Chapter 28Ground 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. 28-1 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 Operation Technology, Inc. 28-2 ETAP PowerStation 4.0 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. Operation Technology, Inc. 28-3 ETAP PowerStation 4.0 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 Operation Technology, Inc. 28-4 ETAP PowerStation 4.0 0 . For more information on conductors see the Conductor/Rod Editor section (for FEM). For more information on grids see the FEM Group Editor section. For more information on rods see the Conductor/Rod Editor section (for FEM). Operation Technology. Click on the Pointer icon to return the cursor to its original arrow shape. 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. 28-5 ETAP PowerStation 4. Conductor Click on the Conductor icon to create a new conductor and to place it in the Top View of the GGS. 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.Ground Grid Systems FEM Edit Toolbar 28.2 FEM Editor Toolbar The FEM Editor Toolbar appears when the FEM Study Model is selected. For more information on grids see the FEM Group Editor section. Rod Click on the Rod icon to create a new rod and to place it in the Top View of the GGS. and when in the Ground Grid Systems Edit mode. 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. Inc. or to move an element placed in the Top View of the GGS presentation. 0 . 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. 28-6 ETAP PowerStation 4. For more information on grids see the FEM Group Editor section. Inc.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. Operation Technology. For more information on grids see the FEM 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 L-Shape icon to create a new IEEE L-shaped grid and to place it in the Top View of the GGS. IEEE L-Shape The IEEE L-Shape grid is valid only for the IEEE Std 80-2000 method. For more information on grids see the IEEE Group Editor section. and when in the Ground Grid Systems Edit mode. Inc. For more information on grids see the IEEE Group Editor section.3 IEEE Edit Toolbar The IEEE Editor Toolbar appears when the IEEE Study Model is selected. 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. 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. or to move an element placed in the Top View of the GGS presentation. Click on the Pointer icon to return the cursor to its original arrow shape. 28-7 ETAP PowerStation 4. 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. Click on the IEEE Triangular Shape icon to create a new IEEE grid of triangular shape and to place it in the Top View.Ground Grid Design IEEE Edit Toolbar 28.0 . IEEE T-Shape The IEEE T-Shape grid is valid only for the IEEE Std. For more information on grids see the IEEE Group Editor section. 80-2000 method. Operation Technology. 0 . 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.Ground Grid Design Study Method Toolbar 28. This optimization function is for IEEE Std methods only. Operation Technology. 28-8 ETAP PowerStation 4. 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. 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. Inc.4 Ground Grid Study Method Toolbar The Ground Grid Study Method Toolbar appears when the GGS Study mode is selected. 28-9 ETAP PowerStation 4. Operation Technology. Inc. 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.0 .Ground Grid Design Study Method Toolbar Plot Selection This function is valid only for the FEM method. and becomes enabled when a Ground Grid Systems Calculation is initiated. A detailed explanation of the Ground Grid Design Report Manager is given in section 28. Inc. Select a plot type and click OK to bring up the output plot. and the output reports will be incomplete. 28-10 ETAP PowerStation 4. Clicking on this button will terminate calculations in progress.13. Output Report files can be selected from the Output Report List Box on the Study Case Toolbar shown below.0 . Study Case Toolbar Stop The Stop Sign button is normally disabled.Ground Grid Design Study Method Toolbar 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. Operation Technology. move the cursor to the GGS presentation. To ungroup. Add Rods Click on the Rod button on the FEM Edit Toolbar. and click to place the element in the Top View. 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. 