AC DISTRIBUTION - ELECTRICAL POWER SYSTEM

That part of power system which distributes electric power for local use is known as DISTRIBUTION.

Electric power distribution is the final stage in the delivery of electricity. Electricity is carried from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 33 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment and household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops.

1. POWERSYSTEMS-I
Dr.G.NageswaraRao
Professor
LakireddyBaliReddyCollegeofEngineering

2. AC DISTRIBUTION & CABLES
03
AC DISTRIBUTION

6. BusBar
Theconductingmaterialoraconductorusedtocollectpower
fromtheinputterminalsofanelectricalsystemanddistribute
ittovariousoutputcircuitsisknownasanelectricalbusbar
orbussystem.Itactsasajunction,wheretheincoming
powerandoutgoingpowermeets.Itisusedtocollectallthe
electricalpowerinoneplace.Itisavailableintheformof
rectangularstrips,roundtubes,roundbars,andsquarebars
madeupofaluminium,copper,andbrass.

7. DistributionSystem
Thepartofpowersystemwhichdistributeselectricpowerforlocal
useisknownasdistributionsystem.

8. Distributionsystemgenerallyconsistsoffeeders,distributorsandtheservice mains.Fig
showsthesinglelinediagramofatypicallowtensiondistributionsystem.
(i)Feeders.Afeederisaconductorwhichconnectsthesub-station(orlocalisedgenerating
station)totheareawherepoweristobedistributed.Generally,notappingsaretakenfrom
thefeedersothatcurrentinitremainsthesamethroughout.Themainconsiderationinthe
designofafeederisthecurrentcarryingcapacity.
(ii)Distributor.Adistributorisaconductorfromwhichtappingsaretakenforsupplytothe
consumers.InFig.AB,BC,CDandDAarethedistributors.Thecurrentthrougha
distributorisnotconstantbecausetappingsaretakenatvariousplacesalongitslength.
Whiledesigningadistributor,voltagedropalongitslengthisthemainconsideration
sincethestatutorylimitofvoltagevariationsis±6%ofratedvalueattheconsumers’
terminals.
(iii)Servicemains.Aservicemainsisgenerallyasmallcablewhichconnectsthedistributorto
theconsumers’terminals.

9. ClassificationofDistributionSystems
(i)Natureofcurrent.Accordingtonatureofcurrent,distributionsystemmaybeclassifiedas
(a)d.c.distributionsystem(b)a.c.distributionsystem.
Now-a-days,a.c.systemisuniversallyadoptedfordistributionofelectricpowerasitissimpler
andmoreeconomicalthandirectcurrentmethod.
(ii)Typeofconstruction.Accordingtotypeofconstruction,distributionsystemmaybe
classifiedas
(a)overheadsystem(b)undergroundsystem.
Theoverheadsystemisgenerallyemployedfordistributionasitis5to10timescheaper
thantheequivalentundergroundsystem.Ingeneral,theundergroundsystemisusedatplaces
whereoverheadconstructionisimpracticableorprohibitedbythelocallaws.
(iii)Schemeofconnection.Accordingtoschemeofconnection,thedistributionsystemmaybe
classifiedas
(a) Radialsystem(b)ringmainsystem(c)inter-connectedsystem.
Eachschemehasitsownadvantagesanddisadvantages.

10. A.C. distribution calculations differ from those of d.c.distribution
in the following respects :
(i) In case of d.c.system, the voltage drop is due to resistance alone. However, in
a.c. system, the voltage drops are due to the combined effects of resistance,
inductance and capacitance.
(ii) In a d.c.system, additions and subtractions of currents or voltages are done
arithmetically but in case of a.c. system, these operations are done vectorially.
(iii) In an a.c. system, power factor (p.f.) has to be taken into account. Loads
tapped off form the distributor are generally at different power factors. There are
two ways of referring power factor viz
(a) It may be referred to supply or receiving end voltage which is regarded as the
reference vector.
(b) It may be referred to the voltage at the load point itself.

