Module (05) High-Voltage A.C.pdf

7/29/2019 Module (05) High-Voltage A.C.pdf

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Module(05)HighVoltageA.CCircuitBreakers Page1of35

Module

05High-Voltage A.C

Circuit Breakers


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5.1 INTRODUCTION

In this chapter, the constructional aspectsof circuit breakers have been briefly discussed. The

theoreticalaspectsregardingtransientvariationofcurrentandvoltage,arcextinctionprocessand

thevarious typesofcircuitbreakershavebeendescribed indetail inotherchapters.Thecircuitbreakersareautomaticswitcheswhichcaninterruptfaultcurrents.Insomeapplicationslikesingle

phase traction system, single pole circuit breakers are used. The part of the circuit breakers

connected inonephase iscalled the 'pole'.Acircuitbreakersuitable for threephasesystem is

calleda'triplepolecircuitbreaker'.

Each pole of the circuit breaker comprises one or more 'interrupter' or 'arcextinguishing

chambers'.Theinterruptersaremountedonsupportinsulators.Theinterrupterenclosesasetof

'fixedandmovingcontacts'.Themovingcontactscanbedrawnapartbymeansoftheoperating

linksoftheoperatingmedium.Theoperatingmechanismofthecircuitbreakergivesthenecessary

energyforopeningandclosingofcontactsofthecircuitbreakers.

The arc produced by the separation of current carrying contacts is interrupted by a suitable

medium and by adopting suitable techniques for arc extinction. The circuit breaker can be

classifiedonthebasisofthearcextinctionmedium.

5.2THE

FAULT

CLEARING

PROCESS

During thenormaloperatingcondition thecircuitbreakercanbeopenedorclosedbyastation

operatorforthepurposeofswitchingandmaintenance.Duringtheabnormalorfaultyconditions

the relays sense the faultandclose the tripcircuitof thecircuitbreaker.Thereafter thecircuit

breakeropens.Thecircuitbreakerhastwoworkingpositions,openandclosed.Thesecorrespond

toopencircuitbreakercontactsandclosedcircuitbreakercontactsrespectively.Theoperationof

automaticopeningandclosingthecontact isachievedbymeansoftheoperatingmechanismof

the circuit breaker. As the relay contacts close, the triple circuit is closed and the operating

mechanismofthecircuitbreakerstartstheopeningoperation.

The contacts of the circuit breaker open and an arc is drawn between them. The arc is

extinguished at some natural current zero of a.c. wave. The process of current interruption is

completedwhenthearcisextinguishedandthecurrentreachesfinalzerovalue.Thefaultissaid

tobecleared.Theprocessoffaultclearinghasthefollowingsequence:

FaultOccurs:As the faultoccurs, the fault impedancebeing low, thecurrents increaseand therelaygetsactuated.Therelaytakessometimetocloseitscontacts.


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Relay contacts close the trip circuitof the circuitbreaker closes and trip coil is energized. The

operating mechanism starts operating for the opening operation. The circuit breaker contacts

separate.

Arc is drawn between the breaker contacts. The arc is extinguished in the circuit breaker by

suitabletechniques.Thecurrentreachesfinalzeroasthearcisextinguishedanddoesnotrestrike

again.

5.3 TRIPCIRCUIT

The basic connection of the circuitbreaker control, for the opening operation, is illustrated in

Figure5.1.

Figure(5.1)SimplifiedDiagramofCircuitBreakerControlforOpeningOperation

Theprotectedcircuit is shownbydashed line.Whena faultoccurs in theprotectedcircuit, the

relayconnectedtotheCTandPTactuatesandclosesitscontacts.Currentflowsfromthebattery

(source) in the trip circuit.As the trip coilof the circuitbreaker (C.B.) is energized, the circuit

breaker operating mechanism is actuated and it operates for the opening operation. Auxiliary

switchisanimportantiteminthecircuit.


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5.4 HISTORICALREVIEW

Long ago, before 1875, the problem of the circuit breaking was relatively simple. The power

systemswerebelow15kVandfaultMVA1wasoftheorderof250.Mercuryswitchesandknife

switcheswereused forswitching.Fusesgaveshortcircuitprotection.Today thepowersystemshavebecomecomplexandhuge.Thevoltagesusedfortransmissionareoftheorderof220kV,

400kV,500kVand750kV.Thecapacityof thecircuitbreakersonEHV lines isof theorderof

35,000MVA.Today'scircuitbreakersarefaster(2cycles,2.5cycles).

Theoilcircuitbreakerwasdevelopedaround1885.Intheoilcircuitbreakers,dielectricoilisused,

forinsulatonandforarcextncton.Later(1900),arcextnctondeviceswereintroduced.Thearc

extinctiondevicesaresemiclosedchambersofdielectricmaterial,whicharefixedtothecontacts

forassistingthearcextinctioninoilcircuitbreakers.Minimumoilcircuitbreakersweredeveloped

toreplacethebulkoilcircuitbreakers.Today theminimumoilcircuitbreakersarebeingwidely

used.

The firstshortcircuit testingplantwasbuilt in1916 inEurope.Theairblastcircuitbreakerwas

developed in 1930's. In air blast circuit breakers, high pressure air is forced on the arc at the

instantofthecontactseparationandarcgetsextinguishedbytheblasteffect.Theairblastcircuit

breakershaveseveraladvantagessuchasfastoperation,highrupturingcapacityetc.TheAirBlast

CircuitBreakerbecamequitepopularby1950.

The inventon of SF6 circuit breakers is a very important development in the circuit breaker

technology. SF6 (sulphurhexafluoride) gas has good dielectric and arc quenching properties.

DoublepressuretypeoutdoorSF6circuitbreakersweredevelopedbyWestnghouse,USAduring

1950's. This was followed by development of single pressure puffer type SF6 circuit breakers

(1970).TodaySF6circuitbreakersandSF6 insulatedmetalenclosed switchgearareverywidely

usedforvoltagesfrom72.5kVto760kV.

Vacuumwasknowntobeagooddielectricmedium.Buttocreatevacuumandmaintainvacuum

wasaproblem.Vacuum interruptershavebeendevelopedduring1960'sandarebeingusedfor

applicatonsbelow36kV.

Otherinterestingdevelopmentsforlowandmediumvoltageapplicationsincludeminiaturecircuit

breakers,mouldedcasecircuitbreakers,contactors,solidstateswitchingdevices.Miniaturecircuit

breakersandmouldedcasecircuitbreakershavecertainadvantagesoverconventionalHRCfuses.

1 Fault MVA =610

3 IV whereV is the service voltage in volts and I is the fault current in amperes


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Contactorshavebeendeveloped forawide rangeofa.c.andd.c. ratings.Solid state switching

devicesarebeingincreasinglyusedinindustrialcontrols.

5.5CLASSIFICATION

BASED

ON

ARC

QUENCHING

MEDIUM

Thecircuitbreakerscanbeclassifiedonthebasisofratedvoltages:Circuitbreakersbelowrated

voltageof1000Varecalledlowvoltagecircuitbreakersandabove1000Varecalledhighvoltage

circuitbreakers.

Thetypeofthecircuitbreakerisusuallyidentifiedaccordingtothemediumofarcextinction.The

classificationofthecircuitbreakersbasedonthemediumofarcextinctionisasfollows:

AirBreakCircuitBreaker.

MiniatureCircuitBreaker.

OilCircuitBreaker(TankTypeorBulkOil).

MinimumOilCircuitBreaker.