28.1 Select Elements Place the cursor on an element located on the Edit toolbar and click the left mouse button. Place the selected element by clicking the mouse anywhere in the Top View section of the GGS presentation.2 Add Elements To add a new element to the GGS presentation. PowerStation creates the new grid shape using default values. rods. These elements are located on the Edit Toolbar of the GGS module. Inc. Add Grid Shapes Click on the desired Shape button on the FEM Edit Toolbar.0 . and click to place the element in the Top View. 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. and note that the cursor returns to its original shape. 28-11 ETAP PowerStation 4. and click to place the element in the Top View. the shape and its contents will also be deleted or copied.Ground Grid Design Edit A GGS 28. Operation Technology. right-click and select “Ungroup”. PowerStation creates the new conductor using default values. Note that when a grid shape is selected. PowerStation creates the new rod using default values. and grids of various shapes are the elements available for adding to the Top View of the Ground Grid Systems presentation. Notice that the shape of the cursor changes to correspond to that of the selected element. move the cursor to the GGS presentation. 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.5. Add Conductors by Ungrouping FEM Shapes An FEM shape added in the Top View of a GGS presentation can be ungrouped into individual conductors. Press the <Ctrl> key and click on multiple elements to either select or de-select them. move the cursor inside the selected shape. 28. move the cursor to the GGS presentation.5 Edit A GGS Conductors. select a new element from the Edit Toolbar by clicking on the appropriate element button.5. and select Copy. 28. Click and hold the left mouse button. drag the element to the desired position. Ly. Once the cursor changes its form.Ground Grid Design Edit A GRD 28.4 Cut (Delete) Elements Select the element or group of elements and press the Delete key on the keyboard. click and hold the left mouse button.0 . the outside diameter and/or length of a rod can only be changed from the conductor or rod Editor. The width and height of grid shapes and the length of conductors can be graphically changed. Move Conductors/Rods Select the element.5. and Z1 will remain unchanged. 28. click and hold the left mouse button to drag the element to its new size. and Z2 will change accordingly.7 Size of Elements When an element is added to a GGS presentation. drag the shape to the new location and release the left button. Select the shape. Y2. The cross-sectional area of a conductor. # 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. Release the left mouse button once the desired size has been obtained. 28. To drag an element.5. Inc. 28. first select the element to be moved. click and hold the left mouse button.5. Rules • Sizing elements can be done in Edit mode ONLY • Elements cannot overlap each other Operation Technology. Conductor/rod sizes can be change from the spreadsheet or shape editors. 28-12 ETAP PowerStation 4. and X2. Depth.5. click the right mouse button.6 Paste Use the Paste command to copy the selected cells from the Dumpster into the GGS presentation. When the Length is altered.5. Select the element and move the cursor to a corner or edge of the element. Y1. y and z) are updated automatically in the editor/spreadsheet and in the Help line at the bottom of your screen. drag the element to the new position and release the left button. Move Shapes Shapes can be graphically moved within the Top View. Place the cursor on top of the selected element.3 Move / Relocate Elements When an element is added to a GGS presentation its position coordinates (x. The element may be relocated to new coordinates by changing the coordinate values at the editor/spreadsheet (x’s. X1.5 Copy Elements Select an element or group of elements. yes and z’s for conductors/rods. and release the left button. and Lx. its size is set by default. The Study case ID can be up to 25 alphanumeric characters. allowing the user to easily switch between different GGS study cases.6. PowerStation allows for the creation and saving of an unlimited number of study cases for each type of study.. Ambient Temperature. 28-13 ETAP PowerStation 4. and Plot Parameters (for the Finite Element Method only). option to input or compute Fault Current Parameters (i. and X/R ratio). Current Projection Factor. Operation Technology.6 Study Case Editor The GGS Study Case Editor contains Average Weight. Fault Current Durations. This feature is designed to organize the study efforts and to save time. go to the Study Case Menu on the toolbar and select Create New to bring up the GGS Study Case Editor. current division factor.e.0 . 