems are as fellows: Ground Electrode is a conductor embedded in the earth and used for collecting ground current from, or dissipating ground current into, the earth. Ground Grid is a system of horizontal ground electrodes that consists of a number of interconnected, bare conductors buried in the earth, providing a common ground for electrical devices or metallic structures in a specific location. Ground Mat is a solid metallic plate, or a closely spaced bare conductor, that is connected to, and often placed in, shallow depths above a ground grid or elsewhere at the earth surface. Grounded metal gratings placed on or above the soil surface, or wire mesh placed directly under the crushed rock, are common forms of a ground mat.

9.

GROUND POTENTIAL RISE (GPR), STEP, TOUCH, MESH GPR is the maximum voltage that a station grounding grid may attain relative to a distant grounding point assuming to be at the potential of remote earth. GPR is proportional to the magnitude of the grid current, and to the grid resistance. Step Voltage is the difference in surface potential experienced by a person bridging a distance of 1 m with his feet contacting no other grounded object. Touch Voltage is the potential difference between the ground potential rise ( GPR ) and the surface potential at the point where a person is standing, while simultaneously his hands contact a grounded structure. Mesh Voltage is the maximum touch voltage to be found within a mesh of a ground grid.

4


Transferred Voltage: This is a special case of the touch voltage where a voltage is to be transferred into, or out of, a substation, occurring when a person standing within the station area touches a conductor grounded at a remote point, or when a person standing at a remote point touches a conductor connected to the station grounding grid.

10. COMPUTATION OF BODY CURRENT The non-fibrillating current (IB) is related by the shock energy ( SB) absorbed by the human body: IB =

where

k

=

k

(1)

ts SB

IB =

RMS magnitude of the current through the body

ts =

duration of the current exposure in seconds

SB =

special constant related to the electric shock energy tolerated by a certain percent of a given population

For persons of an approximate weight of 50 kg. ( 110 lb. ), k50 = 0.116, and For persons of an approximate weight of 70 kg. ( 155 lb. ), k70 = 0.157. So equation (1) becomes, IB =

IB =

ts =

0.116 ts 0.157 ts

for 50 kg (110 lb) body weight

(2)

for 70 kg (155 lb) body weight

(3)

time in seconds

For a person of average weight who could withstand current (I) without ventricular fibrillation, the equation is: I

p

116.0ma

(4)

ts

11. USE OF CRUSHED ROCK LA YER (

s)

A layer of crushed rock on surface helps in limiting the human body current by adding resistance to the equivalent body resistance. The values of rs vary from 1000-5000 Ω -m.

5


12. RESISTANCE OF THE HUMAN BODY The human body can be represented by a non-inductive resistance (R B) for both DC and AC at normal power frequency. A value of R B = 1000 is selected as a resistance of a human body from hand to both feet, and also from hand to hand, or from one foot to the other (see ANSI/IEEE Std 80-2000). Hence R B = 1000

13.

BASIC SHOCK SITUATIONS; RECTANGULAR GRID

Figure 1: Basic Shock Situations

1.

Step Voltage

2.

Touch Voltage

3.

Mesh Voltage

4.

Transferred Voltage

Figure 2: Example of a Rectangular Grid

6


14. RESISTANCE OF THE GROUND BENEATH THE TWO FEET The resistances of the ground beneath the two feet in series and parallel are:

where:

15.

R 2FS

=

6.0 C s ( h s , k ) ρ s

(5)

R 2FP

=

1.5 C s ( h s , k ) ρ s

(6)

Cs

=

Reduction factor for derating the surface layer resistivity, and C s is a function of ( h s , k)

hs

=

Thickness of crushed rock surface layer in meters

k

=

Reflection factor

k

=

ρ

-

ρ s

ρ

+

ρ s

ρ s

=

Crushed rock resistivity in ohm-meter

ρ

=

Resistivity of the soil in ohm-meter

COMPUTATION OF ALLOWABLE STEP AND TOUCH POTENTIALS The maximum driving voltage of any accidental circuit should not exceed the limits defined below: For step voltage the limit is Estep

=

( R B + R 2FS ) I B

(7)

Combining equations 7, 4, 5 and either 2 or 3 Estep50

=

[ 1000 + 6 C s ( h s , k ) ρ s ] 0.116 / t s

(8)

Estep70

=

[ 1000 + 6 C s ( h s , k ) ρ s ] 0.157 / t s

(9)

or

To ensure safety, the actual step voltage ES should be less than the maximum allowable step voltage Estep. Similarly, the touch voltage limit is: Etouch

=

( R B + R 2FP ) IB

( 10 )

Combining equations 10, 4, 6 and either 2 or 3 Etouch50

=

[ 1000 + 1.5 C s ( h s , k ) ρ s ] 0.116 / t s

7

( 11 )


or Etouch70

=

[ 1000 + 1.5 C s ( h s , k ) ρ s ] 0.157 / t s

( 12 )

To ensure safety, the actual touch voltage, mesh voltage, or transferred voltage, should be less than the maximum allowable touch voltage, Etouch. Note:

Please see ANSI/IEEE Std 80-2000.

