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Line Optimization

Line Optimization is one of the most powerful features of PLS-CADD. It will provide the least-cost solution for spotting a given set of structures for a desired set of design criteria on any desired route. This TechNote is intended to detail the steps that should be done in order to perform an optimization as well as features of the optimization process and even some advanced features after the optimization is complete.

The first step to an optimization is that you must have your route selected in PLS-CADD. Route selection is beyond the scope of this TechNote and is covered in the manual as well as other TechNotes. Please keep in mind though that you could have several projects utilizing the same data in order to optimize numerous routes to get cost comparisons between alternative routes.

Route Selection in PLS-CADD

The next step to optimization preparation is that you should have the clearances required for the various voltages that you will be using on your line in your Terrain / Feature Code Data / Edit tables. This is the only way to insure that the proper clearances will be met after your optimization is complete.

Several items in your in your Criteria settings should be verified. Insure that the Criteria / Survey Point Clearances table has been completed with any and all conditions that you want to insure that the proper clearances as defined in your Feature Code tables above are maintained - don’t forget to include any cold and/or uplift conditions listed in your Survey Point Clearances in the event that your line design will be going under any aerial obstacles. Select and note the four weather conditions in Criteria / Insulator Swing. It is also strongly suggested that the Criteria / Automatic Sagging table be completed with all the possible desired stringing limitations for the conductors on your line. It will be much easier to allow the program to automatically select the tensions during the actual optimization process than trying to figure out an actual stringing tension at that time.

Now for the most important step in line optimization; the designer must model structures for use in the optimization. In order to do this, one must understand the different methods in structure modeling for PLS-CADD.

Again, the full explanation of these various methods is beyond the scope of this TechNote, but they can be found in Section 8.3 of the PLS-CADD Users Manual. But an important concept to understand is that PLS-CADD can currently only optimize with Method 1 or Method 2 structure models. Unfortunately, computers are just not fast enough yet to allow the millions and millions of structure analyses required for the optimization with Method 4 structures. Perhaps one of these years they will be, but for the current capabilities, Method 1 or Method 2 structures must be used. For the remainder of this TechNote, it will be assumed that the designer will be using Method 1 structures for optimization. There will be a brief discussion at the end of the TechNote for those that wish to utilize the more advanced Method 2 structures.

Of course one could just model Method 1 structures directly using the Structures / Create New Structure function in PLS-CADD (see Appendix F in the PLS-CADD Users Manual.) However, this method has many shortcomings as we will see later, and most designers would be calculating their allowable spans from PLS-POLE and/or TOWER anyway. Which finally brings us to the primary purpose of this TechNote; how do we optimize a line with structures that we have modeled in PLS-POLE and/or TOWER? The process to do this will now be outlined step-by-step. In this process, we will assume that the designer is using wood poles in PLS-POLE, but keep in mind that the process will be nearly identical for any other material of poles or even structures modeled in TOWER.

  1. Model your tangent structure in PLS-POLE. It is suggested that you use your shortest and smallest wood pole for this base model. For further discussion purposes, let’s call this model TANGENT.POL.

    Base Tangent Structure in PLS-POLE

  2. In PLS-POLE, go to Geometry / Wood Poles, click on the Multiple Pole Selection button at the bottom of the dialog box, and select the heights and classes of poles that is desired to be used for the entire tangent structure family.

    Selecting the Desired Heights and Classes of Poles

  3. Insure that you have completed the Geometry / PLS-CADD / Insulator Link table with the desired Sets and Phases for the various wires that will be strung on the structures.

    Selecting the Insulator Links

  4. There is no reason to define any loads for this structure, so it is suggested that the designer not even define anything under the Loads menu item.
  5. Save the structure (TANGENT.POL) and close PLS-POLE.
  6. Open PLS-CADD Lite (if you have PLS-CADD, you can just open up PLS-CADD).
  7. Select File / New / PLS-CADD Lite from the menu, and give it a name – for our discussion here, we will call it TANGENT.LOA (don’t forget to add the “LOA” extension when typing the name so that the program will know you are in Lite mode.)
  8. For the Lite Options;
    1. Select the Criteria File (*.CRI) from your PLS-CADD project that you are going to optimize for the Criteria Options.
    2. Select “Use a predefined structure file” for the Structure Options.
    3. Select only the “Enter span Azimuth, Length, and Elevation change” option for the Span Geometry Options.
    4. Select only the “Have program calculate maximum permissible tension based on limits in the criteria library” for the How do you want to Sag wires options.

