Overhead Line Design and Transmission Line Construction

7/25/2019 Overhead Line Design and Transmission Line Construction

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EE35T - Overhead Line Design and

Transmission Line Construction

The fundamental purpose of a Transmission or Distribution Line is to carry the activepower from one point to another.

A Transmission line should possess the following characteristics:

The voltage should e !ept as constant as possile over the entire length of the line"

The line losses must e small so as to otain a high transmission efficienc#

The Copper losses must not overheat the conductor"

Components of a $igh %oltage Transmission Line

1. Conductors

Conductors are alwa#s are

The# are the vital lin! in the transmission s#stem and distriution s#stem

The# must e designed to meet the specified voltage level

The conductor consideration should include the voltage level at which the power istransmitted& the ma'imum allowale losses on the line& the ma'imum thermal capacit# ofthe line& the current carr#ing capacit# and the tension of the line

(actors which affect the location of the line include the climate of the countr#& the

atmospheric conditions and viration of the line

There are several different t#pes of conductors that are used to transmit power and these include:

)i* ACSR - Aluminum Conductor Steel Reinforced. This is the most popular conductor that isused ecause of its high strength and relativel# low cost" +t comprises aluminum strands ound

around a steel core" The most common are ,.& /,0& 510"

)ii* ACSRA! - ACSR Conductor with Aluminum clad steel reinforced core. This is ver#

useful in corrosive environments"

)iii* ACSRSD - ACSR Conductor that is self dampin".+t is more e'pensive than regularAC2& and comprises two trape4oidal la#ers of conductor around a steel core" The strands are


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made of ,/6. Aluminum& and the structure ma!es them self damping against Aeolian %iration"

The# can e strung at ver# high tensions"

)iv* ACAR - Aluminum Conductor Alloy Reinforced. This comprises strands of .356Aluminum around a core made of ,/6. Aluminum" +t is lighter than AC2& ut more e'pensive

and 7ust as strong" +t is used in corrosive environments"

)v* AAC-1#$% - Aluminum Conductor made of &1#$% Strands.+t is used in construction that

re8uires good conductivit# and short spans"

)vi* AAAC-'(%1 - Conductor composed of &'(%1 Aluminum Alloy. +t is stronger than AC2&and lighter& ut more e'pensive" +t is used for long spans in corrosive environments"

2ome factors to e considered when selecting the transmission line conductors include:

e8uired sag and span etween conductors

Tension on the conductors

9hether or not the atmosphere is corrosive

9hether or not the line is prone to viration

ower loss allowed on the line

%oltage loss allowed on the line

Climate at the line location

(inall#& the si4e of the conductor has to e considered" Again& several factors are used in

determining the si4e of the conductor to e used"

)olta"e Drop Considerations: The conductor meets the minimum si4e re8uirement ut

transmits the power with an acceptale loss" +t is often e'pressed as a ma'imum voltage drop of

5;" The total series impedance is e8ual to the ma'imum allowale voltage drop divided # the

ma'imum load current"

Thus:

Thermal Capacity: The conductor should e ale to carr# the ma'imum long term load current

without overheating" The Conductor is assumed to withstand a temperature of 05 degrees celsius

without a decrease in strength" Aove this temperature& the strength decreases"


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*conomic Considerations: The conductor is rarel# si4ed to meet the minimum re8uirements"

The total cost per !ilometer or mile must e ta!en into account as too the present worth of energ#

losses associated with the conductor" There must also e some compensation for load growth"

/" +nsulators

There are two t#pes of insulators: Suspension Typeand +in Type"The function of the insulatoris to support and anchor the insulator" Additionall#& the# also insulate the conductor from ground

and tend to e made of either glass or porcelain and in some cases& ceramic"

3" 2upport 2tructures

These serve the purpose of !eeping the conductors at a safe height from ground as well as at an

ade8uate distance from each other" The construction of the support is dependent on the cost" The

cost ta!es into account the design and the materials as well as transportation and laour"


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The aove factors are determined # whether there is the choice to use ma,imum euipmentand minimum labour or minimum euipment and ma,imum labour"