16. DETERMINATION OF MAXIMUM GRID CURRENT The design value of maximum grid current is defined as follows: IG

=

C p D f I g

( 14 )

IG

=

Maximum grid current in Amperes

D f

=

Decrement factor for entire duration of fault in seconds

C p

=

Projection factor for future system growth

Ig

=

RMS symmetrical grid current in Amperes

Ig

=

S f I f

I f

=

RMS value of symmetrical ground fault current in Amperes

S f

=

Current division factor relating the magnitude of fault current to that of its portion flowing between the grounding grid and surrounding earth

where:

Again,

where:

Approximately 50 - 60% of the fault current (If = 3 I0) flows through the grid to remote earth.

17. SOIL MODEL Within the substation, the grid resistance and the voltage gradients are directly dependent on the soil resistivity influenced by such factors as moisture content and temperature. Also, the soil resistivity may vary depending on weather conditions. The present program models a two-layer soil resistivity model and uses an optimization technique to fit a perfectly horizontal layered soil structure to a set of soil resistivity measurements that are carried out using the “Wenner” method. The principal of Wenner method is shown in the figure below:

8


Figure 3: “Wenner” Soil Resistivity Test Method

The “Wenner” method is one of the widely used methods for measuring soil resistivity. In this method, four test rods are inserted a short distance into the soil in a straight line with equal spacing between the probes. A test current is applied to the outer probes and the resulting potential difference between the inner probes, is measured. The potential difference divided by the test current give an apparent resistance in ohms. The apparent soil resistivity is obtained from the measured resistance. For test configurations where the test probe depth is small compared with the probe spacing so that the test probes appear approximately as point sources, the apparent resistivity is given by the following equation :

ρ = 2πaR  is an Where ρ is the soil resistivity, a is the probe spacing and R is the measured resistance. R apparent resistivity for that particular test probe spacing. In most instances a series of measurements will be taken using different probe spacing. When the apparent resistivity is plotted against the probe spacing, it is often found to vary considerably as the spacing is changed. This is due to non-uniformity of the soil. In reality, the structure of soil may be very complex with vertical and/or horizontal layers and/or pockets of soil having differing resistivities.

9


SAMPLE SOIL RESISTIVITY MEASUREMENTS DATA The following table shows a sample of soil resistivity measurements using Wenner method: Table 1: Sample Soil Resistivity Measurements Using Wenner Method

Probe spacing (a)

Resistance ( R)

1.0 2.0 4.0 6.0 8.0 12.0 16.0 20.0 24.0 30.0 36.0

67.000 25.600 10.270 4.380 2.870 1.629 1.225 0.990 0.831 0.700 0.584

Resistivity (ρ) 423.4 322.2 258.2 165.1 144.3 122.8 123.2 124.4 125.3 131.9 132.1

Based on the method described in Appendix B of IEEE Guide 81 - 1983, upper layer resistivity estimated value is 420.0 and lower layer resistivity is 120.0 ohm-m. The estimated depth of upper layer is 4.0 meters. The results of EDSA’s Substation Grounding Grid Design program are: Upper layer resistivity: 417.74 Ω-m Lower layer resistivity: 123.12 Ω-m Upper layer thickness: 2.14 m Probe spacing (m) 1.0 2.0 4.0 6.0 8.0 12.0 16.0 20.0 24.0 30.0 36.0

Measured Resistivity(Ω -m) 423.4 322.2 258.2 165.1 144.3 122.8 123.2 124.4 125.3 131.9 132.1

Computed Resistivity(Ω -m) 404.3 349.7 230.9 170.8 146.2 131.1 127.1 125.5 124.8 124.1 123.8

Deviance (%) -4.5 8.5 -10.6 3.5 1.3 6.7 3.2 0.9 -0.4 -5.9 -6.3

Comments

Suspicious measurement Suspicious measurement

Suspicious measurement

Suspicious measurement Suspicious measurement

RMS Deviance

4.715%

Standard Deviation

3.242%

In the above, if the Deviance between measured and computed value is greater than 1.5*(Standard Deviation), then, the program assumes that the measurement can be considered suspicious.

10


18. SUMMARY OF THE PROGRAM CAPAB ILITIES This program is capable of analyzing a grounding grid system consisting of: ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾

Rectangular, square or arbitrary shaped grid; Parallel or arbitrary oriented run of conductors; Equal, irregular conductor spacing; Multi layers of earth types (soil and crushed rock or other material as the top layer); Calculation of tolerable step voltage and touch voltage on the basis of the person weighing 50 or 70 kg (110 or 155 lbs) Ground resistance is calculated using conductance matrix. Contrarily to the Sverak's or Schwarz's approximate methods, this method is exact and accurate English or Metric units Development of the Two Layer Soil Resistivity Model based on the Wenner Measurement Techniques using Advanced Optimization Method. Computation of Ground grid assembly resistance, ground potential rise (GPR), Step, Touch, and Mesh Voltages 2-D and 3-D plots of surface potential Plots of touch and step voltages along a user defined path Identifies the maximum potentials within a user-defined area Ground grid and rod properties are remembered, and the last entered value remains until it is changed by the user Touch, step and absolute voltage values can be exported to Excel Non-rectangular areas can be evaluated with EquiPotential lines

