If you can't read please download the document
Upload
theodore-victor-sullivan
View
755
Download
204
Embed Size (px)
DESCRIPTION
Introduction Electric power can be transmitted or distributed either by means of underground cables or by overhead lines. The underground cables are rarely used for power transmission due to two main reasons: Firstly, power is generally transmitted over long distances to load centers. Obviously, the installation costs for underground transmission will be very heavy. Secondly, electric power has to be transmitted at high voltages for economic reasons. It is very difficult to provide proper insulation to the cables to withstand such higher pressures. An overhead line is subjected to uncertain weather conditions and other external interferences. This calls for the use of proper mechanical factors of safety in order to ensure the continuity of operation in the line. In general, the strength of the line should be such so as to provide against the worst probable weather conditions. risemark.pbworks.com
Citation preview
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