In the initial days, Power ControlCentres (PCCs) were connected tothe transformers and throughfeeders of the PCCs, various MotorControl Centres were being fed. Thisled to duplication of feeders,extensive cabling and higher costs.In LV switchgear, most of the powertransformers used to be 1000kVA or1600kVA. Thus, the maximumcurrents were of the order of2000-2500A.

When PMCCs were introduced,larger transformers were connecteddirectly to the Motor Control Centresthrough ACB incomers. Last decadehas seen the transformer ratinggoing up (upto 2500kVA) andthereby the current rating of thebusbars also increasing. For amedium size industrial plant,busbars of 3500A current ratinghave become a normal requirement.

Earlier specifications used to specifyambient temperature as 40 deg. C.Later, it was also realised that thetropical conditions in our countrymay lead to an ambient of 45 deg.C.or even higher at a few places.

This awareness on the part ofcustomers brought temperature rise

July-September 1994

limits on the busbars inswitchboards. It was also realisedthat for paper bonded insulators, theinsulating properties deterioraterapidly at higher temperature,resulting in arcing fault in the busbarzone.

When low temperature rise andhigher current ratings werespecified, switchboardmanufacturers started:

− Increasing the cross section ofbusbar conductors

− trying with different geometry ofconductors for busbars.

Simultaneous study of differentvariables under stipulated conditionsrevealed that:

− ampacity of busbars is governedby:

• material of busbars(aluminium/copper)

• cross section of busbars• profile of the conductors• spacing between the

conductors of same phase• total busbar zone volume• type of ventilation

− beyond a certain cross section,adding to the cross section of theconductor does not increaseampacity of the busbar at all.

− different profiles of conductors ofsame cross section give differentampacity under the sameconditions.

− any profile, other than rectangle,poses difficulty in manufacturing,assembly and maintenance.

− if the busbar thickness isincreased, its current carryingcapacity doesn't increaseproportionately. So, choosing aconductor of rectangular crosssection, with minimum thickness,which is adequate for requiredmechanical strength provides thebest alternative.

− above 1600A rating, ventilatinglouvers help in increasingampacity of busbars.

Inductive reactance of the busbarsresults in higher temperature rise.

Even the conduction losses inswitchboards have a cumulativeeffect. They result in higher busbartemperature, burdening the air-conditioning system. It also meansloss of power for which the user hasto pay. Thus, effective steps taken toreduce these conduction losses willnot only save the user's money but itwill also make the system eco-friendly!

In this issue of L&T Current Trends,we bring you the analysis of thisinductive effect on the ampacity ofbusbars and a probable solution toachieve higher ampacity of thebusbar.

Issued by Switchgear Contracts Division LARSEN & TOUBRO LIMITED Powai Works, Bombay 400 072.

Prospect/Retrospect

Higher Ampacity OfBusbars

Temperature rise (TR) is animportant consideration for busbarinstallations. As far as requiredampacity of busbar system isbelow 1600 A, it can beconveniently achieved by usingone or more conductors of suitablesize and shape. The problem liesin designing an a.c. busbar systemfor higher ampacities. Test resultsshow that increasing cross sectionbeyond certain limits in aconventional manner (say byadding a conductor or increasingthe thickness) gives marginalincreasing in the ampacity. Thereason is skin and proximity effectsor in other words, inductive effectsdue to a.c. currents.

Direct current distribution over across section depends only on theresistance and hence currentdensity is uniform irrespective ofshape, size and arrangement ofconductors. However, a.c.distribution is controlled more byinductive effects than byresistance. Effect of current in theconductor itself is known as skineffect. This causes the current toconcentrate in the outer parts.Effect of current in the adjacentconductor is known as proximityeffect. Inductive effects result innon uniform current distributionover a cross section (i.e. inefficientuse of conducting material);consequently making busbars torun at a higher temperature.

Figure 1 gives current distributionpattern over round conductors invarious configurations where eachdot represents a unit quantity ofcurrent.

Why does non uniform distributionresult into higher losses andthereby high temperature?

2

To understand this, take the caseof a flat shaped conductorassumed to be comprising severalsmaller conductors of equal area inparallel, carrying total current of 10A as shown in figure 2 on page 4.In the case of uniform distribution,watt loss works out to 20 W. Fornon uniform distribution, element-wise currents differ not only in

magnitude but also in phase angle(Note that arithmetic addition is11.2 A). Watt loss in this case is30.8 W. As watt loss is more,steady state temperature will behigher for the same value ofcurrent; or for the same steadystate temperature, it will have lowerapmacity.

The ratio of a.c. to d.c. watt loss isknown as Rac/Rdc ratio. (Hereaftertermed as 'ratio'). This indicateseffective resistance (Rac) offeredby a configuration in relation to theactual resistance (Rdc). Ideal(minimum) value of this ratio is1 i.e. Rac = Rdc.

What is interleaving?For increasing busbar systemampacity, conventionally one tendsto add a conductor or a group ofconductors adjacent to existingconductors / groups and ends upplacing all conductors / groupstogether. (Refer to table 1,configuration 2) This results inwidely non uniform currentdistribution due to skin effect andproximity effect. Proximity effect isdependent on the distancebetween conductors. Hence, thisproblem can be overcome byarranging individual conductors inspecific geometries. Interleaving isone such arrangement. (Refer toconfiguration 5).