19. INPUT DATA A list of general data entries is follows: Input Variable

Metric units

English units

Remarks

Grid length Grid width Conductor spacing Conductor diameter Ground Fault Current Fault duration

Meters Meters Meters Meters Amperes Seconds

Feet Feet Feet Feet Amperes Seconds

Symmetrical 0.5 or less

Resistivity: Soil Crushed rock

ohm-meters ohm-meters

ohm-meters ohm-meters

Depth of burial: Grounding grid Crushed rock

Meters Meters

Feet Feet

Ground rod data: Diameter of rod Length of rod

Millimeters Meters

Inches Feet

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20. MENUS OF SUBSTATION GROUNDING GRID DESIGN PROGRAM In this section we will briefly described the menus of the program. “File” option of the main menu provides the following functionalities (see figure below): 1) 2) 3) 4)

“New” to create a new study case “Open” to open an existing case study “Save” to save all of the data so far in the active case study “Save As” to save all of the data so far in the active case study to a different file than the last opened case study 5) “Print Report” to view and print the text result of grounding resistance, GPR, grounding conductors and rods 6) List of the last four recently accessed study cases 7) “Exit” quitting the program

“Edit” option supports the following shown below:

1) “Edit Master File” takes the user to the general substation data dialog

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“View” option supports the following items as shown below:

1) “General Information”, here information on the substation name, system of units and other general data can be viewed and modified 2) “Soil Model Data” the soil resistivity can either be provided or be computed from a set of measurements using Wenner technique 3) “Ground Grid” the substation grounding grid assembly layout can be viewed using this option 4) “EquiPotential Lines” the surface potential computed within an area can be viewed 5) “3-D Potential Graph” the surface potential computed within an area can be viewed in 3-D 6) “Potential Along an Axis”, the computed absolute potential, touch or step along a path (axis) can be viewed 7) “Toolbar” and “Status Bar” are used to make these tools visible or hide

“Analysis” can be used to either calculate: 1) Allowable touch and step voltages given the surface material, body weight, and fault duration. 2) Calculate or recalculate ground grid resistance, GPR. 3) Identify Max Touch Voltage, is used to find where in a given area the potential is maximum and what the corresponding value of the potential is

13


“Graph” option supports the following shown below: 1) “Print Graph” the active graph/plot shown on the screen can be sent directly to a printer 2) “Copy Graph to Clipboard” is used to copy the active graph to the Clipboard for later insertion in other applications such as Word. 3) “Save Graph as jpeg” is used to save the active graph to a file 4) “Refresh” is used to update the plots/graphs 5) “Export Graph” is used to export the numerical values of the calculated potentials to a comma separated format (csv) used by Excel

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21. HOW TO SETUP A STUDY CASE In the following, we will show all steps required to setup a new case study. To start Substation Grounding Grid Design program select “3-D Substation Grounding” icon as shown below:

Figure 4: Starting Substation Grounding Program from EDSA Technical 2005 Main Menu

Next, the main menu of the substation grounding grid program will appear:

Figure 5: Main Menu of The Substation Grounding Program

15


To build a “New” case study, select File->New or just select the “New” icon as shown in Figure 6. Information related to System, Grid, Soil resistivity (model), and conductors are required in order to build a case. Select “File->New” and enter a file name for the newly created job file, let’s call it “ieee80-2000-example-b1” as shown below:

Figure 6: Opening A New Job file

Next, provide the general data as shown below. In this dialog, the user can specify the preferred system of units (metric or US):

Figure 7: General Data Dialog of Substation Grounding

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The ground fault current is specified next. Note that the program requires substation “Parallel Impedance”. This impedance will be paralleled with the computed ground grid assembly resistance. This is an alternative method of accounting for fault distribution factor defined in the IEEE 80-2000 standard. This parallel impedance is normally represents any grounding resistance in the substation such as transmission line tower footing resistance, substation transformer grounding resistance, etc. For the worst-case condition, enter a high value for the parallel impedance. The Parallel impedance may represent substation transformer grounding resistance, transmission line tower footing resistance and or any other resistance that can be considered to be in parallel to the grounding grid assembly resistance.

Figure 8: General Substation Data Dialog

To compute the maximum allowable touch and step voltages the user should also supply Body weight, fault duration, treated surface (e.g. crushed rock) resistivity and thickness. Next, the soil model and its parameters are required. There are three different ways of providing the soil model data. First based on the available soil data, user should choose one of the following choices:   

Homogenous Soil model Two Layer Soil model Soil model is not known but soil resistivity measurements are available

These choices are shown in the following figure. If the soil model is to be considered homogenous, then, the user just has to supply the value of soil resistivity. If the soil model is a two layers model, then, the user should supply upper layer thickness and resistivity as well as lower layer resistivity. In case the user selects “Compute from Measurements”, then, the user should supply resistivity measurements as a function of probe spacing (at least three measurement points should be given).

17


In case a “Return Ground Grid” Assembly is defined, the return fault current should also be defined (can normally be equal or smaller than the ground fault current). Any grid/rod specified to be passive, then, they can be either at the same voltage or having different voltages. Conductors of a “Passive Grid” assembly can be either at the same voltage or having different voltages. For the case at hand, we have a homogenous soil model even though we have selected “Two Layer Model”. That is why we have entered both upper and lower resistivity to be 400 ohm-m as specified in the IEEE80-2000 standard.

