bus bar design

7/18/2019 Bus Bar Design

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Chapter ( 3 ) Bus Bars design

Chapter ( 5 )

( Bus Bars )

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Contentes.

1. Design ConsiderationsA. Introduction

B. Types o Bus!ar

C. Choice o Bus!ar "ateria#

$. A#ternating Current %ects in Bus!ars

A. &'in %ect

B. Condition or "iniu oss

3. %ect o Bus!ar Arrangeents on *ating

A. ainated copper !ars

B. Inter+#ea,ing o conductors

C. Transposition o conductors

D. -o##o s/uare arrangeent

%. Tu!u#ar !ars

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http://www.cda.org.uk/megab2/elecapps/pub22/sec1.htm#Introduction

http://www.cda.org.uk/megab2/elecapps/pub22/sec1.htm#Types%20of%20Busbar

http://www.cda.org.uk/megab2/elecapps/pub22/sec1.htm#Choice%20of%20Busbar%20Material

http://var/www/apps/conversion/tmp/scratch_2/sec4.htm#Skin%20Effect

http://var/www/apps/conversion/tmp/scratch_2/sec4.htm#Condition%20for%20Minimum%20Loss

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Laminated%20copper%20bars

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Inter-leaving%20of%20conductors

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Transposition%20of%20conductors

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Hollow%20square%20arrangement

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Tubular%20bars














0. Concentric conductors

.Channe# and ang#e !ars

-.Coparison o conductor arrangeents

I. %nc#osed copper conductors

2. Copound insu#ated conductors

.P#astic insu#ated conductors

. Iso#ated phase !us!ars

4. &e#ection o Bas !arsA. Coparison !eteen to types o se#ections

B. "iniu c#earance due to corona

C. &hort circuit heating and Durating Tie

D. 0au#t duration

1.Design Considerations

A. Introduction

B. Types of Busbar

C. Choice of Busbar Material

A. Introduction

The word busbar, derived from the Latin word omnibus (for all!, "ives the

idea of a universal system of conveyance. In the electrical sense, the term busis used to describe a #unction of circuits, usually in the form of a small number

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http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Concentric%20conductors

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Channel%20and%20angle%20bars

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Comparison%20of%20conductor%20arrangements

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Enclosed%20copper%20conductors

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Compound%20insulated%20conductors

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Plastic%20insulated%20conductors

http://www.cda.org.uk/megab2/elecapps/pub22/sec5.htm#Isolated%20phase%20busbars

















of inputs and many outputs. Busbar describes the form the bus system

usually ta$es, a bar or bars of conductin" material.

In any electrical circuit some electrical ener"y is lost as heat which, if not $ept

within safe limits, may impair the performance of the system. This ener"yloss, which also represents a financial loss over a period of time, is

proportional to the effective resistance of the conductor and the s%uare of the

current flowin" throu"h it. A low resistance therefore means a low loss& a

factor of increasin" importance as the ma"nitude of the current increases.

The capacities of modern'day electrical plant and machinery are such that the

power handled by their control systems "ives rise to very lar"e forces.

Busbars, li$e all the other e%uipment in the system, have to be able to

withstand these forces without dama"e. It is essential that the materials used

in their construction should have the best possible mechanical properties andare desi"ned to operate within the temperature limits laid down in B )*+, B

- /01+')2)++0, or other national or international standards.

A conductor material should therefore have the followin" properties if it is to

be produced efficiently and have low runnin" costs from the point of view of

ener"y consumption and maintenance2

a! Low electrical and thermal resistance

b! 3i"h mechanical stren"th in tension, compression and shear

c! 3i"h resistance to fati"ue failure

d! Low electrical resistance of surface films

e! ase of fabrication

f! 3i"h resistance to corrosion

"! Competitive first cost and hi"h eventual recovery value

This combination of properties is met best by copper. Aluminium is the main

alternative material, but a comparison of the properties of the two metals

shows that in nearly all respects copper is the superior material.

B. Types o Bus!ar

Busbars can be sub'divided into the followin" cate"ories, with individual

busbar systems in many cases bein" constructed from several different types2

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a! Air insulated with open phase conductors

b! Air insulated with se"re"atin" barriers between conductors of different

phases.

c! Totally enclosed but havin" the construction as those for (a! and (b!

d! Air insulated where each phase is fully isolated from its ad#acent phase(s!

by an earthed enclosure. These are usually called Isolated 4hase Busbars.

e! 5orce'cooled busbar systems constructed as (a! to (d! but usin" air, water,

etc. as the coolin" medium under forced conditions (fan, pump, etc.!.

f! 6as insulated busbars. These are usually constructed as type (e! but use a

"as other than air such as 5, (sulphur he7afluoride!.

