Electricity is the prime mover oftoday's industry. For becomingglobally competitive, industries areenhancing their capacities andintroducing the highest level ofautomation. In this process, thedemand for electricity is going up.

One way of meeting this increasingpower demand is captive powergeneration which has a highinstallation cost.

On the other hand, many industriesare going ahead with the upgrating oftheir transformers. But if we have acloser look at this concept, we willrealise that the solution is not assimple as just upgrating thetransformer. This calls for redesigningthe entire distribution network forreasons discussed herein.

Issued by : Electrical Syste

Geared uHigher transformer rating increases thecontinuous current being carried in theentire system. Secondly, in order toreduce the transformer losses,transformer specifications demandlower transformer impedance. Thisultimately results in very high current inthe event of a fault.

This fact must to be considered whiledesigning low voltage switchboardsalso. The entire busbar system in theswitchboard must be:

capable of carrying this highcurrent continuously

able to withstand the high faultcurrent.

Another important consideration in theswitchboard is with regard to the type2 co-ordination.

The switchgear selection shouldconsider higher let through energy.

L&T has studied these changingrequirements and has carried out thenecessary upgradation of the LVswitchboards.

The PCC and the MCC busbarsystem, including horizontal andvertical busbars, is tested for shortcircuit withstand capability of:

65 kA 70 kA 80 kA 100 kA 113 kA

The busbars are also designed tocarry a continuous current up to6000A.

ms & Equipment LARSEN & TOUBRO LIMITED, Powai Works, Mumbai 400 072

Prospect / Retrospect

April - June 2001

p to withstand higher fault levels

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When we trace back the history ofLow Voltage (LV) distribution networkin India, it can be observed that, till therecent past, the source transformercapacities were restricted to amaximum of 2MVA. Hence, the LVswitchboards, a vital link between thedistribution transformer and thedownstream loads in the electricalenergy distribution network, weredesigned for a service current ofmaximum 3000A and a fault withstandcapacity of maximum 50kA.

Over the years, industries have grownin size and plants have becom4emore sophisticated, automated andcomplex. This has resulted incorresponding increase in the powerrequirement. It has also created theneed for a reliable electricaldistribution network designed to takecare of effects of higher currents.

This increased power requirement hasnecessitated development of sources(transformers for LV distribution) withhigher capacity. Thus, the use oftransformers with 2.5MVA rating andabove has become a commonpractice. In addition to this, forensuring reliable and continuouspower supply, switchboards aretraditionallsource.

In a nutshnumber ofeeding necessitatand the inc

breaker of the main PCC. Theswitchboards, now, are required to becapable of carrying higher continuouscurrent and be also capable ofwithstanding increased fault currents.A typical single line diagram of adistribution network is shown in Fig. 1.

The devices closest to the distributiontransformer will generally be circuitbreakers fed through duct/busbars.Such devices lie in the high faultl4evel zones of the network.

To understand the requirements of the

equipment in the high fault zone, letus first look at the different types offaults. The commonly occurring faultsin an electrical network are: - Three phase faults Two phase faults Phase to earth faults

These faults can occur either due toan external shorting or due to arcing.An arcing faults much less in intensitythan to the dead short circuit fault.However, an arcing fault mayeventually result into a short circuitbetween phases. The fault currents

FEATURE

LV SWITCHBOARDS DESIGNED TO WITHSTAND HIGHERFAULT CURRENT

C. D. Mehta Switchgear Design & Product Developmenty fed from more than one

ell, the increase in size andf distribution transformers

the switchboards haveed uprating of the busbarsoming/buscoupler circuit

are generally maximum for a threephase fault and hence this faultcurrent is considered while designingthe busbar system and selecting theswitchgear equipment.

The value of this short circuit currentdepends primary on the: - Capacity of the supply source Distance of the fault from the

transformer Source impedance Motor contribution Fault impedance

Three phase short circuit current atthe transformer terminals can becalculated as: -

Where,

Isc = Short circuit currentS = Transformer rating in kVA 103

V2 = System voltageZ = % Imp

In LV sysassume thasource anlimiting thtransformer6.25% impetransformersize 1250 k

level is normally taken as 16 timesthe full load current. This is of coursean approximation based ontransformer impedance. Appropriatecorrection factors will have to be

Applied depending on the type andnumber of parallel sources, motorload contribution, impedance of theconnecting busbars / cables /busduct etc. The short circuit currentincreases with the rating of thedistribution transformer. (Ref. Chartgiven above).