28. To create a new GGS study case. Inc.Ground Grid Design Study Case Editor 28. Use of the Navigator button at the bottom of the Study Case Editor allows the user to go from one study case to another.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. zero-sequence fault current. Report Details Check this box to report intermediate results for an IEEE Std. Operation Technology. The weight is used to calculate the tolerable Step and Touch potentials. Plot Step Plot Step is valid only for the FEM Study Model. tc Enter in seconds the duration of Fault Current for sizing ground conductors. decrease this number. the more calculations are required. 50 kg Check this button to select an average body weight of 50 kg. Inc. Method or voltage profiles for the Finite Element Method. Ambient Temperature Enter the ambient temperature in 0C. unless the Fault duration is the sum of successive shocks.0 . and Shock duration (ts) are normally assumed to be equal. This value is used to extend the grid boundaries inside which the Absolute/Step/Touch potentials need to be computed. Boundary Extension Enter the boundary extension in m/ft.Ground Grid Design Study Case Editor Options In this section. tf Enter the duration of fault current in seconds to determine decrement factor. increasing calculation time. 28-14 ETAP PowerStation 4. Auto Display of Summary & Alert Check this box to automatically show the result window for Summary & Warning. 70 kg Check this button to select an average body weight of 70 kg. and the ambient temperature. The Fault duration (tf). Fault Durations Allows the user to specify Fault Current durations. This parameter is used for checking the size of the ground conductors. The recommended value is 1 meter. Note that the smaller this number. select the average body weight for the person working above the ground grid. If higher resolution is needed. and it is used to find the points (or locations) where Absolute/Step/Touch potentials need to be computed and plotted. Reports & Plots Specify the report/plot parameters. This value is entered in m/ft. tc. but yielding smoother plots. Ground Grid Design Study Case Editor ts Enter in seconds the duration of Shock Current to determine permissible levels for the human body. Current Projection Factor. Ground Short-Circuit Current This section is used to specify the fault current conditions for the GGS. 28-15 ETAP PowerStation 4. accounting for the relative increase of fault currents during the station lifespan. User Specified Check this button to input and display values for 3I0 and X/R specified by the user. Note that the first line of the header information is global for all study cases and is entered in the Project Information Editor. The Maximum Grid Current is determined from this rms value. Cp Enter the Corrective Projection Factor in percent. Update Check this box to update/replace the number of conductors/rods in the Conductor/Rod Editor. X/R Enter the ratio of Inductive Reactance to Resistance.0 . with the number of conductors/rods calculated by using optimization methods. Remarks 2nd Line Up to 120 alphanumeric characters can be entered in this remark box. 3I0 Enter the rms value of the zero-sequence fault current in kA. 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. This value is used to calculate the decrement factor. the Corrective Projection Factor and the Current Division Factor can be specified. and Current Division Factor. This box is only valid with the IEEE methods. Grid Current Factors In this section. Information entered here will be printed on the second line of every output report page header. relating the magnitude of Fault current to that of its portion flowing between the grounding grid and the surrounding earth. For a zero future system growth. These remarks can provide specific information regarding each study case. Cp = 100. Sf Enter the Current Division Factor in percent. the Decrement Factor. Inc. Operation Technology. right-click on the grid of interest.7 Ground Short-Circuit Current Values This feature allows the user to link the Ground Grid Systems module with the One-Line Diagram. for the ANSI and IEC standards specified. select the Short-Circuit Study radio button located in the Ground Short Circuit Current section of the GRD dialog box. and choose the “Update Fault kA” option from the menu. only the results for the bus with the highest total short circuit current will be used. located on this dialog box. Updating Ground-Short Circuit Current To update and use the 3I0 and X/R values. Inc. The following conditions must be met: • • For the ANSI SC Unbalanced Fault Calculation only the half-cycle values are transferred. All the buses covered by the grid are considered by the update function. 