autogrounddesign

8/18/2019 AutoGroundDesign

1/15

AutoGroundDesign: An Overview

AutoGroundDesign is the only fully automated software package that can analyze and design grounding / earthing systems without the intervention of the user between various phases of the design thanks to its intelligent database driven algorithms. It offers powerful and intelligent functions that help electrical engineers design safe grounding installations quickly and efficiently.

A multiple-step approach is used for the automated grounding system design.

First, a grounding system consisting of a buried metallic plate is used as a reference. This gives the minimum ground impedance achievable with a grid of a given size, and determines whether the desired ground impedance and safety limits can be achieved with a solid plate. If not, then the whole process is stopped and the user is informed that the design process is impossible without additional mitigationmeasures.

Second, a grounding system consisting of a minimum number of conductors, for example, the conductors along the grid periphery with or without a few conductors inside the grid is analyzed to see whether the desired ground impedance and safety limits can be achieved with a sparse grid. If yes, the design process is completed quickly without the need to refer to the reference design database and smartterative techniques.

Third, an appropriate preliminary grid design is selected based on SES reference database and other intelligent rules or as specified manually by the user. The use of the reference database is based on the input data provided by the user, such as the size and the geometrical proportions of the grid, the soil structure type, the fault current injected, and the required safety criteria.

Finally, the initial design is refined recursively using rule-based techniques and smart algorithms to improve its performance and meet safety constraints, while reducing the overall cost of the grid.

Practicing electrical engineers are often faced with the necessity of having to design a grounding grid to meet certain protection objectives with regards to personnel safety or to equipment around electrical installations. The design procedure often starts with a preliminary grid which is usually determined based on practical considerations and engineering experience. This grid is then submitted to therounding engineering module to determine if all design requirements are met. If not, then the grid is refined and analyzed again until a satisfactory grid is obtained.

The design of grounding systems is often based on rough guidelines, derived from engineering experience. A grounding system design requires several iterations before obtaining a safe configuration. Therefore, it and can be quite time consuming. It is difficult to account for the large number of variables (topology and dimensions of the grounding system, burial depth, type and characteristics of the soiltructure and material used for the grid’s conductors (horizontal wires and grounding rods, etc. ) that can affect the grounding system performance. Thus, AutoGroundDesign uses a database and rule-based automated grounding system design method to meet design requirements (such as ground potential rise, touch voltage, step voltage, and ground resistance limits), giv en the soil structure, dimensions ofhe grid area, characteristics of conductors, configuration of the grid, and fault current discharged by the grid. The ultimate objective is to complete a grid design that meets the mentioned design requirements.

The goal of AutoGroundDesign is to allow researchers and engineers to quickly and accurately find out which grid (if any) can satisfy the provided constraints. AutoGroundDesign has unique features that set it apart from previous implementations:

Generates grounding system designs based on a simple description of the substation site. The data entry requirements are reduced to a minimum: environment settings, soil data, grounding grid zone, fault current in the grid, safety related data, and automated design parameters & controls.

Models grounding systems and evaluates their performance; it is suited to analyze and design a grounding grid as long as the longitudinal impedances of the ground conductors can be neglected.

Analyzes and designs horizontal arbitrary grounding grids consisting of horizontal and vertical arrangements of bare conductors buried in uniform and multilayered soils.

Carries out automated design with several procedures, such as Automatic, Midpoint, Linear, and User-Defined methods. These procedures will specify the performance and progress of the automated design process appropriately and use ground grid databases, smart search algorithms and techniques, and user-supplied criteria and constraints more efficiently.

Allows users to specify if ground rods are to be used in the design of the final grid and ground rod characteristics. If yes, the rods can be distributed along the edges of the grid or over the whole grid area, as desired.

Computes earth potentials at specific soil locations called observation points that may be determined automatically by the program or user-defined.

Offers three other modes of operations, namely, the Estimator, Configuration and Dimension Predictor modes that allow users to quickly and accurately estimate the resistance of various grounding systems (such as grids, plates array of rods, star electrodes, circular rings, etc.) or predict the size (dimension) or configuration of the grounding system that meets that resistance. Please see the topicentitled Using the Grounding System Estimator and Predictor modes for details.

Please refer to Program Development History for more information on AutoGroundDesign.

Automated System Design Structure

The automated grounding system design software integrates the following modules.

Automated System Design Central Module. This core and controlling module has a simple interface that allows a user to establish an automated grounding system design quickly and efficiently. The ultimate objective of this module is to manage and coordinate input data, safety criteria and progress decisions in order to obtain a grid design that meets all requirements. The overall automated designarameters are controlled by this module to select the methodology used to obtain the initial design of the grounding systems, specify which grid database methodology is to be used for the automated design, and specify the maximum number of design iterations as well as the rate at which the design of the grid evolves.

Grounding Analysis Module. The main mode of operation of this module is used to analyze power system ground networks subjected to DC or AC currents discharged into soil . It computes the safety performance of the grounding grid, in terms of GPR, touch and step voltages. Furthermore, the Estimator, Configuration and Dimension Predictor modes allow users to quickly and accurately estimate theesistance of various grounding systems (grids, plates array of rods, star and hemispherical electrodes, circular rings, etc.) or determine the size (dimension) or configuration that meets a target resistance value.

oil Analysis Module. This module is dedicated to the development of equivalent earth structure models based on measured soil resistivity data. It can generate models with many horizontal layers, as well as vertically and exponentially layered soil models.

Fault Current Distribution and Line Parameter Analysis Module. This module calculates the fault current distribution in multiple terminals, transmission lines and distribution feeders using minimum information and a simple set of data concerning the network. It provides the actual fault current flowing into a grounding grid, as well as currents in the shield wires, tower structures and cable sheaths.elf and mutual impedances of shield wires and cable sheaths are also computed by a built-in line constant module.

afety Module. This module generates safety threshold values based on IEEE Standard 80, IEC Standard 479, user’s own standard or a hybrid combination of these standards. The computed safety voltage limits are used to decide whether to stop or continue the design process. The safety voltage limits are: fault clearing time, earth surface covering layer (e.g., crushed rock) resistivity, earth surfaceovering layer thickness, equivalent subsurface layer resistivity (this is the resistivity of the soil beneath the earth surface covering layer), body resistance, optionally specified foot resistance and resistance of protective wear, such as gloves or boots, and fibrillation current threshold computation method.

View, Plot and Report Tools. A CAD-based module is used to view or edit three-dimensional grounding grids consisting of straight-line segments. The line segments represent either metallic conductors or observation profiles. They can be viewed from any direction, in a variety of ways. A powerful and flexible report and graphics module serves as an integrated output processor to display theomputation results in various graphical or print formats. This module also has the capability to view the input data and even launch the grounding analysis module.

Documentation Road Map

The following topics are available for more information.

Getting Help and Support during your input session.

Selecting your Regional, Environment and Preferences default settings and Using the Windows Interface.

Defining your Working Directory, Job ID and Input data files.

Selecting your System of Units and Reference Database Methodology.

Describing the Grounding Grid Zone in terms of its horizontal shape and depth.

Defining a Soil Model that best fits the soil in which the grid is buried.

Specifying the fault current discharged into the grounding system or defining the Electric Network that applies to determine fault current distribution in multiple terminal transmission and distribution electric line networks.

Specifying the frequency and Computation settings for the observation points.

Specifying the Safety Criteria that apply to determine if the grid performance meets the applicable standards for safe touch and step voltages.

Defining the Parameters and Controls that govern the automated design process.

Viewing the final report and related computation plots pertaining to the grid design and displaying the final grid configuration that was designed and applying changes to it, if appropriate.

Getting Help and Support

You can get help in several ways in AutoGroundDesign.

1. First, you can use the Help | Help Topics menu item to load the help file.

2. You can get context sensitive help from almost anywhere in the program by pressing the F1 key. This will bring up a help topic describing the part of the program that currently has the focus.

3 . T he status bar of the input screen displays helpful messages and the input text color will change based on the validity of the entered data (black if the data is sound, red if the data is not of the expected type, i.e., numeric or integer and dark red if the data is simply invalid). Out of range data brings a suitable message box indicating the acceptable range.

4. You can also obtain help on the SES Input Command language by selecting the Help | Command Mode Help menu item. This help document is useful to advanced users who are familiar with t he SES SICL Commands and prefer to edit directly the input files

5. You can also go to the SES Web site at www.sestech.com. This web site offers a discussion group for grounding-related topics and a lot more information.

