CYMGRD 65 ReferenceManual En

CYMGRD 6.5 Reference Manual and Users Guide

July 2011

©

Copyright CYME International T&D Inc. All 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 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 mentioned in this document are the trademarks or trade names of the respective owners.

CYMGRD 6.5 – Reference Manual and Users Guide

Table of Contents Chapter 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12

Getting Started............................................................................................1 General introduction .....................................................................................1 Software and hardware requirements ..........................................................1 Installing CYMGRD.......................................................................................2 CYMGRD modules .......................................................................................2 First-time user...............................................................................................3 Interactive data entry ....................................................................................3 How to use CYMGRD to design a new grounding grid ................................3 Dividing the grid into elements .....................................................................4 How to use CYMGRD to reinforce and verify existing grounding grids .......5 Creating and opening Projects and Studies .................................................5 The Windows layout of CYMGRD ................................................................7 Default Parameters.......................................................................................9

Chapter 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

Soil Resistivity and Safety Assessement ..............................................11 Soil resistivity measurements and soil models...........................................11 Soil resistivity: Methodology and algorithm ................................................12 How to perform a soil analysis....................................................................13 How to specify the soil model type .............................................................15 How to perform Safety Analysis .................................................................17 Transferring the results of Safety Analysis for danger point evaluation .....19 Importing Projects from the previous version .............................................20 Importing Projects from the previous version – An alternative method......21

Chapter 3 3.1 3.2 3.3

Grid Analysis Module...............................................................................23 General introduction ...................................................................................23 Electrode types and terminology ................................................................23 Electrode Sizing..........................................................................................24 3.3.1 LG fault parameters........................................................................27 3.3.2 Electrode Material ..........................................................................28 3.3.3 Electrode Sizing report ...................................................................28 3.4 Grounding system structure and location...................................................29 3.5 Split-factor (Sf), Decrement- factor (Df) and Definition for RemoteContribution in [%] ......................................................................................31 3.5.1 Decrement Factor (Df)....................................................................32 3.5.2 Split Factor (Sf) ..............................................................................32 3.6 Entering the Grid data.................................................................................34 3.6.1 Symmetrically-arranged grid Conductors.......................................34 3.6.2 Asymmetrically-arranged grid Conductors.....................................36 3.6.3 Symmetrically-arranged ground Rods............................................37 3.6.4 Asymmetrically-arranged ground Rods..........................................38 3.6.5 Rod Encasement............................................................................39 3.6.6 Arc Conductors...............................................................................41 3.7 Modifying and inspecting the station Geometry data .................................42 3.7.1 Enabling and disabling entries .......................................................42 3.7.2 Reviewing and verifying the data ...................................................42 3.8 Importing/Exporting Grid data and Station layouts.....................................43 3.9 Overlapping conductor elements................................................................43 3.10 Grid analysis and reports............................................................................44 3.11 Visualize the station layout in 3-Dimensions. .............................................46 3.12 The station layout and the ‘Installation’ view. .............................................49

TABLE OF CONTENTS

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3.13 A note on modeling Grounding Structures .................................................49 3.14 Soil data from earlier versions of the application........................................50 Chapter 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12

Plotting Module.........................................................................................51 General introduction ...................................................................................51 How to generate ‘Touch’ and ‘Surface’ potential Contours ........................51 Touch and Surface potential contours........................................................54 Contour color coding and Safety Analysis..................................................54 Allowable LG fault current ..........................................................................56 How to generate 3-D contour plots.............................................................57 Contour graph reports ................................................................................58 Contour graph management.......................................................................59 How to perform ‘spot-check’ danger point evaluation ................................59 How to generate Profile voltage plots.........................................................60 Inspecting potential profile plots .................................................................62 Comparing contour plots from two different studies ...................................62

Chapter 5 5.1 5.2 5.3 5.4

Example Studies.......................................................................................65 Example 1: Primary electrode only.............................................................65 Methodology ...............................................................................................66 First step: Soil Analysis and interpretation of resistivity measurements ....66 Second step:Calculation of the maximum permissible touch and step voltages for the soil model ..........................................................................67 Third step: Grounding installation data entry..............................................69 Fourth step: Danger point evaluation and surface analysis .......................73 Example 2: Primary, Return and Distinct electrodes..................................75 Grounding installation data and layout .......................................................75

5.5 5.6 5.7 5.8 Chapter 6 6.1

Comparison with the IEEE80 Guide .......................................................81 Comparison with the IEEE80 Guide ...........................................................81 6.1.1 CASE NAME: Example 1: Preliminary design stage .....................82 6.1.2 CASE NAME: Example 2: Improved design ..................................83 6.1.3 CASE NAME: Example 3: Finalized design ...................................85 6.1.4 CASE NAME: Example 4: L-Shaped with rods ..............................86

Chapter 7 7.1 7.2

CADGRD - The CYMGRD - AutoCAD Interface module........................89 Program summary ......................................................................................89 Drawing a station ground grid with AutoCAD .............................................91 7.2.1 General outline ...............................................................................91 7.2.2 Drawing the Grid Layout using AutoCAD:......................................92 7.2.3 Example..........................................................................................98 Validation & Update of the AutoCAD drawing ..........................................107 7.3.1 Validating the AutoCAD drawing..................................................108 7.3.2 Updating the AutoCAD drawing. ..................................................111 Importing from AutoCAD to CYMGRD .....................................................112 Exporting from CYMGRD to AutoCAD .....................................................114 Working with AutoCAD .............................................................................115

7.3

7.4 7.5 7.6 Chapter 8

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Troubleshooting .....................................................................................117

TABLE OF CONTENTS


Chapter 1

1.1

Getting Started

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 802000. 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

CHAPTER 1 – GETTING STARTED

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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 Profile 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 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:

1.6

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

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:

1.7

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.

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 ‘Userdefined’ 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.


5


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. 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 ‘Nominal Frequency’, when creating a new Study. The Default-Parameters dialog box can be called by clicking the Defaults button in the File > New dialog box.


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Chapter 2

2.1

Soil Resistivity and Safety Assessement

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 ρ=

4πa(V I) ⎡ ⎤ 2a a − ⎢1 + ⎥ ⎢⎣ a 2 + 4 b2 a2 + b2 ⎥⎦

or

ρ = 2πa(V I )

if a >> b

where b is the length of the probe.

CHAPTER 2 – SOIL RESISTIVITY AND SAFETY ASSESSMENT

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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: N

f ( x) = ∑ [( Pmi − P(i )) 2 / Pmi2 ] i =1

where the sum spans all the available measurements.

Pmi = Measured value of earth resistivity at probe distance Di

P (i ) = 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.

•

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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.



2.3

•

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.

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: N

RMS error =

∑ error

2

(i )

i

N

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 two-layer soil model. For a twolayer 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) 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

•

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. 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.

Cs = 1 −

0.09(1 − ρ

ρs )

2hs + 0.09

where: •

hs is the thickness of the high resistivity surface layer material

•

ρs

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 the de-rating factor, and

ρ = ρs

when calculating

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 (ShockDuration) 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

3.1

Grid Analysis Module


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.

CHAPTER 3 - GRID ANALYSIS MODULE

23


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

is the RMS fault current (in A)

LG

K

f

constant dependent of the conductor material ( K = 7.01 for Copper, Soft Drawn) f

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.

24



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 ShockDuration 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.


25


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).


27


3.3.2

•

‘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).

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 ‘Copperclad steel’ is selected for the rod sizing. Note:

3.3.3

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.

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.


29


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;

GPR = S f × I LG × R g •

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

30



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 (User-defined) to 9999 Ω or the Split Factor (Userdefined) 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 now computed as per the following equation of the IEEE STD 80 2000.


31


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

3.5.1

[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).

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.

Df = 1+

(

Ta − 2t / T 1− e f a tf

)

Where: • 3.5.2

tf is the fault duration.

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.

32



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.

The electrical equivalent circuit of the above is as follows:


33


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

3.6

% RC is the Remote Contribution % entered in the Bus data entry parameters.

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, symmetricallyarranged ground rods and asymmetrically 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.

34



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 finiteelements 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.

Diameter

Ground conductor diameter.


35


Note:

3.6.2

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.

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:

36

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 assumes to be horizontal.



3.6.3

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.

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

Number of rods along each row (Defined above).


37


3.6.4

Length

Ground rod length.

Depth

Burial depth (the distance between the soil surface and the top of the rods).

Diameter

Ground rod diameter.

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. The following set of data defines a row of rods: Type

38




3.6.5

[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 finiteelements analysis.

Elements per Rod in lower soil layer

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

Length

The rod length.

Diameter

The rod diameter.

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 Encased for the rod.

box

Material-

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


39


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].

40



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


[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).

Diameter

The arc conductor diameter.


41


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.


