Improving the Goodness of Fit in RESAP (KB - 228)

Computing the Potential at Points Close to a Conductor (KB - 230)

Working with AutoCAD DXF files

Modeling a Grounding Grid with a Known Impedance in HIFREQ (KB - 290)

Editing Conductors that are Part of a Group in SESCAD (KB - 316)

Intelligent Textboxes in SESCAD.

Output Toolbox: MALT, MALZ and HIFREQ

Improving the Goodness of Fit in RESAP (KB - 228)

Question: Which parameters can I change to improve the goodness of fit in RESAP? I tried varying the "ITERATIONS" option, but nothing changed.

Answer: The "ITERATIONS" option in RESAP controls the MAXIMUM number of iterations that the program is allowed to make before stopping. Usually, the computation converges before that limit is reached, therefore increasing this number by itself will not result in the program making more iteration steps. To increase the number of iteration steps, it is best to decrease the step size. This is described in more details in the on-line help for the OPTIMIZATION screen.

The goodness of fit reached by the program should not be affected strongly by those choices (as long as convergence is reached). What really governs the goodness of fit is whether or not the data can be represented by the selected soil model. Often, the best approach to decrease the RMS error is to use more layers in the soil model.

Computing the Potential at Points Close to a Conductor (KB - 230)

Question: When I try to compute the earth potential at points very close to the surface of a conductor, I sometimes get values that are larger than the GPR of the conductor. What is wrong?

Answer: The potential very close to a conductor's surface is strongly affected by the detailed current distribution in the vicinity of the observation point. This, in turn, is usually strongly affected by the length of the conductor segments (especially at higher frequencies).

The boundary condition satisfied by the scalar potential is imposed only at the mid-point of the conductor segments. In between those points, the boundary condition may or may not be well satisfied, depending on the nature of the current distribution. Therefore, the computation of the scalar potential at the surface of a conductor at a point other than its mid-point can be inaccurate in some cases. Note that this does not affect the computation accuracy for points which are located further away from the conductor: the scalar potential at points located at a distance equal to a few conductor radii away from the conductor is mainly affected by the average current distribution in the system, and is not very sensitive to the exact position of the observation point.

The question is, then, how short should the segments be in order for the potential at any point at the surface of a conductor (or close to it) to be accurate? In extreme cases, the length of the segments should be as small as the distance of the observation point from the axis of the conductor. This is usually overkill and is certainly not practical as it would generate too many conductor segments. In practice, the conductor segments can usually be considerably longer than this, but it is difficult to say by exactly how much. Basically, the segments should be short enough that the basic assumption in MALZ (to the effect that the leakage current is uniform along a segment) is satisfied to the desired accuracy. This, in turn, depends strongly on the presence of intersections of conductors or of "floating nodes" (where the current distribution tends to vary more rapidly) close to the observation points as well as on the type of soil (to a lesser degree).

It is difficult to give more explicit guidance without going into a detailed analysis of the conductor network. One approach that can be used (and should always be used whenever a network segmentation issue arises) is to repeat the calculations using a finer segmentation, to verify the stability of the results.

Working with AutoCAD DXF files

Question: How to convert my grounding design from an AutoCAD DXF file to a working MALZ or MALT file?

Answer: The process involves the following two steps:

Step 1: Create a DXF file containing the grounding grid that can be cleanly imported into the SESCAD.

The key factor in this process is to ensure that all grounding grid conductors are drawn as Line or Polyline objects in your AutoCAD file. To avoid unnecessary import errors and cleaning work after importing, create a DXF file (from the AutoCAD drawing) that contains only the grounding grid, remove all other contents in the DWG file before you create the DXF file.

Since the ground rods in the DGW file are often not drawn as Line or Polyline, their locations are lost after the DXF file is imported. To transfer this info into SESCAD, mark the locations of the ground rods in the DWG file by drawing an “X” with Lines at the ground rods location. The ground rods can then be created easily in SESCAD by using the Create Rod under the Power Tool in SESCAD.