You can contact SES?support staff quickly and efficiently by:

1. Access the menu item Help | About AutoGroundDesign ... that provide basic information on the AutoGroundDesign software and contact information (web site, toll free phone, phone, or fax numbers).

2. You can also contact SES by phone or fax using the coordinates available on the Web site atwww.sestech.com/SES/coordinates.asp.

A Typical AutoGroundDesign Session

This section describes briefly what can be done with AutoGroundDesign and how to get started with it. Three sample examples are available to help you get familiar with the AutoGroundDesign program. The associated files are located in the folder "Examples\AutoGroundDesign" in your SESSoftware installation folder.

The main steps of a typical AutoGroundDesign session are summarized below:

1. Starting AutoGroundDesign. To start the program, simply double-click the AutoGroundDesign icon in the tools of the SES Software Program Group.

2. Specifying Your Input Data. One single Windows screen consisting of three tabbed regions allows you to define all the input data and parameters controlling the program's behavior as a whole. These are regrouped in the following categories.

Input session environment, system of units and reference database

Grounding system data

Soil structure data

Electric network data

Computation settings

Page 1 of 15AutoGroundDesign: An Overview

13/02/2016ile://C:\Users\aivanov\AppData\Local\Temp\~hh4231.htm


2/15

Safety criteria selection

Automated design parameters and controls

3. Processing the Grid Design, Electrode Estimation, or Electrode Prediction. When you have finished entering all the necessary input data, the automated grounding design, estimation, or prediction analyses can begin.

4. Viewing the Computations Results and the Final Grid Design . You can use any CDEGS or AutoGrid Pro tools or programs to do this. The plots and reports are displayed using the SESCAD, GRServer and FileView utilities.

5. Ending an AutoGroundDesign Session. To quit the application and terminate the AutoGroundDesign session, click the Exit button.

Starting AutoGroundDesign

To start the program, simply double-click the AutoGroundDesign icon in the SES Software Program Group. The information and data necessary to define the design automation process are described in section entitl ed Specifying Input Data.

Using the AutoGroundDesign Windows Interface

Once you have successfully started a new session or loaded a previous one, you may need to modify or define new options and input data and start or respond to various events during your input session. For example, some error or warning messages that were issued during your session will be stored and can be accessed by pressing the View Message button or by selecting it from the File menu item.

n fact for every command button, there is a corresponding menu item that can be accessed from the menu bar. The following describes the role or function of each button or menu item.

Start Design / Start Automated Design: If all your data are valid, the button will be enabled and by selecting the button or the menu item, the automated design computation process will be launched.

View Designed Grid: The preceding button will change to this one as shown above if the run is successfully launched. In this case, selecting this button or the corresponding menu item will load the SESCAD tool and you will be able to view and modify the final version of the grid design.

View Design Summary File / View Design Summary Output File: This button or menu item will start a file viewer that displays the most relevant design computation information and results.

View Plots and Reports: Pressing this button or selecting this menu item launches the GRSERVER graphics viewer that will allow you to display the computation results as dedicated plots or as specialized reports.

View Messages: Some error or warning messages that were issued during your session will be stored and can be accessed by pressing this button or by selecting the corresponding menu item from the File drop down menu bar.

Reset Session: This button will clear all warning and error flags and will force the session status to become valid although errors are still present. This is sometimes useful to enable buttons or fields that are otherwise inaccessible.

Save Session / Save Session Input File: Pressing this button or selecting this menu item will immediately save all your input session data, selections and settings to the pertinent files or locations.

Restore Default Data: If you wish to populate your input data fields with default values and select default options, just press this button or select the corresponding menu item.

New / New Session: If you wish to start a new session, simply press this button or select the corresponding menu item. This will clear the Job ID and Working Directory path and will populate all data fields with default values. All options will be set to their default values. Note that you must specify a valid Job Id and Working directory before you can save and submit your input session forprocessing.

Open... / Open Input File: This button or menu item allows you to browse to and load any existing AutoGroundDesign input session file.

Folder...: This button allows you to browse and select any folder path that you wish to specify as your Working Directory path. This is particularly convenient when the path name is long and you are starting a new session (i.e., you cannot browse to an existing input file).

View Edit Comments / View and Edit Comment Lines: This button or menu item is normally used to view or edit comment lines that you wish to insert in your input session file for future reference. The rich text box editor is loaded to allow you to edit these comment lines.

Environment and Input Data Files

The following information is required in order to define l ocation of all input, output and database files that are related to the problem being investigated. The required information is listed below.

ob Identification (JobID): This is a text string identifying uniquely the various output files produced by the AutoGroundDesign engineering program run. This string will be appended to the name of the file just before its extension and will be printed on every plot and report. For example the most relevant input, output and database files are the following:

1. AD_ JobID.F05: This is the AutoGroundDesign command input fil e that is produced by your input session and that will be used to start the automated design process. It is an editable file. However, it is strongly recommended that you avoid editing this file if you are not familiar with the SES Input Command Language (SICL) environment.

2. AD_ JobID_Backup.F05: This is a backup copy of the input data file that was loaded at the beginning of your session. It contains a processed input file that is identical to the contents of your input data fil e. If for any reasons you modify and save the data, you can recover your original data by loading or editing the contents of this fil e.

3. AD_ JobID.F09: This is the AutoGroundDesign Summary results file that is produced by the automated design process. It is an editable fil e. That can be opened by any text browser such as Notepad or WordPad. SES FileView utility can be used by pressing the View Design Summary File button.

4. MT_JobID.F05, MT_JobID.F09, MT_JobID.F21: These are the corresponding MALT input, results and database files that are produced by the automated design process. They are identical to what is produced when you use the CDEGS or AutoGrid Pro software packages and can be accessed using the s ame tools and methods.

5. Other files that are normally produced when the MALT, FCDIST, and RESAP engineering modules are used. Please consult the CDEGS or AutoGrid Pro documentation for further details.

Working Directory Path Specification: The Working Directory is the location where these input, output and database results files are produced. You can define the working directory by entering or choosing through the Open... or Folder... (for a new session) button to select the directory of your choice in the Working Directory field.

System of Units

AutoGroundDesign stores all data internally using the metric (SI) system of units. When data is displayed as text (in a dialog or any other place), the data is first converted to the user-specified system of units. The program saves the data in the user-specified system of units.

Choose between the Metric or Imperial System of units. The radius can be specified in centimeters (cm) when using the metric system. When using the British system of units, the radius can be specified in inches.

Metric: This is the default choice. The data is specified in meters (m) or centimeters (cm), as indicated in the screen.

Imperial: This option selects the Imperial (old British) units. The data is specified in feet or inches, as indicated in the screen.

When the system of units is changed, you can choose whether all appropriate values are converted or not. However, the overhead shield/neutral wire characteristics will be converted in any case.

Reference Database Methodology

The AutoGroundDesign databases are, optionally, the starting point of any automated design and cover most practical and reasonable grids that are often designed in practice. Extensive collections of predefined grids have been analyzed, updated and are enhanced regularly. Among these are the collections shown in the Complete and Basic reference databases. The automated design process can use twoifferent database reference files. This radio button option specifies which grid database is to be used for the automated design. Three options are available:

Complete: This is the default and recommended option. This option uses the extended two-layer soil grid database. More...

Basic: This option selects the uniform and equivalent two-layer soil grid database. This is a simplified database that can be used for comparison purposes or in some rare and unusual cases where the Complete database is unable to find a suitable initial grid. More...

None / Manual (User-Defined Initial Grid): The method provides further flexibility to a user that wishes to define an initial grid design based on the number of conductors along the grid sides, characteristics of the conductors and rods. The program will then refine this grid automatically. This is a very useful feature that allows a user to start where a previous automated design was interrupted orfailed rather than starting all over again . When this option is selected, the following two new input fields become enabled and should be defined in the Initial Number of Conductors along Zone panel:

Length

Width

Remaining Issue List

The following Remaining Issue List window provides more detailed remaining issues found in the input data. It will bring you to the location where the error or warning happens if you double click on an item in the list.

For example, there are five remaining issues in the above list: two input errors and three warnings.

elect the first item in the list and double click on it. This will bring you to the location where the input error happens.

Double click on the right bottom of t he status bar on the AutoGroundDesign main screen to open this window. If n o error or warning is detected, the list is empty.