11. A.C.Distribution
Thea.c.distributionsystemisclassifiedinto
(i)primarydistributionsystemand(ii)secondarydistributionsystem
Primarydistributionsystem:Itisthatpartofa.c.distributionsystemwhich
operatesatvoltagessomewhathigherthangeneralutilisationandhandleslargeblocksof
electricalenergythantheaveragelow-voltageconsumeruses.Thevoltageusedfor
primarydistributiondependsupontheamountofpowertobeconveyedandthedistance
ofthesubstationrequiredtobefed.Themostcommonlyusedprimarydistributionvoltages
are11kV,6·6kVand3·3kV.Duetoeconomicconsiderations,primarydistributionis
carriedoutby3-phase,3-wiresystem.Fig.showsatypicalprimarydistributionsystem.
Electricpowerfromthegeneratingstationistransmittedathighvoltagetothesubstation
locatedinornearthecity.Atthissubstation,voltageissteppeddownto11kVwiththe
helpofstep-downtransformer.Powerissuppliedtovarioussubstationsfordistributionorto
bigconsumersatthisvoltage.Thisformsthehighvoltagedistributionorprimarydistribution.

13. Secondarydistributionsystem.
Itisthatpartofa.c.distributionsystemwhichincludestherangeofvoltagesatwhichthe
ultimateconsumerutilisestheelectricalenergydeliveredtohim.Thesecondarydistribution
employs400/230V,3-phase,4-wiresystem.
Fig.showsatypicalsecondarydistributionsystem.Theprimarydistributioncircuitdelivers
powertovarioussubstations,calleddistributionsubstations.Thesubstationsaresituatednear
theconsumers’localitiesandcontainstepdowntransformers.
Ateachdistributionsubstation,thevoltageissteppeddownto400Vandpowerisdelivered
by3-phase,4-wirea.c.system.
Thevoltagebetweenanytwophasesis400Vandbetweenanyphaseandneutralis230V.
Thesinglephasedomesticloadsareconnectedbetweenanyonephaseandtheneutral,
whereas3-phase400Vmotorloadsareconnectedacross3-phaselinesdirectly.

15. OverheadVersusUndergroundSystem
(i)Publicsafety.Theundergroundsystemismoresafethanoverheadsystembecausealldistribution
wiringisplacedundergroundandtherearelittlechancesofanyhazard.
(ii)Initialcost.Theundergroundsystemismoreexpensiveduetothehighcostoftrenching,conduits,
cables,manholesandotherspecialequipment.Theinitialcostofanundergroundsystemmaybefiveto
tentimesthanthatofanoverheadsystem.
(iii)Flexibility.Theoverheadsystemismuchmoreflexiblethantheundergroundsystem.Inthelattercase,
manholes,ductlinesetc.,arepermanentlyplacedonceinstalledandtheloadexpansioncanonlybe
metbylayingnewlines.However,onanoverheadsystem,poles,wires,transformersetc.,canbeeasily
shiftedtomeetthechangesinloadconditions.
(iv)Faults.Thechancesoffaultsinundergroundsystemareveryrareasthecablesarelaid
undergroundandaregenerallyprovidedwithbetterinsulation.
(v)Appearance.Thegeneralappearanceofanundergroundsystemisbetterasallthedistributionlines
areinvisible.Thisfactorisexertingconsiderablepublicpressureonelectricsupplycompaniestoswitch
overtoundergroundsystem.
(vi)Faultlocationandrepairs.Ingeneral,therearelittlechancesoffaultsinanundergroundsystem.
However,ifafaultdoesoccur,itisdifficulttolocateandrepaironthissystem.Onanoverheadsystem,
theconductorsarevisibleandeasilyaccessiblesothatfaultlocationsandrepairscanbeeasilymade.

