CYMGRD 65 ReferenceManual En

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July 2011

CYMGRD 6.5

Reference Manual and

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© Copyright CYME International T&D Inc.

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 is authorized only pursuant to a valid written license agreement from CYME.

CYME makes no warranty, either expressed or implied, including but not limited to any implied warranties of merchantability or fitness for a particular purpose, regarding these materials and makes such materials available solely on an "as-is" basis.

CYME International T&D reserves the right to revise and improve its products as it sees fit. The information in this manual is subject to modification without notice.

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

CYME International T&D Inc.

1485 Roberval, Suite 104 St. Bruno QC J3V 3P8

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

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

Internet : http://www.cyme.com E-mail: [email protected]

Other Trademarks: The names of all products and services other than CYME’s

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CYMGRD 6.5 – Reference Manual and Users Guide

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 of ground potential rise and danger point evaluation within any area of interest.

The program supports soil resistivity analysis taking into account field measurements, an analysis necessary to arrive at a soil model that will subsequently be used for the analysis of the potential 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 a layer 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 rise calculations. CYMGRD can also determine the equivalent resistance of ground grids of arbitrary shapes that are composed of ground conductors, rods and arcs since it employs matrix techniques for resolving the current distribution to ground. Directly energized and/or passive electrodes, 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 of interest within the area of investigation. The program can also generate equipotential contours for surface and/or touch potentials, and potential profiles showing touch and step voltages along any direction. Color-coding is used to view the results. These can be displayed in either two or three dimensions, making it easy to evaluate the safety of personnel and the equipment in and around the 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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1.3 Installing

CYMGRD

The CYMGRD package requires a license to operate. Access is granted with the use of either a physical hardware key (Parallel port / USB) and/or a license string (Added manually or by importing 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 of the 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 using the following modules:

Soil Analysis module (includes Safety Assessment): Defines either a two-layer, a

uniform, or a user-defined soil model CYMGRD plots the measured and calculated resistivity on the same graph to allow easy verification of the quality of the soil model. The maximum allowable 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 the electrode size, CYMGRD uses the parameters of the electrode material and the ambient temperature setting. Users can select one or more of the materials from the CYMGRD library. A number 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 in

the grounding grid. The potential at the soil surface is determined from these results. You may define the grid one conductor at a time and/or by using groups of conductors arranged in rectangular sub-grids. You can define the grounding rods in a similar way. Other buried conductors (such as nearby foundations) and/or neighboring grounding structures may also be defined, to be able to assess the influence of their presence on the surface voltages. These structures 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 on

Potential Contour and/or Potential Profile plots. Potential Contour plots can be used to display both 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, in any desired direction. The voltage variations, along with the corresponding maximum allowable voltages, can be shown simultaneously on the same graph. Both Potential Contour and Potential

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

1.5 First-time

user

If you have not used CYMGRD before, we suggest you read this manual before performing 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 you in the utilization of CYMGRD.

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

1.6 Interactive data entry

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

Note: Besides interactive data entry, the program remains backwards compatible with earlier releases. All cases entered via earlier Windows versions can be directly imported. In the unlikely case where users are interested in importing cases entered with the DOS version of the package, they should contact Customer 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 ‘Study’ within CYMGRD. A ‘Project’ can be viewed as a container of ‘Studies’. The studies may be variations on a design theme towards optimizing a grid design.

The second step is to determine the soil model that will be used for the subsequent analyses. This is done using the Soil Analysis module. It is the same module that performs the Safety Assessment calculations, thus yielding the maximum permissible step and touch voltage for 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 into account the worst single line to ground fault parameters in the substation and material of the electrodes.

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 depth and physical dimensions.

Note: Auto™CAD drawings of the station layout may be directly imported into CYMGRD assuming that certain design rules are followed. Please refer to Chapter 7 CADGRD - The CYMGRD - AutoCAD Interface module for more details.

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The final step is to make certain that the design for the station meets the necessary safety criteria. This can be accomplished through direct inspection of the danger points on the surface. Entire areas may need to be verified by generating Potential Contours plots of the touch voltages, particularly near the grid edges. Finally, Potential Profiles plots should be generated to ascertain 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 this procedure 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 conductors and rods into smaller segments called ‘elements’. These elements are the basic units that diffuse the injected fault current to ground. Using a higher number of smaller elements may give greater precision. 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 message “…element(s) with minimum resolution found” after performing a grid analysis, you will need to reduce the number of elements for each of the conductors shown.