"! Totally enclosed busbars usin" compound or oil as the insulation medium.

The type of busbar system selected for a specific duty is determined by

re%uirements of volta"e, current, fre%uency, electrical safety, reliability, short'

circuit currents and environmental considerations. Table ) outlines how these

factors apply to the desi"n of busbars in electricity "eneration and industrial

processes.

Table ) Comparison of typical desi"n re%uirements for power "eneration andindustrial process systems

5eature 6eneration Industrial 4rocesses

) 8olta"e drop -ormally not important Important

9 Temperature

rise

:sually near to ma7imum

allowable. Capitalisation

becomin" important.

In many cases low due to

optimisation of first cost

and runnin" costs.

1 Current ran"e ;ero to 0/ $ A a .c . with

fre%uencies of <ero to 0//

3<.

;ero to 9// $A a.c. and

d.c.

0 =ointin" and

connections

:sually bolted but hi"h

current applications are

often fully welded. =oint

preparation veryimportant

:sually bolted. =oint

preparation very

important.

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* Cross'

sectional area

:sually minimum.

omewhat lar"er if

optimisation is re%uired.

:sually lar"er than

minimum re%uired due to

optimisation and volta"e

drop considerations.

>elvins Law -ot applied. ?ther forms

of optimi<ation are often

used.

Applies. Also other forms

of optimi<ation and

capitali<ation used

@ Construction :p to 1 $ 8. Individually

en"ineered usin" basic

desi"ns and concepts.

:sually low volta"e.

Individually en"ineered.

tandard products for low

currentvolta"e

applications.

nclosures Totally enclosed with or

without ventilation.

:sually open. nclosed or

protected by screens when

usin" standard products.

+ 5ault capacity :sually lar"e. esi"ned

to meet system

re%uirement.

:sually similar to runnin"

current. tandard products

to suit system short circuit.

)/ 4hasearran"ement

-ormally 1 phase flatthou"h sometimes trefoil.

-ormally flat buttransposition used to

improve current

distribution on lar"e

systems

)) Load factor :sually hi"h. -ormally

)./.

:sually hi"h but many

have widely varyin" loads.

)9 Cost Low when compared with

associated plant.

Ma#or consideration in

many cases. 4articularly

when

optimisationcapitalisation

is used.

)1 ffects of

failure

8ery serious. 3i"h

ener"ies dissipated into

fault.

Limited by low volta"e

and busbar si<e.

)0 Copper type 3i"h conductivity. 3i"h conductivity.

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)* Copper shape :sually rectan"ular. Tubular used for hi"h current force'

cooled. :sually lar"e cross section rectan"ular. Tubular used for some low

current hi"h volta"e applications and hi"h current force'cooled.

C. Choice o Bus!ar "ateria#At the present time the only two commercially available materials suitable for

conductor purposes are copper and aluminum. The table below "ives a

comparison of some of their properties. It can be seen that for conductivity

and stren"th, hi"h conductivity copper is superior to aluminum. The only

disadvanta"e of copper is its density& for a "iven current and temperature rise,

an aluminum conductor would be li"hter, even thou"h its cross'section would

be lar"er. In enclosed systems however, space considerations are of "reater

importance than wei"ht. ven in open'air systems the wei"ht of the busbars,

which are supported at intervals, is not necessarily the decisive factor.

Ta!#e $ Typica# re#ati,e properties o copper and a#uiniu

Copper(CD

//0A!

Aluminium

()1*/!

:nits

lectrical conductivity (annealed! )/) ) E IAC

lectrical resistivity (annealed! ).@9 9.1 cm

Temperature coefficient of

resistance(annealed!

/.//1+ /.//0 F C

Thermal conductivity at 9/FC 1+@ 91/ Dm>

Coefficient of e7pansion )@ 7 )/G 91 7 )/G F C

Tensile stren"th (annealed! 9// G 9*/ */ G / -mm9

Tensile stren"th (halfGhard! 9/ G 1// * G )// -mm9

/.9E proof stress (annealed! */ G ** 9/ G 1/ -mm9

/.9E proof stress (halfGhard! )@/ G 9// / G * -mm9

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lastic modulus )) G )1/ @/ $-mm9

pecific heat 1* +// =$" >

ensity .+) 9.@/ "cm1

Meltin" point )/1 / FC

Ta!#e 3 Copper conductors of rectangular cross section inindoor installations.