      Selecting the PLS-CADD Lite Options

  9. Click the OK button and then find and select the TANGENT.POL model from step 5 above.
  10. Modify the Model Setup table to select the conductors at each attachment point on the structure as well as any other changes that might be desired. It is suggested that the Span Horizontal Projection be modified to the approximate spans anticipated on the line (don’t get too detailed on this – many designers use the anticipated Ruling Span for this number). It is further suggested that the Ruling Span column be set to “0” for all spans (this forces Lite to use the actual span length for the Ruling Span). There are several other TechNotes as well as all of Chapter 15 in the PLS-CADD Users Manual if you have any further questions on using Lite.

    Selecting the Proper Conductors in PLS-CADD Lite Line / Setup

  11. Click OK on the Model Setup dialog box and you will be presented with a Profile and 3D view of the structure with the conductors strung and displayed as defined in the previous step.


  12. Creating Method 1 structures from this model now is as easy as selecting the Structures / Generate Allowable Spans File from the menu.

    Generating Allowable Spans Files in PLS-CADD Lite

  13. In the case of our wood pole example, select “No” when prompted to run all structures in an available structure list. (In the event of using multiple TOWER or PLS-POLE models, you could select “Yes” and select the available structure list from your project to run them all at one time.)
  14. You will then be presented with an explanation that all pole heights and classes selected in the Multiple Pole Selection in PLS-POLE (see Step 2 above.)
  15. The designer should then complete the Wire Loads Parameters as desired.
    1. For a true tangent, the minimum and maximum line angle would be left at 0 degrees.
    2. Some tangents are designed to handle even small line angles, which can be used for the maximum line angle.
    3. Many designers design tangents for some sort of small angle to account for construction tolerances, which can be used for the minimum line angle.
    4. It is very common for designers to use the same numbers for both the minimum and maximum line angles for tangents to address both of these issues (say 0.5 degrees).
    5. In the case of an angle structure, the minimum and maximum angle can be specified, and the increment option will calculate varying spans based on each increment of line angles. It is common for designers to just use the maximum line angle for both the minimum and maximum line angles to duplicate existing design practices.
    6. A major limitation of the archaic wind and weight span concept is that only one Wind to Weight Span relationship can be used. Set too small for the terrain, grading will be very flat and many taller structures will be selected than required due to the limiting vertical factor. Set too large for the terrain, many spans will be limited based on the vertical capacity. The Weight / Wind Span Ratio used is best selected based on the designer’s personal experiences for their regions. In very flat areas, one might want to use a small number such as 1.1 or 1.2 to get the most out of their span lengths, while those in mountainous regions may want to use 1.5 or even 2.0 in order to allow more variations in grading and keeping their structures as short as possible. (Note, creating Method 2 Interaction Diagrams will allow the use of all Wind/Weight Span ratios.)
    7. Minimum weight span is a way to allow the user to define the uplift capacity, if any, of a structure. Leaving it on the default “0” will not allow any uplift on a structure. Some designers like to use a small distance on this to always require some vertical force be used; one example of this might be on a post insulator line where trunion clamps are used. In the case of a dead end structure, a designer would probably want to allow uplift on a structure, in which case a negative distance would be used.
    8. For the Wind span search limit, this should probably be left on “0” for tangent and self-supporting structures. On guyed structures, particularly in-line guyed structures, a change of span length has very little effect on the structure (as the tensions are usually limited), so a reasonable number might be desired to be used here in order to keep the program from spending precious time calculating excessively long spans that will never be used on the line.
    9. Finally, three loading conditions should be selected that will match the three weight spans desired. Keep in mind that the Load Case Conditions that can be selected come from the Criteria / Structure Loads and the three weight span conditions are those that are specified in Criteria / Weight Spans. If there isn’t a Structure Load case that corresponds with a desired Weight Span condition, the designer will need to add one. Additionally, the direction of the structure should be paid particularly close attention to. The Line Angle in the LIC Parameters is assumed to be positive (i.e. to the right). On some structures (such as the one shown in this example), the orientation of the structure can create different allowable spans. The orientation that results in the worst loading condition should be chosen. In our example structure, orienting the structure with the side with the two braced post insulators to the left will actually HELP the structure. Therefore, it should be oriented so that the side with the two braced post insulators are oriented to the right. The optimization will automatically check both positive and negative wind directions, so selecting the positive wind direction Load Case Conditions is sufficient.