Figure 9: Soil Resistivity Model Data Dialog

Now that all of the general substation data is completed we will specify the size of land covering the substation. This data is not used in the comput ation and it is only required for better visualization of the substation grid. Next, we need to provide data of grounding grid and rod assembly if any. Press “Go to Next Window” to proceed.

18


As shown below, define total substation land size (100 x 100 m in this example).

Figure 10: Defining the Substation Grid Land Size

As shown below, the program prompts the user for regular grid data (we will see later how irregular grids can be modeled). For the case at hand, we h ave a regular grid of 70 x 70 m.

Figure 11: Regular Ground Grid Data Dialog

The origin coordinates (X0 and Y0) are only a reference point. We will enter 70 for both Length and Width of the grid. The number of vertical and horizontal grid conductors is 11 for both. Burial depth for the grid conductor is also the same as default value of 0.5 m (this burial depth is commonly used). The conductor diameter is 10 mm. To select a conductor size from the library, press “Lib” button shown in following figure:

19


Figure 12: Entering Regular Grid Data

The user should also specify if the entered grid data should be considered as part of “Main”, “Return”, or “Passive” assembly. Next, the program prompts the user for grounding Rods as shown below. However, since there are no rods to be considered for this grounding grid system we will select the “cancel” button.

Figure 13: Ground Grid Rod Data Dialog

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Finally all of the data entry for this example is complete and program will show the substation grid layout as seen below: If any of the previous data needs to be changed, then icons shown to left of the grid can be selected. Note that in the figure below “Ground Grid” icon is highlighted. If for example, we need to change soil data, we just need to select the “Soil Model Data” icon.

Figure 14: Viewing Substation Ground Grid Layout

Note that several important quantities are listed in the lower part of the screen shown in the above figure. These are, Ground Potetial Rise (GPR), ground grid assembly resistance (RG), Allowable touch and step potentials (Et and Es), depth upper layer soil model (H), resistivities of the upper and lower layer soil (Rho1 and Rho2). Before we start the grounding grid computations, the icons shown just above the ground grid layout will be described.

The below shown icon is used to define/redefine the substation grid land size.

21


The ground grid assembly can be moved up, down, left, and right using the icon shown below:

Once the user selects this option, the following dialog will appear:

The distance of the ground grid move can be specified along with its direction (up, down, left, or right). The below shown icon is used to define a rectangular grid data.

The below shown icon is used to define grounding rods. This can be used to add multiple rods within a defined grid and/or along a path.

To add a group of slanted rods use the icon shown below:

22


The below shown icon is used to add a grounding grid segment. This is mainly used to build irregular shaped grid.

The below shown icon is used to add a grounding rod. This is mainly used to build irregular placed grounding rods.

Also, it is possible to add rods that are slanted. The icon shown below can be used to add slanted rods:

The below shown icon is used to calculate voltage profile along an axis (path). It is also possible to compute touch and step voltages along an axis with this option.

The below shown icon is used to calculate voltage profile within a defined area. With this option user can visualize the equi-potential lines in two or three-dimensional graphs. It is also possible to compute touch voltages within an area with this option.

The below shown icon is used to calculate voltage profile within an irregular defined area. With this option user can visualize the equi-potential lines in two or three-dimensional graphs. It is also possible to compute touch voltages within an area with this option.

23


Zoom facilities. The following functions are implemented: Zoom in an area Return to the original view (zoom out) Zoom in/Scale up the view (magnify) Zoom out/Scale down the view Return to select mode (leaving zoom facilities) Now we will see how the above icons are used to compute different potentials. We will first press the “3D Potential and Equi-Potential Contour Lines”. As shown in the figure below, the mouse will turn into a rectangular shape . Using the mouse, left mouse click to the location where the area of equi-potential should start as shown below.

Figure 15: Selection EquiPotential Calculation Option Icon

24


Figure 16: Defining Area for EquiPotential Calculation (setting the start point)

Figure 17: Defining Area for EquiPotential Calculation (setting the end point)

25


As can be seen in the above figures, the area where equipotential should be computed are selected with only two left mouse clicks. Once, the above selection is completed, the program prompt the user with the coordinates of the selected area as shown below.

The user has another chance to modify the coordinates if necessary. Additional data required are the resolution of computation within the specified area. Number of points of 40 along each x and y axis is the maximum and found to be more than sufficient to obtain clear and concise picture of the equipotential lines. Finally the equipotential lines can be either absolute or touch voltages.

Figure 18: Data Dialog for Defining Area for EquiPotential Calculation

Figure 19: Calculated EquiPotential Lines Plot

The above figure shows the computed touch voltage within the defined area.

26


Graphs can be copied to Clipboard for later inclusion in other documents or can be saved in the form of jpeg to file for later retrieval.

