auto grid pro

MultiGround+

TOUCH VOLTAGES ABOVE GROUND GRID

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HOW TO… Engineering Guide

A Simple Substation Grounding Grid Analysis

Using Autogrid Pro

2012 Release

Page iv

REVISION RECORD

Date Version Number Revision Level

January 2001 9 0

November 2002 10 0

June 2004 11 0

December 2006 13 0

January 2012 14 0

Address comments concerning this manual to:

Safe Engineering Services & technologies ltd.

___________________________________________

3055 Blvd. Des Oiseaux, Laval, Quebec, Canada, H7L 6E8

Tel.: (450) 622-5000 FAX:(450) 622-5053

Email: [email protected]

Web Site: www.sestech.com

Copyright 2000-2012 Safe Engineering Services & technologies ltd. All rights reserved.

SPECIAL NOTE

Due to the continuous evolution of the Autogrid Pro software, you may find that some

of the screens obtained using the present version of the Autogrid Pro package are

slightly different from those appearing in this manual. Furthermore, small differences

in the reported and plotted numerical values may exist due to continuous

enhancements of the computation algorithms.

TABLE OF CONTENTS

Page

Page vii

CCCHHHAAAPPPTTTEEERRR 111 INTRODUCTION ................................................................................................................. 1-1

1.1 OBJECTIVE ............................................................................................................................................... 1-1

1.2 GROUNDING PROBLEM .......................................................................................................................... 1-2

1.3 COMPUTER MODELLING TOOL ............................................................................................................. 1-2

1.4 METHODOLOGY OF THE GROUNDING DESIGN .................................................................................. 1-2

1.5 ORGANIZATION OF THE MANUAL ......................................................................................................... 1-3

1.6 SOFTWARE NOTE .................................................................................................................................... 1-4

1.7 FILE NAMING CONVENTIONS ................................................................................................................ 1-4

1.8 WORKING DIRECTORY ........................................................................................................................... 1-5

1.9 INPUT AND OUTPUT FILES USED IN TUTORIAL .................................................................................. 1-6

CCCHHHAAAPPPTTTEEERRR 222 DESCRIPTION OF THE PROBLEM & DEFINITION OF THE SYSTEM DATA ................. 2-1

2.1 THE SUBSTATION GROUNDING SYSTEM ............................................................................................ 2-1

2.2 THE OVERHEAD TRANSMISSION LINE NETWORK ............................................................................. 2-2

2.3 THE SUBSTATION TERMINALS .............................................................................................................. 2-3

2.4 THE SOIL CHARACTERISTICS ............................................................................................................... 2-3

CCCHHHAAAPPPTTTEEERRR 333 PROGRAM HIGHLIGHTS & USING AUTOGRID PRO ...................................................... 3-1

3.1 PROGRAM HIGHLIGHTS ......................................................................................................................... 3-1

3.2 USING AUTOGRID PRO ........................................................................................................................... 3-2

3.2.1 STARTING AUTOGRID PRO ....................................................................................................... 3-2

3.2.2 WORKING WITH PROJECTS AND SCENARIOS ....................................................................... 3-3

3.2.3 SPECIFYING DATA FOR A SCENARIO ...................................................................................... 3-4

3.2.4 PROCESSING A SCENARIO ....................................................................................................... 3-5

3.2.5 ADDING NEW SCENARIOS ......................................................................................................... 3-5

3.2.6 CLOSING A PROJECT ................................................................................................................. 3-6

3.2.7 ENDING YOUR AUTOGRID PRO SESSION ............................................................................... 3-6

CCCHHHAAAPPPTTTEEERRR 444 CREATING A PROJECT AND SCENARIO ....................................................................... 4-1

4.1 START-UP PROCEDURES ....................................................................................................................... 4-1

4.1.1 CREATING A NEW PROJECT ..................................................................................................... 4-4

4.1.2 OPENING AN EXISTING PROJECT ............................................................................................ 4-6

4.1.3 USING THE PROJECT ................................................................................................................. 4-6

4.1.4 FILES THAT ARE PART OF THE PROJECT ............................................................................... 4-7

TABLE OF CONTENTS (CONT’D)

Page

Page viii

CCCHHHAAAPPPTTTEEERRR 555 SOIL RESISTIVITY DATA ENTRY .................................................................................... 5-1

5.1 A HORIZONTAL TWO-LAYER SOIL MODEL ......................................................................................... 5-1

5.2 SOIL RESISTIVITY DATA ENTRY ........................................................................................................... 5-2

CCCHHHAAAPPPTTTEEERRR 666 INITIAL GROUNDING GRID DESIGN ............................................................................... 6-1

6.1 DATA ENTRY ............................................................................................................................................ 6-1

CCCHHHAAAPPPTTTEEERRR 777 FAULT CURRENT DISTRIBUTION ANALYSIS ................................................................ 7-1

7.1 INTRODUCTION ....................................................................................................................................... 7-1

7.2 PREPARATION OF THE INPUT DATA ................................................................................................... 7-2

7.2.1 DATA ENTRY ............................................................................................................................... 7-3

CCCHHHAAAPPPTTTEEERRR 888 PERFORMANCE EVALUATION OF EAST CENTRAL SUBSTATION ............................ 8-1

8.1 SAFETY CRITERIA ................................................................................................................................... 8-1

8.1.1 TOUCH VOLTAGES .................................................................................................................... 8-2

8.1.2 STEP VOLTAGES ........................................................................................................................ 8-2

8.1.3 GPR MAGNITUDE ....................................................................................................................... 8-3

8.1.4 GPR DIFFERENTIALS ................................................................................................................. 8-3

8.1.5 DETERMINING SAFE TOUCH AND STEP VOLTAGE LEVELS ................................................ 8-3

8.1.6 A SIMPLER WAY TO SPECIFY THE LOCATION OF OBSERVATION POINTS ....................... 8-5

8.2 PLOTS AND REPORTS ........................................................................................................................... 8-5

8.2.1 SELECTING PLOTS AND REPORTS ......................................................................................... 8-5

8.2.2 CUSTOMIZING PLOTS ................................................................................................................ 8-8

8.2.3 CARRYING OUT THE COMPUTATIONS AND PRODUCING THE PLOTS AND REPORTS .................................................................................................................................... 8-9

8.3 ANALYSIS OF THE RESULTS .............................................................................................................. 8-10

8.3.1 USING THE GRAREP UTILITY ................................................................................................. 8-10

8.3.2 GENERAL INFORMATION REPORTS ...................................................................................... 8-11

8.3.3 SOIL RESISTIVITY ANALYSIS .................................................................................................. 8-13

8.3.4 GROUND GRID PERFORMANCE AND SAFETY ANALYSIS .................................................. 8-15

8.3.5 COMPUTATION OF FAULT CURRENT DISTRIBUTION ......................................................... 8-21

CCCHHHAAAPPPTTTEEERRR 999 REINFORCING THE GROUNDING SYSTEM ................................................................... 9-1

9.1 EXPONENTIAL GRID DESIGN ................................................................................................................ 9-1

9.1.1 CREATING THE EXPONENTIAL GRID SCENARIO ................................................................... 9-1

TABLE OF CONTENTS (CONT’D)

Page

Page ix

9.1.2 OPENING THE EXPONENTIAL GRID SCENARIO ..................................................................... 9-2

9.1.3 MODIFYING THE EXPONENTIAL GRID SCENARIO ................................................................. 9-2

9.2 ADDING GROUND RODS ......................................................................................................................... 9-6

9.2.1 THE DETAILS ............................................................................................................................... 9-6

9.3 EXPORT GROUNDING GRID INTO DXF FILE ........................................................................................ 9-8

CCCHHHAAAPPPTTTEEERRR 111000 USING GRSERVER .......................................................................................................... 10-1

10.1 STARTING GRSERVER ............................................................................................................. 10-1

10.2 CREATING 3D PLOTS ............................................................................................................... 10-2

10.3 CREATING 2D PLOTS ............................................................................................................... 10-5

10.4 SAVING AND PRINTING PLOTS .............................................................................................. 10-5

10.5 SUMMARY .................................................................................................................................. 10-6

CCCHHHAAAPPPTTTEEERRR 111111 CONCLUSION .................................................................................................................. 11-1

Chapter 1. Introduction

Page 1-1

CCCHHHAAAPPPTTTEEERRR 111

INTRODUCTION

1.1 OBJECTIVE

This How To… Engineering Guide shows you how to carry out a typical substation grounding

design using the AutoGrid Pro software package. The AutoGrid Pro package combines the

computational power of the engineering modules RESAP, MALT and FCDIST of the CDEGS

software package with a simple, largely automated interface. The result is an-easy-to use, yet

powerful, grounding analysis program. A step-by-step approach is used to illustrate how to use the

program to input your data, carry out the computation and explore the computation results.

Please note that you may press the F1 key

at any time to display context-sensitive

on-line help pertinent to the topic to

which you have given focus with your

mouse. You may also access the complete

help file by selecting Contents from the

Help menu of the main AutoGrid Pro

interface.

If you are anxious to start entering data and running AutoGrid Pro you may do so by reading Section

1.5 of this chapter and skipping the rest of this chapter and Chapter 2. We strongly recommend,

however, that you refer to the skipped sections to clarify items related to input files, system

configuration and data, file sharing and design methodology.

Please call SES’ toll-free support line with any questions you may have, as you work through

this manual. Call us collect at +1-450-622-5000 if you do not have this number handy. You can

also E-mail us questions at [email protected].

mailto:[email protected]


Page 1-2

East CentralSubstation

GreenbayTerminal

HudsonTerminal

NewhavenTerminal

1.2 GROUNDING PROBLEM

The grounding analysis problem

discussed in this manual is

illustrated in Figure 1.1. A new

230 kV Substation (named East

Central) is planned. It will be

interconnected to the rest of the

network via three transmission

lines terminating at three different

substations, namely Terminals

Greenbay, Newhaven and Hudson

respectively. The objective of the

analysis is to provide a new grid design for East Central Substation. The final design is to limit touch

and step voltages to safe levels for personnel within the substation area, based on up-to-date system

data, appropriate measurement techniques and instrumentation, and state-of-the-art computer

modeling methods.

1.3 COMPUTER MODELLING TOOL

SES’ AutoGrid Pro is used to model the field measurements (i.e., soil resistivities and grounding

system impedance) and interpret the measured data, to compute the distribution of fault current

between the transmission line static wires, distribution line neutral wires, and the substation

grounding grid, and to simulate a representative phase-to-ground fault in the substation in order to

compute the ground potential rise and ground resistance, touch voltages, step voltages, and earth

potentials throughout the substation.

This software integrates all the tools required for such an analysis. It includes:

A soil resistivity analysis module to determine the soil structure from soil resistivity

measurements.

A fault current distribution analysis module to compute the fraction of the fault current that is

discharged in the grounding grid.

A grounding module that computes the response of the grounding grid to the fault current.

A safety analysis module that computes the touch and step potentials above the grid and

compares them to safety limits deduced from the relevant standards.

The results are presented in graphical and tabular form; several detailed reports are available.

1.4 METHODOLOGY OF THE GROUNDING DESIGN

A grounding design analysis is normally carried out in six major steps as follows:


Page 1-3

Step 1 The first step of the study is aimed at determining a soil model that is equivalent to the real

earth structure. This is done using the soil resistivity analysis module, RESAP. Any of

several soil type models can be selected by the design engineer as an approximation to the

real soil (uniform, two-layer, multilayer, etc.).

Step 2 Based on experience and on the substation ground bonding requirements, a preliminary

grounding system configuration is developed and a simulation is carried out (initial design).

Step 3 The configuration and characteristics of the transmission lines connecting this substation to

adjacent substations are defined. This allows the program to determine what fraction of the

total fault current actually flows into the grounding grid of the studied substation.

Step 4 The calculated results are analyzed and various computation plots and printout reports are

examined to determine if all design requirements are met. In particular, the safe touch and

step voltage thresholds are determined, based on the applicable standards and regulations,

and are compared to the computed values.

Step 5 If not all design requirements are met or if all these requirements are exceeded by a

considerable margin, suggesting possible significant savings, design modifications to the

grounding system or to the transmission line network are made and the design analysis is

restarted. This normally involves carrying out Step 2, then Steps 4 and 5.

Step 6 If seasonal soil resistivity variations must be taken into account, then the entire analysis is

repeated for every realistic soil scenario and the worst-case scenario is used to develop the

final design.

1.5 ORGANIZATION OF THE MANUAL

In accordance with the design methodology described above, the manual is organized as follows:

Chapter 2 outlines the problem being modeled and defines the system data required for the study.

Chapter 3 briefly introduces the components of the AutoGrid Pro program and also describes in

general how to work with Projects and Scenarios in AutoGrid Pro.

Chapter 4 shows how to get started with the program by creating a project and first scenario.

Chapter 5 describes the data entry for the soil measurements module (RESAP), which is used to

interpret the soil resistivity data based on measurements taken at East Central Substation (Step 1).

Chapter 6 presents the initial design of the grounding system. It describes in detail how to use

AutoGrid Pro to set up the initial design of the grounding grid at East Central Substation (Step 2).

Chapter 7 describes data entry for the fault current distribution module (FCDIST), which is used to

determine the fault current distribution (for the fault current simulations) between the transmission

line static wires, distribution line neutral wires, and the substation grounding grid (Step 3).

Chapter 8 presents the ANSI/IEEE safety criteria applicable to substation grounding. The fault

simulation results are presented in graphical and report formats. Grid potential (GPR), touch

voltages, and step voltages are provided in detail (Step 4).


Page 1-4

Chapter 9 presents the design of the reinforced grounding system. It describes how you can easily

repeat the computations from Chapters 6 and 7 to meet the safety criteria (Step 5).

Chapter 10 shows how to use the GRServer program to examine the computation results of AutoGrid

Pro in greater detail.

In Chapter 11, the conclusions of the study are summarized. Step 6 is not considered in this manual.

1.6 SOFTWARE NOTE

Depending upon your software license terms, some of the options described in this document may

not be available to you. When this is the case, a lock symbol will be displayed next to the

unavailable options in the user-interface screens.

1.7 FILE NAMING CONVENTIONS

It is important to know which input and output files are created by the CDEGS software. All CDEGS

input and output files have the following naming convention:

XY_JobID.Fnn

where XY is a two-letter abbreviation corresponding to the name of the program which created the

file or which will read the file as input. The JobID consists of string of characters and numbers that

is used to label all the files produced during a given CDEGS run. This helps identify the

corresponding input, computation, results and plot files. The nn are two digits used in the extension

to indicate the type of file.

The abbreviations used for the various CDEGS modules are as follows:

Application Abbreviation Application Abbreviation

RESAP RS FCDIST FC

MALT MT HIFREQ HI

MALZ MZ FFTSES FT

TRALIN TR SICL* SC

SPLITS SP CSIRPS* CS

SESTLC TC SESEnviroPlus TR

SESShield LS SESShield-3D SD

GRSPLITS-3D SP ROWCAD RC

* The SICL module is used internally by the Input Toolbox data entry interface. The CSIRPS

module is used internally by the Output Toolbox and GRServer – graphics and report

generating interface.

The following four types of files are often used and discussed when a user requests technical support

for the software:


Page 1-5

.F05 Command input file (for engineering applications programs). This is a text file that can

be opened by any text editor (WordPad or Notepad) and can be modified manually by

experienced users.

.F09 Computation results file (for engineering applications programs). This is a text file that

can be opened by any text editor (WordPad or Notepad).

