CYMGRD 65 ReferenceManual En

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July 2011

CYMGRD 6.5

Reference Manual and

Users Guide


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Copyright CYME International T&D Inc.

Al l Rights Reserved

No part of this publication may be reproduced, or transmitted in any form

or by any means without the written permission of CYME International T&D.

Possession or use of the CYME software described in this publication isauthorized only pursuant to a valid wr itten li cense agreement from CYME.

CYME makes no warranty, either expressed or implied, including but notlimited to any implied warranties of merchantability or fitness for a particularpurpose, regarding these materials and makes such materials available solely onan "as-is" basis.

CYME International T&D reserves the right to revise and improve itsproducts as it sees fit. The information in this manual is subject to modificationwithout notice.

While every precaution has been taken in the preparation of this manual,CYME assumes no responsibility for errors or omissions, or for damages resultingfrom the use of the information contained herein.

CYME International T&D Inc.1485 Roberval, Suite 104St. Bruno QC J3V 3P8

Canada

Tel.: (450) 461-3655Fax: (450) 461-0966

Canada & United States: Tel.:1-800-361-3627

Internet : http://www.cyme.com

E-mail: suppor [email protected]

Other Trademarks: The names of all products and services other than CYMEsmentioned in this document are the trademarks or trade names of the respective owners.


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CYMGRD 6.5 Reference Manual and Users Guide

TABLE OF CONTENTS i

Table of Contents

Chapter 1

Gett ing Started............................................................................................11.1 General introduction .....................................................................................1

1.2 Software and hardware requirements ..........................................................11.3 Installing CYMGRD.......................................................................................21.4 CYMGRD modules .......................................................................................21.5 First-time user...............................................................................................31.6 Interactive data entry ....................................................................................31.7 How to use CYMGRD to design a new grounding grid ................................31.8 Dividing the grid into elements .....................................................................41.9 How to use CYMGRD to reinforce and verify existing grounding grids .......51.10 Creating and opening Projects and Studies .................................................51.11 The Windows layout of CYMGRD................................................................71.12 Default Parameters.......................................................................................9

Chapter 2 Soil Resistiv ity and Safety Assessement ..............................................112.1 Soil resistivity measurements and soil models...........................................112.2 Soil resistivity: Methodology and algorithm ................................................122.3 How to perform a soil analysis....................................................................132.4 How to specify the soil model type .............................................................152.5 How to perform Safety Analysis .................................................................172.6 Transferring the results of Safety Analysis for danger point evaluation.....192.7 Importing Projects from the previous version .............................................202.8 Importing Projects from the previous version An alternative method......21

Chapter 3 Grid Analysis Module...............................................................................233.1 General introduction ...................................................................................233.2 Electrode types and terminology ................................................................233.3 Electrode Sizing..........................................................................................24

3.3.1 LG fault parameters........................................................................273.3.2 Electrode Material ..........................................................................283.3.3 Electrode Sizing report ...................................................................28

3.4 Grounding system structure and location...................................................293.5 Split-factor (Sf), Decrement- factor (Df) and Definition for Remote-

Contribution in [%] ......................................................................................313.5.1 Decrement Factor (Df)....................................................................323.5.2 Split Factor (Sf) ..............................................................................32

3.6 Entering the Grid data.................................................................................343.6.1 Symmetrically-arranged grid Conductors.......................................343.6.2 Asymmetrically-arranged grid Conductors.....................................363.6.3 Symmetrically-arranged ground Rods............................................373.6.4 Asymmetrically-arranged ground Rods..........................................38

3.6.5 Rod Encasement............................................................................393.6.6 Arc Conductors...............................................................................413.7 Modifying and inspecting the station Geometry data .................................42

3.7.1 Enabling and disabling entries .......................................................423.7.2 Reviewing and verifying the data ...................................................42

3.8 Importing/Exporting Grid data and Station layouts.....................................433.9 Overlapping conductor elements................................................................433.10 Grid analysis and reports............................................................................443.11 Visualize the station layout in 3-Dimensions. .............................................463.12 The station layout and the Installation view. .............................................49


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CHAPTER 1 GETTING STARTED 1

Chapter 1 Getting Started

1.1 General introduction

CYMGRD assists engineers to design grounding facilities for substations and buildings.The program can be used to perform soil resistivity measurement interpretations, elevation ofground potential rise and danger point evaluation within any area of interest.

The program supports soil resistivity analysis taking into account field measurements, ananalysis necessary to arrive at a soil model that will subsequently be used for the analysis of thepotential elevations. The module supports both single-layer and two-layer soil model analysis.The same module also computes the tolerable Step and Touch Voltages per IEEE Standard 80-2000. The user defines the prospective fault current magnitude, the thickness and resistivity of alayer of material (such as crushed rock) applied to the soil surface, the body weight and the

anticipated exposure time.

CYMGRD is capable of performing ground-electrode sizing and ground potential risecalculations. CYMGRD can also determine the equivalent resistance of ground grids of arbitraryshapes that are composed of ground conductors, rods and arcs since it employs matrixtechniques for resolving the current distribution to ground. Directly energized and/or passiveelectrodes, not connected to the energized grid, can be modeled to assess proximity effects.

CYMGRD calculates surface voltage and touch voltage potential gradients at any point ofinterest within the area of investigation. The program can also generate equipotential contours forsurface and/or touch potentials, and potential profiles showing touch and step voltages along anydirection. Color-coding is used to view the results. These can be displayed in either two or threedimensions, making it easy to evaluate the safety of personnel and the equipment in and aroundthe grounding grid.

The results of alternative grid designs may be displayed simultaneously for comparison.

1.2 Software and hardware requirements

CYMGRD can be used with Windows NT or Windows 9X platforms.

The minimum hardware requirements are:

Pentium computer

64 MB RAM

20 MB free memory on the hard disk

A Microsoft mouse or equivalent;

A color monitor with Super VGA and a graphic card supporting 256 colors or more

Any printer or plotter supported by Windows


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2 CHAPTER 1 GETTING STARTED

1.3 Installing CYMGRD

The CYMGRD package requires a license to operate. Access is granted with the use ofeither a physical hardware key (Parallel port / USB) and/or a license string (Added manually or byimporting a license file). You may, however, install the CYMGRD package independent of the

license.

Installation steps:

1. Start Microsoft Windows.

2. Insert the CYME CD into the CD-ROM reader. If installing the WEB based package,open the executable and proceed to step 7.

3. The installation program should start automatically after a few seconds.

If it does not start by itself, use Windows Explorer to inspect the main directory ofthe CYME CD. Locate the icon Setup32 and double-click on it.

4. Click on the option to Install Products or Demos.

5. Choose English and then your version of Windows.

6. Choose CYMGRD from the list of software names.

7. Follow the prompts and screen instructions.

1.4 CYMGRD modules

The functions outlined in the General introduction (section 1.1) can be performed usingthe following modules:

Soil Analysis module (includes Safety Assessment): Defines either a two-layer, auniform, or a user-defined soil model CYMGRD plots the measured and calculated resistivity onthe same graph to allow easy verification of the quality of the soil model. The maximumallowable step and touch voltages are calculated according to IEEE Standard 80-2000. The

results are automatically communicated to the other modules.

Electrode Sizing module: Determines the minimum required ground electrode(conductor and/or rod) size in accordance with the IEEE 80-2000 standard. To determine theelectrode size, CYMGRD uses the parameters of the electrode material and the ambienttemperature setting. Users can select one or more of the materials from the CYMGRD library. Anumber of parameters for the materials can be modified and retained on a per-study basis.

Grid Analysis module: Calculates the current diffused by every element of conductor inthe grounding grid. The potential at the soil surface is determined from these results. You maydefine the grid one conductor at a time and/or by using groups of conductors arranged inrectangular sub-grids. You can define the grounding rods in a similar way. Other buriedconductors (such as nearby foundations) and/or neighboring grounding structures may also bedefined, to be able to assess the influence of their presence on the surface voltages. Thesestructures may be included in the analysis or excluded at any time for comparison purposes.