Location of +oles and Structures: oles and 2tructures have to e located in oservance of theright of wa# )2ee Definitions elow*" The initial step when locating the poles is to estalish a

plan-profile drawing" These drawings show a topographical contour map of the terrain along theright of wa#& and a sideview profile of the line& showing elevations and towers" The plan profile

drawing acts as a wor!sheet as to what needs to e done& in dealing with the prolems that areposed" The# are used to complete the wor! with respect to structure spotting"

2tructure spotting is a process that determines the height& location and t#pe of consecutive

structures on the plan profile drawing" 2tructure spotting should closel# conform to the designcriteria estalished for the line" The following steps should e ta!en when spotting structures:

Estalish the plan profile drawing on a fi'ed scale

Estalish the sag template on the same scale as the plan profile drawing

>a!e a tale showing the conductor clearances to ground as well as relative to other

overhead lines

Decide on the hori4ontal and vertical span limitations due to clearance and strength

re8uirements

Towers have to e uried at a certain depth to ensure that the# do not collapse" The depth ma# efrom , feet up to the height of the tower"

Two t#pes of towers are used:." Towers used for straight runs

/" Towers used when ends have to e made in the path of the line )Deviation Towers*

+n putting down deviation towers& gu#ed wires and gu#ed loc!s have to e used to alance thetensile forces on the tower" 9hen two forces act on a tower )which is usuall# the tension of the

line*& a resultant force is produced" A gu#ed wire is used to counteract this resultant force so as to

prevent the tower from collapsing" The gu#ed loc! is also used& and this is the uried loc! towhich the gu#ed wire is connected" The loc! is usuall# uried at an angle to negate the resultant

force on the line" The aove description is figurativel# shown elow"


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(igure ." This is the diagrammatic representation of the use of the deviation tower with the


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(igure /" This is the diagrammatic representation of the use of the


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of the area through which the line passes" +n locating towers and stringing the lines& the

electricit# commission has to determine the route of the line" Once this is estalished& then it is

necessar# to determine the right of wa#" +n some cases& the right of wa# cannot e otained& andas a result& alternate routes& in which the right of wa# can e otained must e devised" ight of

9a# must e clear of trees& or an# ostructions which ma# cause the line to fault& or touch& or

even result in the tower collapsing"

Sa"/

2ag is defined as the vertical distance etween the point where the line is 7oined to the tower and

the lowest point on the line"

(igure 3" Diagram showing the definition of sag"

The sag is as a result of the tensioning of the line and must not e too low otherwise the safet#clearances ma# not e met" Also& the sag had to e such that it caters for ice loading in the winter

of temperate climates" +f the sag is large& and the line ecomes heavil# loaded& then the sag will

further increase and reach the safet# clearances" 2imilarl#& if the sag is low& then when the linecontracts in the winter& a low sag will indicate a high tension& and as a result of this contraction&

the line ma# snap" 2ag is inversel# proportional to the tension of the line& and is given # the

formula elow"


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Overhead Transmission Lines @ rinciples of Engineering 2tatic >echanical 2upport

3ntroduction to the desi"nin" principles

9hen designing an overhead transmission line&we should pa# attention to ensure that the

tension forcedoes not e'ceed& in an# case& the limit of the mechanical strength of theconductor"

The ma'imum stress occurs at the lower temperature&when the line is su7ected to contraction&

and apossile ice coating">oreover& it should e considered that can simultaneousl# e andwind pressure on the line" To address these conditions& a re8uirement& is !nowledge of the arrow

of the conductor"

?esides& the arrow determines the height and strength of the supporting towers& as well as the

span length)distance between two towers*"

1. Static vision of the transmission line

Even:
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l span length in m )distance etween two support points*"

L conductor length in m& corresponding to the opening l"

w conductor weight in =pper meter"

T tensile strength of the transmission line& in =p"

D ma'imum arrow& in m"

(. Transmission line coated with ice 4 under the effect of wind

Even:

d conductor diameter in cm"

i radial ice thic!ness in cm"

wi ice weight per meter"

+ wind pressure at speed of B6 !mhr"

! resultant force )weight and wind pressure*"

The vertical component of the arrow:

#. Calculation of the arrow 5transmission line construction6

We accept as unfavorable conditions the following:

Amient temperature -.6C


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adial ice coating . cm

9ind speed B6 !mhr

9ith these conditions& we choose 2nddegree safety factor& so that the tension force should not

e'ceed half the rea!ing load of the conductor"