A!ient teperature 356C.Conductor teperature 756C.Conductor idth ,ertica# c#earance !eteen conductors e/ua# to

conductor thic'ness8 ith a#ternating current8 c#earance !eteenphases9 :.; < phase centre #ine distance.Bare conductor part#y o=idi>ed8 gi,ing a radiation coeicient o :.4(cu).Conductor painted (on#y the outside suraces in the case o coposite!us !ars)8 gi,en a radiation coeicient o appro=. :.?.

@idth<

Thic'ness

""

Cross&ection""$

"ateria#3 Continuous current in Aa. c.up to 5: ->Paintedo. o conductors per ph.

1 $ 3 4

bare No. of conductors per phase

1 2 3 4

1$< 5 5?.5 %Cu 0 3 $:3 345 411 177 312 3981$ <1: 11?.5 %Cu 0 3 3$7 7:5 ;? 285 553 811$:< 5 ??.1 %Cu 0 3 31? 57: $; 274 500 690$:< 1: 1?? %Cu 0 3: 4? ?$4 13$: 427 825 1180

3:< 5 14? %Cu 0 3 44 7: ?44 379 672 896 3:<1: $?? %Cu 0 3: 77 1$:: 17: 573 1060 1480

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4:<5 1?? %Cu 0 3 53 ?5$ 114: 482 836 10904:<1: 3?? %Cu 0 3: ;5: 14: $::: $5;: 715 1290 1770 22805:<: $4? %Cu 0 3 7? 114: 133: $:1: 583 994 1240 19205:< 1: 4?? %Cu 0 3: 1:$: 1$: $3$: $?5: 852 1510 2040 26007:< 5 $?? %Cu 0 3: ;$7 133: 151: $31: 688 1150 1440 2210

7:< 1: 5?? %Cu 0 3: 11;: 1?7: $71: 3$?: 985 1720 2300 2900;: <5 3?? %Cu 0 3: 1:: 17;: 1;3: $;3: 855 1450 1750 2720;: <1: ?? %Cu 0 3: 15:: $41: 31: 3?3: 1240 2110 2790 34501:: <5 4?? %Cu 0 3: 13:: $:1: $15: 33:: 1080 1730 2050 31901:: <1: ?;; %Cu 0 3: 1;1: $;5: 3$: 453: 1490 2480 3260 39801$: <1: 1$:: %Cu 0 3: $11: 3$;: 4$: 513: 1740 2860 3740 4500 17:< 1: 17:: %Cu 0 3: $:: 413: 537: 73$: 2220 3590 4680 5530200 10 2000 !"Cu # 30 3290 4970 6430 7490 2690 4310 5610 6540

$. A#ternating Current %ects in Bus!ars

A. &'in %ectB. Pro=iity %ect

C. Condition or "iniu oss

A. &'in %ect

The apparent resistance of a conductor is always hi"her for a.c. than for d.c.

The alternatin" ma"netic flu7 created by an alternatin" current interacts with

the conductor, "eneratin" a bac$ e.m.f. which tends to reduce the current in

the conductor. The centre portions of the conductor are affected by the

"reatest number of lines of force, the number of line lin$a"es decreasin" as

the ed"es are approached. The electromotive force produced in this way byself'inductance varies both in ma"nitude and phase throu"h the cross'section

of the conductor, bein" lar"er in the centre and smaller towards the outside.

The current therefore tends to crowd into those parts of the conductor in

which the opposin" e.m.f. is a minimum& that is, into the s$in of a circular

conductor or the ed"es of a flat strip, producin" what is $nown as s$in or

ed"e effect. The resultin" non'uniform current density has the effect of

increasin" the apparent resistance of the conductor and "ives rise to increased

losses.

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http://var/www/apps/conversion/tmp/scratch_2/sec4.htm#Proximity%20Effect



http://var/www/apps/conversion/tmp/scratch_2/sec4.htm#Proximity%20Effect





The ratio of the apparent d.c. and a.c. resistances is $nown as the s$in effect

ratio2

where Hf a.c. resistance of conductor

Ho d.c. resistance of conductor

s$in effect ratio

The ma"nitude and importance of the effect increases with the fre%uency, and

the si<e, shape and thic$ness of conductor, but is independent of thema"nitude of the current flowin".

