ETAP Ground Grid Systems

Chapter 28 Ground Grid Systems Since the early days of the electric power industry, the safety of personnel in and around electric power installations has been a primary concern. With ever increasing fault current levels in today’s interconnected power systems, there is renewed emphasis on safety. The safety of personnel is compromised by the rise in the ground potential of grounded structures during unbalanced electric power faults. At such times, humans touching grounded structures can be subjected to voltages. However, the magnitude and duration of the electric current conducted through the human body should not be sufficient to cause ventricular fibrillation Years of research on the effects of electric current on the human body have lead to the development of standards of permissible values to avoid electrocution. The Ground Grid Systems program utilizes the following four methods of computation: • • • •

FEM - Finite Element Method IEEE 80-1986 IEEE 80-2000 IEEE 665-1995

Operation Technology, Inc.

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ETAP PowerStation 4.0

Ground Grid Systems

Introduction

The Ground Grid Systems program calculates the following: • • • • •

•

The Maximum Allowable Current for specified conductors. Warnings are issued if the specified conductor is rated lower than the fault current level. The Step and Touch potentials for any rectangular/triangular/L-shaped/T-shaped configuration of a ground grid, with or without ground rods (IEEE Std 80 and IEEE Std 665). The tolerable Step and Mesh potentials and compares them with actual, calculated Step and Mesh potentials (IEEE Std 80 and IEEE Std 665). Graphic profiles for the absolute Step and Touch voltages, as well as the tables of the voltages at various locations (Finite Element Method). The optimum number of parallel ground conductors and rods for a rectangular/triangular/L-shaped/Tshaped ground grid. The cost of conductors/rods and the safety of personnel in the vicinity of the substation/generating station during a ground fault, are both considered. Design optimizations are performed using a relative cost effectiveness method (based on the IEEE Std 80 and IEEE Std 665). The Ground Resistance and Ground Potential rise (GPR).

Some of the main features of the Ground Grid Systems Analysis Study are summarized below: • • • • • • • • • • • • • • • • •

Calculate the tolerable Step and Touch potentials Compare potentials against the actual, calculated Step and Touch potentials Optimize number of conductors with fixed rods based on cost and safety Optimize number of conductors & rods based on cost and safety Calculate the maximum allowable current for specified conductors Compare allowable currents against fault currents Calculate Ground System Resistance Calculate Ground Potential Rise User-expandable conductor library Allow a two-layer soil configuration in addition to the surface material Ground grid configurations showing conductor & rod plots Display 3-D/contour Touch Voltage plots Display 3-D/contour Step Voltage plots Display 3-D/contour Absolute Voltage plots Calculate Absolute, Step & Touch potentials at any point in the configuration Conductor/Rod can be oriented in any possible 3-Dimensional direction Handle irregular configurations of any shape


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Ground Grid Systems

Ground Grid Systems Presentation

28.1 Ground Grid Systems Presentation The GGS presentation is composed of the Top View, Soil View, and 3D View. The Top View is used to edit the ground conductors/rods of a ground grid. The Soil View is used to edit the soil properties of the surface, top, and lower layers of soil. The 3D View is used for the three-dimensional display of the ground grid. The 3D View also allows the display of the ground grid to rotate, offering views from various angles. The GGS presentation allows for graphical arrangement of the conductors and rods that represent the ground grid, and to provide a physical environment to conduct ground grid design studies. Each GGS presentation is a different and independent ground grid system. This concept is different from the multi-presentation approach of the One-Line Diagram, where all presentations have the same elements. There is no limit to the number of GGS presentations that can be created.

28.1.1 Create a New Ground Grid Presentation To create a GGS presentation, a ground grid must first be added to the One-Line Diagram. Click on the Ground Grid component located on the AC toolbar, and drop the GGS symbol anywhere on the OneLine Diagram. Right-click on any location inside the ground grid box, and select Properties to bring up the Grid Editor. The Grid Editor dialog box is used to specify grid information, grid styles, equipment information, and to view calculation results. Click on the Grid Presentation button to bring up a GGS presentation.

Double-clicking on the ground grid box located on the One-Line Diagram will bring up the Ground-Grid Project Information dialog box, used to select an IEEE or FEM - Finite Element Method Study Model.


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Ground Grid Systems

Ground Grid Systems Presentation

After selecting the IEEE or FEM Study Model, the Ground Grid Systems graphical user interface window will be displayed. Below is a GGS presentation of a ground grid for the FEM Study Model case. Soil View

3D View

Top View


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Ground Grid Systems

FEM Edit Toolbar

28.2 FEM Editor Toolbar The FEM Editor Toolbar appears when the FEM Study Model is selected, and when in the Ground Grid Systems Edit mode. This toolbar has the following function keys: Pointer Conductor Rod FEM Rectangular Shape FEM T-Shape FEM L-Shape FEM Triangular Shape

Pointer The cursor takes the shape of the element selected from the Edit Toolbar. Click on the Pointer icon to return the cursor to its original arrow shape, or to move an element placed in the Top View of the GGS presentation.