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

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

An increased number of elements does not necessarily mean a more accurate estimate of neither the station resistance nor the ensuing surface potentials. A general rule of thumb is to begin by creating a study using one or two elements per grid conductor (assuming the conductors physical length does not exceed 1 meter). If greater accuracy is desired, a new study with further conductor/rod subdivisions may be carried out to see if there is indeed a significant change in the results.

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1.9 How to use CYMGRD to reinforce and verify existing grounding

grids

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

1. To take the existing soil model into account, choose the ‘User-defined’ model for soil analysis type in the Soil Parameters dialog box and enter the required information for the 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 to determine the maximum permissible touch and step voltages.

2. Verify the station conductor and rod data entries and make certain any reinforcements and/or additions are included in the station data. Determine the Ground Potential Rise (GPR) and station resistance using the Grid Analysis module. 3. Use the plotting facilities, potential contours and/or profiles, to visualize touch and

step potentials in selected areas of interest.

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

5. If the grid is not adequate, return to Step 2 and make the necessary changes to the grid 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 a design 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 this case, the dialog box shown provides the possibility to define a new Study, as well as a new Project that will contain the study. If, a new study is desired within the active project, click on the check box “Insert into the active project” and the lower project-related prompt will no longer be accessible.

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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 in the 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 will need to begin entering your substation data. The CYMGRD interface is sub-divided into dedicated sections that occupy specific regions within the overall display.

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

The middle-left section is the ‘Installation’ view. It displays a condensed view of the station grounding grid layout (NOT UNDER SCALE AND WITHOUT TAKING INTO ACCOUNT THE ASPECT RATIO OF THE MAIN ‘GRID LAYOUT’ WINDOW). The Installation View contents appear 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 main display area of the application. The ‘Soil Model’ tab displays a visual representation of all the soil measurement data and possibly any calculated results due any soil analysis. The ‘Grid Layout’ tab 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 tab named ‘Soil measurements’ is reserved for soil measurement data entry. The tab named ‘Asymmetrical 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 to all 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 contour or profile plots shown in the ‘Workbook’ view will also have a corresponding report shown here.

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1.12 Default

Parameters

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

The Default-Parameters dialog box can be called by clicking the Defaults button in the

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Chapter 2 Soil Resistivity and Safety

Assessement

2.1 Soil resistivity measurements and soil models

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

The Two-layer model has an upper layer of a definite depth and a lower layer of an infinite depth and with a different resistivity. The approach is a practical one and has been followed for many years in substation grounding practice. Of the various soil measurement techniques, 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. The apparent or measured resistivity is given by

( )

⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ + − + + π = ρ b a a b a a I V a 2 2 2 2 ₄ 2 1 4 or

ρ

=

2 π

a

( )

V

I

if

a

>>

b

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2.2 Soil resistivity: Methodology and algorithm

Let ρa be the apparent earth resistivity as computed by a two-layer model, ρ1 and ρ2 the

resistivity 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 ρ1, ρ2

and h according to the mathematical equations described below. The results will be automatically communicated to the Grid Analysis module, which calculates the surface potentials.

K = reflection coefficient = (ρ2 - ρ1) / (ρ2 + ρ1)

n = integer varying from 1 to

∝

h = upper layer thickness a = electrode spacing

ρ1, ρ2 = upper & lower soil layer resistivity

By finding ρ1, ρ2, and h, CYMGRD minimizes the following function:

]

/

))

(

[(

)

(

2 2 1 mi N i mi

P

i

P

x

f

=

∑

−

=

where the sum spans all the available measurements. mi

P

= Measured value of earth resistivity at probe distance Di

)

(i

P

= Computed value of earth resistivity at probe distance Di

Note: CYMGRD uses reduced gradient techniques to calculate the optimal model and to minimize the RMS error. The term ‘optimal’ signifies that the soil model that will be deduced will be the one that best fits the available measurements. CYMGRD identifies measurements that do not seem to fit very well the computed resistivity function. In order to try to improve the accuracy of the soil model, you may remove one or more such measurements from the input data and run the analysis again. These ‘Suspect measurements’ can be found in the Soil analysis report and are also shown in the graphical representation of the soil model marked with a ‘cross’ and labeled ‘Doubtful points’.

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 other words, any local soil resistivity discontinuities, like regions of very high conductivity surrounded by the native soil are not accounted for.