Plotting module: Generates a visual representation of the grid analysis results onPotential Contour and/or Potential Profile plots. Potential Contour plots can be used to displayboth touch and surface voltages. Both representations can be color-coded in 2 or 3 dimensions.Potential Profile plots can be used to display both step and touch voltages along a straight line, inany desired direction. The voltage variations, along with the corresponding maximum allowablevoltages, can be shown simultaneously on the same graph. Both Potential Contour and PotentialProfile graph types allow for easy identification of hazardous areas (i.e. areas where tolerable


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voltages are exceeded). These graphics can be sent to a printer, a plotter or copied to theWindows clipboard.

1.5 First-time user

If you have not used CYMGRD before, we suggest you read this manual beforeperforming a grounding study, to familiarize yourself with the capabilities of the program.Illustrated step-by-step examples have been included in Chapter 5 Example Studies to assist youin the utilization of CYMGRD.

Note: The ReadMe file includes important information as well. Please refer to thecontents of this file before operating the program.

1.6 Interactive data entry

CYMGRD features a modern multi-window interface for data entry. A spreadsheet isused to enter the data about station layout, soil resistivity, bus, and electrode sizing. Anyremaining data is provided via standard dialog box entries.

Note: Besides interactive data entry, the program remains backwards compatiblewith earlier releases. All cases entered via earlier Windows versions can bedirectly imported. In the unlikely case where users are interested in importingcases entered with the DOS version of the package, they should contactCustomer Support for further assistance.

1.7 How to use CYMGRD to design a new grounding grid

The first step in performing a grounding study is to define a Project and then a Studywithin CYMGRD. A Project can be viewed as a container of Studies. The studies may bevariations on a design theme towards optimizing a grid design.

The second step is to determine the soil model that will be used for the subsequentanalyses. This is done using the Soil Analysis module. It is the same module that performs theSafety Assessment calculations, thus yielding the maximum permissible step and touch voltagefor particular surface and exposure conditions as defined in IEEE Standard 80-2000.

The third step is to determine the electrode sizing (conductors and rods) taking intoaccount the worst single line to ground fault parameters in the substation and material of theelectrodes.

The fourth step is to actually enter the geometrical configuration of the station layout. All

electrodes (conductors and rods) need to be entered with their exact coordinates, burial depthand physical dimensions.

Note: AutoCAD drawings of the station layout may be directly imported intoCYMGRD assuming that certain design rules are followed. Please refer toChapter 7 CADGRD - The CYMGRD - AutoCAD Interface module for moredetails.


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The final step is to make certain that the design for the station meets the necessarysafety criteria. This can be accomplished through direct inspection of the danger points on thesurface. Entire areas may need to be verified by generating Potential Contours plots of the touchvoltages, particularly near the grid edges. Finally, Potential Profiles plots should be generated toascertain that touch and step potentials are not exceeded. If any of the safety criteria is not met,the grid design may need to be reinforced or modified. This is accomplished by repeating thisprocedure from the third step until acceptable results are obtained.

1.8 Dividing the grid into elements

The Grid Analysis module calculates the surface potentials by dividing the conductorsand rods into smaller segments called elements. These elements are the basic units that diffusethe injected fault current to ground. Using a higher number of smaller elements may give greaterprecision. However, the total number of elements in any grounding study cannot exceed 3500,including the main Primary electrode and any Return or Distinct electrode.

Note: You must select the number of elements so that the length of each element is

greater than 0.275 meters. So if you are presented with the error messageelement(s) with minimum resolution found after performing a grid analysis,you will need to reduce the number of elements for each of the conductorsshown.

The number of elements defined is not necessarily related to the number ofconductors in the grid or to the number of meshes the grid features.

How many elements per conductor/rod the program uses does not appear inany graphical representations and is solely related to the desired accuracy ofthe numerical simulations. There are cases for which increasing the numberof elements may result in higher accuracy. This is not, however, necessarilythe case despite the fact that the computational burden increasesconsiderably whenever the number of elements is increased.

An increased number of elements does not necessarily mean a moreaccurate estimate of neither the station resistance nor the ensuing surfacepotentials. A general rule of thumb is to begin by creating a study using oneor two elements per grid conductor (assuming the conductors physical lengthdoes not exceed 1 meter). If greater accuracy is desired, a new study withfurther conductor/rod subdivisions may be carried out to see if there is indeeda significant change in the results.


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1.9 How to use CYMGRD to reinforce and verify existing groundinggrids

For existing grids, soil measurements may be available from the original design. If thesoil model has already been determined and remains valid, it is not necessary to enter the soilmeasurements.

1. To take the existing soil model into account, choose the User-defined model for soilanalysis type in the Soil Parameters dialog box and enter the required information forthe upper, the lower and the surface layers. If desired, you may also enter User-defined data for use with the safety assessment data, which will be used todetermine the maximum permissible touch and step voltages.

2. Verify the station conductor and rod data entries and make certain anyreinforcements and/or additions are included in the station data. Determine theGround Potential Rise (GPR) and station resistance using the Grid Analysis module.

3. Use the plotting facilities, potential contours and/or profiles, to visualize touch andstep potentials in selected areas of interest.

4. Based on the results, judge the adequacy of the existing or reinforced groundingsystem.

5. If the grid is not adequate, return to Step 2 and make the necessary changes to thegrid layout by adding or removing conductors and/or rods.

1.10 Creating and opening Projects and Studies

A Project can be viewed as a container of Studies, which may be variations on adesign theme towards optimizing a grid design. The real container of data and results, however,remains the Study. Defining a project and a study is done via the Files menu, as shown below,from the menu bar of CYMGRD.

To define a new project, the New option needs to be chosen for the File menu. In thiscase, the dialog box shown provides the possibility to define a new Study, as well as a newProject that will contain the study. If, a new study is desired within the active project, click on thecheck box Insert into the active project and the lower project-related prompt will no longer beaccessible.


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To open an existing Project,click on the Open Project command of the File menu.

The browse function is activated that lets you see the various Projects already created inthe active folder.


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1.11 The Windows layout of CYMGRD

Once a Project has been created and a new Study generated within that Project, you willneed to begin entering your substation data. The CYMGRD interface is sub-divided intodedicated sections that occupy specific regions within the overall display.

The upper-left section is referred to as the Workspace view. It is reserved for theStudies and the corresponding Project file, shown in a tree structure. If more Studies wereincluded, they would be shown as part of the root Project. The active Study is shown using a redcheckmark as part of its icon.Note that this window features 3 tabs. The tab named Studiesshows the Project/Study tree structure. The tab named Contours shows the various potentialcontour plots generated for the active Study. The tab named Profiles shows the potential profileplots generated within the active Study. Thus, the second and third tabs are context-sensitive anddependent on the first tab.

The middle-left section is the Installation view. It displays a condensed view of thestation grounding grid layout (NOT UNDER SCALE AND WITHOUT TAKING INTO ACCOUNTTHE ASPECT RATIO OF THE MAIN GRID LAYOUT WINDOW). The Installation View contentsappear only when data is has been entered for the station layout. Gradual station data entry

enriches the view accordingly.

The upper-right section is the Workbook view. It is reserved to show the Grid Layout,Soil Model and Potential Contour and Profile plots generated during the simulation. It is the maindisplay area of the application. The Soil Model tab displays a visual representation of all the soilmeasurement data and possibly any calculated results due any soil analysis. The Grid Layouttab displays a visual representation of all the conductor data representing the station geometry.


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The lower-left section is the Data Entry view. It is used for data tabular input. The tabnamed Soil measurements is reserved for soil measurement data entry. The tab namedAsymmetrical Conductors is reserved for the grid conductor asymmetrical data, and so on.