Conductor Click on the Conductor icon to create a new conductor and to place it in the Top View of the GGS. For more information on conductors see the Conductor/Rod Editor section (for FEM).

Rod Click on the Rod icon to create a new rod and to place it in the Top View of the GGS. For more information on rods see the Conductor/Rod Editor section (for FEM).

FEM Rectangular Shape Click on the FEM Rectangular Shape icon to create a new FEM grid of rectangular shape and to place it in the Top View of the GGS. For more information on grids see the FEM Group Editor section.

FEM T-Shape Click on the FEM T-Shape icon to create a new FEM T-shaped grid and to place it in the Top View of the GGS. For more information on grids see the FEM Group Editor section.


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Ground Grid Systems

FEM Edit Toolbar

FEM L-Shape Click on the FEM L-Shape icon to create a new FEM L-shaped grid and to place it in the Top View of the GGS. For more information on grids see the FEM Group Editor section.

FEM Triangular Shape Click on the FEM Triangular Shape icon to create a new FEM grid of triangular shape and to place it in the Top View. For more information on grids see the FEM Group Editor section.


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Ground Grid Design

IEEE Edit Toolbar

28.3 IEEE Edit Toolbar The IEEE Editor Toolbar appears when the IEEE Study Model is selected, and when in the Ground Grid Systems Edit mode. This toolbar has the following function keys:

Pointer IEEE Rectangular Shape IEEE T-Shape IEEE L-Shape IEEE Triangular Shape

Pointer The cursor takes the shape of the element selected from the Edit Toolbar. Click on the Pointer icon to return the cursor to its original arrow shape, or to move an element placed in the Top View of the GGS presentation.

IEEE Rectangular Shape Click on the IEEE Rectangular Shape icon to create a new IEEE grid of rectangular shape and to place it in the Top View of the GGS. For more information on grids see the IEEE Group Editor section.

IEEE T-Shape The IEEE T-Shape grid is valid only for the IEEE Std. 80-2000 method. Click on the IEEE T-Shape icon to create a new IEEE T-shaped grid and to place it in the Top View of the GGS. For more information on grids see the IEEE Group Editor section.

IEEE L-Shape The IEEE L-Shape grid is valid only for the IEEE Std 80-2000 method. Click on the IEEE L-Shape icon to create a new IEEE L-shaped grid and to place it in the Top View of the GGS. For more information on grids see the IEEE Group Editor section.

IEEE Triangular Shape The IEEE Triangular Shape grid is valid only for the IEEE Std 80-2000 method. Click on the IEEE Triangular Shape icon to create a new IEEE grid of triangular shape and to place it in the Top View. For more information on grids see the IEEE Group Editor section.


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Ground Grid Design

Study Method Toolbar

28.4 Ground Grid Study Method Toolbar The Ground Grid Study Method Toolbar appears when the GGS Study mode is selected. This toolbar has the following function keys:

Ground-Grid Calculation Optimized Conductors Optimized Conductors and Rods Summary and Warning

Plot Selection Report Manager Stop

Ground-Grid Calculation Click on the Ground-Grid Calculation button to calculate: • • • • •

Step and Touch (mesh) Potentials Ground Resistance Ground Potential Rise Tolerable Step and Touch Potential Limits Potential Profiles (only for the FEM method)

Optimized Conductors Click on the Optimized Conductors button to calculate the minimum number of conductors (that satisfy the tolerable limits for the Step and Touch potentials) for a fixed number of ground rods. This optimization function is for IEEE Std methods only.

Optimized Conductors and Rods Click on the Optimized Conductors button to calculate the optimum numbers of conductors and ground rods needed to limit the Step and Touch potentials. This optimization function is for IEEE Std methods only.

Summary and Warning Click on this button to open the GRD Analysis Alert View dialog box of Summary and Warning for the Ground Grid Systems Calculation.


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Ground Grid Design


Plot Selection This function is valid only for the FEM method. Click on this button to open the Plot Selection dialog box to select a variety of potential profile plots to review, and click OK to generate the output plots.


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Ground Grid Design


Report Manager Click on this button to open the Ground Grid Design Report Manager dialog box to select a variety of pre-formatted output plots to review. Select a plot type and click OK to bring up the output plot. A detailed explanation of the Ground Grid Design Report Manager is given in section 28.13.

Output Report files can be selected from the Output Report List Box on the Study Case Toolbar shown below.

Study Case Toolbar

Stop The Stop Sign button is normally disabled, and becomes enabled when a Ground Grid Systems Calculation is initiated. Clicking on this button will terminate calculations in progress, and the output reports will be incomplete.