Figure 20: Copying EquiPotential Plots to the Clipboard

The text report of calculated ground resistivity and GPR can be viewed or saved to file by selecting “Print Report” from the File option as shown in the figure below:

Figure 21: Printing Text Result Report for EquiPotential Calculation

Sample text report is shown below. As shown below, the total grounding grid resistance is 2.67 ohms. The value computed by IEEE 80-2000 method is 2.78 ohms as report on page 133. However, IEEE80-

27


2000 shows that using an accurate computer program, the computed resistance will be 2.67 (see page 134 of IEEE 80-2000) that agree 100% with the result o f EDSA’s Substation Grounding Grid Design program. EDSA Advanced Gr ound Mat Pr ogr am v4. 80. 00 ============================================================ Proj ect No. : Pr oj ect Name: Ti t l e : Dr awi ng No. : Revi si on No. : J obf i l e Name:

123 ABC ABC 123 1 i eee80- 2000- exampl e- b1

Dat e Ti me Company Engi neer

: : : EDSA : AN

Thi s exampl e i s based on t he I EEE80- 2000 St andar d "I EEE Gui de f or Saf et y i n AC Subst ati on Gr oundi ng" PP. 129- 134, Exampl e B. 1 System I nf or mat i on ============================================================ Subst at i on Name = TEST Uni t Sys t e m = Met r i c Faul t Curr ent = 1908. 00 ( Amp) Paral l el I mpedance = 10000. 00 ( ohm) F aul t Cur r ent Di vi s i on F ac t o r , Sf = 1. 00 Body Wei ght = 70. 00 ( Kg) Faul t Durat i on = 0. 50 ( second) Surf ace Mater i al Descri pti on = Cr ushed Rock Thi cknes s of Sur f ace Mat er i al = 0. 10 ( m) Resi st i vi t y of Sur f ace Mater i al = 2500. 00 ( ohm- m) Upper Layer Mater i al Descri pt i on= Upper Layer Thi ckness = 100. 00 ( m) Resi st i vi t y of t he Upper Layer = 400. 00 ( ohm- m) Lower Layer Mater i al Descri pt i on= Resi st i vi t y of t he Lower Layer = 400. 00 ( ohm- m) Al l owabl e Touch Vol t age = 840. 548 ( Vol t ) Al l owabl e Step Vol t age = 2696. 097 ( Vol t )

Mai n Gr ound Gr i d Axi s # 0001 0002 0003 0004 0005 0006 0007 0008 0009 0010 0011 0012 0013 0014 0015 0016 0017 0018 0019 0020 0021

X1

Y1

Z1

X2

Y2

Z2

Lengt h

0. 0 7. 0 14. 0 21. 0 28. 0 35. 0 42. 0 49. 0 56. 0 63. 0 70. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0

0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 7. 0 14. 0 21. 0 28. 0 35. 0 42. 0 49. 0 56. 0 63. 0

0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5

0. 0 7. 0 14. 0 21. 0 28. 0 35. 0 42. 0 49. 0 56. 0 63. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0

70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 70. 0 0. 0 7. 0 14. 0 21. 0 28. 0 35. 0 42. 0 49. 0 56. 0 63. 0

0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5

70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00 70. 00

28

Di amet er 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100 0. 0100

Cur r ent 165. 32 95. 53 72. 10 61. 04 55. 77 54. 19 55. 77 61. 04 72. 10 95. 53 165. 32 165. 32 95. 53 72. 10 61. 04 55. 77 54. 19 55. 77 61. 04 72. 10 95. 53


0022

0. 0

70. 0

0. 5

70. 0

70. 0

0. 5

Tot al Conduct or Lengt h

=

1540. 0 met er

Gr ound Pot ent i al Ri se Gr oundi ng Gr i d & Rod Resi st ance Tot al I mpedance of I nst al l at i on

= = =

5086. 3 Vol t s 2. 6665 Ohms 2. 6658 Ohms

70. 00

0. 0100

165. 32

Figure 22: Sample Text Report

22. EXAMPLE OF SUBSTATION GROUNDING GRID ASSEMBLY HAVING SLA NTED RODS In this section we will briefly describe how to model grids having slanted rods. This will be shown with help of an example jobfile named “ieee80-2000-example-b2-SLANTED”. This example is identical to the jobfile “ieee80-2000-example-b2” with the exception that we will use a slanted rod in upper right corner as shown in the following figure:

Figure 23: Sample Jobfile using Grounding Grid Assembly Having Slanted Rod

Two icons can be selected to define either multiple slanted rods and/or single slanted rod as shown in the fifth and the eighth icons below.

Figure 24: Icons for Defining Multiple Slanted Rods and/or Single Slanted Rod

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Data dialog for entering either single slanted rod and/or multiple slanted rods are shown in Figure 25 and Figure 26 respectively. It can be seen that a slanted rod data consists of simply providing X, Y and Z coordinates of the rod extremes (beginning and ending coordinates). Defining multiple slanted rods requires specification of the rods area, similar to the vertically driven rods, with the addition of data for extremes of a sample slanted rod in the area as shown in Figure 26.

Figure 25: Data Dialog for Defining Single Slanted Rod

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Figure 26: Data Dialog for Defining Multiple Slanted Rods

Figure 27: Multiple Slanted Rods Added

31


Figure 28: EquiPotential Lines Plot for Sample Jobfile With Slanted Rods

The previous figure shows the computed touch voltage within the defined area. It can be seen that due to installation of the slanted rod, the area in the upper right corner experiences lower touch potential than other corners of the ground grid assembly.