AirBlastCircuitBreaker.

SulphurHexafluorideCircuitBreaker

(SinglePressureorDoublePressure).

Vacuumcircuitbreaker.

Eachcircuitbreakerwillbestudiedthoroughlyinthesubsequentsections.Thesecircuitbreakers

employvarioustechniquestoextinguishthearcresultingfromseparationofthecurrentcarrying

contacts. The mode of arc extinction is either 'high resistance interruption' or 'zeropoint

interruption'.

HighResistanceInterruption:Inthisprocesstheresistanceofthearc is increasedbylengthening

andcoolingittosuchanextentthatthesystemvoltageisnolongerabletomaintainthearcand

thearcgetsextinguished.Thistechniqueisemployedinairbreakcircuitbreakersandd.c.circuit

breakers.

LowResistanceor ZeroPoint Interruption: In thisprocess, the arc getsextinguished atnatural

currentzeroofthealternatingcurrentwaveandispreventedfromrestrikingagainbyrapidbuild

upofdielectric strengthof thecontact space.Theprocess isemployed inalmostalla.c.circuit

breakers.

Each leadingmanufacturerofcircuitbreakerdevelops twoormore typesofcircuitbreakers for

everyvoltageclass.Theconstructionofthecircuitbreakerdependsupon itstype(arcquenching

medium),voltageratingandstructuralform.


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Airbreak circuitbreakers:Utilizeair at atmosphericpressure forarcextinction.Airblast circuit

breakers:Utilizehighpressurecompressedairforarcextinction.Theyneedcompressedairplant.

BulkoilandMinimumoilcircuitbreakers:UtilizeDielectricoil(Transformeroil)forarcextinction.

InBulkoilcircuitbreakers,thecontactsareseparatedinsideasteeltankfilledwithdielectricoil.Inminimumoil circuitbreakers the contracts are separated in an insulating housing (interrupter)

filledwithdielectricoil.

SF6circuitbreakers:SulphurHexaFluoridegasisusedforarcextinction.Therearetwotypes:

SinglepressurepuffertypeSF6circuitbreakers,inwhichtheentrecircuitbreakerisfilledwithSF6

gasat singlepressure (4 to6kg/cm2).Thepressureandgasflow required forarcextncton is

obtainedbypistonaction.

DoublepressuretypeSF6circuitbreaker inwhichthegasfromhighpressuresystem isreleased

into low pressure system over the arc during the arc quenching process. This type has been

supersededbyputtertype.

Vacuum circuit breakers: In vacuum circuit breaker the fixed and moving contacts are housed

inside a permanently sealed vacuum interrupter. The arc is quenched as the contacts are

separatedinhighvacuum.

AbriefcomparisonbetweenthedifferenttypesofcircuitbreakersisgiveninTable5.1.

5.6 TECHNICALPARTICULARSOFACIRCUITBREAKER

Acircuitbreakerisidentifiedbythefollowingparticulars:Typeofmediumforarcextinction.

Ratedvoltage.Thiscorrespondstohighestpowerfrequencyvoltagebetweenphasetophase,e.g.

3.6kV,7.2kV,12kV,36kV,72.5kV,145kV,245kV.

Ratedbreakingcurrent.

Otherratedcharacteristics.

Typeofconstruction.

Indoormetalcladtype

Outdoortype

MetalcladSF6gasinsulatedtype.

Typeofoperatingmechanism.

Totalbreaktme,e.g.2cycle,3cycle,5cycle.

Structuralform:

LivetanktypeDeadtanktype.


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Table(5.1)ComparisonofCircuitbreakers

Type Medium Voltage, breaking capacity Design Features Remarks

1. Air-break

circuit-

breaker

Air at atmospheric

pressure

430-600 V, 5-15-35 MVA

recently 3.6-12 kV, 500

MVA

Incorporates: Arc

runners, arc

splitters, magnetic

coils

Used for

medium/low

voltage, A.C.,

D.C., Industrial

circuit-breakers.

Have current

limiting features.

Miniature

C.B.

Air at atmospheric

pressure

430-600 V small size, current

limiting feature

Used for low and

medium voltages.

2. Tank type

oil circuit-

breaker

Dielectric oil 12-36 kV One tank up to 36

kV, 3 tanks above

36 kV, fitted with

arc control

devices

Getting obsolete,

used up to 12 kV,

500 MVA

3. Minimum-

oil circuit-

breaker

Dielectric oil 3.6-245 kV The circuit

breaking chamber

is separate from

supporting

chamber. Small

size, arc control

device used

Used for metal

enclosed

switchgear up to

36 kV. Outdoor

type between 36

and 245 kV

4. Air-blast

circuit-

breaker

Compressed air

(20-30 kgf/cm2)

245 kV, 35,000 MVA up

to1100 kV, 50,000 MVA,

also 36 kV, 500 MVA

Unit type

construction,

several units per

pole, auxiliary

compressed air

system required

Suitable for all

EHV applications,

fast opening-

closing. Also for

arc furnace duty

5. SF6

circuit-

breaker-

Single-

pressurepuffer type

SF6 gas

(5 kgf/cm2)

145 kV, 7500 MVA

245 kV, 10,000 MVA

12 kV, 1000 MVA

36 kV, 2000 MVA

Live tank/Dead

tank design,

single pressure

type preferred

Suitable for SF6

switchgear, and

medium voltage

switchgear. EHV

circuit breaker.Maintenance free

6. Vacuum

circuit-

breaker

Vacuum Preferred for indoor

switchgear rated up to 36

kV, 750 MVA

Variety of

designs, long life,

modest

maintenance

Suitable for a

variety of

applications from

3.6 kV up to 36 kV

7. H.V.D.C.

circuit-

breaker

Vacuum or SF6 +500 kV DC, 15 kA/20 kA Artificial current

zero by switching

in capacitors

Recently

developed, used

in HVDC systems.

Installed in USA


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5.7 ASSEMBLYOFOUTDOORCIRCUITBREAKERS

Thedesignfeaturesofanindividualcircuitbreakerdependuponitsvoltage,otherratingsandthe

type.Thecircuitbreakersmanufacturedbydifferentcompaniesmayhavequitedifferentdesign

patterns. However, a general description of an EHV circuit breaker can be given to cover thevarious types. The low voltage circuit breakers have different design features as the voltage,

capacityandfrequencyofoperationisdifferentfromthatoftheEHVcircuitbreakers.

Figure5.2 shows three identcalpolesofa circuitbreakerassembledonacommon frame.The

distancebetween thepoles isdeterminedby the voltagebetween their conductingparts. The

currentcarryingpartsaresupportedbydielectricmaterials.Thecurrent is interrupted inclosed

chamberknownasarcextnctonchamber(item3)orinterrupter.Thecontacts(10)aregenerally

inpairsoffixedcontactandmovingcontact.

Circuitbreakerpole

Operatingmechanism

Interrupter

Supportporcelain

Conductor

Terminals

Operatingrod

Insulatingoperatingrod

Frame

Contacts

Transfer contacts between

movingcontactsandterminal

Linkages

Figure(5.2)DiagramIllustratingtheAssemblyofanOutdoorCircuitBreaker

Themovingcontact ismovedmechanically.Toachievethisoperationofclosingandopening,an

OperatingMechanismisnecessary.Thefunctionofoperatingmechanismistoopenandclosethe

contactswhendesired. Theoperatingmechanismmaybecommonforthethreepolesormaybeseparateoneforeachpole.


