Mechanical Considerations

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MECHANICAL

CONSIDERATIONSIN

LINEPERFORMANCE


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Introduction

Electric power can be transmitted or distributed either by means ofunderground cables or overhead lines

The underground system is much more expensive than overheadsystem. Therefore, it has limited use for distribution purposes incongested areas where safety and good appearances are the main

considerations

In overhead lines bare conductors are used and air acts as theinsulation. The necessary insulation between the conductor can beprovided by adjusting the spacing between them.

Electric power has to be transmitted at high voltages for economicreasons so its dicult to provide proper insulation


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Factors to be considered

The successful operation of an overhead line greatly depends onthe mechanical design of the line so, the overhead line shouldhave good mechanical strength against worst probable weatheconditions

The overhead line is subjected not only to uncertain weatherconditions but also to other external interferences


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echanical !omponents of "verheadlines

# !onductors

# $upports

# Insulators

# !ross arms

# iscellaneous items


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!onductor

%igh electrical conductivity, %igh tensile strength in order to withstandmechanical stresses, &ow cost ,&ow speci'c gravity so that weight perunit volume is small

In standard conductors there is one central wire and round this,successive layers of wires containing (,)*,)+ wires , thus total number

of individual wires is n-n)/) Types 00!,000!,0!$1,0!01


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00! has a minimum conductivity of ().*2 I0!$. It has limited use intransmission lines and distribution side for long distances butextensively seen in urban areas where spans are short

The excellent corrosion resistance of aluminum has made 00! aconductor of choice in coastal areas.

0!$130luminium !onductor $teel 1einforced, a standard of theelectrical utility industry since the early )4556s, consists of a solid orstranded steel core surrounded by one or more layers of strands of)75 aluminium

0!$1 is available in a wide range of steel content 3 from 82 by weightfor the (9) stranding to :52 for the 598 stranding. Early designs of0!$1 such as (9), 598, 59)4, 7:9)4 and 7:98 strandings featured high

steel content, *(2 to :52, with emphasis on strength perhaps due tofears of vibration fatigue problems.

Today, the most used strandings are )+9), :798, 8*98, and +:9)4,comprising a range of steel content from ))2 to )+2. For themoderately higher strength 7:9)4, 7:98, and *(98 strandings, the steelcontent is *(2, *(2 and )2


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6201 "AAAC" 3 0 high strength 0luminium3agnesium3$ilicon 0lloy!able was developed to replace the high strength (9) 0!$1 conductors

"riginally called 000!, this alloy conductor o;ers excellent electricalcharacteristics with a conductivity of 7*.72 I0!$, excellent sag3tensioncharacteristics and superior corrosion resistance to that of 0!$1

0!01 combines )75 and (*5) aluminium alloy strands to provide atransmission conductor with an excellent balance of electrical andmechanical properties. This conductor consists of one or more layers of)753%)4 aluminium strands helically wrapped over one or more (*5)3

T+) aluminium alloy wires.

(*5) 0luminium 0lloy !onductor $teel 1einforced/ 3 Is an 0!$1 with th)75 aluminium wires replaced by (*5)3T+) aluminium alloy wire

00!$1 conductors have approximately :52 to (52 more strength thancomparable standard 0!$1 conductors of e


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!onductors and its dimensions

(i) For 132 KVlines

: 'Panther' ACSR having 7-strands of steel of dia 3!! "" and 3!Strands of Al#"ini#" of dia 3!! ""

(ii) for 22! KVlines

: '$e%ra' ACSR having 7-strand of steel of dia 31& "" and -Strands of Al#"ini#" of dia 31& ""

(iii) for !! KVlines

: *in '+oose' ACSR having 7-Strands of steel of dia 33 "" an-Strands of Al#"ini#" of dia 33 ""


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&ine $upports


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Insulators

=in Type insulator

$uspension Type insulator

$train Type insulator

$hac>le Insulator


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=in Type insulator

Pin Insulatoris earliest developed o!r!a# insulator, but stillpopularly used in power networ> up to ?@ system

0s the lea>age path of insulator is through its surface,it is desirable to increase vertical length of the insulator

surface area for lengthening lea>age path

These rain sheds or petticoats are so designed, that duringraining the outer surface of the rain shed becomes wet butthe inner surface remains dry and non3conductive. $o therewill be discontinuations of conducting path through the wet

pin insulator surface


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$uspension Type insulator

In higher voltage, beyond ?@, it becomes uneconomical to use pin inbecause siAe, weight of the insulator

In sus$!nsion insulatornumbers of insulators are connected form a string and the line conductor is carried by the bottom most Each insulator of a suspension string is called disc insulatortheir disc li>e shape

Each suspension disc is designed for normal voltage rating ))?@-%igher volta)7?@/, so by using di;erent numbers of discs, a suspension string can be mafor any voltage level

0s the current carrying conductors are suspended from supporting structure suspension string, the height of the conductor position is always less than theheight of the supporting structure. Therefore, the conductors may be safe fro


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$train Type Insulator

Bhen suspension string is used to sustain extraordinary tensile load of condreferred as string insulator

Bhen there is a dead end or there is a sharp corner in transmission line, thesustain a great tensile load of conductor or strain.