      Selecting the Wind and Weight Span Parameters in PLS-CADD Lite

    10. Click OK and you will now be presented with a dialog box with the various poles selected in Step 2 as well as the ability to save the various structure models with a desired name and location. The code options will allow you to use various parameters in the resulting structure file names. It is merely this authors suggestion that the height of the structure be used as the extension and the option to add .POL to all the files unchecked. Note the folder that you are saving of these structures to as they will need to be later picked for the optimization process (you can change the folder by simply clicking on the folder name button).

      Selecting the Structure File Names in PLS-CADD Lite

    11. Click OK and go get a cup of coffee. Depending on many parameters (the number of structures, the number of load cases, the range of angles selected, linear or nonlinear structure analysis method chosen, etc.), this could take just a few seconds or many minutes. A progress bar will indicate the stage in the process that it is in.
  16. When complete, save the Lite project so that if there is a change in the weather or loading conditions during the project, the allowable spans files can then be recreated very easily.
  17. Repeat the above steps for each of the members of the structure family that will be used on the project.
  18. Close out of PLS-CADD Lite and all PLS-POLE models that are open.

Now, go to PLS-CADD, go to Structures / Available Structure List / Add/Delete Structure to add all of the Method 1 structures and then Structures / Available Structure List / Edit to assign costs and to determine the other variables for optimization. The four variables that must be determined for each structure are the cost to use for optimization, whether or not use the structure for optimization, the set for optimization, and the minimum line angle.

Adding Structures to the Available Structure List in PLS-CADD Structures / Available Structures List / Add/Del Structures

Editing the Available Structure List in PLS-CADD Structures / Available Structures List / Edit Structures

If you want to automate the pricing, make sure in PLS-POLE you go to Geometry / PLS-CADD / Material Options (and Edit User Entered Material) to build your materials. Then, back in PLS-CADD, in Structures / Available Structure List / Edit, the automated cost from the materials will be available and you can just copy/paste that column to use the developed cost for the optimization cost.

Now that we have told PLS-CADD what structures are available for the spotting and the cost basis for doing so, the second process that needs to be completed before optimization is determining where structures can and cannot be placed, and extra costs associated with doing so, and where any specific structures must be used. These constraints can all be added (and deleted) under Structures / Automatic Spotting / Spotting Constraints. It is the authors opinion that the / Add (graphical) function should be used as this allows the constraints to be determined much quicker than the alternate Edit (table based) method. (Hint; don’t forget to use the Dynamic Pan/Zoom function (hold the Shift key down and move the mouse) to “fly” around the project to quickly add your constraints.) It is also suggested that constraints be added in both the Plan view to find the obvious constraints such as roads, but also in the Profile view to look for areas where constructability of the line should be considered. A tip that is often not thought of is the use of negative costing for Extra Cost Zones; using a negative price for a region will make that region appear more favorable for spotting. This will not necessarily force a structure to be spotted in this region like the Required Structure constraint will, but it will dictate that structures will gravitate to this area if that is the most economical option. A common misconception is that Optimization is only for rural projects; this couldn’t be further from the truth. With the ability to quickly determine constraints, optimizing in urban areas not only still develops the least cost option, but is also much easier than manually determining the structure locations and heights in these highly constrained areas.

Adding Constraints in PLS-CADD

TIP! It might be a good idea to add the Graphical Constraint functions to a Custom Toolbar Button by using View / Edit Customizations / Custom Toolbar. Make sure you have the Custom Toolbar turned on under View / Toolbars / Custom Toolbar.

Custom Toolbars for Constraints in PLS-CADD

Once the Allowable Structures and the Spotting Constraints have been determined, you are now ready to optimize. Chapter 14 of the PLS-CADD manual covers the actual optimization process in depth, so it will not be covered here in this TechNote. However, a couple of extra “tips” will be discussed here;