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Inadditiontotheoperatingmechanism,there isa 'ControlCabinet'orwhat isknownas 'Switch

Cabinet'.Thevariouscontrol,interlocking,indicatingconnectionsarethroughthiscontrolcabinet

placednear thebreaker.Thus a complete threephase circuitbreaker consistsof the following

subassemblies:

ThreePoles

OperatingMechanism

ControlCabinet

Auxiliaries

5.8 STRUCTURALFORMOFCIRCUITBREAKERS

Thestructuralformofacircuitbreakerdependsonitstype,ratedvoltage,typeofdesign,typeof

operatingmechanismetc.Inindoor,metalcladswitchgear,thethreepolesofthecircuitbreaker

aremountedonawithdrawaltruck.Suchconfigurationiscommonlyusedforratedvoltagesupto

36kV.

For36kVandabove,outdoorcircuitbreakersarepreferred.Thestructuralformofoutdoorcircuit

breakers depends on rated voltage, number of interrupters per pole and type of operating

mechanism.Circuitbreakersofratedvoltagesuptoand145kVgenerallyhaveasingleinterrupter

per pole. In such a structural form, the interrupter porcelain and support porcelain should

withstandthepowerfrequencyandimpulsetestvoltagesinternallyandexternally.245kVcircuit

breakers have two or more identical interrupter units (elements) per pole. The number of

interruptersperpoledependsupon the ratedvoltageand ratedbreaking currentof the circuit

breaker.Suchcircuitbreakerpolecomprises identicaltwininterrupterunitsmountedonasingle

support porcelain column in T or Y formation. Such a structural form is preferred in outdoor

minimum oil circuit breakers, airblast circuit breakers and live tank type outdoor SF6 circuit

breakers. The configuration with modular construction has advantages with reference to the

shortcircuit testing and the interchangeability of interrupter modules. The single twin

interruptermodulecanbetestedforshortcircuitduties insteadoftestingafullpolecomprising

severaltwininterruptermodules.Thisiscalled'UnitTesting'or"ElementTesting'.

Inmultibreak typeconstruction,voltagegradingcapacitor isconnectedacrosseach interrupter

for equalizing the voltage shared by the interrupter during interruption process. Preclosing

resistors are also connected in parallel whenever necessary. The preinsertion resistors (Pre

closing resistors) are necessary to limit overvoltages occurring during closing unloaded

transmissionlines.


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Circuitbreakersforratedvoltageabove145kVgenerallyhave independentpoleoperaton.The

operatingmechanismofeachpoleisindependentandeachpolecanbetrippedindependentlyby

aseparaterelay.Independentpoleoperationisdesirableforimprovingthestabilityofthepower

system. Structural form of EHV metalclad SF6 insulated switchgear is quite different thanconventionalequipmentdiscussedabove.

5.9 ELECTRICALCONTACTSINCIRCUITBREAKERS

Thepowercurrentpassesthroughtheconductngmaterialintheinterruptngchamber(Fig.5.3).

Variouspartsthatarejoinedtogetherformtheconductingmaterial.Thedifferentjunctionsform

theelectricalcontacts.Electricalcontactisobtainedbyplacingtwoconductingobjectsinphysical

contact.Thiscanbedoneinseveralways.Eventhoughthereisawiderangeofcontactdesignsin

interruptingchambers,theymaybegroupedinfourmajorcategories:

Makebreakcontacts whichmaymakeorbreakunderload;

Slidingcontacts whichmaintaincontactduringrelativemovement;

Fixedcontacts whichmaybeclampedtogetherpermanentlyforyearsandneveropened.

Demountablecontacts whichmakeorbreakoffload.Usuallyseeninmetalcladmedium

voltageswitchgear.

Figure(5.3)FlowofCurrentthroughConductingMaterialinInterruptingChamber

Figure5.4 isasymbolicschematcofatypicalcontactarchitectureandclearlyshowsthecurrent

flow through three of the main types of contacts during the sequence of events of an open

operation.Inallthreetypes,thecontactismadebythetouchingsurfacesofeachcomponent.


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Figure(5.4)Schematicdiagramoftypicalcontactarchitecture

5.9.1 MAKEBREAKCONTACTS

The typesofmakebreak contacts canbe subdividedbypower rating, startingwith the

highest:

Highcurrent,highvoltagecircuitbreakercontacts,whichdisconnectlargeelectricalloads,

andproduce arcs, are containedwithin special arcing chambers.Thesemaybe in air at

normal pressure or in a blast of air, in Sulfur Hexafluoride (SF6), in oil or another arc

extinguishingmedium,includingavacuum.

It includes a moving contact and a stationary contact. Usually one of them is a ring of

sprungcoppercontactfingers(insertontype,Fig.5.5orbu type),ortheother isasolid

rodofcopper.Thecontactsmaybetippedwithanarcresistantmaterialtoresisterosion

from the highpower arc, and the surfaces may be plated (e.g. with silver) to improve

conductivity. Themechanicalpropertiesof copper combinedwith itsexcellentelectrical

conductivity and good arcing endurance in oilhavemade it thepreferred metal in this

application.


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Figure(5.5)InsertionTypeContact

In vacuum circuitbreakers, the contactsare also generally copper,mixedwith tungsten

andspeciallyshaped toensureproperdistributionof theelectric fieldandmovementof

thearcroot.Smallerairbreakcircuitbreakers(mediumvoltage),usecopperinallinternal

conducting parts, but the contacts are often faced with a silver based alloy to resist

welding.Suchcircuitbreakers,beingprotectivedevices,rarelyopenorclose.

5.9.2 SLIDINGCONTACTS(FIG.5.6)

Thesecanbeofverydifferentnature.High speed,heavycurrent types, theyareusually

found inpower interrupterchambers.Thesecontactsmusthaveaveryhighresistanceto

mechanicalwear,astheirrelatvespeedmayreach10meterspersecondormore.

5.9.3 FIXEDCONTACTS

These include awide rangeofbolted and crimped contacts.A clampedjoint avoids the

reduction in cross section caused by drilling to insert bolts, and gives a more uniform

distribution of the contact force, making the contact more efficient and hence running

cooler.Boltingisusedbecauseitischeapandconvenient.

Figure(5.6)SlidingTypeContact


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Crimpedjointsemploytheultimateextremeforceofcontactmaking,causingthemetalto

flowandmakeapermanentconnection.The troublefreenatureofthesejoints,andthe

simplicityandrapidityofthecrimpingoperationmakesthistypeofjointveryattractivefor

permanentconnections.Boltedorcrimpedcontactsareused in interruptingchamberstosecureandtomaintaintheintegrityoftheelectricalcomponent.

5.9.4 DEMOUNTABLECONTACTS

Found in medium voltage metal clad breakers. It helps in taking the breaker off the

networkbyeasilyslidingitoffthebusbarsformaintenancepurposes.Thishastobedone

offload.

Figure(5.7)Demountabletypecontact

These contacts, like the makebreak contacts, may be carrying high currents at high

voltages(e.g.highvoltageisolatorsorhighormediumvoltagefusecontacts).Theyhaveto

carrycurrentreliablyfor longperiods,withoutoverheatingor lossofcontact,butdonot

makeorbreakcurrent.Theyarenotsubjectedtothestressofarcing;hencetheydonot

get the inherentcleaningactionassociatedwith it.Theyarefrequentlydesignedtohave

somefrictionalactiononclosingtoremovesuperficialoxideorcorrosionfilmswhichmight

impedecontact,andcopperand itsalloysarethemostfrequentlyusedmaterialsforthe

bulkofdemountablecontacts.

