Chapter # 8 Mechanical Design of Overhead Lines Introduction Electric power can be transmitted or distributed either by meansof underground cables or by overhead lines. The underground cables are rarely used for power transmissiondue to two main reasons: Firstly, power is generally transmitted over long distances to loadcenters. Obviously, the installation costs for undergroundtransmission will be very heavy. Secondly, electric power has to be transmitted at high voltages foreconomic reasons. It is very difficult to provide proper insulationto the cables to withstand such higher pressures. An overhead line is subjected to uncertain weather conditions andother external interferences. This calls for the use of proper mechanical factors of safety inorder to ensure the continuity of operation in the line. In general,the strength of the line should be such so as to provide against theworst probable weather conditions. risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com Sag The difference in level between points of supports andthe lowest point on the conductor is called sag. risemark.pbworks.com risemark.pbworks.com Calculation of Sag When supports are at equal levels:
risemark.pbworks.com Calculation of Sag When supports are at unequal levels:
risemark.pbworks.com Some Mechanical Principles
Mechanical factors of safety to be used in transmission linedesign should depend to some extent on the importance ofcontinuity of operation in the line under consideration. Ingeneral, the strength of the line should be such as to provideagainst the worst probable weather conditions. We now discusssome important points in the mechanical design of overheadtransmission lines. Tower Height Conductor clearance to ground Sag and tension Stringing charts Conductor spacing Conductor vibration risemark.pbworks.com Tower Height Tower height depends upon the length of span. Withlong spans, relatively few towers are required butthey must be tall and correspondingly costly. It isnot usually possible to determine the tower heightand span length on the basis of direct constructioncosts because the lightning hazards increase greatlyas the height of the conductors above ground isincreased. This is one reason that horizontal spacingis favored in spite of the wider right of way required. risemark.pbworks.com Conductor Clearance to Ground
The conductor clearance to ground at the time ofgreatest sag should not be less than some specifieddistance (usually between 6 and 12 m), depending onthe voltage, on the nature of the country and on thelocal laws. The greatest sag may occur on the hottestday of summer on account of the expansion of thewire or it may occur in winter owing to the formationof a heavy coating of ice on the wires. Specialprovisions must be made for melting ice from thepower lines. risemark.pbworks.com Sag and tension When laying overhead transmission lines, it isnecessary to allow a reasonable factor of safety inrespect of the tension to which the conductor issubjected. The tension is governed by the effects ofwind, ice loading and temperature variations. Therelationship between tension and sag is dependenton the loading conditions and temperaturevariations. For example, the tension increases whenthe temperature decreases and there is acorresponding decrease in the sag. Icing-up of theline and wind loading will cause stretching of theconductor by an amount dependent on the linetension. risemark.pbworks.com Stringing charts For use in the field work of stringing the conductors,temperature-sag and temperature-tension chartsare plotted for the given conductor and loadingconditions. Such curves are called stringing charts. risemark.pbworks.com Conductor spacing Spacing of conductors should be such so asto provide safety against flash-over whenthe wires are swinging in the wind. Theproper spacing is a function of span length,voltage and weather conditions. The use ofhorizontal spacing eliminates the dangercaused by unequal ice loading. Small wiresor wires of light material are subjected tomore swinging by the wind than heavyconductors. Therefore, light wires shouldbe given greater spacing. risemark.pbworks.com Conductor vibration Wind exerts pressure on the exposed surface of the conductor.If the wind velocity is small, the swinging of conductors isharmless provided the clearance is sufficiently large so thatconductors do not approach within the sparking distance ofeach other. A completely different type of vibration, calleddancing, is caused by the action of fairly strong wind on a wirecovered with ice, when the ice coating happens to take a formwhich makes a good air-foil section. Then the whole span maysail up like a kite until it reaches the limit of its slack, stopswith a jerk and falls or sails back. The harmful effects of thesevibrations occur at the clamps or supports where the conductorsuffers fatigue and breaks eventually. In order to protect theconductors, dampers are used. risemark.pbworks.com risemark.pbworks.com risemark.pbworks.com