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.Ground Grid Design Ground Short-Circuit Current Editor 28. The new Short-Circuit Current values will be used only if the user clicks on the Replace button. 28-16 ETAP PowerStation 4.0 . 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. For the IEC SC Unbalanced Fault Calculation only the IEC 909 values are transferred. with values obtained by performing an Unbalanced Fault Short Circuit Study on a One-Line Diagram. Operation Technology. However. This option is only available when successful Unbalanced Fault Current Calculation results are obtained. Select a Grid to be Updated At the One-Line Diagram. The Grd Short–Circuit Current Updating dialog box will be displayed. Operation Technology. New SC kA Displays the new Short-Circuit kA value to be updated for the selected grid. Fault Type Displays the type of fault used to calculate the New SC kA value. 28-17 ETAP PowerStation 4.Ground Grid Design Ground Short-Circuit Current Editor 28.0 . but additional fault types will be added in future versions of PowerStation. New SC kA X/R Displays the new short-circuit kA X/R value to be updated for the selected grid. GRD ID Displays the ID of the selected grid. Currently only Line-Ground faults are used. Existing SC kA X/R Displays the current short-circuit kA X/R value used with the selected grid. Existing SC kA Displays the current short-circuit kA value used with the selected grid. Inc.1 GRD Short-Circuit Current Updating All the fields in this dialog box are for display only. The Range and format are the same as those for the X/R field in the Induction Motor Editor.7. Faulted Bus Displays the ID of the faulted bus used for the study case. Cancel Click on this button to close the dialog box and retain the existing SC kA and X/R values.Ground Grid Design Ground Short-Circuit Current Editor Replace Click on this button to update the New SC kA and X/R values for the selected grid. Operation Technology. 28-18 ETAP PowerStation 4. Inc.0 . depth. Resistivity Enter the Resistivity of the Surface Material in Ohms-m. and type of material for the Top Layer soil.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. Material Select the type of material of the Top Layer soil. Top Layer Used to specify the resistivity. Depth Enter the Surface Material depth in m/ft. Surface Material Used to specify the resistivity. Material Select the type of Surface Material. Inc.Ground Grid Design Soil Editor 28. and material type for the surface layer.0 . Operation Technology. depth. Resistivity Enter the material resistivity of the Top Layer soil in Ohms-m. 28-19 ETAP PowerStation 4. Inc. 28-20 ETAP PowerStation 4. Resistivity Enter the resistivity of the material of the Lower Layer soil in Ohms-m.Ground Grid Design Soil Editor Depth Enter the depth of the Top Layer soil in m/ft.0 . Material Select the type of material of the Lower Layer soil. Operation Technology. Lower Layer Used to specify the Resistivity and type of material used for Lower Layer soil. 28-21 ETAP PowerStation 4. show Ly.long if the L-shape or T-shape are selected. The Editor is used to specify conductor/rod parameters for the grid shape. Show Ly if the rectangular/triangular shape is selected.1 Conductors Page Within the Conductor Page. Inc.long Enter the long length of the grid in the Y direction in m/ft. Ly/Ly. Operation Technology. show Lx.9 IEEE Group Editor When an IEEE Study Model is used.9.0 . Grid Size Lx/Lx. Show Lx if the rectangular/triangular shape is selected.long Enter the long length of the grid in the X direction in m/ft.long if the L-shape or T-shape are selected.Ground Grid Design IEEE Group Editor 28. double-click on any location inside the selected grid shape in the Top View of the GGS to bring up the IEEE Group Editor. specify the parameters of the conductors and the grid size. 28. and the Thermal Capacity Factor in J/cm3/0C.short Enter the short length of the grid in the Y direction in m/ft.short Enter the short length of the grid in the X direction in m/ft.short appears only if the L-shape or T-shape are selected. Resistivity of the Ground Conductor at 20 0C in µΩ•cm. Operation Technology. Inc. Thermal Coefficient of Resistivity at 20 0C (1/0C).Ground Grid Design IEEE Group Editor Lx.mdb which can be modified using Microsoft Access).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. 28-22 ETAP PowerStation 4. Lx. Cost Enter the cost of the conductor in $/m or $/ft. Ly.0 . 