The characteristicof these contacts is that theyhaveahigh contact force,muchhigher

thanforcircuitbreakersofsimilarcurrentrating,butnotsohighasthecontactforceina

boltedcontact,becauseof theexcessivemechanicalwearwhichwouldbecausedwhen

separatingthecontacts.


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5.9.5 CONTACTRESISTANCE

Aswe said, thecontactoccurswhen two surfaces touch.For theelectriccurrent, if it is

conductivematerial, itmeans apath for it to flow. Observation on a microscopic scale

shows that the contact surface is actually rough even though it seems smooth to theunaided eye. In fact, as the microscope shows, the real contact between two surfaces

happens through a number of small surfaces, called micro contacts (Fig. 5.8), spread

randomly insidethe limitsofthevisiblecontactarea. It isthesumoftheareasofallthe

microcontactsthatconstitutestheeffectivecontactarea.

Figure(5.8)SchematicDiagramofMicroContactAreas

Since theresistanceofanelectricalcontact is inverselyproportional to thecontactarea,

thesmallertheeffectveareathegreatertheresistance(Fig.5.9).

Figure(5.9)SchematicDiagramoftheEffectiveAreainaContact

5.9.6 EFFECTOFCONTACTRESISTANCE

WhenacurrentIpassthroughanareaAthathasaresistanceR,theEnergyEabsorbedbyA

is:

E=RI2t

wheretisthetimedurationofI.


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WeknowthattemperatureTisdirectlyrelatedtoEbythefollowingequation:

E= T

where isafunctionoftheheatdissipationrate.

ForaconstantcurrentIo,ifRincreases,Ethenincreases,leadingtoincreasingtemperature

of thecontact. IfTcontinues to increase thematerialof thecontactcanreach itsmelting

point,leadingtoitsdestructon(Fig.5.10).

Figure(5.10) Contactdestruction

5.9.7 ELEMENTSAFFECTINGTHECONTACTRESISTANCE

5.9.7.1 OXIDATION

Athinlayerofinsulatingoxidecoveringtheareaofasinglemicrocontactwouldhavelittle

effectontheconductivityofthecontactasawhole.Assoonastheoxidelayerextendstoa

significant number of microcontacts, the currentbearing area would reduce, thus

increasing its resistance. Increased resistance will increase the contact temperature,

leadingtoitsdestruction.

Allambientatmospheresthatcontainsgasescapabletoreactwiththecontact'smaterial,

suchasO2,SO2,H2O,H2S,etc.,wouldbefavorabletoproducingoxidelayerseventhough

thecontact isclosed.Withtime, thegaswouldsucceed inpenetratingandreactingwith

thecontactsurfacetodegradeitscharacteristicsandtoincreaseitsresistance.

Figure5.11showstheresistancevalueincreasingwith tme.Aswecansee,theresistance

changeisnotsignificantuntilacertainpointintimewherethedegradationincreasesfast.

Similarresultsareobtainedforcoppercontactsinoil.


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These results show interesting behavior and indicate the urgency of a maintenance

interventionwhenacontact'sresistancestartstoincrease.

5.9.7.2CONTACT

WEAR

Mechanically, itcanbeduetothemovementandfrictionofthecontactsandelectrically

due to thearceffect (mainly themakebreak contact).Contactweardirectlyaffects the

contact resistanceandmakes it increasedramatically if thewear is inanadvancedstate

(Fig.5.12).

Figure(5.11)ChangeofContactResistancewithTime

Figure(5.12)AdvancedWearinaContact

5.9.7.3 FRETTING

A formof acceleratedoxidation ispossible, if the contact surfaces experience a cycling

movementrelativelytoeachother.Forexample,thecontactswouldnotcloseatthesameareaeach tme(Fig.5.13).


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Figure(5.13)Changeincontactarea

Thisphenomenonwasnoticed longagobut itsmagnitudewas recognizedonly recently.

When a contact moves from its previous position, a part is exposed to the ambientatmosphere.Anoxidationlayerthenforms.Whenthecontactgoesbacktothisposition,it

breaksthethin layerandpushes itaside.Thisphenomenonrepeatsmanytimesuntilthe

oxidation layerbecomesofasignificant thickness,enough to increase its resistance.The

resistanceincreasesrapidlyrightaferitstartstochange.Figure5.14showssimilarcaseto

figure5.11,butaccelerated.

Figure(5.14)Changeofcontactresistancewithtimeincaseoffrettingphenomenon

5.9.7.4CONTACT

FORCE

Asknown,theresistanceRisfunctionofthecontactmaterial'sresistivity andareaS,(R=

/S).

Sisthesumofallcontactpointsareas.Thecontactpointsareasarefunctionoftheapplied

forceFandthematerialhardnessH,(kisaconstant).


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IfFdecreases,SdecreasesaswellandR then increases.Fcandecreasedue todifferent

factors,forexample:

Excessivewearofcontactsurface;

Fatigueofcontactspringsovertime;Chemicalreactionofspringmaterialwithambientatmosphere;

Looseormisalignedcontact,etc.

Springmaterialsare thusan importantelement to take intoconsideration.By the same

logic,animportantprecautiontotakeistoavoidlettingthespringbeacurrentpath,asthe

increaseinitstemperaturewouldcauseaweaknessoftheresultantforceF.

5.9.7.5TEMPERATURE

ForanincreasingtemperatureTofthecontacts,thematerialofthecontactsmaysoftento

thepointwhereitwillreducethecontactforce,leadingtoaquickincreaseofthecontact

resistance.

5.10 OPERATINGMECHANISMS

5.10.1 GENERAL

Circuitbreakershavetwoworkingpositionsopenandclose.Duringtheclosingoperation,

thecircuitbreakercontactscloseagainstopposing forces.During theopeningoperation,

theclosedcontactsareseparatedasearlyaspossible.Operatingmechanismsareprovided

to achieve the opening and closing operations. The main requirement of the operating

mechanismistoopenandclosethecontactsofthecircuitbreakerwithinaspecifiedtime.

Theoperatingmechanismshallprovidethefollowingconsecutivefunctions:

Chargingandstoringofenergy

Releaseofenergy

Transmissionofenergy

Operationofthecontacts

In addition, an operating mechanism shall provide control and signaling interface to a

networks control and protection system. Operating mechanisms are also necessary for

isolators.Figure5.15andFigure5.16showthelocatonoftheoperatngmechanism.


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Figure5.15

Threepole operated circuit breaker with one

interrupterperpole.

Figure5.16

One pole of a singlepole operated circuit breaker with

twointerruptersperpole.

1.Breakingunit 5.Tripmechanism 9.Gradingcapacitor(ifrequired)

2.Supportinsulator 6.Gassupervision(onoppositeside) 10.Preinsertionresistor(PIR)(ifrequired)

3.Supportstructure 7.Pullrodwithprotectivetube 11.Primaryterminals

4.Operatngmechanism 8.Positonindicator

Thecircuitbreakeroperatingmechanismsmustbecapableofdealingwith large forces,athigh

speeds, with complete reliability even if the circuit breaker has remained idle for a prolonged

duration.Theopeningshouldbefast, inordertoreducecircuitbreakertime.Theoperatingtimebetween

instantofreceivingtripsignalandfinalcontactseparaton isoftheorderof0.03second, i.e.1.5

cyclesinmodernEHVcircuitbreakers.Inslowcircuitbreakersusedindistributionsystemtimecan

beabout3cycles.