43


3.10 Grid analysis and reports The Grid analysis can be performed in the same manner that the ‘Soil’ analysis was invoked. Run the ‘Grid Analysis’ by clicking on the lightning bolt button. The time bar at the bottom 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’ view of the application, the first part of which is illustrated below. It is seen that, at first, the soil model used in the calculations is echoed in the report. It is important to verify that the soil model used in the grid analysis is indeed the one obtained from the soil analysis results. Otherwise, the grid analysis results may not be relevant.

Next, the coordinates of the grid elements and the current every each element diffuses to ground are listed. Note that for each element, a column indicates whether it belongs to a symmetrical or an asymmetrical assembly and a second one indicates the reference number of the assembly it belongs to. The reference number is the row item number of spreadsheet data entry. This way, it is easy to track the elements even if they might represent subdivisions of original 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 ‘Return’ electrode results and finally, by ‘Distinct’ electrode results, if any.

44



The Ground potential rise and the station resistance is displayed at the beginning of the report. Note:

The various elements of the grounding installation, as resulted from the partitioning of the grid conductors and ground rods diffuse a positive current into the ground. The simulation may however indicate that the current diffused to ground by one or more elements is zero. This indicates that CYMGRD found that each such element diffused a small negative current and consequently set it to zero. This situation is due to numerical instability. To avoid this problem, change the number of elements in the affected conductors or rods so that these elements are about as long as other elements in other conductors in the grid. If negative currents are found for some of the elements of the Primary electrode during the analysis, CYMGRD will indicate these elements in the grid report flagging them in Red. This may be the result of false numerical representation, since currents from all elements should be positive (diffused to ground). If the negative current, from one or more elements, adds up to more than a few percent of the totally injected current, a new simulation should be performed with the number of elements changed as explained above. The same considerations apply if a positive current is found for any of the elements of the Return electrodes. No such considerations apply to the Distinct electrodes. Experience has shown that the negative current is a very small fraction of the injected fault current and that the error introduced in calculating the station resistance and GPR is negligible. Simulations performed after changing the number of elements in conductors should indicate no change in the overall results, apart from correcting the negative currents. It is always advisable to verify that strictly positive currents are diffused to the ground by all the elements.


45


3.11 Visualize the station layout in 3-Dimensions. The station layout shown in previous illustrations was a 2-D representation (Default). It is however, possible to view the station layout in three dimensions as well. The 3-D view is often useful, 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 to locate these inconsistencies via a simple inspection. To generate a 3-D representation, position the mouse on the window containing the grid layout and right-click.

This provides access to the “Chart Settings” dialog that allows access to the actual graph settings.

By default, the Style setting “Area” is selected, which means that the grid layout will be shown as a function of the dimensions and aspect ratio of the display window. If the option “Scaled Area” is selected, the grid will be drawn to scale with proper consideration of its actual dimensions.

46



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

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


47


Note:

48

The Grid Analysis calculations are not affected even if an electrode component is made invisible. It is solely a method for the visual examination of the Grid layout.



3.12 The station layout and the ‘Installation’ view. A representation of the station layout is always shown in the ‘Installation’ view of the application. 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 the ‘Grid 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 basic analytical assumption CYMGRD makes, in compliance with International Standards, is that the entire grounding system that absorbs the fault current, and diffuses it to the ground, is elevated to a single potential. This is the Ground Potential Rise of the Primary electrode (i.e. the calculated GPR). Thus, voltage drop along the grid electrodes is not modeled. Furthermore, the ground structures are assumed to contain only resistance (i.e. no reactive component for the grounding grids and structures is modeled by CYMGRD). Modeling of the reactive component of the grid impedance may be necessary for stations possessing either a resistance of less than 0.5 Ohms or if they extend over an unusually large area. Any metallic structures bonded and/or connected to the primary electrode by accident or with purpose such as fences, building foundations etc, and that help in reducing the GPR should be modeled as part of the Primary electrode. The Return electrode should only be used in the case where a grid absorbing current from the ground exists and is located in the vicinity of the energized Primary electrode. Both Primary 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 are adjacent 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 altering the surface potentials in the vicinity.


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When a simulation comprises Primary, Return and Distinct electrodes, all electrodes are assumed to be within the same soil featuring the soil model obtained from the Soil analysis module of CYMGRD. Neither the Return nor the Distinct electrode can feature a galvanic connection with the Primary electrode.

3.14 Soil data from earlier versions of the application The application provides dedicated embedded safeguards against inconsistent data and is less permissive than earlier versions. It has already been mentioned that soil model values can no longer be entered separately within the grid parameters. Earlier versions, however, did permit this. As a result difficulties may be experienced when importing studies generated with previous versions. If inadvertent program termination or inconsistent results are seen, an effective remedy will be to re-affirm a slight modification to the soil data. Re-typing in the same value should be sufficient. This will simply invalidate all analysis results. Simply perform a new set of analysis to obtain new and consistent results.

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Chapter 4

4.1

Plotting Module


The Plotting module is used to calculate and view the results of the surface potential analysis. With this module the user can perform danger-point evaluation on various surface points and/or areas of the substation. 2-D and 3-D contour plots illustrating the equipotential touch 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 the beginning 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 been performed. A graph containing equipotential contours is a graph that pertains to a particular portion of the station layout (that can be the entire grid) and that shows the variations of the touch or surface potential. When the area of interest is specified, CYMGRD performs calculations taking into account 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. The crosshair that appears determines one corner of the area. Hold and drag the mouse to select the area, as shown below.

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

CHAPTER 4 – PLOTTING MODULE

51


potentials. By default, the program considers 20 intervals in both X and Y directions. More can be specified 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 that portrays the potential gradient in the selected area.

The generated graph is solid-filled. Actual equipotential contours can be seen along with their associated levels if the appropriate options are applied. Position the mouse on the contour graph and right click. Select the “Settings…” item, which will open the Chart settings dialog. By highlighting ‘Contours’, it is seen that different options are available for drawing contours in the ‘Style’ dropdown box.

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The default setting is ‘Solid-filled’. If another option is selected, ‘Lines with labels’ for example, then the equipotentials appear along with their levels indicated in the form of labels as shown in the following image. If, for some reason, the number of generated equipotentials is too large generating a graph 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 of ‘Levels’ 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 Touch and Surface potential contours. In fact, both are contained within the same graph. Right-click on the 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 high touch potential are actually characterized by low surface potentials. Note:

It is not necessary to access the contour settings to ascertain whether an equipotential contour plot pertains to Touch or Surface potentials. When moving over the chart with the mouse, Touch contour plots display the mouse cursor in the shape of a hand instead of an arrow to designate the touch 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 analysis calculations as conducted within the Soil Analysis module. Whenever a safety analysis is conducted, maximum permissible touch and step voltages are calculated. The program considers 4 thresholds for the touch potentials and another 4 for the surface potentials. The thresholds considered for the touch potentials are 25%, 50%, 75% and 100% of the maximum tolerable touch voltage. Anything above the maximum permissible touch voltage is shown in various shades of red to signify dangerous areas. The user can define the colors although default settings are provided by CYMGRD. As shown below the threshold colors can be accessed under the tab thresholds in the appropriate Chart settings. In this example, for the ‘Touch potentials’ Chart settings, the first threshold region is defined to be between 0 and 25% of the maximum permissible touch voltage, and the default color is coded as green.

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The last threshold region is defined such that everything above 100% will be red, while everything 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 defined here as well; the difference being that the maximum threshold is set to the Ground Potential Rise as calculated by the Grid analysis module. The thresholds are accessed in the same way as with the touch equipotentials. The last threshold is shown here for illustration.

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


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4.5

Allowable LG fault current

Based on the grounding analysis result, CYMGRD calculates the maximum allowable LG fault for each contour of the grounding system under the study. CYMGRD calculates and reports ‘Allowable LG Current’ for the selected area in each contour plot. The ‘Allowable LG Current’ is maximum LG fault current that causes safe touch voltage 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 the unsafe 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 layout plots were generated in 3-D. Right click on the 2-D graph, select ‘Settings…’, and under ‘Graph’ check 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 graph for better inspection.


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4.7

Contour graph reports

Whenever a contour plot is generated the program produces a corresponding tabular report. This report contains among other things, the point of maximum potential found within the area 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 on the 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 in the ‘Contours’ tab of the ‘Workspace’ view. The contour plots shown in the ‘Workbook’ view pertain 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 any contour 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 of interest, 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 tip showing 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 is a touch or a surface contour plot.


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

4.10 How to generate Profile voltage plots Profile plots are useful whenever an analysis along an axis is desired in order to assess the touch and surface potential instead of an entire grid area or single coordinate. Another important use for generating these graphs is the evaluation of step potentials. In order for CYMGRD to generate profile plots, the appropriate option must be selected in the same manner as for performing Soil analysis, Grid analysis etc. The ‘Profile Plot’ item can be selected from the drop-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 the coordinates appear in the ‘Profile Parameters’ dialog box. In the same dialog box, the step size is specified for the step potential evaluations. The step interval defines the distance between the two 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 any manual selection inaccuracies, a graph is generated by clicking on the ‘OK’ button.