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Ground Grid Design

Edit A GGS

28.5 Edit A GGS Conductors, rods, and grids of various shapes are the elements available for adding to the Top View of the Ground Grid Systems presentation. These elements are located on the Edit Toolbar of the GGS module.

28.5.1 Select Elements Place the cursor on an element located on the Edit toolbar and click the left mouse button. Note that when a grid shape is selected, regardless of the number of conductors or rods it contains, the shape is considered to be one element. If a selected shape is deleted or copied, the shape and its contents will also be deleted or copied. Press the key and click on multiple elements to either select or de-select them.

28.5.2 Add Elements To add a new element to the GGS presentation, select a new element from the Edit Toolbar by clicking on the appropriate element button. Notice that the shape of the cursor changes to correspond to that of the selected element. Place the selected element by clicking the mouse anywhere in the Top View section of the GGS presentation, and note that the cursor returns to its original shape. Double-click on any element in the Edit Toolbar to place multiple copies of the same element in the Top View section of the GGS presentation. Rules • Elements can be added ONLY in Edit mode • Two conductors/rods cannot be added on top of each other • Elements cannot be added in the Study mode • Only one IEEE shape can be added in the Top View • FEM group shapes can overlap each other

Add Conductors Click on the Conductor button on the FEM Edit Toolbar, move the cursor to the GGS presentation, and click to place the element in the Top View. PowerStation creates the new conductor using default values.

Add Rods Click on the Rod button on the FEM Edit Toolbar, move the cursor to the GGS presentation, and click to place the element in the Top View. PowerStation creates the new rod using default values.

Add Grid Shapes Click on the desired Shape button on the FEM Edit Toolbar, move the cursor to the GGS presentation, and click to place the element in the Top View. PowerStation creates the new grid shape using default values.

Add Conductors by Ungrouping FEM Shapes An FEM shape added in the Top View of a GGS presentation can be ungrouped into individual conductors. To ungroup, move the cursor inside the selected shape, right-click and select “Ungroup”.


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Ground Grid Design

Edit A GRD

28.5.3 Move / Relocate Elements When an element is added to a GGS presentation its position coordinates (x, y and z) are updated automatically in the editor/spreadsheet and in the Help line at the bottom of your screen. The element may be relocated to new coordinates by changing the coordinate values at the editor/spreadsheet (x’s, yes and z’s for conductors/rods, and Lx, Ly, Depth, # of Rods and # of Conductors in X/Y Directions for various typical grid shapes) or by dragging the element and watching the Help line change to the desired position. To drag an element, first select the element to be moved. Place the cursor on top of the selected element, Click and hold the left mouse button, drag the element to the desired position, and release the left button.

Move Conductors/Rods Select the element, click and hold the left mouse button, drag the element to the new position and release the left button.

Move Shapes Shapes can be graphically moved within the Top View. Select the shape, click and hold the left mouse button, drag the shape to the new location and release the left button.

28.5.4 Cut (Delete) Elements Select the element or group of elements and press the Delete key on the keyboard.

28.5.5 Copy Elements Select an element or group of elements, click the right mouse button, and select Copy.

28.5.6 Paste Use the Paste command to copy the selected cells from the Dumpster into the GGS presentation.

28.5.7 Size of Elements When an element is added to a GGS presentation, its size is set by default. The width and height of grid shapes and the length of conductors can be graphically changed. Select the element and move the cursor to a corner or edge of the element. Once the cursor changes its form, click and hold the left mouse button to drag the element to its new size. Release the left mouse button once the desired size has been obtained. Conductor/rod sizes can be change from the spreadsheet or shape editors. When the Length is altered, X1, Y1, and Z1 will remain unchanged, and X2, Y2, and Z2 will change accordingly. The cross-sectional area of a conductor, the outside diameter and/or length of a rod can only be changed from the conductor or rod Editor. Rules • Sizing elements can be done in Edit mode ONLY • Elements cannot overlap each other


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Ground Grid Design

Study Case Editor

28.6 Study Case Editor The GGS Study Case Editor contains Average Weight, Ambient Temperature, Current Projection Factor, Fault Current Durations, option to input or compute Fault Current Parameters (i.e., zero-sequence fault current, current division factor, and X/R ratio), and Plot Parameters (for the Finite Element Method only). PowerStation allows for the creation and saving of an unlimited number of study cases for each type of study, allowing the user to easily switch between different GGS study cases. This feature is designed to organize the study efforts and to save time. To create a new GGS study case, go to the Study Case Menu on the toolbar and select Create New to bring up the GGS Study Case Editor.

28.6.1 Study Case Page

Study Case ID A study case can be renamed by simply deleting the old Study Case ID and entering a new one. The Study case ID can be up to 25 alphanumeric characters. Use of the Navigator button at the bottom of the Study Case Editor allows the user to go from one study case to another.