.F21 Computation database file (for engineering applications programs). This is a binary file

that can only be loaded by the CDEGS software for reports and graphics display.

.F33 Computation database file (for engineering applications programs MALZ and HIFREQ

only). This is a binary file that stores the current distribution to recover.

For further details on CDEGS file naming conventions and JobID, please consult CDEGS Help

under Help | Contents | File Naming Conventions.

1.8 WORKING DIRECTORY

A Working Directory is a directory where all input and output files are created. In this tutorial, we

recommend the following Working Directory:


Page 1-6

C: (or D:)\Projects\AutoGrid Pro\

You may prefer to use a different working directory. Either way, you should take note of the full

path of your working directory before running AutoGridPro, as you will need this information to

follow this tutorial.

1.9 INPUT AND OUTPUT FILES USED IN TUTORIAL

There are two ways to use this tutorial: by following the instructions to enter all input data manually

or by loading the input files provided with the tutorial and simply following along.

All input files used in this tutorial are supplied on your DVD. These files are stored during the

software installation under documents\Howto\AutoGrid Pro (where documents is the SES software

documentation directory, e.g., C:\Users\Public\Documents\SES Software\version, and version is the

version number of your SES Software) Note that this folder is a distinct folder than the SES software

installation directory, e.g, C:\Program Files\SES Software\version (where version is, again, the

version number of your SES Software).

Copying Input Files to Working Directory

For those who prefer to load the input files into the software and simply follow the tutorial, you can

copy all of the files from the documents\Howto\AutoGrid Pro directory to your working directory.

After the tutorial has been completed, you may wish to explore the other How To… Engineering

manuals which are available as PDF files on the SES Software DVD in the folder \PDF\HowTo.

If the files required for this tutorial are missing or have been modified, you will need to manually

copy the originals from the SES Software DVD.

Both original input and output files can be found in the following directories on the SES Software

DVD:

Project Files: Examples\Official\HowTo\AutoGrid Pro

Note that the files found in project directory should be copied directly into the working directory.

Chapter 2. Description of the Problem & Definition of the System Data

Page 2-1


DESCRIPTION OF THE PROBLEM &

DEFINITION OF THE SYSTEM DATA

The system being modeled is located in an isolated area (i.e., not in an urban area and not close to

any pipelines), where there are no

major geological disturbances

(ocean, rivers, valleys, hills, etc.). It

consists of the following three major

components (see Figure 2.1):

1. The substation and

associated grounding system of the

substation under study;

2. An overhead transmission

line network;

3. Various substations

(terminals) from which power is fed

to the transmission line network.

Soil resistivity measurements have

been carried out at the substation site

under study and are available.

Figure 2.1 Schematic of System under Study

2.1 THE SUBSTATION GROUNDING SYSTEM

Figure 2.2 shows the configuration of the initial design of the East Central grounding grid, which

consists of a 100 m by 60 m (328 feet by 197 feet) rectangular grid buried at a depth of 0.5 m (1.64

feet). Each conductor has a radius of 0.6 cm (0.02 feet or 0.23"): these are 4/0 copper conductors.

There are 9 equally spaced conductors along the X axis and 7 equally spaced conductors along the Y

axis. The perimeter of the grid was defined such that the outermost conductors are located 1 m ( 3.3

ft) outside the edge of the fence to protect people standing outside the substation from excessive

touch voltages. The fence is regularly connected to the outermost conductors. The fence posts,

however, (which are metallic) have been omitted for simplicity. It is an easy task to add the fence

posts using the “Create Rods” tool in AutoGrid Pro, as explained later. The initial ground resistance

of the East Central Substation is 0.538 as will be determined by the MALT engineering module.

Greenbay

(R = 0.2 )

Terminal

g A

Hudson

(R = 0.3 )

Terminal

g A

Newhaven

(R = 0.3 )

Terminal

g A

East Central

(Re to be computed)Substation

230 kV

230 kV

230

kV

(6.8

mile

s or

11

km)


Page 2-2

(0,0,0.5)

60 m

100 m(100,60,0.5)

Figure 2.2 Initial Design of the Grounding System at the East Central Substation

Note that more complex grid shapes can easily be created: conductors may be modeled in any 3-

dimensional orientations.

2.2 THE OVERHEAD TRANSMISSION LINE NETWORK

There are three double-circuit transmission

lines leaving the East Central substation. The

average span length of the transmission lines is

330 m (1083'). The first transmission line is 64

spans long (21 km or 13 miles) and is

connected to the Greenbay substation

(terminal). Another transmission line is 33

spans long (11 km or 6.8 miles) and is

connected to the Newhaven substation. The

remaining transmission line is connected to the

Hudson substation and is 25 spans long (8.3 km

or 5.2 miles). Each tower has two 7 No. 8

Alumoweld type shield wires and the phase

wires are 795 MCM Drake. The GMR and the

average DC resistance of the shield wires are

0.00064 m (0.0021 feet) and 1.76 /km (2.83

/mile), respectively. Figure 2.3 shows a cross

section of the transmission line used in this

study.

The ground resistances of the transmission line

towers in the Greenbay - East Central arm of

the network, which are 330 m (1083') apart, are

all estimated to be equal to 10 . The towers in

the Hudson - East Central and Newhaven - East

Central arms, which are also 330 m apart, have

a higher estimated resistance of 28

Figure 2.3 Transmission Line Configuration

Y

X


Page 2-3

2.3 THE SUBSTATION TERMINALS

The ground resistances of the terminals are

equal to 0.2 , 0.3 and 0.3 for

Greenbay, Hudson and Newhaven

terminals, respectively. Figure 2.4 illustrates

a circuit diagram of the power system under

study during a phase-to-ground fault on

Phase B2 at East Central Substation.

Figure 2.4 Power System Network Analyzed in Example

In this study, we assume that the highest fault current discharged into the earth by the East Central

Substation grid occurs for a 230 kV single-phase-to-ground fault at East Central Substation on Phase

B2 of Circuit 21. Let us suppose that short-circuit calculations carried out by the power utility

provide the following fault current contributions from Phase B2 of each terminal substation for a

fault at East Central:

Greenbay: 1226 - j 5013 A

Hudson: 722 - j 6453 A

Newhaven: 745 - j 5679 A

2.4 THE SOIL CHARACTERISTICS

Detailed soil resistivity measurements have been carried out at the substation site, using the Wenner

4-pin technique (i.e., the distances between adjacent electrodes are equal). Table 2-1 gives the

apparent resistance values measured at the substation site. Note the exponentially increasing pin

spacings and extent of the largest spacings. This is of capital importance to achieve a reliable

grounding grid design. In fact, usually more than one set of measurements are made in different

directions and at different locations throughout the substation site, as well. Each set of measurements

is then interpreted independently.

1 Note that it is usually conservative to model a fault occurring on the phase furthest from the static wires, since this

results in the lowest current pulled away from the substation grounding grid by means of magnetic field induction

between the faulted phase and the static wires. Other scenarios can of course be investigated with the software.


Page 2-4

Separation Depth of Depth of Apparent

Between

Adjacent

Probes1

Current

Probes2

Potential

Probes2

Resistance

(V/I)

(meters) (meters) (meters) (ohms)

0.3 0.1 0.05 152.300

1 0.1 0.05 48.160

2 0.1 0.05 6.120

5 0.1 0.05 3.340

7 0.15 0.05 1.760

10 0.15 0.05 1.110

15 0.3 0.05 0.692

25 0.3 0.05 0.441

35 0.3 0.05 0.320

50 0.6 0.1 0.218

65 0.6 0.1 0.156

90 0.6 0.1 0.106

120 1 0.1 0.079

150 1 0.1 0.064

Table 2-1 Apparent Resistances Measured at Substation Site Using the Wenner Method

1 Also known as the “a” spacing associated with the Wenner technique.

2 These values are used to determine soil resistivities close to the surface with better accuracy. Knowing these values is

therefore important only for the first few pin spacings. At larger spacings, as a practical matter, the current probes should

be driven deeper in order to increase the strength of the signal measured between two potential probes.

Chapter 3. Program Highlights & Using Autogrid Pro

Page 3-1


PROGRAM HIGHLIGHTS & USING

AUTOGRID PRO

In this chapter, we will briefly describe the highlights and major functions of the program. A more

detailed description of the program’s capabilities will be given in the chapters that follow. The on-

line help provides further detailed descriptions about each module.

3.1 PROGRAM HIGHLIGHTS

With AutoGrid Pro, the data entry requirements are reduced to a minimum. The input data includes:

Soil resistivities: specify the measured resistivities or the soil layer resistivities and thicknesses

directly (if they are already known).

The grounding grid: use a grid creation wizard, import a preliminary design from a DXF file

created by a CAD package, or else draw the grid directly using the graphical tools of SESCAD

or combine these three methods.

Fault currents: directly specify the component of the fault current injected into the earth by the

grounding grid or let the program compute it based on the network specification. Use the

transmission line databases to quickly describe the network for this calculation.

Safety-related data: specify at what locations earth potentials (and therefore touch and step

voltages) should be computed or let the program decide automatically.

Desired reports and plots: select which reports and plots the program should generate, from an

extensive, predefined list.

Once the data has been entered, simply click Process and let the program do the rest. The program

will compute everything that is necessary, produce the requested reports and plots and display them.

AutoGrid Pro computes only what is needed. Changing the network configuration, for instance, can

affect the fault current component injected into the earth by the grounding grid and therefore the

safety-related quantities. Only these quantities will be recomputed before producing the output. On

the other hand, adding conductors to the grid can affect the grid’s impedance, the fault current

component injected into the earth and the earth potentials, in addition to the safety-related quantities.

All of these quantities are therefore recomputed before producing the output in such a case.


Page 3-2

3.2 USING AUTOGRID PRO

This section briefly describes what can be done with AutoGrid Pro and how to get started with

it. The sections that follow will give more details about the user interface of the program.

3.2.1 Starting AutoGrid Pro

To start the program, simply double-click the AutoGrid Pro icon in your SES Software Program

Group. See Chapter 4 for illustrations of this and the following steps. You will be presented with the

following screen. Users of other software from SES may recognize this screen, which is similar to

the SESCAD program interface. In fact, AutoGrid Pro inherits most of the functionality of SESCAD.

Do not worry if you are not familiar with the SESCAD program, however: this manual does not

assume any prior knowledge of SESCAD.

Figure 3.1 The Main Screen of Autogrid Pro and Some Auxiliary Screens.

The AutoGrid Pro screen also displays a Project toolbox, floating on the right-hand side of the

screen, which is not available in SESCAD. The Project menu item at the top of the screen also gives

access to this new functionality of AutoGrid Pro. (Note: this section will show how to use the

Project menu item to control the application; the same functionality is available most of the time,

from the Project Toolbox.)

The main screen is used to create, modify and view the grounding grid and acts as a controller for

the program. Several other screens are available, coordinated by the AutoGrid Pro Project Toolbox.


Page 3-3

3.2.2 Working with Projects and Scenarios

The design of new grounding systems or the enhancement of existing ones is often an iterative

process in which the design is modified and refined until the goals of the design engineer are

attained. In AutoGrid Pro, these alternative designs are known as Scenarios. A scenario in AutoGrid

Pro contains all of the input data necessary to specify a design, as well as the corresponding

computation results, plots and reports. A Project in AutoGrid Pro is simply a collection of related

scenarios.

Before anything can be done with AutoGrid Pro, a project must be created (or an existing one must

be opened).

To create a new project, select Project | New Project. You will be prompted for the location and

name of the project as well as for the name and location of the first scenario of this project. The new

project is created under the filename ‘Project Name’.agp and the scenario under the filename

‘Scenario Name’.ags where ‘Project Name’ and ‘Scenario Name’ are the names provided for the

project and scenario, respectively. You may drag and drop existing directory to new project and

scenario file location text-boxes. (Note: Experienced CDEGS users may wonder what are the JobID

and Working Directory for the scenario. The answer is that the ‘Scenario Name’ will be used as the

JobID and the selected location for the scenario will be used as working directory. While the concept

of JobID and Working Directory is no longer used in AutoGrid Pro, it may help to know that the

database and output files are still produced using the traditional conventions. For example, the

database file for Malt will be produced in the scenario directory under the name mt_‘Scenario

Name’.f21.)

To open an existing project, select Project | Open Project. This will bring up a file browser that

allows you to select an existing project file (with extension “AGP”). You also can drag and drop

existing project directory to Project File Location or File Name text-boxes. Note that a demo project

(called ‘Demo 1’) is available in the folder ‘SES Software\<Version>\Examples\Autogrid

Pro\Demo 1’ (where <Version> is the version number of your SES Software) in your SES Software

Document Folder directory, e.g., C:\Users\Public\Documents. It is also available on your SES

SOFTWARE DVD: \Examples\Standard\Autogrid Pro\Demo 1.

Only a single project can be opened at a given time. Therefore, if you attempt to create a new project

or open an existing one while a project is currently open, you will be prompted to save changes to

this last project and the project will be closed before opening the new one.

To save a project, select Project | Save Project or Project | Save Project As. Note that a backup of

the original file is created under the name “Backup of ‘Project Name’”.agp.

Note that you can easily see the contents of the folder containing your project files and of several

other important folders by clicking on Browse to Project Folder on the main toolbar.


Page 3-4

3.2.3 Specifying Data for a Scenario

When a project is open, you always have access to at least one scenario. The data in this scenario can

be edited in the following way.

To specify the soil structure or the soil resistivity measurement data, select Project | Define Soil

Characteristics. This brings up a dialog that allows you to define the structure of the soil

(number of layers, resistivities of the layers, etc…) if it is known or to specify resistivity

measurement data and have the program deduce the soil structure.

To specify the grounding grid and (optionally) the location of the computation points, use the

functionality of the Edit and Tools menus of the main interface. The dialog obtained from

Advanced | Network Energizations and Buried Structures is also useful to specify the fault

current directly (or other forms of grid energization) and to create other structures besides the

main grounding grid.

To specify the circuit and fault current distribution data, use Project | Define Circuit

Characteristics. The data entered in the resulting screens will allow the program to determine

how much current should be injected in the main grounding grid as a result of the fault. You can

use the computed value of the main grounding grid’s resistance or a user specified resistance for

the impedance of the central site of the circuit.

To specify the safety criteria to be used when analyzing the grounding grid, select Project |

Define Safety Criteria. The safety screens allow you to enter the threshold values for safety

when analyzing touch and step voltages as well as some parameters defining the region around

the main grid that should be assessed for safety.

To define which reports and plots the program should produce, use Project | Report

Preferences. The resulting screen offers a wide variety of reports and plots that can be produced

whenever the scenario is processed, including safety reports, touch and step voltage plots, etc…

To control the appearance of the plots, use Project | Graphics Preferences. This allows you to

specify colors, font types and size, etc… that are used when plotting.

To save the scenario, select Project | Save Scenario. Note that a backup of the original file is

created under the name “Backup of ‘Scenario Name’.ags”. To save the scenario under a different

filename, choose Project | Save Scenario As and select an appropriate filename (with the AGS

extension). Note that this will automatically change the scenario name.


Page 3-5

3.2.4 Processing a Scenario

Once the data for a scenario is specified, the grounding safety analysis can begin. To do this,

simply select Project | Process. The program will compute all necessary quantities in the

background, prepare the requested plots and reports and display them. Depending on the input

data entered in the scenario, the processing may include the following steps:

Saving of the scenario’s data

Computation of an appropriate soil structure from the measured soil resistivities

Determination of an appropriate safety zone where the analysis should be conducted

Computation of the main grid resistance

Determination of the distribution of the fault current throughout the network

Computation of the earth potentials and grid GPR at the fault site

Computation of the safety limits for touch and step voltages

Computation of touch and step voltages, and safety analysis

Production of reports and plots

Other optional steps may include ampacity assessment, etc…

When the processing begins, a window appears and displays messages regarding the progress of the

computations.