The lower-right section is the Reports view. It is used to display the reports pertinent toall analysis options. The tab named Soil Analysis contains the report of soil analysis module,

while the tab named Grid Analysis contains the report of the Grid analysis module. Any contouror profile plots shown in the Workbook view will also have a corresponding report shown here.

The default view of a study with actual data is shown below to illustrate these principles:


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1.12 Default Parameters

The user can set the default parameters values, such as Shock Duration and NominalFrequency, when creating a new Study.

The Default-Parametersdialog box can be called by clicking the Defaultsbutton in theFile > Newdialog box.


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CHAPTER 2 SOIL RESISTIVITY AND SAFETY ASSESSMENT 11

Chapter 2 Soil Resistiv ity and SafetyAssessement

2.1 Soil resist ivi ty measurements and soil models

The ambient soil may contain a uniform resistivity to a significant depth. It is howevermore common to find that soils are stratified (i.e. composed of layers having differentresistivities). In general, to identify the exact soil stratification is a difficult problem. Manyapproaches have been suggested over the years, both graphical and analytical, but on manyoccasions, a judgment call will need to be made in order to arrive at practical soil models. Thereare currently techniques to interpret a set of soil resistivity measurements as a multi-layer soilmodel. CYMGRD offers a choice between Uniform and Two-layer soil models. Multi-layer soilmodels are not currently supported by CYMGRD.

The Two-layer model has an upper layer of a definite depth and a lower layer of aninfinite depth and with a different resistivity. The approach is a practical one and has beenfollowed for many years in substation grounding practice. Of the various soil measurementtechniques, CYMGRD supports only the Wenner technique, in which the distance (a)between

each pair of probes is equal.

A current (I) is injected and the resulting voltage (V)is measured by the voltmeter. Theapparent or measured resistivity is given by

( )

+

++

=

ba

a

ba

a

IVa

2222 4

21

4 or ( )IVa 2= if ba>>

where bis the length of the probe.


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12 CHAPTER 2 SOIL RESISTIVITY AND SAFETY ASSESSMENT

2.2 Soil resistivity: Methodology and algorithm

Let abe the apparent earth resistivity as computed by a two-layer model, 1and 2theresistivity of the upper and lower soil layers, and h the thickness of the upper soil layer

(CYMGRD assumes that the thickness of the lower layer is infinite). The module will find 1, 2

and haccording to the mathematical equations described below. The results will be automaticallycommunicated to the Grid Analysis module, which calculates the surface potentials.

K= reflection coefficient = (2 - 1) / (2+ 1)

n= integer varying from 1 to

h= upper layer thickness

a= electrode spacing

1, 2= upper & lower soil layer resistivity

By finding 1, 2, and h, CYMGRD minimizes the following function:

]/))([()( 22

1

mi

N

i

mi PiPPxf ==

where the sum spans all the available measurements.

miP = Measured value of earth resistivity at probe distance Di

)(iP = Computed value of earth resistivity at probe distance Di

Note: CYMGRD uses reduced gradient techniques to calculate the optimal modeland to minimize the RMS error. The term optimal signifies that the soil modelthat will be deduced will be the one that best fits the available measurements.

CYMGRD identifies measurements that do not seem to fit very well thecomputed resistivity function. In order to try to improve the accuracy of thesoil model, you may remove one or more such measurements from the inputdata and run the analysis again. These Suspect measurements can befound in the Soil analysis report and are also shown in the graphical

representation of the soil model marked with a cross and labeled Doubtfulpoints.

CYMGRD interprets either resistivity measurements or resistance values.

When the soil model is determined, all subsequent electrodes (no matter the type)and grounding structures analyzed by CYMGRD will assume the same soil model.


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No pockets of soil discontinuity are supported by the embedded technique. In otherwords, any local soil resistivity discontinuities, like regions of very high conductivitysurrounded by the native soil are not accounted for.

Only horizontal soil stratification type is supported by CYMGRD. No verticalstratification is taken into account.

Whenever two sets of soil measurements with identical probe spacing are entered,the program will not interpret the soil measurements and a warning will be generatedin the Soil Analysis report. This will be the case even if the two sets of measurementsfeature different resistivities.

Whenever measurement sets along different search directions are made for thesame site, it is not advisable to enter the various measurements as one set, not onlybecause duplicate probe spacing is not permitted but, more importantly, because, adistorted soil model may result.

You must enter at least one measurement for uniform soil. You must enter at leastthree measurements for two-layer soil. CYMGRD can accept a maximum of 100measurements.

2.3 How to perform a soil analysis

Soil resistivity and/or safety assessment analysis are done within the Soil Analysis module, which is activated by selecting the Soil Analysis engine from the drop-down list thatcontains all available analysis modules.


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The available data is shown in the Data Entry view window at the Soil Measurementstab that uses a spreadsheet-like interface as shown above. Note that any of the measurementscan be disabled using the checkmark in the dedicated column. This is where you can remove anysuspect measurements before recalculating the soil model.

The calculation is performed by clicking on the Run Engine button, which is the button

that has the lighting bolt as a symbol, next to the drop down list for the selection of the analysismodule.

The soil model is seen graphically in the Workbook view. Any measurements that thesimulation found departing from the average RMS errors that resulted from the optimization fit aremarked with an X on the graphic. The RMS error is computed to indicate the degree ofcorrespondence between the calculated soil model and the measured values, and is calculatedas follows:

RMS errorN

iN

ierror

=)(

2

The user will need to decide either to retain or to discard them by performing a newsimulation with a reduced set of measurements.

You can track the curve values with the mouse. Select any point on the curve with thecursor to see the probe distance and the calculated apparent resistivity values.

The text results of the soil analysis simulation can be seen in the Report view, within theSoil Analysis tab. The measured and calculated resistivities for the provided probe spacing arelisted along with the associated errors. The same measurements marked with an X in theWorkbook View are shown in red in the Report view. You can enlarge the Report view section bydragging the split bar to the position you want. The reports are shown here for illustration. Thecalculated soil model results are translated in the written report. This, actually, is a very good wayof verifying the soil model that the program has in memory before proceeding with the potential

rise calculations.


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2.4 How to specify the soil model type

The report shown in the illustration above pertains to a two-layer soil model. For a two-layer soil model, the program calculates the resistivity of the upper and of the lower layers of soil,along with the thickness of the first layer (or upper layer). The second layer (or lower layer) isassumed infinitely thick and the program simply calculates a resistivity for it.

To specify the soil model desired, select the Parameters option in the Soil menu item.

The module provides the options of interpreting the soil measurements as a two-layer soilmodel or as a uniform model. It also gives the possibility of entering any soil model desired(user-defined). If a uniform soil model is selected, the program will provide only one soilresistivity value, which is the average of all the entered measurements.


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Note: CYMGRD no longer supports the function of entering the Soil data as part ofthe Grid analysis as some earlier versions did. Thus, the Soil data can nolonger be bypassed if new soil data are to be used for analyzing the samegrid. ALL SOIL DATA NEEDS TO BE DEFINED AS PART OF THE SOIL

ANALYSIS. However, once analyzed, the Soil data results are stillcommunicated to the Grid module.

Whenever a User-defined model is selected, the results are calculated andtransferred automatically to Grid module without requiring the user to performan analysis.

Whenever one or more measurements are changed a new calculation must

be performed. The calculation will assure that the new soil model is used bythe program for subsequent analysis.


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2.5 How to perform Safety Analysis

This option allows the user to estimate the maximum permissible touch and step voltagesunder specific surface and exposure conditions. The safety assessment calculations comply withstandard North American practice as described in the IEEE Guide for Safety in AC Substation

Grounding, 2000 edition.