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In the example above, surface, touch and step potentials are indicated with red, blue and green curves respectively. The dashed curves of the same colors pertain to the limiting values for each (i.e. the GPR along with the maximum permissible values, as resulted from the safety analysis). It is clearly seen that violations for the step potential are recorded in the area where no grid conductors are placed, as expected. Note:

The same philosophy for tabular report generation applies to the profile plots as seen in the appropriate tab of the ‘Reports’ view. The same chart management principles for the generated profile plots can be seen in the appropriate tab of the ‘Workspace’ view. Graphs can be tracked with the mouse to visualize potentials along the search direction. Activating the “Scroll-Lock” button will restrict movement of the mouse to one of the graph curves. Moving the mouse up or down switches between curves. The ‘Installation’ view indicates the actual position of mouse on the station layout when moving over an active profile plot window.


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4.11 Inspecting potential profile plots You can track the profile plot curves with the mouse. Move the mouse along the curve to see the distance and the voltage between a selected point and the starting point. You can also show the absolute coordinates of a specific point along the axis. In order to do this, track the curve with the mouse and every time the X-Y coordinates are needed, simply left-click the mouse and hold. The same applies for the step potential curve. However, since the step potential is the difference between the surface potentials of two consecutive points, the coordinates displayed define the second point only.

4.12 Comparing contour plots from two different studies It is common to compare two contour plots from different studies in order to ascertain station design improvement and/or danger point elimination. While any two graphs from the same study can be concurrently visualized, graphs from different studies first need to be exported to CYMVIEW for concurrent visualization. CYMVIEW is a general-purpose chart viewer that is provided with the CYMGRD application. For purposes of illustration, assume that we want to assess the difference the addition of an arc conductor makes in the local touch potential distribution of a station. First, generate the graph from within the study that contains no arc in the station layout and export it to CYMVIEW.

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Then generate the same graph from the study that contains the arc conductor in the station layout and export it to CYMVIEW as well. Tile the windows within the CYMVIEW application to readily view the two situations.

It is seen that adding an arc conductor significantly reduces the risk of shock exposure by diminishing the touch potentials.


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Chapter 5

Example Studies

In this chapter, two step-by-step sample studies are detailed: A primary electrode only model, below, and a Primary, Return and Distinct electrodes model. The descriptions of the two examples show how the most commonly used functions of the application are actually utilized.

5.1

Example 1: Primary electrode only

In this example, the grounding grid is square and symmetrical (meshes of equal area). It is 76.2 meters long and 76.2 meters wide. All conductors are buried at a depth of 0.5 meters. Nine conductors lie parallel to the X-axis and Nine parallel to the Y-axis. The diameter of all the conductors is 19.1 millimeters. Finally, 25 grounding rods are connected to the grounding grid at the perimeter. The rods are 10.9 meters long with diameter 2.858 centimeters (1-1/8 inches). There are no auxiliary grounds in the vicinity and the fence of the station is to be disregarded, for now. The grounding installation is in parallel with a resistance of 25 ohms, simulating the presence of overhead sky wire and counterpoise resistance. The total fault current is 4000 Amperes, but since the equivalent impedance of the sky wires is not infinite (9999 ohms), not all of that current will contribute to the station potential rise. This example shows how to build the station from scratch with all resistivity measurements taken along the same direction, using the Wenner technique, in order to determine the soil characteristics. In order to test the Soil Analysis module in CYMGRD, the following measurements were obtained from one of IEEE sample. (IEEE 80-2000, page 168 & 169. Soil type 2)

The station surface is to be backfilled with crushed rock of 2500 Ohm-meter resistivity at a thickness of 0.2 meters. Safety design considerations focus concerns on an exposure duration of 500 milli-seconds and a weight of 70 Kilograms for the potential shock victim.

CHAPTER 5 – EXAMPLE STUDIES

65


5.2

Methodology

The first step is to interpret the soil resistivity measurements and arrive at a soil model for the subsequent analysis. It is at this point that CYMGRD is used to calculate a two-layer soil model from the measurements. The second step is done automatically by CYMGRD as part of the soil model calculations. The maximum permissible touch and step voltages for the soil model is determined according to the IEEE Std 80-2000 and in accordance with the station surface treatment conditions and safety requirements. The third step is to enter the dimensions of the grounding assemblies and perform station potential rise analysis as well as to determine the station resistance. The fourth step is to carry out a danger point evaluation.

5.3

First step: Soil Analysis and interpretation of resistivity measurements

Activate CYMGRD and define a new Project and Study. Enter the soil measurements in the appropriate ‘Data Entry’ view tab. You will notice that as soon as the measurements are entered, they are reflected as dots on the ‘Soil Model’ chart in the ‘Workbook’ view. Continue by opening the ‘Soil Parameters’ dialog box by selecting the ‘Parameters’ item in the ‘Soil’ menu, to define the safety analysis settings.

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5.4

Second step:Calculation of the maximum permissible touch and step voltages for the soil model

Click the lightning bolt symbol on the main toolbar to perform the analysis. The results will be shown in the ‘Soil Analysis’ tab of the ‘Reports’ view. This is the second step of the process.


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Despite the fact that the results show a considerable RMS error of around 14 percent, the user can accept the soil model as is without discarding any measurements. But, should the user decide to reject some of the measurements in order to improve (i.e. reduce) the RMS error, the third or fourth items in the list can be good choices. For this example, we decided to disable the first four measurements, which produces the following results:

The following table shows the comparison between CYMGRD results and the values obtained from IEEE. (See IEEE 80-2000 pages 168 and 169)

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5.5

Upper Layer

Upper Layer

Lower Layer

Thickness [m]

Resistivity [Ohm-m]

Resistivity [Ohm-m]

CYMGRD

6.11

298.24

99.98

IEEE 80-2000

6.1

300.0

100.0

Third step: Grounding installation data entry

In this example, all conductors are buried at a depth of 0.5 meters. Nine conductors lie parallel to the X-axis and nine are parallel to the Y-axis. For analysis purposes, the conductors parallel to the X-axis are subdivided into 16 elements and the conductors parallel to the Y-axis into 24 elements.

We can enter all pertinent general type data in the dialog box shown below, which is accessed from ‘Parameters’ item under the ‘Grid’ menu. Since there is no return electrode, the return current is 0.


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The geometrical data can now be entered. Since the conductor assembly is a symmetrical arrangement (i.e. it can be defined as equally spaced and equidistant both X and Y directions), we will use the ‘Symmetrical Conductors’ tab in the ‘Data Entry’ view to supply the data. As soon as the data is entered the station layout is shown on the screen.

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We will now add the ground rods to the conductor assemblies. We can enter the grid and the rods in any order, but it is better to enter the grid layout first and then the ground rods. Similarly the symmetrical ground rod layout allows us to use the ‘Symmetrical Rod’ tab for entering the data. The rods are now shown superimposed on the grid conductors. The rod depth is defined as the distance from the surface of the earth to the top of the rods and is always entered as a positive value.


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For illustration purposes, the station layout is shown in 3-D in the following image.

Perform a grid analysis by selecting the Grid Analysis engine and clicking on the lightning bolt icon.

The results are shown in the ‘Report’ view under the ‘Grid Analysis’ tab. The station Ground-resistance is found to be Rg=0,72 ohms (The same as IEEE Std 80-2000 result in Table E.1) and the ground potential rise is approximately equal to 2886 [kV].

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5.6

Fourth step: Danger point evaluation and surface analysis

To plot a contour of grid voltages (touch or surface), choose the corresponding engine as shown below, and then click on the lightning bolt icon.

The following dialog box is displayed. You can select the desired area to plot and the accuracy of the contour.

Touch voltages contours will be generated for the entire grid area. They can be shown in both 3-D and 2-D in the images that follow.


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A profile along the main diagonal of the grid (i.e. from the lower left to the upper right corner) will be now be analyzed using a step value of 0.22 meters (the minimum step value). Select ‘Profile Plot’ from engine selection box on the main toolbar. Select the start and end coordinates of the profile using the mouse which will then open the ‘Profile Parameters’ dialog box. You can use this dialog box to refine the final coordinates for the analysis, and then click “Ok”.

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The surface touch and step potentials are shown once the analysis in complete. The results show that no cause for concern exists since all calculated values are well below their corresponding maximum values. If more details are desired, the profile plot can be tracked with the mouse, and the exact movement monitored in the ‘Installation’ view.

5.7

Example 2: Primary, Return and Distinct electrodes

If a ground fault occurs within the substation, and that another electrode at a certain distance from the grid absorbs the current injected to the grid, this electrode becomes a Return electrode. Also, if adjacent building foundations are present, but are not energized, they will need to be modeled as a distinct electrode.