3.2.5 Adding New Scenarios

Once the processing is complete, the results can be reviewed to determine if the design is

satisfactory. If not, the design can be modified and the above steps repeated. There are two ways to

modify the design: you can modify the existing scenario directly, in which case the original data is

lost, or you can create a new scenario and modify that one.

To create a new scenario, select Project | New Scenario. You will be prompted to provide a name

for the new scenario as well as (optionally) the name of an existing scenario to be used as a

reference. When a valid reference scenario is provided, the program creates a copy of that scenario

under the new name. This is convenient when you want to examine small design variations from one

scenario to the next.

You can also open one of the project’s existing scenarios by choosing Project | Open Scenario. You

will be presented with a list of the scenarios that are presently in the project, from which you can

select the desired one.


Page 3-6

3.2.6 Closing a Project

To close a project, select Project | Close Project. This will close the current project, but not the

program itself. Closing the window containing the drawing of the main grid is also interpreted by

AutoGrid Pro as a signal to close the project.

When the project is closed, you can still use AutoGrid Pro much as you would SESCAD, i.e., the

project functionality is disabled but everything else is available. Use Project | Open Project to open

another project.

The most recently used projects are listed at the end of the Project menu and in the Open Project

dialog, for quick access.

3.2.7 Ending Your AutoGrid Pro Session

To quit the application and terminate the AutoGrid Pro session, use File | Exit. The program will

optionally prompt for those files that need saving before terminating.

Chapter 4. Creating a Project and Scenario

Page 4-1


CREATING A PROJECT AND SCENARIO

In this chapter, we will describe in detail how to get started by creating a new project which contains

a first scenario.

4.1 START-UP PROCEDURES

In the SES Software <Version> group folder, where <Version> is the version number of the

software, you should see the icons representing Autogrid Pro, AutoGroundDesign, CDEGS,

Right-of-Way, SESEnviroPlus, SESShield-3D and SESTLC software packages, as well as four

folders. The Documentation folder contains help documents for various utilities and software

packages. The Program Folders provides shortcuts to programs, installation and projects folders.

The System folder allows you to conveniently set up security keys. Various utilities can be found in

the Tools folder. The main function of each software package and utility is described hereafter.

SOFTWARE PACKAGES

Autogrid Pro provides a simple, integrated environment for carrying out detailed grounding

studies. This package combines the computational powers of the engineering programs RESAP,

MALT and FCDIST with a simple, largely automated interface.

AutoGroundDesign offers powerful and intelligent functions that help electrical engineers

design safe grounding installations quickly and efficiently. The time devoted to design a safe and

also cost-effective grounding grid is minimized by the use of automation techniques and

Click here


Page 4-2

appropriate databases. This module can help reduce considerably the time needed to complete a

grounding design.

Right-of-Way is a powerful integrated software package for the analysis of electromagnetic

interference between electric power lines and adjacent installations such as pipelines and

communication lines. It is especially designed to simplify and to automate the modeling of

complex right-of-way configurations. The Right-of-Way interface runs the TRALIN and SPLITS

engineering modules and several other related components in the background.

SESEnviroPlus is a sophisticated program that evaluates the environmental impact (radio

interference, audio-noise, corona losses, and electromagnetic fields) of AC, DC or mixed

transmission line systems.

SESShield-3D is a powerful graphical program for the design and analysis of protective

measures against lightning for substations and electrical networks. Its 3D graphical environment

can be used to model accurately systems with complex geometries.

SESTLC is a simplified analysis tool useful to quickly estimate the inductive and conductive

electromagnetic interference levels on metallic utility paths such as pipelines and railways

located close to electric lines (and not necessary parallel to them), as well as the magnetic and

electric fields of arbitrary configurations of parallel transmission and distribution lines. It can

also compute line parameters.

CDEGS is a powerful set of integrated engineering software tools designed to accurately analyze

problems involving grounding, electromagnetic fields, electromagnetic interference including

AC/DC interference mitigation studies and various aspects of cathodic protection and anode bed

analysis with a global perspective, starting literally from the ground up. It consists of eight

engineering modules: RESAP, MALT, MALZ, SPLITS, TRALIN, HIFREQ, FCDIST and

FFTSES. This is the primary interface used to enter data, run computations, and examine results

for all software packages other than Right-of-Way, Autogrid Pro, AutoGroundDesign, SESTLC,

SESShield-3D and SESEnviroPlus. This interface also provides access to the utilities listed

below.

TOOLS

AutoTransient automates the process required to carry out a transient analysis with the HIFREQ

and FFTSES modules

CETU simplifies the transfer of Right-of-Way and SPLITS output data to MALZ. A typical

application is the calculation of conductive interference levels in an AC interference study.

FFT21Data extracts data directly from FFTSES’ output database files (File21) in a spreadsheet-

compatible format or in a format recognized by the SESPLOT utility.

GraRep is a program that displays and prints graphics or text files. For more information on

GraRep see Chapter 6 of the Utilities Manual or invoke the Windows Help item from the menu

bar.


Page 4-3

GRServer is an advanced output processor which displays, plots, prints, and modifies

configuration and computation results obtained during previous and current CDEGS sessions.

GRSplits plots the circuit models entered in SPLITS or FCDIST input files. This program

greatly simplifies the task of manipulating, visualizing and checking the components of a

SPLITS or FCDIST circuit.

GRSplits-3D is a powerful interactive 3D graphical environment that allows you to view and

edit the circuit data contained in SPLITS input files and to simultaneously visualize the

computation results.

ROWCAD is a graphical user interface for the visualization and specification of the geometrical

data of Right-of-Way projects. Its 3D graphical environment can be used to visualize, specify

and edit the path data of Right-of-Way, and to define the electrical properties of those paths.

SESAmpacity computes the ampacity, the temperature rise or the minimum size of a bare buried

conductor during a fault. It also computes the temperature of bare overhead conductors for a

given current or the current corresponding to a given temperature, accounting for environmental

conditions.

SESBat is a utility that allows you to submit several CDEGS engineering program runs at once.

The programs can be run with different JobIDs and from different Working Directories.

SESCad is a CAD program which allows you to create, modify, and view complex grounding

networks and aboveground metallic structures, in these dimensions. It is a graphical utility for the

development of conductor networks in MALT, MALZ and HIFREQ.

SESConductorDatabase gives access to the SES Conductor Database. It allows you to view the

electrical properties of conductors in the database, and to add new conductors to the database or

modify their properties.

SESEnviroPlot is a graphical display tool is an intuitive Windows application that dynamically

displays arrays of computation data produced by the SESEnviro software module.

SESGSE rapidly computes the ground resistances of simple grounding systems, such as ground

rods, horizontal wires, plates, rings, etc, in uniform soils. SESGSE also estimates the required

size of such grounding systems to achieve a given ground resistance.

SESPlot provides simple plots from data read from a text file.

SESScript is a simple programming language that automatically generates input files for

parametric analyses.

SESShield provides optimum solutions for the protection of transmission lines and substations

against direct lightning strikes and optimizes the location and configuration of shield wires and

masts in order to prevent the exposure of energized conductors, busses and equipment. It can also

perform risk assessment calculations associated with lightning strikes on various structures.


Page 4-4

SESSystemViewer is a powerful 3D graphics rendition software that allows you to visualize the

complete system including the entire network and surrounding soil structure. Furthermore,

computation results are displayed right on the system components.

SoilModelManager is a software tool that automates the selection of soil model structures that

apply during various seasons.

SoilTransfer utility allows you to transfer the soil model found in several SES files into several

MALT, MALZ or HIFREQ input (F05) files.

TransposIT is a tool for the analysis of line transpositions on coupled electric power line

circuits. To ensure that voltage unbalance is kept within predefined limits, it allows the user to

determine the optimal number of power line transpositions and their required locations.

WMFPrint displays and prints WMF files (Windows Metafiles) generated by CDEGS or any

other software.

The Open Project dialog screen shown below is the starting point of every AutoGrid Pro session.

As mentioned in Chapter 3, a project must be created (or an existing one must be opened), before

anything can be done with AutoGrid Pro. The Open Project dialog allows you to open an existing

project (either from a list of recently used projects, by browsing to an AGP project file, or by

dragging directory to Project File Location text-box) or to create a new project. In this case, the

name and path of the project should be provided, as well as the name and path of its first scenario.

The program suggests default values for all these fields.

At this time, a new project should be created. If you prefer to load the finished project and follow

along, please follow the instructions of Section 4.1.2 instead.

4.1.1 Creating a New Project

Click on the New tab to request a new

project workspace. You will first see

the following screen in which a Project

Name Project1 and a Scenario Name

Scenario1 is automatically assigned by

the program.

In this tutorial, we will create a project

called AGP Tutorial under the folder

D:\Projects\AGP Tutorial (you can use

any existing folder on your PC, or

create a new one). We will first change

the default Project File Location to

D:\Projects, and then we enter AGP

Tutorial in the Project Name field. A sub-folder called AGP Tutorial under D:\Projects is

automatically offered while you are typing the project name AGP Tutorial. You can manually

rename the project folder name (under Project File Location) if you wish this name to be different


Page 4-5

from the Project Name. However, to keep things simple, it is usually recommended to keep the

same name for the Project File Location and the Project Name.

To create a scenario called Initial Design,

enter Initial Design under Scenario Name.

Again, a sub-folder called Initial Design

under D:\Projects\AGP Tutorial is

automatically offered while you are typing

the scenario name Initial Design. It is also

recommended to keep the same name for

the Scenario File Location and the

Scenario Name.

Click the Create button. The AGP Tutorial

project and the Initial Design scenario are

created and ready for use. You can now

proceed to Section 4.1.3.


Page 4-6

4.1.2 Opening an Existing Project

Click on the Existing tab to browse for an existing project file. Navigate to the AGP Tutorial folder

under your Project folder, then double-click the file AGP Tutorial.agp. This will load the project.

4.1.3 Using the Project

The buttons on the Project Toolbox are now active for you to enter data. The AutoGrid Pro project

toolbox acts as a quick launch pad for the other data entry screens of the application.

Project: allows you to open new or existing projects and scenarios: analogous to the File | Open

and File | New commands in Microsoft® Applications.

Reports: Allows you to select which reports and plots you wish to generate.

Setup: Customizes the appearance of plots.

Wizard: Loads the AutoGrid Pro Wizard that guides you through a typical session (not yet

available).

Settings: Specify your general personal preferences here.

Soil: Enter soil description or measurements.

Grid: Enter grid data that you have not specified graphically.

Circuit: Specify the power lines connected to the substation if you wish to have the program

calculate the split of fault current between the grounding grid and earth return conductors such as


Page 4-7

static wires and neutral conductors. Note that conductive earth return wires can decrease fault

current injected into the earth by the grid by 50% or more.

Safety: Specify the criteria to be used in the safety analysis.

GRServer: Start the GRServer program (an advanced graphics processor program) to display

graphically the results of a scenario in greater detail.

Process: Initiate the computations, which end with the production of all requested plots and

reports.

4.1.4 Files that Are Part of the Project

When you create a new project, several files and folders are automatically created on your hard disk.

The folders and files created in the previous sections can be viewed in the following Windows

Explorer screen.

The file AGP Tutorial.agp is a project file for the AGP Tutorial project. It contains the information

regarding all the scenarios defined under this project. Under the scenario subfolder Initial Design,

you will find a file Initial Design.ags, and two other files MT_Initial Design.F05 and FC_Initial

Design.F05. These files were created the moment the scenario Initial Design was created. The file

Initial Design.ags stores the data for the Scenario Initial Design. For those who have used CDEGS

software before, you may recognize that the two files MT_Initial Design.F05 and FC_Initial

Design.F05 are the input files for the MALT and FCDIST programs, respectively.

With a project and scenario defined, we are now ready to enter soil resistivity data (Chapter 5),

define the initial design of the grounding grid (Chapter 6), prepare the fault current data (Chapter 7),

evaluate the performance of the initial grid design (Chapter 8), and make final refinements to the

design.

Chapter 5. Soil Resistivity Data Entry

Page 5-1


SOIL RESISTIVITY DATA ENTRY

5.1 A HORIZONTAL TWO-LAYER SOIL MODEL

The data values listed in Table 2-1 at the end of Chapter 2 were entered as input to the soil resistivity

analysis module of the AutoGrid Pro package. This consists of the following information:

Spacing Between Probes: The distance between adjacent measurement probes.

Apparent Resistance (V/I): The apparent resistance measured at each probe spacing.

Current Probe Depth: The depth to which the current injection electrodes were driven

into the earth. This value influences the interpretation of soil resistivities at short

electrode spacings. It is an optional field data.

Potential Probe Depth: The depth to which the potential probes were driven into the

earth. This value also influences the interpretation of soil resistivities at short electrode

spacings. It is an optional field data.

The soil resistivity interpretation module RESAP is used to determine equivalent horizontally

layered soils based on the site measurements. Although RESAP is capable of producing multi-

layered soil models, it is preferable to try to fit the measured results to the simplest soil structure

(i.e., a two-layer model), at least initially. This minimizes the time required for the computations.

When a two-layer soil model is selected, the computation results lead to an equivalent two-layer soil

structure such as the one shown in Table 5-1. The “RMS Error” computed by RESAP (see Section

8.3.3) provides a quantitative indication of the agreement between the measurements and the

proposed soil model. The grounding system resistance computed by the grounding module MALT

(see Sections Chapter 6 and 8.3.4) is also shown. Note that the resistance shown here was computed

for the initial design of the grounding system of the East Central Substation.

Layer Resistivity

(-m)

Thickness

(Meters)

Top

Bottom

297.08

65.85

0.67

Table 5-1 Two-Layer Soil Model Computed Using Data from Table 2-1

RMS Error: 16.95%

Grid Impedance: 0.538

The following section describes the steps required to determine the soil model shown in Table 5-1.


Page 5-2

5.2 SOIL RESISTIVITY DATA ENTRY

Click the Soil button located in the

Project Toolbar to define the soil

model. The Soil Structure screen will

appear, without data and you are now

ready to enter it.

The soil model can be defined by

specifying measured resistivity data

(default setting shown above), in which

case the program will compute an

appropriate soil structure, or by

specifying the soil structure explicitly.

When the soil structure is specified

explicitly by selecting the Use Specified

Soil Structure Characteristics option at

the very top of the screen, the following

data entry screen is shown; use it to enter

details of the soil model that is desired.

The earth structures that can be analyzed include:

Uniform soil model (default)

Horizontally layered soil (any

number of layers)

Vertically layered soil (two or

three layers)

Hemispherical soil model: three

regions delimited by two concentric

hemispherical boundaries

Cylindrical soil model: two

regions delimited by a vertical or

horizontal cylindrical boundary

Arbitrary heterogeneity soil

model: any number of regions of

any shape formed by 6 surfaces and

8 vertices

In this tutorial, the soil model will

be deduced based on the soil

resistivity measurements presented in Section 2.4.

In the following, it will be assumed that the reader is entering the data as indicated in the

instructions. Note that it is advisable to save your work regularly by selecting Project | Save


Page 5-3

Project, or by pressing the Ctrl + S key combination as a shortcut. The data, entered up to that

point, will be saved in the RESAP input file RS_Initial Design.F05 in the “Initial Design” folder, in

addition to the project and scenario files.