This standard requires the following data:

Body weight of the shock victim (by default equal to 70 kg, with an alternative of 50kg).

The thickness and resistivity of the material (i.e. crushed rock) on the surface of thestation native soil.

Soil resistivity of the upper and lower layers, and thickness of the upper layer of thenative soil (additional surface material excluded).

Shock duration (0.1 seconds by default). Protection reaction time.

CYMGRD uses the following equations, taken from IEEE 80-2000, to calculate themaximum permissible touch and step voltages.

For a 50 kg body weight:

E touch = (1000+1.5CsPs) 0.116/ t

E step = (1000+6.0CsPs) 0.116/ t

For a 70 kg body weight: E touch = (1000+1.5CsPs) 0.157/ t

E step = (1000+6.0CsPs) 0.157/ t


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where:

t is shock duration in sec.

Cs is the de-rating factor when high resistivity surface material is present. Thereduction factor Cs is a function of the reflection factor k and the thickness of theupper layer h.

Ps is the resistivity of the surface material in ohm-m.

This safety assessment data is defined in the same dialog box that specifies the soilmodel data. The purpose of the calculation is to arrive at a de-rating factor that will permit totake advantage of the high resistivity surface layer, thus permitting a higher touch voltage to betolerated. The de-rating factor Cs can either be calculated or obtained from graphs according tothe IEEE 2000 Guide. CYMGRD calculates the de-rating factor Cs according to Equation 27 ofIEEE Std 80-2000, i.e.

09.02

)1(09.0

1+

=

s

s

hCs

where:

sh is the thickness of the high resistivity surface layer material

s is the resistivity of the surface material

is the resistivity of the earth below the high resistivity surface material.

Note: For metal-to-metal calculations, of this kind assume s= when calculating

the de-rating factor, and 0== s , when calculating maximum permissibletouch and step voltages (IEEE Std 80, 2000).

The safety calculations are the only part of CYMGRD that uses the surface

layer high resistivity and it does so for the sole purpose of calculating themaximum permissible touch and step voltages. Actual potential rise analysisof the grounding assemblies takes into account only the native soil resistivitymodel reported by the Soil analysis.

The results of the Safety Analysis are included in the Soil Analysis report.

When User-Defined Safety is selected, CYMGRD will use the Maximum PermissibleTouch and/or Step as constant value to determine the Maximum Permissible Shock Duration.

When touch and/or step voltage must be limited by the specified value for MaximumPermissible Touch and/or Step, this feature helps user to determine protection speed (Shock-Duration) to achieve the specified values for the voltage limits.

The calculation can be based on touch and/or step voltages limits. But when both areselected, CYMGRD reports only the minimum calculated value for Shock-Duration based on theMaximum Permissible Touch or Step voltage.


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By choosing the above option, Shock-Duration will be reported as one of the outputresults under the Soil Analysistab in the report view.

2.6 Transferring the results of Safety Analysis for danger pointevaluation

Once the Safety Analysis has been performed, or, if user-defined safety thresholds areentered, maximum permissible touch and step voltages have been established, the results areautomatically transferred to the Plotting module. (See Chapter 4 Plotting Module)

Note: The Plotting module will only permit the utilization of the maximumpermissible step and touch voltages as calculated by the Soil analysis ordefined by the user.


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2.7 Importing Projects from the previous version

A Project may be imported from a previous version of CYMGRD by using the Importoption found in File menu.

Once this is selected you will need to specify the directory in which the projects that areto be updated reside. Click on the (i.e. Browse) button to change directories and navigate.


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Once a directory is selected, any projects found are listed by name.

Continue by selecting the project you want to import, followed by clicking the OK button.

Note: Only one project at a time can be imported.

All studies within the selected project will be automatically imported as well.

If a project has already been imported into version 6.00 or higher, OR hasbeen constructed using the version 6.00 or higher of the application, anasterisk will be shown under Exists to show that there is no need for theimport operation to take place for this particular project. You do, however,retain the option to overwrite it by rebuilding it from the older version.

2.8 Importing Projects from the previous version An alternativemethod

A Project may be imported from a previous version of CYMGRD (prior 6.0) using thefollowing alternative procedure. Start by running the old version of CYMGRD and open theProject you wish to import. Then, verify the Project number indicated at the right of the Projecttitle on the status bar at the bottom of the application window. The number in question is shownin white with a gray background. This value represents the extension of the project file on yourhard drive (i.e. grdprj.001). It will also be necessary to note the working directory for the Projecton the title bar at the top of the application window. Start the new version of CYMGRD and selectthe 'Open' item from the File menu. Change the working directory to that of the old Project asoutlined previously and select the file extension 'grdprj.*' in the Open dialog window. You shouldsee one or more files with the name 'grdprj' but with different extensions. Selecting and openingthe one with the same extension as the Project number from the old version of CYMGRD, shouldimport the contents of your Project into the application. At the same time, a file with the samename as the Project name from the previous version of CYMGRD, but with the extension 'cgp',

will be created in the working directory. From now on, when you wish to open this Project fromthe new version of CYMGRD, you need only select this 'cgp' file using the Open item from theFile menu.

Note: This alternative technique can be used if, for any reason, the directory cannotbe scanned with the previously described technique.

Only one project can be imported at the time, importing along all the studieswithin that project.


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24 CHAPTER 3 - GRID ANALYSIS MODULE

Note: Within CYMGRD, Conductor means horizontal ground-electrodes, and Rodmeans vertical or none-horizontal ground-electrodes.

No Return electrode should be modeled in the absence of a Primaryelectrode.

By using a Split-factor, CYMGRD takes into account Return current via thelocally grounded transformers, transmission line and distribution feeders.

If the substation fence is not bonded to the grounding grid, model the fenceposts as parts of a Distinct electrode. Otherwise, model them as part of thePrimary electrode.

You must define whether or not all elements of the Distinct electrode have thesame potential. They have the same potential if they are connected together.If the Distinct electrode is comprised of insulated sections, they do not havethe same potentials. This will have a bearing on the simulation and needs tobe specified as part of the Grid data.

3.3 Electrode Sizing

If desired, prior to designing the grounding grid, the minimum required conductor and/orrod size can be determined. Simply enable one or more electrode types provided in theElectrodes tab of the Data Entry view. CYMGRD calculates the minimum required groundconductor or rod size in accordance with IEEE 80-2000.

The selection of the suitable conductor material and size should satisfy the followingcriteria: electrical conductivity, corrosion resistance, current carrying capacity and mechanicalstrength.

Any conductor should be capable of conducting the entire ground fault current withoutexceeding a specified temperature.

As per ANSI/IEEE Std. 80-2000: ,

A Is the conductor section (in cmils)

ILG

is the RMS fault current (in A)

Kf constant dependent of the conductor material

( Kf= 7.01 for Copper, Soft Drawn)

tc fault duration (in sec.)

The size of the ground electrode must be specified prior to the grounding system design.CYMGRD calculates the minimum required size of the ground conductor or rod in accordance toIEEE standards.

To determine the minimum required electrode size, the constant parameters of thematerial of the electrode (conductor/rod), the Ambient-temperature, the Maximum fault-currentand the Fault-duration are required.


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CHAPTER 3 - GRID ANALYSIS MODULE 25

The default value for the fault current is 1000 [amps], and the Fault-duration is equal tothe Shock-duration as default. However the user should change the values to the desired valuesin the Busestab in the Data Entrywindow. (See below)

In order to consider auto-recloser reaction if any the Fault-Duration is assumed to beequal to the summation of the Shock-Durations.

Notes: The Fault-Duration in the Buses tab cannot be less than the Shock-Duration in the Soil Parameter dialog box.

Ambient temperature can be specified in the Soil Parametersdialog box.

In order to specify the electrode material, the user can choose one of the materials fromthe CYMGRD library in the Electrodes tab. (See below). In addition, the user can change thematerial parameters in the CYMGRD library to specify a user-defined material.

