Electrical Power Systems

(f ) Vertical sag = dcos q
cos q = 297 .
3078 .
\ vertical sag = 1.342 × 297 .
m
3078 .
15.8 LOCATION O LINE
= 1.295 m.
(see ig. 15.12)
Analysis of Sag and Tension 393
The routing of a transmission or distribution lines requires thorough investigatians and for
selecting the most desirable and practical route, following points should be considered:
1. cost of construction
2. cost of easements
3. cost of clearing
4. cost of maintenance.
15.9 SAG TEMPLATE
or correct design and economy, the location of structures on the profile with a template is very
essential. Sag template is a convenient device used in the design of a transmission line to
determine the location and height of stuctures. Sag template can be relied upon to provide the
following:
1. Economic layout
2. Minimum errors in design and layout
3. Proper grading of structures
4. Prevention of excessive insulator swing
Generally two types of towers are used:
1. The standard or straight run or intermidiate tower.
2. The angle or anchor or tension tower.
The straight run towers are used for straight runs and normal conditions. The angle towers
are designed to withstand heavy loading as compared to standard towers because angle towers
are used at angles, terminals and other points where a large unbalanced pull may be thrown on
the supports.
or standard towers, for normal or average spans, the sag and the nature of the curve
(Catenary or Parabola) that the line conductor will occupy under expected loading conditions is
evaluated and plotted on template. Template will also show the required minimum ground
clearance by plotting a curve parallel to the conductor shape curve. or the standard tower and
same height, the tower footing line can also be plotted on the template. Tower footing line is
used for locating the position of towers and minimum ground clearance is maintained throughout.
igure 15.13 shows the sag template used for locating towers. In fact there are no clear-cut
guide lines for locating the tower positions and sesveral other alternatives may be examined.
Ground clearance depends on voltage level and Table-15.1 gives the span length and ground
clearance at different voltage levels.