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Ground Grid Design

Study Case Editor

Options In this section, select the average body weight for the person working above the ground grid, and the ambient temperature. The weight is used to calculate the tolerable Step and Touch potentials.

50 kg Check this button to select an average body weight of 50 kg.

70 kg Check this button to select an average body weight of 70 kg.

Ambient Temperature Enter the ambient temperature in 0C. This parameter is used for checking the size of the ground conductors.

Reports & Plots Specify the report/plot parameters.

Report Details Check this box to report intermediate results for an IEEE Std. Method or voltage profiles for the Finite Element Method.

Auto Display of Summary & Alert Check this box to automatically show the result window for Summary & Warning.

Plot Step Plot Step is valid only for the FEM Study Model. This value is entered in m/ft, and it is used to find the points (or locations) where Absolute/Step/Touch potentials need to be computed and plotted. Note that the smaller this number, the more calculations are required, increasing calculation time, but yielding smoother plots. The recommended value is 1 meter. If higher resolution is needed, decrease this number.

Boundary Extension Enter the boundary extension in m/ft. This value is used to extend the grid boundaries inside which the Absolute/Step/Touch potentials need to be computed.

Fault Durations Allows the user to specify Fault Current durations.

tf Enter the duration of fault current in seconds to determine decrement factor. The Fault duration (tf), tc, and Shock duration (ts) are normally assumed to be equal, unless the Fault duration is the sum of successive shocks.

tc Enter in seconds the duration of Fault Current for sizing ground conductors.


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Ground Grid Design

Study Case Editor

ts Enter in seconds the duration of Shock Current to determine permissible levels for the human body.

Grid Current Factors In this section, the Corrective Projection Factor and the Current Division Factor can be specified.

Cp Enter the Corrective Projection Factor in percent, accounting for the relative increase of fault currents during the station lifespan. For a zero future system growth, Cp = 100.

Sf Enter the Current Division Factor in percent, relating the magnitude of Fault current to that of its portion flowing between the grounding grid and the surrounding earth.

Update Check this box to update/replace the number of conductors/rods in the Conductor/Rod Editor, with the number of conductors/rods calculated by using optimization methods. This box is only valid with the IEEE methods.

Ground Short-Circuit Current This section is used to specify the fault current conditions for the GGS.

User Specified Check this button to input and display values for 3I0 and X/R specified by the user.

Short-Circuit Study Check this button to use and display the 3I0 and X/R values obtained from a short circuit study performed on a One-Line Diagram.

3I0 Enter the rms value of the zero-sequence fault current in kA. The Maximum Grid Current is determined from this rms value, the Decrement Factor, Current Projection Factor, and Current Division Factor.

X/R Enter the ratio of Inductive Reactance to Resistance. This value is used to calculate the decrement factor.

Remarks 2nd Line Up to 120 alphanumeric characters can be entered in this remark box. Information entered here will be printed on the second line of every output report page header. These remarks can provide specific information regarding each study case. Note that the first line of the header information is global for all study cases and is entered in the Project Information Editor.


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Ground Grid Design

Ground Short-Circuit Current Editor

28.7 Ground Short-Circuit Current Values This feature allows the user to link the Ground Grid Systems module with the One-Line Diagram, to update and use the total 3I0 and the equivalent X/R values obtained directly from the One-Line Diagram representation of the power system.

Updating Ground-Short Circuit Current To update and use the 3I0 and X/R values, with values obtained by performing an Unbalanced Fault Short Circuit Study on a One-Line Diagram, select the Short-Circuit Study radio button located in the Ground Short Circuit Current section of the GRD dialog box, and follow these steps: Perform an Unbalanced Fault Short Circuit Study An Unbalanced Fault Short Circuit Study must be performed on the One-Line Diagram power system representation. The following conditions must be met: • •

For the ANSI SC Unbalanced Fault Calculation only the half-cycle values are transferred. For the IEC SC Unbalanced Fault Calculation only the IEC 909 values are transferred.

Select a Grid to be Updated At the One-Line Diagram, right-click on the grid of interest, and choose the “Update Fault kA” option from the menu. This option is only available when successful Unbalanced Fault Current Calculation results are obtained, for the ANSI and IEC standards specified. All the buses covered by the grid are considered by the update function. However, only the results for the bus with the highest total short circuit current will be used. The Grd Short–Circuit Current Updating dialog box will be displayed. The new Short-Circuit Current values will be used only if the user clicks on the Replace button, located on this dialog box.


