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Transformers:coreofelectricitysupply

Transformer:mostvitalequipmentofthesupplysystem

Types: EHVTransformer HTTransformer LTTransformer InstrumentTransformers

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Toprovethatthedesignmeetsthespecifiedjobrequirementsandtoobtaintransformercharacteristics.

Tocheckthatthequalityrequirementshavebeenmetandthatperformanceiswithinthetoleranceguaranteed.

TestsperformedfortheformerpurposearereferredtoasTypetests andthatforthelaterpurposearereferredtoasRoutinetests (carriedoutoneveryunitmanufactured).

InAdditiontothesetwocategoryoftests,specialtestsMayalsobeperformedtoobtaininformationusefultotheUserduringoperationormaintenanceofthetransformer.

NECESSITYOFTESTS

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TransformerTests

EXAMPLE:ROUTINETESTS Measurementofwindingresistance Measurementofvoltageratio,polarityandcheckofvoltagevectorrelationship Measurementofnoloadlossandexcitationcurrent Measurementofshortcircuitimpedanceandloadloss Measurementofinsulationresistance Switchingimpulsevoltagewithstandtest Lightningimpulsevoltagewithstandtest Magneticcircuit(isolation)test

EXAMPLE: TYPETESTS

Meaurementofpowertakenbycoolingcircuits

TempRiseTest

Reference standard : IEC 60076

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ADDITIONALTESTS

TESTWITHLIGHTNINGIMPULSECHOPPEDONTHETAIL

MAGNETICCIRCUIT(ISOLATION)TEST

MAGNETICBALANCETESTONTHREEPHASETRANSFORMERS

DISSOLVEDGASANALYSIS(DGA)OFOILFILLEDINTHETRANSFORMER

DETERMINATIONOFCAPACITANCESANDTANDELTABETWEENWINDINGTOEARTHANDBETWEENWINDINGS

MECHANICALTESTS

MEASUREMENTOFVIBRATIONONTRANSFORMERTANK

VACUUMTESTONTRANSFORMERTANK

OILPRESSURETESTONCOMPLETELYASSEMBLEDTRANSFORMER

JACKINGTESTANDDYEPENETRATIONTEST

PRESSURERELIEFDEVICETEST

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TestsonTransformers

Erectiontests(installationTests) Precommissioningtests CommissioningTests

Erection tests: Thorough checking for any damage Verification of technical specifications Foundation etc

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ERECTIONATSITE:INSPECTION

y Makevisualinspectionforanytransitdamagey Checknitrogenpressure.y Checkvariousaccessoriesforanytypeoftransitdamage.y Makeinternalinspectionofthetransformertotheextent

possibleforanyvisiblediscrepancy.

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OILFILLING

y Beforefillingtheoilintotransformerchecktheoilcarefullyfory Breakdownvoltagey Moisturecontenty Tandeltaandcapacitance

y Iftheoilisnothavingthepropertiesasrecommendedbythestandards,itmustbefilteredwithfilterswithbuiltinheaters andvacuumpumpsforimprovingthequalityofoil.

y Fortransformersdispatchedgasfilled,oilfillingmustalwaysbedoneundervacuum.

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GENERALTESTSAFTERTRAFOINSTALLATION

(GetClearanceforTesting&Commissioningactivities) Physicalinspection Fireprotectionsystemavailable Oillevelchecking,nooilleakage Powerandcontrolcablingchecking, CoolerfanandPumpcontrolcktchecking Buch.Relaycorrectlymounted,airreleasedfromBuchrelay, Coolingsystem&radiators,OLTC BreatherinstallationOk,silicagelcolourblue OTI&WTICalibration

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IRvalueoftransformerandcables WindingResistance TransformerTurnsRatio PolarityTest MagnetizingCurrent VectorGroup MagneticBalance Bushing&WindingTanDelta(HV) Protectiverelaytesting Transformeroiltesting Hipottest

PrecommissioningTests

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CommissioningTests:

RemoteAnnunciationandcontrolschemes PowercableIdentification&Tests Chargingthetransformer:noloadandfullloadoperationandObservationforanyabnormality

Confirmingtheoperationofallprotection,interlocksandmeteringschemes

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1.ChecktheoperationoftheBuchholtzalarmandtripbyinjectingairthroughthetestpetcock.2.TesttheOTIforalarmandtrip.3.TesttheWTIforalarmandtrip.4.ChecktheworkingoftheWTI/RTD(ResistanceTemperatureDevice)repeatersatthecontrolroom.5.TesttheOLTC Oilsurgerelayfortrip.6.Checkalarmforlowoillevel.7.Checkvariousrelaysforcurrent&timesettings8.Checkthecoolerunitfor

overcurrentsettingoffansandoilpumps coolersupplyfailurealarm fan/pumptripalarm.anymal operationofthetransformerBuchholtzrelaywhenalltheoilpumpsareswitchedonsimultaneouslyinforcedoilcooledtransformers

FIELDTESTS:

NoLoad Windingresistance TurnsRatio. Polarity, connectiongroupand Excitationcurrent.Onload shortcircuittest, Impedancemeasurement

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InsulationResistance

y Thistesthasbeenthemostusualhistorically,beingcalledtomegger thetransformer(thetermcomesfromthefirmofthefirstIRsystems).