If you intend to enter the data manually, proceed with this section; otherwise, you can import all the

data by proceeding as follows:

Importing Data

In the Soil Structure screen, click the Import button. Select the file “\Files For Import\RS_Initial

Design.F05” in the dialog, and then click OK. The data described in the next section will be loaded

and you will not have to enter it yourself.

In the Soil Structure screen, enter

the measured apparent resistances

at the substation site (see Table

2-1). This screen also allows you to

specify the Measurement Method

used to gather the data (the default

setting is Wenner), the Type of

data recorded, and the field

measurement data obtained. Click

the radio buttons General, Wenner

and Schlumberger to understand

the differences between them. You

can immediately plot the raw data

in a linear/linear, log/linear or

log/log fashion to examine the

shape of the curve and spot

irregularities in the measurements.

Click the Properties button to bring

up the screen shown in the

following page. This screen allows you to provide comments under Case Description and to select

the System of Units. You can provide comments that apply to the entire project (the Project Level

comments), to the current scenario (the Scenario Level comments), or to any individual module. In

this case, we enter a description of the soil resistivity analysis under Measured Soil. These

comments are echoed in the output file of the RESAP program.

You can enter comments for the other modules of the software, if you wish to, by clicking on the

buttons of the top of the screen; we will do this later, in other parts of the tutorial. (Here, enter “A

simple substation grounding grid analysis using AutoGrid Pro.” in the Project Level comment and

“Initial Design: a linearly spaced grid, without rods.” in the Scenario Level comment.)

A Run-ID Initial Design is automatically entered in the Run-Identification data entry field. The

Run-ID is useful in identifying all the plots which will be made later in Section 8.3.3.


Page 5-4

In this tutorial, the Metric System of Units is used.

The system of units selected here applies to the entire

scenario, and all the computation modules. When

changing the units, you have the option to convert the

data to the new system of units or to leave the data as

is. By default, the data is left as is. To convert the data,

click on “Data Conversion Options”, and follow the

instructions in the resulting screen.

Focusing on any field in this screen (e.g., by clicking

on the field or by clicking on a screen button, without

releasing the mouse button, then dragging the mouse

off the screen button, then releasing the mouse button),

then pressing the F1 key will bring a help text related to

the focused field, or to the screen as a whole. This is

true of all screens in AutoGrid Pro.

Click the OK button to return to the Soil Structure

main screen. By default, the RESAP program will

provide a soil model which is the best fit to your data.

If you wish to see how sensitive the computed resistivity

curve is to changes in soil layer thicknesses or resistivities

then click on the Advanced button and suggest your own

soil model. Select the User-Defined option in the resulting

screen if you wish to specify the number of layers and,

optionally, the characteristics of selected layers. In this

tutorial, we select the two-layer soil model.

You can actually Lock or Unlock the resistivity and the

depth of each layer. Note that when the soil characteristics

of one layer (resistivity or depth or both) are locked, these

values will not be altered during the least-square iterative

minimization process. By leaving the Locking Options

column blank (or setting it to Unlock), you are leaving the

program free to determine suitable values for the associated

soil layer characteristics. If you prefer to specify your own

values, you should use the Lock option, then select which

item(s) to lock under Lock/Unlock Item.

In this tutorial, we will select a User-Defined soil with 2

layers, and let the program determine the properties of the layers automatically. Click the OK button

to return to the Soil Structure main screen, then click OK in that screen to complete the data entry

for the soil structure specification.

Chapter 6. Initial Grounding Grid Design

Page 6-1


INITIAL GROUNDING GRID DESIGN

In this chapter, we will show how to create a detailed computer model of a grounding system.

The determination of the grounding grid performance is carried out by the MALT engineering

program, which computes the grounding grid resistance, ground potential rise, earth potentials, and

thereby touch and step voltages. In fact, you can model several distinct grounding systems at the

same time, each energized with a different current or voltage: MALT will determine how they all

influence one another, allowing you to determine transferred potentials, touch voltages and step

voltages at any location. Each grounding grid (or “electrode”) consists of a group of cylindrical

conductors with any orientations and positions, although they must all be buried. All conductors you

identify as belonging to given grounding grid are automatically interconnected for you (by means of

invisible cables) and all conductors are assumed to have negligible longitudinal impedance - a fair

assumption for typical substation. As a rule of thumb, a computed ground resistance less than 0.5

indicates a possible need for modeling by other software which does account for conductor

impedance, such as SES’ MALZ software module.

At least one electrode (called “MAIN”) must be defined. It is normally used to model the main

grounding grid of the system under study (most studies only do examine a single grid). Other

electrodes (called “RETURN Ground” and “BURIED Structures”) can be defined; although they can

be used in many different ways, they are typically used to model a return electrode that collects all

the current injected in the main, (e.g., when simulating a ground impedance field test) and passive

buried structures (such as pipes or floating fences not connected to the main substation grid) which

are within the zone of influence of the main grounding grid.

In this tutorial, we will only need to define a MAIN electrode. It will be used to model the main

grounding grid at East Central Substation. For the initial design, a 100 m by 60 m grid will be

studied.

6.1 DATA ENTRY

As for the soil resistivity data entry, you can enter the data manually by following the steps

described in this section; or if you do not wish to do so, you can import all the data required for this

tutorial by proceeding as follows:

Importing Data

Select Project | Import File and select the file “\Files For Import\MT_Initial Design.F05”. The

data described in the next section will be loaded and you will not have to enter it.


Page 6-2

While most of the data entry regarding

the grid will be carried out using the

graphical tools available from the main

screen of AutoGrid Pro, some extra

data can be specified in the screen

below. For those who have used

SESCAD before, you have probably

already noticed that the graphical tools

are simply the SESCAD program of

the CDEGS package. Chapter 10 of

Utilities Manual is devoted to

describing in detail how to use

SESCAD. This manual is available in

the PDF folder on your SES

SOFTWARE DVD under the filename

Utilities.PDF. You are strongly

encouraged to read Chapter 10 of this

manual, in order to learn how to

unleash the full power of this graphical

interface.

We will begin by defining some properties of the main grid. Select Project | Define Grounding

System Energization and Buried Structures to load the Grounding System Energization screen.

The data to be provided on this screen consists mainly of the current or voltage magnitude to be

impressed upon each grounding system modeled. It is also used to specify the presence of buried

metallic structures other than the MAIN grounding grid. Note that there is no fundamental difference

between the MAIN grounding grid, the RETURN ground, and any Buried Structures you may wish

to model. The only real differences are that the MAIN grounding grid must be defined, whereas the

other buried system are optional; furthermore this MAIN grounding grid must be energized by a

non-zero voltage or current, whereas the other buried systems, if they exist, may be either energized

or left floating.

By default, the option Use Value Calculated by the Fault Current Distribution Module (if

available) is selected. This instructs the program to use the fault current calculated in the Fault

Current Analysis module as the energization current for the grid. If the fault current distribution

calculation is not required, you can enter the magnitude of the fault current component to be injected

into the earth by the grounding grid: do this under the option Use Specified Value. The value

specified in the Magnitude field will also be used when the calculated value of the fault current is

not available yet (usually because the data specification for the fault current distribution module is

incomplete).


Page 6-3

Click the Properties button to bring up the screen

shown below. This screen allows you to provide

comments under Case Description and to select the

System of Units, as was done in the Soil Resistivity

Data Entry in Section 0.

The comments are echoed in the output file of the

MALT program. Again, a Run-ID Initial Design is

automatically entered in the Run-Identification field

and the Metric System of Units is selected. Click the

OK button, then the OK button to return to the Auto

Grid Pro main screen.

Next, we complete our description of the system under

study with the graphical tools of AutoGrid Pro: these

will allow us to rapidly describe the grounding grid and

the points at which earth potentials (and therefore touch

and step voltages) are to be computed. In our initial

design shown in Figure 2.2, we require a 100 m x 60 m

(328 feet x 197 feet) rectangular grid, buried at a depth

of 0.5 m (1.64 feet), and made of 9 linearly spaced

conductors parallel to the y-axis and 7 linearly spaced conductors parallel to the x-axis. Each

conductor has a radius of 0.6 cm (0.24 inches) and will be subdivided into 10 sub conductors or

“segments” for improved accuracy of the results (see below for further details on conductor

segmentation). The origin of the coordinate system used to specify the grounding grid is chosen to be

at the bottom left-hand corner of the grid.

To enter this data, select Create Object from the Edit menu. The Create Object dialog allows you

to define Conductors as well as observation Profiles (or observation points; more about those later).

Select the Detailed Grid option under Conductors and enter the coordinates of three corners of the

grid: a, b and c (as identified in the figure below). Nab indicates the total number of conductors

parallel to the x-axis, and Nac indicates the total number of conductors parallel to the y-axis. Note

that you should specify the Z coordinate as positive, to indicate that they are buried. This is generally

true in AutoGrid Pro, which considers the positive Z direction to be going down into the earth.


Page 6-4

Conductor Subdivision

As with many computer models of physical systems, the theory behind the MALT

program requires a discrete representation of a continuous phenomenon, namely the

distribution of the current discharged to earth by the grid’s conductors. The assumption

made by the program is that every conductor segment discharges current uniformly along

its length. In order for this to represent reality accurately, the conductor segments must

be small enough.

There are several ways to generate conductor segments from the specified grid

conductors. First, the program automatically breaks all conductors at every conductor

intersection. This is called the node subdivision process. Usually, this is already enough

to guarantee accurate results, so that further intervention is unnecessary. A second,

simple way to generate the conductor segments is to enter the total number of segments

to be generated by the program in the Desired Number of Segments field of the

Grounding System Energization screen. If the total number of segments obtained after

the node subdivision is smaller than the desired number of segments, the program will

break the longer conductors into two equal length pieces, until the desired number of

segments is reached. Another way is to proceed as above, by specifying explicitly the

number of segments desired for each conductor individually. This method gives a very

fine control over the segmentation process.

Note also that it is often a good idea to do two sets of computations, the second one using

a larger number of segments (but otherwise identical to the first). The results should not

change by more than a few percent. This verifies explicitly if the number of segments

used in the computations is adequate.

Click the Characteristics button and assign a radius of 0.006 m (0.019 ft) to all conductors. The

value in Subdivision # specifies a minimum segmentation number for all the conductors. In this

tutorial, we leave the value of this field at 1 since the conductor segmentation generated by the node

subdivision feature is already adequate. This is explained in greater detail in the “Conductor

Subdivision” inset (see below).

The grid is now created. Click on Apply to transfer the grid to the main drawing window, then click

on Close to close the Create Object window. Note that, for simplicity, we have defined a grid with

uniformly spaced conductors in this example. However, in many cases, grids with uniformly spaced

conductors are not as efficient as grids with conductors more closely spaced towards the edge of the

grid than at its center as will be demonstrated later in this tutorial.

To examine the touch and step voltages in and around the substation, the earth potentials should be

computed at observation points covering an area extending about 3 meters outside the substation. As

will be shown in the next section, you can let the program determine the location of suitable

observation points. On the other hand, for finer control you can also specify the observation points

explicitly. This is what we will do here.


Page 6-5

We will define a profile containing evenly spaced points and replicate this profile using the surface

entry fields. This will produce a grid of observation points at the surface of the earth above the

grounding system. Note that if you have already imported the data from a file, the computation

profiles are already specified in the Auto Grid Pro screen. Avoid generating a duplicate set of

profiles.

As for the grounding grid, the observation points can be defined using the Create Object command

under the Edit menu. Select the Detailed Surface option in the Create Object screen and enter the

data as shown in the screen below, then click on Apply and Close. This defines a rectangular surface

of observation points centered above the grid. The points are evenly spaced, being 1 meter apart both

along the x and y axes.

The data is specified by defining an observation profile, that is a linear group of NPoints evenly

spaced observation points, then by replicating this profile NProfiles times along the direction

defined in Profile Step. The original profile starts at (-3, -3, 0) and the profile points are separated

by 1 meter along the X axis.

Note that the values of NPoints and NProfiles always include the starting point and profile,

respectively. With a total of 107 points per profile and 67 profiles, the observation surface extends

from x = -3 m to x = + 103 m and from y = -3 m to y = 63 m, and therefore extends past the

perimeter of the grid by 3 meters.

Once the profiles are created, they are superposed on top of the grid already defined, which provides

a convenient way to check the positions of the observation points with respect to the grid. You can

also turn off the display of the observation points by unchecking the Profiles options in the View

menu.


Page 6-6

At this point, you have completed the data entry for the grid specification.

Chapter 7. Fault Current Distribution Analysis

Page 7-1


FAULT CURRENT DISTRIBUTION ANALYSIS

7.1 INTRODUCTION

The touch and step voltages associated with the grounding network are directly proportional to the

magnitude of the fault current component discharged into the soil by the grounding network1. It is

therefore important to determine how much of the fault current returns to remote sources or external

grounding via the shield wires and neutral wires of the transmission lines and distribution lines

connected to the substation under study, in this case, East Central Substation. In other words, the

current discharged into the East Central Substation grounding system is smaller than the maximum

available fault current, because a portion of the fault current returns via the shield wires and neutral

wires of the power lines connected to the East Central Substation and local transformer contributions

are disregarded. In order to be able to determine the actual fault current split, a model of the

overhead transmission line network (and, when present, distribution neutrals and associated

grounding) must be built. Before this, however, it is necessary to calculate transmission and

distribution line parameters such as self and mutual inductive impedances, at representative

locations.

This work is described in the present chapter. In this study, we assume a single-phase-to-ground

fault. The engineering module FCDIST is used to compute the fault current distribution. For more

complicated fault scenarios, the engineering modules TRALIN and SPLITS of the CDEGS package

can be combined to complete the task: in this case, the line parameters are computed using the

TRALIN module, then the resulting parameters are used by the SPLITS module to compute the fault

current distribution. The How To… Engineering Guide entitled “Analysis of AC Interference

Between Transmission Lines and Pipelines” gives a detailed example on how to use TRALIN and

SPLITS to compute the fault current distribution.

As mentioned in Chapter 2, we assume that the highest fault current discharged by the East Central

grounding grid occurs for a 230 kV single-phase-to-ground fault at the East Central Substation on

Phase B2 of Circuit 2. Note that if autotransformers are involved it becomes particularly important to

examine the currents flowing into the substation in all phases of all circuits (at all voltage levels) for

the 230 kV fault in order to correctly assess the situation.

1 Strictly speaking, circulating currents flowing in grounding grid conductors from the fault location to local transformer

ground connections and to static and neutral wire ground connections also contribute to touch voltages, particularly in

large grounding grid in low resistivity soils. For typical substation applications, however, this component is relatively

small.


Page 7-2

7.2 PREPARATION OF THE INPUT DATA

The fault current distribution is computed using the FCDIST (Fault Current Distribution)

engineering module. The goal of this analysis is to find the fraction of the total fault current that is

discharged in the grounding grid under study. The important data for such an analysis consists of:

The impedance of the grounding grid under study. In the program, this grounding grid is

referred to as the Central Site.

The fault current sources: these are called Terminals in the program. The data to be specified

includes the magnitude and phase angle of the contribution of each terminal to the fault

current, as well as the impedance of the grounding grid at each terminal.

The electrical characteristics of the transmission lines connecting the Terminals to the

Central Site. This normally includes the geometrical configuration of the faulted phase

conductors and of the shield wires as well as the type of shield wires used; alternatively, the

line impedances can be specified explicitly. In addition, representative ground resistances of

the transmission, and distribution line towers and poles must be specified, in order to take

credit for the full benefit provided by these.