The following figure shows the CYMGRD library (Electrodes data entry tab), whichincludes the list of the most common grounding electrode materials and corresponding parametervalues.

After all the required parameters are specified, the result will appear in the Outputwindow under the Electrode Sizingtab. There is no need to run electrode-sizing analysis. Thefollowing figure shows an electrode-sizing result.


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After the electrode material and size have been chosen by the user, the diameters of theelectrodes are required. CYMGRD has a feature to help entering the diameter of the electrodes.When one or more Conductor and/or Rod items are selected in the Electrodesdata entry taband that the Electrode Sizingreport has been generated (a valid Soil Model analysis must beavailable for the active study), a list of corresponding Materials and Sizes will be available forselection in the data entry windows for all matching Electrode types.

By picking a Material from the list, the Nominal Size (this is the default setting asreported in the Electrode Sizing results) for the Conductor will be set and its Diameter will beadjusted accordingly.

Proceeding to change the Size will alter the Conductor Diameter. Modifying theDiameter directly will cancel both the Material and Size selections.


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3.3.1 LG fault parameters

LG fault current and corresponding X/R are the results of fault analysis and are requiredfor Electrode Sizing analysis.

In the Buses tab of Data Entry view, the user must enter data for all the buses in thesubstation. CYMGRD will automatically choose the bus that requires the thickest electrode andapply it towards the Electrode Sizing analysis.

As shown under the Buses data entry tab above:

When the Enabled box is checked, it means that the bus data will be considered inthe analysis.

Usually a substation has two or more buses. CYMGRD identifies each bus and thecorresponding parameters by a Bus ID. The results of the analysis appear in theElectrode Sizing tab in the Reports view with corresponding Bus ID (See followingimage).

LG Fault Current is the total single line- to-ground fault current in amperes.

Remote Contribution is the summation of the contributions (of the LG Fault Current)from the transmission lines (not the local transformers within the substation) dividedby total fault current and multiplied by 100.

LG X/R is (2x1+Xo)/(2R1+Ro) for the corresponding single line-to-ground faultcurrent.

Note: CYMGRD does not use the following parameters for Electrode Sizing,however, in order for the bus data as a whole to be saved, they must besupplied. CYMGRD uses this additional data for grid analysis when a CurrentSplit Factor needs to be determined.

Transmission Lines is the number of the lines connected to the bus.

Rtg is the ground electrode resistance of the above transmission line (Default = 100Ohms).


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Distribution Feeders is the number of the feeders connected to the other side of thetransformers which, in turn, is connected to the bus.

Rdg is the ground electrode resistance of the above feeders (Default = 200 Ohms).

3.3.2 Electrode Material

To determine the minimum required electrode size, a correction factor (i.e. Decrementfactor), the constant parameters for the electrode material and ambient temperature value arerequired:

The ambient temperature is defined in the Grid Parameters dialog box (Default = 40degrees Celsius). The Grid Parameters dialog box can be accessed under theParameters item of Grid menu.

The type of the material along with its parameters is specified in the Electrodes tabof the Data Entry view (See below).

CYMGRD uses the information in the Buses tab to calculate the Decrement factor inaccordance with the standard. This factor is used to take into account the DCcomponents, resulting in the asymmetrical fault current for the corresponding faultduration.

The following image shows the CYMGRD ground conductor library (Electrodes tab). Inthis example, Copper commercial hard-drawn is selected for the conductor sizing and Copper-clad steel is selected for the rod sizing.

Note: Certain parameters, such as the Melting Temperature (Tm) can be modifiedin order to better define the materials in use. Any altered values will be savedonly as part of the active study.

3.3.3 Electrode Sizing repor t

After all the required data for the Electrode Sizing has been specified, the result of theanalysis automatically appears in the Electrode Sizing tab of the Reports view.


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3.4 Grounding system structure and location

CYMGRD is capable of analyzing grounding systems of either symmetrical orasymmetrical configuration. A grounding system is made of electrodes, which the programdivides into elements for calculation purposes. If a two-layer soil model is used, then the grid

conductors must be located in the upper layer. Grid rods may cross the two-layers boundary.Important factors for the calculation of station resistance are the station geometry and the soilmodel as determined from the Soil analysis. When calculating the Ground Potential Rise, theinjected current needs to be known as well.

While the station geometry data is entered in the Data Entry view, the remaining datacan be entered through the Grid Parameters dialog box, which can be accessed under theParameters item of Grid menu.

That same dialog box allows the user to specify the attributes of the Distinct electrodeand specify the current for the Return electrode.

The single line-to-ground fault current (LG) at the fault location produced by thesubstation, does not necessarily flow to the ground via the grid. Some of it may be diverted backto the system through line-to-ground wires, cable sheaths and/or tower counterpoises. The factthat only a part of the total fault current usually flows between the grounding system and thesurrounding earth has implications on both personnel safety and equipment requirements.


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To calculate that portion of the fault current, CYMGRD presents three options in the GridParameters dialog box.

Infinite Z: CYMGRD considers that total LG current goes to the surrounding earth viathe ground grid.

Current Split Factor: CYMGRD estimates the current split factor (Sf) in accordanceto IEEE Std-80. The current split factor is a ratio based on the portion of the LGcurrent that goes back to the remote sources via the ground grid. Thus;

gLGf RISGPR =

User Defined (Split Factor or Parallel Z): When you choose this option, you candirectly enter your desired Splitting Factor or Parallel-Z.

Note: The check box Include Local Contribution accounts for the case where alocal source is solidly grounded to the ground grid. This is option is availablefor the Infinite Z and User Defined options.

When this option is checked then the % Remote Contribution (% RC) can be

entered in the Bus Data entry field.

If it is unchecked the field to enter the Remote Contribution is defaulted to 100% and can not be edited.

The equivalent resistance in parallel with the grounding grid, Parallel R(Rp p is the totalequivalent resistance (in ohms) of the sky wires and counterpoises of all the lines connected tothe substation. The LG fault current is divided between these two resistances (Rg and Parallel-

Zp).

The following equation shows the relationship between Split Factor (Sf), Parallel-Z (Rp p)and Ground resistance.


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The same dialog box allows the activation or deactivation of entire sets of electrodecomponents to assess their effect on the performance of the grid grounding design withoutresorting to extensive editing of the station data.

Note: To direct the entire ground fault current into the grid, without any current

division, set the Parallel-Z (User-defined) to 9999 or the Split Factor (User-defined) to 1.

For a Return electrode enter the return electrode current. If not, the current is0.

If you change any of the electrodes after performing an analysis, you will haveto re-analyze the ground potential rise and grid resistance

Grid conductors cannot bridge two soil layers if a two-layer soil model is used.However, Rods can bridge the two layers of the soil model.

3.5 Split-factor (Sf), Decrement- factor (Df) and Defini tion forRemote-Contribution in [%]

To avoid overdesigning in substation grounding systems, CYMGRD takes into accountthe correction factors (Split factor and Decrement factor) in accordance with IEEE 80-2000.

IEEE Standards emphasis is on the determination of the actual fault-current flowing,between the substation grounding system and the surrounding earth.

The fact that only a part of the total fault current usually flows between the groundingsystem and the surrounding earth has implications on both personnel safety and equipmentrequirements. (See figure below)

To account for both the Decrement (Df)and Split (Sf) Factors, the Ground Currentisnow computed as per the following equation of the IEEE STD 80 2000.


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For conservative and desired approximation of the above mention correction factors, thefollowing parameters are required in the Busestab.

[Total fault current] The single phase to fault (LG) current at the Buses.

[Remote Contribution

(%)]: =

(Summation of the contributions from the lines)/(LG fault

current) X 100.

[LG X/R] = (2X1+Xo)/(2R1+Ro) from the bus fault analysis result.

