help manual

K:\

CD

EG

S\H

OW

TO

\AU

TO

GR

ID_P

RO

\CO

VE

R_A

GP.

CD

R

How To...Engineering Guide

How To...Engineering Guide

MultiGround+

TOUCH VOLTAGES ABOVE GROUND GRID

Blank Page

HOW TO… Engineering Guide

A Simple Substation Grounding Grid Analysis

Using Autogrid Pro 2006 Release

Blank Page

REVISION RECORD

Date Version Number Revision Level

January 2001 9 0

November 2002 10 0

June 2004 11 0

December 2006 13 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 © 2000-2009 Safe Engineering Services & technologies ltd. All rights reserved.

Blank Page

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

1 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..................................................................................1-3 1.5 ORGANIZATION OF THE MANUAL ........................................................................................................1-3 1.6 NOTE 1-4 1.7 PRELIMINARIES.......................................................................................................................................1-4

2 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

3 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

4 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............................................................................................4-5 4.1.3 USING THE PROJECT.................................................................................................................4-6 4.1.4 FILES THAT ARE PART OF THE PROJECT ..............................................................................4-7

5 SOIL RESISTIVITY DATA ENTRY.....................................................................................5-1 5.1 A HORIZONTAL TWO-LAYER SOIL MODEL..........................................................................................5-1 5.2 SOIL RESISTIVITY DATA ENTRY ...........................................................................................................5-2

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

Page iii

TABLE OF CONTENTS (CONT’D)

Page

6.1 DATA ENTRY............................................................................................................................................ 6-1 7 FAULT CURRENT DISTRIBUTION ANALYSIS................................................................ 7-2

7.1 INTRODUCTION ....................................................................................................................................... 7-2 7.2 PREPARATION OF THE INPUT DATA ................................................................................................... 7-3

7.2.1 DATA ENTRY............................................................................................................................... 7-4 8 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 ........................................................................................................................... 8-6 8.2.1 SELECTING PLOTS AND REPORTS ......................................................................................... 8-6 8.2.2 CUSTOMIZING PLOTS................................................................................................................ 8-9 8.2.3 CARRYING OUT THE COMPUTATIONS AND PRODUCING THE PLOTS AND

REPORTS .................................................................................................................................... 8-9 8.3 ANALYSIS OF THE RESULTS .............................................................................................................. 8-11

8.3.1 USING THE GRAREP UTILITY ................................................................................................. 8-11 8.3.2 GENERAL INFORMATION REPORTS...................................................................................... 8-12 8.3.3 SOIL RESISTIVITY ANALYSIS.................................................................................................. 8-14 8.3.4 GROUND GRID PERFORMANCE AND SAFETY ANALYSIS .................................................. 8-16 8.3.5 COMPUTATION OF FAULT CURRENT DISTRIBUTION ......................................................... 8-23

9 REINFORCING THE GROUNDING SYSTEM ................................................................... 9-1 9.1 EXPONENTIAL GRID DESIGN ................................................................................................................ 9-1

9.1.1 CREATING THE EXPONENTIAL GRID SCENARIO................................................................... 9-2 9.1.2 OPENING THE EXPONENTIAL GRID SCENARIO..................................................................... 9-2 9.1.3 MODIFYING THE EXPONENTIAL GRID SCENARIO................................................................. 9-3

9.2 ADDING GROUND RODS........................................................................................................................ 9-7 9.2.1 THE DETAILS............................................................................................................................... 9-7

10 USING GRSERVER ......................................................................................................... 10-1 10.1 STARTING GRSERVER............................................................................................................ 10-1 10.2 CREATING 3D PLOTS .............................................................................................................. 10-3 10.3 CREATING 2D PLOTS .............................................................................................................. 10-6 10.4 SAVING AND PRINTING PLOTS.............................................................................................. 10-7

Page iv


Page

10.5 SUMMARY..................................................................................................................................10-8 11 CONCLUSION..................................................................................................................11-1

Page iv


Page

Blank Page

Page iv

Chapter 1. Introduction

Chapter 1

1 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.

