TUTORIAL EXAMPLE

In this section we will show how to build a case and exercise all of the functionalities supported by the substation grounding grid design program. For this purpose, we will use the example shown on Page 139 of the IEEE 80-2000 standards.

The substation to be analyzed is a L-shaped grid shown below:

Start the substation ground grid design program as described in the previous section. Select “New” and give a file name to the project, let’s use “ieee80-2000-example-b4”.

Figure 30: Opening a New Job File, IEEE80-2000 -- example b-4

Next, let’s provide general substation data for this example. Here we have selected “Metric” system of  units as shown below:

The fault current given in the IEEE standard is 1908 amps (after considering current division factor). The surface material is 4 in crushed rock with resistivity of 2500 ohm-m.

Figure 32: General Substation Data Dialog, IEEE80-2000-example-b4 Based on the IEEE standard, the soil is homogenous with resistivity of 400 ohm-m.

Next, we specify a land size of 75 x 108 that is sufficient for better visibility of the L-shape grid.

Figure 34: Defining the substation Grid Land Size, IEEE80-2000-example-b4

The substation grounding grid will be built in two steps. First we will build lower part of the L-shaped using “Regular Grid Data”. The size of the first part of the grid is provided below.

Next, the program will prompt for data entry for grounding rods. At this time, we will select “Cancel” since the ground grid is not yet completed. We will come back to adding rods later.

Figure 36: Entering Regular Grid Data – Screen 1, IEEE80-2000-example-b4

Figure 37: Entering Rectangular Grid– Screen 2, IEEE80-2000-example-b4

Now we will build the upper part of the L-shaped grid. Please note by using the data entered in the dialog shown below, one grid conductor will be superimposed on a segment previously entered. This is a segment running from (0,35) to (70,35). This is not allowed, the program does support grid segments/rods that are partly/completely superimposed. Fortunately, the user interface detects this condition and ignores the latest superimposed segment.

Figure 38: Entering Rectangular Ground Grid Data – Screen 3, IEEE80-2000-example-b4

Figure 39: Entering Rectangular Ground Grid Data – Screen 4, IEEE80-2000-example-b4

 As seen in the figure below, the L-shaped ground grid was assembled in two easy steps. Therefore, it is easy to build irregular shaped grid using several “Rectangular Grid Data”.

Next, we will add the ground rods. Select “Add Multiple Ground Rods” icon as shown below. This works very similar to the “Rectangular Grid Data” but instead rods will be places at the corners of the grid defined, let’s see how.

Figure 41: Adding Multiple Ground Rods- Screen 1

 Again, we will place the ground rods in several steps. There are many ways to achieve this. We proceed as follows. First let’s place the rods in the lower right corners (please see Figure 29 or IEEE 80-2000 page 140 for reference). Note that the rod length is 7.5 meters given in the standard.

The result of adding rods based on the above data is shown below:

Figure 43: Adding Multiple Ground Rods- Screen 3

Next, we will show how a single rod can be added. Select “Add Single Ground Rod” icon as shown below:

Figure 45: Adding Multiple Ground Rods- Screen 5

The program will read the location of the mouse automatically and rod data dialog will appear (see figure below). If the rod coordinates are not exactly pickup by the program, the user has another chance to make modification to the rod coordinates. In our case, the program pick up exact location as desired. However, we need to modify rod Z coordinate and length.