The user will need to clear all errors/warnings before the design can be started.

This list will be updated automatically whenever there is any change made by the user in the input data.

Grounding System Design Parameters

AutoGroundDesign uses the computation power of the MALT engineering module to analyze power system grounding systems. Typically, MALT is used to analyze and design grounding systems for HVAC and HVDC power stations, substations or transmission towers. MALT can analyze complex ground networks consisting of arbitrary arrangements of bare conductors buried in vertically, horizontally,pherically, or cylindrically layered soils. It can also account for the presence of arbitrarily shaped regions of varying resistivities embedded in uniform and two-layer soils. The grounding grid constitutes an equipotential structure (i.e., all conductors are at the same potential called Ground Potential Rise or GPR). This is because MALT assumes that all metallic conductors are perfect, i.e., lossless.

A complete specification of the data related to a grounding grid requires the following data.

Grounding System

Soil Data. Two options are possible, the Specify Soil Characteristics option where you must specify the structure and characteristics of the soil directly and the Determine Soil Characteristics option where you specify the measured apparent soil resistivity (or resistance) values in order to determine the equivalent soil structure model and characteristics




3/15

Fault Current Contribution Data. Two options are possible, you can specify the magnitude of this current in amperes by selecting the option Specify Grounding System Current, or you can simply select the Determine Grounding System Current option to calculate the appropriate current based on the fault current distribution in the multiple terminal electric network connected to the target groundingsystem.

Computations

Safety Criteria. Two options are possible, the Specify Safe Voltages option where you must specify the Touch Voltage and Step Vol tage and the Determine Safe Voltages option where you specify the network fault, surface layer data, and equivalent human circuit safety thresholds to compute touch and step voltages.

Design and Control Parameters

The type of automated analysis and the type of grounding system design should be determined first before any grounding system specifications.

The types of automated analyses include:

Automated Grounding Design

Ground Resistance Estimator

Electrode Configuration Predictor

Electrode Dimension Predictor

The types of automated grounding designs include:

Horizontal Arbitrary Shape Grid and Vertical Ground Rods

Horizontal Rectangular Grid and Vertical Ground Rods

The types of grounding system used in Estimator and Predictor include:

Horizontal Rectangular Grid and Vertical Ground Rods

Horizontal Arbitrary Shape Grid and Vertical Ground Rods

One Horizontal Wire

Array of Horizontal Parallel Wires (2 or more)

Array of Horizontal Radial Wires (2 or more)

One Ground Rod (vertical wire)

Linear Array of Ground Rods (2 or more)

Rectangular Array of Ground Rods (vertical wires)

Circular Array of Ground Rods

Horizontal Rectangular Plate (approximation)

Vertical Rectangular Plate (approximation)

Horizontal Rectangular Plate

Vertical Rectangular Plate

Circular Ring (approximation)

Circular Plate (approximation)

Hemispherical Electrode (approximation)

Grounding System Specifications

A complete specification of the data related to a grounding grid includes the following data. However, some data may be inapplicable depending on the type of grounding system selected.

Main Grounding System Specification

Ground Rod Specification and Options

Main Grounding System Specification

Earth Current Flowing from Grounding System (A): This field specifies the fault current discharged by the grid. It excludes any current carried away by alternate metallic paths such as overhead ground and shield wires and cable shields and armors. You must specify the magnitude of this current in amperes. This field is unavailable when the Type of Automated Analysis selected is Ground ResistanceEstimator, Electrode Configuration Predictor, or Electrode Dimension Predictor or when the Determine Grounding System Current is selected from the Electric Network tab.

Grounding Zone Specification: The available grounding system area or grounding zone (typically the fenced area and the outer ground loop connected to the fence if any) where it is possible to install the grounding grid that is going to be built must be specified. This area is specified as follows.

If a Horizontal Arbitrary Shape Grid and Vertical Ground Rods is to be analyzed, the grounding zone vertices are specified on the following screen by clicking on the Define ... button.

More information on the above screen is available in the Horizontal Arbitrary Shape Grid Vertex Definitions topic.

For other selections of analysis and grounding grid types, you need to specify the following parameters:

Length: This is the length, in m or feet, of the grounding zone. This dimension is assumed to be along the X axis.

Width: This is the width, in m or feet, of the grounding zone. This dimension is assumed to be along the Y axis.

Depth: This is the depth, in m or feet, of the grounding grid in that zone. This dimension is assumed to be along the Z axis.

Conductor Radius: This is the radius of the grid conductor, in cm or inches.

Initial) Number of Conductors along Zone: This defines the (initial) number of conductors along both zone directions of

Length

Width

Compression Ratio along Zone (p.u.):

Factor multiplying the spacing between conductors (or rods) when moving away from the center of the zone. A compression ratio of 1 gives linearly spaced conductors (or rods). A compression factor smaller than 1 yields a grid with a progressively smaller mesh size or an array of ground rods with progressively smaller spacings as we move towards the edge. They can be defined along both zoneirections:

Length

Width

Add Rods: This option specifies if ground rods are to be used in the design of the final grid and if required, the way they will be distributed over the whole grid area. The available options are:

No Yes

If Yes is selected, then the following Rod Placement and Distribution and Rod Specification and Options are available in the Ground Rod Specification and Options tab.

Ground Rod Specification and Options

Length: This is the length of the ground rod in m or feet. This value must be positive and must be larger than the radius of the ground rod.

Depth: This is the depth of the ground rod in m or feet. This value must be positive.

Radius: This is the radius of the ground rod in cm or inches. This value must be positive and must be smaller than the radius of the ground rod.

Initial) Number of Rods along Zone: This defines the (initial) number of rods along both zone directions of

Length

nmlkj nmlkj




4/15


5/15

Uniform

Two-Layer

Three-Layer

MultiLayer (4 Layers)

MultiLayer (5 Layers)

Automatic

The last option Automatic indicates that the total number of layers in a horizontally layered soil is determined by the program based on the measured apparent resistances/resistivities. For more information on this method, see the topic entitled Automatic Determination of Total Number of Layers in Horizontally Layered Soils .

The initial values of soil characteristics can be specified in the Resistivity and Thickness fields:

Resistivity of Layer n: . The initial resistivity of t he layer n which will be used in the interpretation of measured apparent earth resistivity (or resistance). The program will automatically select the initial resistivity of layer n if a zero value is assigned to this variable.

Thickness of Layer n: . The initial thickness of the l ayer n which will be used in the interpretation of measured apparent earth resistivity (or resistance). The program will automatically select the initial thickness of layer n if a zero value is assigned to this variable.

Soil Characteristics Analysis

n AutoGroundDesign you can specify directly the soil structure model and its characteristics or you can let AutoGroundDesign interpret directly the measured apparent earth resistivity (or resistance) data to determine an equivalent earth structure model which can be used to analyze grounding systems.

From the resistivity measurement data, obtained using arbitrarily spaced 4 electrode configuration methods (including Wenner or Schlumberger methods), the program determines an equivalent horizontal layered earth.

For more information, refer to:

The Concept of Earth Resistivity

Measurement Methods

Types of Earth Structures

Preparing Input Data for the Soil Measurements

Resistivity Measurement Configuration

Use the Resistivity Measurement Configuration options to specify the method that was used to measure the earth's resistivity.

Possible options:

Wenner: . The electrode configuration consists of two outer current injection electrodes and two inner potential probes, all collinear and all equally spaced apart. This is the standard Wenner 4-pin method.

General: Similar to Wenner, only none of the inter-pin spacings need to be equal. For limited-layer soil types, General invokes the multilayer computation algorithm.

chlumberger: Similar to Wenner, except that the distance between the inner potential probes need not be the same as the distance between each potential probe and its adjacent current injection electrode.

Unipolar: The electrode configuration consists of three fixed electrodes (the two outer current injection electrodes, C1 and C2, and one potential probe, P2) and one moving potential probe (P1), all collinear. In a measurement traverse, the moving potential electrode P1 starts near C1 (the starting current electrode) and moves towards P2, normally stopping halfway between C1 and P2.

Dipole-Dipole: In this configuration, the electrodes follow the sequence C2, C1, P1, and P2. The distance between the current electrodes is normally small, and kept fixed. The current electrodes therefore form an electric dipole. Likewise, the distance between the potential electrodes is normally small, and kept fixed. The distance between the current dipole and the potential dipole (the C1 - P1 distance)aries with each measurement.