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Ground Grid Design


28.7.1 GRD Short-Circuit Current Updating All the fields in this dialog box are for display only.

GRD ID Displays the ID of the selected grid. Existing SC kA Displays the current short-circuit kA value used with the selected grid. Existing SC kA X/R Displays the current short-circuit kA X/R value used with the selected grid. New SC kA Displays the new Short-Circuit kA value to be updated for the selected grid. New SC kA X/R Displays the new short-circuit kA X/R value to be updated for the selected grid. The Range and format are the same as those for the X/R field in the Induction Motor Editor. Faulted Bus Displays the ID of the faulted bus used for the study case. Fault Type Displays the type of fault used to calculate the New SC kA value. Currently only Line-Ground faults are used, but additional fault types will be added in future versions of PowerStation.


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Ground Grid Design


Replace Click on this button to update the New SC kA and X/R values for the selected grid. Cancel Click on this button to close the dialog box and retain the existing SC kA and X/R values.


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Ground Grid Design

Soil Editor

28.8 Soil Editor Double-click at any location inside the Soil View to bring up the Soil Editor to specify earth/surface materials, their soil resistivities and depths.

Surface Material Used to specify the resistivity, depth, and material type for the surface layer.

Resistivity Enter the Resistivity of the Surface Material in Ohms-m.

Material Select the type of Surface Material.

Depth Enter the Surface Material depth in m/ft.

Top Layer Used to specify the resistivity, depth, and type of material for the Top Layer soil.

Resistivity Enter the material resistivity of the Top Layer soil in Ohms-m.

Material Select the type of material of the Top Layer soil.


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Ground Grid Design

Soil Editor

Depth Enter the depth of the Top Layer soil in m/ft.

Lower Layer Used to specify the Resistivity and type of material used for Lower Layer soil.

Resistivity Enter the resistivity of the material of the Lower Layer soil in Ohms-m.

Material Select the type of material of the Lower Layer soil.


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Ground Grid Design

IEEE Group Editor

28.9 IEEE Group Editor When an IEEE Study Model is used, double-click on any location inside the selected grid shape in the Top View of the GGS to bring up the IEEE Group Editor. The Editor is used to specify conductor/rod parameters for the grid shape.

28.9.1 Conductors Page Within the Conductor Page, specify the parameters of the conductors and the grid size.

Grid Size Lx/Lx,long Enter the long length of the grid in the X direction in m/ft. Show Lx if the rectangular/triangular shape is selected; show Lx,long if the L-shape or T-shape are selected.

Ly/Ly,long Enter the long length of the grid in the Y direction in m/ft. Show Ly if the rectangular/triangular shape is selected; show Ly,long if the L-shape or T-shape are selected.


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Ground Grid Design

IEEE Group Editor

Lx,short Enter the short length of the grid in the X direction in m/ft. Lx,short appears only if the L-shape or T-shape are selected.

Ly,short Enter the short length of the grid in the Y direction in m/ft. Ly,short shows only if the L-shape or T-shape are selected.

# of Conductors X Direction Enter the number of the conductors in the X direction

Y Direction Enter the number of the conductors in the Y direction

Conductors Depth Enter the depth of the conductor grip in m/ft.

Type Select the type of the conductor material.

Size Select the conductor size in AWG/kcmil or mm2.

Cost Enter the cost of the conductor in $/m or $/ft.

Material Constants This information is displayed on the Conductors Page to reflect the selected conductor type (the conductor constants are from an internal conductor library/file GRDLib.mdb which can be modified using Microsoft Access). It includes Material Conductivity (%), Thermal Coefficient of Resistivity at 20 0C (1/0C), K0 Factor (0C), Fusing Temperature (0C), Resistivity of the Ground Conductor at 20 0C in µΩ•cm, and the Thermal Capacity Factor in J/cm3/0C.


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Ground Grid Design

IEEE Group Editor

28.9.2 Rods Page Within the Rods Page, specify the parameters of the Rods.

Rods # of Rods Enter the number of rods.

Diameter Enter the diameter of the rod in inch/cm.

Length Enter the length of the rod in m/ft

Arrangement Select the arrangement of the rods throughout the grid area.

Type Select the type of rod material.


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Ground Grid Design

IEEE Group Editor

Cost Enter the cost of the rod in $/rod.

Material Constants This information is displayed on the Rods Page to reflect the selected rod type (the conductor constants are from an internal conductor library/file GRDLib.mdb which can be modified using Microsoft Access). It includes Material Conductivity (%), Thermal Coefficient of Resistivity at 20 0C (1/0C), K0 Factor (0C), Fusing Temperature (0C), Resistivity of the Ground Conductor at 20 0C in µΩ•cm, and the Thermal Capacity Factor in J/cm3/0C.


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Ground Grid Design

FEM Group Editor

28.10 FEM Group Editor When an FEM Study Model is used, double-click on any location inside the selected grid shape in the Top View of the GGS to bring up the FEM Group Editor. The Editor is used to specify conductor/rod parameters and grid size for the shape.