16. (vii)Currentcarryingcapacityandvoltagedrop.Anoverheaddistributionconductorhasa
considerablyhighercurrentcarryingcapacitythananundergroundcableconductorofthe
samematerialandcross-section.Ontheotherhand,undergroundcableconductorhasmuch
lowerinductivereactancethanthatofanoverheadconductorbecauseofcloserspacingof
conductors.
(viii)Usefullife.Theusefullifeofundergroundsystemismuchlongerthanthatofanoverhead
system.Anoverheadsystemmayhaveausefullifeof25years,whereasanunderground
systemmayhaveausefullifeofmorethan50years.
(ix)Maintenancecost.Themaintenancecostofundergroundsystemisverylowascompared
withthatofoverheadsystembecauseoflesschancesoffaultsandserviceinterruptionsfrom
wind,ice,lightningaswellasfromtraffichazards.
(x)Interferencewithcommunicationcircuits.Anoverheadsystemcauseselectromagnetic
interferencewiththetelephonelines.Thepowerlinecurrentsaresuperimposedonspeech
currents,resultinginthepotentialofthecommunicationchannelbeingraisedtoanundesirable
level.However,thereisnosuchinterferencewiththeundergroundsystem.

17. RequirementsofaDistributionSystem
(i)Propervoltage.Oneimportantrequirementofadistributionsystemisthatvoltagevariationsatconsumer’s
terminalsshouldbeaslowaspossible.Thechangesinvoltagearegenerallycausedduetothevariationofload
onthesystem.Lowvoltagecauseslossofrevenue,inefficientlightingandpossibleburningoutofmotors.High
voltagecauseslampstoburnoutpermanentlyandmaycausefailureofotherappliances.Therefore,agood
distributionsystemshouldensurethatthevoltagevariationsatconsumersterminalsarewithinpermissiblelimits.The
statutorylimitofvoltagevariationsis±6%oftheratedvalueattheconsumer’sterminals.Thus,ifthedeclared
voltageis230V,thenthehighestvoltageoftheconsumershouldnotexceed244Vwhilethelowestvoltageofthe
consumershouldnotbelessthan216V.
(ii)Availabilityofpowerondemand.Powermustbeavailabletotheconsumersinanyamountthattheymayrequire
fromtimetotime.Forexample,motorsmaybestartedorshutdown,lightsmaybeturnedonoroff,without
advancewarningtotheelectricsupplycompany.Aselectricalenergycannotbestored,therefore,thedistribution
systemmustbecapableofsupplyingloaddemandsoftheconsumers.Thisnecessitatesthatoperatingstaffmust
continuouslystudyloadpatternstopredictinadvancethosemajorloadchangesthatfollowtheknownschedules.
(iii)Reliability.Modernindustryisalmostdependentonelectricpowerforitsoperation.Homesandofficebuildings
arelighted,heated,cooledandventilatedbyelectricpower.Thiscallsforreliableservice.Unfortunately,electric
power,likeeverythingelsethatisman-made,canneverbeabsolutelyreliable.However,thereliabilitycanbe
improvedtoaconsiderableextentby(a)interconnectedsystem(b)reliableautomaticcontrolsystem(c)providing
additionalreservefacilities.

18. DesignConsiderationsinDistributionSystem
Goodvoltageregulationofadistributionnetworkisprobablythemost
importantfactorresponsiblefordeliveringgoodservicetotheconsumers.Forthis
purpose,designoffeedersanddistributorsrequirescarefulconsideration.
(i)Feeders.Afeederisdesignedfromthepointofviewofitscurrentcarrying
capacitywhilethevoltagedropconsiderationisrelativelyunimportant.Itis
becausevoltagedropinafeedercanbecompensatedbymeansofvoltage
regulatingequipmentatthesubstation.
(ii)Distributors.Adistributorisdesignedfromthepointofviewofthevoltage
dropinit.Itisbecauseadistributorsuppliespowertotheconsumersandthereis
astatutorylimitofvoltagevariationsattheconsumer’sterminals(±6%ofrated
value).Thesizeandlengthofthedistributorshouldbesuchthatvoltageatthe
consumer’sterminalsiswithinthepermissiblelimits.