Specifying Soil Resistivity Measurement Data

The Determine Soil Characteristics screen is used to specify whether the user will enter apparent resistance or apparent resistivity values as the soil resistivity data and to select the module in which the soil resistivity measurement data is specified.

Possible settings for Data Type options:

Resistance: . Indicates that soil data will be specified as apparent resistance measurements.

Resistivity: Indicates that soil data will be specified as apparent resistivity measurements.

Probe Spacing Considerations

elect Determine Soil Characteristics to specify the measured earth resistivity data for electrode spacing. Enter into the Measurements grid, as many rows of data as necessary to specify all earth resistivity data for a full traverse.

When selecting this option, the values to be filled in the Measurements grid are:

Spacing C1-P1: Distance between current electrode C1 and adjacent potential probe P1 (in meters or feet). For Wenner measurements, this is the distance between any pair of adjacent pins.

Spacing P1-P2: Distance between potential electrode P1 and adjacent potential probe P2 (in meters or feet). For Wenner measurements, this is the distance between any pair of adjacent pins.

Spacing P2-C2: Distance between potential electrode P2 and adjacent current probe C2 (in meters or feet). For Wenner measurements, this is the distance between any pair of adjacent pins.

Spacing C1-C2: Distance between the two current electrodes C1 and C2 (in meters or feet). Required only for Unipolar and Dipole-Dipole measurements.

Apparent Resistance or Resistivity: . Measured apparent resistivity (in ohm-m) or apparent resistance (in ohms) depending on the Type option selected.

Convert to General: Click this button to convert measurements specified using the Wenner, Schlumberger or Unipolar method into equivalent measurements specified with the General method. Note that this operation is not reversible, in general.

Notes:

1. Only one traverse or set of measurements should be specified per run, resulting in one apparent resistivity or resistance value per electrode spacing.

2. The apparent resistance is the voltage difference between the potential probes divided by the injection current. The apparent resistivity can be calculated from the following general formula:

R = (K *V / I )/(1/ C 1P

1 + 1/ C

2P

2 - 1/ C

1P

2 - 1/ C

2P

1),

where R is the apparent resistivity (in ohm-m), K is 6.283... (i.e., 2 pi), V / I is the apparent resistance (in ohms), and C iP j is the distance (in meters) between current electrode i and potential probe j.

The Concept of Earth Resistivity

The purpose of the earth resistivity test related to power system design is to assist in the determination of an appropriate soil model which can be used to predict the effect of the underlying soil characteristics on the performance of a grounding system or a power delivery system.

Usually, the electrical characteristics of the earth are sufficiently uniform over horizontal distances to permit the assumption that the soil beneath typical sites is uniform over horizontal dimensions. In such cases, vertical variations in resistivity can often be described by one, two or more frequently, three or more distinct horizontal layers of earth.

ometimes, however, earth resistivity variations over horizontal dimensions are significant and can therefore not be neglected. In such instances, the horizontal variations in resistivity can often be modelled by two or more distinct vertical layers of earth.

Measurement Methods

The measurement configuration most widely used in the electric power industry is a four-electrode (probe) method developed by Wenner.

As shown in the figure below, four electrodes (probes) are used, with the outer pair being used as current input probes and the inner pair as potential references.

Four-Probe Earth Resistivity Measurement Test Set

The Wenner geometry uses equally-spaced probes. In this case the apparent measured resistivity is: Rho=2(pi)aR where

Rho = apparent soil resistivity, in ohm-meters.

pi = 3.1415926...

= electrode spacing, in meters (a = Se1 = Si = Se2 in the figure).

R = ratio of measured voltage to test current, in ohms.

Considering electrode penetration depth to be small compared to electrode spacing, the preceding equation effectively describes the variation in measured resistivity as a function of electrode separation a. Physically, the greater the electrode spacing, the greater the volume of earth encompassed by the test current in its traverse from C1 to C2 and hence, the greater depth of earth involved in themeasurement.

t is important to note that the equation is valid for electrode spacings much larger than electrode length (or burial depth, if spherical sources are used). Wenner has developed an equation that takes into account the depth of a point source, but is not applicable to spikes that are commonly used as electrodes. The program will optionally use an exact equation that takes the lengths of the probes intoonsideration.

An important variation of the equal-spaced four probe method, which is widely used in geophysical prospecting, is the unequal-spaced symmetrical or Schlumberger arrangement. This method circumvents a shortcoming of the Wenner method often encountered at large probe spacings whereby the magnitude of the potential between the potential probes becomes too small to give reliable measurements.By moving the potential wires closer to the outer current electrodes, the potential value is increased and the sensitivity limitations encountered using the Wenner method may be overcome. For large probe spacings, the apparent resistivity according to the Schlumberger method is given by: Rho=(pi)Rc(c+d)/d where

= spacing between adjacent potential and current electrodes (inner and outer electrodes: c = Se1 = Se2 in the figure).

pi = 3.1415926...

d = spacing between potential electrodes (inner electrodes: d = Si in the figure).

R = measured apparent resistance.

The preceding equation is not valid for short electrode spacings that are comparable to electrode lengths. The program will use an exact equation if the lengths of the current and/or potential electrodes are specified. Note that the ratio d/(c+d) is often referred to by geophysicists as the eccentricity of the symmetrical traverse configuration. This eccentricity is usually very small in most geophysicalesistivity soundings carried out based on the Schlumberger configuration.

n order to provide complete flexibility to the user, The program offers a General method which can interpret measurements made with completely arbitrary electrode spacings: i.e., Se1. Si, and Se2 can all be unequal. This can be helpful if difficult field conditions make it impractical to respect the symmetrical electrode positions required by the Wenner and Schlumberger methods.

Types of Earth Structures

The earth structures that can be analyzed include soils with horizontal layers, soils with vertical layers, and soils having resistivities that vary exponentially with depth as illustrated hereafter. Presently, AutoGroundDesign supports only horizontally layered soil models.

n AutoGroundDesign, you may specify not only the type of soil model, but also the number of layers and even the initial resistivities and thicknesses of any layers that the user believes are close to those that should be obtained in the computed soil model. In this way, the user can sometimes guide the program to a more satisfactory solution.

On the other hand, it is rare that you will need to specify any parameters other than the soil type (i.e., Horizontally Layered Soils versus Exponential Soils versus Vertically layered Soils ) and the number of layers desired in the computed soil model.

Uniform Soil Type

This option instructs AutoGroundDesign to best match the measured apparent resistances/resistivities with a uniform soil type. A uniform soil model is shown in the following figure.




6/15

n the process of soil measurement interpretation, AutoGroundDesign will adjust the characteristics (resistivity) of the uniform soil so that the computed apparent resistivity curve matches the measured apparent resistivities.

Resistivity: . This value is an initial estimate of the resistivity of the uniform soil (ohm-meters). If this value is not specified (default setting), the resistivity is best averaged by the program based on the measured apparent resistances / resistivities.

Horizontal Soil Type

This option instructs AutoGroundDesign to best match the measured apparent resistances/resistivities with a horizontal layered soil type. A horizontally layered soil model is shown in the following figure.

n the process of soil measurement interpretation, AutoGroundDesign will adjust the characteristics (resistivities and thicknesses) of horizontally layered soils so that the computed apparent resistivity curve matches the measured apparent resistivities. If the real earth structure can be approximated by a multilayer soil model then the total number of layers of the target multiplayer soil and, optionally, initialuesses of the soil layer characteristics must be specified unless the Automatic layer selection option is selected to determine automatically the most appropriate values. If the automatic determination of the number of soil layers is selected, then the initial guessed values of the layer characteristics are also automatically performed by the program. Otherwise, the user must specify how many layers the targetquivalent (or reconstructed) soil model should have and optionally an initial estimate of the characteristics of each layer. The program will then optimize the soil layer characteristics of the target equivalent soil model.

Possible settings:

Air: . The characteristics of the air above the earth's surface are being specified.

Top: . The characteristics of the surface earth layer are being specified.

Central: The characteristics of one of the central earth layers, if any, are being specified.

Bottom: The characteristics of the bottom layer (which extends to infinite depth) are being specified.

The variables are as follows:

Resistivity: . Initial resistivity of the specified layer (ohm-meters). If this data is omitted or values are set to 0, then the program determines suitable initial values based on the measurement data.