Page 1-1


Please call SES’ toll-free support line with any questions you may have, as you work through this manual. Call us collect at +1-514-336-2511 if you do not have this number handy. You can also E-mail us questions at [email protected].

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 Centrasafe levels for personnel withinmeasurement techniques and inst

1.3 COMPUTER MODSES’ AutoGrid Pro is used to msystem impedance) and interprebetween the transmission line grounding grid, and to simulate compute the ground potential ripotentials throughout the substati

This software integrates all the to

• A soil resistivity analysmeasurements.

• A fault current distributiodischarged in the groundi

• A grounding module that

• A safety analysis modulcompares them to safety l

East CentralSubstation

GreenbayTerminal

HudsonTerminal

NewhavenTerminal

Figure 1.1 Illustration of the Grounding Analysis Problem

l Substation. The final design is to limit touch and step voltages to the substation area, based on up-to-date system data, appropriate rumentation, and state-of-the-art computer modeling methods.

ELLING TOOL odel the field measurements (i.e., soil resistivities and grounding

t the measured data, to compute the distribution of fault current static wires, distribution line neutral wires, and the substation a representative phase-to-ground fault in the substation in order to se and ground resistance, touch voltages, step voltages, and earth on.

ols required for such an analysis. It includes:

is module to determine the soil structure from soil resistivity

n analysis module to compute the fraction of the fault current that is ng grid.

computes the response of the grounding grid to the fault current.

e that computes the touch and step potentials above the grid and imits deduced from the relevant standards.

Page 1-2

mailto:[email protected]


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:

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 (the 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 which is 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).

Page 1-3


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).

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 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 PRELIMINARIES 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. Either way, you should create a folder on a local drive (i.e., C: or D:) that has sufficient free space to serve as a work directory for the tutorial (the complete tutorial requires about 16 MB). You can call this folder anything you like; it will be referred to as the “Project” folder in the remainder of this manual. This manual was created using “D:\Projects” as the Project folder, and this will be reflected in the various screen shots shown in the manual. Make sure to note the complete path to your Project folder, since you may want to explore this folder to see the files created by the program or use them with other software.

If you want to load the existing input files, you must first copy them from your CD-ROM: they are located in the folder named “\Examples\Official\HowTo\AGP Ground\”. You should copy all the files and folders found in this directory to your Project folder.

Even if you plan to enter the data manually, you may still want to copy the folder “Files For Import” (and its content) to your Project folder. This folder contains files that can be interactively imported into the program instead of entering the data for some of the lengthier data entry tasks in the tutorial.

Page 1-4


Once the tutorial is completed, you may wish to explore the other How To…engineering manuals that are available in Acrobat electronic format on the CD.

Page 1-5


Blank Page

Page 1-6

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

Chapter 2

2 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):

Figure 2.1 Schematic of System under Study

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.

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)

Page 2-1


100 m(100,60,0.5)

60 m

(0,0,0.5)

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

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 Ω as will be determined by the MALT engineering module.

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 Ω/km (2.83 Ω/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

Page 2-2

Y

X Figure 2.3. Transmission Line Configuration


estimated to be equal to 10 Ω. 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 Ω.

2.3 THE SUBSTATION TERMINALS The ground resistances of the terminals are equal to 0.2 Ω, 0.3 Ω and 0.3 Ω 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.

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 2 1 . 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

Figure 2.4 Power System Network Analyzed in E l

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

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 2-3


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

Separation Depth of Depth of Apparent

Between

Adjacent

Probes1

Current

Probes2Potential

Probes2Resistance

(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.

Page 2-4

Chapter 3. Program Highlights & Using Autogrid Pro

Chapter 3

3 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 3-1


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.)

Page 3-2


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.

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. (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”). Note that a demo project (called ‘Demo 1’) is available in the folder ‘SESSoftware 2006\Examples\Autogrid Pro\Demo 1’ in your SES Software installation directory.

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 3-3


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 3-4


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 3-5


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.