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23. TUTORIAL EXAMPLE

In this section we will show how to build a case and exercise all of the functionalities supported by the substation grounding grid design program. For this purpose, we will use the example shown on Page 139 of the IEEE 80-2000 standards. The substation to be analyzed is a L-shaped grid shown below:

Figure 29: Substation Layout of a L-Shaped Grid with Ground Rods, IEEE80-2000

33


Start the substation ground grid design program as described in the previous section. Select “New” and give a file name to the project, let’s use “ieee80-2000-example-b4”.

Figure 30: Opening a New Job File, IEEE80-2000 -- example b-4

Next, let’s provide general substation data for this example. Here we have selected “Metric” system of units as shown below:

Figure 31: General Data Dialog, IEEE80-2000 –example-b4

34


The fault current given in the IEEE standard is 1908 amps (after considering current division factor). The surface material is 4 in crushed rock with resistivity of 2500 ohm-m.

Figure 32: General Substation Data Dialog, IEEE80-2000-example-b4

Based on the IEEE standard, the soil is homogenous with resistivity of 400 ohm-m.

Figure 33: Soil Resistivity Data Dialog, IEEE80-2000-example-b4

35


Next, we specify a land size of 75 x 108 that is sufficient for better visibility of the L-shape grid.

Figure 34: Defining the substation Grid Land Size, IEEE80-2000-example-b4

The substation grounding grid will be built in two steps. First we will build lower part of the L-shaped using “Regular Grid Data”. The size of the first part of the grid is provided below.

Figure 35: Entering Ground Grid Data – Step 1, IEEE80-2000-example-b4

36


Next, the program will prompt for data entry for grounding rods. At this time, we will select “Cancel” since the ground grid is not yet completed. We will come back to adding rods later.

Figure 36: Entering Regular Grid Data – Screen 1, IEEE80-2000-example-b4

From the icons shown below the menu bar, we select “Rectangular Grid Data” (see figure below):

37


Figure 37: Entering Rectangular Grid– Screen 2, IEEE80-2000-example-b4

Now we will build the upper part of the L-shaped grid. Please note by using the data entered in the dialog shown below, one grid conductor will be superimposed on a segment previously entered. This is a segment running from (0,35) to (70,35). This is not allowed, the program does support grid segments/rods that are partly/completely superimposed. Fortunately, the user interface detects this condition and ignores the latest superimposed segment.

Figure 38: Entering Rectangular Ground Grid Data – Screen 3, IEEE80-2000-example-b4

The message shown below is a result of having a segment superimposed an already existing segment.

38


Figure 39: Entering Rectangular Ground Grid Data – Screen 4, IEEE80-2000-example-b4

As seen in the figure below, the L-shaped ground grid was assembled in two easy steps. Therefore, it is easy to build irregular shaped grid using several “Rectangular Grid Data”.

Figure 40: Entering Rectangular Ground Grid Data –Screen 5, IEEE80-2000-example-b4

39


Next, we will add the ground rods. Select “Add Multiple Ground Rods” icon as shown below. This works very similar to the “Rectangular Grid Data” but instead rods will be places at the corners of the grid defined, let’s see how.

Figure 41: Adding Multiple Ground Rods- Screen 1

Again, we will place the ground rods in several steps. There are many ways to achieve this. We proceed as follows. First let’s place the rods in the lower right corners (please see Figure 29 or IEEE 80-2000 page 140 for reference). Note that the rod length is 7.5 meters given in the standard.


40


The result of adding rods based on the above data is shown below:


Next, we will show how a single rod can be added. Select “Add Single Ground Rod” icon as shown below:


Once this icon is selected the mouse will turn into a rod shaped (see figure below) having a cross. Place the cross on the desired location where the rod should be added and release the mouse button.

41



The program will read the location of the mouse automatically and rod data dialog will appear (see figure below). If the rod coordinates are not exactly pickup by the program, the user has another chance to make modification to the rod coordinates. In our case, the program pick up exact location as desired. However, we need to modify rod Z coordinate and length.


42


As shown below, we were able to add one rod at a specified location. Next let’s add rods to top and bottom rows. Again, we select “Add Multiple Ground Rods” and rods data dialog for adding these rods is shown below:


As seen in the figure figure below, with one easy step the six rods were added to the top and bottom rows.

Figure 48: Adding Multiple Ground Rods-Screen 8

43


The rods in the middle of the L-shaped grid, are also added in a similar fashion as seen in the rods data dialog shown in the figure below:

Figure 49: Adding Multiple Ground Rods - Screen 9

The eight rods, in the middle, are added as specified and are shown below:


44


Now we will see how “Add Multiple Ground Rods” can be used to place rods along an axis (path). Note that to add two rods in the lower left side of the grid, we have provided the following data. It is important to note that we have entered Length to be zero since the rods needs to be added along y-axis only (to place rods along an axis which is parallel to x-axis, then, the Width should be zero)


The two added rods are shown in the figure below.


45


There is only one more rod left to be added. This rod should be placed just below the upper right corner. Again, let’s select “Add Single Ground Rod” and click right mouse button at the rod location (see figure below):


The program picks up the coordinates of the rod correctly as shown below. Note that the Z coordinate of the rod and its length should be modified as shown below.