It should be noted that as the conductor temperature increases the s$in effect

decreases "ivin" rise to a lower than e7pected a.c. resistance at elevated

temperatures. This effect is more mar$ed for a copper conductor than an

aluminium conductor of e%ual cross'sectional area because of its lower

resistivity. The difference is particularly noticeable in lar"e busbar sections.

• Copper rodsThe s$in effect ratio of solid copper rods can be calculated from the formulae

derived by Ma7well, Haylei"h and others ( Bulletin of the Bureau of

Standards, 1912!2

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where $in effect ratio

d diameter of rod, mm

f fre%uency, 3<

J resistivity, J cm

K permeability of copper ()!

where A cross'sectional area of the conductor, mm9

• Copper tu!es

$in effect in tubular copper conductors is a function of the thic$ness of the

wall of the tube and the ratio of that thic$ness to the tube diameter, and for a

"iven cross sectional area it can be reduced by increasin" the tube diameter

and reducin" the wall thic$ness.

5i"ure *, 5i"ure , and 5i"ure @, which have been drawn from formulae

derived by wi"ht ()+99! and Arnold ()+1!, can be used to find the value of

s$in effect for various conductor sections. In the case of tubes (5i"ure *!, it

can be seen that to obtain low s$in effect ratio values it is desirable to ensure,where possible, low values of td and (fr!. 5or a "iven cross'sectional area

the s$in effect ratio for a thin copper tube is appreciably lower than that for

any other form of conductor. Copper tubes, therefore, have a ma7imum

efficiency as conductors of alternatin" currents, particularly those of hi"h

ma"nitude or hi"h fre%uency.

The effect of wall thic$ness on s$in effect for a )// mm diameter tube

carryin" a */3< alternatin" current is clearly shown in 5i"ure *.

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0igure 5 *esistance o -C copper tu!es8 1:: outside diaeter8 d.c.and 5: -> a.c.

0igure 7 &'in eect or rods and tu!es

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• 0#at copper !ars

The s$in effect in flat copper bars is a function of its thic$ness and width.

Dith the lar"er si<es of conductor, for a "iven cross'sectional area of copper,

the s$in effect in a thin bar or strip is usually less than in a circular copper rod

but "reater than in a thin tube. It is dependent on the ratio of the width to the

thic$ness of the bar and increases as the thic$ness of the bar increases. A thincopper strip, therefore, is more efficient than a thic$ one as an alternatin"

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current conductor. 5i"ure @ can be used to find the s$in effect value for flat

bars.

0igure &'in eect or rectangu#ar conductors

• &/uare copper tu!es

The s$in effect ratio for s%uare copper tubes can be obtained from 5i"ure .

0igure ; &'in eect ratio or ho##o s/uare conductors

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B. Condition or "iniu oss

Both s$in and pro7imity effects are due to circulatin" or eddy currents caused

by the differences of inductance which e7ist between different elements of

current'carryin" conductors. The necessary condition for avoidance of both

these effects (and hence for minimum loss! is that the shapes of each of the

conductors in a sin"le'phase system appro7imates to e%ui'inductance lines.

Arnold ()+1@! has shown that for close spacin", rectan"ular section

conductors most closely approach this ideal. uch an arran"ement is alsoconvenient where space is limited and where inductive volta"e drop due to

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busbar reactance must be reduced to a minimum. In the case of heavy current

sin"le'phase busbars and where space is sli"htly less restricted, the sin"le

channel arran"ement "ives the closest appro7imation to the e%ui'inductance

condition, the channels of "o and return conductors bein" arran"ed bac$'to'

bac$, while for wider spacin" a circular section is preferable.

3. %ect o Bus!ar Arrangeents on *ating



C. Transposition o conductors

. -o##o s/uare arrangeent

. Modified hollow s%uare

5. Tu!u#ar !ars

6. Concentric conductors

3. Channe# and ang#e !ars

I. Coparison o conductor arrangeents

=. %nc#osed copper conductors

>. Copound insu#ated conductors

L. P#astic insu#ated conductors

M. Iso#ated phase !us!ars

The efficiency of all types of heavy current busbars depends upon careful

desi"n, the most important factors bein"2

a! The provision of a ma7imum surface area for the dissipation of heat.

b! An arran"ement of bars which cause a minimum of interference with the

natural movements of air currents.

c! An appro7imately uniform current density in all parts of the conductors.

This is normally obtained by havin" as much copper as possible e%uidistant

from the ma"netic centre of the busbar.

d! Low s$in effect and pro7imity effect for a.c. busbar systems.