The model allows only a single-phase wire per power line; therefore, only the faulted phase and the

neutral conductors (or shield wires) are represented; the other phases are ignored. You may,

however, include an approximate of the contributions of the other phases by specifying the vector

sum of the currents flowing in the three phases of the circuit of interest as the current flowing in the

faulted phase. The average height and lateral position of the conductor bundle associated with the

faulted phase are specified in terms of their Cartesian coordinates. The positions of up to two static

or neutral wires per power line are specified in a similar manner. A concentric neutral can be

modeled instead of simple static or neutral conductors: this shield is modeled as a bundle of small

conductors arranged to form a cylinder resembling the concentric neutral.

The following input data can be extracted from the description of the circuit in Section 2.2.

Central Site

Name: East Central

Ground Impedance: To be supplied automatically by the grounding module each time the grounding

system is modified and upon its initial creation.

Terminals

Static Wires: 7 No. 8 Alumoweld


Page 7-3

Terminal (Source Substation) Characteristics Transmission Line Characteristics

Name Fault Current

Contribution

(Amps)

Ground

Impedance ()

Span

Length (m) Number of

Spans

Tower Ground

Resistance ()

Greenbay 1226 – j 5013 0.2 330 64 10

Hudson 722 – j 6453 0.3 330 25 28

Newhaven 745 – j 5679 0.3 330 33 28

Table 7-1 Terminal Information for the Example Study: Single-Phase-to-Ground Fault

at East Central Substation

In this study, the static wires are located symmetrically with respect to the center line of the tower, at

a height of 35 m (115 feet) and at a distance of 7.3 m from the center of a tower (see Figure 2.3).

You will note that the primary purpose of the fault current analysis is to determine how much of the

fault current flows into the grounding system of the substation under study (i.e., the Central Station)

during a fault at that location and how much does not, because of alternate ground return paths

provided by static and neutral wires. The analysis will also determine the magnitude of the current

that returns to each power source, through the earth, via the terminal grounds and determine the

influence of the mutual impedances between the phase and static/neutral wires. This latter effect

manifests itself as a “trapped” current in the static/neutral wires. The computation results provide the

self-impedances of the static/neutral wires as well as the mutual impedances between the phase and

static/neutral wires for each power line modeled.

In this chapter, we will show how to set up the computer model of the transmission system

connected to East Central Substation

7.2.1 Data Entry

On the Project toolbar, click on the Circuit button. This will bring the Network Fault Current

Distribution window. As for the other parts of this tutorial, you can manually enter the data

associated with this tutorial by following the steps described in this section; or, if you do not wish to

do so, you can import all the data by proceeding as follows.

Importing Data

In the Network Fault Current Distribution screen, click on the Import button. Select the File

Name “\Files For Import\FC_Initial Design.F05”, then click OK. The data described in the

remainder of this chapter will be loaded and you will not have to enter it.


Page 7-4

The Network Fault Current

Distribution screen contains two tabs

(Central Site and Terminals) dedicated,

as their names indicate, to the data entry

for the Central Site and Terminals,

respectively. We will begin by entering

the Central Site data.

After specifying the name of the Central

Site (“East Central”), the impedance of

the grounding grid must be provided. At

this point, this impedance is unknown,

since it depends on the actual design of

the grounding grid. In fact, as the design

of the grounding grid is refined in the

course of the study, the value of this

impedance will change. The simplest way

to specify the impedance is to let the

program compute it from the data

defining the grid: this option can be

selected by choosing Deduce from Grounding Computations.

This screen also allows you to specify the

computation frequency (typically 60 or

50 Hz) and the average electrical

characteristics of the soil in the region

covered by the electrical network. These

properties are used when computing the

self and mutual impedances of the

transmission lines connecting the

terminals to the central site. These are not

highly sensitive to the soil resistivity, so

an order of magnitude estimate of the

average soil resistivity usually suffices.

The soil’s relative permeability is usually

equal to 1.0.

The Terminals tab (shown next) is used

to define the properties of all terminals.

We will show how to specify the data for

one terminal completely, and then show

some shortcuts to rapidly create the other

terminals.

To begin entering the data, first type the name of the first terminal (“Greenbay”) in the Name field

and press Enter. The remaining fields on the screen should become active, allowing you to enter the

values listed in Table 7-1 for this terminal. First, enter a ground impedance of 0.2 + j 0.0 . Next, a


Page 7-5

current of a 1226 – j 5013 Amps should be entered under Current Source, and 64 sections with a

length of 330 m and a tower ground resistance of 10 should be defined under Sections (see the

next illustration).

To specify the characteristics of the

transmission lines connecting this

terminal to the central site, click on

Define circuit. The resulting screen,

shown further below, is separated

vertically into two parts: the left side is

used to define the geometry of a cross

section of the line (the Conductor

Structure) and the right side is used to

define the electrical characteristics of the

static or neutral wires of the line.

The geometry of a cross section of the

transmission line is shown in Figure 2.3.

There are two static wires located at a

height of 35 m and at a distance of 7.3 m

from the center of the tower, on both

sides. To minimize the mutual

interactions between the phase wire and

the static wires and thus obtain the worst-

case scenario, the fault is assumed to

occur on the phase furthest away from

the static wires, namely Phase B2

(Phase C1 would have been just as

bad) in the figure. The coordinates of

this phase wire are X = 12 m and Y =

21.5 m. This can be specified by

entering the data shown on the screen

entitled “Conductor Specification”

below.


Page 7-6

You can verify visually that the positions of the wires are correct by clicking on the Display button

on this screen. The resulting drawing shows a cross section of the power line defined so far. Use the

Zoom button to look at finer details of

the picture. You can click on Illustrate

to turn off the display of the power line

and return to the explanatory

illustration.

The simplest way to define the

electrical characteristics of the static

wires is to import the information from

the conductor database. To do this,

click on Import from Conductor

Database. The following screen

should appear.

Select “ALUMOWELD” in the

Conductor Class dropdown menu,

then scroll to the desired conductor (7

No. 8), click on it to select it, then

click on Export. The data for this

conductor will be exported to the

Conductor Specification screen.

This completes the data specification for Terminal Greenbay. Click OK to return to the Network

Fault Current Distribution screen.

Since the other two terminals are very similar to the first, the simplest way to enter the

corresponding data is to create a copy of Terminal Greenbay and modify the copy to account for the

differences between the terminals. To do this, click on Copy, enter “Hudson” under Copy Terminal


Page 7-7

To, then click OK. We must then correct the source current (722 – j 6453 Amps), grid impedance

(0.3 + j0.0 ) and number of sections for this terminal (25), as well as the ground resistances of the

towers (28 ); the other characteristics of the terminal are identical to those of Terminal Greenbay.

The remaining terminal (“Newhaven”) can be handled in a similar way.

From the Network Fault Current Distribution

screen, it is possible to view a schematic

representation of the circuit at any time by clicking

on View Circuit. This invokes the GRSplits screen,

which offers a subset of the standalone utility

GRSplits, which is shipped with AutoGrid Pro. You

can use the Help menu on this screen to obtain

more information about the plotting options.

Selecting all three terminals, then using Plot | Plot

circuit, yields the following plot, displayed in the

GraRep utility. Select File | Exit on this screen to

return to the Network Fault Current Distribution

screen.

The Display button on the Network Fault Current Distribution screen provides a quicker way to

obtain a plot of the circuit. When you click this button, a plot of the circuit appears directly on the

screen. This plot is generated using default settings for all plotting options. You can click on

Illustrate to recover the

original circuit illustration.


Page 7-8

Finally, you can click the Properties button to bring up the screen shown below. This screen allows

you to provide comments under Case Description and to select the System of Units, as was the case

for the Soil Resistivity Data Entry in Section 0.

This concludes the data entry session for the Fault Current Analysis module. Click on OK to save

your changes.

Chapter 8. Performance Evaluation of East Central Substation

Page 8-1


PERFORMANCE EVALUATION OF EAST

CENTRAL SUBSTATION

Now that we have specified an initial design for our grounding grid and that we have entered the

data defining the soil structure and the characteristics of the circuit connected to the main grounding

grid, we are ready to evaluate if our proposed design is safe and adequate.

In this chapter, we will demonstrate how to carry out the computations and how to extract the

computation results. The first step consists in determining suitable safety criteria to evaluate the

performance of the grounding grid. Then, we will show how to select a few representative reports

and graphics among those offered by the program, and how to generate those reports and graphics.

These reports and graphics will then be analyzed in order to determine at what locations, if any,

mitigative measures are required.

8.1 SAFETY CRITERIA

Before describing the steps to extract the computation results, let us first identify the safety

objectives.

One of the main concerns when designing grounding systems is to ensure that no electrical hazards

exist outside or within the substation during normal and fault conditions. In most cases, there are no

safety concerns during steady-state normal conditions because very little current flows in the neutral

and grounding system. This current, called residual current, rarely exceeds 10% of the nominal load

current in most electrical distribution systems. Therefore, safety is usually a concern only during

phase-to-ground faults.

In practice, most electric substations are fenced and the fence is quite often placed 1 m (3.28 feet)

inside the outer conductor loop of the grounding system. This way, a person contacting the fence

from the outside will be standing above or close to a ground conductor which will normally result in

lower touch voltages than in the case where the fence is not surrounded by such a ground conductor

loop. In this study, the fence at the East Central Substation is located 1 m inside the outer loop of the

grounding system. Furthermore a large portion of the fence is not metallic (concrete or bricks).

Therefore, unless there are concerns for transferred potentials to remote locations via overhead or

metallic paths, such as gas, oil or water pipes, railway tracks, etc., only the area delimited by the

grounding system outer loop conductor needs to be examined with respect to unsafe touch voltages.

However, step voltages must be explored everywhere inside and outside the substation site. In

general, however, step voltages are rarely a concern inside electric power substation grounding grids,

when touch voltages have been made satisfactory; outside the grid perimeter, however, step voltages

need to be checked.


Page 8-2

8.1.1 Touch Voltages

The first safety criterion used for the evaluation of the grounding system performance is the touch

voltage limit. The touch voltage is usually defined as the difference in potential between a point on

the earth’s surface, where a person is standing, and an exposed metallic structure (present or future)

within reach of that person. Since all metallic structure within a substation should be bonded to the

grounding grid, touch voltages are calculated by computing the difference in potential between the

grounding grid and earth surface points.

ANSI/IEEE Standard 80-2000 (North America) and IEC 479-1 (Europe) provide methodologies for

determining maximum acceptable touch and step voltages, based on the minimum current required

to induce ventricular fibrillation in a human subject. The touch and step voltage limits are a function

of shock duration (i.e., fault clearing time), system characteristics (for short fault clearing times),

body weight, and foot contact resistance (which depends on the electrical resistivity of the material,

such as crushed rock or soil, on which the person is standing, its thickness, and the subsurface soil

resistivity). The table below shows how the touch voltage limit computed in accordance with

ANSI/IEEE Standard 80 varies as a function of earth surface covering material, for a 0.3 s fault

clearing time, a system X/R ratio of 20, and a 50 kg body weight.

Surface Layer Touch Voltage Limit (V)

Type Resistivity (-m)

Native Soil 297 286

15 cm Crushed Rock 3000 933

The crushed rock surface layer installed on the surface of the East Central Substation is 15 cm (6)

thick and has an estimated resistivity (when wet) of 3000 -m. The maximum total clearing time of

the backup relays and circuit breakers in this example is 0.3 s. The crushed rock surface layer

overlies a soil with a resistivity of 297 -m (as will be determined in Section 8.3.3). Resistivity

varies as a function of the type of rock, the size of the stones, the moisture content and the degree of

contamination (e.g., filling of the voids between stones by finer lower resistivity material). The

above table and similar ones can be produced using the Safety module, as explained in Section 8.1.5.

8.1.2 Step Voltages

A similar table can be compiled for step voltages, defined as the difference in potential between two

points spaced 1 m (3.28 ft) apart at the earth’s surface.

Surface Layer Step Voltage Limit (V)

Type Resistivity ( -m)

Native Soil 297 558

15 cm Crushed Rock 3000 3147

Inside a substation and within 1 m (3.28 ft) outside the perimeter fence, step voltages are usually

lower than touch voltages; furthermore, the maximum acceptable values are higher than for touch


Page 8-3

voltages. Consequently, satisfying the touch voltage safety criteria in this area normally ensures that

that the step voltage safety criteria will also be satisfied. The step voltages in the substation and in an

area extending 3 m (about 10 feet) outside the substation grounding grid will be examined. Outside

the extended area of the substation, no computations will be performed. However, it is unlikely for

hazardous step voltages to exist at such remote locations when they are safe closer to the substation.

8.1.3 GPR Magnitude

Sometimes, the absolute magnitude of the GPR of the grid can be a concern. This is particularly the

case for the rating of equipment installed to isolate telecommunications lines from equipment inside

the substation. The program allows you to specify a maximum value for the GPR of the grid; a

warning is issued when the maximum GPR exceeds this value.

8.1.4 GPR Differentials

Significant potential differences between distant parts of the grounding system can give rise to local

touch voltages or equipment stress voltages when low voltage insulated conductors connect

equipment at two such locations. Appropriate protection must be in place at such locations, rated for

the GPR differentials that can arise. The GPR differentials are not going to be a concern in this study

since the grounding grid is small. If the grid is extensive or if there are buried metallic structures

connected to the grounding system, the GPR differentials should be examined. This can be done, for

instance, using the MALZ or HIFREQ Engineering Modules of the CDEGS package.

8.1.5 Determining Safe Touch and Step Voltage Levels

The various parameters governing

the safety limits for touch and step

voltages can be defined in the

Safety screen. This module also

provides a quick way to specify a

surface of observation points

covering the grounding grid.

Select Project | Define Safety

Criteria to open the Safety screen

shown in the following page. As

discussed in the previous sections,

there are several parameters that

govern the magnitude of the safety

limits for the touch and step

voltages. Two of the more

important parameters can be

defined directly on the main

Safety screen, namely the

Insulating Surface Layer

Resistivity and the Fault Clearing Time. Input 0.3 as the Fault Clearing Time and 3000 as the

Insulating Surface Layer Resistivity here (the correct safety limits will be obtained later). The

remaining parameters can be defined using the Standard and Advanced safety screens. The

Click here to define

safety criteria and

border offset for

observation points


Page 8-4

Advanced safety screen (shown in the following page) allows you

to define all of the safety parameters while the Standard safety

screen contains only the most often used parameters. The Sub-

Surface Uniform Soil Layer Resistivity should be set to a

representative value of the resistivity of the soil close to the earth’s

surface. Normally, this should be equal to the resistivity of the top

soil layer, although there may be cases where you want to specify a

different value (such as when the top soil layer is very thin).

Selecting the Use top soil layer resistivity option instructs the

program to always use the resistivity of the top soil layer. When

this option is selected, the program will automatically re-compute

the safety limits whenever there are changes made to the soil

model. Since we haven’t obtained a soil model yet from the soil

resistivity measurement data, the program doesn’t know the value

of the resistivity of the top soil layer. The computed values for the

touch and step voltage limits are therefore (probably) incorrect at

this stage, but will be automatically corrected the moment the soil

resistivity analysis is complete. (If no soil data is given, the program uses 100 -m as a default

value.)

To help in selecting appropriate values for the

Fault Clearing Time and Insulating Surface

Layer Resistivity on the main Safety screen,

the Safety (Advanced) screen allows you to

examine several scenarios at once by

specifying up to 3 different fault clearing

times as well as any number of equally spaced

resistivity values for the insulating layer.