• Only horizontal soil stratification type is supported by CYMGRD. No vertical stratification 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 generated in the Soil Analysis report. This will be the case even if the two sets of measurements feature different resistivities.

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

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

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 that contains all available analysis modules.

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The available data is shown in the Data Entry view window at the ‘Soil Measurements’ tab that uses a spreadsheet-like interface as shown above. Note that any of the measurements can be disabled using the checkmark in the dedicated column. This is where you can remove any suspect 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 analysis module.

The soil model is seen graphically in the Workbook view. Any measurements that the simulation found departing from the average RMS errors that resulted from the optimization fit are marked with an “X” on the graphic. The RMS error is computed to indicate the degree of correspondence between the calculated soil model and the measured values, and is calculated as follows: RMS error

N

i

N i

error

∑

=

)

(

2

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

You can track the curve values with the mouse. Select any point on the curve with the cursor 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 the ‘Soil Analysis’ tab. The measured and calculated resistivities for the provided probe spacing are listed along with the associated errors. The same measurements marked with an ‘X’ in the Workbook View are shown in red in the Report view. You can enlarge the Report view section by dragging the split bar to the position you want. The reports are shown here for illustration. The calculated soil model results are translated in the written report. This, actually, is a very good way of 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 layer soil model. For a two-layer soil model, the program calculates the resistivity of the upper and of the lower two-layers of soil, along with the thickness of the first layer (or upper layer). The second layer (or lower layer) is assumed ‘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 soil model 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 soil resistivity 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 of the Grid analysis as some earlier versions did. Thus, the Soil data can no longer be bypassed if new soil data are to be used for analyzing the same grid. ALL SOIL DATA NEEDS TO BE DEFINED AS PART OF THE SOIL ANALYSIS. However, once analyzed, the Soil data results are still communicated to the Grid module.

Whenever a ‘User-defined’ model is selected, the results are calculated and transferred automatically to Grid module without requiring the user to perform an 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 by the 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 voltages under specific surface and exposure conditions. The safety assessment calculations comply with standard 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 50 kg).

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

• Soil resistivity of the upper and lower layers, and thickness of the upper layer of the native 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 the maximum 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

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where:

• t is shock duration in sec.

• Cs is the de-rating factor when high resistivity surface material is present. The reduction factor Cs is a function of the reflection factor k and the thickness of the upper 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 soil model data. The purpose of the calculation is to arrive at a “de-rating” factor that will permit to take advantage of the high resistivity surface layer, thus permitting a higher touch voltage to be tolerated. The de-rating factor Cs can either be calculated or obtained from graphs according to the IEEE 2000 Guide. CYMGRD calculates the de-rating factor Cs according to Equation 27 of IEEE Std 80-2000, i.e.

09 .

0

2 )

1 (

09 .

0

1 +

−

=

s s

h

Cs

ρ

where:

•

h

_s is the thickness of the high resistivity surface layer material •

ρ

_s is the resistivity of the surface material

•

ρ

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

Note: _{For metal-to-metal calculations, of this kind assume}

s

ρ

=

when calculating the de-rating factor, and

ρ

=

ρ

s

=

0

, when calculating maximum permissible

touch 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 the maximum permissible touch and step voltages. Actual potential rise analysis of the grounding assemblies takes into account only the native soil resistivity model 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 Permissible Touch 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 Maximum Permissible 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 are selected, CYMGRD reports only the minimum calculated value for Shock-Duration based on the Maximum Permissible Touch or Step voltage.

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By choosing the above option, Shock-Duration will be reported as one of the output results under the Soil Analysis tab in the report view.

2.6 Transferring the results of Safety Analysis for danger point

evaluation

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

Note: The Plotting module will only permit the utilization of the maximum permissible step and touch voltages as calculated by the Soil analysis or defined 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 ‘Import’ option found in ‘File’ menu.

Once this is selected you will need to specify the directory in which the projects that are to 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 has been constructed using the version 6.00 or higher of the application, an asterisk will be shown under “Exists” to show that there is no need for the import 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 alternative

method

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

Note: This alternative technique can be used if, for any reason, the directory cannot be scanned with the previously described technique.

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

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Chapter 3 Grid Analysis Module

3.1 General

introduction

The GRID module is used to calculate the grounding system’s resistance, the ground potential rise (GPR) and the potential gradients at the soil surface. These results are needed to assess the adequacy of the grid design and to evaluate the safety of the personnel working at the site.