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To meet these re%uirements there are many different arran"ements of copper

busbars usin" laminations, as well as copper e7trusions of various cross'

sections.

0igure ? Bus!ar arrangeents


To obtain the best and most efficient ratin" for rectan"ular strip copper

conductors they should be mounted whenever possible with their ma#or cross'sectional a7es vertical so "ivin" ma7imum coolin" surfaces.

Laminations of or .1 mm thic$ness, of varyin" widths and with or .1

mm spacin"s are probably the most common and are satisfactory in most a.c.

low current cases and for all d.c. systems.

It is not possible to "ive any "enerally applicable factors for calculatin" the

d.c. ratin" of laminated bars, since this depends upon the si<e and proportions

of the laminations and on their arran"ement. A "uide to the e7pected relative

ratin"s are "iven in Table below for a */ 3< system. The ratin"s for sin"le bars can be estimated usin" the methods "iven in ection 1 and ection 0.

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Ta!#e ; "u#tip#ying actors or #ainated !ars

Table )1 (Appendi7 9! "ives a.c. ratin"s for various confi"urations of

laminated bars based on test measurements.

5or all normal li"ht and medium current purposes an arran"ement such as that

in 5i"ure +a is entirely satisfactory, but for a.c. currents in e7cess of 1/// A

where lar"e numbers of laminations would be re%uired it is necessary to

rearran"e the laminations to "ive better utilisation of the copper bars.

The effect of usin" a lar"e number of laminations mounted side by side is

shown in 5i"ure )/ for a.c. currents. The current distribution is independent of

the total current ma"nitude.

0igure 1: A#ternating current distri!ution in a !ar ith ten #ainations

This curve shows that due to s$in effect there is a considerable variation in the

current carried by each lamination, the outer laminations carryin"

appro7imately four times the current in those at the centre. The two centre

laminations to"ether carry only about one'tenth of the total current.

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The currents in the different laminations may also vary appreciably in phase,

with the result that their numerical sum may be "reater than their vectorial

sum, which is e%ual to the line current. These circulatin" currents "ive rise to

additional losses and lower efficiency of the system. It should also be noted

that the curve is non'symmetrical due to the pro7imity effect of an ad#acent phase.

5or these reasons it is recommended that alternate arran"ements, such as those

discussed in the followin" sections, are used for heavy current a.c. svstems.


Dhere lon" low'volta"e a.c. bars are carryin" heavy currents, particularly at a

low power factor, inductive volt drop may become a serious problem with

laminated bars arran"ed as in 5i"ure +a. The volta"e drop for any "iven si<eof conductor is proportional to the current and the len"th of the bars, and

increases as the separation between conductors of different phases increases.

In the case of laminated bars the inductive volt drop can be reduced by

splittin" up the bars into an e%uivalent number of smaller circuits in parallel,

with the conductors of different phases interleaved as shown in 5i"ure +b.

This reduces the avera"e spacin" between conductors of different phases and

so reduces the inductive volt drop.

C.Transposition o conductorsThe unbalanced current distribution in a laminated bar carryin" a.c. current

due to s$in and pro7imity effects may be counteracted by transposin"

laminations or "roups of laminations at intervals. Tappin"s and other

connections ma$e transposition difficult, but it can be worthwhile where lon"

sections of bars are free from tappin"s. The arran"ement is as shown in 5i"ure

+e.

D.-o##o s/uare arrangeentTo obtain a ma7imum efficiency from the point of view of s$in effect, as

much as possible of the copper should be e%uidistant from the ma"netic centre

of a bar, as in the case of a tubular conductor. This can reduce the s$in effect

to little "reater than unity whereas values of 9 or more are possible with other

arran"ements havin" the same cross'sectional area.

Dith flat copper bars the nearest approach to a unity s$in effect ratio is

achieved usin" a hollow s%uare formation as shown in 5i"ure +c, thou"h the

current arran"ement is still not as "ood as in a tubular conductor. The heat

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dissipation is also not as "ood as the same number of bars arran"ed side by

side as in 5i"ure +b, due to the hori<ontally mounted bars at the top and

bottom.

%. "odiied ho##o s/uareThis arran"ement (5i"ure +d! does not have as "ood a value of s$in effect

ratio as the hollow s%uare arran"ement, but it does have the advanta"e that the

heat dissipation is much improved. This arran"ement can have a current'

carryin" capacity of up to twice that for bars mounted side by side, or

alternatively the total cross'sectional area can be reduced for similar current'

carryin" capacities.

