How to Use Sag Template for Tower Spotting – Part 3

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Tower or Structure spotting is a term used for the process of determining the optimum location, height, and type of transmission line structures on the plan and profile drawings.

The main goal of design engineer is to select the optimal design out of all the possible options. The optimal design is the most economical alternative of all feasible options. Today, optimization is easily done using computer software.

Characteristic of a well-designed and economical plan

  • Spans should be approximately uniform in length, equal to or slightly less than the ruling span.
  • Maximum usage of structure of equal height and type.
  • Smooth shape of the conductor profile. If the shape is smooth-flowing curve, the loadings are equalized on successive structures.

Step by Step Tower Spotting Procedure

The process of spotting begins at a known or established conductor attachment point such as a substation take-off structure. The process usually progresses from left to right on the profile. The sag template is applied to the profile by moving the same horizontally while always ensuring that the vertical axis is held vertical.

  1. Identify the position of Terminal Tower or Structure.
  2. Start from the Terminal Tower.
  3. Match the tower footing curve with the position of Terminal Tower.
  4. Adjust the position of the Sag Template. Always ensure that its vertical axis is held vertical.
  5. Ensure Ground Clearance. This is done by ensuring the ground clearance curve is tangent or barely touching the ground profile.
  6. Identify the position of the next Tower. This is the point where the tower footing curve intersects the ground line. The height of the tower or pole can be determined using a proper structure height template, usually etched on the sag template.
  7. Move the sag template to the next Tower.
  8. Repeat the procedure.

However, the above procedure can be followed only on lines that are approximately straight and which cross relatively flat terrain with the basic ground clearances. When line angles, broken terrain, and crossings are encountered, it may be necessary to try several different arrangements of structure locations and heights at increased clearances to determine the arrangement that is most satisfactory.

This is done by shifting the sag template until the ground profile touches or is below the clearance curve with the previously established conductor attachment point is positioned on the conductor curve. The maximum sag curve would then indicate the required conductor height at any selected span. Structure height may be determined by scaling or by use of the proper structure height template. Design limitations due to clearance or structure strength should be observed.

Other Design Considerations

Crossings

Although the process of spotting structures usually progresses from left to right, it is best to examine the profile for several spans ahead because there may be conditions which will require special consideration and affect the location of the structure. Examples:

  • Line angle points
  • Highway or railroad crossings
  • Powerline or communication crossings
  • River Crossings

Such conditions often fix the location of a transmission line structure, and it is usually a matter of determining the most desirable arrangement of the structures between these fixed locations. These conditions may require additional clearance aside from the basic clearance. Sometimes it is desirable to move ahead to one of the fixed structure locations and work backward.

Weight Span

The weight span of a structure is a measure of the vertical force a structure must be able to withstand. The weight span is equal to the horizontal distance between the conductor lowest points in the back and ahead span, on two adjacent spans.

Wind Span

The wind span of any structure is equal to the distance measured between the center points of two adjacent spans supported by that structure. On other words, wind span is simply half of ahead span plus half of back span length. The wind span is used to determine the transverse force a structure must withstand under high wind conditions.

Wind span is not dependent on conductor sag or tension, only on horizontal span length. When the elevation of adjacent structures is the same, the wind and weight spans are equal.

Cold Uplift

On steeply inclined span when the cold sag curves show the low point to be above the lower support structure, the conductors in the uphill span exert upward forces on the lower structure.

Uplift has to be avoided for suspension, pin-type, and post insulators construction. For structures with suspension insulators, the check for allowable insulator swing is usually the controlling criteria on vertical span.

Cold Uplift could be avoided by:

  1. Adjusting structure locations on the plan-profile drawing
  2. By using a higher structure at the point of uplift
  3. Attaching weights to the conductor
  4. Dead End Structure, if all other method fails.

Sag Template can quickly check for the uplift of the structure in question. For example, we will determine if Str# 8 will undergo uplift.

  1. Place the cold curve on the alternate structures which is Str# 7 and Str# 9.
  2. Note that Str# 8 is just within the cold curve.
  3. Suppose, however, that Str# 8 is replaced by a 50-footer structure. Then this structure will be below the cold curve causing an upward pull on the structure.

Conclusion

The process of Tower Spotting is iterative. The initial first-pass design is revised repeatedly until it complies with all regulations and stakeholder requirements and is optimal in terms of cost, reliability and practically for construction, maintenance and operations.


Sources:

  • The planning, design and construction of overhead power lines.
  • Central Board of Irrigation and Power. Construction of Transmission Lines. Technical Report No.9
  • Rajasthan Rajya Vidyut Prasaran Nigam Ltd. Construction Manual for Transmission Lines
  • Farr, Holand. Transmission Line Design Manual. 1980
  • Setayeshgar, Armin. Principles of Mechanical Design in Overhead Transmission Lines. 2016
  • RUS, USDA. Design Manual for High Voltage Transmission Lines. Bulletin 1724E-200
  • Phaloprakarn, Somkiet. Transmission Line Tower Location on Nonflat Terrain by Digitial Computer Methods. 1964