Ptd Lect 5

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  • 8/9/2019 Ptd Lect 5


    Mechanical Design

    ofOverhead Lines

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    Main Components of Overhead Lines

    main components of an overhead line are:

    1. Conductors : carry electric power from thesending end station to the receiving endstation.

    2. Supports : poles or towers.

    3. nsulators: Attached to supports andinsulate the conductors from the ground.

    !. Cross arms : Provide support to theinsulators.

    ". Miscellaneous items: danger plates,lightning arrestors, anti-climbing wires etc.

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    Conductor Materials

    erial should have the follo#ing properties :

    . high electrical conductivity.!. high tensile strength in order to withstand

    mechanical stresses.". low cost so that it can be used for long distances.#. low specific gravity so that weight per unit volume

    is small.mmonl$ used conductor materials.

    1. Copper

    2. %luminum3. Steel&cored aluminum

    !. 'alvani(ed steel

    ". Cadmium copper

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    . Copper:

    $igh electrical conductivity

    %reater tensile strength

    $igh current density

    Metal is &uite homogeneous, durable and has highscrap value

    )igher cost and non&availa*ilit$

    rend is to use aluminum in place of copper

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    2.%luminum:Cheap and light as compared to copper

    &'maller conductivity and tensile strength comparedto copper+elative comparison of the t#o materialsis as:


    Conductivit$ of aluminium is ,-that ofcopper. /or the same resistance) the diameter of

    aluminium conductor is about 102, times thediameter of copper conductor.

    ( The specific gravit$ of aluminium 201 gmcc4is lower than that of copper 506 gmcc4.

    Aluminium conductor has almost one&half theweight of e&uivalent copper

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    ( Aluminium conductor being light, is liable togreater swings and hence larger cross&arms arere7uired.

    ( *ue to lower tensile strength and higher co-efficient of linear e+pansion of aluminium, the sagis greater in aluminium conductors.

    %luminum has an edge over

    copper8 9);

    Com*ined properties of cost,conductivit$, tensile strength, and #eightmaes aluminium profitable for widely uses.

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    3. Steel coredaluminium

    &Aluminium conductor is reinforced with a core of

    galvanised steel #ires.

    'teel core taes greaterpercentage ofmechanical strength.

    Aluminium strandscarry the bul ofcurrent.

    esult of this composite conductor is that:

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    The steel cored aluminium conductorshave the follo#ing advantages :

    . he reinforcement with steel increasesthe tensile strengthbut at the sametime

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    !. 'alvanisedsteel:&/sed for e+tremely long spans.


    Also for short line sections e+posed to abnormallyhigh stresses due to climatic conditions

    &0ot suitable for transmitting large power over a longdistance

    ". Cadmiumcopper:&Copper alloy with cadmium

    &Addition of 1 or !1 cadmium to copper increases

    the tensile strength by about 231.&Conductivity is only reduced by 21 below that ofpure copper.&/seful for e+ceptionally longspans.&$igh cost of cadmium, economical only for lines of sma


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    Line 'upports

    &he supporting structures for overhead line

    conductors. various t$pes of poles and to#ers

    Line supports should have the follo#ingproperties :

    . $igh mechanical strength

    !. Light in weight

    ". Cheap in cost

    #. Longer life

    2. 5asy accessibility

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    he line supports used for transmissionand distri*ution of electricpower are of

    various types including)

    1. #ooden poles

    2. Steel poles

    3. +.C.C. poles

    !. steel to#ers

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    . 6ooden poles)

    &Cheap, easily available, provide insulatingproperties.&

    6idely used for distirbution purposes in ruralareas as an economical proposition

    &the portion of the pole below the ground level isimpregnated with preservative compounds lie

    creosote oilain o*=ections to #ooden supports are :

    1. Tendenc$ to rot *elo# the ground level

    2. Comparativel$ smaller life 2-&2" $ears43. Cannot *e used for voltages higher than 2-

    !. Less mechanical strength and

    ". +e7uire periodical inspection

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    9ooden poles Dou*le pole structures of the?%@ or ?)@ t e4

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    . 'teel poles)

    ubstitute for wooden poles

    %reater mechanical strength, longer life and permitlonger spans

    /sed for distribution purposes

    %alvanised or painted in order to prolong its life

    e steel poles are of three t$pes

    1. +ail poles

    2. Tu*ular poles

    3. +olled steel =oints

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    ". 7CCpoles)

    &%reater mechanical strength, longer life and permitlonger spans than steel poles.

    &7e&uire little maintenance and have goodinsulating properties.&)oles in the poles facilitate the climbing of poles

    and at the same time reduce the weight of linesupports

    ain difficult$ #ith the use of these pol

    $eavy weight and high cost oftransport

    (Often manufactured at the site in order toavoid heavy cost of transportation.

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    +.C.C. poles

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    #. 'teeltowers)

    &/sed for long distance transmission at highervoltage.'teel towers have)

    %reater mechanical strength

    Longer life

    6ithstand most severe climaticconditions

    Permit the use of longer spanso'ingle circuit tower o*ouble circuit tower

    The dou*le circuit has the advantage that:

    8t ensures continuity of supply.

