Chapter ( 3 )
Bus Bars design
Chapter ( 5 )
( Bus Bars )
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Chapter ( 3 )
Bus Bars design
Contentes. 1. Design Design Conside Considerati rations ons A. Introduction
B. Types o Bus!ar C. Choice o Bus!ar "ateria#
$. A#ternating A#ternating Curren Currentt %ects in in Bus!ars Bus!ars A. &'in %ect B. Condition or "iniu oss 3. %ect %ect o Bus!ar Bus!ar Arrange Arrangeents ents on *ating *ating
A. ainated copper !ars Inter+#ea,ing #ea,ing o conductors B. Inter+ C. Tr Transposition ansposition o conductors D. -o##o s/uare arrangeent %. Tu Tu!u#ar !u#ar !ars Project – P1
Chapter ( 3 )
Bus Bars design
0. Concentric conductors . Channe# and ang#e !ars -. Coparison o conductor arrangeents I. %nc#osed copper conductors 2. Copound insu#ated conductors . P#astic insu#ated conductors . Iso#ated phase !us!ars 4. &e#ection o Bas !ars
A. Coparison !eteen to types o se#ections B. "iniu c#earance due to corona C. &hort circuit heating and Durating Tie D. 0au#t duration
1.Design Considerations A. Introduction B. Types of Busbar C. Choice of Busbar Material
A.
Introduction
The word busbar, derived from the Latin word omnibus (for all!, "ives the idea of a universal system of conveyance. In the electrical sense, the term bus is used to describe a #unction of circuits, usually in the form of a small number
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of inputs and many outputs. Busbar describes the form the bus system usually ta$es, a bar or bars of conductin" material. In any electrical circuit some electrical ener"y is lost as heat which, if not $ept within safe limits, may impair the performance of the system. This ener"y loss, which also represents a financial loss over a period of time, is proportional to the effective resistance of the conductor and the s%uare of the current flowin" throu"h it. A low resistance therefore means a low loss& a factor of increasin" importance as the ma"nitude of the current increases. The capacities of modern'day electrical plant and machinery are such that the power handled by their control systems "ives rise to very lar"e forces. Busbars, li$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 and are 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 and industrial process systems 5eature
6eneration
Industrial 4rocesses
)
8olta"e drop
-ormally not important
Important
9
Temperature rise
:sually near to ma7imum In many cases low due to allowable. Capitalisation optimisation of first cost becomin" important. and runnin" costs.
1
Current ran"e
;ero to 0/ $ A a .c . with ;ero to 9// $A a.c. and fre%uencies of
0
=ointin" and connections
:sually bolted but hi"h current applications are often fully welded. =oint preparation very important
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:sually bolted. =oint preparation very important.
Chapter ( 3 )
Bus Bars design
*
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
Applies. Also other forms of optimi
@
Construction
:p to 1 $ 8. Individually :sually low volta"e. en"ineered usin" basic Individually en"ineered. desi"ns and concepts. 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.
)/
4hase arran"ement
-ormally 1 phase flat -ormally flat but thou"h sometimes trefoil. transposition used to improve current distribution on lar"e systems
))
Load factor
:sually hi"h. -ormally )./.
)9
Cost
Low when compared with Ma#or consideration in associated plant. 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
)0
Copper type
3i"h conductivity.
3i"h conductivity.
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:sually hi"h but many have widely varyin" loads.
Chapter ( 3 )
Bus Bars design
)* 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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Chapter ( 3 )
Bus Bars design
lastic modulus
)) G )1/
@/
$-mm9
pecific heat
1*
+//
=$" >
ensity
.+)
9.@/
"cm1
Meltin" point
)/1
/
FC
Ta!#e 3 Copper conductors of rectangular cross section in indoor 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 !eteen phases9 :.; < 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 A bare a. c.up to 5: -> No. of conductors per Painted phase o. o conductors per ph. 1
1$< 5 1$ <1: $:< 5 $:< 1: 3:< 5 3:<1: Project – P1
5?.5 11?.5 ??.1 1?? 14? $??
%Cu 0 3 %Cu 0 3 %Cu 0 3 %Cu 0 3: %Cu 0 3 %Cu 0 3:
$:3 3$7 31? 4? 44 77
$
3
345 7:5 57: ?$4 7: 1$::
411 ;? $; 13$: ?44 17:
4
1
2
3
177 285 274 427 379 573
312 398 553 811 500 690 825 1180 672 896 1060 1480
4
Chapter ( 3 ) 4:<5 4:<1: 5:<: 5:< 1: 7:< 5 7:< 1: ;: <5 ;: <1: 1:: <5 1:: <1: 1$: <1: 17:< 1: 200 10
1?? 3?? $4? 4?? $?? 5?? 3?? ?? 4?? ?;; 1$:: 17::
2000
Bus Bars design %Cu 0 3 %Cu 0 3: %Cu 0 3 %Cu 0 3: %Cu 0 3: %Cu 0 3: %Cu 0 3: %Cu 0 3: %Cu 0 3: %Cu 0 3: %Cu 0 3: %Cu 0 3:
!"Cu # 30
53 ?5$ ;5: 14: 7? 114: 1:$: 1$: ;$7 133: 11;: 1?7: 1:: 17;: 15:: $41: 13:: $:1: 1;1: $;5: $11: 3$;: $:: 413:
114: $::: $5;: 133: $:1: $3$: $?5: 151: $31: $71: 3$?: 1;3: $;3: 31: 3?3: $15: 33:: 3$: 453: 4$: 513: 537: 73$:
3290 4970 6430 7490
482 836 1090 715 1290 1770 2280 583 994 1240 1920 852 1510 2040 2600 688 1150 1440 2210 985 1720 2300 2900 855 1450 1750 2720 1240 2110 2790 3450 1080 1730 2050 3190 1490 2480 3260 3980 1740 2860 3740 4500 2220 3590 4680 5530 2690 4310 5610 6540
$. A#ternating Current %ects in Bus!ars A. &'in %ect B. Pro=iity %ect C. Condition or "iniu oss
A. &'in %ect The apparent resistance of a conductor is always hi"her for a.c. than for d.c. The alternatin" ma"netic flu7 created by an alternatin" current interacts with the conductor, "eneratin" a bac$ 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$a"es decreasin" as the ed"es are approached. The electromotive force produced in this way by self'inductance varies both in ma"nitude and phase throu"h the cross'section of the conductor, bein" lar"er in the centre and smaller towards the outside. The current therefore tends to crowd into those parts of the conductor in which the opposin" e.m.f. is a minimum& that is, into the s$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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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$in effect ratio The ma"nitude and importance of the effect increases with the fre%uency, and the si
Copper rods
The s$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$in effect for a )// mm diameter tube carryin" a */3< alternatin" current is clearly shown in 5i"ure *.