Interleaving means arrangingconductors groups in the alternatefashion (R Y B R Y B). In theconfiguration 5, two groups ofthree conductors each per phasehave been arranged in this fashion.One can go even beyond this andmake three groups of twoconductors each to increase therating further.

Table 1 gives ratios for variousconfigurations of three phasebusbar systems with wide andmoderate/close interphasespacings. One can note thedifference between the ratios forwide spacings and close spacingsfor configurations 2, 3 & 4.Effective resistance (relative) isgiven in the last column.

Interleaved busbar system for a.c. applicationsAn effective solution for achieving higher ratings

- U. V. Raut & S. R. ShetyeSwitchgear Design & Product Development Department

L&T Current Trends, July-September 1994

F e a t u r e

Discussion on Table 1

(1) Effect of increase in busbarcross-section in a conventionalmanner: Configuration 2 is havingdouble the cross-section as that ofconfiguration 1. However, effectiveresistance for middle phase ismore and so is the heat producedat same current. Thus, increasing

the cross-section has not givenany benefits. Configuration 2 willcarry more current only becausemore number of flats are availablefor dissipating the heat.

(2) Effect of changing the materialfrom aluminium to copper: Forconfiguration 2, copper is havinghigher ratio as compared to that of

aluminium. This is because forcopper, inductive effects distortcurrent more due to lowerresistance. This reflects in theeffective resistance (Rac) which isnot lower in the same proportionas that of Rdc. Considering thefact that heating is proportional tothe square of the current, ratingwith copper would be only 19%

more. Thus, for large cross-sections, changing the materialfrom aluminium to copper givesmarginal increase in the ampacityat a very high cost.

(3) Use of Tubes/Channel section(configurations 3 & 4): These areoften suggested for busbarsystems as they are ideal shapesto counter skin effect. This is

evident from ratios for widespacings at which interphaseproximity effects are negligible.However, for close spacings,configurations 3 and 4 aremarginally better than configuration2. These sections are also notdesirable as processing onmachines, making joints etc. isdifficult.

(4) Interleaved System(configuration 5):For same cross sectional area,interleaved configuration 5 enjoysmuch better rations thanconfigurations 2, 3 & 4. This isdue to substantial improvement incurrent distribution as proximityeffects tend to cancel each other.

Comparing this with configuration

3L&T Current Trends, July-September 1994

• RESISTANCE VALUES ARE RELATIVE WITH THE BASE VALUE OF 9 SQ. IN. ALUMINIUM AS 1

Table 1

2, effective resistance for middlephase is less than half. Therefore,ampacity would be more byaround 35 to 40% for aluminiumand 40 to 45% for copper or inother words, the losses will be lessin the same proportion.

One can also note that forinterleaved system, ratios for allphases are uniform; while for otherconfigurations, ratios for middlephase are much higher. Thus,ampacity is limited by TR on thisphase even though outer phasesrun cooler.

The switchgear and petrol pump -related operations of Larsen &Toubro Limited have been awardedISO 9001 certificate by BVQI,London - one of the leadingaccreditation agencies in theworld - following comprehensiveaudit of quality systems at theCompany's factories at Powai,Madh and Ahmednagar as well asits marketing offices.

The ISO 9001 certification covers

Results of TR test

A test was carried out on a 2.5metre long busduct (IP54) withconfigurations 2 & 5 withaluminium at 3150 A. Average risefor configuration 2 was 66°C andthat for configuration 5 was 36°C.

Other advantages of interleavedsystem

(i) It offers lower and balancedimpedance. This becomesimportant for long busducts wherevoltage drop is critical.

design, manufacture, marketing-installation and servicing ofswitchgear and controlgear pro-ducts; switchboards; and petrolmetering and dispensing units. Theproduct range covered includesabout 550 catalogued and price-listed standard products, motor andpower control centres and varioustypes of petrol pumps with mechani-cal and electronics register. L&Tholds a leadership position in Indiafor these products.

(ii) Figure 3 gives instantaneous(at the instant of 105 kA peakcurrent in the R phase) values ofshort circuit current and force forconfigurations 2 & 5. For theinterleaved arrangement, forcesdue to short circuit current aremuch lower. This enables redesignand reconsideration of the busbarsupport system. Also, it canenable the existing support systemto withstand much higher faultlevels.

Disadvantages of the interleavedsystem(i) Phases see each other atgreater number of places. Thismay increase size of busbarchamber.

(ii) Putting equalizers betweengroups is difficult. Howeverequalizers are not required if eachload is tapped from both packets.

(iii) For busduct applications, phasecross-over chamber may becomeessential to facilitate passing allbars (of a phase) through CTs.

Conclusion

Considering all pros and cons,optimum solution for reducing lossof energy and achieving highampacity busbar system would beinterleaving. As interleaved systemneeds less material and use ofaluminium is possible for very highratings, this system is better suitedfor Indian conditions.

Edited, printed and published by Cerena de Souza for Larsen & Toubro Limited. Published from L&T House, Ballard Estate,Bombay 400 001 and printed at Vakil & Sons Ltd., 18 Ballard Estate, Bombay 400 001. Associate Editor : Luis S. R. Vas.

ISO 9001 for L&T's Switchgearand

Petrol Pumps