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. Size Select the conductor size in AWG/kcmil or mm2. K0 Factor (0C). Type Select the type of the conductor material. Fusing Temperature (0C). Ly. It includes Material Conductivity (%). 0 . 28-23 ETAP PowerStation 4.Ground Grid Design IEEE Group Editor 28. specify the parameters of the Rods. Diameter Enter the diameter of the rod in inch/cm. Rods # of Rods Enter the number of rods.9. Inc. Operation Technology. 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.2 Rods Page Within the Rods Page. Thermal Coefficient of Resistivity at 20 0C (1/0C). Operation Technology. Resistivity of the Ground Conductor at 20 0C in µΩ•cm. It includes Material Conductivity (%).Ground Grid Design IEEE Group Editor Cost Enter the cost of the rod in $/rod. Inc.mdb which can be modified using Microsoft Access). 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. and the Thermal Capacity Factor in J/cm3/0C. Fusing Temperature (0C). K0 Factor (0C).0 . 28-24 ETAP PowerStation 4. The Editor is used to specify conductor/rod parameters and grid size for the shape.long Enter the long length of the grid in the Y direction in m/ft. Ly/Ly. Show Ly if the rectangular/triangular shape is selected. show Ly. 28-25 ETAP PowerStation 4.long if the L-shape or T-shape are selected. Inc. Operation Technology.long if the L-shape or T-shape are selected. double-click on any location inside the selected grid shape in the Top View of the GGS to bring up the FEM Group Editor. Show Lx if the rectangular/triangular shape is selected.1 Group Conductors Page Grid Size Lx/Lx. 28. show Lx.Ground Grid Design FEM Group Editor 28.long Enter the long length of the grid in the X direction in m/ft.10.10 FEM Group Editor When an FEM Study Model is used.0 . Cost Enter the cost of the conductor in $/m or $/ft. Ly.Ground Grid Design FEM Group Editor Lx. Operation Technology.short appears only if the L-shape or T-shape are selected. Insulation Select the type of conductor insulation (Bare or Insulated). Lx.0 . If Insulated is selected.short Enter the short length of the grid in the X direction in m/ft. Type Select the type of conductor material. Y Direction Enter the number of conductors in the Y direction. Size Select the conductor size in AWG/kcmil or mm2. this grid group will not be reconsidered for calculation/plotting.short Enter the short length of the grid in the Y direction in m/ft. Inc. Ly. Conductors Depth Enter the depth of conductor grip in m/ft. 28-26 ETAP PowerStation 4.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. To edit the data for a conductor/rod.Ground Grid Design Conductor/Rod Editor (FEM) 28. Operation Technology. Each conductor/rod record (row) is a unique set of data.11 Conductor/Rod Editor (FEM) The Conductor/Rod Editor is used with the FEM study model only.0 . Each conductor/rod record must have a unique identifier: ConID. 28-27 ETAP PowerStation 4. Inc. Duplicate records with the same data are overwritten. The Material Constants of the conductor/rod are displayed in the top section of the spreadsheet according to the material type. Double-click on a conductor/rod in the Top View to bring up the Conductor/Rod Spreadsheet Editor. a conductor/rod must be selected from the FEM Edit Toolbar and placed in the Top View of the GGS. Cost Cost in $/m or $/ft for a conductor.0 . Y2 and Z2 are changed accordingly. Inc. If the length is altered. Insulation Conductor insulation type. If X1. 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.Ground Grid Design Conductor/Rod Editor (FEM) Label Symbol representing a conductor/rod. used only in the Conductor Editor. Y2. the length is changed accordingly. Operation Technology. and Z2 values are entered. X2. Type Type of conductor/rod material. used only in the Conductor Editor. 28-28 ETAP PowerStation 4. Size Conductor cross-sectional area in AWG/kcmil or mm2. Length Length of the conductor/rod in m/ft. used only in the Rod Editor. Z1. Y1. X2. cost in $/rod for a rod. 28. or T-Shaped ground grids. 28-29 ETAP PowerStation 4.12. IEEE Std 80-1986. Tolerable Step and Touch Potential Limits. 28. Ground Potential Rise. and assumes that the grounding system is an equipotential structure. the program determines the minimum number of conductors that satisfy the tolerable limits for the Step and Touch potentials. L-Shaped.1 Finite Element Method The Finite Element method (FEM) is based on a method of images. IEEE Std 80-1986. This optimization function is for IEEE Std methods only.3 Optimization of Conductors For a fixed number of ground rods. and increases the number of conductors (keeping the mesh almost square) until a solution is reached. and 4 rods. The uniform or twolayer soil view is also used with the FEM method.2 IEEE Std Methods IEEE Std 80-2000.0 . Operation Technology. Ground Resistance. or IEEE Std 665-1995 is used only for the Square/Rectangular shapes of ground grids. Triangular.12. This optimization function is for IEEE Std methods only. the number of rods and conductors is increased based on their cost effectiveness in reducing unwanted potential levels. The GGS program begins calculations with a grid consisting of only two conductors on each side.