5.8

Grounding installation data and layout The following installation data and layout apply to our example: •

The soil is to be uniform with a resistivity of 100 Ohm-meters.

•

The station surface is reinforced with a 10 cm thick layer of material and has a resistivity of 1000 Ohm-meters. A body weight of 50 Kilograms is required for the shock calculations, along with an exposure time of 0.1 seconds.

•

The Primary grounding grid is square (10 meters by 10 meters), with its origin at 0, 0. The grid conductors are buried at a depth of 0.5 meters, with four parallel conductors along the X-axis partitioned into four elements each and five parallel conductors along the Y-axis partitioned in five elements each. The diameter of the (#4/0 AWG) grid conductors is 1.34 cm (0.528 in.)

•

A fault current of 300 Amps is injected into the grid, all of which contributes to the station potential rise (Parallel impedance of 9999 Ohms).


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•

The return electrode is also a square symmetrical electrode equipped with a rod at its lower right corner. The rod, with a diameter of 0.14 meters and a length of 2 meters, is positioned 25 meters away from the origin coordinates of the primary grid, along the X-axis, and is buried at a depth of 0.5 meters. The return electrode is assumed to collect all of the injected current diffused to the ground by the primary electrode (i.e. the Return electrode current will be entered in the program as –300 Amperes). The symmetrical Return electrode is a 7.5 by 4 m square grid with four parallel conductors on both the X and the Y-axes, buried at a depth of 0.9 meters. The same material was used for its conductors as for the primary grid.

•

The distinct electrode, representing a simplified version of the adjacent building foundations will also be modeled for illustration purposes as a symmetrical electrode. It will be defined as a 5 meter by 10 meter rectangle with it lower-left corner located 15 meters away form the primary electrode origin in the X direction and 5 meters in the Y direction. Again, a burial depth of 0.9 meters is assumed. The conductor diameter is assumed to be 15 centimeters. Since the building foundations are galvanically connected, all the distinct electrode elements will be assumed to be at the same potential.

The following illustration shows the corresponding data entry dialog to enter the soil and safety parameters.

The illustration below shows the grid layout for the installation..

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If you generate a contour plot for an area encompassing the entire installation, you will see the distortion of the equipotential contours due to the presence of the return and distinct electrode.

For comparison purposes, the same plot can be generated in the absence of the distinct electrode. First, disable the distinct electrode entries in the ‘Grid Parameters’ dialog box.


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Then, generate the same area contour graph using the new design elements.

In the absence of both the Return and Distinct electrodes, and following the same procedure, we obtain:

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Finally, a surface plot in 3-D is also shown, portraying all 3 electrodes:


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Chapter 6

6.1

Comparison with the IEEE80 Guide

Comparison with the IEEE80 Guide

We have used the following four IEEE cases in order to provide a comparison with the results obtained using CYMGRD. Example 1: Preliminary design stage •

IEEE80 GUIDE, 2000 Edition, Page 129.

•

Square grid 70m x 70m, 100 meshes with no ground rods.

Example 2: Improved design •


•

Square grid 70m x 70m, 100 meshes with ground rods placed along the perimeter.

Example 3: Finalized design •


•

Rectangular grid 63m x 84m, 108 meshes with ground rods placed along the perimeter and at selected places in the gird in an effort to further minimize surface touch potentials.

Example 4: L-Shaped Grid with Ground Rods •


CYMGRD utilizes a finite element analysis algorithm, which is more accurate than the approximate formulas provided in the IEEE80 GUIDE. The finite element analysis algorithm enables CYMGRD to analyze grounding systems of either symmetrical or asymmetrical configuration of ground conductors and rods.

CHAPTER 6 – COMPARISON WITH THE IEEE80 GUIDE

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6.1.1

CASE NAME: Example 1: Preliminary design stage REFERENCE

: IEEE80 GUIDE, 2000 Edition, Page 129.

SOIL MODEL : Uniform. Soil resistivity = 400 ohm-meters. A. SAFETY CALCULATIONS: Input data: Body weight Crushed rock surface layer resistivity Crushed rock surface layer thickness Clearing time Uniform soil resistivity Max LG fault current, X/R (Local) LG fault current (Remote) Split factor Sf Conductor material Ambient temperature

70 Kg 2500 Ω-m 0.102 m 0.50 sec 400 Ω-m 6814 A, 16.2 3180 0.6 Copper hard-drawn 40 Celsius

Results: REFERENCE

CYMGRD IEEE Guide 80

Conductor Sizing 2/0 AWG 2/0 AWG

MAX. ALLOWABLE TOUCH 840.55 Volts 838.20 Volts

MAX. ALLOWABLE STEP 2,696.10 Volts 2,686.00 Volts

REDUCTION FACTOR CS 0.740 0.740

B. GRID DESIGN ASPECTS: Input data: •

Square grid, 70m x 70m, 100 meshes with no ground rods as shown in the IEEE 80 EXAMPLE 1 STATION LAYOUT figure shown below.

Square grid Grid conductor diameter Burial depth Remote contribution of the LG-fault current Split factor Uniform soil resistivity

70m x 70m, 100 meshes 0.01 m 0.5 m 3180 Amps 0.6 (User defined) 400 Ω-m

Results: •

In the table that follows ‘Rg’ and ‘GPR’ respectively mean the station resistance and the ground potential rise.

REFERENCE CYMGRD IEEE Guide 80

82

RG 2.675 Ohms 2.780 Ohms

GPR 5,105.61 Volts 5,304.00 Volts

CHAPTER 6 - COMPARISON WITH THE IEEE80 GUIDE


Note:

6.1.2

For Example 1, the RG calculated with the EPRI TR-10062 computer program, is 2.67 Ohms (see IEEE Guide 2000, page 134).

CASE NAME: Example 2: Improved design REFERENCE


SOIL MODEL : Uniform. Soil resistivity = 400 ohm-meters. A. SAFETY CALCULATIONS: Input data: Body weight Crushed rock surface layer resistivity Crushed rock surface layer thickness Clearing time Uniform Soil Resistivity

70 Kg 2500 Ω-m 0.102 m 0.50 sec 400 Ω-m

Results: REFERENCE






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Square grid 70m x 70m, 100 meshes with ground rods placed along the perimeter as shown in the IEEE 80 EXAMPLE 2 STATION LAYOUT figure shown below.

Square Grid Grid conductor diameter Length of Ground rods Ground rod diameter Burial Depth Injected ground current Uniform soil resistivity

70m x 70m, 100 meshes 0.01 m 7.50 m 0.01 m 0.5 m 1,908 Amps 400 Ω-m

Results: •


REFERENCE

RG

GPR


2.500 2.750

4,780.00 Volts 5,247.00 Volts

Note:

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6.1.3

CASE NAME: Example 3: Finalized design REFERENCE


SOIL MODEL : Uniform. Soil resistivity = 400 ohm-meters. A. SAFETY CALCULATIONS: Input data: Body weight Crushed rock surface layer resistivity Crushed rock surface layer thickness Clearing time Uniform Soil Resistivity

70 Kg 2500 Ω-m 0.102 m 0.50 sec 400 Ω-m

Results: REFERENCE






Rectangular grid 63m x 84m, 108 meshes with ground rods placed along the perimeter and at selected places in the gird in an effort to further minimize surface touch potentials, as shown in the IEEE 80 EXAMPLE 3 STATION LAYOUT figure shown below.

Rectangular Grid Grid conductor diameter Length of Ground rods Ground rod diameter Burial Depth Injected ground current Uniform soil resistivity

70m x 70m, 100 meshes 0.01 m 10.0 m 0.01 m 0.5 m 1,908 Amps 400 Ω-m

Results: •


REFERENCE

RG

GPR


2.278 Ohms 2.620 Ohms

4,348.00 Volts 4,998.96 Volts


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Note:

6.1.4


CASE NAME: Example 4: L-Shaped with rods REFERENCE


SOIL MODEL : Uniform. Soil resistivity = 400 ohm-meters. SAFETY CALCULATIONS: Same as CASE NAME: Example 3: Finalized design (see 6.1.3. A. GRID DESIGN ASPECTS: •

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L-Shaped Grid, with the same effective grounding area as before, as shown in IEEE Std 2000, page 140.



Results: •


REFERENCE CYMGRD IEEE Guide 80

Note:

RG 2.330 Ohms 2.740 Ohms

GPR 4,562.49 Volts 5,227.92 Volts


The 12 IEEE samples study files included in the above analysis cases are available upon request. Please contact CYME and ask for the “CYMGRD Benchmark cases.”