Whileclosing,thecontactclosureshouldbefast,sure,withouthesitation,withadequatecontact

pressureat theendofcontact travel. If theseconditionsarenot satisfied, contactwelding can

result.


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Theoperatingmechanismsshouldbecapableofgivingthespecifieddutyofthebreaker(sequence

ofopeningandclosingas specified instandard specifications).A requirementcommon tomost

circuitbreakers, regardlessof the typeofoperatingmechanisms, is tocarryoutanopenclose

open (O 0.3s CO)sequencewithnoexternalpowersupplytotheoperatingmechanism.Thecircuit breaker shall, after a closing operation, always be able to trip immediately without

intentionaltimedelay.Forcircuitbreakers intendedforrapidautoreclosing,theoperatingduty

cycleinaccordancewithIEC62271100is:

O 0.3s CO 3min CO

The tmeof3ministhe tmeneededfortheoperatngmechanismtorestoreitspoweraferaO

0.3s CO.Modernspringandhydraulicoperatngmechanismsdonotneed3mintorestoretheir

power,asanalternative IECspecifies that the tmevalues15s.or1min.canalsobeused.The

dead tmeof0.3 s isbasedon the recovery tmeof the air surrounding anexternal arc in the

system(i.e.ashortcircuit).SometimestheoperatingsequenceCO 15s COisspecified.

Thebreakershouldalsopasstheoperationaltestswhichascertainthecapabilityoftheoperating

mechanisms.The interlocksareprovidedbetweenbreaker isolatorandearthingswitch,soasto

avoid wrong operation and to assure operation in a correct sequence. The functions of the

operatingmechanismscanbesummarizedasfollows:

Toprovidemeanswherebythecircuitbreakercanbeclosedrapidly,withouthesitationat

allcurrentsfromzerotoratedmakingcurrentcapacity.

Toholdthecircuitbreakerinclosedpositionbytogglesorlatchestillthetrippingsignalis

received.

Toallowthecircuitbreakertoopenwithoutdelayimmediatelyonreceivingtrippingsignal.

Toperformtheautoreclosurecycle.

Toperformtherelatedfunctionssuchasindication,control.

5.10.2 CLOSINGOPERATION(C)

Normally,closingthecircuitbreakercontactsduringnormalloadconditionsdoesnotcause

any difficulty. The operating mechanism has to overcome friction and accelerate the

moving masses. However, when the circuit breaker has to close against a short circuit,

additionalthermalstressesandelectromagneticstressesareinvolved.

InEHVcircuitbreakers,thearcisestablishedpriortofinalcontacttouch.Thisisknownas

prearcing.Prearcingcauseshighertemperaturestresses.Thecontactsshouldclosewithsufficientspeedtominimizetheprearcing.


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Assoonas thecontactscloseonanexistingshortcircuit,breaker issubjected tomaking

current. The electromagnetic forces, setup by the making current, tend to repel the

contacts. The circuit breaker should have rated making capacity, i.e. the highest peak

current against which the circuit breaker canbe closed at a given voltage. The makingcapacityofthecircuitbreakerdependsupon the forceandspeedwithwhich theclosing

operationiscarriedout.

Whileclosingthecircuitbreaker,theoperatingmechanismshouldhaveenoughpowerto

overcometheopposingforcesandacceleratethemovingcontactassemblyrapidlywithin

specifiedshorttime.

Theopposing

forces

during

closing

operation:

Electromagnetic forces between contacts: When the contacts touch during the closing

operation,electromagneticforcesappearatthe instantofcontacttouch,theirmagnitude

being proportional to square of the current and the direction being opposite to the

directionclosing.These forcesare large if thebreaker isclosingonexistingshortcircuit.

Breakershouldbecapableofclosingonshortcircuit.

Actionofoperatingspring:Themovingcontactsofcircuitbreakersareopenedbyspring

pressure.Whileclosing,thesespringsopposetheclosure.Inertiaofmovablesubassembly:

The movable subassembly contacts, their holders, tension rods, operating links of

operatingmechanisms,etc.Themassof thesubassemblies isquite large inEHVcircuit

breakers. And their inertia tries to oppose rapid acceleration. In modern EHV circuit

breakers,thesepartsaremadeaslightaspossible.

Opposing forcesdue tomedium such asoil, SF6 gas:Themovable subassemblyhas to

moveindielectricmediumwhichis,insomecases,compressedoratarehighpressureand

density.Thetotalforcesoftheoperatingmechanismshouldbemorethanthesumofthe

abovementionedopposingforces.Friction:Staticanddynamic.

5.10.3 OPENINGOPERATION(O)

The opening operation is significant in the faultclearing process. As the trip coil is

energized, the opening operation is initiated. The energy required for the opening

operationisobtainedfromoneofthefollowingmethods:

Openingspringschargedduringtheclosingoperation Highpressurehydraulicoilstoredinaccumulators


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Highpressurecompressedairstoredinauxiliaryairreceivers.

Thefunctionalrequirementsoftheopeningmechanismareasfollows:

Toacceleratethemovingmassesincludingcontactsandlinkagesrapidlytoachieve

desiredopeningcharacteristic.

Toachievedesiredspeedofcontactatcontactseparationandduringtheopening

stroke(3to7m/s).

Todampthespeedattheendofthetravelbydampers.

Theforcesandenergyshouldbeadequatetocaterofthefollowing:

Opposingforcesduringopeningoperating

Electromagneticforcesduetocontactgrip:Thefingercontactsarespringloadedandtheir

grip opposes the movement of moving contract. During the shortcircuit condition, the

electromagneticforcestendtoincreasethegripofthefingercontactassembly.Theforce

ofcontactgrip increases inproportiontosquareofcurrent.Hence it issignificantduring

highershortcircuitcurrents.

Friction:Thevariousoperating links,bearingsurfaces,matingsurfacesbetweenmovable

and fixed parts, etc. offer static friction. The frictional component depends upon the

coefficientof friction, smoothnessofmating surfaces,configurationofmovingpartsetc.

Highfrictioncanreducetheinitialspeedofmovingcontactwhichmayresultindisastrous

consequenceoffailureofthecircuit breakertoquenchthearc.

Inertiaofmovableparts:Energyintheoperatingmechanismisutilizedinacceleratingthe

movablesubassembliestorequiredspeed.Opposingforcesduetoquenchingmedium:The

quenching medium (compressed air, dielectric oil, SF6 gas) itself may offer substantal

opposingforcestothemovement.

Theoperatingmechanisms shouldbe capableofovercoming theseopposing forcesend

shouldachievedesiredopening characteristicofcontact travelduringnormaland short

circuitopeningoperations.

5.10.4 CLOSINGFOLLOWEDBYOPENINGOPERATING(CO)

The rated operating sequence of the circuit breaker demands the operation 'CO'. The

operating mechanism should have enough stored energy and capability to perform COoperationandratedoperatingsequenceundershortcircuitcondition.


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5.10.5TYPESOFMECHANISMS

Theoperatingmechanisms in circuitbreakersareeither 'dependable'or 'storedenergy'

type.Dependentoperatingmechanismsdependoncontinuityofpowersupplyormanual

forcesduringclosing.Theyareaccordinglycalledas:

Dependablemanualoperatingmechanisms

Dependablepowermechanisms

The stored energy type operating mechanisms are called independent operating

mechanisms as they are independent of continuity of power supply or the skill of the

operator.Insuchmechanisms,theenergyrequiredforclosingisstoredinachargedspring

orincompressedgas/hydraulicoil.