19. ConnectionSchemesofDistributionSystem
Alldistributionofelectricalenergyisdonebyconstantvoltagesystem
(i)RadialSystem.Inthissystem,separatefeedersradiatefromasinglesubstationand
feedthedistributorsatoneendonly.Fig.1showsasinglelinediagramofaradialsystemfor
d.c.distributionwhereafeederOCsuppliesadistributorABatpointA.Obviously,the
distributorisfedatoneendonlyi.e.,pointAisthiscase.Fig.2showsasinglelinediagramof
radialsystemfora.c.distribution.Theradialsystemisemployedonlywhenpoweris
generatedatlowvoltageandthesubstationislocatedatthecentreoftheload.Thisisthe
simplestdistributioncircuitandhasthelowestinitialcost.However,itsuffersfrom
thefollowingdrawbacks:
(a)Theendofthedistributornearesttothefeedingpointwillbeheavilyloaded.
(b)Theconsumersaredependentonasinglefeederandsingledistributor.Therefore,anyfault
onthefeederordistributorcutsoffsupplytotheconsumerswhoareonthesideofthefault
awayfromthesubstation.
(c)Theconsumersat thedistant endof thedistributor would be subjectedto seriousvoltage
fluctuationswhentheloadonthedistributorchanges.Duetotheselimitations,thissystemis
usedforshortdistancesonly.

21. (ii)Ringmainsystem.Inthissystem,theprimariesofdistributiontransformersforma
loop.Theloopcircuitstartsfromthesubstationbus-bars,makesaloopthroughtheareatobe
served,andreturnstothesubstation.Fig.showsthesinglelinediagramofringmainsystem
fora.c.distributionwheresubstationsuppliestotheclosedfeederLMNOPQRS.
ThedistributorsaretappedfromdifferentpointsM,OandQofthefeeder
through distributiontransformers.Theringmainsystemhasthefollowingadvantages:
(a)Therearelessvoltagefluctuationsatconsumer’sterminals.
(b)Thesystemisveryreliableaseachdistributorisfedviatwofeeders.Intheeventoffault
onanysectionof thefeeder, thecontinuityof supplyismaintained. Forexample,suppose
thatfaultoccursatanypointFofsectionSLMofthefeeder.ThensectionSLMofthe
feedercanbeisolatedforrepairsandatthesametimecontinuityofsupplyismaintainedto
alltheconsumersviathefeederSRQPONM.

23. (iii)Interconnectedsystem.Whenthefeederringisenergisedbytwoormorethantwo
generatingstationsorsubstations,itiscalledinter-connectedsystem.Fig.showsthesingleline
diagramofinterconnectedsystemwheretheclosedfeederringABCDissuppliedbytwo
substationsS1andS2atpointsDandCrespectively.DistributorsareconnectedtopointsO,P,
QandRofthefeederringthroughdistributiontransformers.Theinterconnectedsystemhasthe
followingadvantages:
(a)Itincreasestheservicereliability.
(b)Anyareafedfromonegeneratingstationduringpeakloadhourscanbefedfromthe
other generating station.Thisreducesreservepower capacity and increasesefficiencyof the
system.

25. SELF-TEST
1.Fillintheblanksbyinsertingappropriatewords/figures.
(i)Theundergroundsystemhas.............initialcostthantheoverheadsystem.
(ii)Aringmainsystemofdistributionis.............reliablethantheradialsystem.
(iii)Thedistributiontransformerlinkstheprimaryand.............distributionsystems
(iv)Themostcommonsystemforsecondarydistributionis............3-phase,.............wiresystem.
(v)Thestatutorylimitforvoltagevariationsattheconsumer’sterminalsis.............%ofrated
value.
(vi)Theservicemainsconnectthe.............andthe.............
(vii)Theoverheadsystemis.............flexiblethanundergroundsystem.
ANSWERSTOSELF-TEST
(i)more(ii)more(iii)secondary(iv)400/230V,4(v)=6(vi)distributor,consumer
terminals(vii)more