3.2 Electrode types and terminology

CYMGRD supports three types of electrodes also referred to in this guide as ‘grounding systems’, since they may be composed of both conductors and ground rods. The first type is the

’Primary’ electrode and is the electrode that absorbs the grounding current. The second type is

called the ’Return’ electrode and is used to model electrodes. It there is no Return electrode all the current absorbed by the primary electrode would have been diffused to ground. Finally, the third type, the ’Distinct’ electrode, is not connected to the primary or the return electrode but may be subjected to the influence of their electric fields. Although Return and Distinct electrodes are not often found as components of a grounding system, it is sometimes necessary to represent them.

The ‘Primary’ electrode

This is the grounding grid that absorbs the fault current. You may build it up out of conductors and rods. The vast majority of grounding studies will consider only the Primary electrode.

The ‘Return’ electrode

If two grounding grids are in the vicinity of each other, and current injected to ground at the first grid returns to the system via the second, then the second grid is a Return electrode. The presence of a Return electrode will alter the surface potential distribution.

You can model the Return electrode in the same way that you model the primary electrode. Even a single rod can serve as a Return electrode. In addition, you must enter the current absorbed by the return electrode, in Amperes. This value must be negative.

The ‘Distinct’ electrode

Conductive structures like pipelines and building foundations, which are near a grounding installation, but not connected to the electric network (not energized), are Distinct electrodes.

You model the Distinct electrode in the same way that you model the Primary electrode. Even a single rod or buried conductor can act as a Distinct electrode.

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

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

By using a Split-factor, CYMGRD takes into account Return current via the locally grounded transformers, transmission line and distribution feeders. If the substation fence is not bonded to the grounding grid, model the fence posts as parts of a Distinct electrode. Otherwise, model them as part of the Primary electrode.

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

3.3 Electrode Sizing

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

The selection of the suitable conductor material and size should satisfy the following criteria: electrical conductivity, corrosion resistance, current carrying capacity and mechanical strength.

Any conductor should be capable of conducting the entire ground fault current without exceeding a specified temperature.

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

A Is the conductor section (in cmils)

I

LG is the RMS fault current (in A)

K

f constant dependent of the conductor material

( K

f = 7.01 for Copper, Soft Drawn)

t

c 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 to IEEE standards.

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

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

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

Notes: y The Fault-Duration in the Buses tab cannot be less than the

Shock-Duration in the Soil Parameter dialog box.

y Ambient temperature can be specified in the Soil Parameters dialog box. In order to specify the electrode material, the user can choose one of the materials from the CYMGRD library in the Electrodes tab. (See below). In addition, the user can change the material parameters in the CYMGRD library to specify a user-defined material.

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

After all the required parameters are specified, the result will appear in the Output window under the Electrode Sizing tab. There is no need to run electrode-sizing analysis. The following 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 the electrodes 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 Electrodes data entry tab and that the Electrode Sizing report has been generated (a valid ‘Soil Model’ analysis must be available for the active study), a list of corresponding ‘Materials’ and ‘Sizes’ will be available for selection 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 as reported in the Electrode Sizing results) for the Conductor will be set and its ‘Diameter’ will be adjusted accordingly.

Proceeding to change the ‘Size’ will alter the Conductor ‘Diameter’. Modifying the ‘Diameter’ 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 required for Electrode Sizing analysis.

In the “Buses” tab of ‘Data Entry’ view, the user must enter data for all the buses in the substation. CYMGRD will automatically choose the bus that requires the thickest electrode and apply 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 in the analysis.

• Usually a substation has two or more buses. CYMGRD identifies each bus and the corresponding parameters by a ‘Bus ID’. The results of the analysis appear in the ‘Electrode Sizing’ tab in the ‘Reports’ view with corresponding Bus ID (See following image).

• ‘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) divided by total fault current and multiplied by 100.

• ‘LG X/R’ is ‘(2x1+Xo)/(2R1+Ro)’ for the corresponding single line-to-ground fault current.

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 be supplied. CYMGRD uses this additional data for grid analysis when a ‘Current Split 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 = 100 Ohms).

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• ‘Distribution Feeders’ is the number of the feeders connected to the other side of the transformers 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. Decrement factor), the constant parameters for the electrode material and ambient temperature value are required:

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

• The type of the material along with its parameters is specified in the “Electrodes” tab of the ‘Data Entry’ view (See below).