Page 3-6

Chapter 4. Creating a Project and Scenario
Chapter 4
4 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

Click here

Page 4-1


In the SES Software 2006 group folder, you should see the icons representing the Autogrid Pro, AutoGroundDesign, CDEGS, Right-of-Way, SESTLC and SESEnviroPlus software packages, as well as five folders. The Documentation folder contains help documents for various utilities and software packages. The System folder allows you to conveniently set up security keys, etc. Various utilities can be found in the Tools folder. The main function of each software package and utility is described hereafter.

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 appropriate databases. This new module should help reduce considerably the time needed to complete a grounding design.

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 and Autogrid Pro. This interface also provides access to the utilities listed below.

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.

SESTLC is a simplified analysis tool useful to quickly estimate the line parameters, magnetic and electric fields of arbitrary configurations of parallel transmission and distribution lines. As well, it will estimate the inductive interference level on other metallic utility paths such as pipelines and railways running parallel to the electric lines.

Ampacity computes the following quantities: Minimum conductor size, Ampacity (i.e., maximum fault current rating), and temperature rise during a fault (computed in terms of the final temperature after the fault).

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

Page 4-2


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 new 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.

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.

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.

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.

SES-Shield is a software package aimed at providing 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.

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 new software tool that automates the selection of soil model structures that apply during various seasons.

Page 4-3


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 above 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 or by browsing to an AGP project file) 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 0 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 you PC, or create a new one). We will first enter AGP Tutorial in the Project Name field. A sub-folder called AGP Tutorial under D:\Projects\AGP Tutorial 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 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.

Page 4-4


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.

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.

Page 4-5


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.

Page 4-6


• 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 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 are 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_InitialDesign.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.

Page 4-7


Blank Page

Page 4-8

Chapter 5. Soil Resistivity Data Entry

Chapter 5

5 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 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 (Ω-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 Ω

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

Page 5-1


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.

Page 5-2


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 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.

Page 5-3


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.

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.

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 Close 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.

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.

Page 5-4


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.

Page 5-5


Blank Page

Page 5-6

Chapter 6. Initial Grounding Grid Design

Chapter 6

6 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 Ω 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:

Page 6-1


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.

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 CD-ROM 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 of 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

Page 6-2


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).

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 5.2

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 Close 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 6-3


Click the Characteristics button and assign a radius of 0.006 m (0.019 ft) to all conductors. The value 10 in Subdivision # specifies a segmentation number of 10 for all the conductors. In this tutorial, this was done for illustration purposes only: we could have left 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.

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

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.

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.

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

Page 6-5


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.

Page 6-7


Blank Page

Page 6-8

Chapter 7. Fault Current Distribution Analysis
Chapter 7
7 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.

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


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.

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

Page 7-3


Static Wires: 7 No. 8 Alumoweld

Terminal (Source Substation) Characteristics Transmission Line Characteristics

Name Fault Current Contribution

(Amps)

Ground Impedance (Ω)

Span Length (m) Number of

Spans

Tower Ground Resistance (Ω)

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 Ω. 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 Ω 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 Specify Conductors. 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 Configuration) 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

Page 7-7


on Copy, enter “Hudson” under Copy Terminal To, then click OK. We must then correct the source current (722 – j 6453 Amps), grid impedance (0.3 + j0.0 Ω) and number of sections for this terminal (25), as well as the ground resistances of the towers (28 Ω); 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.

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 5.2.

Page 7-8


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

Page 7-9


Blank Page

Page 7-10

Chapter 8. Performance Evaluation of East Central Substation
Chapter 8
8 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,

Page 8-1


when touch voltages have been made satisfactory; outside the grid perimeter, however, step voltages need to be checked.

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 (Ω-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 Ω-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 Ω-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.

Page 8-2


Surface Layer Step Voltage Limit (V)

Type Resistivity (Ω -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 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. The remaining parameters can be defined using the Standard and Advanced safety screens. The 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.

Page 8-3


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 and uses 100 Ω-m as a default value. 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.

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 8-4


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

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.

Page 8-5


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 summary of the plots and reports that were requested in this screen.

• 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.

Page 8-6


• 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. 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 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 elect 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

Page 8-7


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. 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.

Page 8-8


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.

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

Page 8-9


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

Page 8-10


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.

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.

