HOW TO DESIGN TRANMISSION LINES – Engineering Basics

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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