If this information is entered in the Safety

(Advanced) screen as shown here and if you click

the Get Initial Safety Values button, the Safety

Table screen shows up. This screen shows the safety

limits for touch and step voltages for all the

combinations of fault clearing times and insulating

surface layer resistivity that were specified in the

Safety (Advanced) screen.


Page 8-5

As the screen indicates, the values shown on screen at this point will be modified later once the sub-

surface resistivity is known.

8.1.6 A Simpler Way to Specify the Location of Observation Points

As mentioned in the previous section, the main Safety screen also offers a simpler way to specify the

location of observation points for the computation of touch and step voltages. When the option

“Automatic Generation of Observation Points” is checked, the program will ignore any

observation points explicitly specified above the grid (as was done in Section 6.1) and will instead

automatically generate a rectangular surface covering the entire grid. This is shown in yellow in the

screen. The rectangular area extends beyond the region covered by the grid’s conductors by the

maximum of the amounts specified in Grid Border Offset for Touch Voltages and Grid Border

Offset for Step Voltages. By specifying 3 meters for the Grid Border Offset for Step Voltages,

we are guaranteed that the observation points will extend 3 meters outside the area of the grid, as is

desired. The spacing between the observation points can be controlled from the Automatic

Observation Points Options screen obtained by clicking on Advanced.

Moreover, the program will restrict the analysis of the touch and step voltages to the area defined by

the corresponding border offset. This means that, for the data as defined in the above screen, the step

voltages will be analyzed up to 3 meters outside the grid while the touch voltages will be analyzed

up to the edge of the grid, generally located 1 m outside the fence line.

Another advantage of using this automatic mode for the specification of the location of observation

points is that the program will automatically adjust the location of the points and of the safety

analysis areas whenever the grid is modified.

8.2 PLOTS AND REPORTS

We have now completed the data entry. What remains

is to select which reports and plots the program should

produce once the computations are complete, and to

customize the plots and reports.

8.2.1 Selecting Plots and Reports

Use Project | Report Preferences to load the Report

and Graphics Specifications screen. This screen

allows you to select which plots and reports are to be

produced once the computations are complete. These

plots and reports are produced every time the

computations are carried out.

The following reports are selected.

System Data Summary: A summary of the input

data provided to the program.

Requested Computation Reports and Plots: A


Page 8-6

summary of the plots and reports that were requested in this screen.

Resistivity Comparison: A report that lists the comparison of measured vs. computed apparent

resistivities (click the Advanced button to see this option).

List of Materials: A bill of materials report listing the characteristics of the conductors in the

grid.

Soil Resistivity Measurement Interpretation: A report showing the soil model deduced from

the provided measurements.

Ground Grid Performance: A report showing the resistance and other characteristics of the

main grounding grid and other grounding structures.

Fault Current Distribution: A report showing the results of the fault current distribution

analysis.

Safety Assessment: A report showing the safety table generated in Section 8.3.4.

Other reports are available, such as an ampacity assessment report, and a report listing the

computed self and mutual impedances of the conductors in the circuit (click the Advanced button

to see the options). They will not be generated in this tutorial.

On the Graphics tab, plots of touch and step voltage are selected, as well as a plot of the computed

and measured soil resistivities (as obtained from the Soil Resistivity analysis) and plots of various

quantities obtained from the Fault Current Distribution analysis. Other plots are available: the

Click on the Touch

Voltages and then

click on the View

Options…


Page 8-7

Scalar Potential plot shows the earth potentials above the grid, the Grounding System

Configuration plot shows the grounding system itself and the Electric Network Configuration

plot shows a schematic representation of the circuit (as was generated in Section 7.2.1). These plots

have not been requested in this tutorial.

Some of these selections can actually generate more than one plot. This, and other attributes of the

plots, can be controlled by clicking on View Options after having clicked on the pertinent item in

the window in the upper half of the screen: in this

case, click on the wording describing the option of

interest, not on the check box to the left of the

wording. For example, the options for touch voltage

plots are shown here. You can select to produce a

plot of all values of the touch voltages (Show All

Values) or a plot of only those values that are

above the safety limit for touch voltages (as defined

in the Safety screen, Section 8.3.4) or both of these

plots. For the latter plot, the regions where the

touch voltages are below the safety limits are left

transparent, making it easy to identify the

troublesome areas. Similar options are available for

step voltages. Click on Back to return to the

previous screen.

Several options are available for Fault Current

Distribution plots. You can choose to plot any

combination of the Section Span Current (i.e., the

current flowing in the shield and neutral wires

along the power lines modeled), the current

discharged into the earth by every tower and pole

along the power lines (Shunt Tower Currents)

and the ground potential rise of every tower and

pole (Shunt Tower Potentials). You can plot up to

two terminals at a time, and you can restrict the

plots to a range of towers and poles by defining the

Beginning Section and Ending Section fields

(leave these fields blank to plot all sections): recall

that each “section” represents one power line span.

Note that these plots can be helpful in gaining

insight into the behavior of the power system

modeled; however, the key data required for the

design study is to be found in the corresponding

report, as will be seen.

The Type of Plots tab allows you to select the chart

types of the plots that should be produced. All the

plots selected in the Graphics tab will be produced

(if possible) using all of the selected chart types.


Page 8-8

The 2D Spot plots are generally the most useful for a safety analysis, since they display the touch

and step voltages above the grid as a color intensity plot superposed on a plan view of the grid:

these plots constitute contour plots, with the regions between the contour lines shaded in different

colors to make the voltage levels clearer.

Note that this selection of plot types applies only for some of the plots: those available under

Computation Plots apply only to plots of touch voltages, step voltages and scalar potentials, while

those under Configuration Plots apply only to plots of the grounding grid itself.

The plot and reports selection is now complete. Click OK to close this screen and return to the main

screen of AutoGrid Pro.

8.2.2 Customizing Plots

You can customize the appearance of the

plots generated by AutoGrid Pro using the

functionality of the Setup screen, available

from Project | Graphics Preferences.

In the Printer Attributes and Screen

Attributes tabs, you can select the type of

font to use in the plots, and whether color

plots should be rendered in shades of gray.

These settings can be selected

independently for printed plots and plots

displayed on screen.

The Configuration tab allows you to

control certain aspects of the plots displaying the grounding grid, such as the scaling factors to

employ when drawing the grid, and whether or not

to show the coordinate axes on the plot.

The Computations tab can be used to customize

all other plots. You can select a display threshold

value for spot plots (or instruct the program to use

the Safe Allowable Values, as was done in this

case) and you can also set the range of the

coordinate axes for 3D perspective plots.

For greater control over the appearance of the

plots, you can use the GRServer graphics

processor (available at Project | Advanced

Output Processor). This program gives you full

control over the display of plots. Chapter 10

shows how to use this program in greater detail.


Page 8-9

8.2.3 Carrying Out the Computations and Producing the Plots and Reports

Select Project | Process (or click Process on the Project Toolbar) to start the computations and

create the requested plots and reports. The program will analyze the changes you have made to the

data to determine which computations should be carried out. The following confirmation screen will

appear. This screen shows which computations the program thinks it should perform. You can

override the program’s decisions by explicitly selecting or deselecting the options. You can also use

this screen to restrict the production of plots and reports to a subset of those that were selected in the

Reports screen in the previous sections.

The computations begin once you click OK on this screen. A message screen details the progress of

the computations and shows any errors or warnings generated in the run.

You can cancel the run at any time by clicking Cancel on that screen or by choosing Project |

Cancel Processing. Once the computations are complete, the requested plots and reports are

produced and displayed in the GraRep utility, as illustrated in the next section. Copies of the plots

and reports are also stored in the ‘Results’ folder that can be found in the ‘Initial Design’ sub-folder

of your project folder. The following figure shows the content of this folder after the run. The files

with a “rep” extension are the report files and those with an “EMF” extension are the plot files. The

names of the report and plot files are pretty much self-explanatory. In case more details are desired,

the file “Results_Initial Design.txt” gives a short description of each file.


Page 8-10

The following section will examine each one of these files in greater detail.

8.3 ANALYSIS OF THE RESULTS

At this stage, the GraRep (Graphics and Reports Viewer) utility contains all the plots and reports that

were produced in the computations phase. The following screen automatically appears, giving you

access to the results. The plots are stored in the View Plots tab of the utility, and the reports in its

View Reports tab.

In this section, we will examine these

reports and plots and draw conclusions as

regards the safety aspects of our initial

ground grid design.

8.3.1 Using the GraRep Utility

Before we proceed with a detailed

examination of the results, a few words

should be mentioned about the GraRep

utility. This utility displays all the

computation results produced by

AutoGrid Pro. It can be used to print

these results, both in text and graphical

formats. The following paragraphs

summarize the most important features of the utility. Use GraRep’s on-line help (Help | Help

Topics, or press the F1 key) to obtain more details.


Page 8-11

GraRep displays graphical output in its View Plots tab and textual output in its View Reports tab

(there is a third tab that can be used to display messages, but is not used in AutoGrid Pro).

Several plots can be displayed in the View Plots tab, although only one can occupy the main

viewing area. The other plots can be activated by clicking on the corresponding item in GraRep’s

Icon Queue, which appears along the right-hand edge of the screen. You can select several plots

simultaneously, and print them using File | Print Selected Plots. You can also preview the printing

using File | Print Preview.

You can zoom on any part of the plot by holding the Shift key down while dragging the mouse in

the main viewing area to create a rectangle enclosing the area of interest. To restore the picture to its

original size, select Options | Fit to Size. (Do not try to magnify any part of the plot too much: at

some point the magnification process will refuse to continue, a limitation of the operating system

control used by the software.)

The View Reports tab can also hold several text reports simultaneously. The reports are all

displayed in the same window, separated by markers like Report # 1, End Report # 1. The reports

can be printed (File | Print) and previewed before printing (File | Print Preview).

8.3.2 General Information Reports

In GraRep, select the View Reports tab and scroll the window to the top (you can do this quickly by

first clicking in the window, then pressing Ctrl + Home.) The first report is a summary of the plot

and report selections for the run:

----------------------------------------------------------------

Input Data Summary Reports

----------------------------------------------------------------

System Data Summary

D:\Projects\AGP Tutorial\Initial Design\Results\System Input.rep

Requested Computation Reports and Plots

D:\Projects\AGP Tutorial\Initial Design\Results\User Input.rep

----------------------------------------------------------------

Graphics option chosen

----------------------------------------------------------------

Computation Plots

Touch Voltages

Show All Values

Show Unsafe Values Above Selected Safety Threshold

Step Voltages

Show All Values

Show Unsafe Values Above Selected Safety Threshold

Soil Resistivity Measurement Interpretation

Fault Current Distribution

Section Span Currents

Shunt Tower Currents

Shunt Tower Potentials

One Terminal Plot

Terminal Number ................... 1

All Sections Selected


Page 8-12

Configuration Plots

Grounding System Configuration

----------------------------------------------------------------

Types of plot selected

----------------------------------------------------------------

Computation Plots

2D Spot

Configuration Plots

Top View

Report #1: User Input Data

The second report summarizes the input data entered for the computations. The Safety section shows

the computed touch and step voltage safety limits.

-----------------------------------------------------------------------------------------------------

--------------------------

Project Summary

-----------------------------------------------------------------------------------------------------

--------------------------

Case Description ............................ A simple substation grounding grid analysis using

AutoGrid Pro.

Run Identification .......................... Initial Design

System of Units ............................. Metric

Radius Measured in .......................... Meters

Frequency ................................... 60 Hz

-----------------------------------------------------------------------------------------------------

--------------------------

Soil Structure (deduce soil structure from field resistivity measurements)

-----------------------------------------------------------------------------------------------------

--------------------------

Measurement method...........................Wenner

Type of measurement..........................Resistance

Probe depth option...........................Account for Probe Depth

Measurement Spacing S Apparent Depth of Depth of

Number (Meters) Resistance Current Potential

R (Ohms) Probes Do Probes Di

(Meters) (Meters)

--------------------------------------------------------------------------

R1 0.3 152.3 0.1 0.05

R2 1 48.16 0.1 0.05

R3 2 6.12 0.1 0.05

R4 5 3.34 0.1 0.05

R5 7 1.76 0.15 0.05

R6 10 1.11 0.15 0.05

R7 15 0.692 0.3 0.05

R8 25 0.441 0.3 0.05

R9 35 0.32 0.3 0.05

R10 50 0.218 0.6 0.1

R11 65 0.156 0.6 0.1

R12 90 0.106 0.6 0.1

R13 120 0.079 1 0.1

R14 150 0.064 1 0.1


Page 8-13

-----------------------------------------------------------------------------------------------------

--------------------------

Network Fault Current Distribution

-----------------------------------------------------------------------------------------------------

--------------------------

Average soil characteristics along electric lines:

Resistivity(Ohm-m) .......................... 100

Relative Permeability (p.u.) ................ 1

Central site definition:

Name ........................................ East Central

Ground Impedance (To be deduced from grounding computations)

-----------------------------------------------------------------------------------------------------

--------------------------

Safety

-----------------------------------------------------------------------------------------------------

--------------------------

Determine Safety Limits for Touch and Step Voltages

Safety Threshold for Touch Voltages ......... 933.1 V

Safety Threshold for Step Voltages .......... 3146.7 V

Automatic Generation of Observation Points.

Grid Border Offset for Touch Voltages ....... 0 m

Grid Border Offset for Step Voltages ........ 3 m

----------------------------------------------------------------

The computation results are written in the following reports:

----------------------------------------------------------------

Soil Resistivity Measurement Interpretation

D:\Projects\AGP Tutorial\Initial Design\Results\Soil Structure.rep

Ground Grid Perfomance

D:\Projects\AGP Tutorial\Initial Design\Results\Ground Grid Performance.rep

Fault Current Distribution

D:\Projects\AGP Tutorial\Initial Design\Results\Fault Current.rep

Safety Assessment

D:\Projects\AGP Tutorial\Initial Design\Results\Safety.rep

Resistivity Comparison

D:\Projects\AGP Tutorial\Initial Design\Results\Resistivity Comparison.rep

List of Materials

D:\Projects\AGP Tutorial\Initial Design\Results\Bill of Materials.rep Report #2: System Input Data

8.3.3 Soil Resistivity Analysis

The third report in GraRep summarizes the results of the soil resistivity analysis. This report shows

the computed soil structure and gives the RMS error between the computed and measured

resistivities. In our case, the soil is a two-layer soil: the top layer has a resistivity of 297 –m and a

thickness of 0.67 m. The bottom layer has a resistivity of 66 –m.

=========< R E S I S T I V I T Y ( SYSTEM INFORMATION SUMMARY ) >=========

Run ID......................................: Initial Design

System of Units ............................: Meters

Soil Type Selected..........................: Multi-Layer Horizontal

RMS error between measured and calculated...: 16.9464 in percent

resistivities (Note RMS=SQRT(average(Di**2)).