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Chapter 7

7.1

CADGRD - The CYMGRD - AutoCAD Interface module

Program summary

CYMGRD has a bi-directional interface with AutoCAD to import ground grid layouts already entered via AutoCAD and to export grid layouts to AutoCAD whenever ground grids have been entered within CYMGRD. The improvement that was introduced in version 6 of the CYMGRD application is that this interface utility has now been merged with the CYMGRD program. CYMGRD, therefore, now features the additional capability of allowing the user to alternate easily between the AutoCAD and the CYMGRD environments. This CYMGRD-AutoCAD interface allows the user to: •

Enter a ground grid via the CYMGRD drawing/design facilities and then export it to AutoCAD.

•

Import to the CYMGRD a ground grid already entered in the AutoCAD drawing environment, an operation normally performed to either verify/reinforce existing grid designs.

•

Enter a grid layout directly in AutoCAD and prepare it for import, for further engineering analysis into CYMGRD.

In order, however, for the Import facility to work properly, certain rules must be followed when entering the station layout in the AutoCAD environment. The description of these rules is the very purpose of this chapterand they are examined in detail further below. The following important points should be noted: •

Due to the fact that the ground grid contains many different entities (i.e. ground conductors, ground rods, arc-type conductors etc), and that these very variants can belong to different electrode types, the notion of reserving dedicated AutoCAD layers for every type of these components is crucial in order for the CYMGRD AutoCAD Interface to work.

•

The AutoCAD interface is conceived to accommodate ONLY the geometrical aspects of the station grid layout. In other words, no soil data can be communicated from the AutoCAD module to the CYMGRD application data structures. Therefore, the soil model that CYMGRD can export to the AutoCAD information structures is meant only to be informative data.

•

AutoCAD is not a firm software requirement for CYMGRD. In fact, the application can be operated totally independently from AutoCAD if so desired. The standalone drawing facilities imbedded within CYMGRD support very adequately both grid design and/or grid component editing. The AutoCAD interface is provided as an add-on facility intended to facilitate grid data entry, thus providing a more rapid alternative to the interactive data entry supported by the CYMGRD application.

CHAPTER 7 - CADGRD - THE CYMGRD - AUTOCAD INTERFACE MODULE

89


AutoCAD and CYMGRD produce the necessary *.DWG and/or *.DXF files that contain the information to convey the geometrical description of the substation ground grid. Additional properties for the grid components like electrode sizes are also supported within the AutoCAD data structures in order to avoid further editing of the entire grid upon importing it to the CYMGRD application. All data structures within the AutoCAD environment follow the same philosophy and input data patterns of the CYMGRD data input interface in order to ascertain continuity of information exchange. This is done by properly assigning attributes to the AutoCAD drawn entities using special data blocks supported by CYMGRD. Whenever this function is to be exercised, AutoCAD becomes a firm software requirement for CYMGRD. General functionality for the AutoCAD interface can be seen as: •

CYMGRD importing the station data from *.DXF and/or *.DWG files.

•

CYMGRD exporting the station data (already entered, designed/optimized within CYMGRD), by producing the relevant *.DWG and/or *.DXF files necessary for describing the grounding assemblies in the AutoCAD environment.

The advantages are as follows: •

CYMGRD has now access to the powerful drawing facilities of the AutoCAD environment with full support of its GUI data structures.

•

Grounding grid layouts can be entered independently and communicated to CYMGRD so that an engineering analysis can proceed for verification, correction or further design optimization.

•

The engineering analysis results for designing a new or reinforcing/optimizing the design of existing ground grids can now be efficiently exported in the AutoCAD environment without any loss of information.

•

Exchange of information between the AutoCAD and CYMGRD environment is rendered transparent and seamless, even when different persons service either end of the link.

•

All the above advantages are possible with drawing packages other than AutoCAD, at the condition that the same *.DWG or *.DXF files are supported.

The following pages describe the CYMGRD-AutoCAD interface. A working level of familiarity with AutoCAD is assumed in order to present this information in a concise manner and avoid duplicating AutoCAD user manual details. This user’s guide should not therefore be used as an aid to comprehend the hereby implicated AutoCAD functions but, instead, as a means to efficiently use CYMGRD within the AutoCAD environment.

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7.2

Drawing a station ground grid with AutoCAD

7.2.1

General outline

To draw a grid layout, CYMGRD always starts from the “CYMDEF.dwg” file. No matter how many times the export module of CYMGRD is activated, it is this template it uses to create a new drawing. This file is the default template used by CYMGRD to start the AutoCAD drawing and should never be overwritten. Furthermore, this file should reside in the same directory as the CYMGRD program and should never be deleted. When the station drawing is finalized within AutoCAD, make sure it is saved using a different name. The “CYMDEF.dwg” file contains seven layers, which are used to draw the GRID layout and define the data for the CYMGRD analysis. These layers are also reserved in name and function for CYMGRD and should not be modified in any fashion. In fact, the very functionality of CYMGRD depends on them when exporting the grid design to AutoCAD.

These seven layers are defined as: Layer Name

Layer Use

DISTINCT DATA:

Store data for the Distinct electrodes.

DISTINCT:

Define the layout of the Distinct electrodes.

PRIMARY DATA:

Store data for the Primary electrodes.

PRIMARY:

Define the layout of the Primary electrodes.

RETURN DATA:

Store the data of the Return electrodes.

RETURN:

Define the layout of the Return electrodes

GENERAL:

Define the system units (Imperial or Metric)

Note:

The layout data should be drawn in the appropriate layers. If they are represented in any other layers they will be ignored.


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7.2.2

Drawing the Grid Layout using AutoCAD:

The Grid Layout should be drawn in AutoCAD without the use of a scaling factor. One unit in AutoCAD drawing represents either 1 meter or 1 foot in CYMGRD, depending upon the system unit you define in the ‘General Data Block’. The coordinates (0, 0, 0) in AutoCAD correspond to the coordinates (0, 0, 0) in the CYMGRD grid layout. Note:

CYMGRD recognizes both Imperial and Metric unit systems. Thus, a grid entered via AutoCAD say, in Metric units, is imported into CYMGRD as such respecting the system of units selected within AutoCAD. The same data could however be converted to Imperial units within CYMGRD once imported. Upon exporting the same grid layout to AutoCAD, the data will now be represented using Imperial units.

7.2.2.1 Entities The Grid Conductor entity: Grid Conductors are represented as line objects in AutoCAD via their end points coordinates (X1, Y1, Z1) and (X2, Y2, Z2). The burial depth of the Conductor ends, Z1 and Z2, are represented by negative Z coordinates. Note:

In CYMGRD, the burial depth is always entered as a positive value. This apparent inconsistency, however, is automatically taken into account when data is transferred back and forth to and from CYMGRD. The diameter of the Grid Conductor is defined in the ‘Conductor Data Block’, which is discussed later.

The Arc Conductor entity: Arc Conductors are represented as arc objects in AutoCAD via their center point (X1, Y1, Z1), starting angle and ending angle. The two latter assume a counter-clockwise sense of direction. The burial depth of the Arc is represented by a negative Z coordinate. Note:

In CYMGRD, the burial depth is always entered as a positive value. This apparent inconsistency, however, is automatically taken into account when data is transferred back and forth to and from CYMGRD. The diameter of the Arc Conductor is defined in the ‘Arc Data Block’, which is discussed later.

The Grid Rod entity: Grid Rods are represented as Circles in AutoCAD via the coordinates (X, Y, Z) of their center. The Rod length is defined by the ‘Thickness’ of the Circle. A negative value for the Thickness represents the Rod pointing downwards from the center point (X, Y, Z). The diameter of the Circle in the AutoCAD drawing is only used for display purposes. The actual diameter of the Rod used by CYMGRD is defined in the ‘Rod Data Block’, which is discussed later.

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Note:

In CYMGRD, the Rod length is always entered as a positive value and it is implicitly assumed that the Rod points downwards. This apparent inconsistency however, is automatically taken into account when data is transferred back and forth to and from CYMGRD.

7.2.2.2 Data Blocks for the entities Data blocks are used to define the data for the Conductors, Rods, Arcs and System Units. The AutoCAD ‘INSERT’ command can be invoked to bring up the data ‘Insert’ dialog box as shown below. There are four insertion blocks supported by CYMGRD with AutoCAD.

Data Block #1: General Data Block (CY_GEN)

The ‘General Data Block’ is used to define the global attributes for the Grid design.


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Click on ‘Next’ to view the remaining attributes: Note:

Only the ‘System Units’ from the “CY_GEN” data block is communicated to CYMGRD. This value is either “M” for metric or “I” for imperial. All the other values of this data block are ignored. In other words, even if data relevant to Soil resistivity is entered here it will not be communicated to CYMGRD. This is intentional since the very same station layout may be analyzed under different soil models, within the CYMGRD application.

Block #2: Grid Conductor Data Block (CY_GRID) The ‘Grid Conductor Data Block’ is used to assign data to a ‘Grid Conductor’ entity:

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# of elements

Number of elements in the Grid Conductor.