Thickness: . Initial thickness of the layer specified (meters or feet). If this data is omitted or values are set to 0, then the program determines suitable initial values based on the measurement data.

Preparing Input Data for the Soil Measurements

The main input data required by the Determine Soil Characteristics consists of the electrode spacings and apparent earth resistance or resistivity values obtained using the Wenner, Schlumberger, or the general 4-electrode configurations as shown in the following figures.

1. Wenner Method

2. Schlumberger Method

3. General Method

4. Unipolar Method

5. Dipole-Dipole Method

Optionally, the lengths of the current and potential electrodes may also be specified. The main block of data consists of a sequence of n data lines (j=1,n) containing the following values:

S, R, Do, Di (Wenner Method)

Se, R, Do, Di, Si (Schlumberger Method)

Se1, R, Do, Di, Si, Se2 (General Method)

Se1, R, Do, Di, C1-C2, Se2 (Unipolar Method)

Se1, R, Do, Di, Si, Se2 (Di pole-Dipole Method)

where

, Se, Se1: Spacing between a current (outer) electrode C1 and its adjacent potential (inner) electrode P1 (see above figure). For the Dipole-Dipole method, this is the spacing between the current electrodes (C1 and C2).

R: Apparent soil resistance in ohms (i.e., R = V / I) or apparent soil resistivity in ohm-m corresponding to the electrode spacings (i.e., (2piV/I) (1/C1P

1+1/C

2P

2-1/C

1P

2-1/C

2P

1) with all spacings specified in meters). When the RESISTIVITY setting is specified in the Measurements module, the measured data should be entered as apparent resistivities. When the RESISTANCE setting or no setting is

pecified, the apparent resistance values should be entered.

Do: Is the average length of current electrodes (meters or feet). If this field is blank or zero, then the program assumes a negligible length and will use appropriate formulas (depending on the length of the potential electrodes) to compute the apparent resistivity. Otherwise, it will compute the apparent resistivity based on a more accurate formulation for handling cylindrical spikes.

Di: Is the average length. of potential electrodes (meters or feet). If this field is blank or zero, then the program assumes a negligible length of the current electrodes) to compute the apparent resistivity. Otherwise, it will compute the apparent resistivity based on a more accurate formulation for handling cylindrical spikes.

i: Spacing between two potential (inner) electrodes (see figure above). For the Dipole-Dipole method, this is the spacing between the inner current and potential electrodes (C1 and P1).

e2: Spacing between the remaining (outer) electrode C2 and its adjacent potential (inner) electrode (see figure). For the Dipole-Dipole method, this is the spacing between the two potential electrodes (P1 and P2).

C1-C2: Spacing between the two current electrodes C1 and C2.

Automatic Determination of Total Number of Layers in Horizontally Layered Soils

A user usually does not know the total number of layers before the computed apparent resistivity curve versus the measured resistivity data is obtained by the program. To make the analysis of the soil resistivity data more efficient, an improvement was introduced to determine the total number of layers in horizontally layered soils if a user does not explicitly specify a horizontally layered soil model.

Properties of the apparent resistivity curve and correspondence between the total number of layers and measured resistivity data

Based on the study of theoretical resistivity curves (also known as sounding curves) of Wenner or Schlumberger configuration, the following properties are generally observed [1,2], regardless of the number of layers or resistivity distribution with depth. For the convenience of the reader, the properties in [1] are included as follows:

1. Computed apparent resistivity are always positive;

2. The form of a sounding (measured) resistivity curve follows the form of the true resisti vity-depth curve. As the true resistivity increase (or decrease) with greater depth, the apparent resistivity increase (or decrease) with greater electrode spacing. This is particularly evident for layers whose thickness increases logarithmically with depth;

3. The maximum change in apparent resistivity always occurs at an electrode spacing that is larger than t he depth at which the corresponding change in true resistivity occurs. This is a sounding curve is "out of p hase" with the resistivity-depth curve and is always shifted to the right of the resistivity-depth curve (delayed effects).

4. The amplitude of a sounding curve is always less th an or equal to the amplitude of the true resistivity-depth curve. The apparent resistivity asymptotically approaches the true resistivity at electrode spacings that are very small with respect to the thickness of the first l ayer or very large with respect to the depth to an infinitely thick last layer.

5. In a multilayer model, if the true resistivity of a thick layer is changed, the apparent resistivity along a corresponding segment of the soundi ng curve also changes accordingly. Furthermore, the maximum change in apparent resistivity is approximately equal to the net change in true resist ivity.

6. A severe K-type (three-layer horizontal soil) sounding curves ( ) is a curve that rises steeply at an angle of nearly 45 degrees, continues to rise for nearly two logarithmic cycles, forms a somewhat sharp maximum, then falls to low resistivity values.

7. An incomplete sounding curve is one in which th e left and (or) right asymptotes to the resistivity of the top and (or) bottom layer, respectively, are not measured. That is, there exists great uncertainties on the so unding curve for the top and bottom layer resistivity.

Figure 1, Figure 4 and Printouts 1 to 4 better illustrate these properties (in particular No. (4)). Figure 1 to Figure 4 show four typical measured resistivity points (dots) superposed with the resulting computed apparent resistivity curve (line) which corresponds to horizontal two-layer, three-layer, four-layer and five-layer soil models, respectively. Printouts 1 to 4 provide their corresponding soil models that

re obtained by the program.

For example, Figure 1 shows a two-layer soil model in which the soil resistivities vary from high to low. The asymptotical value at smaller spacings corresponds truly to the top layer resistivity, while the asymptotical value at larger spacings corresponds truly to the bottom layer resistivity. Such one-to-one correspondence does not apply to any other layers between the top and bottom layers. The reason isery simple: the measured apparent resistivities corresponding to any layers between the top and bottom layers are influenced by the top and bottom layer resistivities during the measurement (assuming a 4-pin electrode method). For a three-layer soil shown in Figure 2, the middle layer soil resistivity 792 W-m (see Printout 2) is slightly higher than the highest measured apparent resistivity data 780 W-m,imply because the top and bottom layer resistivities are lower. For the same reason, the true middle layer resistivity is always lower than the measured apparent resistivity data if a High-Low-High three-layer soil type is encountered. Such effects are even more pronounced in Figure 3 and Figure 4 and their corresponding soil models in Printouts 3 and 4. Furthermore, a minimum total number of layers

an be obtained by counting the number of extreme (maximum or minimum) on the measured apparent resistivity curve. Figure 3 is a four-layer soil since the data exhibit one peak and one valley, while Figure 4 exhibits one peak and two valleys which leads to a five-layer soil model, and so on.

t is obvious that for a given set of soil resistivity measurement, there are a number of electrically equivalent soil models. For instance, a five-layer or six-layer soil models would also fit the measured data in Figure 3. However, a smaller number of layers will consume usually less computation time in MALT or MALZ. Besides, since they are all electrically equivalent, the computation results from MALTr MALZ should be very similar. Therefore, for a given set of soil measurements, we should look for the electrically equivalent soil model with the least number of layers possible.

Figure 1: Computed Versus Measured Resistivities for a Limited-Layer (Two-Layer) Soil Model




7/15

Figure 2: Computed Versus Measured Resistivities for a Three-Layer Soil Model

Figure 3: Computed Versus Measured Resistivities for a Four-Layer Soil Mode

Figure 4: Computed Versus Measured Resistivities for a Five-Layer Soil Model

Basis of Method

The task of the automatic determination of the total number of layers is to determine the total number of layers based on measured resistivity data. As demonstrated in the preceding section, the basis of the method lies in a fact that the total number of layers is best reflected by the number of extreme or bends on measured apparent resistivity curves. The key point of the method is thus to determine theumber of extreme or bents on the measured resistivity curve which always contains noise on the data points. To overcome this difficulty, a data-smoothing step is carried out first using a Windows Median technique [3]. Using the smoothed data, the number of extremes is then obtained using the modified Brent's method [3]. The properties of sounding curves listed in the preceding section have also beenncorporated as guidelines in both the data smoothing and the second step that determines the number of extremes. Note that the smoothed data is only used to determine the total number of layers. The original measurement data is still used to obtain the final soil model.