  1. Automatic Sagging should ALWAYS be used. If you don’t have the proper constraint determined yet, go add it under Criteria / Automatic Sagging.
  2. Optimization is an iterative process. It is suggested that a Station Spacing of 100 feet (the default) be used to get the first “rough” optimization and to aid the designer in quickly determining if any design mistakes have been make (i.e. one of the most common being that you don’t have any structures capable of spanning a Prohibited Zone or not having any structures capable of a certain line angle.) Once the designer is comfortable with the “rough” optimization, the “final” optimization can be performed and a Station Spacing of 25 or even 10 feet could be used. Keep in mind though that the Station Spacing does not greatly impact the final spotted cost of the line. Cutting it down to say 1 foot may seem like the best thing to do, but the designer must determine if it is feasible to spend hours in the design process to save an additional $100 over say a 25 foot Station Spacing solution.
  3. It is suggested that only the Forward solution be selected unless the “rough” optimization is not selecting a desired spotting pattern in a difficult area. Then, the Reverse option could be selected. If you desire to spot Both and then compare the two optimizations, be sure and deselect the “Merge” option so that the Reverse does not overwrite the Forward solution. You will then have to use the Lines / Edit function to make either line active and then hide or even delete the undesired spotting solution.
  4. It is strongly suggested that the option to “Respot if ruling span more than 5% off” is selected. It is not necessary that this be done during one of the first “rough” optimizations, but it should definitely be selected when the final optimization is being performed. If this option is not selected, the optimization will be based solely on the user entered Ruling Span and not the actual Ruling Span after the optimization is complete. While other programs will make this appear that this solution is valid, PLS-CADD actually recalculates the actual Ruling Span for each section, which will lead to conductors being slightly too tight and some minor clearance violations (depending on whether the actual Ruling Span was longer or shorter than the assumed Ruling Span.) Selecting the Respot option will ensure that your final design meets all of the proper design criteria.

Selecting the Spotting Parameters in PLS-CADD

Once the optimization is complete, you can then use the Sections / Automatic Stringing to string all of the other sections of wire, and then the Sections / Table to modify them. It is suggested in the Sections / Table that the Sort Sections By be set to "Attachment Set Section Starts Upon" for easier editing. If this is done, the properties for the first wire in each Attachment Set can be input, and then that row can easily be copied and then pasted for the remaining Attachment Sets in the line. This means a 1000 span line could be completed nearly as fast as just a 1 span line. (It is assumed that the designer has followed the recommendations of using the same set number for all like wires for all structures in a family in order for these options to be used most effectively. If this has not been done, then Sections / Modify / Edit Stringing might need to be used to ensure that the wires are connecting properly to all the structures on the line.)

Note that you can select the "AutoSag" command in the Command To Apply column to have all wires sagged automatically so that they reflect any changes you made in the Sections Table.

Finalizing all sections of conductors in Sections / Table Edit in PLS-CADD

If you have used PLS-POLE and PLS-CADD Lite as described above to create your Method 1 structures for optimizing, you can hit the F1 key, choose Use Method 4 Structure Check/Edit When Possible, and you can then check your structures via PLS-POLE instead of the simplified wind-weight spans. Running a Lines / Reports / Structure Usage before doing this will show you the structure usage based on Wind and Weight Span limits. Switching to Method 4 and rerunning this report will show you how conservative the Wind and Weight Span design method is. If the designer feels that this is too conservative for their project, then one might consider using Method 2 (Interaction Diagrams) structures. This is a bit more complex and takes additional time to calculate all of the Interaction Diagrams, but the process is nearly identical as above and can easily be done with both TOWER and PLS-POLE structures.

Finally, as discussed in the Wind & Weight Spans technical note, Wind and Weight span designs have many problems. Using PLS-POLE (and TOWER) with PLS-CADD Lite to develop the Method 1 (or Method 2) structures for PLS-CADD optimization allows the designer to take advantage of the best that both methods have to offer and really completes the "design circle". Toggling to the Method 1 side, one can quickly optimize a line, and toggling to Method 4 will allow a full finite element structural analysis of the structures to be completed. In the event of a conductor change out, this can simply be done (using Sections / Table is the easiest way) and then a full structural check be performed again. Since a conductor or even just a wire change out voids the Wind and Weight span limits of a structure, the Method 1 checks can now be ignored, or, the conductor change out can be made in the PLS-CADD Lite models for each structure type and the Method 1 structures can quickly be regenerated for reoptimizing the line with the revised conductor.

Optimization will easily beat any manual spotting process in overall project costs. In many comparisons done by knowledgeable utilities and consultants, the PLS-CADD Optimization provided the absolute least cost constructed line alternative that met the project design criteria when compared to any other spotting optimization program. Not only does PLS-CADD provide the least cost constructed line cost, the few hours spent setting up the optimization process as described above will save a significant amount of engineering and design time (and therefore money) over any manual spotting process.

While Optimization can appear to be quite complex on the surface, it is actually quite simple once the process is observed. This was covered quite extensively at the last User Group meeting, and will be covered again in future meetings. The User Groups really are free "advanced training” sessions, so we hope that you can make the next one.

Optimization Complete in PLS-CADD

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