• CYMGRD uses the information in the ‘Buses’ tab to calculate the Decrement factor in accordance with the standard. This factor is used to take into account the DC components, resulting in the asymmetrical fault current for the corresponding fault duration.

The following image shows the CYMGRD ground conductor library (“Electrodes” tab). In this 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 modified in order to better define the materials in use. Any altered values will be saved only as part of the active study.

3.3.3 Electrode Sizing report

After all the required data for the Electrode Sizing has been specified, the result of the analysis 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 or asymmetrical configuration. A grounding system is made of electrodes, which the program divides 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 soil model as determined from the Soil analysis. When calculating the Ground Potential Rise, the injected current needs to be known as well.

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

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

The single line-to-ground fault current (LG) at the fault location produced by the substation, does not necessarily flow to the ground via the grid. Some of it may be diverted back to the system through line-to-ground wires, cable sheaths and/or tower counterpoises. The fact that only a part of the total fault current usually flows between the grounding system and the surrounding 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 ‘Grid Parameters’ dialog box.

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

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

g LG

f

I

R

S

GPR

=

×

• User Defined (Split Factor or Parallel Z): When you choose this option, you can directly enter your desired ‘Splitting Factor” or ‘Parallel-Z’.

Note: The check box “Include Local Contribution” accounts for the case where a local source is solidly grounded to the ground grid. This is option is available for 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 total equivalent resistance (in ohms) of the sky wires and counterpoises of all the lines connected to the 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 electrode components to assess their effect on the performance of the grid grounding design without resorting 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 defined) to 9999 Ω or the Split Factor (User-defined) to 1.

For a Return electrode enter the return electrode current. If not, the current is 0.

If you change any of the electrodes after performing an analysis, you will have to 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 Definition for

Remote-Contribution in [%]

To avoid overdesigning in substation grounding systems, CYMGRD takes into account the 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 grounding system and the surrounding earth has implications on both personnel safety and equipment requirements. (See figure below)

To account for both the Decrement (Df) and Split (Sf) Factors, the Ground Current is

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For conservative and desired approximation of the above mention correction factors, the following parameters are required in the Buses tab.

[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 (the conservative default value is Rtg=100 Ohm).

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

[Rdg]: the ground electrode resistance of a distribution feeder neutral. (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 Decrement

factor (Df) must be included in the calculation of the Ground Current. This factor is used to take

into account the DC components, resulting in the asymmetrical fault current, for corresponding fault duration.

(

tf Ta

)

f a f

e

t

T

D

=

1 +

1 −

−2 / Where:

• tf is the fault duration.

3.5.2 Split Factor (Sf)

In order to take into account that portion of the fault current, the Split factor (current division 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. This translates to savings when designing the grounding system.

In order to estimate and take into account the Split Factor in the analysis, choose the option ‘Current Split Factor’ in the Grid Parameters dialog box.

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

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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 geometrical coordinates or can be imported from an AutoCAD file formatted for use with CYMGRD. This section describes data entry for the case where AutoCAD data files are not available. In CYMGRD, the ‘grid components’ data is classified into five categories: Symmetrically arranged grid conductors, asymmetrically arranged grid conductors, arc conductors, symmetrically-arranged ground rods and asymmetrically symmetrically-arranged ground rods. All are explained in the following 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 and short axes, creating a grid. CYMGRD assumes that symmetrically-arranged grid conductors are buried horizontally and are oriented along two perpendicular axes (the X and Y axes in the graphic 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 for symmetrically-arranged components is entered using the ‘Symmetrical Conductors’ tab of the ‘Data Entry’ view.

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Symmetrical Conductor data is shown above. Note that the check box ‘Enabled’ is selected, which means that it will be considered in the Grid Analysis. Furthermore, the ‘Primary’ electrode type is selected (default). The drop-down box allows modifying that default to ‘Return’ or ‘Distinct’.

For this example, we have used the symmetrical conductor arrangement to represent the lower rectangular part of an L-shaped grid.

The following set of data is used to define a symmetrically-spaced grid:

Type Primary, Return or Distinct.

[X1, Y1] and

[X2, Y2]

Coordinates of two opposite corners of the rectangular array.

Grid conductors parallel to X

The number of grid conductors parallel to the X-axis.