Conductor diameter

Diameter of the Grid Conductor used for CYMGRD analysis.

Conductor group no

0 for Asymmetrical, 1 to 9999 for Symmetrical.

Entity Handle

Unique AutoCAD ID to associate the Grid Conductor with its Data Block.

Grid Conductor group number

Each Grid Conductor is assigned a group number based on whether the conductor is part of a Symmetrical Conductor assembly or not. This notion is borrowed from CYMGRD, which permits entry of many conductors exhibiting a certain pattern of symmetry. Normally, a grounding Grid will feature symmetrically-spaced as well as stand-alone Conductors that do not necessarily belong to a symmetrical pattern. It is the latter that will feature a group number of 0 and are hereby referred to as ‘asymmetrical’ Conductors. A set of Conductors with the same group number, any number between 1 and 9999, is treated as a group of symmetrical Conductors.



Note:

The symmetry of Conductor assemblies entered in AutoCAD will be retained as entered in CYMGRD, as long as this symmetrical pattern is not disturbed in any way within AutoCAD. If, for instance, a symmetrical assembly of three conductors parallel to the Xaxis has been defined in CYMGRD, this assembly will be retained within AutoCAD and transferred back to CYMGRD if left as is. If, however, any spacing of the original coordinates or even the number of elements for any one of the three Conductors is disturbed, the symmetrical assembly will be broken down into its individual components and communicated back to CYMGRD as a new set of asymmetrical Conductors. The number of elements per conductor is another value used for electrical analysis within CYMGRD. These values are crucial for actual CYMGRD simulations. A value of 1 is suggested when data for the grid is entered for the first time via AutoCAD, and it remains up to the CYMGRD analyst to redefine this parameter. If, however, data are communicated to AutoCAD from CYMGRD, this parameter should not be modified because the modified value will be passed once again to CYMGRD. A symmetrical group of Conductors should feature at least a minimum order of 2 by 2 (equal to 4 Conductors).

Block #3: Conductor Data Block: (CY_ARC) The ‘Arc Conductor Data Block’ is used to assign data to an ‘Arc Conductor’ entity: # of elements

Number of elements in the Arc Conductor.

Conductor diameter

Diameter of the Arc Conductor used for CYMGRD analysis.

Entity Handle

Unique AutoCAD ID to associate the Arc Conductor with its data block.


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Block #4: Grid Rod Data Block: (CY_ROD) The ‘Grid Rod Data Block’ is used to assign data to a ‘Grid Rod’ entity:

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# of upper layer elements

Number of elements per Grid Rod in the upper soil layer.

# of lower layer elements

Number of elements per Grid Rod in the lower soil layer.

Rod diameter

Diameter of the Grid Rod used for CYMGRD analysis.

Enter group no

0 for Asymmetrical, 1 to 9999 for Symmetrical.

Entity Handle

Unique AutoCAD ID to associate the Grid Rod with its data block.

Grid Rod Group Number

Each Grid Rod is assigned a group number based on whether the rod forms part of a Symmetrical Conductor assembly or not. This notion is borrowed from CYMGRD, which permits entry of many rods exhibiting a certain pattern of symmetry. Normally, a grounding Grid will feature symmetrically-spaced as well as stand-alone Rods that do not necessarily belong to a symmetrical pattern. It is the latter that will feature a group number of 0 and are hereby referred to as ‘asymmetrical’ Rods. A set of Rods with the same group number, any number between 1 and 9999, are treated as a group of symmetrical Rods.



Note:

Equidistantly-spaced Rods along the same line can still be entered with the same group number in AutoCAD as if they were part of a symmetrical assembly. CYMGRD will properly support this data structure. The symmetry of Rod assemblies entered in AutoCAD will be retained as entered in CYMGRD, as long as this symmetrical pattern is not disturbed in any way within AutoCAD. If, for instance, a symmetrical assembly of three rods along the X-axis has been defined in CYMGRD, this assembly will be retained within AutoCAD and transferred back to CYMGRD if left as is. If, however, any spacing of the original coordinates for any one of the three Rods is disturbed, the symmetrical assembly will be broken down into its individual components and communicated back to CYMGRD as a new set of asymmetrical Rods A symmetrical group of Rods should feature at least a minimum order of 2 by 2 (equal to 4 Rods).

7.2.2.3 AutoCAD Entity Handle The ‘entity handle’ is a unique ID (Conductor, Rod, or Arc) with its associated because vital data for the entity are contained handle must be present for every data block. In and unique handle.

used to couple the drawing element Data Block. This coupling is important in the Data Block. That is why an entity fact, every data block demands a distinct

When the user places the data block for an entity anywhere in the drawing area, the handle must be explicitly defined. This can be laborious particularly if the drawing contains a large number of entities, a very likely situation for sizeable transmission ground grids. In order to circumvent this difficulty and ease the data entry, CYMGRD can automatically assign the entity handle assuming the Data blocks are inserted at the correct “insertion points” in the AutoCAD drawing. A ‘proper insertion” point for the data block is considered for the purposes of automated entity handle assignment to be any point of the entity itself. In order for CYMGRD to automatically assign the entity handles, the ‘Update Drawing…’ function needs to be invoked from within CYMGRD for the drawing at hand. If the grid data is imported into CYMGRD, it will take care of both assigning the entity handles and positioning the data blocks automatically. Note:

If, for any reason, the entity handle is not assigned and the data block is present at the proper insertion point, the entity handle will be assigned automatically by CYMGRD.


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7.2.3

Example

This following example illustrates the basic procedure to draw the station grounding grid layout using AutoCAD. The following hypothetical data apply to this installation: •

The grounding grid is square (10 meter by 10 meters), with its origin at (Xo = 0 meters and Yo = 0 meters)

•

The grid Conductors are buried at 0.5 meters, with five parallel Conductors along the X-axis partitioned in three elements each and five parallel Conductors along the Yaxis partitioned in four elements each. The diameter of the #4/0 AWG grid Conductor wire is 1.34 centimeters (0.528 inches).

•

There is a Return electrode composed of a single grounding Rod, with a diameter of 0.02 meters and a length of 1 meter, positioned 45 meters away from the grid. We will assume the top of the Rod is at the surface of the earth (Z1 = 0.0).

To draw the above-depicted grounding system, start with the template file ‘CYMDef.dwg’. This file, as pointed out earlier, is a reserved file and should only be used to initialize a drawing. It should always be resident in the directory of CYMGRD program and should never be modified, overwritten or moved. 1

Open file Open the ‘CYMDef.dwg’ file using AutoCAD and save the file under a new name: “PROJ2.dwg”.

2

Draw the Primary electrode

2.1

Set the Layer to ‘Primary’. Draw one Primary conductor parallel to X-axis and one parallel to the Y-axis: Command: line • • •

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Specify first point: 0,0,-0.5 Specify next point or [Undo]: 0,10,-0.5 Specify next point or [Undo]: , for the X-axis



Command: line • • •

Note:

2.2

Specify first point: 0,0,-0.5 Specify next point or [Undo]: 10,0,-0.5 Specify next point or [Undo]: , for the Y-axis

When drawing the lines, you may also want to use the standard command ‘Zoom’ with a convenient associated option like ‘All’ or ‘Extend’ to have a clear view of the entities drawn so far. This may be necessary when elements distant from the coordinates ‘0.0, 0.0’ are drawn first, because the ‘CYMDef.dwg’ file is configured to make the point (0.0, 0.0) visible right from the beginning.

Specify the data blocks for the two entities already entered At this point we need to specify the data blocks for the two entities already entered. They will both be used later by the ‘array’ command to initialize copies of any data blocks generated. By specifying their data blocks, we will also properly duplicate the attributes of the entities.

2.3

•

Set the Layer to ‘Primary data’

•

Insert the Conductor data block on the Conductor parallel to the Y-axis.

Specify Insertion Point The insertion point can be any point along the Conductor, but it must be on the Conductor (See note below). As you move the cursor close to the conductor, the cursor will be highlighted with a yellow square to indicate the insertion point. The reason the insertion point must be on the conductor is that the entity handle can automatically be assigned by CYMGRD for all entities in the group thus avoiding specifying the handle for a great number of entities. Do not insert the data block at the END points of the line.