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0igure 5 *esistance o -C copper tu!es8 1:: outside diaeter8 d.c. and 5: -> a.c.
0igure 7 &'in eect or rods and tu!es
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•
Bus Bars design
0#at copper !ars
The s$in effect in flat copper bars is a function of its thic$ness and width. Dith the lar"er si
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current conductor. 5i"ure @ can be used to find the s$in effect value for flat bars.
0igure &'in eect or rectangu#ar conductors
•
&/uare copper tu!es
The s$in effect ratio for s%uare copper tubes can be obtained from 5i"ure .
0igure ; &'in eect ratio or ho##o s/uare conductors Project – P1
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B. Condition or "iniu oss Both s$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 also convenient where space is limited and where inductive volta"e drop due to
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Bus Bars design
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 A. B.
ainated copper !ars Inter+#ea,ing o conductors
C.
Transposition o conductors
.
-o##o s/uare arrangeent
. Modified hollow s%uare 5.
Tu!u#ar !ars
6.
Concentric conductors
3. I.
Channe# and ang#e !ars 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$in effect and pro7imity effect for a.c. busbar systems.
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To meet these re%uirements there are many different arran"ements of copper busbars usin" laminations, as well as copper e7trusions of various cross' sections.
0igure ? Bus!ar arrangeents
A. ainated copper !ars To obtain the best and most efficient ratin" for rectan"ular strip copper conductors they should be mounted whenever possible with their ma#or cross' sectional a7es vertical so "ivin" ma7imum coolin" surfaces. Laminations of or .1 mm thic$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
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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$in effect there is a considerable variation in the current carried by each lamination, the outer laminations carryin" appro7imately four times the current in those at the centre. The two centre laminations to"ether carry only about one'tenth of the total current.
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The currents in the different laminations may also vary appreciably in phase, with the result that their numerical sum may be "reater than their vectorial sum, which is e%ual to the line current. These circulatin" currents "ive rise to additional losses and lower efficiency of the system. It should also be noted that the curve is non'symmetrical due to the pro7imity effect of an ad#acent phase. 5or these reasons it is recommended that alternate arran"ements, such as those discussed in the followin" sections, are used for heavy current a.c. svstems.
B. Inter+#ea,ing o conductors Dhere lon" low'volta"e a.c. bars are carryin" heavy currents, particularly at a low power factor, inductive volt drop may become a serious problem with laminated bars arran"ed as in 5i"ure +a. The volta"e drop for any "iven si
C.Transposition o conductors The unbalanced current distribution in a laminated bar carryin" a.c. current due to s$in and pro7imity effects may be counteracted by transposin" laminations or "roups of laminations at intervals. Tappin"s and other connections ma$e transposition difficult, but it can be worthwhile where lon" sections of bars are free from tappin"s. The arran"ement is as shown in 5i"ure +e.
D.-o##o s/uare arrangeent To obtain a ma7imum efficiency from the point of view of s$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
%. "odiied ho##o s/uare This 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 into account 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$in" #oints and connections which are more difficult than those for laminated bars. These problems have now been reduced by the introduction of copper weldin" and e7othermic copper formin" methods. Copper tubes are particularly suitable for hi"h current applications, such as arc furnaces, where forced li%uid coolin" can be used to "reat advanta"e. The tube can also be used in isolated phase busbar systems due to the ease with which it can be supported by insulators.
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.Concentric conductors This arran"ement is not widely used due to difficulties of support but has the advanta"e of the optimum combination of low reactance and eddy current losses and is well suited to furnace and weld set applications. It should be noted that the isolated phase busbar systems are of this type with the current in the e7ternal enclosure bein" almost e%ual to that in the conductor when the continuously bonded three'phase enclosure system is used.
-.Channe# and ang#e !ars Alternative arran"ements to flat or tubular copper bars are the channel and an"le bars which can have advanta"es. The most important of these shapes are shown in the dia"rams below. These are easily supported and "ive "reat ri"idity and stren"th while the ma$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
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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
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 arrangeents each 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
0igure 1$ Coparison o appro=iate current ratings or !us!ars in dierent 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 is conducted 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
. 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 opposite current 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/ $A. To obtain the hi"her currents forced coolin" is used, the most commonly used coolin" media bein" air and water thou"h other coolin" "ases or li%uids can be used. The use of these coolin" systems usually creates much
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Bus Bars design
increased heat losses and so their use must be #ustified by benefits in other areas, e."., reduced civil costs, reduced physical si
4.$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
Project – P1
Chapter ( 3 )
Bus Bars design
'.Co(parison bet)een t)o t*pes of selections&
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ne conductor per phase (!are8 rectangu#ar) 14?:
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Chapter ( 3 )
Bus Bars design
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Project – P1