Elements per conductor parallel to X

CYMGRD considers this number of elements in finite-elements analysis, for conductors parallel to the X-axis

Grid conductors parallel to Y

The number of grid conductors parallel to the Y-axis.

Depth The distance between the soil surface and the center of the conductor.

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

3.6.2 Asymmetrically-arranged grid Conductors

An asymmetrically-arranged conductor is a single straight conductor stretched between two points defined by two coordinates (X1, Y1, Z1) and (X2, Y2, Z2). Asymmetrical conductors that are slanted may be represented in the model (Z coordinate), which is not the case for the symmetrical 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 symmetrical arrangements with a common diameter for all conductors.

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

For this example, we have used the asymmetrical conductor arrangement to represent the 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 may be inclined with respect to the soil surface, which CYMGRD

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Number of

conductor elements

CYMGRD considers this number of elements for conductors parallel 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 in rows parallel to the X-axis with all rods in a row equally spaced. All rods defined in the same array 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 ‘Primary’ electrode Type is selected (default). The drop-down box allows modifying the default to ‘Return’ or ‘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 the rods are placed.

Rod rows parallel to the X-axis

Number of the horizontal rod rows on the display.

Number of ground rods per row

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Length Ground rod length.

Depth Burial depth (the distance between the soil surface and the top of 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 position of the first rod is given by the coordinates (X1, Y1, Z1) and the position of the last rod in the row is given by the coordinates (X2, Y2, Z2). The upper end of each rod lies on the straight line between these two points. All rods defined in the same array have the same length and diameter. If a single rod is specified (Number of Rods along axis = 1), then only the starting point coordinates (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 rods placed in the grid were strategically positioned at specific coordinates. It is seen in the data that we have entered the rods one at a time using different coordinates for the beginning and the end points.

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

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

Number of rods along axis

Number of rods in the row.

Elements per Rod in upper soil layer

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

Elements per Rod in 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 of semiconductor material buried in the soil. See the following picture from IEEE 80.

This is of particular interest in medium and highly resistive soils.

To enter a rod encase in CYMGRD:

1) Activate the check box Material-

Encased for the rod.

2) Enter the Material Thickness (the cylinder radius).

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3) In the Grid Parameters dialog box,

enter the Resistivity of the material around 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, which means 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:

Type Primary, Return or Distinct electrode.

[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-clockwise rotation.

Radius The radius of the arc.

Number of

conductor elements

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

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

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Note: A positive value of Z denotes a position below the surface of the soil for all electrode 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 approximated to is 3.

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

3.7 Modifying and inspecting the station Geometry data

3.7.1 Enabling and disabling entries

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

Data Entry view. If a check mark is shown the component is enabled. To disable it remove the

check mark.

3.7.2 Reviewing and verifying the data

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

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3.8 Importing/Exporting

Grid

data and Station layouts

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

More details about the preparation of the data in AutoCAD, the import/export mechanism of 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 such a case arises, please contact CYME International T&D Customer Support for instructions.

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

3.9 Overlapping conductor elements

CYMGRD cannot perform a station analysis if conductor elements are found to overlap each 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 during execution the calculations will stop and an appropriate error message will be generated indicating which components overlap. Common errors causing that condition are duplicates of either asymmetrical conductor elements or grounding rods that are placed one on top of another. When the duplicate is disabled or removed from the grid design, the problem should be alleviated.

CYMGRD 65 ReferenceManual En

CYMGRD 6.5

Reference Manual and

© Copyright CYME International T&D Inc.

Table of Contents

Chapter 1

Getting Started

1.1 General

introduction

1.2

Software and hardware requirements

1.3 Installing

CYMGRD

1.4 CYMGRD

modules

1.5 First-time

user

1.6

Interactive data entry

1.7

How to use CYMGRD to design a new grounding grid

1.8

Dividing the grid into elements

1.9

How to use CYMGRD to reinforce and verify existing grounding

grids

1.10 Creating and opening Projects and Studies

1.11 The Windows layout of CYMGRD

1.12 Default

Parameters

Chapter 2

Soil Resistivity and Safety

Assessement

2.1

Soil resistivity measurements and soil models

( )

ρ

=

2

π

a

( )

V

I

a

>>

b

2.2

Soil resistivity: Methodology and algorithm

∝

]

/

))

(

[(

)

(

P

i

P

P

x

f

=

∑

−

P

)

(i

P

2.3

How to perform a soil analysis

N

i

error

∑

=

)

(

2.4