SpringOperatedMechanism:

In the spring mechanism, the energy for open and closeoperation is stored in springs.

When themechanisms control system receives anopenor close command, theenergy

stored inthespringwillbereleasedandtransmittedthroughasystemof leversand links

andthecontactswillmovetotheopenorclosedposition.

Inmostdesigns theclosing springhas two tasks: toclose thecontacts,andat the same

timetochargetheopeningspring(orsprings).Thusthecriteriastatedabovearefulfilled;

thecircuitbreakerinclosedpositionisalwaysreadytotrip.

AftertheO 0.3s COoperation,theclosingspringwillberechargedbyanelectricmotor,

aprocedurethatlasts1020seconds.ThecircuitbreakerwillthenbereadyforanotherCO

operation.

OneexampleofaspringmechanismisshowninFigure5.17.Thistypeofmechanismhasa

setofparallelhelicalwound springswith linearmotion. The electricmotor charges the

springsviaanendlesschain.Whentheclosing latch isreleased, theenergystored inthe

springsistransmittedviaarotatingcamdiscandasystemofleversandlinkstothecircuit

breaker pole or poles. The trip spring is in this case located outside the operating

mechanism. Insteadofhelical springs, clock springsmaybe applied.The function is the

same as for thehelical springsdescribed above. Figure 5.18 shows a systemwith clock

springforclosingoperation.Theadvantageofthespringoperatedmechanism isthatthe

systemispurelymechanical;thereisnoriskofleakageofoilorgas,whichcouldjeopardize

thereliability.Awellbalancedlatchingsystemprovidesstableoperatingtimes.


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Furthermore, the spring system is less sensitive to variations in temperature than

pneumatic or hydraulic mechanisms are. This ensures stability even at extreme

temperatures.Thespringmechanismhasfewercomponentsthanhydraulicandpneumatic

mechanisms,whichimprovesitsreliability.

MotorDrive:

Oneof themost recentlydevelopedoperatingmechanisms is the electricalmotordrive

(Fig.5.19).Themotordriveusesaservomotortoperformasmoothandsilentoperatonof

the circuitbreaker. The operation is actively controlled by a sensor which continuously

readsthepositionofthemotorandadjuststhemotorcurrenttoobtainanoptimaltravel

curve.Theenergyisstoredinacapacitorbankandcanbedeliveredinstantaneouslytothe

converter,whichtransformsdcfromthecapacitorsandfeedsthemotorwithregulatedac.

Figure5.17

Springoperatingmechanismwithhelicalwound

springs.Tripandclosespringsinchargedposition.

ABBtypeBLG.

Figure5.18

Springoperatingmechanismwithclock

spring.ABBtypeBLK.

1.Linkgear 5.Closingdamper 1.Tripspring 5.Closinglatchwith

coil

2.Tripspring 6.Trippinglatchwithcoil 2.Linkgear 6.Motor

3.Closinglatchwith

coil

7.Openingdamper 3.Closespring 7.Openingdamper

4.Motor 8.Closesprings 4.Trippinglatchwith

coil


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Figure(5.19)BlockdiagramshowingfunctionoftheMotorDrive

1. Charger unit that converts supply voltage and feeds the capacitors and internal

electronics.

2. Capacitorunitstorestheenergyforoperation.

3. Theconverterunittransformsdcfromthecapacitorstoacforthemotor.

4. Servomotor thatdelivers the force tomove the contacts, integratedposition sensor

givesinformationtothecontrolunit.

5. The I/O unit takes care of signaling between the station control system and the

operatingmechanism.

6. Control unit that controls and regulates the motion of the contacts. The interlock

functionsarealsohandledbythismodule.

7. Linkgearthattransformsrotarymovementtolinearmovement.

The major advantage of the motor drive is the minimized mechanical system, which

reduces theserviceneed toaminimumandmakes the technology ideal forapplicationswithfrequentoperation.

PneumaticOperatedMechanism:

The pneumaticoperated mechanism uses compressed air as energy storage, and

pneumatic cylinders for operation. Solenoid valves allow the compressed air into the

actuatingcylinderforclosingorforopening.Thecompressedairtank isreplenishedbya

compressorunit.Theuseofpneumaticoperatingmechanisms isdecreasing.Due to the

high operating pressure, there is always a risk of leakage of air, particularly at lowtemperatures.Thereisalsoariskofcorrosionduetohumidityinthecompressedair.


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HydraulicOperatedMechanism:

Thehydraulicmechanismusuallyhasoneoperatingcylinderwithadifferentialpiston.The

oil is pressurized by a gas cushion in an accumulator, and the operating cylinder is

controlledbyamainvalve.

The hydraulic mechanism has the advantages of high energy and silent operation.

However,therearealsosomedisadvantages.Thereareseveralcriticalcomponentswhich

require specialized production facilities. The risk of leakage cannot be neglected as the

operatngpressure is intherangeof3040MPa(300400bar). It isnecessarynotonlyto

checkthepressureassuchbutalsotosupervisetheoillevelintheaccumulatoror,inother

words,thevolumeofthegascushion.Largevariationsintemperatureleadtovariationsin

operatingtime.

Untl recently several manufacturers used hydraulic mechanisms for their SF6 circuit

breakers.However,withtheintroductionofselfblastcircuitbreakers,therequirementof

highenergyforoperationisdecreasingandthehydraulicmechanismsarelosinggroundto

springoperatedmechanisms.

HydraulicSpringOperatedMechanism:

Thehydraulicspringoperatedmechanismisanoperatingmechanismcombininghydraulics

andsprings.Energy isstored inaspringset that is tensionedhydraulically.Adifferential

piston,poweredbyoil that ispressurizedby the springpackage, isused tooperate the

circuit breaker during opening and closing. Figure 5.20 gives an example of such

mechanism.

Figure(5.20)HydraulicspringOoperatedMechanism.ABBtypeHMB


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OtherTypesofOperatingMechanisms:

In addition to the types of operating mechanisms mentioned above, there are other

variants, e.g. a design which basically applies the same technology as the pneumatic

mechanisms but with SF6 gas instead of air. Another design is the magnetic actuatormechanism,whichisappliedonlyforcertainmediumvoltagecircuitbreakers.

5.11 INTERLOCKS,INDICATIONANDAUXILIARYSWITCH

Interlockingdevicesarethosewhichmaketheoperationoftheswitchingdevicedependentupon

thepositionoroperationofotherequipment.Interlocksareprovidedasasafetymeasureagainst

erroneousoperationof a switchingdevice.The interlocks areof the following forms: Electrical

interlock, Mechanical Interlock. Electrical interlock can be used between remote equipment.

Mechanicalinterlockcanbeprovidedwheretheoperatingmechanismsofthetwoequipmentsare

inneighborhood.Theelectrical interlockcomprisescoilandbolt.Whenthecoil isenergized,the

bolt is drawn by magnetic attraction and the interlocking is achieved. Interlocks are provided

betweencircuitbreaker,isolatorandearthingswitchtoensurethefollowingsequence:

Whileopening:

Firsttoopen:Circuitbreaker

Nexttoopen:Isolato

Thentheearthingswitch(ifany)toclose

Whileclosing:

Openearthingswitch

Close:Isolator

Thenclosecircuitbreaker.