 As

 As shown shown below, below, we we were were able able to to add add one one rod rod at at a a specified specified location. location. Next Next let’s let’s add add rods rods to to top top andand bottom rows. Again, we select “Add Multiple Ground Rods” and rods data dialog for adding these rods is bottom rows. Again, we select “Add Multiple Ground Rods” and rods data dialog for adding these rods is shown below:

shown below:

Figure 47: Adding Multiple Ground Rods- Screen Figure 47: Adding Multiple Ground Rods- Screen 77  As seen in the fi

The rods in the middle of the L-shaped grid, are also added in a similar fashion as seen in the rods data The rods in the middle of the L-shaped grid, are also added in a similar fashion as seen in the rods data dialog shown in the figure below:

dialog shown in the figure below:

Figure 49: Adding Multiple Ground Rods - Screen 9 Figure 49: Adding Multiple Ground Rods - Screen 9 The eight rods, in the middle, are added as specified and

The eight rods, in the middle, are added as specified and are shown below:are shown below:

Figure 50: Adding Multiple Ground Rods - Screen 10 Figure 50: Adding Multiple Ground Rods - Screen 10

Now we will see how “Add Multiple Ground Rods” can be used to place rods along an axis (path). Note Now we will see how “Add Multiple Ground Rods” can be used to place rods along an axis (path). Note that to add two rods

that to add two rods in the lower left side of the grid, we have in the lower left side of the grid, we have provided the following data. It is important toprovided the following data. It is important to note that we have entered

note that we have entered Length to be zeroLength to be zero since the rods needs to be added along y-axis only (tosince the rods needs to be added along y-axis only (to place rods along an axis which is parallel to x-axis, then, the

place rods along an axis which is parallel to x-axis, then, the Width should be zero)Width should be zero)

Figure 51: Adding Multiple Ground Rods - Screen 11 Figure 51: Adding Multiple Ground Rods - Screen 11

The two added rods are shown in the

There is only one more rod left to be added. This rod should be placed just below the upper right corner.  Again, let’s select “Add Single Ground Rod” and click right mouse button at the rod location (see figure

below):

Figure 53: Adding Multiple Ground Rods - Screen 13

The program picks up the coordinates of the rod correctly as shown below. Note that the Z coordinate of  the rod and its length should be modified as shown below.

The last rod is now added as seen in the figure below:

Figure 55: Adding Multiple Ground Rods - Screen 15

Now, we have completed building of the ground grid assembly (grid conductors and rods).

The ground surface potentials can be calculated by selecting the “3-D Potential and EquiPotential Contour Lines” icon as shown below:

Figure 57: Plotting 3-D Potential and Equipotential Couture lines – Screen 2

The computed equipotential lines are shown below. Note that we selected that equipotential to be computed for touch voltage and not absolute voltages.

To obtain a 3-D view just select click left mouse button on the “3-D Potential Graph” icon as shown below:

Figure 59: Touch Potential and 3D Graph

 At this point before we see how potentials along an axis can be computed, let’s see how maximum allowable touch and step voltages can be computed. Select “Analysis->Allowable Touch and Step Voltages” as shown below (from the menu bar items):

Based on the general information provided earlier (body weight, fault duration, surface resistivity, upper  layer resistivity) the program can compute allowable touch and step voltages. Press “Calculate” button and these quantities will be calculated and displayed. The maximum allowable touch in our case is 840 and maximum allowable step is 2696 volts. The reported values by IEEE 80-2000 are 838 and 2686 that are in excellent agreement (see page 132 of IEEE standard).

Figure 61: Calculate Allowable Touch and Step Voltages

Now that we have computed the maximum allowable touch and step voltages, the computation of voltage profile along an axis can be directly compared with these values. To compute this profile, select “Potential along an Axis” icon as shown below:

Once this icon is selected, the mouse will turn into a pointer (pen like) object as seen in the below figure.

Figure 63: Potential Along the Axis Calculation – Mouse Shape

Click mouse button at beginning of the axis and drag the mouse to the end of the desired axis and then release the mouse button.

potential should be absolute, touch or step. If step potential along the specified axis to be computed, then, it is necessary to enter “Spacing”. Normally this spacing for step voltage calculation is 1.0 meter.

Figure 65: Potential Along the Axis Calculation – Define an Axis Continuation

The computed step voltage profile along the specified axis is shown below. Notice that maximum allowable touch and step voltages are also shown in this figure for easy identification of violations (where voltage exceed the allowable value).