0 strain insulator must have considerable mechanical strength as well as theelectrical insulating properties.


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$hac>le Insulator

The sa%&l! insulatoror s$ool insulatoris usually used inlow voltage distribution networ>. It can be used both in

horiAontal and vertical position

The use of such insulator has decreasedrecently after increasing

the using of underground cablefor distributionpurpose


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E;ects on vibrations

0s the number of sub3conductors used in bundle increases, thesevibrations ,countermeasures and spacing6s of sub3conductors will alsoa;ect the electrical design

Thus a mechanical designer has to consider the tower dimensions,phase spacing6s, conductor height, sub3conductor spacing6s, etc. from

which the electrical designer has to commence his calculations ofresistance, inductance, capacitance, electrostatic 'eld, corona e;ects,and all other performance characteristics.

The sub3conductors in a bundle are separated by spacers of suitabletype, which bring their own problems such as fatigue to themselves an

to the outer strands of the conductor during vibrations


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# 0eolian vibrations and galloping are present for both single3and multi3conductor bundles, while the wa>e3induced oscillation is con'ned to abundle only

# $tandard forms of bundle conductors have sub3conductors ranging from*.7: to 7cm diameters with bundle spacing of :5 to 75 cm betweenadjacent conductors

# For E.%.@ Transmission, the number ranges from * to + sub3conductorsfor transmission voltages from :55 >v to )*55 >v, and up to )* or even)+ for higher voltages which are not yet commercially in operation.

# The charges on the sub3conductors are of the same polarity, there exist

electrostatic repulsion among them. on the other hand, since they carrycurrents in the same direction, there is electromagnetic attraction


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Factors for @ibrations C


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# !onductor Tensions

# $pan &ength

# !onductor $iAe

# Type of !onductor

# Terrain of line

# Direction of prevailing winds

# Types of $upporting clamp of conductors3insulator assemblies from thetower

# Tower type

# %eight of tower type of spacers and dampers

# The vegetation in the vicinity of the line


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Types of vibrations and

oscillations)/0E"&I0 @I10TI"*/G0&&"=IG

/ B0?E3IDH!ED "$!I&&0TI"$


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0eolian @ibration

B%E 0 $""T%J $T1E0 "F 0I1 =0$$E$ 0!1"$$0 !K&ID1I!0& $%0=E, $H!% 0$ 0 !"DH!T"1 "1"%$B, @"1TI!E$ -EDDIE$/ 01E F"1ED " T%E&EEB01D $IDE -0!? $IDE/. T%E$E @"1TI!E$0&TE10TE F1" T%E T"= 0D "TT" $H1F0!E$,0D !1E0TE 0&TE10TIG =1E$$H1E$ T%0T TED T"

=1"DH!E "@EET 0T 1IG%T 0G&E$ T" T%EDI1E!TI" "F T%E 0I1 F&"B. T%I$ I$ T%E E!%0I$T%0T !0H$E$ 0E"&I0 @I10TI"


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0eolian @ibration

Bhen a conductor is under tension and a comparatively steady wind blowsacross it small vortices are formed on the leeward side called ?arman@ortices

The fre


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# They also give rise to wave e;ects in which the vibration travels alongthe conductor su;ering reMection at points of di;erent mechanicalcharacteristics

# Thus dampers are designed to provide negative reMections to reduce

the wave amplitudes

# 0eolian vibrations are not observed at wind velocities in excess of *7>m9hour.

# Flexible spacers are generally provided which may or may not bedesigned to o;er damping.

# In cases where they are purposely designed to damp the sub3span

oscillations, they are >nown as spacer3dampers# The 0eolian vibrations depend upon the power imparted by the wind of

the conductor. the fre


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Galloping

Galloping of a conductor is a very high amplitude, low3frem9hour, which may normallythan that ree the form of wedi;erent locations on the span.

Galloping may not be a problem in a hot country li>e India where temperatures arabove freeAing in winter


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Ba>e3Induced "scillation

The wa>e3induced oscillation is peculiar to a bundle conductor, andsimilar to 0eolian vibration and galloping occurring principally in Matterrain with winds of steady velocity and low turbulence

The free3induced oscillation must be normally in therange *7 to (7 >m9hour.

Ba>e3induced oscillation, also called NMutter instabilityN, is caused whenone conductor on the windward side aerodynamically shields the leewardconductor


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# The oscillation occurs when the bundle tilts 7 to )7O with respect to aMat ground surface

# The conductor spacing to diameter ratio in the bundle is also critical

# If the spacing b is less than )7d, d being the conductor diameter, a

tendency to oscillate is created while for b9d P )7 the bundle is found tobe more stable


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Dampers and $pacers


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Twin $pacers


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$toc>bridge Damper


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$pacer !on'guration


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Mechanical Considerations

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