26. CHAPTERUNITENDQUESTIONS
1.Whatdoyouunderstandbydistributionsystem?
2.Drawasinglelinediagramshowingatypicaldistributionsystem.
3.Defineandexplaintheterms:feeder,distributorandservicemains.
4.Discusstherelativemeritsanddemeritsofundergroundandoverheadsystems.
5.Explainthefollowingsystemsofdistribution:
(i)Radialsystem
(ii)Ringmainsystem
(iii)Interconnectedsystem
6.Discussbrieflythedesignconsiderationsindistributionsystem.
7.Withaneatdiagram,explainthecompletea.c.systemfordistributionof
electricalenergy.

27. Methods of Solving A.C. Distribution Problems
(i)w.r.t.receivingorsendingendvoltage
(ii)w.r.t.toloadvoltageitself.

28. (i)Power factors referred to receiving end voltage

39. Example:A3-phase,400VdistributorABisloadedasshowninFig.14.8.The3-phaseloadat
pointCtakes5Aperphaseatap.f.of0·8lagging.AtpointB,a3-phase,400Vinduction
motorisconnectedwhichhasanoutputof10H.P.withanefficiencyof90%andp.f.0·85
lagging.IfvoltageatpointBistobemaintainedat400V,whatshouldbethevoltageat
pointA?Theresistanceandreactanceofthelineare1Ωand0·5Ωperphaseperkilometre
respectively
Solution.Itisconvenienttoconsideronephaseonly.Figshowsthesinglelinediagramofthedistributor.
Impedanceofthedistributorperphaseperkilometre=(1+j0·5)

51. CHAPTER REVIEW TOPICS
1. How does a.c. distribution differ from d.c.distribution ?
2. What is the importance of load power factors in a.c. distribution ?
3. Describe briefly how will you solve a.c. distribution problems ?
4. Write short notes on the following :
(i) Difference between d.c.and a.c. distribution
(ii) Systems of a.c. distribution
5. Discuss about feeder, distributor and service main.

52. 1. Fill in the blanks by inserting appropriate words/figures.
(i) The most common system for secondary distribution is 400/..... V, 3-phase, ......... wire
system.
(ii) In a 3-phase, 4-wire a.c. system, if the loads are balanced, then current in the neutral wire
is .........
(iii) Distribution transformer links the ............ and ........... systems.
(iv) The 3-phase, 3-wire a.c. system of distribution is used for .......... loads.
(v) For combined power and lighting load, .............. system is used.
2. Pick up the correct words/figures from brackets and fill in the blanks.
(i) 3-phase, 4-wire a.c. system of distribution is used for .............. load. (balanced,
unbalanced)
(ii) In a.c. system, additions and subtractions of currents are done ..............
(vectorially, arithmetically)
(iii) The area of X-section of neutral is generally .............. that of any line conductor. (the
same, half)
(iv) For purely domestic loads, .............. a.c. system is employed for distribution

53. ASSIGNMENT QUESTIONS
i)Whatarethedifferentdistributionsystemadoptedinpowersystem?
ii)Whataretheadvantagesofringmaindistributionsystem?
iii)Whatarethetypesofdcdistributionsystemarethere?Explain.
iv)Asinglephasedistributor2km.longsuppliesaloadof120Aat0.8p.f.lagging
atitsfarendandaloadof80Aat0.9p.f.laggingatitsmidpoint.Bothpowerfactor
arereferredtothevoltageatthefarend.Theresistanceandreactanceperkm.go
andreturnare0.05Ωand0.1Ωrespectively.Thevoltageatthefarendismaintained
at230v,calculate
i)Voltageatthesendingend.
ii)Phaseanglebetweenvoltageatthetwoend.
ANSWERS TO SELF-TEST
1. (i) 230, 4 (ii) zero (iii) primary, secondary (iv) balanced (v) 3-phase 4-wire.
2. (i) unbalanced (ii) vectorially(iii) half (iv) single phase 2-wire.