[Transmission Lines]: Number of lines (which has sky-wire) connected to the bus.

[Rtg]: Ground electrode resistance of the transmission line (theconservative default value is Rtg=100 Ohm).

[Distribution feeders]: Number of grounded neutrals at the other sides oftransformers.

[Rdg]: the ground electrode resistance of a distribution feederneutral. (The conservative default value is Rdg=200 Ohm).

3.5.1 Decrement Factor (Df)

To complete the calculation correction in accordance with the standard, the Decrementfactor (Df)must be included in the calculation of the Ground Current. This factor is used to takeinto account the DC components, resulting in the asymmetrical fault current, for correspondingfault duration.

( )af Ttf

a

f et

TD

/211 +=

Where:

tfis the fault duration.

3.5.2 Split Factor (Sf)

In order to take into account that portion of the fault current, the Split factor (currentdivision factor) must be used.

This implies that the GPR, touch, and step voltages are also lower than might be

expected. Thus, substation and personnel require less or lower rated protective equipment. Thistranslates to savings when designing the grounding system.

In order to estimate and take into account the Split Factor in the analysis, choose theoption Current Split Factor in the Grid Parametersdialog box.


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The following diagram is a detailed illustration of how the line to ground current isdistributed between the Ground Grid, Tower Footings, Sky Wires, Local and RemoteContributions.

The electrical equivalent circuit of the above is as follows:


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The above equations should always be valid and therefore the GPR is computed as:

Or

Note: % RC is the Remote Contribution % entered in the Bus data entry

parameters.

3.6 Entering the Grid data

Ground Grid data can be entered by either specifying directly their geometricalcoordinates or can be imported from an AutoCAD file formatted for use with CYMGRD. Thissection describes data entry for the case where AutoCAD data files are not available. InCYMGRD, the grid components data is classified into five categories: Symmetrically arrangedgrid conductors, asymmetrically arranged grid conductors, arc conductors, symmetrically-arranged ground rods and asymmetrically arranged ground rods. All are explained in thefollowing sub-sections. Section Symmetrically-arranged grid Conductors explains the

import/export of AutoCAD data.

3.6.1 Symmetrically-arranged grid Conductors

This type of array is rectangular, with a number of conductors laid out along the long andshort axes, creating a grid. CYMGRD assumes that symmetrically-arranged grid conductors areburied horizontally and are oriented along two perpendicular axes (the X and Y axes in thegraphic window). The spacing between the conductors is assumed to be equal along each axis,but the spacing along the Y-axis can be different from the spacing along the X-axis. The data forsymmetrically-arranged components is entered using the Symmetrical Conductors tab of theData Entry view.


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Note: If the electrodes (Conductors or rods) placed in the grid cannot satisfy aplacement pattern with some symmetry, then they should be defined usingasymmetrical electrodes.

3.6.2 Asymmetrically-arranged grid Conductors

An asymmetrically-arranged conductor is a single straight conductor stretched betweentwo points defined by two coordinates (X1, Y1, Z1) and (X2, Y2, Z2). Asymmetrical conductorsthat are slanted may be represented in the model (Z coordinate), which is not the case for thesymmetrical arrangements, which are entered using a common burial depth (X,Y). Furthermore,each conductor may have a different diameter, which is not the case for the symmetricalarrangements with a common diameter for all conductors.

Asymmetrical conductor data is shown above. Note that the check box Enabled isselected, which means that it will be considered in the Grid Analysis. Also, the Primary electrodeType is selected (default). The drop-down box allows modifying that default to Return orDistinct.

For this example, we have used the asymmetrical conductor arrangement to representthe upper left protruding part of an L-shaped grid.

The following set of data is used to define an asymmetrical grid:

Type Primary, Return or Distinct.

[X1, Y1, Z1] and[X2, Y2, Z2]

Coordinates of two ends of each conductor. Conductors maybe inclined with respect to the soil surface, which CYMGRDassumes to be horizontal.


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Number ofconductor elements

CYMGRD considers this number of elements for conductorsparallel to the X (or Y-axis) in finite-elements analysis.

Diameter Ground conductor diameter.

3.6.3 Symmetrically-arranged ground Rods

A symmetric array of ground rods covers a rectangular area in which rods are located inrows parallel to the X-axis with all rods in a row equally spaced. All rods defined in the samearray have the same burial depth, length and diameter.

Symmetrical rod data is shown above. Note that the check box Enabled is selected,which means that it will be considered in the Grid analysis. In this example, the Primaryelectrode Type is selected (default). The drop-down box allows modifying the default to Returnor Distinct.

The following set of data is used to define symmetrically-arranged rods:

Type Primary, Return or Distinct electrode.

[X1, Y1]and[X2, Y2]

Coordinates of the two opposite corners of the area where therods are placed.

Rod rows parallelto the X-axis

Number of the horizontal rod rows on the display.

Number of groundrods per row

Number of rods along each row (Defined above).


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Length Ground rod length.

Depth Burial depth (the distance between the soil surface and the topof the rods).

Diameter Ground rod diameter.

3.6.4 Asymmetrically-arranged ground Rods

An asymmetric array of ground rods is a single row of equally spaced rods. The positionof the first rod is given by the coordinates (X1, Y1, Z1) and the position of the last rod in the rowis given by the coordinates (X2, Y2, Z2). The upper end of each rod lies on the straight linebetween these two points. All rods defined in the same array have the same length anddiameter. If a single rod is specified (Number of Rods along axis = 1), then only the starting pointcoordinates (X1, Y1, Z1) need to be entered.

Asymmetrical rod data is shown above. Note that the check box Enabled is selected,which means that it will be considered in the Grid analysis. The Primary electrode Type is

selected (default). The drop-down box allows modifying the default to Return or Distinct.

For this example, we have used the asymmetrical rod arrangement because all the rodsplaced in the grid were strategically positioned at specific coordinates. It is seen in the data thatwe have entered the rods one at a time using different coordinates for the beginning and the endpoints.

The following set of data defines a row of rods:



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[X1, Y1, Z1]and [X2,Y2, Z2]

Coordinates of the two end points of the row of rods.

Number of rodsalong axis

Number of rods in the row.

Elements per Rodin upper soil layer

Number of elements for rods in upper soil layer for the finite-elements analysis.

Elements per Rodin lower soil layer

Number of elements for rods in lower soil layer for the finite-elements analysis.

Length The rod length.

Diameter The rod diameter.

3.6.5 Rod Encasement

In order to improve the impact of a rod in

the grid, the rod may be installed in a cylinder ofsemiconductor material buried in the soil. See thefollowing picture from IEEE 80.

This is of particular interest in medium andhighly resistive soils.

To enter a rod encase in CYMGRD:

1) Activate the check box Material-Encasedfor the rod.

2) Enter the Material Thickness (thecylinder radius).


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3) In the Grid Parametersdialog box,enter the Resistivityof the materialaround the rod in the encasement(cylinder). The default value is 100[Ohm-m].


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3.6.6 Arc Conductors

An arc conductor is a circular or arced conductor laid in the ground.

Arc conductor data is shown above. Note that the check box Enabled is selected, whichmeans that it will be considered in the Grid analysis. The Primary electrode Type is selected(default). The drop-down box as allows modifying that default to Return or Distinct.

The following set of data defines an arc conductor:


[X1, Y1] Coordinates of the arc center.

Starting angle Beginning of the arc in degrees.

Ending angle End of the arc in degrees, assuming a counter-clockwiserotation.

Radius The radius of the arc.

Number ofconductor elements

Number of conductor elements the arc is to be approximatedwith as an inscribed polygon.

Depth The arc burial depth (common for both ends).

Diameter The arc conductor diameter.


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Note: A positive value of Z denotes a position below the surface of the soil for allelectrode types and arrangements. No negative Z is permitted.