Page 8-14

<--- LAYER CHARACTERISTICS --> Reflection Resistivity

Layer Resistivity Thickness Coefficient Contrast

Number (ohm-m) (Meters) (p.u.) Ratio

====== ============== ============== ============ ============

1 Infinite Infinite 0.0 1.0

2 297.0809 0.6741050 -1.0000 0.29708E-17

3 65.84766 Infinite -0.63713 0.22165

**WARNING** MORE THAN ONE SOIL MODEL CAN PRODUCE SIMILAR APPARENT RESISTIVITY

MEASUREMENT CURVES. IF YOU USE THE DEFAULT STEEPEST-DESCENT METHOD,

THEN YOU WILL MOST OFTEN OBTAIN DECENT AGREEMENT BETWEEN MEASURED

VALUES AND THE COMPUTED CURVE, WITH A REALISTIC SOIL MODEL; HOWEVER,

THE FIT MAY OCCASIONALLY BE SUB-OPTIMAL. IN SUCH CASES, THE MARQUARDT

METHOD WILL USUALLY YIELD AN EXCELLENT FIT, BUT MAY SOMETIMES SUGGEST

EXTREME RESISTIVITY VALUES. NOTE THAT DIFFERENT SOIL MODELS WILL USUALLY

YIELD SIMILAR RESULTS FOR YOUR GROUNDING SYSTEM MODELS (I.E., GPR, TOUCH &

STEP VOLTAGES), PROVIDED THAT THE GROUNDING SYSTEM IS LOCATED CLOSE TO

THE EARTH SURFACE. IF IN DOUBT, CHECK YOUR RESULTS WITH BOTH SOIL MODELS.

Report #3: Soil Resistivity Analysis

We have also requested a plot of the measured and computed resistivity values. This plot is the

second one displayed in the View Plots tab in GraRep (the first one shows the circuit, and was

produced earlier in Section 7.2.1). It shows a few measurement points that differ markedly from the

computed ones, a situation that explains the RMS error of 17% that was obtained in the run. Aside

from these discrepancies, the computed soil model fits the measured data quite well. This visual

check is important to evaluate the nature of the agreement between the measured data and the

computed soil model.

Report #4 provides a comparison of measured & computed apparent resistivities.

Comparison of Measured & Computed Apparent Resistivities

========================================================

C1-C2 SPACING APPARENT RESISTIVITY DISCREPANCY

POINT (meters) MEASURED COMPUTED Di (percent)

===== ============= ======== ======== ===========

1 0.900000 298.2 287.3 3.67

2 3.00000 303.7 179.5 40.91

3 6.00000 76.98 94.44 22.69

4 15.0000 104.9 68.16 35.05

5 21.0000 77.42 66.95 13.52

6 30.0000 69.75 66.32 4.91

7 45.0000 65.23 66.05 1.26

8 75.0000 69.28 65.93 4.83

9 105.000 70.37 65.87 6.40

10 150.000 68.49 65.85 3.86

11 195.000 63.71 65.85 3.35

12 270.000 59.94 65.85 9.85

13 360.000 59.57 65.84 10.54

14 450.000 60.32 65.84 9.14

=========

Average discrepancy: 12.14%

RMS ERROR BETWEEN MEASURED AND CALCULATED RESISTIVITIES :

16.95 percent


Page 8-15

*NOTE* RMS = SQRT( average(Di**2) )

Report #4: Soil Resistivity Comparison

8.3.4 Ground Grid Performance and Safety Analysis

The 5th

report displayed in GraRep (the List of Materials report) gives some information about the

physical characteristics of the grid and its surrounding. This includes:

The quantity and size of grid conductors and rods that were used.

The number of interconnections in the grid at which bonding will be required.

The characteristics of the insulating layer (surface area, thickness, volume, resistivity)

****************************************************************

List of Materials

Creation Date/Time: 7 Jan 2012/13:23:26

****************************************************************

Interconnection / Bonding Nodes ....................... 63

Extent of Grounding System ............................ 6000 (Square Meters)

Surface Layer Thickness ............................... 15 (Centimeters)

Volume of Insulating Layer ............................ 900 (Cubic meters)

Wet Resistivity of Insulating Surface Layer ........... 3000 (Ohm-m)


Page 8-16

Grounding System Data

Number of Rods Length (m) Diameter (m)

----------------------------------------------------------------

None - -

Number of Grid Conductors Length (m) Diameter (m)

----------------------------------------------------------------

7 100 0.012

9 60 0.012

Total Length of Grid Conductors (m) Diameter (m)

----------------------------------------------------------------

1240 0.012

Report #5: List of Materials

The next report shows a table of computed safety limits corresponding to different values of

resistivity of the insulating layer and different fault clearing times. Note the “Equivalent Sub-Surface

Layer Resistivity” entry. The value used here is the value that was computed for the resistivity of the

top soil layer, as shown in the previous section. The remaining data reflects the data we entered in

Section 8.1.5.

Report #6:

Date of run (Start) = Saturday,07 January 2012

Starting Time = 1:23:26 PM

>>Safety Calculation Table

System Frequency....................................: 60.000(Hertz)

System X/R..........................................: 20.000

Surface Layer Thickness.............................: 15.000(cm)

Number of Surface Layer Resistivities...............: 10

Starting Surface Layer Resistivity..................: NONE

Incremental Surface Layer Resistivity...............: 500.00(ohm-m)

Equivalent Sub-Surface Layer Resistivity........... .: 297.08(ohm-m)

Body Resistance Calculation.........................: IEEE Std.80-2000

Fibrillation Current Calculation....................: IEEE Std.80-2000 (50kg)

Foot Resistance Calculation.........................: IEEE Std.80-2000

User Defined Extra Foot Resistance..................: 0.0000 ohms

==============================================================================

Fault Clearing Time (sec) | 0.100 | 0.200 | 0.300 |

+----------------------------+---------------+---------------+---------------+

Decrement Factor | 1.232 | 1.125 | 1.085 |

Fibrillation Current (amps)| 0.367 | 0.259 | 0.212 |

Body Resistance (ohms)| 1000.00 | 1000.00 | 1000.00 |

==============================================================================

==========================================================================

| Fault Clearing Time | |

Surface |-----------------+-----------------+-----------------| Foot |

Layer | 0.100 sec. | 0.200 sec. | 0.300 sec. | Resist |

Resist |-----------------|-----------------|-----------------| ance |

ivity | Step | Touch | Step | Touch | Step | Touch | 1 Foot |

(ohm-m) |Voltage |Voltage |Voltage |Voltage |Voltage |Voltage | (ohms) |

|(Volts) |(Volts) |(Volts) |(Volts) |(Volts) |(Volts) | |

==========================================================================


Page 8-17

NONE | 850.5| 435.9| 658.8| 337.7| 557.7| 285.8| 928.4|

|---------+--------+--------+--------+--------+--------+--------+--------+

500.0| 1159.3| 513.1| 898.1| 397.5| 760.3| 336.5| 1447.1|

|---------+--------+--------+--------+--------+--------+--------+--------+

1000.0| 1896.9| 697.5| 1469.4| 540.3| 1244.0| 457.4| 2685.9|

|---------+--------+--------+--------+--------+--------+--------+--------+

1500.0| 2625.2| 879.6| 2033.6| 681.4| 1721.6| 576.8| 3909.2|

|---------+--------+--------+--------+--------+--------+--------+--------+

2000.0| 3350.7| 1060.9| 2595.5| 821.8| 2197.3| 695.7| 5127.6|

|---------+--------+--------+--------+--------+--------+--------+--------+

2500.0| 4074.9| 1242.0| 3156.5| 962.1| 2672.2| 814.5| 6343.9|

|---------+--------+--------+--------+--------+--------+--------+--------+

3000.0| 4798.4| 1422.9| 3717.0| 1102.2| 3146.7| 933.1| 7559.0|

|---------+--------+--------+--------+--------+--------+--------+--------+

3500.0| 5521.5| 1603.7| 4277.1| 1242.2| 3620.9| 1051.6| 8773.6|

|---------+--------+--------+--------+--------+--------+--------+--------+

4000.0| 6244.4| 1784.4| 4837.1| 1382.2| 4094.9| 1170.2| 9987.6|

|---------+--------+--------+--------+--------+--------+--------+--------+

4500.0| 6967.1| 1965.0| 5396.9| 1522.2| 4568.9| 1288.6| 11201.4|

|---------+--------+--------+--------+--------+--------+--------+--------+

* Note * Listed values account for short duration asymmetric waveform

decrement factor listed at the top of each column. Report #6: Safety Limits Calculation Table

This report depicts the safety threshold values applicable to various scenarios of clearing times and

surface layer resistivities. It indicates that touch voltages of 933 V or less and step voltages of 3147

V or less are safe if a 15 cm (6) crushed rock layer with a resistivity of 3000 -m is overlying a

native soil with a resistivity of 297 -m, for a 0.3 s fault clearing time. Touch voltages of 286 V or

less and step voltages of 558 V or less are safe if no surface crushed rock is present at East Central

Substation. Note that outside the substation where there is no crushed rock, step voltages must not

exceed 558 Volts.

The next report summarizes the performance aspects of the grounding grid.

DATE OF RUN (Start)= DAY 7 / Month 1 / Year 2012

STARTING TIME= 13:23:26:67

===========< G R O U N D I N G ( SYSTEM INFORMATION SUMMARY ) >===========

Run ID......................................: Initial Design

System of Units ............................: Metric

Earth Potential Calculations................: Single Electrode Case

Type of Electrodes Considered...............: Main Electrode ONLY

Soil Type Selected..........................: Multi-Layer Horizontal

SPLITS/FCDIST Scaling Factor................: 9.3560

1

1

MULTI-LAYER EARTH CHARACTERISTICS USED BY PROGRAM

LAYER TYPE REFLECTION RESISTIVITY THICKNESS

No. COEFFICIENT (ohm-meter) (METERS)

----- ------ ------------- ------------- -------------

1 Air 0.00000 0.100000E+21 Infinite

2 Soil -0.999990 297.081 0.674105

3 Soil -0.637132 65.8477 Infinite

1


Page 8-18

CONFIGURATION OF MAIN ELECTRODE

===============================

Original Electrical Current Flowing In Electrode..: 1000.0 amperes

Current Scaling Factor (SPLITS/FCDIST/specified)..: 9.3560

Adjusted Electrical Current Flowing In Electrode..: 9356.0 amperes

Number of Conductors in Electrode.................: 16

Resistance of Electrode System....................: 0.53773 ohms

SUBDIVISION

===========

Grand Total of Conductors After Subdivision.: 110

Total Current Flowing In Main Electrode......: 9356.0 amperes

Total Buried Length of Main Electrode........: 1240.0 meters

EARTH POTENTIAL COMPUTATIONS

============================

Main Electrode Potential Rise (GPR).....: 5031.0 volts

Report #7: Grounding Grid Performance

This report shows that the ground resistance of the East Central substation grid is 0.538 , and that

the fault current injected into the grid is 9.36 kA, for a ground potential rise (GPR) of 5.03 kV.

We also requested 5 plots related to the performance of the grounding grid. The first one (and the

next in the queue in GraRep’s View Plots tab) is a plan view of the grounding grid itself.

The following plot shows the touch voltages throughout the grid. Recall from Section 8.1 that the

touch voltages are computed up to a distance of 1 meter outside the fence line, i.e. up to the edge of

the grid. The touch voltages can reach values as large as 2.28 kV. The larger values occur in the

corners of the grid, which is typical. These values are very much above the safety limit for touch

voltages (933 Volts) that was displayed in the safety report.


Page 8-19

This can be verified in the next plot, which shows the “unsafe” values of the touch voltages. In this

plot, any value of the touch voltages falling below the 933 Volts limit is left transparent; therefore all

colored areas are above the limit and should be considered unsafe. Most of the plot is colored for this

initial design: we will therefore have to reinforce the grid substantially to eliminate this problem.


Page 8-20

The next plot shows the step voltages over an area extending up to 3 meters outside the grid. The

maximum step voltage reached is 525 Volts. This is below the safe step voltage limit computed in

the presence of crushed rock (3147 Volts). This can be verified in the step voltage plot following the

first one: only “unsafe step voltages” are colored. Since there are no colored areas over the grid, the

grid is completely safe from the point of view of step voltages. Note that the step voltages are also

safe even when there isn’t any crushed rock, since in this case the safety limit for step voltages is

558 Volts.

The situation encountered here is quite common: the touch voltages over the grid are unsafe but the

step voltages are fine. The safety requirements for step voltages are usually easier to meet than those

for touch voltages.

It is important to mention that computations of step voltages are sensitive to the location of

observation points. When the maximum computed step voltage is on the border line as compared

with the safe step voltage limit (525 V vs. 558 V for native soil in this case), it is recommended to

reduce the spacing between observation points to less than 1 m in order to capture the worst case

step voltage which are usually at the corners of a substation where earth potentials drop quickly.


Page 8-21

8.3.5 Computation of Fault Current Distribution

The next report in GraRep’s View Reports tab summarizes the results of the fault current

distribution analysis. This report shows the computed value of the current injected into the East

Central substation grid (the “Total Earth Current”) as well as a wealth of information regarding the

various terminals of the circuit. Note that the computations used the computed value of the grid’s

“Ground Resistance”, namely 0.538 .

DATE OF RUN (Start)= DAY 7 / Month 1 / Year 2012

STARTING TIME= 13:23:26:69

=======< FAULT CURRENT DISTRIBUTION ( SYSTEM INFORMATION SUMMARY ) >=======

Run ID.......................................: Initial Design

Central Station Name.........................: East Central

Total Number of Terminals....................: 3

Average Soil Resistivity.....................: 100.00 ohm-meters

Printout Option..............................: Detailed

1

Central Station: East Central

Ground Resistance........................: 0.53773 ohms

Ground Reactance.........................: 0.0000 ohms

1

Terminal No. 1 : Greenbay

Number of Sections..............: 64

Ground Impedance................: 0.20000 +j 0.0000 ohms

Source Current..................: 5160.7 Amps / -76.257 degrees

Neutral Connection Impedance....: 0.0000 +j 0.0000 ohms

Span Length.....................: 330.00 m


Page 8-22

1

Terminal No. 2 : Hudson





Span Length.....................: 330.00 m

1

Terminal No. 3 : NewHaven





Span Length.....................: 330.00 m

1

TERMINAL GROUND SYSTEM (Magn./Angle)

Term: 1 Total Earth Current...: 3869.0 Amps / 93.292 deg.

Earth Potential Rise..: 773.80 Volts / 93.292 deg.





Average Resistivity...........: 100.00 Ohm-meters

Grid Impedance................: 0.53773 +j 0.0000 Ohms

< Magnitude / Angle >

Total Fault Current...........: 17355. Amps / -81.073 degrees

Total Neutral Current.........: 8068.5 Amps / -75.565 degrees

Total Earth Current...........: 9356.0 Amps / -85.821 degrees

Ground Potential Rise (GPR)...: 5031.0 Volts / -85.821 degrees

Report #8: Fault Current Distribution Calculation

The next plot on the View Plot tab of GraRep shows the “Section Current” (i.e., the current flowing

in the shield wires of the transmission line) for the first terminal of the circuit. The plot shows that a

considerable portion of the fault current (about 3000 A) leaves the fault site via the shield wires. This

current, however, quickly flows into the earth via the first few towers and then levels off to a

constant “trapped” current maintained by induction from the faulted phase.


Page 8-23

The next plot shows the “Shunt Current”, i.e. the current flowing in every tower structure of the

transmission line. It confirms that most of the current is discharged close to the fault site: we see that

the values drop off quite abruptly and then increase again as the terminal station is approached.

This is also reflected in the following plot (“Shunt Potential”), which shows the GPR of the tower

structures along the transmission line.