During the construction of the transmission line&with the conditions prevailing at the time)higher temperature and without ice*& the tension must e done in such a wa# that& at the most

unfavorale conditions& the transmission line to have a /nddegree safet# factor"

According to Rapson:

when:

T tensile strength during the construction& inKp)is considered constant along the transmission

line*"

A conductor cross section& in cm/

E #ield strength factor& in =pcm/

e'pansion factor per C"

t amient temperature aove -.6C"

Tc tensile strength in adverse conditions& in =p)Tcobtained half of the breaking load*"

?# solving the aove formula& we otain the value of T" Then& the arrow& during the construction&is:

7. Transmission line based on different levels

onsider !"# the imaginary lowest point of the transmission line$

%& is the hori4ontal distance etween the lowest support point and the O"%2 is the hori4ontal distance etween the highest support point and the O"

'& is the imaginar# arrow from the lowest support point"

'2 is the imaginar# arrow from the highest support point"

With these data, we have:
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We observe that:

From the above, we have:

9ith the values of '.& '/& the arrows D.& D/ can e calculated& as well as the height of an# point

on the transmission line from the ground"

*,ample

An overhead transmission line crosses a river and it is ased on the two an!s # two towers atheight h&()&$*mand h2(*+$,maove the water surface" The hori4ontal distance etween thetowers is --+$- m" The ma'imum tensile force is T ( &)-2$- Kpand the weight of the conductor

is w ( .$//*Kp0m"

Determine the height !h#of the line over the water& midwa# etween the two towers"


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Consider " the imaginar# lowest point of the transmission line and D.& D/ the arrows from

the lower and the tallest tower respectivel#"

Transmission line ased on different levels

However:

Therefore:


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Tower foundation

Thefoundationis the name given to the s#stem which transfers to the ground the various stead#

state )dead* and variale )live* loads developed # the transmission towerand conductors"

(oundations ma# e variousl# su7ected to compressive or earing forces& uplift and shearforces& either singl# or as a result of an# comination of two or three of the forces"

1sually the limiting design load with transmission line foundations is the uplift load$

2unrise owerlin! 2teel Cap >icropile (oundation )atent ending*
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+n this respect& there is a ma7or difference etween the design of foundations for transmission

linescompared to the design of foundations for most normal civil engineering structures"

Accordingl#& the amount of literature descriing design techni8ues for overhead line foundationsis relatively smallcompared to the literature availale for more traditional civil engineering

foundation design practice"

The selected foundation design for a particular tower must provide an economical& reliale

support for the life of the line" The foundation must e compatile with the soil and must not losestrength with age"

9ith the progressive increase in transmission s#stem voltages there has een a related increase in

foundation si4es and it is worth noting that with a t#pical 3uad conductor +.. 45 line& single leg

uplift and ultimate compression loads of 06 or B6 tonnes are usual for suspension towers"

With tension towers ultimate loads of 2.. or -.. tonnes are often developed$

+n ground of poor load-earing capacit# the dimensions of foundations ecome considerale"

+n the past& it was often acceptale to Fover-designG foundations to allow for uncertainties in the

soil characteristics" 9ith the large si4es of foundations for E$% and H$% transmissionit is

ovious that significant economies can e made in producing foundation designs to e'actl#match the soil conditions"

+ncreasingl#& transmission lines are routed through areas ofpoor ground conditions& often for

reasons of amenit#" This results in the need for the use of special& generall# larger& foundations"

The logistical prolems of installing large foundations& often in difficult ground conditions& muste ta!en into account when considering foundation design"

Types of "round

>icro pile (oundation for Transmission Line
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The ground in which the foundations are installed can var# from igneous& sedimentar# or

metamorphic roc!& noncohesive soils& sand or gravel to cohesive soil& usuall# cla#s" E8uall#&

soils with a high organic content& for e'ample peat& can also prevail" Composite soils will also efound& and e'amples of these are sand# gravels and silt# sand or sand# peat"

(undamental to the proper design of foundations is an accurate series of soil teststo determinethe range of soil t#pes for which the foundation designs will e re8uired" +t is good practice to

carr# out soil tests at a rate of . in 5 tower sites"

This is generall# sufficient to enale an accurate forecast of the range of soil t#pes to e

estalished"

+t should e pointed out& however& that with large towers having .5 or /6 m s8uare ases&

occasionall# each of the four legs of a tower may be founded in four different types of ground"