References

1. Zohdy, A. A. R., "A new method for the automatic interpretation of Schlumberger and Wenner sounding curves", Geophysics, Vol. 54, No. 2, pp.245-253, 1989

2. Esparza, F. J., "1-D inversion of resistivity and induced polarization data for the least number of layers", Geophysics, Vol. 62, No. 6, pp.1724-1729, 1997

3. Press, W. H., Flannery, B. P., Teukolsky, S. A., and Vetterling, W. T., Numerical Recipes, Cambridge University Press, Cambridge, 1989

Electric Network Analysis

AutoGroundDesign calculates fault current distribution in multiple terminal transmission and distribution electric line networks. AutoGroundDesign uses a simple set of data concerning the network and has been implemented with ease of use as an important design criterion.

AutoGroundDesign makes a computer model of the electric line network incorporating a single faulted phase (which can be used to represent one or any number of phases), and a single ground wire group (which can represent one or more ground or neutral or static wires). The transmission line parameters are assumed to be the same in all the spans in any given arm of the transmission line network. This

model accounts for coupling between the phase wire and the ground (or neutral or static) wire. Structure ground impedances are assumed to be equal for all structures in any arm of the network.

Fault current distribution is determined by the grounding of the various sections of the electric line and associated substations, and also the inductive coupling between the phase wire and the ground (or neutral or static) wires groups. The model created by the program, which accounts for inductive coupling, and assigns unique ground impedances to the central site and terminal stations along theransmission line, and accounts for the grounding of all structures along the transmission line, takes into consideration all the key elements necessary to calculate fault current distribution accurately. By using a single phase and a single ground (or neutral or static) wire group, and constant parameters for all sections, the amount of necessary input data is maintained at a relatively low and easily manageableevel. Although this approach is not as flexible or as powerful as the complete circuit model approach taken by SPLITS , it is much easier to gather and prepare the input data.

For more information, refer to:

Representation of an Electric Network

Conductor Data Required by Electric Network

Currents Sources Specification

Define Electric Network

Define Electric Network

Use the Electric Network section to specify the fault current discharged by the grounding system. It excludes any current carried away by alternate metallic paths such as overhead ground, shield wires, and cable shields and armors. You must specify the magnitude of this current in amperes by selecting the option Specify Grounding System Current, or you simply select the Determine Grounding SystemCurrent option to calculate the appropriate current based on the fault current distribution in the multiple terminal electric network connected to the target grounding system.

Printout 1: Two-Layer Soil Model

Layer Number Resistivity (ohm-m) Thickness (Meters)

1 364.5703 0.6672840

2 63.76699 Infinite

- - -

- - -

Printout 2: Three-Layer Soil Model

Layer Number Resistivity (ohm-m) Thickness (Feet)

1 273.8638 2.317178

2 792.3849 114.4890

3 269.4607 Infinite

- - -

Printout 3: Four-Layer Soil Model

LayerNumber

Resistivity(ohm-m)

Thickness(Feet)

1 206.2624 2.806843

2 3239.253 2.179909

3 67.99847 7.562907

4 582.0868 Infinite

Printout 4: Five-Layer Soil Model

LayerNumber

Resistivity (ohm-m)

Thickness(Meters)

1 612.9487 1.013675

2 212.9404 5.708103

3 429.4193 19.789554 119.9847 36.22168

5 1555.584 Infinite




8/15

Specify Grounding System Current

Determine Grounding System Current

For each new terminal, you may type new terminal name in the New Terminal Name and choose the type for this new terminal from the following terminals:

Distribution Line (DL) with 1 Neutral

Overhead Transmission Line (or DL) with 1 Shield Wire

Overhead Transmission Line (or DL) with 2 Shield Wires

Overhead Transmission Line (or DL) with a Bundle of Shield Wires

Above Ground or Buried Power Cable

The Add Terminal button is used to create the new terminal. The Edit Terminal button is used to input or edit the characteristics of the terminal.

The Copy Terminal button will copy the chosen one of the existing terminals to the new terminal. This new terminal will automatically inherit the characteristics of the former. In order to erase all data you have entered for the terminal currently being displayed and start over, click the Delete Terminal button.

The Editing Terminal screen is used to introduce a terminal whose data is to be specified.

Representation of an Electric Network

AutoGroundDesign can create circuit models to represent a transmission line network with an arbitrary number of terminal substations (arms) connected to a transmission line emanating from a Central Substation (Site) where the phase-to ground fault (involving one or more phases) occurs as illustrated in the following figure.

The circuit model of a transmission line network consists of three basic elements: i) a central site, ii) terminal stations, and iii) transmission lines connecting the central site to the terminal stations. These elements, and the data required by the program to specify them, are explained in more detail below:

Central Site. This is a node that connects to all arms of the network. The user must specify the central site ground impedance (i.e., shunt impedance). Typically this site represents a faulted substation or transmission line structure and the shunt impedance is set equal to the ground impedance (resistance) of the faulted site. Only one central site can exist in a given circuit run.

Terminal Stations. At least one, and possibly several, terminal stations may exist in each circuit model. The terminal station consists of a ground impedance representing the impedance of the terminating station ground network and a current source that energizes the transmission line. The data specified by the user are as follows:

Terminals: each arm of the transmission line ends in a terminal section. You must create the terminal by specifying the terminal name. In this version, you can not select an arbitrary ground impedance for the terminal. Only a small or very large value can be selected by opening or closing a switch.

Sources: each arm of the network is energized by a current source whose current can be specified with a user-defined magnitude and phase angle (or as a complex number) relative to the current sources of the other terminals. Each source is connected to the transmission line ground (or neutral or static) wire group through a switch that can be opened or closed. In this version this connection can nothave a user-defined mutual impedance with respect to the phase wire (the default value of the mutual is set to 0 ohm).

Transmission Lines or Power Cables. The transmission line is modeled as a single faulted phase and a single ground (or neutral or static) wire group. Each transmission line or cable is made up of sections (i.e., spans) that are all of the same length. Each span is terminated by a ground shunt connected to the ground wire group. This represents the ground impedance of the transmission line towers (or cableman holes) connected to the ground wire group (or sheath and armour group). Each section of the ground wire group has a series impedance and mutual impedance with respect to the faulted phase. Data entered by the user is given below:

Faulted Phase: the location of the faulted phase is specified in terms of its cross-sectional location in the right-of-way.

Ground (or Neutral or Static) Wire Group: the circuit model also includes a single ground (or neutral or static) wire group. It can be composed of several conductors regularly arranged on the perimeter of a circle. Its location is specified in terms of the position of the group's center and the number of conductors, and the position of the first conductor. The ground wire group has a series impedanceand a mutual impedance with respect to the faulted phase that can be calculated by the program or specified by the user. More details are provided in the conductor-data help topic.

Section (Span) Shunt: each section (or span) of the transmission line has a ground shunt impedance located at its extremity which is furthest away from the central site. The section shunt represents the ground impedance of the transmission line structures. This ground shunt is a complex-valued quantity which is the same for all structures in a given arm of the transmission line network.

The user must also specify the span length and the total length of all spans in each transmission line arm.

Note that terminal, source, ground wire group, section and phase parameters must be specified separately for each transmission line arm, i.e. for each specified terminal.

To fully describe the transmission line network, it is also necessary to specify the electrical characteristics (resistivity and permeability) of the soil in which the network is located. A uniform soil model is used. The user must also specify the power system frequency. This value is specified in the Safety specification tab.

Conductor Data Required by Electric Network

Required Network Data:

The span (section) length is the same for all sections in a given arm of the transmission line (or power cable) network. When the impedances are specified directly, the span length is not required.

Required data for the ground wire group (or bundle):

X-coordinate of the center of the ground wire group.

Y-coordinate of the center of the ground wire group.

Distance between the ground wire group center to the center of each ground wire.

Starting angle of the first conductor in the ground wire group.

Number of conductors in the ground wire group (see Note 1).

Relative permeability of members of the ground wire group. Alternatively, this value can also be specified as the geometric mean radius or the 60 hertz reactance at 1 foot spacing of the conductor (the option selected depends on the setting of a flag specified by the user). This value can be retrieved automatically from a conductor database (see Note 2).

Relative resistivity of members of the ground wire group. Alternatively, the user can specify this parameter in terms of the conductor dc resistance or the conductor ac resistance (the option selected depends on the setting of a flag specified by the user). This value can be retrieved automatically from a conductor database (see Note 2).

External radius of members of the ground wire group. This value can be retrieved automatically from a conductor database (see Note 2).