0. Tu!u#ar !ars

A tubular copper conductor is the most efficient possible as re"ards s$in

effect, as the ma7imum amount of material is located at a uniform distance

from the ma"netic centre of the conductor. The s$in effect reduces as the

diameter increases for a constant wall thic$ness, with values close to unity

possible when the ratio of outside diameter to wall thic$ness e7ceeds about

9/.

The natural coolin" is not as "ood as that for a laminated copper bar system of

the same cross'sectional area, but when the pro7imity effects are ta$en intoaccount the one'piece tube ensures that the whole tube attains an even

temperature ' a condition rarely obtained with laminated bar systems.

Tubular copper conductors also lend themselves to alternative methods of

coolin" by, for e7ample, forced air or li%uid coolin" where heat can be

removed from the internal surface of the tubes. Current ratin"s of several

times the natural air cooled value are possible usin" forced coolin" with the

lar"est increases when li%uid coolin" is employed.

A tubular bar also occupies less space than the more usual copper laminated

bar and has a further advanta"e that its stren"th and ri"idity are "reater and

uniform in all deflection planes. These advanta"es are, however, somewhat

reduced by the difficulty of ma$in" #oints and connections which are more

difficult than those for laminated bars. These problems have now been

reduced by the introduction of copper weldin" and e7othermic copper formin"

methods. Copper tubes are particularly suitable for hi"h current applications,

such as arc furnaces, where forced li%uid coolin" can be used to "reat

advanta"e. The tube can also be used in isolated phase busbar systems due to

the ease with which it can be supported by insulators.

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.Concentric conductors

This arran"ement is not widely used due to difficulties of support but has the

advanta"e of the optimum combination of low reactance and eddy current

losses and is well suited to furnace and weld set applications. It should benoted that the isolated phase busbar systems are of this type with the current in

the e7ternal enclosure bein" almost e%ual to that in the conductor when the

continuously bonded three'phase enclosure system is used.

-.Channe# and ang#e !ars

Alternative arran"ements to flat or tubular copper bars are the channel and

an"le bars which can have advanta"es. The most important of these shapes are

shown in the dia"rams below.

These are easily supported and "ive "reat ri"idity and stren"th while the

ma$in" of #oints and connections presents no serious difficulty.

The permissible alternatin" current density in free air for a "iven temperature

rise is usually "reater in the case of two an"le'shaped conductors (dia"ram

(a!! than in any other arran"ement of conductor material.

5or low volta"e heavy current sin"le'phase bars with narrow phase centres,

sin"le copper channels with the webs of the "o and return conductors

towards one another "ive an efficient arran"ement. The channel si<es can be

chosen to reduce the s$in and pro7imity effects to a minimum, "ive ma7imum

dissipation of heat and have considerable mechanical stren"th and ri"idity.

Dhere hi"h volta"e busbars are concerned the phase spacin" has to be much

lar"er to "ive ade%uate electrical clearances between ad#acent phases with best

arran"ement bein" with the channel webs furthest apart. 5or hi"h'capacity"enerators which are connected to transformers and allied e%uipment by

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se"re"ated or non'se"re"ated copper busbars, the double an"le arran"ement

"ives the best combination with the copper bar si<es still bein" readily

manufactured. The current ratin"s of these arran"ements are "iven in Table )*

(Appendi7 9!. The ratin"s "iven are the ma7imum current ratin"s which do

not ta$e the cost of losses into account and hence are not optimised.

I. Coparison o conductor arrangeents

The e7tent to which the a.c. current ratin" for a "iven temperature rise of a

conductor containin" a "iven cross'sectional area of copper depends on the

cross'section shape. The appro7imate relative a.c. ratin"s for a typical cross'

sectional area of )/ /// mm9 are shown in 5i"ure )). 5or cross'sectional

areas "reater than )/ /// mm9 the factors are "reater than those shown, and

are smaller for smaller cross'sections. In the case of double'channel busbars,

the ratio of web'to'flan"e len"ths and also the web thic$ness have a

considerable effect on the current carryin" capacity.

0igure 11 Coparati,e a.c. ratings o ,arious conductor arrangeentseach ha,ing a cross sectiona# area o 1:8::: $ o -C copper

2. %nc#osed copper conductors

In many cases busbars are surrounded by enclosures, normally metallic, which

reduce the busbar heat dissipation due to reduction in coolin" air flow and

radiation losses and therefore "ive current ratin"s which may be considerably

less than those for free air e7posure. 8entilated enclosures, however, provide

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mechanical protection and some coolin" air flow with the least reduction in

current ratin".