It is clear that the short circuitperformance of the switchgear is animportant consideration whileselecting the switchgear for anyspecific application. The prospectiveor the maximum fault current at thepoint of installation will decide thebreaking capacity and the short timewithstand capacity requirement of thecircuit breakers and related devices inthe installation. Type 2 co-ordinationessential for reliable and safeoperation of the switchgear also

ulttheof

will

transformer, is required to becapable of withstanding muchhigher normal and fault currents. Inaddition to the transformerimpedance, only the impedance ofthe short length of the ducts/cableslimits the fault current in this case.

With the upward shift in thetransformer, it has become afrequent requirement for the mainPCCs to be suitable for a currentrating of up to 6000A with a faultwithstand capacity of up to 80kA.

Two factors should be taken care ofin the switchboards for this highrating: -

Thermal effect Electrodynamic forces

The heat loss in the busbars isproportional to the square of thecurrent. If the fault current increasesfrom 50kA to 80kA, the loss willincrease to around 2.5 times. All thisheat generated results in very hightemperature on the busbars duringfault, due to its adiabatic nature.

The electrodynamic forces, similarly,are also proportional to the squareof the current. However, in thiscase, the peak current will be thedeciding factor. In case of faultcurrent of more than 50kA, thepower factor reduces from 0.25 to0.2. This further increases theforces as the peak of the faultcurrent goes up. The busbar system

Transformer Rating Typical Short CircuitCurrent

Full LoadCurrent

1250 kVA

1500 kVA

2000 kVA

2500 kVA

3150 kVA

28 kA

33 kA

45 kA

56 kA

70 kA

1739 A

2087 A

2782 A

3478 A

4382 A

Typical fault levels and full load currents for a 415V distribution systemedance of transformer

tems, it is common tot the system is an infinite

needs to be verified at this higher falevel. The short time rating and electrodynamic withstand strength the busbars in the switchboards d the only impedancee fault current is the impedance. Assumingdance for the distribution

(for the transformers ofVA and above), the fault

also depend on the fault level of thesystem.

The main distribution board, due to itsproximity to the distribution

will have to be supported properly towithstand these high forces. Anotherimportant point to note is that theseforces are of impact nature and theytend to severely stress thesupporting structure. Hence, the

material of the supports will haveto be strong enough to withstandthe impact forces.

All this calls for a through designconsideration in case of thebusbar system.

The sub-distribution boards aregenerally placed nearer to theload, away from the main PCC.The length of the cables orbusduct connections between themain PCC and the sub-distributionboard is relatively great. Thishelps in reducing the fault currentsat the sub distribution boardpermitting use of busbars anddevices of lower short circuitwithstand capacity. It may benoted that a 100kA fault at thePCC can get reduced to 53kAlevel at the end of a 20m longcopper cable of 120sq.mm.

The conventional way of meeting therequirement of higher continuouscurrent capacity is to provide highercross section by increasing thenumber of flats for the busbarsassuming that the current densityremains constant. But in reality,increase in cross section does notgive the required current ratingproportionately. Higher currentsresult in non-uniform currentdistribution in the busbars (due toskin effect and proximity effect). Thisconsequently results into highpower/energy losses, higher voltagedrops, high temperature rise and veryhigh and non-uniform electrodynamicforces.

It is proved that the interleavedbusbar system is best suited for therequirements of higher fault levelsand higher currents. It results in moreuniform distribution of the currentthereby reducing the losses and theshort circuit forces.

A mathematical calculation wascarried out considering a threephase busbar system where a 50kARMS fault where the peak is 105kAon the R phase. Results of theexperiment are shown in Fig. 2. Aswe observe, the electrodynamicforces, in case of the conventionallyarranged busbar system (R-R-Y-Y-B), are as high as 12.6kN.Whereas,in case of the interleaved busbarsystem (R-R-Y-Y-B), the maximumforce experienced by the busbars is4 kN.

Conclusion

With the present trend of usinghigh capacity transformers, LVswitchboards, including horizontaland vertical busbars, must bedesigned to withstand higher faultcurrents viz. 80kA. The samesystem should also be able to carrycurrent as high as 6000Acontinuously.

Printed by Printania Offset Pvt. Ltd., D 20/21, Shalimar Industrial Estate, Matunga (east), Mumbai 400 019. Tel: 407 7996/8866/4540Fax: 402 4703 Email: pds6@bol.net.in Edited by Dr. Arun C. Vakil for larsen & toubro Limited, from L & T House, Narottam MorarjiMarg, Ballard Estate, Mumbai 400 001. The views expressed in this magazine are not necessarily those of the management of larsen& Toubro Limited. The contacts of this magazine should not be reproduced without the written permission of the Editor. Not for sale -only for Circulation among the customers. Associate Editor: Luis S. R. Vas

For further details on this subject, please contact:Electrical Systems & Equipment Division, Larsen & Toubro Limited, Saki-Vihar Road, P. O. Box 8901, Powai, Mumbai 400 072

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