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Note: It will help to properly configure AutoCAD, at least for the CYMGRD session, to conveniently display the insertion point along the Conductor. To do that, use the command ‘OSNAP’ and make sure that the boxes ‘ENDPOINT’, ‘CENTER’ and ‘NEAREST’ are checked. Command: insert Select ‘CY_GRID’ on the Insert window •

Specify insertion point or [Scale/X/Y/Z/Rotate/PScale/PX/PY/PZ/PRotate]: Enter attribute values: • • • •

2.4

# of elements <1>: 4 Conductor diameter <.01>: 0.0134 Enter group no (0 = none) <0>: 1 Enter Entity handle:

Insert the Conductor data block on the Conductor parallel to X-axis. Command: insert Select ‘CY_GRID’ on the Insert window •

Specify insertion point or [Scale/X/Y/Z/Rotate/PScale/PX/PY/PZ/PRotate]: Enter attribute values • • • •

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# of elements <1>: 3 Conductor diameter <.01>: 0.0134 Enter group no (0 = none) <0>: 1 Enter Entity handle:



2.5

Make arrays of the Primary conductors & Data blocks Complete the Primary Grid layout by making arrays of the Primary conductors & Data blocks. It is well known that by exploiting this AutoCAD option, we enter more efficiently symmetrical structures as the one we mean to emulate in this example. Command: array Select the conductor parallel to Y-axis along with its Data block • • • • • • •

Select objects: 1 found Select objects: 1 found, 2 total Select objects: (to denote end of selected objects) Enter the type of array [Rectangular/Polar] : Enter the number of rows (---) <1>: Enter the number of columns (|||) <1> 5 Specify the distance between columns (|||): 2.5


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Command: array Select the conductor parallel to X-axis along with its Data block • • • • • • •

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Select objects: 1 found Select objects: 1 found, 2 total Select objects: (to denote end of selected objects) Enter the type of array [Rectangular/Polar] : Enter the number of rows (---) <1>: 5 Enter the number of columns (|||) <1>: Enter the distance between rows or specify unit cell (---): 2.5



2.6

We will now enter the Arc entity. Command: arc • • •

2.7

Specify start point of arc or [CEnter]: 10,0,-0.5 Specify second point of arc or [CEnter/ENd]: 11,-0.5 Specify end point of arc: 12,0

Insert the CY_ARC data block Then insert the CY_ARC data block at the center point of the arc with the following command. Command: insert Select ‘CY_ARC’ on the Insert window Specify the center point of the arc for the insertion point. •

Specify insertion point or [Scale/X/Y/Z/Rotate/PScale/PX/PY/PZ/PRotate]: Enter attribute values

• •

•

# of elements <3>: Conductor diameter <.01>: 0.0134 Enter Entity handle:

At this point all relevant data for the primary electrode have been entered. The next step is to proceed by entering the data for the return electrode.


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3

Enter the data for the Return electrode

3.1

Set the layer to ‘Return Data’. Draw the Return Rod at a distance 45 meters away from the grid. Command: circle • •

Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: 55,5,0 Specify radius of circle or [Diameter] <0.1000>: 0.5

Set the layer to ‘Return Data’. Insert the Rod Data block. Command: insert Select ‘CY_ROD’ on the Insert window. •

Specify insertion point or [Scale/X/Y/Z/Rotate/PScale/PX/PY/PZ/PRotate]: The insertion point must be at the center of the circle. Move the cursor close to the center of the Rod; the cursor will be highlighted with a yellow square to indicate the insertion point. Enter attribute values • • • • •

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# of upper layer elements <1>: 1 # of lower layer elements <1>: 1 Rod diameter <.01>: 0.02 Enter group no (0 = none) <0>: 0 Enter Entity handle:



3.2

Specify the depth of the Rod Change the Thickness of the Circle to –1.0 (Rod Length = 1 meter). The Thickness is the way to specify the depth of the Rod without resorting to full 3-D description. The Thickness can be modified in two ways: 1. By the ‘ddmodify’ command and then selecting the circle. 2. By selecting our circle first, then right click on it, and select properties from the menu. Command: ddmodify Click on the Circle and change the thickness to -1.0.

This completes the Return Rod data entry.


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4

Specify the General Data Block The next entry will be for the ‘General Data Block’. This block must be inserted in the drawing even if only the System Units are retained. The rest of the data is ignored when importing into CYMGRD. The rest of the values can be entered and modified in CYMGRD. The data contained in this data block, except for the System Units, pertain to the engineering analysis performed by CYMGRD and have no direct relevance to the drawing itself.

4.1

Set the Layer to ‘General’. Insert the ‘General Data Block’. Command: insert Select ‘CY_GEN’ on the Insert window. •

Specify insertion point or [Scale/X/Y/Z/Rotate/PScale/PX/PY/PZ/PRotate]: Enter attribute values • • • • • • • • •

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Title: Example: System units (M=Metric or I=Imperial) : M Distinct electrode flag (0 or 1) <0>: Return electrode current (A) <0>: Upper layer depth (m) <1.0>: Upper layer resistivity (Ohm-m) <100.00>: Lower layer resistivity (Ohm-m) <100.00>: Primary electrode current (A) <1.0>: Parallel impedance (Ohm) <9999.0>:



The completed Grid layout with its Data blocks is shown here:

7.3

Validation & Update of the AutoCAD drawing

Once the AutoCAD drawing is completed and before importing it into CYMGRD, the Grid drawing must be ‘Validated’ and subsequently ‘Updated’ using the appropriate CYMGRD commands. It is important to run these functions, since they supplement the drawing and render the AutoCAD drawing ready to be imported into CYMGRD for analysis. Neither one should be omitted. These commands can be accessed from the CYMGRD menu: Grid > Electrodes > Cad Editor. Under this Cad Editor sub-menu, you have two items, ‘Validate Drawing…’ and ‘Update Drawing…’.


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7.3.1

Validating the AutoCAD drawing

The ‘Validation’ option is used to verify the AutoCAD drawing. This will only highlight the errors caused by the Grid layout, Data insertion points, etc. More specifically. the validation will address the following salient aspects: •

It verifies that all entities have an associated data block.

•

If a data block is not defined for a stand-alone entity (i.e. an entity that has not been defined via the command ‘array’), CYMGRD assigns a data block to that entity, identical to the last valid data block of a similar stand-alone entity. In other words, if a data block is missing for an individual Rod, the data block for the last valid individual Rod will be in effect. This practice assures that once data for an individual Rod, Conductor, or Arc entity is entered, there is no need to enter them again for a good number of like entities. Following this practice can be very efficient but it could also lead to inadvertently assuming wrong data for a good number of entities.

•

In the case a data block has not been assigned to a stand-alone entity and if there is no similar entity entered previously, the internal default values of CYMGRD for data blocks are used. These defaults are as follows: Stand-alone Conductor •

# of elements in Conductor = 1

•

Conductor diameter = 0.01

Stand-alone Rod •

# of rod elements in upper soil layer = 1

•

# of rod elements in lower soil layer = 1

•

Rod diameter = 0.01

Stand-alone Arc

•

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•

# of elements in Conductor = 3

•

Conductor diameter = 0.01

If entities have been generated using the ‘array’ command and if no data block has been assigned to the ‘seed entity’ (i.e. the entity that was used as a template to generate copies), CYMGRD does not retain the structure as symmetrical. The symmetrical structure of entities is broken down into individual entities while their geometry is retained and the internal default values are assigned to each of them. The disadvantage of this is that when the .DWG file is imported into CYMGRD, the symmetrical structure for this particular set of entities will be lost and they will be shown within CYMGRD as individual entities. This, in turn, may have a considerable bearing on the flexibility of the modifying data very conveniently within CYMGRD. It is therefore preferable before using the array command to make certain that a data block is assigned to the ‘seed entity’ so that the symmetrical structure is retained for all future data exchanges with CYMGRD.



•

CYMGRD verifies that all entities and associated data blocks are within acceptable coordinate limits. The boundary limits are set to be 10,000 meters in all directions for Metric units and 32,820 feet for Imperial. This check is carried out once the units of the drawing are defined in the ‘General data block’. That is why a ‘General data block’ needs to be inserted in the drawing.

•

CYMGRD imports the units in Metric if no ‘General data block’ is inserted in the XXX.DWG file representing the station grid. Note:

If no ‘General data block‘ has been entered, CYMGRD generates a warning that the ‘General data block’ is missing when the drawing is being validated. When the DWG file is simply saved from session to session without being validated, no general data block is put in it. If the DWG file is not validated and is sent to CYMGRD, the default General data block contents will be assumed.

•

All entities must be assigned to any of the seven reserved layers. Note:

•

Verify that the length of the Rods is not positive. If positive values were accidentally entered, they will be converted to negative for the sake of consistency. Note:

•

CYMGRD functions by assuming a positive rod length, since it implies that the rod always points downwards. Consistent with this convention, the interface of CYMGRD shows all rod lengths as positive in its dialog boxes. It is CYMGRD that will convert all negative Z’s coming from the XXX.DWG drawing to positive for interface and calculations within CYMGRD. Similarly it is also CYMGRD that will convert all Z’s to negative when generating the equivalent XXX.DWG file.

Verify that the Z coordinates are entered as negative. Positive Z’s coordinates will be converted to negative Z’s to assure information consistency. Note:

•

Ideally, all entities belonging to a given layer should have all their corresponding data blocks in the corresponding data layer. For instance primary electrode entities should find their correspondent data blocks in the primary data layer. There are reserved color codes for every layer and its correspondent data blocks.

