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.
Higher 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.
Issued by : Electrical Systems & Equipment LARSEN & TOUBRO LIMITED, Powai Works, Mumbai 400 072
Prospect / Retrospect
April - June 2001
Geared up to withstand higher fault levels
Visit us at www.LNTEBG.com
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FEATURE
LV SWITCHBOARDS DESIGNED TO WITHSTAND HIGHERFAULT CURRENT
C. D. Mehta Switchgear Design & Product Development
trace back the history ofge (LV) distribution networkcan be observed that, till thest, the source transformer were restricted to aof 2MVA. Hence, the LV
rds, a vital link between the transformer and them loads in the electricalistribution network, werefor a service current of3000A and a fault withstandf maximum 50kA.
ears, industries have grownnd plants have becom4ehisticated, automated and
This has resulted inding increase in the powernt. It has also created ther a reliable electrical network designed to take
ects of higher currents.
sed power requirement hased development of sourcesers for LV distribution) withpacity. Thus, the use ofrs with 2.5MVA rating and
as become a commonIn addition to this, forreliable and continuous
upply, switchboards arey fed from more than one
ell, the increase in size andf distribution transformersthe switchboards haveed uprating of the busbarsoming/buscoupler circuit
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
are generally maximum for a threephase fault and hence this faultcurrent is considered while designingthe busbar system and selecting theswitchgear equipment.
The value odepends pr
Capacit Distanc
transfor Source Motor c Fault im
Three phasthe transfocalculated a
Where,
Isc = Short S = TransfV2 = SystemZ = % 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
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 the
Transfor
1250 kV
1500 kV
2000 kV
2500 kV
3150 kV
Typical faul
mer Rating Typical Short CircuitCurrent
Full LoadCurrent
f this short circuit currentimary on the: -y of the supply sourcee of the fault from themerimpedanceontributionpedance
e short circuit current atrmer terminals can bes: -
circuit currentormer rating in kVA ∆103
voltageedance of transformer
tems, it is common tot the system is an infinited the only impedancee fault current is the impedance. Assumingdance for the distribution
(for the transformers ofVA and above), the fault
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 alsoneeds to be verified at this higher faultlevel. The short time rating and theelectrodynamic withstand strength ofthe busbars in the switchboards willalso depend on the fault level of thesystem.
The main distribution board, due to itsproximity to the distribution
transformer, 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 systemwill 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
A
A
A
A
A
28 kA
33 kA
45 kA
56 kA
70 kA
1739 A
2087 A
2782 A
3478 A
4382 A
t levels and full load currents for a 415V distribution system
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: [email protected] 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
Fax: 022-8581024 & e-mail: [email protected]
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