The reduction in ratin" for a "iven temperature rise will vary considerably

with the type and si<e of bar and enclosure. The "reatest decrease in currentratin" occurs with bars which depend mainly on free air circulation and less

on uniform current distribution such as the modified hollow s%uare

arran"ement (5i"ure +d!. In these cases the ratin" may be reduced to between

/ and *E when the conductors are enclosed in non'ma"netic metal

enclosures. In the case of tubular conductors or those of closely "rouped flat

laminations, which are normally not so well cooled by air circulation, the

ratin"s may be reduced to about @*E of free air ratin"s for normal

temperature rises.

Dhere the busbar system is enclosed in thic$ ma"netic enclosures, such as inmetal'clad switch"ear, the reduction is appro7imately a further )*E. The

effect of thin sheet'steel enclosures is somewhat less. These additional

reductions are due to the heat "enerated by the alternatin" ma"netic fields

throu"h hysteresis and eddy current losses. Besides the deratin" caused by

enclosure conditions, other limitations on ma7imum wor$in" temperature are

often present, such as when the outside of enclosures should not e7ceed a

"iven safety value. These deratin"s are affected by the electrical clearances

involved and the de"ree of ventilation in the enclosure. The above fi"ures and

the curves shown in 5i"ure )9 should only be ta$en as a rou"h "uide to there%uired deratin"& an accurate fi"ure can only be obtained by testin".

All parts such as conductor and switch fittin"s, enclosures and interphase

barriers may be sub#ect to appreciable temperature rise due to circulatin" and

eddy current losses when close to the heavy current bars and connections.

These losses can be reduced to a minimum by ma$in" these parts from hi"h

conductivity non'ma"netic material such as copper or copper alloy.

0igure 1$ Coparison o appro=iate current ratings or !us!ars indierent enc#osures

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.Copound insu#ated conductors

The current ratin" of copper immersed in oil or compound depend upon a

number of factors which may vary widely with desi"n, and can normally only

be confirmed by carryin" out temperature rise tests on the complete assembly.

The ratin"s of enclosed bars are nearly always much lower than the free air

ratin"s. The temperature rise is dependent on the rate at which heat isconducted throu"h the insulatin" media and dissipated from the outside casin"

by radiation and convection. There is nearly always a closer phase spacin"

between conductors "ivin" hi"h pro7imity effects and hi"her heat losses in the

ma"netic outer casin"s and so "ivin" rise to hi"her temperature rises.

4ro7imity effect is often more important for insulated bars than those in air.

Laminated bars have fewer advanta"es when immersed in oil or compound

and circular copper conductors either solid or hollow thou"h are often

preferred particularly for hi"h'volta"e "ear and hi"h current "enerators,

transformers, etc., where more effective coolin" such as water coolin" can be

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employed to improve conductor material utilisation and hence reduce the

overall si<e of plant.

. P#astic insu#ated conductors

There is a widenin" use of plastic continuous insulation as the primary

insulation for low current and volta"e busbars. This insulation is usually of the

shrin$'on 4.8.C. type thou"h wrap'on tape is sometimes used. This method is

used for volta"es up to about )* $8, thou"h much hi"her levels can be

attained when specialised insulation systems such as epo7y resin or similar

based tapes and powders are employed. These systems are particularly useful

where hi"h atomic radiation levels, or hi"h temperatures (up to )1/FC! are

encountered, althou"h account must be ta$en of the possibility of halo"en

"assin" from 4.8.C. insulations at temperatures around )//FC. Modified

4.8.C. materials with improved hi"h'temperature performance are available.

". Iso#ated phase !us!ars

solated phase busbars consist of a metallic enclosed conductor where each

individual phase or pole is surrounded by a separately earthed sheath which is

connected at its ends by a full short'circuit current rated bar. The sheath is

intended primarily to prevent interphase short'circuit currents developin".

They have the further advanta"e that the hi"h ma"netic fields created by the

conductor current are almost completely cancelled by an e%ual and oppositecurrent induced in the enclosure or sheath with reductions of +*E or better in

the e7ternal ma"netic field bein" possible. An important result is that the

li$elihood of steelwor$ overheatin" when ad#acent to the busbar system is

considerably reduced e7cept where the sheath short'circuit bars are located.