Thissequencemustbefollowedbecauseisolatorsarenoloaddisconnectingdevices.Theydonot

havebreakingcapacity,nordotheyhavemakingcapacity.Hencebreakerperformstheopeningandclosingduty.

Indicator or indicating device indicates whether the switching device is in 'open' or 'closed'

position.Suchindicationisavailableontheglasswindowonthecontrolcabinetnearthebreaker,

in formofa flagmarkedopen/close.Onbreakerpanel, the indication isobtainedbymeansof

lamps.Thus, from thecontrolroom, theoperatorcanknow thepositionofcircuitbreakersand

isolators.(BreakerPanelisinstalledincontrolroom.)


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Auxiliaryswitcheshavestandardnumberofpairsofcontracts(6,8,and12).Auxiliaryswitchhas

two positions 'open' and 'close' corresponding to the position of the circuit breaker. In each

position, some auxiliary circuits are opened and some are closed. The auxiliary circuits serve

severalpurposessuchas:

Indication:Breakeropenorclosedbylamps,nearcircuitbreakerandataremoteplace.Electrical

Interlocks:Thebreaker is interlockedelectricallywith isolators.Theconnections to solenoids in

operatingmechanismaremade through theauxiliary switch.Connections for relaying,auxiliary

circuitsofoperatingmechanisms.Thevariousterminalsareconnectedinaterminalblocksinthe

operatingcubical.

5.12AUTO

RECLOSURE

Manyfaultsonoverheadtransmissionlinesaretransientinnature.Statisticalevidenceshowsthat

about90%offaultsonoverheadtransmissionlinesarecausedbylightningorbypassingofobjects

nearorthroughlines(birds,vines,treebranchesetc.).Theseconditionsresultingarcingfaultsand

the arc in the fault can be extinguished by deenergizing the line by simultaneous opening of

circuitbreakersonbothendsof the lineorononeendof the line.Since thecauseof transient

faultsmentionedabovedisappearsafterashorttimethecircuitbreakerscanbereclosedassoon

as the arc in fault has been extinguished and the path has regained its dielectric strength.

Reclosing of lines restores the supply. Continuity of service is the major advantage of

autoreclosure.Ifthefaultistransientonethenormalconditionisrestoredbyautoreclosure.

Figure(5.21)SequenceofAutoreclosureforEHV,bulk

PowerTransmissionLines,SingleShotScheme


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Highspeedtrippingandhighspeedreclosing improvesthestabilityofpowersystem.Hencethe

circuitbreakersandrelayingonEHVlinesareprovidedwithautoreclosingfeature.Testsonhigh

voltage systems have shown that a reclosure in 12 cycles (0.24 sec) is practcal, the period

dependinguponthetimenecessarytodissipatetheionizedairofarcpath.TheAutoreclosingofEHVlinesisofhighspeedandsingleshot,i.e.onlyonereclosingisaempted(Fig.5.21).

5.13 AUTORECLOSUREOFEHVCIRCUITBREAKERSFORTRANSMISSION

[Voltages36to245kV,Breakingcapacites:25to40kA]

Experienceshowsthatgenerallyminimum tmeof0.2secondmustbeallowedtoelapsetoenable

thefaultzonetobecomedeionizedcompletely,henceadead tmeof0.3secondhasbeenchosen

asasafereclosure tme.Figure5.22illustratesatypicalcircuitbreakercontacttravelcharacteristc

andalsogivesascheduleoftypicaloperatingtimesoftheabovecircuitbreaker.Incaseofsome

otherrelayingthecyclemaydiffer.

Figure(5.22)FaultClearingandReclosingTimeSequence, ContactTravel.

LookinginFig.5.22,thefollowingsequencecanbeobserved:

Sequence

No.

Time in

1/100sec. Operation Remarks

1 0 Faultoccurs

Circuitbreaker closed. Protective gear

startsoperating

2 04 Relaytime Fastrelaying

3 4 Tripcircuitclosed Operatingmechanismstartstoopen


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4 49 Openingtimeofbreaker

5 912 Totalbreaktime Breakerisof4cycles

6 1236 Deadtime 12cycles,fordeionization.CBremainsopen

7 27 Contactsstartclosing8 36 Contacttouchforreclose

9 40 Circuitbreakerreclosed

Will be opened again if fault persists and

willlockopen

10 Singleshot iscomplete,thecircuitbreakerwillremainclosed iffaulthasvanished.

CBwillopenagainiffaultpersistsandwillremainlockedopen

Several aspects need careful consideration while selecting autoreclosure schemes of EHV

transmissionlines.Someoftheseaspectsarediscussedhereinbrief.

Thefaultlevelsonsuchlinesarehigh(ascomparedwiththoseondistributionlines).Switchingof

suchlinesresultsinovervoltagetransientswhichareharmfultoinsulationandshould,therefore,

avoidedasfaraspossible.Thesynchronismbetweentwosidesofthecircuitbreakershouldnotbe

lost.Asregardsthesystemconsiderations,thestabilityofthesystem(conditionofsynchronism)

shouldnotbelostbythetimeofreclosing.Duetotheseaspects,theautoreclosureofEHVlineis

generallySINGLESHOTAUTORECLOSURE.Thebreakersused for suchautoreclosureare fast in

opening,closing. Further, the circuit breakers at both ends of the line should reclose almost

simultaneously.

ThetimingofEHVautoreclosureisbasedonthefollowingrequirements:

itshouldbeless,intheordertoavoidlossofsynchronismbetweeneithersidesoftheC.Bs

thearcinC.B.andthatinthefaultshoulddeionizedbeforeallowingreclosure

theoperatingmechanismsofC.B.toopenandtocloseshouldbecapableofachievingthedesired

timeschedule

C.Bsatbothendsofthelineshouldreclosesimultaneously.

Deionization time for arc space in fault on overhead linedepends on several aspects such as

magnitudeof faultcurrent,servicevoltage, lengthof linewindcondition,spacingofconductors

etc.Generallythetimeallowedisbasedonratedvoltageoflineandisasfollows:


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VoltageofTransmission line,

(kV)

RatedVoltageofC.B.,

(kV)

MinimumDeionizationtime

necessary,Hz

66 72.5 5

132 145 9

220 245 14

400 420 18

Thecircuitbreakersshouldbecapableofwithstandingtheelectrodynamicstressincasetheyare

reclosingonanexistingshortcircuit.Thepressureinthereservoirgenerallyreducesafterthefirst

opening;therebythereisareductioninbreakingcapacityforthesubsequentopening.Thisaspect

shouldbetakencareofwhiledesigningthecircuitbreakerssuitableforautoreclosure.

5.14 AUTORECLOSUREFORDISTRIBUTIONLINES

Inruraldistribution,overhead linesareused.Thespacingbetweenconductors isrelativelyclose.

The disturbances on such lines are generally transient, as described earlier. Autoreclosure is

therefore, suitable in improving the continuity of service. The usual procedure was to reclose

circuit breaker three tmes between 15 to 120 seconds. If the breaker trips afer the third

reclosure, itopensandremainsopen.Theattendant therebyknows that thefault ispermanent

and sends electricians to locate and correct the fault. The autoreclosure cycle is illustrated in

Fig.5.23,but thesequencemayvary inothercases.Thispractce isnomore favored inmodern

distributionsystems.