28.10.1 Group Conductors Page

Grid Size Lx/Lx,long Enter the long length of the grid in the X direction in m/ft. Show Lx if the rectangular/triangular shape is selected; show Lx,long if the L-shape or T-shape are selected.

Ly/Ly,long Enter the long length of the grid in the Y direction in m/ft. Show Ly if the rectangular/triangular shape is selected; show Ly,long if the L-shape or T-shape are selected.


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Ground Grid Design

FEM Group Editor

Lx,short Enter the short length of the grid in the X direction in m/ft. Lx,short appears only if the L-shape or T-shape are selected.

Ly,short Enter the short length of the grid in the Y direction in m/ft. Ly,short appears only if the L-shape or T-shape are selected.

# of Conductors X Direction Enter the number of conductors in the X direction.

Y Direction Enter the number of conductors in the Y direction.

Conductors Depth Enter the depth of conductor grip in m/ft.

Size Select the conductor size in AWG/kcmil or mm2.

Type Select the type of conductor material.

Insulation Select the type of conductor insulation (Bare or Insulated). If Insulated is selected, this grid group will not be reconsidered for calculation/plotting.

Cost Enter the cost of the conductor in $/m or $/ft.


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Ground Grid Design

Conductor/Rod Editor (FEM)

28.11 Conductor/Rod Editor (FEM) The Conductor/Rod Editor is used with the FEM study model only. To edit the data for a conductor/rod, a conductor/rod must be selected from the FEM Edit Toolbar and placed in the Top View of the GGS. Double-click on a conductor/rod in the Top View to bring up the Conductor/Rod Spreadsheet Editor. The Material Constants of the conductor/rod are displayed in the top section of the spreadsheet according to the material type. Each conductor/rod record (row) is a unique set of data. Each conductor/rod record must have a unique identifier: ConID. Duplicate records with the same data are overwritten.


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Ground Grid Design

Conductor/Rod Editor (FEM)

Label Symbol representing a conductor/rod.

Length Length of the conductor/rod in m/ft. If the length is altered, X2, Y2 and Z2 are changed accordingly. If X1, Y1, Z1, X2, Y2, and Z2 values are entered, the length is changed accordingly.

X1 X coordinate of One End of the conductor/rod in m/ft

Y1 Y coordinate of One End of the conductor/rod in m/ft

Z1 Z coordinate of One End of the conductor/rod in m/ft

X2 X coordinate of the Other End of the conductor/rod in m/ft

Y2 Y coordinate of the Other End of the conductor/rod in m/ft

Diameter Rod diameter in cm or inches, used only in the Rod Editor.

Type Type of conductor/rod material.

Size Conductor cross-sectional area in AWG/kcmil or mm2, used only in the Conductor Editor.

Insulation Conductor insulation type, used only in the Conductor Editor.

Cost Cost in $/m or $/ft for a conductor, cost in $/rod for a rod.


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Ground Grid Design

Calculation Methods

28.12 Calculation Methods The Ground Grid Systems Program includes the following methods of computation: • • • • • •

Finite Element Method ANSI/IEEE Std 80-1986 IEEE Guide for Safety in AC Substation Grounding ANSI/IEEE Std 80–2000 IEEE Guide for Safety in AC Substation Grounding ANSI/IEEE Std 665 – 1995 IEEE Guide for Generating Station Grounding Optimization of Conductors ANSI/IEEE Std Based Methods Optimization of Conductors and Rods ANSI/IEEE Std Based Methods

28.12.1 Finite Element Method The Finite Element method (FEM) is based on a method of images, and assumes that the grounding system is an equipotential structure. It gives accurate results for small (around 50 m by 50 m) and medium (around 250 m by 250 m) size grounding networks at low frequencies. The uniform or twolayer soil view is also used with the FEM method.

28.12.2 IEEE Std Methods IEEE Std 80-2000, IEEE Std 80-1986, or IEEE Std 665-1995 is optional for the calculation of Step and Touch (mesh) Potentials, Ground Resistance, Ground Potential Rise, Tolerable Step and Touch Potential Limits. IEEE Std 80-1986, or IEEE Std 665-1995 is used only for the Square/Rectangular shapes of ground grids; IEEE Std 80-2000 can be used for Square/Rectangular, Triangular, L-Shaped, or T-Shaped ground grids.

28.12.3 Optimization of Conductors For a fixed number of ground rods, the program determines the minimum number of conductors that satisfy the tolerable limits for the Step and Touch potentials. The GGS program begins calculations with a grid consisting of only two conductors on each side, and increases the number of conductors (keeping the mesh almost square) until a solution is reached. This optimization function is for IEEE Std methods only.