46


The last rod is now added as seen in the figure below:


Now, we have completed building of the ground grid assembly (grid conductors and rods). The ground surface potentials can be calculated by selecting the “3-D Potential and EquiPotential Contour Lines” icon as shown below:

Figure 56: Plotting 3-D Potential and Equipotential Couture lines – Screen 1

The user can specify the area of surface potential computation by dragging the mouse to the desired section. Once the mouse button is released the program will prompt the user to confirm/modify the area coordinates or other data as shown in the following figure:

47


Figure 57: Plotting 3-D Potential and Equipotential Couture lines – Screen 2

The computed equipotential lines are shown below. Note that we selected that equipotential to be computed for touch voltage and not absolute voltages.

Figure 58: Touch Potential and Equipotential lines

48


To obtain a 3-D view just select click left mouse button on the “3-D Potential Graph” icon as shown below:

Figure 59: Touch Potential and 3D Graph

At this point before we see how potentials along an axis can be computed, let’s see how maximum allowable touch and step voltages can be computed. Select “Analysis->Allowable Touch and Step Voltages” as shown below (from the menu bar items):

Figure 60: Allowable Touch and Step Voltages

49


Based on the general information provided earlier (body weight, fault duration, surface resistivity, upper layer resistivity) the program can compute allowable touch and step voltages. Press “Calculate” button and these quantities will be calculated and displayed. The maximum allowable touch in our case is 840 and maximum allowable step is 2696 volts. The reported values by IEEE 80-2000 are 838 and 2686 that are in excellent agreement (see page 132 of IEEE standard).

Figure 61: Calculate Allowable Touch and Step Voltages

Now that we have computed the maximum allowable touch and step voltages, the computation of voltage profile along an axis can be directly compared with these values. To compute this profile, select “Potential along an Axis” icon as shown below:

Figure 62: Potential Along the Axis Calculation – Icon

50


Once this icon is selected, the mouse will turn into a pointer (pen like) object as seen in the below figure.

Figure 63: Potential Along the Axis Calculation – Mouse Shape

Click mouse button at beginning of the axis and drag the mouse to the end of the desired axis and then release the mouse button.

Figure 64: Potential Along the Axis Calculation – Define an Axis

The program will show the coordinates of the axis as read by the mouse movement. These coordinates are shown in the data dialog below. Notice two important items in this dialog. First, we need to select if

51


potential should be absolute, touch or step. If step potential along the specified axis to be computed, then, it is necessary to enter “Spacing”. Normally this spacing for step voltage calculation is 1.0 meter.

Figure 65: Potential Along the Axis Calculation – Define an Axis Continuation

The computed step voltage profile along the specified axis is shown below. Notice that maximum allowable touch and step voltages are also shown in this figure for easy identification of violations (where voltage exceed the allowable value).

Figure 66: Potential Along the Axis Calculation – Step Potential Plot

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24. A FEW IMPORTANT NOTES

WHAT IS THE GROUNDING GRID PARALLEL IMPEDANCE The grounding grid parallel impedance can be used to represent a number of scenarios. Most common application of it is to represent the substation transformer grounding. The transformer grounding resistance in this case should be considered to be parallel to the ground grid assembly resistance. The total resistance is therefore,

Rtot =

Rg * Rtrans Rg + Rtrans

Where Rg is the ground grid assembly resistance and Rtrans is the substation transformer grounding resistance. In this case, the ground potential rise is: GRP=Rtot*Ig Where Ig is the ground fault current. Other situation for consideration of the parallel impedance is to represent transmission line tower footing resistance. HOW TO MODEL GROUNDING GRID WITH HORIZONTALLY CHANGING SOIL MODEL The present program, like almost all other software, assumes that the soil model (two layer model) is the same horizontally. Accurate modeling of soil models that are changing horizontally requires utilization of the finite element techniques. Using EDSA’s grounding program, one can approximate this complex case by assuming several grounding grids assemblies that are placed in different horizontal soil models. First, the grounding grid assembly resistance for one of the horizontal soil model grid is computed. Then, the next grounding grid assembly will be analyzed while specifying the Parallel Impedance for this assembly to be grid assembly resistance for the first grid model. Alternative method is to consider the soil model not to change horizontally and compute the resistance for the worst possible soil model.

HOW TO MODEL SYSTEMS WITH MULTIPLE GROUNDS These are situations where two or more ground grids that are not bonded together. Each grid is electrically connected to other only through conductive soil. Some examples are:  

Grounding Grid and buried parts of substation fences poles that are not bonded Grounding Grid in the vicinity of the buried pipes not bonded together

A simple example is shown below (see reference 12, pages 176-178).