    8f there is breadown of one circuit, the continuity

    of supply can be maintained by the other circuit.

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    Steel To#ers'ingle circuit tower double circuit tower

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    Dou*le circuit to#er

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    A Stoc

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    Space Dampers8n very high voltage transmission lines, conductor

    bundles are used for each phase to fulfil electricalre&uirements. 'pacing devices are installed on thebundles to maintain subconductor separation. 5lasticand damping elements are introduced into the spacingdevices for distributing the damping effect along thespans, which prevents entrapment of vibrationbetween spacers and avoid bending stress at thespacer clamps.

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    8 l

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    &8nsulators provide necessary insulation *et#een

    line conductors and supportsand thus preventan$ lea

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    ypes of 8nsulators

    he successful operation of an overhead line depends

    to a considerable e+tent upon the proper selection ofinsulators.

    There are several t$pes of insulators *utthe most

    commonl$ used are:Ain t$pe insulators

    Suspension t$pe insulators

    Strain insulators


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    orcelain nsulator

    Aropert$ >alue%pproBimate4


    'traingth93 :; < cm


    'trength=3,333 :g < cm!

    ensile 'trength 233 :g < cm!

    Aroperties of Aorcelainnsulator

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    'lass nsulatorNow days glass insulatorhas become popular in transmission and

    distribution system.

    Annealed tough glass is used for insulating purpose.Glass insulator has numbers of advantages over conventional porcelain


    Aropert$ >alue%pproBimate4


    'trength#3 :; < cm


    'trength3,333 :g < cm!

    ensile 'trength "2,333 :g < cm!

    Aroperties of 'lassnsulator

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    Ain t$pe insulators:

    &Pin type insulators are used for transmission and

    distribution of electric power at voltages upto33 .

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    ses of insulator failure:&8nsulators are re&uired to withstand both

    mechanical and electrical stresses.

    &he electrical breadown of the insulator can occureither by flash&over or puncture.

    case of flash&over:8nsulator will continue to act in its proper capacityunless e+treme heat produced by the arc destroys theinsulator.

    n case of puncture:*ischarge occurs from conductor to pin through thebody of the insulator

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    Arcing distance is a * c for the

    insulator.he ratio of puncturestrength to flashovervoltage is nown as

    safet$ factor

    Safet$factor of

    insulator8t is desirable that the value ofsafety factor is high so that flash-

    over taes place before theinsulator gets punctured.


    /lash & overvoltage

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    uspension t$pe insulators:

    &Pin insulator is not economicalbeyond "" ;.

    &$igh voltages >"" ; suspensiontype insulators is used.

    &0umber of porcelain discsconnected in series by metallins in the form of a string.

    5ach unit or disc is designedfor low voltage, say ;, for,, voltage siB discs inseries #ill *e provided onthe string.

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    1. Cheaperthan pin type insulators for voltages

    beyond "" ;.

    !. he desired num*er of discs can be connected

    in series.

    ". 8f any one disc is damaged, can be replaced



    'reater fleBi*ilit$ to the line, string is free toswing in any direction where mechanical stresses

    are minimum.

    ". The additional insulation re7uired for the

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    Strain insulators:

    &/sed at dead end of the line or there is cornerorsharp curve.

    &/or eBcessivetension, straininsulators are used.

    &/or high voltagetransmission lines,strain insulator consistsof an assembly ofsuspension insulators.

    &At long river spans, twoor more strings are usedin parallel

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    & ?or low voltage lines E 11 48 shacle

    insulators are used as strain insulators.


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    Potential *istribution over 'uspension 8nsulator

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    Potential *istribution over 'uspension 8nsulator'tring

    &he porcelain portion of each disc is in between twometal lins forms a capacitor C .

    Charging current same through all the discs.;oltage across each unit 'ame.

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    &Capacitance also e+ists @shunt capacitance Cbetween metal fitting of each disc and tower orearth.*ue to shunt capacitance, charging current

    is not the same through all the discs of thestring.herefore, voltage across each disc will bedifferent.

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    0on uniform voltage distribution across string.

    !. *isc nearest to the conductor has ma+imum


    ". Probability of disc puncture is more in nearest

    disc due to ma+ electrical stress.

    #. 8n case of *C, voltage across each unit of string

    remain same.

    'tring 5fficiency

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    'tring 5fficiency

    he ratio of voltage across the #holestring to the product of num*er of discsand the voltage across the discnearest to the conductor.

    'tringefficiency B

    ;oltage across the string

    n ;oltage across disc nearest to conduct

    n B number of discs in the string

    %reater the string efficiency, the moreuniform is the voltage distribution.

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    athematical eBpression:

    @5&uivalent circuitfor a "-disc string

    Assume, C B :C

    ppl$ing FCL to node %

    ppl$ing FCL to node G

    ;! B ; @ D :E. @i

    6e getE.

    6e getE.