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. With each iteration. 28. two parallel conductors vertically. The program begins the optimization routine with a minimum of two parallel conductors horizontally. IEEE Std 80-2000 can be used for Square/Rectangular. Inc.12. or IEEE Std 665-1995 is optional for the calculation of Step and Touch (mesh) Potentials.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.Ground Grid Design Calculation Methods 28. 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. Surface Layer Derating Factor. Cs For IEEE Std 80-2000 Cs = 1 − 0. Estep and Etouch For body weight of 50 kg Estep 50 = (1000 + 6Cs ρ s ) Operation Technology. For IEEE Std 80-1986.96 ∞ Kn 1 + 2∑ 2 n =1 1 + (2nh / 0. Tolerable Step Potential. IEEE Std 665-1995 Cs = 1 0. Inc. 0. Decrement Factor.09 Where hs is the thickness of the surface layer in meter.0 . K K= ρ − ρs ρ + ρs Where ρ is the resistivity of the earth beneath the surface material in ohm-m. Reflection Factor.08) s Cs is 1 when K=0.5 Fundamental Formulas Some fundamental formulas are given below.Ground Grid Design Calculation Methods 28.116 ts 28-30 ETAP PowerStation 4. ρs is the surface layer soil resistivity in ohm-m.12.09(1 − ρ / ρ s ) 2hs + 0. Df Df = 1+ ( Ta − 2t / T 1− e f a tf ) Where Ta is the equivalent system subtransient time constant in seconds. IG I G = S f C p D f (3I 0 ) Operation Technology.5Cs ρ s ) 0.0 .Ground Grid Design Calculation Methods Etouch50 = (1000 + 1.116 ts For body weight of 70 kg Estep 70 = (1000 + 6Cs ρ s ) 0. 28-31 ETAP PowerStation 4.157 ts Etouch 70 = (1000 + 1. Inc.5Cs ρ s ) 0.157 ts Maximum Grid Current. Inc.’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 Operation Technology.Ground Grid Design Required Data 28. A summary of these data for different types of calculation methods is given in this section. the following related data is necessary: Soil Parameters. Grid Data. 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.0 . and System Data. 28-32 ETAP PowerStation 4.13 Required Data To run a Ground Grid Systems study. 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 Operation Technology. Y and Z Coordinates of One End of Rod X. Y and Z Coordinates of Other End of Conductor Cost Rod Data (FEM model only) • • • • • • Material Type Insulation Diameter X.0 .’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. Inc. 28-33 ETAP PowerStation 4.Ground Grid Design Required Data Rod Data (IEEE Std. 0 . 28. Input Page Provides the format for different input data. Complete Page Selects a report format that provides the complete output report. 28-34 ETAP PowerStation 4.14 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.14. The Ground Grid Systems Report Manager consists of four pages and provides different formats for the Crystal Reports.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. Inc. Operation Technology.Ground Grid Systems Output Reports 28. Ground Grid Systems Output Reports Result Page Provides the format for different calculation results.0 . The Ground Grid Systems study Crystal Report contains the following major sections: Operation Technology.2 Ground Grid Systems Crystal Report After running the Ground Grid Systems study. 28-35 ETAP PowerStation 4. to open and view the Crystal output report. click on the Report Manager button located on the Study Case Toolbar. or select the Crystal Report format from the Ground Grid Systems Toolbar. Inc. 28.14. Summary Page Provides the summary from the calculation results. project file name.0 . Plot Step (for FEM model only). Short Circuit Current Projector Factor. System X/R Ratio. to unit system. Durations of Fault. Soil.Ground Grid Systems Output Reports Cover Page This is the first page of the Ground Grid Systems study Crystal Report. 28-36 ETAP PowerStation 4. and the output file name and its location. It includes information from the number of conductors and rods. Short Circuit Current. Ambient Temperature. System Input Data This section reports the input data related to the system including the System Frequency. and Conductor Library. and Boundary Extension (for FEM model only). Input Data This section reports the input data related to the System. Operation Technology. Short Circuit Current Division Factor. Average Weight of Worker. Inc. Grid. and Thermal Capacity Factor. Maximum Length of the Grid in the X Direction. 