CYMGRD functions by assuming that a positive Z coordinate points downwards. That is why when calculations are performed within CYMGRD the Z’s are considered positive. Consistent with this convention, the interface of CYMGRD shows all Z’s as positive in its dialog boxes. It is CYMGRD that will convert all negative Z’s coming from the XXX.DWG drawing to positive for interface and calculations within CYMGRD. Similarly it is also CYMGRD that will convert all Z’s to negative when generating the equivalent XXX.DWG file

To Validate the drawing click on ‘Grid > Electrodes > Cad Editor > Validate drawing…’.


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110

•

Then select the drawing (Proj2.dwg) file and click open.

•

The Report window lists the errors/warnings (if any) as shown below.



7.3.2

Updating the AutoCAD drawing.

Once the Validating of the AutoCAD drawing is done, the ‘Update Drawing…’ option is used to assign the proper Entity Handles to the Conductor, Rod, and Arc data blocks and associate the symmetrical groups of Conductors and Rods in the drawing. This option re-builds the AutoCAD drawing file (*.DWG, *.DXF) and updates the drawing with default data for undefined parameters. Before using this option, the AutoCAD drawing must be closed. This option also keeps a backup of the original drawing as “*.BK2” before it updates the drawing. •

To Update the drawing click on ‘Grid menu > Electrodes > Cad Editor > Update drawing…’.

•

Select the drawing (Proj2.dwg) file and click open.

•

The Report window lists the errors/warnings (if any) as shown below.


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7.4

Importing from AutoCAD to CYMGRD

To import a drawing from AutoCAD (*.DWG, *.DXF) file, one must be positioned within CYMGRD. The following steps illustrate the procedure:

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•

Open a new Project and an associated Study using CYMGRD.

•

Click on the ‘Grid > Electrodes > Import from…’. This will open the file selection dialog box.

•

Select the AutoCAD file name (Proj2.dwg) and click ‘Open’. The AutoCAD drawing will be validated and updated.



•

The GRID layout will appear in CYMGRD after conversion as shown below:


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7.5

Exporting from CYMGRD to AutoCAD The Export of a Grid layout from CYMGRD, is done from within the CYMGRD package. The following steps illustrate the procedure:

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•

Open the Project, Study using CYMGRD.

•

From the menu, select ‘Grid > Electrodes > Export to… ‘.This will open the file selection dialog box.

•

Specify a file name (proj updated.dwg), which you want to save to:

•

The exported AutoCAD file with the Grid layout is shown below.



7.6

Working with AutoCAD

When entering data for a station grid using AutoCAD, it is often necessary to maximize the efficiency of data entry particularly when large grids are an issue and when the data attributes for a large number of components needs to be entered. Tip #1: Make full use of the symmetrical structure of groups of elements by utilizing the ‘array’ command of AutoCAD, particularly for symmetrical arrays of Conductor assemblies and Rod structures. When, however, the ‘array’ command is used, the data block containing the ‘seed’ entry,(a Conductor or a Rod), must be entered so that for the rest of the Conductors and Rods within the symmetrical structure of the data block attributes are properly duplicated. Tip #2: Whenever data are exported from CYMGRD and the station layout comprises symmetrical structures, make certain that no coordinate displacement or change in any of the associated data blocks is modified within AutoCAD. If this is the case, CYMGRD will decompose the symmetrical structure into elementary non-symmetrical structures, and data modification within CYMGRD will be far more laborious. For example if an array of 6 by 6 Conductors is exported as a symmetrical structure to AutoCAD and the coordinates or data block of any of the 36 Conductors is modified the whole assembly will be broken down to 36 individual Conductors upon importing the same grid back to CYMGRD.


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Tip #3: Whenever a large number of asymmetrical (stand-alone) Conductors and Rods is to be entered, entering the data block for one of them only may be sufficient because CYMGRD will assign the missing data blocks to the values of the last like data block entered. Make certain, however, that the last valid data block does reflect the desired data attributes. Tip #4: The system units (Metric or Imperial) must be entered when a station grid is entered in AutoCAD. The rest of the parameters for the ‘General Data Block’ will not be retained when importing a DWG file. They must be entered in CYMGRD. If no system unit is specified, it will be defaulted to ‘Metric’. Tip #5: For simplicity of data entry, the number of elements for Conductors can be set to 1 when a station grid is entered in AutoCAD. It will be up to the analyst working with CYMGRD to assess whether an increased number of elements per Conductor is needed for the finite elements simulation performed within CYMGRD. It is important to realize however that if the number of elements per Conductor have already been entered within CYMGRD, they should not be modified within AutoCAD. If this is the case, they will be exported back as such to CYMGRD and important data having a detrimental effect on simulation integrity may be lost. By virtue of the same arguments, the same logic applies to the number of elements in the upper and lower soil layer entered for the grounding Rods.

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Chapter 8

Troubleshooting

If CYMGRD crashes, especially when you are trying to open a project file: y

Go to the option, Start > Run and select regedit to open Registry Editor of the MS window.

y

Delete the CYMGRD folder in the Software > CYME path.

CHAPTER 8 - TROUBLESHOOTING

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INDEX Allowable LG fault current .........................56 Arc Conductors..........................................41 Asymmetrically-arranged grid Conductors.............................................36 Asymmetrically-arranged ground Rods .....38 AutoCAD Entity Handle .............................97 AutoCAD Interface module........................89 Comparing contour plots ...........................62 Comparison with the IEEE80 Guide..........81 Contour color coding .................................54 Contour graph management .....................59 Contour graph reports ...............................58 Creating and opening Projects and Studies .....................................................5 CYMGRD modules ......................................2 Danger point evaluation and surface analysis ..................................................73 Data Blocks for the entities........................93 Decrement Factor (Df)...............................32 Design a new grounding grid.......................3 Dividing the grid into elements ....................4 Drawing a station ground grid with AutoCAD ................................................91 Drawing the Grid Layout using AutoCAD ..92 Electrode Material......................................28 Electrode Sizing.........................................24 Electrode Sizing report ..............................28 Electrode types and terminology ...............23 Enabling and disabling entries ..................42 Entering the Grid data ...............................34 Entities .......................................................92 Example 1: Preliminary design stage ........82 Example 2: Improved design .....................83 Example 3: Finalized design......................85 Example 4: L-Shaped with rods ................86 Example Studies........................................65 Exporting from CYMGRD to AutoCAD ....114 First-time user..............................................3 General introduction ....................................1 Generate 3-D contour plots .......................57 Generate Profile voltage plots ...................60 Getting Started.............................................1 Grid analysis and reports...........................44 Grid Analysis Module.................................23

INDEX

Grounding installation data and layout ..... 75 Grounding installation data entry.............. 69 Grounding system structure and location .............................................................. 29 Importing from AutoCAD to CYMGRD ... 112 Importing Projects............................... 20, 21 Importing/Exporting Grid data and Station layouts....................................... 43 Inspecting potential profile plots ............... 62 Installing CYMGRD..................................... 2 Interactive data entry .................................. 3 LG fault parameters .................................. 27 Methodology and algorithm ...................... 12 Modeling Grounding Structures................ 49 Modifying and inspecting the station Geometry data ...................................... 42 Overlapping conductor elements.............. 43 Perform a soil analysis.............................. 13 Plotting Module ......................................... 51 Primary electrode...................................... 65 Primary, Return and Distinct electrodes ... 75 Program summary .................................... 89 Reinforce and verify existing grounding grids......................................................... 5 Reviewing and verifying the data.............. 42 Rod Encasement ...................................... 39 Safety Analysis ................................... 17, 54 Second step:Calculation of the maximum permissible touch and step voltages for the soil model .................... 67 Software and hardware requirements ........ 1 Soil data .................................................... 50 Soil models ............................................... 11 Soil Resistivity and Safety Assessment.... 11 Specify the soil model type ....................... 15 Split Factor (Sf) ......................................... 32 Split-factor (Sf), Decrement- factor (Df) and Definition for RemoteContribution in [%]................................. 31 Spot-check’ danger point evaluation ........ 59 Station layout and the ‘Installation’ view... 49 Symmetrically-arranged grid Conductors . 34 Symmetrically-arranged ground Rods ...... 37 Touch and Surface potential contours...... 54

119


Touch’ and ‘Surface’ potential Contours ...51 Transferring the results..............................19 Troubleshooting.......................................117 Updating the AutoCAD drawing ..............111 Validating the AutoCAD drawing .............108

120

Validation & Update ................................ 107 Visualize the station layout in 3Dimensions ........................................... 46 Windows layout of CYMGRD.................. 7, 9 Working with AutoCAD ........................... 115

INDEX

CYMGRD 65 ReferenceManual En

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