    ;" B ;F D ": D:!G [email protected]

    >oltage *et#eenconductor and is; B ; D ;! D ;"

    ; B ;@ D : @" D : [email protected]

    ;oltage across top unit ;, ;oltage across !nd unit

    ;!, ;oltage across "rd unit ;"

    Ho#8 #e cancalculate

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    1age 'tringefficiency B

    ;oltage across the string

    n ;oltage across disc nearest to conduct




    " + ;"B

    Aoints from the a*ovemathematical anal$sis :. 8f : B 3H! then we get, ;! B H! ; and ;" B

    H9# ;voltage across other discs decreasing

    progressively as the cross-arm in approached.!. %reater the value of : @B C

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    Methods of 8mproving 'tring 5fficiency

    various methods of improving string efficiency are )

    . Iy using longer cross-arms

    !. Iy grading the insulators

    ". Iy using a guard ring

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    he phenomenon of violet glo#, hissing

    noise and production of o(one gas in anoverhead transmission line is nown ascorona.

    ritical disruptive voltage:

    he value of voltage at which corona occurs.

    &The higher the voltage is raised

    the larger and higher the luminous envelope becomesgreater are the sound, the power loss and the radionoise

    f f ti

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    or$ of corona formation.

    &8oniKation in the air due to cosmic rays and /;radiation etc&/nder the influence of potential gradient, thee+isting free electrons ac&uire greater velocities.

    &At "3 ; per cm @ma+. value, the velocity ac&uired

    is sufficient to strie a neutral molecule to dislodgeone or more electrons from it.

    &he result of this ioniKation is that either corona is

    formed or spar taes place between the conductors.

    ?actors Affecting Corona

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    ?actors Affecting Corona

    he following are the factors upon which coronadepends )1. %tmosphere:

    8n the storm$ #eather, the number of ions ismore than normal and as such corona occurs atmuch less voltage as compared with fair weather.

    . Conductor si(e:

    he rough and irregular surface will give rise tomore corona.

    Spacing *et#een conductors:

    8f the spacing between the conductors is made ver$

    large ascompared to their diameters, there may not beany corona effect.!. Line voltage:

    8f it is low, no corona is formed.

    Advantages and *isadvantages of Corona

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    Advantages and *isadvantages of Corona

    . >irtual diameterof the conductor is increased.he increased diameter reduces the electrostaticstresses between the conductors

    !. Corona reduces the effects of transientsproduced by surges.



    .Loss of energy

    !.OKone production causes conductor corrosion.

    ". 8nterference with neighbouringcommunication lines.

    'ag in Overhead Lines

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    'ag in Overhead Lines

    he difference in level between points of supports%8G4 and the lo#est pointo4 on the conductor is

    called sag.

    .wo supports at the same level, conductortaes catenary shape.

    !. ension o at the lowest point O acts horiKontally.". he horiKontal component of tension is constantthroughout the length of the wire.

    . 8f is the tension at the support I, then B o

    d t d t i

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    ductor sag and tension:

    &Conductor 'ag should be minimum to avoid materialre&uired and e+tra pole height.

    &ension in the conductor should be lo# to avoid themechanical failure of conductor and to permit the useof less strong supports.

    Gut8 is it possi*le at thesame time ;Lo# conductor tension and minimum sag

    are not possi*leLow sag means a tight wire and high

    tension.Low tension means a loose wire andincreased sag.

    n actual practice8 a compromise in made*et#een the t#o.

    Calculation of 'ag

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    Calculation of 'ag

    The tension isgoverned *$:

    Conductor weight5ffects of wind8ce loadingemperature variations.

    &Conductor tension should be less than "- of itsultimate tensile strength.

    Calculation of sag and tension of aconductor #hen:

    1. supports are at e7ual levels

    2. supports are at une7ual levels.

    h t t l l l

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    hen supports are at e7ual levels:

    I Length of span# 9eight per unit length ofconductor

    T Tension in the conductor.

    he two forces acting on the portion OP of the

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    he two forces acting on the portion OP of theconductor are )1. The #eight #B of conductor acting at a

    distance B2 from O.

    2. The tension T acting at O.


    y5&uating the moments of above twoforces about point O

    5&uating the

    horiKontal andvertical components.B+ andyB6+$oriKontalcomponent of

    tension isconstant.6hen OPB+ , thenBy

    y B w + +

    y B w +!

    he ma+imum dip @sag isrepresented by the value of y at

    either of the supports A and I.+ B

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    pp 7

    I Span lengthh Difference in levels *et#een t#osupportsB1 Distance of support at lo#er leveli.e.8 %4 from OB2 Distance of support at higher level

    ct of #ind and ice loading:

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    ct of #ind and ice loading:

    al weight of conductor per unit length is

    B weight of conductor per unit lengthi B weight of ice per unit length

    w B wind force per unit length

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    +O9s Management ofline

    A 7O6 is a largely passive but critical component

    of a transmission line. 8t provides a safety marginbetween the high-voltage lines and surroundingstructures and vegetation. he 7O6 also providesa path for ground-based inspections and access to

    transmission towers and other line components, ifrepairs are needed.;oltage @:; 7ange of [email protected]

    !"3 2 to !2

    "#2 >!2

    233 >=2

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