28.12.4 Optimization of Conductors and Rods The GGS program performs a cost optimization routine to determine the optimum number of conductors and ground rods needed to limit the Step and Touch potentials. The program begins the optimization routine with a minimum of two parallel conductors horizontally, two parallel conductors vertically, and 4 rods. With each iteration, the number of rods and conductors is increased based on their cost effectiveness in reducing unwanted potential levels. This optimization function is for IEEE Std methods only.


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Ground Grid Design

Calculation Methods

28.12.5 Fundamental Formulas Some fundamental formulas are given below.

Reflection Factor, K K=

ρ − ρs ρ + ρs

Where ρ is the resistivity of the earth beneath the surface material in ohm-m; ρs is the surface layer soil resistivity in ohm-m.

Surface Layer Derating Factor, Cs For IEEE Std 80-2000

Cs = 1 −

0.09(1 − ρ / ρ s ) 2hs + 0.09

Where hs is the thickness of the surface layer in meter. For IEEE Std 80-1986, IEEE Std 665-1995 Cs =

1 0.96

 ∞ Kn 1 + 2∑ 2  n =1 1 + (2nh / 0.08) s 

   

Cs is 1 when K=0.

Decrement Factor, Df Df = 1+

(

Ta − 2t / T 1− e f a tf

)

Where Ta is the equivalent system subtransient time constant in seconds.

Tolerable Step Potential, Estep and Etouch For body weight of 50 kg

Estep 50 = (1000 + 6Cs ρ s )


0.116 ts

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Ground Grid Design

Calculation Methods

Etouch50 = (1000 + 1.5Cs ρ s )

0.116 ts

For body weight of 70 kg Estep 70 = (1000 + 6Cs ρ s )

0.157 ts

Etouch 70 = (1000 + 1.5Cs ρ s )

0.157 ts

Maximum Grid Current, IG

I G = S f C p D f (3I 0 )


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Ground Grid Design

Required Data

28.13 Required Data To run a Ground Grid Systems study, the following related data is necessary: Soil Parameters, Grid Data, and System Data. A summary of these data for different types of calculation methods is given in this section.

System Data • • • • • • • • • •

System Frequency Average Weight of Worker Ambient Temperature Short Circuit Current Short Circuit Current Division Factor Short Circuit Current Projector Factor Durations of Fault System X/R Ratio Plot Step (for FEM model only) Boundary Extension (for FEM model only)

Soil Parameters • • • • •

Surface Material Resistivity Surface Material Depth Upper Layer Soil Resistivity Upper Layer Soil Depth Lower Layer Soil Resistivity

Ground Conductor Library • • • • • •

Material Conductivity Thermal Coefficient of Resistivity K0 Factor Fusing Temperature Ground Conductor Resistivity Thermal Capacity Factor

Grid Data (IEEE Std.’s Only) • • • • • • • • • • •

Shape Material Type Conductor Cross Section Grid Depth Maximum Length of the Grid in the X Direction Maximum Length of the Grid in the Y Direction Minimum Length of the Grid in the X Direction (for IEEE Std 80-2000 L-Shaped or T-Shaped Grids Only) Minimum Length of the Grid in the Y Direction (for IEEE Std 80-2000 L-Shaped or T-Shaped Grid Only) Number of Conductors in the X Direction Number of Conductors in the Y Direction Cost


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Required Data

Rod Data (IEEE Std.’s Only) • • • • • •

Material Type Number of Rods Average Length Diameter Arrangement Cost

Conductor Data (FEM model only) • • • • • •

Material Type Insulation Cross Section X, Y and Z Coordinates of One End of Conductor X, Y and Z Coordinates of Other End of Conductor Cost

Rod Data (FEM model only) • • • • • •

Material Type Insulation Diameter X, Y and Z Coordinates of One End of Rod X, Y and Z Coordinates of Other End of Rod Cost

Optional FEM Model Grid Group Data • • • • • • • • • • •

Shape Material Type Conductor Cross Section Grid Depth Maximum Length of the Grid in the X Direction Maximum Length of the Grid in the Y Direction Minimum Length of the Grid in the X Direction (for L-Shaped or T-Shaped Grids) Minimum Length of the Grid in the Y Direction (for L-Shaped or T-Shaped Grids) Number of Conductors in the X Direction Number of Conductors in the Y Direction Cost


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Output Reports

28.14 Output Report Output reports for the Ground Grid Systems studies are available in different levels and are arranged in two formats: Crystal Output Report and Pop-Up Window display.

28.14.1 Ground Grid Systems Report Manager Click on the Report Manager button on the Ground Grid Study Method Toolbar to open the Ground Grid Systems Report Manager dialog box. The Ground Grid Systems Report Manager consists of four pages and provides different formats for the Crystal Reports.

Complete Page Selects a report format that provides the complete output report.

Input Page Provides the format for different input data.


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Output Reports

Result Page Provides the format for different calculation results.