Internal radius of the members of the ground wire group, in the case of hollow conductors (this is 0 for solid conductors). This value can be retrieved automatically from a conductor database (see Note 2).

Notes:

1. All members of a ground wire group must be identical and arranged along the perimeter of a circle at regular intervals (i.e., at the vertices of a regular polygon).

2. If the conductor does not exist in the supplied default databases, a new database can be used to add the conductor data for subsequent use.

Required data for the faulted phase:

For the faulted phase, the coordinates must be specified in order to calculate the mutual impedance between the faulted phase and the ground wire group.

Current Sources Specification

The Fault Current field is used to specify the current flowing in the equivalent faulted phase wire.

Fault Current specification:

Active or Magnitude : Real part or magnitude (depending on the radio button selection) of the current (in amperes) in the phase wire.

Reactive or Angle : Imaginary part or angle (depending on the radio button selection) of the current (in amperes or degrees) in the phase wire.

Possible Selections:

Cartesian : Indicates that the current specified is in Cartesian form.

Polar: Indicates that the current specified is in polar form.

Further Details

The user must specify the magnitude and phase angle of the power source current energizing each terminal (arm) of the transmission line network. Typically, this value represents the current in the faulted phase. If the currents in the non-faulted phases are not negligible, the current flowing in the phase conductor is specified as the total vector sum of the three phase currents. This improves the accuracy ofhe calculation of the currents in the central site and in each tower.

The program accepts source currents in Cartesian or Polar notation. By default, the program uses the Cartesian notation.

Network Configuration

The Shield Wire Coordinates and Phase Wire Coordinates are used to specify the characteristics of neutral (or ground or shield) and phase conduct ors belonging to a given terminal.

Phase Wire Coordinates:

Yp : Y-coordinate of the center of the phase conductor bundle, in meters or feet.

Zp : Z-coordinate of the center of the phase conductor bundle, in meters or feet.

hield Wire Coordinates:

There are four ways of defining the Metallic path conductors that carry back the fault current to the source.

1. Distribution Line (DL) with 1 Neutral or Overhead Transmission Line (or DL) with 1 Shield Wire

Ys : Y-coordinate of the neutral/ground/shield wire, in meters or feet.

Zs : Z-coordinate of the neutral/ground/shield wire, in meters or feet.

2. Distribution Line (DL) with 2 Neutral Wires

Y1s : Y-coordinate of the first neutral/ground/shield wire, in meters or feet.

Z1s : Z-coordinate of the first neutral/ground/shield wire, in meters or feet.

Y2s : Y-coordinate of the second neutral/ground/shield wire, in meters or feet.

Z2s : Z-coordinate of the second neutral/ground/shield wire, in meters or feet.

3. Overhead Transmission Line (or DL) with a Bundle of Shield Wires

Yc : Y-coordinate of the geometric center of the neutral/ground/shield wire(s), i.e., conductor center (if only one) or midpoint between conductors (if two) or bundle center (if one bundle), in meters or feet.

Zc : Z-coordinate of the geometric center of the neutral/ground/shield wire(s), in meters or feet.

Number of Wires (N) : Number of neutral or shield or overhead ground conductors.

tarting Angle S (Degrees) : Angle (in degrees) between the positive y-axis and a ray traced from the geometric center of the neutral/shield overhead ground conductors to one of these conductors. Angle is always positive and is measured counterclockwise from the positive y-axis.

Bundle Radius : Distance of neutral or shield or overhead ground conductors from their geometric center, in meters or feet.

Note:

pecify the external (re) and internal (ri) radii of each individual neutral or shield or overhead ground wire conductor. For a solid conductor, specify 0. These values are automatically retrieved when the wire is selected from the conductor database.

4. Above Ground or Buried Power Cable




9/15

Yc : Y-coordinate of the cable shield center, i.e., conductor center (if only one) or midpoint between conductors (if two) or bundle center (if one bundle), in meters or feet.

Zc : Z-coordinate of cable shield center, in meters or feet.

Further Details

The coordinate system used assumes the z-axis to be vertical, the y-axis to be at ground level, and the y-z plane to be such that the transmission line conductors pierce it at right angles.

Sections or Spans Specification

The section portion of the System screen is used to specify the number of spans or sections in the radial power line associated with the terminal (arm), along with the length of these spans and the tower or pole ground impedance in each line. The required data are shown as follows.

Sections

Total Line Length (Dt) : Total line length of the power line associated with the terminal. The units are in meters or feet depending on the Units selected in the Main screen.

ection Length (Ds) : Section or span length. This is a constant (average) value for all sections. The units are in meters or feet depending on the Units selected in the Main screen.

Tower Ground Impedance or Power Cable Man Hole Ground Impedance (Ohms):

Rt : Resistive component of the (average) shunt impedance to earth of the neutral (or shield or overhead ground) wires per section (in ohms). Typically, this value represents the structure ground resistance.

Further Details

The shunt impedance is modeled as a lumped impedance at one end of each section (at the end furthest away from the central station).

Line Cross Section

Line Cross Section

The following parameters for the characteristics of the neutral conductors must also be defined or imported automatically from the conductor database.

nternal Reactance (select and enter one of the three following values):

1. Relative permeability (relative to 1.2566 E-06 henries/meter - free space)

2. Geometric mean radius (in feet or meters)

3. 60 hertz reactance at 1 foot spacing (in ohms/mile or ohms/km)

nternal Resistance (select and enter one of the three following values):

1. Relative resistivity (relative to 1.7241 E-08 ohm-meters - annealed copper)

2. DC resistance (in ohms/mile or ohms/km)

3. AC resistance (in ohms/mile or ohms/km). This resistance value should correspond to the specified power s ystem frequency. The AC resistance should only be used for frequencies below 2 kHz.

Note that the above data can be automatically imported from the built-in database, rather than being typed in. Click the Define... button to do so.

Computations

The Computations tabbed region specifies the frequency and settings for a group of equally spaced observation points where earth potentials are to be computed.

Frequency: The system frequency in Hertz is used for calculating the electric line parameters. It is also used to calculate the Decrement Factor at the corresponding fault duration, which accounts for the asymmetrical current component. The default value is 60 Hertz.

Observation Points: Specific soil locations where the program carries out earth potential calculations. The following options are used to select the observation point settings:

Automatic: Instructs the program to select the most appropriate observation point settings.

User-Defined: The observation point preferred settings are defined by users. They must be equally spaced and lying on the same line profile. This option is available only for horizontal arbitrary shape Grid.

Preferred Point Spacing during Iteration: Defines the preferred spacing between the points in the potential profiles during iterations (meters or feet).

Preferred Point Spacing for Final Results: Defines the preferred spacing between the points in the potential profiles for the final computation (meters or feet).

Depth of Profile: Defines the Z-coordinate of the point (meters or feet).

Notes:

Efficient and intelligent techniques have been developed to generate different types of observation points in order to accurately carry out computations and at the same time minimize the computation time during the iterative design steps and the final results computation steps. There are two ways to create observation points.

. Design Horizontal Arbitrary Shape Grid

During the iterations, three different groups of observation points have been created. The first group of observation points is within the horizontal arbitrary grounding zone and is placed along one horizontal direction. The second group of observation points is along the edge of the grid and inside the zone. The last group of observation points is outside the zone occupied by the grounding system and islaced along its edge. In the final results, the observation points occupy a rectangular area covering the bounding box of the horizontal arbitrary grounding system (i.e., the smallest rectangle that completely encloses the horizontal arbitrary grounding system zone). This provides a complete set of observation points that produces conventional rectangular 3D and spot 2D plots that are useful to explore theomputation results.

2. Design Horizontal Rectangular Grid

The observation points are determined automatically. No data are required to specify these points. During the iterations, the automatically generated surface of observation points is sufficiently large to cover a quarter of the entire symmetric grounding grid or simply a profile located from the grid center to the outside of the grid through the grid corner, accounting for the grid border offsets for touch andtep voltages. In the final results, the observation points occupy a rectangular area covering the entire grounding system. This provides a complete set of observation points that produces conventional rectangular 3D and spot 2D plots that are useful to explore the computation results.

Safety Criteria

Use the Safety Criteria Specification tabbed section to define the data that control the safety analysis carried out by the program. It allows you to select which quantities should be analyzed, the region where they should be analyzed and the values that are considered safe for these quantities.

The automated design process requires the following three values in order to decide if a specific grid design meets the target safety criteria.