You should also note that units specified in the DXF file are recognized during the import and converted to your preferred units specified in SESCAD.

Now you can easily import your DFX file by selecting Import… under the File menu in SESCAD.

Step 2: Finalize the grid for MALZ or MALT.

After the DXF file is imported, perform the following steps to restore the integrity of the grid, i.e., make sure all grid conductors are connected properly and remove any overlapping and short conductors:

1. The lines representing the grid conductors drawn in AutoCAD are often not correctly snapped together. You will find gaps or very short extensions at points where two lines should be connected. The Power Tool (in particular, Extend and Join to end) and the Network Diagnostic Tool in SESCAD are excellent tools for resolving these problems.
2. Assign conductor characteristics and adequately subdivide the grounding conductors. The characteristics and subdivision of grouped conductors can be assigned by simply right-click on the grouped conductors and choose the Characteristics.
3. Scale the grid according to its actual length. The grid can be scaled or moved by using the Scale Objects and Move Objects under the Tools menu in SESCAD.
4. Move the grounding grid to the actual burial depth;

Although the Network Diagnostics in the SESCAD has reported zero overlapped and short conductors, you may still have run-time errors after submitting your MALZ or MALT run, which states that overlapping or very short conductors detected. The short conductors generated after the conductor subdivision in the MALZ or MALT run cause these errors. Go back to the grounding grid and make sure the conductors in error are properly connected to other conductors by using the Extend or Join to end under the SESCAD Power Tool. Note that the “bad” connection may look good on a large scale, but it becomes obvious once you zoom into the area.

Use the Find Object By Index under the Tools to locate the conductors in error. Be sure all conductors are not grouped, otherwise the conductor number in the SESCAD file does not correspond to the one in the MALZ or MALT file (you can work with a temporary copy of your original file with all conductors ungrouped).

You may also wish to label all conductors by selecting the Show Conductor and Profile Number under Options/Active View Options in the SESCAD.

If you are working with MALT, keep in mind that all conductors under Main Ground, Return Ground or Buried Structures are electrically connected even though there are gaps between them (unlike MALZ program). There is no need to physically close the gaps in the SESCAD, but you still need to remove overlapping and short conductors.

Details of the process described above will be provided through an example in the upcoming CDEGS Users’ Group Handbook.

Modeling a Grounding Grid with a Known Impedance in HIFREQ (KB - 290)

It is often necessary in HIFREQ (for instance, when modeling long transmission lines) to create several low impedance grids in a network. Often, the impedance of each grid is known, but their precise geometrical configuration is unknown. While it is possible to simulate the grids using conductor models, this requires some guesswork to find a grid that yields the correct impedance for the type of soil under study. This can also require a lot of conductor segments, and could quickly exceed the program’s capacity.

There is another way to model a grid of known impedance. It is possible to simulate it using a combination of a GPR energization and a "lumped" impedance. Such a model does not reproduce the earth potentials and electromagnetic fields correctly in the area immediately surrounding the grid: the grid will effectively be reduced to a single point. Moreover, the current injected into this "grid" will simply disappear, and will therefore have no effect on surrounding buried structures. This approximation should therefore be used only to model a "distant" grounding grid which is expected to have no effect on the potential and electromagnetic fields near the structures of interest.

The approach consists in using two conductors, the first one energized with a GPR energization at 0 Volt, to simulate a remote ground, and the second one carrying a lumped impedance with the desired value. As usual for a GPR energization, the origin node of first conductor should be floating. Then, one end of the conductor carrying the impedance (as specified using its CONDUCTOR-TYPE) should be connected to the end node of the first segment, and the other should be connected to the network.

Note that it isn't necessary that these two conductors be buried: they can very well be located above ground.