GraRep displays graphical output in its View Plots tab and textual output in its

Page 8-11


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:

********************************************************************************** AUTOGRID PRO USER INPUT DATA REPORT Creation Date/Time: 31 Oct 2006/13:26:50 ********************************************************************************** ---------------------------------------------------------------- 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

Page 8-12


Section Span Currents Shunt Tower Currents Shunt Tower Potentials One Terminal Plot Terminal Number ................... 1 All sections selected 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. ***************************************************************************************************** AUTOGRID PRO SYSTEM INPUT DATA REPORT Creation Date/Time: 31 Oct 2006/13:26:51 ***************************************************************************************************** -----------------------------------------------------------------------------------------------------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 Inner 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

Page 8-13


R14 150 0.064 1 0.1 ----------------------------------------------------------------------------------------------------- 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 computation) ----------------------------------------------------------------------------------------------------- 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 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 Ω–m and a thickness of 0.67 m. The bottom layer has a resistivity of 66 Ω–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...: 16.9465 in percent resistivities (Note RMS=SQRT(average(Di**2)). <--- LAYER CHARACTERISTICS --> Reflection Resistivity Layer Resistivity Thickness Coefficient Contrast Number (ohm-m) (Meters) (p.u.) Ratio ====== ============== ============== ============ ============ 1 infinite infinite 0.0 1.0

Page 8-14


2 297.0809 0.6741049 -1.0000 0.29708E-17 3 65.84742 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.

Page 8-15


8.3.4 Ground Grid Performance and Safety Analysis

The 4th 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: 31 Oct 2006/14:39:55 **************************************************************** 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) 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 #4: 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.

DATE OF RUN (Start)= DAY 23 / Month 7 / Year 2001 STARTING TIME= 13:26:51:35 >> Safety Calculations Table System Frequency............................(Hertz).: 60.000

Page 8-16


System X/R..........................................: 20.000 Surface Layer Thickness.....................( cm ).: 15.000 Number of Surface Layer Resistivities...............: 10 Starting Surface Layer Resistivity..........(ohm-m).: NONE Incremental Surface Layer Resistivity.......(ohm-m).: 500.00 Equivalent Sub-Surface Layer Resistivity....(ohm-m).: 297.08 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) | | ========================================================================== | 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 #5: 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 Ω-m is overlying a native soil with a resistivity of 297 Ω-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 23 / Month 7 / Year 2001 STARTING TIME= 13:26:51:66

Page 8-17


===========< 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 Mutual Resistance Calculations..............: NO Main Ground Resistance and GPR Calculations.: Representative Point Method Type of Electrodes Considered...............: Main Electrode ONLY Soil Type Selected..........................: Multi-Layer Horizontal SPLITS/FCDIST Scaling Factor................: 9.3596 1 1 MULTI-LAYER EARTH CHARACTERISTICS USED BY PROGRAM ------------------------------------------------- Common layer height : 0.674105 METERS LAYER TYPE REFLECTION RESISTIVITY HEIGHT No. COEFFICIENT (ohm-meter) METERS ----- ------ ------------- ------------- ------------- 1 Air 0.00000 0.100000E+21 100000. 2 Soil -0.999990 297.081 0.674105 3 Soil -0.637133 65.8474 0.100000E+11 1 CONFIGURATION OF MAIN ELECTRODE =============================== Original Electrical Current Flowing In Electrode..: 1000.0 amperes Current Scaling Factor (SPLITS/FCDIST/specified)..: 9.3596 Adjusted Electrical Current Flowing In Electrode..: 9359.6 amperes Number of Conductors in Electrode.................: 16 Resistance of Electrode System....................: 0.53714 ohms SUBDIVISION =========== Grand Total of Conductors After Subdivision.: 110 Total Current Flowing In Main Electrode......: 9359.6 amperes Total Buried Length of Main Electrode........: 1240.0 meters EARTH POTENTIAL COMPUTATIONS ============================ Main Electrode Potential Rise (GPR).....: 5027.5 volts (based on two representative points)

Report #6: Grounding Grid Performance

This report shows that the ground resistance of the East Central substation grid is .538 Ω, 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 8-18


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.