Figure5.23Autoreclosurecycleofa12kVC.B.forruraldistributon


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5.15 TRIPFREEFEATURE

Suppose the breaker has been instructed to close by manual instruction by pushing of push

button.Theoperatingmechanismwillstartoperatingforclosingoperation.Meanwhileafaulthas

takenplaceandarelayclosesthetripcircuitofthebreaker.TheTripFreemechanismpermitsthecircuitbreakertobetrippedbytheprotectiverelayevenifitisundertheprocessofclosing.This

feature is called Trip Free feature. Another feature of operating mechanisms is to prevent

Pumping,i.e.alternatetrippingandclosingiftheclosingbuttonisheldclosedduringafault.Inoil

circuitbreakersandpuffertypeSF6circuitbreakers,thecontactsmaybeallowedtotouchduring

theendoftheclosingstroke,beforethestartoftheopeningoperation.

5.16 MATERIALSANDSOMEDESIGNASPECTS

Thematerialsareimportantinswitchgearmanufacturing.Normallyalltheincomingmaterialsare

tested in the factorybeforeacceptance.Themanufacturermaintainswithhimall thenecessary

standardsofmaterialspecifications.Thesestandardsgivethespecificationsofchemical,electrical,

metallurgical and mechanical properties of the corresponding materials. The heat treatment,

surface treatment is equally important in the manufacturing process. The materials used in

switchgearsingeneralandcircuitbreakerinparticularcanbeclassifiedinausualway:

Conductingmaterials

Dielectricorinsulatingmaterials

Othermaterialsformanufacturing

Circuitbreakerdesignisgenerallybackedbythevastdesigndata,theexperienceandtheresearch

results.Beforedetaildesignoffunctionalsubassemblies,thestructuralformofthecircuitbreaker

isdecided.Thedimensiondrawingsaremadethis isfollowedbydetaildesignofsubassemblies

and components. Each subassembly and component isdesigned on the basisof its functional

requirementsandtheinformationalreadyavailablewiththedesigners.Aftercompletionofdesign

theprototypes aremanufactured.Theprototypes are subjected to rigorousdevelopment tests

andfullseriesofTypeTests.


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Table(5.2)MaterialsusedinCircuitBreakers

Material Applications Remarks

Porcelain Enclosures for Interruptersupport, support for busbars

insulating tubes solid rods,

etc.

Compressionstrength6000kgf/cm2.Tensilestrength 3000 kgf/cm2. Ceramic material

madeby firingclay,glazingand firingagain.

Suitableforoutdooruse.

EpoxyResin Support insulators for indoor

applications, enclosures cover

encapsulation,etc.

Used insolidform.Obtainedbymixingwith

suitable hardener and curing a suitable

temperature, suitable fillers used. Not

suitableforoutdooruse.

Glass fibre reinforced

syntheticresin

Insulating drive rods,

insulating tubes for

interrupters

High tensile strength, withstand pressure,

dielectricstrength.

Polytetra fluroethelene

PTFE

Nozzles for SF6 Breakers,

bearings,Pistonrings,etc.

Low friction; arc resistant; can be

moulded/machined. Pure PTFE is insulating

usedwithvariousfillers.

Electrolytic Copper

(99.9%purity)

Busbars, main contacts

conductingparts,terminals

Electrical grade

aluminum

Busbars, conducting parts,

casting, terminals, enclosures

ofSF6GIS,enclosuresofbus

bars,enclosuresofbusducts

TungstenCopper Arcingcontacts 80% Tungsten, 20% copper, sintered

material

StainlessSteel Enclosures of SF6 GIS pars

enclosedcircuits

Copperbismuth

CopperChromium

Copperberrylium

Main contacts of Vacuum

interrupters,contactors

Highconductivity,lowweldingtendency

CURRENTSCARRYINGPARTS:

These includecontacts,contactstems,flanges,busbarsbushingconductors,connectorsetc.The

designofconductingpartsisbasedonthefollowingrequirements:

Temperature rise during normal continuous currenttemperature stresses during shorttime

current(rateddurationofshortcircuits.

Mechanicalstressduringopeningandclosingoperation.Mechanicalstressesduetoelectromagneticforcesundershortcircuitconditions.


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INSULATINGPARTS:

These include interrupterenclosures insulating supports for interrupters, insulating supports to

busbars, insulating pullrods, connecting the operating mechanism to the moving contracts,

insulatingtubesenclosingthearccontroldevicesetc. Insulatingpartsaresubjectedtodielectricstressesandmechanicalstresses.

5.17 DESIGNANDDEVELOPMENT

Circuit breaker design and development is relatively longer and costlier process as it involves

shortcircuit type testsandhighvoltage type tests.Circuitbreakersaredevelopmentbyvarious

leadingmanufacturers.Presently theresearchanddevelopment is focusedonSF6switchgear in

EHV range and vacuum switchgear in medium voltage range. Besides product development,

research is also in progress regarding system studies, testing procedures, reliability studies,

materials etc. Details about design and development are beyond the scope of this manual;

howeverthegeneralapproachismentionedinvariouschapters.Thedevelopmentofanewcircuit

breakercomprisesthefollowingmajoractivities:

Research: The research on arc quenching techniques, various thermal, electrical, mechanical

stressesundervariousswitchingconditions,designprincipleforarcquenchingetc.This iscarried

outinresearchlaboratories.

Design and development of Prototypes: The structural configuration is decided first. Then the

various subassemblies are designed and finally the complete breaker is designed. Full scale

prototypes aremanufactured.DevelopmentTesting:Variousdevelopment testsare carriedout

subassemblies,poles,mechanismandcompletebreaker.

TypeTests forCertifications: These areexhaustive tests asper standards.Actual Installation in

systemforobservingperformance.

5.18 SUMMARY

The circuit breaker assembly consists of switch cubicle, operating mechanism, poles, current

carryingparts,dielectricpartsandotherpartsoftheassembly.

The types of a.c. circuit breakers include (1) Airbreak circuit breaker, (2) Tank type oil circuit

breaker, (3)Minimumoilcircuitbreaker, (4)Airblastcircuitbreaker, (5)SF6circuitbreaker, (6)

Vacuuminterrupter.Thesearewidevarietiesofdesignsineachtype.

Airbreakcircuitbreakersemployairatatmosphericpressureforarcextinction.Theyincorporate


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arcrunnersandblowoutsforlengtheningthearc.Theyarewidelyusedforlow/mediumvoltages

andalsoforvoltagesupto12kV.Miniaturecircuitbreakersareusedfordomesticandcommercial

applications.Theyare finding increasinguse inmotorprotection, railway applications and low

voltage switchgear. Minimum oil circuit breaker has been developed for wide range of ratingbetween3.6to245kV.ABCBandSF6CBarepreferredforEHVapplicatons.Vacuuminterrupters

andcircuitbreakersarefinding increasinguseforawiderangeofapplicationsforratedvoltages

upto36kV.

SF6circuitbreakershavetwotypesofdesigns (1)doublepressuretype, (2)singlepressure type

(PufferType).

The operating mechanisms play an important role in the operation of circuit breakers and

isolators. The operating mechanisms are either 'dependent' or 'stored energy' type. Manual,

springloadedmanual,motorwoundspringtype,pneumatic,hydraulic,etc.arethevarioustypes

ofoperatingmechanism.Theoperatingmechanisms forEHVcircuitbreakersshouldbesuitable

forrapidautoreclosing.

Thematerialsused inswitchgear includeconductingmaterial likeelectrolyticcopper,aluminum;

insulatingmaterial likeporcelain, epoxy resins.Arcing contactsof circuitbreakers aremadeof

coppertungstenalloy.Themaincontactsaremadeofelectrolyticcopper.

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