Types of foundation

There are seven basic types of tower foundations:

." 2teel grillage

/" Concrete spread footing

3" Concrete auger or caisson

1" ile foundation

5" oc! foundation

," aft foundation

0" Iovel foundations"

Foundation calculations

There are a numer of methods of calculation of foundation uplift and earing capacit#" (or the

purposes of this article& however& we will confine ourselves to a simple approachwhich must e

treated with care" Ievertheless& the methods indicated will give reasonal# accurate results forthe relativel# shallow foundations which are normall# emplo#ed with transmission line towers"

A shallow foundation is usuall# defined as one in which the readth of the pad is greater than the

setting depth"


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It is usual to calculate the uplift capacityof a foundation as being equal to the

mass of soil contained in the frustum developed between the base of the foundation

pad and the soil surface.

The angle of the face of the frustum to the vertical is usuall# designated J and will var# from

-+6 to *.6 in roc4& to /5 in good homogeneous hard cla# to 4ero in saturated noncohesiveground" The soil densit# will var# from 7ust over 2... 4g0m-for homogeneous roc! to aout

.,66 !gm3for soil with normal moisture content to aout /.. or ).. 4g0m-in the case of

ground su7ected to water uplift"

>ethods of calculation of uplift capacit# are shown elow"


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Undercut Pyramid Foundation

Undercut pyramid foundation

calculation


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Concrete Auger Foundation

Concrete auger foundationcalculation

On hill# terrains the spans would differ consideral# and also u ma# encounter negative wt"span)

alread# discussed in the forum* which is an up ward pull on the supports"Kou have to carr# out a

surve# of the route and profile e ta!en of the line"Once this is done u can decide the span"

temperature determines the ma'" sag which occurs at ma' temp and conductor must e strung

ta!ing this into account"2ag temp curves will e availale"in addition with regard to ice loading

the wind on ice coated conductors ma!es it severe with more area for wind force to act and

there# on the support"Hnder these conditions what is termed as


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L conductor clerence aove the ground in metre

C ruling span in metre

D conductor sag at rulin span C in mtre

uling span 28 oot Of sig L cue sig ma L

+t re8uire more detailing and not so eas# to present here"$ope u cam ma!e something out of the

aove"

/"

COOIA LO22

Corona phenomenon is the ioni4ation of air surrounding the power conductor

1

M " +t is a phenomena of violent

glow

& production of o4one gas and hissing noise in an overhead power conductor

5

M" (ree air surrounding the power

conductor is not perfect insulator and contains some ioni4ed particles )i"e" free electrons

and positive ions and

neutral molecules due to cosmic

ra#s& radioactivit#& ultraviolent radiation& sun& etc

,


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M when ac p"d is applied across

two conductors whose spacing is large compared to the diameters

0

M& potential gradient is set up in the air etween

the two parallel conductors& and increase the co

nducting surface and the velocit# of the free electrons& there#

increasing the electrostatic stress in the air" 9hen the potential gradients reaches or

e'ceeds a critical disruptive

value of 36!v cm or /.".!v )rms*cm

3& ,

&

0

M the ions attain a sufficientl# high velocit# and on stri!ing another

neutral molecule& disintegrates one or more electrons from the neutral molecule" This produces anew electrons and

positive ion which are in turn accelerated until a complete electric

rea!down occurs and an arc is estalished

etween the electrodes" Thus& the ioni4ation process is cumulative to form corona

1& 5

&

,

M" Corona

is noted #


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a

faint luminous luish discharge along the length of the conductors and at the same time

a hissing

sound is heard"

Corona discharge is alwa#s accompanied # production of o4one& which is dictated # its odour"

+ncrease in the p"d

increases the intensit# of the glow and hissing and conse8uentl# cause spar!

-

over etween the conductors" The

corona will

e uniform along the length of the conductor if the# are smooth and polished" +f the

conductors are

rough the glow will e relativel# righter" 9hen dc is applied instead of ac there will

e a difference in the

appearance of the two conductors" The positi

ve conductor will have a smooth glow while the negative conductor

will e spott#" +f the spacing is shorter as to compared with the diameters& the spar!ing

will start without visile

glow BM

...!

!

!" # $%&'&(...!

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Overhead Line Design and Transmission Line Construction