Editing Conductors that are Part of a Group in SESCAD (KB - 316)

When modeling large conductor networks in SESCAD, it is often convenient to group related conductors together to facilitate editing operations. It is not uncommon for large network files to contain dozens of groups. Eventually it becomes more and more difficult to select an individual conductor that is part of a group, especially if that group is itself part of a group, etc…

There are two techniques that you can use to select individual conductor segments that are part of a group:

1. Ungroup the conductors. After selecting the group, use the Edit, Ungroup operation. If the desired conductor is still part of a group, you will have to repeat this operation until you can select the conductor individually. A disadvantage of this method is that the grouped objects become ungrouped, and it becomes a lot more difficult to manipulate them as a unit.
2. Change the Active Object. Another option to select the conductors of the group is to change the "Active Object". SESCAD can only edit one object at a time. In its normal mode of operation, this object is the "MAIN" electrode. However, when a group is selected you make it the object to be edited by using Advanced, Set As Active Object. When this option is used, the content of the selected group becomes editable, while the rest of the document becomes un-editable (it appears in grey). You can repeat this operation whenever you encounter another group. To return to the previous "Active Object", use Advanced, Set Parent As Active Object.

Another option that will be available in the coming release of SESCAD is to temporarily suspend the interpretation of groups by the program. This new option will allow you to temporarily interact with the network as though no groups were defined. Once the required operations are complete, you can turn off this option to restore the groups. Could this be referred to as turning grouping on or off rather than turning the feature off to turn on grouping? Also, is this feature available in GDEGS 2002 April release, if so we should state this explicitly?

Note that it is often unnecessary to go through the above procedures if all you want is to access some basic information about the conductors or profiles, such as their length, coordinates, or other characteristics. Much of this information is available in the Quick Info tool (Tools, Quick Info). With this tool activated, simply move the mouse pointer over the conductor or profile of interest, and wait. After a short delay, the program will highlight the conductor or profile in a different color, and display the some information about it in the Quick Info window.

Intelligent Textboxes in SESCAD.

In the SESCAD program, all the text-boxes can act as simple calculators: they can be used to perform basic arithmetic operations. This can be helpful when the data you have to enter can be more easily expressed as a formula than as a number.

One example that occurs frequently is the specification of a scaling factor (“I have a conductor that is 7 m long and I need it to be 12 m long.”). The necessary scaling factor can be specified as 12 / 7 in the textbox.

The text boxes understand the basic arithmetic operations (+, -, *, /) and arbitrary levels of parentheses. The calculation will be performed, and the result displayed when the cursor is moved away from the text box. When a number is entered in this form, it will be computed (and the computation results displayed) the moment the cursor is moved to another part of the screen. You can also force the computation of the formula by pressing the Ctrl + R key combination.

In the next version of SESCAD, the textboxes will also be able to understand units. When entering data in the SESCAD program, you are often expected to provide the information in specific units. For instance, you may be expected to enter the X coordinate of an observation point in meters. What if you have this information in feet?

If all of the data you have is in Imperial units, then you can use “Advanced | Units and Other Settings” to define Imperial as the default system of units. But if only some data is in a different system of units then the default one, then you can simply type the name of the units after the numerical value to have the program convert it to the default units.

For example, if ‘243 ft’ is entered in a textbox expecting the data in meters, the value will be converted to meters as soon as the cursor moves to a different part of the screen, or when you use the Ctrl + R keyboard shortcut.

Output Toolbox: MALT, MALZ and HIFREQ

Configuration plot, color labelling: When configuring your plot in the output toolbox, it is possible to label certain quantities, like current and GPR, with color and/or text. The new color labelling option displays conductor segments using different colors, where each color corresponds to a range of values. The number of color levels and their values can be customized.

Quantities such as conductor radius, conductor and coating types also can be displayed using color labelling. The number of color levels in this case corresponds exactly to the number of distinct conductor radii/conductor types/coating types defined in the model.

Computation Spot-2D plot: The Spot-2D plot engine has been improved considerably. It allows users to create their plot with great flexibility by defining:

• Differently spaced and differently oriented observation surfaces
• Many separately defined profiles
• Many individually defined observations points.

In other words, all possible combinations of observation profiles can be displayed on the same plot as long as they lie in the same geometrical plane.