This current flowin" in the enclosure ma$es the method of estimatin" the

performance of the busbar system much more complicated and can only be

resolved by obtainin" a heat balance between conductor and enclosure usin"

an interactive calculation method.

These busbars are used normally for operatin" volta"es of between )) $8 and

1 $8 thou"h e%uipment usin" much lower volta"es and hi"her volta"es are

increasin"ly chan"in" to this system. 7amples of such e%uipment are e7citer

connections, switch"ear interconnections, "enerator to transformer

connections, hi"h volta"e switch"ear usin" 5 (sulphur he7afluoride! "as

insulation (this "as havin" an insulation level many times better than air!. The

current flowin" in the conductor ran"es from as little as )/// A to in e7cess of

0/ $A. To obtain the hi"her currents forced coolin" is used, the most

commonly used coolin" media bein" air and water thou"h other coolin" "ases

or li%uids can be used. The use of these coolin" systems usually creates much

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increased heat losses and so their use must be #ustified by benefits in other

areas, e."., reduced civil costs, reduced physical si<e where space is at a

premium or reduction in si<e to enable normal manufacturin" methods be

used both for the basic busbar material and also the complete busbar system.

Another factor which influences the method chosen for forced coolin" is the

naturally cooled ratin" of the busbar system and also its ability to sustain

overload conditions. The busbars are usually manufactured in sin"le'phase

units of transportable len"th and consist of a central conductor usually tubular

of round, s%uare or channel cross'section, supported by porcelain or epo7y

resin insulators. The insulators are located by the e7ternal metallic sheath

throu"h which they are normally removed for servicin".

4.$election of %as bars&

Bus bar connected each transformer and main distribution board.

5or each transformer

Total >8A // >8A

Total current )9)*.* A

Total len"th * m

5rom this data, we can use copper conductor in door installation at

ambient temperature 1*C, conductor temperature *C painted bus bar 5rom tables above for copper conductor ('Cu 51/!

>) ) correction factor for load variations relatin" to conductivity,

>9 ) correction factor for other air and or busbar temperatures

(*C for Cu !

>1 /.*correction factor for thermal load variations due to differences

in layout.

>0 ) correction factor for electrical load variations (with alternatin"

current ! due to differences in layout ,Current carryin" capacity )9)*.*/.* )01/ A

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'.Co(parison bet)een t)o t*pes of selections&

ne conductor per

phase (!are8rectangu#ar)

To conductor per

phase(!are8 rectangu#ar)ContinuousCurrent (A) 14?: 1$:

@idth <thic'ness() 1:: E 1: 7: E 1:

Cross section($) ?;; 5?? E$

*esistance (F) 1.:54E1:G(+4) ;.7?5E1:G(+5)@eight gH ;.;? 5.33 E $

Cost @ -I-Poer #osses (@) $15.7 1.;

0ro ta!#e a!o,e se#ect one conductor per phase (!are8 rectangu#ar)

B. +ini(u( clearance due to corona & @

The iniu distance !eteen conductor centers (s) is

estiated ro h

J 1$5 E E K E #og (&Hr)&

@here+ J *s ,o#tage to neutra# in "J sursace actor J :.?7 K J air density actor J (3.? E Pair)H($3LMair) 8 0or Pair J 7 c -g8 Mair J 45 6 C 8

K J :.?5 r J 1.$5 (H$) J 1.$5 E (H$) J 1.$5 (1::H$) J 7$.5 J (.3;HN$) &J 73

C.$hort circuit heating and ,urating -i(e&

Oau#t J OIB. L O"ca!#e L O Tr. LOB.B.

J :.:3?5Lj:.:?$5L 1.5E1:G(+5) L1.1;E1:G(+4) J :.:3?5Lj:.:?4pu

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Oau#t J :.:;; pu"As.c J 1 H Oau#t J 11.3Is.c J 11.3H(:.3;EN3) J 1.14 Ap.

. #ault duration&

0or cu !as !ar e ha,e

sc pea/ 22104 a tlogf233i233:@here

o Qi J initia# conductor (B.B) tep. !eore au#to Q J ina# B.B tep. ater au#t.o a J area in inch$o tJ duration o au#t in sec.o Iscpea' J pea' short ciruit current J $EN$ E Isc J 4;.4; Ao Qi J 556C ( a##oing 156C tep. rise at nora# condition)

o QJ ;56C (a##oing 456C tep. rise during s.c)o aJ1.513 in$

tJ $ sec

bus bar design

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