Summary Page Provides the summary from the calculation results.

28.14.2 Ground Grid Systems Crystal Report After running the Ground Grid Systems study, click on the Report Manager button located on the Study Case Toolbar, or select the Crystal Report format from the Ground Grid Systems Toolbar, to open and view the Crystal output report. The Ground Grid Systems study Crystal Report contains the following major sections:


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Output Reports

Cover Page This is the first page of the Ground Grid Systems study Crystal Report. It includes information from the number of conductors and rods, to unit system, project file name, and the output file name and its location.

Input Data This section reports the input data related to the System, Soil, Grid, and Conductor Library.

System Input Data This section reports the input data related to the system including the System Frequency, Average Weight of Worker, Ambient Temperature, Short Circuit Current, Short Circuit Current Division Factor, Short Circuit Current Projector Factor, Durations of Fault, System X/R Ratio, Plot Step (for FEM model only), and Boundary Extension (for FEM model only).


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Output Reports

Soil Input Data This section reports the input data related to Soil including the Surface Material Resistivity, Surface Material Depth, Upper Layer Soil Resistivity, Upper Layer Soil Depth, and Lower Layer Soil Resistivity.

Conductor Library This section reports Conductor Library information. It shows the Material Conductivity, Thermal Coefficient of Resistivity, K0 Factor, Fusing Temperature, Ground Conductor Resistivity, and Thermal Capacity Factor.

Grid Data (for IEEE Std’s) This section reports the input data related to the grid including the Shape, Material Type, Conductor Cross Section, Grid Depth, Maximum Length of the Grid in the X Direction, Maximum Length of the Grid in the Y Direction, Minimum Length of the Grid in the X Direction (Only for IEEE Std 80-2000 L-Shaped or TShaped Grid).

Rod Data (for IEEE Std’s) This section reports the input data related to the grid including the Material Type, Number of Rods, Average Length, Diameter, Arrangement, and Cost.


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Output Reports

Conductor Data (for FEM model) This section reports the conductor input data for the FEM model including Material Type, Insulation, Cross Section, X, Y and Z Coordinates of One End of Conductor, X, Y and Z Coordinates of Other End of Conductor, and Cost.

Rod Data (for FEM model) This section reports the rod input data for the FEM model including the Material Type, Insulation, Diameter, X, Y and Z Coordinates of One End of Conductor, X, Y and Z Coordinates of Other End of Conductor, and Cost.

Cost Data Lists the cost data of conductors/rods.

Result This section reports the results related to Intermediate Constants, Potential Profiles, S ummary, and Warning.


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Output Reports

Report of Intermediate Constants for IEEE Std.’s In this section the intermediate results Kim, Kis, Km, Ks, Kii, K1, K2 are reported, if the Report Details box in the Study Case Editor dialog box is checked.

Summary for IEEE Std’s In this section, the Ground Resistance Rg, GPR, Step and Touch potentials, Reflection Factor K, Derating Factor Df, Maximum Grid Current and Warning information are reported.


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Output Reports

Report of Potential Profiles for the FEM Model In this section, the three Potential Profiles are reported if the Report Details box in the Study Case Editor is checked.


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Output Reports

Summary for the FEM Model In this section, the Ground Resistance Rg, GPR, Step and Touch Potentials, Reflection Factor K, Derating Factor Df, Maximum Grid Current and Warning information are reported.

28.14.3 Summary and Warning After running the Ground Grid Systems study, click on the Summary and Warning button located on the Ground Grid System Toolbar, to open the GRD Analysis Alert View dialog box. If the Auto Display of Summary and Warning box located on the Study Case Editor dialog box is checked, this view will open automatically after the Ground Grid Systems calculations are executed.


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Plot Selection

28.15 Plot Selection Plots are used only with the FEM method, and are available for Absolute/Step/Touch Voltages. To select a plot, open up the Plot Selection dialog box by clicking on the Plot Selection button located on the Ground Grid Systems Toolbar.

Plot Selection The following 3-D Potential profiles are available for analysis of GGS study case results:

Absolute Voltage Select to plot an Absolute Potential profile.

Touch Voltage Select to plot a Touch Potential profile.

Step Voltage Select to plot a Step Potential profile.

Plot Type The following plot types are available for analysis of GGS study case results:

3-D Plot a 3-D Potential profile for the Absolute/Touch/Step voltage.

Contour Plot a Contour Potential profile for the Absolute/Touch/Step voltage.

Display Over Limit Voltage Show areas with potentials exceeding the tolerable limits for 3-D Touch/Step Potential profiles. This function is disabled when the Contour plot type is selected. A set of sample plots is shown below.


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Plot Selection

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Ground Grid Systems


Plot Selection

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Ground Grid Systems


Plot Selection

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ETAP Ground Grid Systems

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