28-37 ETAP PowerStation 4. Material Type. Thermal Coefficient of Resistivity. Number of Rods. Minimum Length of the Grid in the X Direction (Only for IEEE Std 80-2000 L-Shaped or TShaped Grid). Operation Technology. It shows the Material Conductivity. Conductor Library This section reports Conductor Library information. Diameter. Inc. Maximum Length of the Grid in the Y Direction. Average Length. and Cost. Grid Data (for IEEE Std’s) This section reports the input data related to the grid including the Shape. and Lower Layer Soil Resistivity. Ground Conductor Resistivity.0 . Conductor Cross Section. Upper Layer Soil Resistivity. Arrangement. K0 Factor. Surface Material Depth. Grid Depth. Upper Layer Soil Depth. Fusing Temperature.Ground Grid Systems Output Reports Soil Input Data This section reports the input data related to Soil including the Surface Material Resistivity. Rod Data (for IEEE Std’s) This section reports the input data related to the grid including the Material Type. Inc. Potential Profiles. Y and Z Coordinates of One End of Conductor. Diameter. Y and Z Coordinates of Other End of Conductor. X. X. Cost Data Lists the cost data of conductors/rods.0 . and Warning. and Cost. Cross Section. X. Rod Data (for FEM model) This section reports the rod input data for the FEM model including the Material Type. and Cost.Ground Grid Systems Output Reports Conductor Data (for FEM model) This section reports the conductor input data for the FEM model including Material Type. Y and Z Coordinates of Other End of Conductor. Result This section reports the results related to Intermediate Constants. 28-38 ETAP PowerStation 4. Operation Technology. Insulation. X. Insulation. Y and Z Coordinates of One End of Conductor. S ummary. Maximum Grid Current and Warning information are reported. Summary for IEEE Std’s In this section.0 . Derating Factor Df. Step and Touch potentials. Inc. GPR. K2 are reported. Ks. Km. the Ground Resistance Rg. 28-39 ETAP PowerStation 4. Kis. K1.Ground Grid Systems Output Reports Report of Intermediate Constants for IEEE Std.’s In this section the intermediate results Kim. Kii. Reflection Factor K. Operation Technology. if the Report Details box in the Study Case Editor dialog box is checked. 28-40 ETAP PowerStation 4. the three Potential Profiles are reported if the Report Details box in the Study Case Editor is checked.Ground Grid Systems Output Reports Report of Potential Profiles for the FEM Model In this section. Operation Technology. Inc.0 . 0 . GPR. If the Auto Display of Summary and Warning box located on the Study Case Editor dialog box is checked. Step and Touch Potentials.14. Derating Factor Df. click on the Summary and Warning button located on the Ground Grid System Toolbar. this view will open automatically after the Ground Grid Systems calculations are executed. to open the GRD Analysis Alert View dialog box.3 Summary and Warning After running the Ground Grid Systems study. Reflection Factor K.Ground Grid Systems Output Reports Summary for the FEM Model In this section. Maximum Grid Current and Warning information are reported. Inc. the Ground Resistance Rg. Operation Technology. 28-41 ETAP PowerStation 4. 28. 28-42 ETAP PowerStation 4. Display Over Limit Voltage Show areas with potentials exceeding the tolerable limits for 3-D Touch/Step Potential profiles.Ground Grid Systems Plot Selection 28. Touch Voltage Select to plot a Touch Potential profile. 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. open up the Plot Selection dialog box by clicking on the Plot Selection button located on the Ground Grid Systems Toolbar. and are available for Absolute/Step/Touch Voltages. Inc.15 Plot Selection Plots are used only with the FEM method. Operation Technology. This function is disabled when the Contour plot type is selected. Contour Plot a Contour Potential profile for the Absolute/Touch/Step voltage. A set of sample plots is shown below. Step Voltage Select to plot a Step Potential profile. To select a plot.0 . 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. Ground Grid Systems Operation Technology. Plot Selection 28-43 ETAP PowerStation 4.0 . Inc. Inc. Plot Selection 28-44 ETAP PowerStation 4.Ground Grid Systems Operation Technology.0 . Plot Selection 28-45 ETAP PowerStation 4. Inc.0 .Ground Grid Systems Operation Technology.