Both ends of an asymmetrical grid conductor must be in the same soil layer.Only ground rods are permitted to bridge two separate soil layers.

The minimum number of conductor elements that an arc can be approximatedto is 3.

Electrodes are color-coded in the graphic window. Primary electrodes arered, Return electrodes are blue and Distinct electrodes are green.

3.7 Modifying and inspecting the station Geometry data

3.7.1 Enabling and disabl ing entries

Click on the Enabled check box located in the dedicated spreadsheet column of the

Data Entryview.If a check mark is shown the component is enabled. To disable it remove thecheck mark.

3.7.2 Reviewing and verifying the data

Any spreadsheet entry can be highlighted on the station layout drawing for verificationand inspection. In order to do that, the appropriate cell on the far left column needs to behighlighted. It is the column that shows the entry number of the component. When you select aconductor in this fashion, it is highlighted in yellow on the grid layout, so that you may see whichelectrode you have selected. This is particularly useful when erroneous coordinates have beenentered and you wish to correct them.


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3.8 Import ing/Exporting Grid data and Station layouts

These commands allow you to import from or export to an AutoCAD drawing the gridlayout design. The menu commands are listed under Grid > Electrodes.

More details about the preparation of the data in AutoCAD, the import/export mechanismof CYMGRD and its CAD Editor function is detailed in Chapter 7 CADGRD - The CYMGRD -

AutoCAD Interface module.

Note: Data files from earlier DOS versions of CYMGRD can still be imported. If sucha case arises, please contact CYME International T&D Customer Support forinstructions.

CYMGRD does not save station data in dedicated files. Instead, theyconstitute an integral part of the entire study.

3.9 Overlapping conductor elements

CYMGRD cannot perform a station analysis if conductor elements are found to overlapeach other. The term elements pertains to the subdivision of ground conductors and rods in

order to increase the accuracy of the calculations. If overlapping elements are found duringexecution the calculations will stop and an appropriate error message will be generated indicatingwhich components overlap. Common errors causing that condition are duplicates of eitherasymmetrical conductor elements or grounding rods that are placed one on top of another. Whenthe duplicate is disabled or removed from the grid design, the problem should be alleviated.


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3.10 Grid analysis and reports

The Grid analysis can be performed in the same manner that the Soil analysis wasinvoked. Run the Grid Analysis by clicking on the lightning bolt button. The time bar at thebottom of the desktop provides an estimate of the time required to complete the analysis.

The results of the simulation are shown in the Grid Analysis tab within the Reports viewof the application, the first part of which is illustrated below. It is seen that, at first, the soil modelused in the calculations is echoed in the report. It is important to verify that the soil model used inthe grid analysis is indeed the one obtained from the soil analysis results. Otherwise, the gridanalysis results may not be relevant.

Next, the coordinates of the grid elements and the current every each element diffuses toground are listed. Note that for each element, a column indicates whether it belongs to asymmetrical or an asymmetrical assembly and a second one indicates the reference number ofthe assembly it belongs to. The reference number is the row item number of spreadsheet dataentry. This way, it is easy to track the elements even if they might represent subdivisions oforiginal data entities.

The last part of the Grid Analysis report is shown below. Conductor data is listed first,followed by the rod data. Similarly, Primary electrode results will be followed by any Returnelectrode results and finally, by Distinct electrode results, if any.


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The Ground potential rise and the station resistance is displayed at the beginning of thereport.

Note: The various elements of the grounding installation, as resulted from thepartitioning of the grid conductors and ground rods diffuse a positive currentinto the ground.

The simulation may however indicate that the current diffused to ground byone or more elements is zero. This indicates that CYMGRD found that each

such element diffused a small negative current and consequently set it tozero. This situation is due to numerical instability. To avoid this problem,change the number of elements in the affected conductors or rods so thatthese elements are about as long as other elements in other conductors inthe grid.

If negative currents are found for some of the elements of the Primaryelectrode during the analysis, CYMGRD will indicate these elements in thegrid report flagging them in Red. This may be the result of false numericalrepresentation, since currents from all elements should be positive (diffusedto ground). If the negative current, from one or more elements, adds up tomore than a few percent of the totally injected current, a new simulationshould be performed with the number of elements changed as explainedabove. The same considerations apply if a positive current is found for any of

the elements of the Return electrodes. No such considerations apply to theDistinct electrodes.

Experience has shown that the negative current is a very small fraction of theinjected fault current and that the error introduced in calculating the stationresistance and GPR is negligible. Simulations performed after changing thenumber of elements in conductors should indicate no change in the overallresults, apart from correcting the negative currents.

It is always advisable to verify that strictly positive currents are diffused to theground by all the elements.


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3.11 Visualize the station layout in 3-Dimensions.

The station layout shown in previous illustrations was a 2-D representation (Default). It ishowever, possible to view the station layout in three dimensions as well. The 3-D view is oftenuseful, since a common error when entering input data is to introduce disparities in the burial

depth of both conductors and grounding rods. A 3-D view of the station layout usually helps tolocate these inconsistencies via a simple inspection.

To generate a 3-D representation, position the mouse on the window containing the gridlayout and right-click.

This provides access to the Chart Settings dialog that allows access to the actual graphsettings.

By default, the Style setting Area is selected, which means that the grid layout will beshown as a function of the dimensions and aspect ratio of the display window. If the optionScaled Area is selected, the grid will be drawn to scale with proper consideration of its actualdimensions.


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More options are available for the Graph and other components of the Chart from withinthe Chart Settings dialog.

Similar settings can also be applied to all other electrode types.. The same settings areused for both the 2-D and 3-D representations.


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Note: The Grid Analysis calculations are not affected even if an electrodecomponent is made invisible. It is solely a method for the visual examinationof the Grid layout.


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3.12 The station layout and the Installation view.

A representation of the station layout is always shown in the Installation view of theapplication. The station layout can be visualized in 2-D, 3-D or Auto mode. When in 2-D mode,the station layout is always shown in 2-D. When in 3-D mode, the station layout is always shown

in 3-D. With the Auto mode, the station layout is shown using the opposite mode defined for theGrid Layout in the adjacent Workbook view.

3.13 A note on modeling Grounding Structures

The Primary electrode is the grounding structure that absorbs the fault current. The basicanalytical assumption CYMGRD makes, in compliance with International Standards, is that theentire grounding system that absorbs the fault current, and diffuses it to the ground, is elevated toa single potential. This is the Ground Potential Rise of the Primary electrode (i.e. the calculatedGPR). Thus, voltage drop along the grid electrodes is not modeled. Furthermore, the groundstructures are assumed to contain only resistance (i.e. no reactive component for the groundinggrids and structures is modeled by CYMGRD). Modeling of the reactive component of the gridimpedance may be necessary for stations possessing either a resistance of less than 0.5 Ohmsor if they extend over an unusually large area.

Any metallic structures bonded and/or connected to the primary electrode by accident orwith purpose such as fences, building foundations etc, and that help in reducing the GPR shouldbe modeled as part of the Primary electrode.

The Return electrode should only be used in the case where a grid absorbing currentfrom the ground exists and is located in the vicinity of the energized Primary electrode. BothPrimary and Return electrodes can only be energized by virtue of currents, not voltages.

The Distinct electrode should be used to model underground metallic structures that areadjacent to the Primary electrode but are inert (i.e. they are not energized by any current).Despite the fact of being inert, they should be modeled since they absorb currents, thus alteringthe surface potentials in the vicinity.


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When a simulation comprises Primary, Return and Distinct electrodes, all electrodes areassumed to be within the same soil featuring the soil model obtained from the Soil analysismodule of CYMGRD. Neither the Return nor the Distinct electrode can feature a galvanicconnection with the Primary electrode.

