Sag Template and Tower Spotting

Sag Template and Tower Spotting

Generally two types of towers are used to support the overhead transmission lines which are,

1. The standard or straight run or intermediate tower.

2. The angle or anchor or tension tower.

The standard towers are used for straight rim of the line under normal conditions. While the positions where line is to be turned, curves and other points where support will be under high mechanical stress, the angle or tension towers are used. Thus design of tension towers is such that they can withstand large mechanical stress. The height of the towers and position of the towers is an important part of the mechanical design of the transmission lines.

For locating the tower positions, for normal spans and for standard towers, the sag and the nature of the conductor curve are calculated, under expected load conditions and plotted on a thin stiff plastic sheet. Such a graph is called sag template. The minimum ground clearance curve can be plotted parallel to the conductor curve on the template. The tower footing line can also be drawn on the template which helps to locate the towers, so that minimum ground clearance is maintained. Various other curves at some other temperatures like 20 °C and 50 °C are also plotted on the template. The axis of the template is placed vertically between the two tower lines. The horizontal distances give span lengths while vertical distances give the sags.

For preparing the sag template, the table for the sag tension is prepared as shown below,

A typical sag template is shown in the Fig. 4.9.1.

The ground clearance curve is tangential to the ground level at point Q. The tower footing line intersects the points on the ground where towers are situated. This line helps to fix up position of towers, with minimum ground clearance of 'C' is maintained throughout. The clearance to ground depends on the operating voltage and specified as per the Indian electricity rules. Taking the rules into consideration, the template is prepared. 

While using the sag template, the profile of the land is drawn on the paper. The spans are indicated by tentative vertical lines. Then the transparent sag template is placed on the route drawn. It is then adjusted such that the ground clearance curve just touches the profile. At this position, the tower footing line gives the positions of the towers. The actual conductor curve is then represented by lowest conductor curve on the template. The template can be used for any shape of ground profile and not only for plane ground. The Fig. 4.9.2 shows the use of sag template to locate the towers.

The points A and B, which are the intersection points of tower footing line with land profile, give the positions of the towers.

Following objectives can be achieved using the sag template,

1. Safe ground clearance is maintained throughout.

2. Choice of span and height of towers with good accuracy.

3. Positions of towers.

4. Towers with almost equal heights, weight and hence cost.

5. Towers which carry almost equal loads.

6. Economical layout of the system.

It should be noted that for each new line conductor, a new template is preferred. 

Review Question

1. What is sag template ? What objectives can be achieved using the sag template?

AU : Dec.-17, Marks 6