Once the route and length of a transmission or distribution
line has been decided upon and the correct conductor size and type selected to
carry the system load safely and economically, there are still several
mechanical considerations which will have an effect on installation practices
and may influence the final choice of conductor.
The line designer must consider such factors as tower and
pole locations and heights, span lengths, conductor tension and sags, ground
clearances, etc. Technically, this means that he must have detailed knowledge
of conductor sag-tension characteristics as a function of span length,
temperature, and weight loading. Much of this information is applied by wire
and cable manufacturers in the form of tables and graphs that are to be used by
the line designer.
Supplementing these, the line designer prepares other
graphs, tables, templates, etc., that are related to a specific installation.
Thus, there are two distinct types of study:
(1) That which is ordinarily
performed by the engineers of the wire and cable manufacturers, and
(2) that
which is performed by the line-design engineer to utilize the manufacturer
supplied information to best advantage.
An overhead conductor suspended between insulator supports
assumes the shape of a catenary curve provided the conductor is of uniform
weight per ft. Usually it is convenient, without significant error, to regard
the curve as a parabola.? A family of such curves exists for a given conductor and span.
The mid-point sag depends on tension in the conductor; the
greater the tension the less the sag. To distinguish between span length and
conductor length, the latter is usually designated arc length.
Anything that increases arc length after initial stringing
increases the sag. Factors that may bring this about are ( 1 ) thermal
expansion of the conductor because of increase of temperature above that during
stringing, (2) increase of conductor apparent weight because of wind and/ or
ice load, (3) creep gradually lengthening the conductor wires as a result of
tension being applied over a period of many years, (4) stressing of wires beyond their elastic
limits.
Though it might appear that sag-tension problems relating to
these subjects could be solved in a simple manner, there are interrelated
factors that must be taken into account. For example, ACSR has components that
have differing stress-strain characteristics, differing coefficients of thermal
expansion, and they normally undergo differing unit tensile stresses.
Thus, it is evident that proper selection of span length and
sags for a given profile and conductor in order to minimize installation and
operational costs requires a high order of engineering skill. However, for many
applications, the required sag-tension analysis has been made by others, and
the results are available in tables and graphs supplied by wire and cable
manufacturers for all commercially offered conductors. Only a moderate amount
of additional work is necessary to utilize them for specific applications.
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