1. Maximum acceptable GPR: This is the maximum acceptable ground potential rise of the grid in volts. The default value is 5000 V. This value must be positive.

2. Touch Voltage: This is the maximum allowable safe touch voltage within the grid zone in volts. This value must be positive.

3. Step Voltages within and outside Grid: These are the maximum allowable safe step voltages within the substation area and outside where the surface insulating material (crushed rock or asphalt). These values must be positive. More ...

The user should specify a very large value for any of these parameters if they are not to be used during the automated design process.

The Maximum acceptable GPR must be specified directly by typing the applicable value if it is relevant in the context of the automated design session. The last two values, namely the allowable touch and step voltages can be specified directly as well or can be determined based on the IEEE or IEC standards. The two options are:

Determine Safe Voltages

Specify Safe Voltages

Network Fault and Surface Layer Data

The following voltage safety criteria are available when the Determine Safe Voltages radio option is selected. Depending on the options that are specified, some fields or options may not be accessible for editing.

urface Layer Data: The resistivity and thickness (in cm or inches) of the layer covering the s oil surface above the grid is specified in this panel. Thi s high resistivity material is used to provide a semi-insulating layer (i.e., earth surface covering layer) to protect the assumed bare-footed person. The applicable data fields are:

Resistivity: This is the surface layer resistivity (ohm-m) that will be used to determine the safe voltage values. This value must be positive.

Thickness: This is the surface layer thickness (cm or inches) that will be used to determine the safe voltage values. This value must be positive.

Within Grid: This indicates the resistivity and thickness (in cm or inches) of the surface layer within the substation area.

Outside Grid: This indicates the resistivity and thickness (in cm or inches) of the surface layer outside the substation area (where it may be an insulating material like crushed rock or asphalt).

Network Fault Data: The parameters in this panel represents data used to calculate the asymmetrical magnitude of the fault current. The available fields are described hereafter.

Default Value (based on X/R=20): The decrement factor is automatically calculated on the assumption that the system X/R ratio equals 20.0.

Computed from X/R Ratio: The decrement factor is automatically calculated based on the specified system X/R ratio.

User Defined: The decrement factor specified by the user.

nmlkji

nmlkj




10/15

X/R Ratio: The computed decrement factor that derates the fibrillation current threshold to account for the asy mmetrical current component associated with short duration faults is based on this ratio. The decrement factors are automatically calculated on the basis of a user-specified system X/R Ratio (the default value is 20.0). This value must b e positive. It is accessible only if Computed from X/R Ratios selected.

Decrement Factor: This value represents the decrement factor used to derate the fibrillation current threshold to account for the asymmetrical current component associated with short duration faults is a computed value based on a user-specified X/R ratio. This value must be greater than 1. It varies normally between 1 and 2. It is accessible only if User Defined is selected.

Note that the system frequency in Hertz is used for calculating the Decrement Factor at the corresponding fault duration, which account for the asymmetrical current component. The default value is 60 Hertz.

Please refer to the topic entitled Equivalent Human Circuit Safety Thresholds for more options.

Equivalent Human Circuit Safety Thresholds

The following voltage safety criteria are available when the Determine Safe Voltages radio option is selected. Depending on the options that are specified, some fields or options may not be accessible for editing.

Fibrillation Current Calculation Method

Body Resistance Data

IEC Body Path Percentage Reduction

Foot Resistance Calculation Method

Fibrillation Current Calculation Method

The fibrillation current threshold can be automatically determined by the program or you can specify its value directly. The available options can be grouped in the following four categories:

The ANSI/IEEE Standard 80 formula suggested for 50 kg body weight, based on Dalziel's studies, is used to calculate the fibrillation current threshold

The ANSI/IEEE Standard 80 alternative formula for 70 kg body weight, based on Dalziel's studies, is used to calculate the fibrillation current threshold.

The fibrillation current threshold is calculated according to fibrillation current versus current flow duration curves published in IEC Report 479-1.

The User-Defined fibrillation current threshold.

Use the Fibrillation Current Calculation Method dropdown box to select the calculation method for the maximum acceptable current (typically the fibrillation current). Six methods are available as well as a manual definition method as listed hereafter.

50KG-IEEE: This is the default value. It selects the IEEE Standard 80 method for a 50 kg human to calculate the fibrillation current based on Dalziel’s formula.

70KG-IEEE: This option selects the IEEE Standard 80 method for a 70 kg human to calculate the fibrillation current based on Dalziel’s formula.

C1-IEC: This option selects the IEC (curve C1) method to calculate the fibrillation current. The C1 curve suggests a negligible probability of ventricular fibrillation to occur. The more rigorous definition is that the probability of fibrillation between the C1 and C2 curves increases progressively from a very low value (about 0.5% or less) to 5%, once C2 is reached.

C2-IEC: This option selects the IEC (curve C2) method to calculate the fibrillation current. The C2 curve represents the limit at which the probability of ventricular fibrillation reaches 5%.

C3-IEC: This option selects the IEC (curve C3) method to calculate the fibrillation current. The C3 curve represents a 50 % probability of ventricular fibrillation to occur.

B-IEC: This option selects the IEC (curve b) method to calculate the maximum body current for long fault exposures. The "b" curve suggests a likelihood for muscular contractions, difficulty in breathing and transient cardiac arrest without ventricular fibrillation.

User-Defined: This option uses the user-defined fibrillation current (in A). When this selection is picked, a text box (with the Current label) appears to let the user enter the desired value in amps. This value must be positive.

More information on the safety parameters related to the equivalent electrical model of a person and methods to determine this equivalent model is given hereafter.

Determine Body Resistance Based on

The parameters in this panel represents data used to estimate the resistance of the body path subjected to the stress voltages. The available fields are described hereafter.

IEEE Std. 80 Method: This default option selects the IEEE 80 Standard as the reference for calculating the body resistance.

IEC 479 Method: This option selects the IEC 479 standard to select the suggested body resistance value of 1000 ohms. The body resistance is calculated according to stress voltage versus body resistance curves published in IEC Report 479-1. The applicable body resistance as a function of the computed or estimated applied voltage is determined. The user selects one of the three following curves

that encompass different proportions of the population.

1. Body Resistance of 95% of Population Exceeds Curve. This is the default value that encompasses the majority of the population.

2. Body Resistance of 50% of Population Exceeds Curve. This value encompasses half of the population.

3. Body Resistance of 5% of Population Exceeds Curve. This value encompasses a minority of the population.

User-Defined: This option allows you to specify your own value of body resistance of the prospective victim. This option is useful to study problems involving a typical scenarios and environment conditions. When this option is selected, a textbox field is enabled to allow you to enter the applicable value in ohms.

IEC Body Path Percentage Reduction

When the body resistance calculation is selected based on IEC method, the IEC Body Percentage Reduction is enabled that allows you to select among a number of additional body path resistance reduction options that can affect the calculated safe voltage values. In particular, you will be able to specify the percentage of the hand-to-hand body resistance that is to be used in the calculation of the safeouch and step voltages. If none is entered, then a value of 75% is assumed, which corresponds to a single-hand-to-two-foot contact. For a two-hand-to-two-foot contact, a value of 50% should be specified. A value of 100% corresponds to a hand-to-hand contact or a single-hand-to-single foot contact. Note that computed step voltage limits are based on this same body resistance percentage. The availableercentage options are:

100% (hand-to-hand): This, typically corresponds to a hand-to-hand contact or a single-hand-to-single foot contact

75% (hand-to-2 feet): This typically corresponds to a single-hand-to-two-foot contact

50% (2 hands-to-2 feet): This, typically corresponds to a two-hand-to-two-foot contact

User Defined: This option allows you to specify any percentage that corresponds to your scenario. In this case, a text box field, Percentage, is enabled that allows you to enter the appropriate percentage value (in %)

Foot Resistance Calculation Method

Use the Foot Resistance Calculation Method dropdown box to select the foot resistance calculation method. Three methods are available, including a manual definition method (user-defined option).

IEEE Std.80-2000: This is the default method. It selects the IEEE Standard 80 (2000 edition) method which uses an exact series expansion that calculates the foot resistance (and the Cs correction factor, see the following for more information) based on a two-layer soil model consisting of the surface covering layer and the native top soil layer (assumed to be of infinite thickness).

IEEE Std.80-

autogrounddesign

Documents