3.14 Soil data from earlier versions of the application

The application provides dedicated embedded safeguards against inconsistent data andis less permissive than earlier versions. It has already been mentioned that soil model values canno longer be entered separately within the grid parameters. Earlier versions, however, did permitthis. As a result difficulties may be experienced when importing studies generated with previousversions. If inadvertent program termination or inconsistent results are seen, an effective remedywill be to re-affirm a slight modification to the soil data. Re-typing in the same value should besufficient. This will simply invalidate all analysis results. Simply perform a new set of analysis toobtain new and consistent results.


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CHAPTER 4 PLOTTING MODULE 51

Chapter 4 Plotting Module

4.1 General introduction

The Plotting module is used to calculate and view the results of the surface potentialanalysis. With this module the user can perform danger-point evaluation on various surfacepoints and/or areas of the substation. 2-D and 3-D contour plots illustrating the equipotentialtouch and/or surface contours can be generated. These can be color-coded for easy evaluation.Finally, touch and step potential graphs can be generated for linear directions by specifying thebeginning and end points and the step size.

4.2 How to generate Touch and Surface potential Contours

Equipotential contours can be generated only after the Grid analysis has beenperformed. A graph containing equipotential contours is a graph that pertains to a particularportion of the station layout (that can be the entire grid) and that shows the variations of the touchor surface potential.

When the area of interest is specified, CYMGRD performs calculations taking intoaccount the various surface points and the current diffused to the ground from all grid elements.To specify the area of interest, position the mouse on the station layout graph and left click. Thecrosshair that appears determines one corner of the area. Hold and drag the mouse to select thearea, as shown below.

Once the mouse button is released, the program displays the coordinates of theencompassed area. At this point, the number of intervals in the X and Y directions can bespecified. CYMGRD uses these values to divide up the area before calculating the surface


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52 CHAPTER 4 PLOTTING MODULE

potentials. By default, the program considers 20 intervals in both X and Y directions. More can bespecified for higher accuracy since more intersections will result for the area under consideration.

Once the area and the resolution are defined the program generates a graph thatportrays the potential gradient in the selected area.

The generated graph is solid-filled. Actual equipotential contours can be seen along withtheir associated levels if the appropriate options are applied. Position the mouse on the contourgraph and right click.

Select the Settings item, which will open the Chart settings dialog. By highlightingContours, it is seen that different options are available for drawing contours in the Style drop-down box.


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The default setting is Solid-filled. If another option is selected, Lines with labels forexample, then the equipotentials appear along with their levels indicated in the form of labels asshown in the following image.

If, for some reason, the number of generated equipotentials is too large generating agraph that looks too busy, less can be plotted so the graph will be amenable to inspection. The

number of equipotentials Levels can be controlled from within the Chart settings dialog.

As shown above, it can be controlled by increasing or decreasing the desired number ofLevels that are to be drawn.


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4.3 Touch and Surface potential contours

Once the equipotentials have been generated it is very easy to switch between Touchand Surface potential contours. In fact, both are contained within the same graph. Right-click onthe generated contour plot to access the Contours sub-menu. Switching from Touch to Surface

creates the reverse stress patterns as shown below. This is due to the fact that areas with hightouch potential are actually characterized by low surface potentials.

Note: It is not necessary to access the contour settings to ascertain whether anequipotential contour plot pertains to Touch or Surface potentials.

When moving over the chart with the mouse, Touch contour plots display themouse cursor in the shape of a hand instead of an arrow to designate thetouch nature of the potentials shown.

4.4 Contour color coding and Safety Analysis

Equipotential contours plots are color-coded based on the results of the safety analysiscalculations as conducted within the Soil Analysis module. Whenever a safety analysis isconducted, maximum permissible touch and step voltages are calculated. The program considers4 thresholds for the touch potentials and another 4 for the surface potentials. The thresholdsconsidered for the touch potentials are 25%, 50%, 75% and 100% of the maximum tolerabletouch voltage. Anything above the maximum permissible touch voltage is shown in variousshades of red to signify dangerous areas.

The user can define the colors although default settings are provided by CYMGRD. Asshown below the threshold colors can be accessed under the tab thresholds in the appropriateChart settings. In this example, for the Touch potentials Chart settings, the first threshold regionis defined to be between 0 and 25% of the maximum permissible touch voltage, and the defaultcolor is coded as green.


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The last threshold region is defined such that everything above 100% will be red, whileeverything between 75% and 100% of the maximum permissible touch will be plotted in blue.

Thresholds are defined for the surface potentials as well. Four thresholds are definedhere as well; the difference being that the maximum threshold is set to the Ground Potential Riseas calculated by the Grid analysis module. The thresholds are accessed in the same way as withthe touch equipotentials. The last threshold is shown here for illustration.

The color-coding of the Surface Potential thresholds are by default the reverse of thetouch potentials.


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4.5 Allowable LG fault current

Based on the grounding analysis result, CYMGRD calculates the maximum allowable LGfault for each contour of the grounding system under the study.

CYMGRD calculates and reports Allowable LG Current for the selected area in eachcontour plot. The Allowable LG Current is maximum LG fault current that causes safe touchvoltage in the entire selected area in the contour.

If the LG fault current in the Buses tab is more than this value, the contour shows theunsafe area in the plot.


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4.6 How to generate 3-D contour plots

The plots shown in 2-D can also be generated in 3-D, in the same way that station layoutplots were generated in 3-D. Right click on the 2-D graph, select Settings, and under Graphcheck 3-D. All contours, both touch and surface, are now shown in 3-D.

3D graphs can be rotated with the mouse (left click, hold and move) to position the graphfor better inspection.


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4.7 Contour graph reports

Whenever a contour plot is generated the program produces a corresponding tabularreport. This report contains among other things, the point of maximum potential found within thearea selected. This point may be of interest since it represents, for touch voltage contours, the

steepest gradient found during the analysis. The same point is shown with a yellow cross-hair onthe contour graph both in the 2-D and the 3-D views.


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4.8 Contour graph management

All contour plots generated within a study are saved as part of the study and displayed inthe Contours tab of the Workspace view. The contour plots shown in the Workbook viewpertain only to the active study and are identified with a user-definable title. Charts can be deleted

and renamed using the facilities shown in the illustration below. Right click with the mouse on anycontour chart and select an activity to either rename or delete it all together.

4.9 How to perform spot-check danger point evaluation

Besides generating contour plots that, essentially, assess the safety of certain areas ofinterest, specific points can be checked for their potential values using the mouse.

As we move the mouse within the contour graph., the program generates a tool tipshowing the coordinates (the first two numbers) and the voltage value (the last number). At each

location, that last value is the touch or surface voltage depending on whether the contour graph isa touch or a surface contour plot.


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Another important feature of this facility is that whenever the mouse is moved within thecontour plot, a cross hair appears in the Installation view indicating the actual position of thesearched point with respect to the entire grid. This option may prove useful, when the contourgraph encompasses only a small region of the grid area as opposed to the entire grid area, whichis always shown in the Installation view.

4.10 How to generate Profile vol tage plots

Profile plots are useful whenever an analysis along an axis is desired in order to assessthe touch and surface potential instead of an entire grid area or single coordinate. Anotherimportant use for generating these graphs is the evaluation of step potentials. In order forCYMGRD to generate profile plots, the appropriate option must be selected in the same manneras for performing Soil analysis, Grid analysis etc. The Profile Plot item can be selected from thedrop-down menu of the engine selection list.

A starting and an end point can be defined using the mouse (left-click, hold and move),as shown below.

Once the two points have been identified, and the mouse is released, and thecoordinates appear in the Profile Parameters dialog box. In the same dialog box, the step size isspecified for the step potential evaluations. The step interval defines the distance between thetwo feet of a potential shock victim for the purpose of displaying the step voltage between two

points along the profile.

Once the step size is specified and the coordinates are re-entered to eliminate anymanu

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