Page 8-21


Page 8-22


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 Ω.

DATE OF RUN (Start)= DAY 25 / Month 7 / Year 2001 STARTING TIME= 10:47:38:25 =======< 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 Central Station .............................East Central Resistance....................................0.53714 ohms Reactance.....................................0.0000 ohms Terminal No. 1 Greenbay Number of Sections............................64 Resistance....................................0.20000 +j 0.0000 ohms Source Current................................5160.7 Amps / -76.257 deg. Neutral Connection Impedance..................0.0000 +j 0.0000 ohms Span Length...................................330.00 m

Page 8-23


Terminal No. 2 Hudson Number of Sections............................25 Resistance....................................0.30000 +j 0.0000 ohms Source Current................................6493.3 Amps / -83.616 deg. Neutral Connection Impedance..................0.0000 +j 0.0000 ohms Span Length...................................330.00 m Terminal No. 3 NewHaven Number of Sections............................33 Resistance....................................0.30000 +j 0.0000 ohms Source Current................................5727.7 Amps / -82.526 deg. Neutral Connection Impedance..................0.0000 +j 0.0000 ohms Span Length...................................330.00 m TERMINAL GROUND SYSTEM (Magn./Angle) 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. Term: 2 Total Earth Current...: 4854.6 Amps / 87.618 deg. Earth Potential Rise..: 1456.4 Volts / 87.618 deg. Term: 3 Total Earth Current...: 4340.2 Amps / 87.807 deg. Earth Potential Rise..: 1302.0 Volts / 87.807 deg. Average Resistivity...........: 100.00 Ohm-meters Grid Impedance................: 0.53752 +j 0.0000 Ohms < Magnitude / Angle > Total Fault Current...........: 17355. Amps / -81.073 degrees Total Neutral Current.........: 8065.1 Amps / -75.555 degrees Total Earth Current...........: 9359.6 Amps / -85.826 degrees Ground Potential Rise (GPR)...: 5027.5 Volts / -85.826 degrees

Report #7: 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 8-24


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 8-25


Page 8-26


Page 8-27


Blank Page

Page 8-28

Chapter 9. Reinforcing The Grounding System

Chapter 9

9 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.

Page 9-1


9.1.1 Creating the Exponential Grid Scenario

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 Open 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.

Page 9-2


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).

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:

Page 9-3


Parameter Old Value New Value

Nab 7 13

Nac 9 17

Compression Ratio 1 0.8

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

Page 9-4


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.

The data entry is complete. Select Project | Process to start the computations and click OK on the confirmation screen. The program will automatically recompute 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 1.16 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 9-5


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, for simplicity.

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 red circle. It should also indicate with red and 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.

Page 9-7


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 926 Volts, and the analysis is complete.

Page 9-8


Page 9-9


Blank Page

Page 9-10

Chapter 10. Using GRServer

Chapter 10

10 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 10-1


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.

Page 10-2


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).

Note that the maximum touch voltage displayed on the plot is about 1.92 kV, as opposed to 926 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.

Page 10-3


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. 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).

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 |

Page 10-4


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, 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.

Page 10-6


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 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.

Page 10-7


10.5 SUMMARY This chapter 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.

Page 10-8

Chapter 11. Conclusion

Chapter 11

11 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 AutoGrid Pro software package. Consult the RESAP, MALT, and FCDIST User’s Manuals for more information about the capabilities of the engineering modules. As mentioned in the introduction, you may press the F1 key at any time to display a specific topic that is context sensitive. Your CD ROM also contains a wealth of information stored in pdf format (files that can be viewed with the Acrobat Reader utility) located in the pdf directory There you will find user’s manual on all CDEGS utilities, How To… Engineering Guides, Annual Users’ Group Meeting Manuals and much more.

Page 11-1

Chapter 11. Conclusion

Blank Page

Page 11-2

Notes

NOTES

N-1

Notes

NOTES

N-2

help manual

Documents

autogrid pro package

engineering guidehow

autogrid pro software

substation grounding

grounding problem

grounding design

ground gridblank page

blank page special note