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Figure 67: Multiple grounding systems

In the above example, the conductor shown on right is modeled as “Passive” and the conductor shown to the left is the “Main”. The jobfile for this case is “gbook5.2”. The results obtained with EDSA grounding program as compared to the reference 12 are shown in the following table:

GPR (Main) GPR(Passive) Total Resistance

EDSA

Reference 12

2116 173.8 28.29

2197 187.65 29.29

54

% Deviance 3.6 7.3 3.4


The text result for the above jobfile is also shown below. EDSA Advanced Gr ound Mat Pr ogr am v4. 80. 00 ============================================================ Pr oj ect No. : Pr oj ect Name: Ti t l e : Dr awi ng No. : Revi si on No. : J obf i l e Name: gbook5. 2

Dat e Ti me Company Engi neer

: : : :

System I nf or mat i on ============================================================ Subst at i on Name = Uni t Sys t e m = Met r i c Faul t Curr ent = 75. 00 ( Amp) Paral l el I mpedance = 10000. 00 ( ohm) F aul t Cur r ent Di vi s i on F ac t o r , Sf = 1. 00 Body Wei ght = 70. 00 ( Kg) Faul t Durat i on = 0. 50 ( second) Surf ace Mater i al Descri pti on = Cr ushed Rock Thi cknes s of Sur f ace Mat er i al = 0. 10 ( m) Resi st i vi t y of Sur f ace Mater i al = 3000. 00 ( ohm- m) Materi al Descri pt i on = Mater i al Resi st i vi t y = 200. 00 ( ohm- m) Al l owabl e Touch Vol t age = 931. 767 ( Vol t ) Al l owabl e Step Vol t age = 3060. 973 ( Vol t ) Mai n Gr ound Gr i d Axi s # 0001 0002 0003

X1

Y1

Z1

X2

Y2

Z2

Lengt h

1. 0 2. 0 9. 0

10. 0 10. 0 10. 0

0. 8 0. 8 0. 8

2. 0 9. 0 11. 0

10. 0 10. 0 10. 0

0. 8 0. 8 0. 8

1. 00 7. 00 2. 00


=

Di amet er 0. 0110 0. 0110 0. 0110

Cur r ent 8. 98 49. 39 16. 41

10. 0 met er

Passi ve Gr i d i s assumed t o be at t he same vol t age Axi s # 0004 0005 0006

X1

Y1

Z1

X2

Y2

Z2

Lengt h

16. 0 18. 0 25. 0

10. 0 10. 0 10. 0

0. 8 0. 8 0. 8

18. 0 25. 0 26. 0

10. 0 10. 0 10. 0

0. 8 0. 8 0. 8

2. 00 7. 00 1. 00


=

Gr ound Pot ent i al Ri se Gr oundi ng Gr i d & Rod Resi st ance Tot al I mpedance of I nst al l at i on Passi ve Gr i d Potent i al

= = = =

Di amet er 0. 0110 0. 0110 0. 0110

20. 0 met er

2116. 0 28. 2937 28. 2138 173. 80

Vol t s Ohms Ohms Vol t s

Figure 68: Report for Multiple Grounding

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Cur r ent - 0. 66 0. 38 0. 28


25. SUPPLIED EXAMPLES WITH THE SUBSTATION GROUNDING GRID DESIGN PROGRAM

The substation grounding grid design program package contains five examples. These example are as follows: Table 2: List of Supplied Examples (Jobfiles)

Example

Appearing on IEEE80-2000 Pages

Grid Resistance (EDSA)

Grid Resistance (IEEE80-2000)

ieee80-2000example-b1 ieee80-2000example-b2 ieee80-2000example-b3 ieee80-2000example-b4

129-134

2.67

2.78

Grid Resistance (EPRI quoted in IEEE80-2000) 2.67

135-137

2.52

2.75

2.52

137-139

2.29

2.62

2.28

139-142

2.34

2.74

2.34

The above grounding grid assembly resistances obtained from the EDSA program which case agrees 100% with the EPRI program quoted in the IEEE80-2000. However, as expected, the results of IEEE802000 can be as much as 20 % in deviance.

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26. SOIL MODEL IN EDSA’s ADVANCED GROUNDING GRID DESIGN PROGRAM EDSA grounding program supports up to three-layer soil structure. The three-layer model is shown below: Layer 1 (normally crushed rock), height ~ 6”, Resistivity=ρs

Layer 2: Resistivity=ρ1, Height=h

Layer 3: Resistivity=ρ2 , Height=

∞

In the above three-layer structure, normally the upper layer is a thin layer of crushed rock. A layer of crushed rock on surface helps in limiting the human body current by adding resistance to the equivalent body resistance. The value of ρs varies from 1000-5000 Ω -m. Therefore, the surface layer influence the maximum touch and step potential that an average adult can tolerate. Now the actual soil model starts just below the surface layer (layer 1) described above. If the soil is uniform, then, ρ1 = ρ2 and the height of layer 2 (h) has no influence on the result, therefore it can be set to any value (normally 100 m). If the soil is not uniform, then, it normally can be represented by so called two-layer soil model. When the measured apparent resistivity (for example in the “Wenner” method) is plotted against the probe spacing, it is often found to vary considerably as the spacing is changed. This is due to non-uniformity of the soil. In reality, the structure of soil may be very complex with vertical and/or horizontal layers and/or pockets of soil having differing resistivities. The EDSA advanced grounding program finds the best two-layer soil resistivity model by utilizing an optimization technique to fit a perfectly horizontal layered soil structure to a set of soil resistivity measurements that are carried out. It is therefore important to note that in EDSA grounding program a soil can have up to three layer structure and when soil resistivity measurements are provided, the program obtains the best possible two-layer soil model that minimizes the differences between computed and measured resistivity values.

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Ground Grid Design

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