Page 8-24

Chapter 9. Reinforcing The Grounding System

Page 9-1


REINFORCING THE GROUNDING SYSTEM

The touch voltages obtained in Chapter 8 indicate that our initial design is quite far from providing a

safe ground grid design: touch voltages exceed the safe limit at most locations throughout the

substation. The highest values occur in the corner meshes of the grounding grid, which suggests that

there is a need to have more conductors towards the edge of the grounding system than towards the

central portion. This observation is consistent with analytical and experimental results. However, the

optimum or most efficient conductor compaction at the periphery of a grounding system depends on

many factors, particularly on earth structure characteristics. Moreover, practical considerations often

introduce additional constraints, which must be accounted for. In general, however, the following

crude rules of thumb can be used as a preliminary set of guidelines:

When the surface (shallow depth) soil resistivity is small compared to that of the deeper

layers (those which are not in contact with the grounding system), use grids with more

conductors at the edge than in the central area (exponentially-spaced conductors). The

degree of conductor clustering (compactness) at the periphery of the grid should increase

with an increase in the contrast between the surface and deep layer resistivities.

When the surface soil resistivity becomes larger than that of the deeper soil layers, the

clustering (compactness) ratio should decrease towards a uniform distribution of conductors

in the case where the contrast ratio is significant (5 or more) and the thickness of surface

layers is small compared to the size of the grounding system (1/5 or less).

Finally, when the surface soil resistivity is quite large compared to that of the deeper layers

and its thickness is small enough so that use of ground rods penetrating into the deeper layer

is efficient, a number of ground rods should be installed wherever possible to reduce the

GPR, touch and step voltages instead of using unequally spaced conductors.

Based on the soil model and the initial design, we will combine the first and the third methods in this

study. We will also increase the total number of conductors in the grid. The improvements will be

carried out in two steps: first, the grid itself will be modified to use a denser exponential design, then

some grounding rods will be added.

9.1 EXPONENTIAL GRID DESIGN

At this point, we want to make some modifications to the grounding grid. We could modify the data

directly in the “Initial Design” scenario we have been working with so far. If we do this, the “Initial

Design” scenario will be lost. Instead, we will use a different scenario to make the changes. If you

have chosen to enter the data manually, proceed to the next section to create a new scenario.

Otherwise, go to Section 9.1.3 that shows how to open an existing scenario.

9.1.1 Creating the Exponential Grid Scenario


Page 9-2

Select Project | New Scenario. In the

resulting dialog, specify “Exponential

Grid” for the Scenario Name. The

program will automatically define a

Scenario File Location, which you can

override. Make sure to select Based On

Existing Scenario for the Reference

Scenario and to use Initial Design as a

reference.

Click Create on the New Scenario

screen. This instructs the program to

create a copy of the Initial Design

scenario and give it the name “Exponential Grid”. You can now proceed to Section 9.1.3 to make the

necessary changes to this scenario.

9.1.2 Opening the Exponential Grid Scenario

To open an existing scenario, select

Project | Open Scenario. Make sure that

the Existing tab is active, then select the

Exponential Grid scenario from the list.

Click Open to open this scenario.

9.1.3 Modifying the Exponential Grid Scenario

At this point, your AutoGrid Pro screen should look as follows. The Active Scenario, that is, the

scenario that can presently be edited, is Exponential Grid. The other scenario (Initial Design) will no

longer be used in this tutorial and can be closed by closing the window containing the drawing of the

grid (the one that shows Scenario Initial Design in its title bar).


Page 9-3

The simplest way to modify the grid is to use the Edit | Edit Object command. To do this, the grid

should first be selected. To select the grid, click on any point on the grid; the grid should turn red to

confirm your selection. This operation will be easier to perform if you first turn off the display of the

observation profiles. To do this, uncheck the option View | Profiles.

Once the grid is selected, select Edit | Edit Object. The screen should appear in the following page.

This screen is very similar to the Create Object screen used in Section 6.1 and is used in the same

way. Make the following changes:

Parameter Old Value New Value

Nab 7 13

Nac 9 17

Compression Ratio 1 0.8


Page 9-4

We have increased the number of conductors substantially and defined a compression ratio different

than 1. This last option instructs the program to bunch the conductors located towards the edge of the

grid more closely together. The ratio of the distance between successive pairs of conductors

decreases by the factor entered in this field.

The resulting screen is shown in the following page. Click OK to confirm the changes. The modified

grid will be shown in the main drawing.

Since we know that the step voltages are safe already, we will turn off the production of the step

voltage plots. Select Project | Define Safety Criteria and uncheck the Step Voltages option under

Determine Safety For. Click OK in the Safety screen to return to the main screen. (If the safety

limits are not correct, they will be updated after computations.)


Page 9-5

The data entry is complete. Select Project | Process to start the

computations and click OK on the confirmation screen. The

program will automatically compute the grid impedance, the fault

current and the touch voltages.

Once the computations are complete, the analysis of the results can

proceed as in Section 8.3. Here, we will focus only on the touch

voltages, since we know them to be problematic.

The results are shown in the following page. The maximum touch

voltage has been reduced to 1.15 kV, which is still above the safety

limit. The plot of unsafe touch voltages shows that the large values

of touch voltages are concentrated at the periphery of the grid. It is

therefore likely that adding ground rods at the corners of the grid

will fix this problem. This is what we will attempt in the next

section.


Page 9-6

9.2 ADDING GROUND RODS

As noted above, adding ground rods to the grid should fix the last problem with the touch voltages.

We will briefly describe how to do this in this section. The steps are very similar to what was done in

the previous section, namely create a new scenario based on the current one, modify the data by

adding rods, run it and analyze the results.

9.2.1 The Details

If you are entering the data manually, create a new scenario as was done in Section

9.1.1, using the name Exponential Grid With Rods for the scenario and using the

Exponential Grid scenario as a reference. If you are following the pre-made tutorial,

then simply open the existing Exponential Grid With Rods scenario as was done in

Section 9.1.2.

To add rods to the corners of the grid, select Options | Pointer Mode | Power Tool, or simply click

on the button on the toolbar at the left of the main screen. This will load the Power Tool.

Make sure to select the Create Rod option. By default, the program will create 10 meter long rods

with a radius of 0.01 meters. We will keep these default settings, but will change the length from 10

m to 20 m.


Page 9-7

With the Power Tool still

loaded, click successively

at the four corners of the

grid. As you do this, the

picture should change to

show the newly created

rod as an empty circle. It

should also indicate with

magenta circles the

location of the nodes in the

grid that are connected to

the one you clicked. This

makes it easier to verify

that the desired point was

clicked on, and not a

neighboring one. You

don’t have to be very

precise with the mouse

clicks, since the program

will automatically snap to

the closest conductor. In fact, if you don’t click close enough to a conductor, the program will refuse

to create a rod and warn you about it. Note also that you can easily undo any undesired action using

Edit / Undo.

To close the Power Tool, select any other option under Options | Pointer Mode, typically Select

Objects.

Once all four rods are created, select Project | Process. Once the computations are complete, we can

look at the touch voltage plot. This time, all values are safe, the maximum value being 874 Volts,

and the analysis is complete.

Further design iterations may be required to remove or reposition some conductors to more practical

locations. Extra ground rods may be added to account for winter soil freezing or summer extreme

drought conditions. In some cases, other soil structure models may need to be analyzed to account

for inherent data uncertainties or known soil characteristic variations. In such cases, the worst-case

scenario should be retained as a reference for the final recommended grounding design

configuration.


Page 9-8

9.3 EXPORT GROUNDING GRID INTO DXF FILE

At this stage, it is possible to send directly the grid configuration to a DXF compatible CAD drawing

system. For example if your CAD system is AUTOCAD (or DXF compatible), you may proceed as

follows:

In the AutoGrid Pro, select Save Document As… under the Files menu. Select the CAD Files from

the Files of types, change the file name to “Exponential Grid With Rods.DXF” and click on OK.

The file “Exponential Grid With Rods.DXF” is created.

On a final note, it is worthwhile mentioning that when redesigning an existing grounding system

(update, upgrade, etc.), you could import the actual system configuration from a DXF-compatible


Page 9-9

CAD file by clicking the Import… under the Files menu in the AutoGrid Pro (after a New

Document is created in AutoGrid Pro). It is however important to note here that some drawings may

contain overlapping lines which will ultimately result in invalid overlapping conductors in MALT.

Furthermore, too many details such as short wire connections and bonding conductors have a

minimal impact on the grounding design performance but a significant negative impact on the

computations in terms of run time and run accuracy. One way to remove this kind of problem is to

use the minimum conductor length threshold to ignore such non-significant short conductors. This

strategy, however, can be used in MALT only and may have a negative impact on the node

subdivision process.

Chapter 10. Using GRServer

Page 10-1


USING GRSERVER

This chapter shows how to use the GRServer program to examine the computation results of

AutoGrid Pro in greater detail. This program allows you to produce high-quality 2D and 3D plots of

the results, and to save those plots to disk or print them. It also allows you to manipulate the plots

(e.g., rotate, translate and scale them) for better viewing. While this program is somewhat more

difficult to use than the GraRep utility (which is used by default in AutoGrid Pro), the greater quality

of the graphics it generates may well be worth the extra effort.

This chapter is not an essential part of the tutorial, and can be skipped if you are not interested in

creating plots of higher quality.

10.1 STARTING GRSERVER

To start the GRServer program, use Project | Advanced Output Processor or click on the

GRServer button on the Project toolbar in AutoGrid Pro. This starts the program, and loads the

computation databases for the active scenario in AutoGrid Pro. Note that these computation

databases, which are created when processing a scenario, should be available before starting

GRServer. This is the case here, since we created the databases for scenario Exponential Grid With

Rods in the previous step.

The screen shown in the following page should appear when the program first loads. The main

screen of the program displays an empty plot window, and three options (Soil, Grid and Circuit) are

available in the toolbar to the left of the screen, corresponding to the computation options in

AutoGrid Pro. These options are available as the first, second and last buttons in the toolbar. The

other buttons in that toolbar allow you to create plots for the MALZ, HIFREQ and SPLITS programs

of the CDEGS package; these are not available in AutoGrid Pro.

The Plot Options window should also be visible. This window is the main interface to create and

customize plots in GRServer. This window may disappear during the execution the program. If this

happens, you can use Plot | Options to bring it back.


Page 10-2

The program is presently ready to create soil

resistivity plots. Click on the Grid button

(the second button from the top) on the

toolbar to activate the grid plotting module,

which is used to plot touch and step

voltages. A second blank plot should

appear.

In the Plot Options window, click on the

More (>>) button to open the

Computations Setup screen. Select Touch

Voltages under Determine. You can then

collapse this screen by clicking on the Less

(<<) button in the Plot Options window. At

this point, we are ready to create touch

voltages plots using GRServer.

10.2 CREATING 3D PLOTS

By default, the program creates a 3D perspective plot of the touch voltages. Click on Draw in the

Plot Options window to generate the plot. The screen should look as follows (the touch voltage plot

window was maximized, for easier viewing).


Page 10-3

Note that the maximum touch voltage displayed on the plot is about 1.92 kV, as opposed to 929 V

when plotted directly with AutoGrid Pro. The reason for this difference is that AutoGrid Pro restricts

the computation of the touch voltages to a region just covering the grid. This is explained in Section

8.1.6, where the Grid Border Offset for Touch Voltages is specified as 0. This information is not

transmitted to GrServer, which examines the touch voltages over the entire area covered by

observation points, namely up to 3 meters outside the grid.

You can restrict the analysis of the touch voltages to points that are located only above the grid using

the Zoom Polygon feature of GrServer. This feature allows you to restrict the display of computed

quantities (touch or step voltages) to points that lie inside a specified polygon. To restrict the

analysis to points located directly above the grid, the polygon should be a rectangle of the same size

as the grid (60 m by 100 m), with one corner at the origin of the coordinate system.

To specify this zoom polygon, first click on the More (>>) button in the Plot Options window to

open the Computations Setup screen, and select the Zoom & Report tab (on the left side of the

window). Enter the following numbers in the Search Zone Vertices Table:

No X Pos Y Pos Z Pos

1 0 0 0

2 0 60 0

3 100 60 0

4 100 0 0

Finally, click Draw. The following plot should be produced. This plot agrees with the result

obtained previously with AutoGrid Pro (Section 9.2.1).


Page 10-4

Several aspects of the plot can be customized. For example, the plot can be easily rotated. To do that,

select Plot | Rotate | Free or click on the last button in the program’s main toolbar, and select the

Free option. Then, drag the mouse (i.e., move the mouse while the mouse button is pressed) in the

plot area: a cube indicating the new position of the plot follows the movements of the mouse. Once

you release the mouse button, the plot is redrawn in the new position.

There are many other options to control the rotation of the plots. For instance, you can restrict the

rotation to be around one the axes of the coordinate system. You can also move the plot (Plot |

Move), scale it up and down (Plot | Scale) or zoom on any region of interest in the plot (Plot |

Zoom).

Other options are available to control the

appearance of the plots. These are regrouped in the

3D Advanced Plot Setup window. Click on the

Plots button in the Plot Options window to get to

that screen. You can control the color of certain plot

elements, whether or not the legend is displayed,

etc… For example, the following figure shows what

happens when a “Spot” ceiling is requested by

selecting the Spot option under Ceiling Projection

Types.


Page 10-5

10.3 CREATING 2D PLOTS

The GRServer program can also

be used to create 2D plots. To

do this, select 2D under Plot

View in the Plot Options

window, select Touch

Voltages, and click on Draw.

(Note that the plot below was

generated with the Zoom

Polygon option turned off.)

When moving the mouse cursor

in the resulting plot, the value

of the X coordinate (here, the

distance from the starting point

of the computation profile) and

of the Y coordinate (here, the

touch voltage at that point)

corresponding to the location of

the mouse in the plot are shown

in the program’s status bar.

10.4 SAVING AND PRINTING PLOTS

You can save a plot produced in GRServer by selecting File | Save As when that plot is the active

plot. The following screen should appear. By default, the program offers to save the file as

‘mt_Exponential Grid With Rods.emf’ in the Exponential Grid With Rods scenario folder. If this


Page 10-6

isn’t satisfactory, you can click on Save As again, then browse to the desired filename. Click OK in

the Save As screen to complete the operation.

You can also print the current plot, or even print all open plots or only the selected plots. To print the

currently selected plots, use File | Print (or File | Print Selected Plots if more than one plot is

currently selected) and follow the instructions in the ensuing dialogs. To print all the plots, select

File | Print All Plots.

10.5 SUMMARY

This chapter has described briefly the main features of the GRServer plotting program. The

capabilities of the program were illustrated by producing 2D and 3D plots of the touch voltage above

the grounding grid studied in scenario Exponential Grid With Rods. The program could also be used

to generate the soil resistivity plots and the fault current analysis plots for that scenario (or any

AutoGrid Pro scenario).

Only a few of the program’s options were explored. Consult GRServer’s on-line help for more

details on the options available in that program.

To exit the GRServer program, select File | Exit.

Chapter 11. Conclusion

Page 11-1


CONCLUSION

This concludes our concise step-by-step instructions on how to prepare, submit and examine results

for a simple grounding analysis problem using AutoGrid Pro. You can use File | Exit to quit the

program. Accept to save the changes when prompted to do so.

Only a few of the many features of the software have been used in this tutorial. You should try the

many other options available to familiarize yourself with the CDEGS software package. Your SES

Software DVD also contains a wealth of information stored under the PDF directory. There you will

find the Getting Started with SES Software Packages manual (\PDF\getstart.pdf) which contains

useful information on the CDEGS environment. You will also find other How To…Engineering

Guides, Annual Users’ Group Meeting Proceedings and much more. All Help documents are also

available online.

Notes

NOTES

auto grid pro

Documents

release page

grounding problem

substation grounding

autogrid pro software

grounding design

ground gridthis page

autogrid pro package

substation terminals