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InsulationResistanceandPolarizationIndex

Equivalent diagram of the dielectric circuit of a transformer. Ri is the insulation resistance, Cg the geometric capacitanceand the different Ra/Ca emulate the equivalent dielectric absorption circuit.

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WindingResistance

y Thisisnothingbuttheresistancemeasurementofthewindingsbyapplyingasmalld.cvoltagetothewindingandmeasuringthecurrentthroughthe same

y Themeasuredresistanceshouldbecorrectedtoacommontemperaturesuchas75Cor85Cusingthefollowingformula:

where

y RCisthecorrectedresistance,RMisthemeasuredresistancey CFisthecorrectionfactorforcopper(234.5)oraluminum(225) windingsy CTisthecorrectedtemperature(75Cor85C)y WTisthewindingtemperature(C)attimeoftest

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WindingResistance.

y Withthistestwedeterminatethepureohmicresistancefromeachphasewindingsbothinhighandlowvoltageandeachpositionoftapchanger.

y inafirstapproachseemseasytomeasure,itsnotso,becauseitisnecessarytomakeflowrelativelyhighcurrentstoregistertheusuallowresistancevalues/m / withtherequiredprecision.Thiscurrentsmustalsoflowthroughtheequivalentinductanceofthetransformer.

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TransformerTurnsRatio(TTR):

Transformer Turns Ratio (TTR): quotient between high voltage / low voltage. Must match with protocol /nameplate values.

In the power transformer with tap changer, take readings for each position (giving extra information on its status and that of the On Line Tap Changer (OLTC)

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Transformer Turns Ratio: Doble kit

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PolarityTest

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Opencircuittest&Excitationcurrent

y Excitationcurrentisthecurrentflowingintothehighvoltagewindingwiththelowvoltagesideopen.

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FIELDTESTS:Loadtest

y shortcircuittest,y Impedancemeasurement

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ShortCircuittest

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ShortCircuittest

y shortcircuittest isbasedintheapplicationofavoltageinonewinding(highvoltageone)withtheotherwindingshortcircuited.Itisusualtoregisternominalandextremepositionsifthetransformerhadan OLTC.

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Impedancemeasurement

y Shortcircuitvoltage:voltagerequiredtocirculatefullloadcurrent.y shouldbeneartheprotocol/nameplatevaluefromthetransformer.y Itschangewillindicateirregularitiesinthemagneticcore,winding,

displacement,shortcircuits,mechanicaldeformations

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FrequencyResponseAnalysis

y Themainpurposeofthistestistodeterminatethefrequencyresponsegraphoftheequivalentelectric/dielectric/magnetic/mechanicaltogetherevaluated.

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FrequencyResponseAnalysis

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DIELECTRICSYSTEMTESTS.Liquiddielectric(usuallymineraloil)ORsoliddielectric(usuallypaper)

Dielectriccircuittests.y InsulationResistanceandPolarizationIndex(IR,PI).y Capacitance,Dissipation(power)factor(tan)andinsulationlosses

indielectricbushing.y PartialDischarge(PD).

Oilsampling.y DielectricStrength.y Moisturecontent.y DissolvedGases.y FuranAnalysis.

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Oiltestingandmaintenance

Therearethreeimportantpurposesoftheoilinatransformer:

1.Gooddielectricstrength2.Efficientheattransferandcooling3.Topreservethecoreandassemblyy Byfillingvoids(toeliminatepartialdischarge)y Bypreventingchemicalattackofcore,copperandinsulationbyhaving

y lowgascontentandnaturalresistancetoageing.

OilTesting

y Visualchecks

y OilBDVtest

y Powerfactor(tandelta)test

y Aciditytest

y Moisture(ppm)

y DGA

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Transformer BDV test set

Breakdownvoltage.

y Oildegradationcanbeeasilyappreciatedwiththisparametertested.Thetestisbasedintheinsertionofelectrodesimmersedinoilandincreasingvoltageuptobreakdown.

y Testisrepeatedsixtimestogetarepeatablemeasurement.y Theonlydisadvantageisthatitisnecessarytoextractfromthe

transformerasignificantsample(testcellwillhave350...600ml).

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IEC 156

TransformerOilSpecificationsTransformerOilSpecifications

Characteristics Requirementasper

IS:18661983

Breakdownvoltage 50KV(min)for>145KV

40KV(min)for72.5KVto145KV

30KV(min)for145KV

35ppm(max)for145KV

1.0(max)for0.1X10E12

NeutralizationvaluemgKOH/gm 140

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Capacitance/Tandelta(DissipationFactor)

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Capacitance/Tandelta(DissipationFactor)

y Anotherusualapproachwhenperformingdielectricevaluationistandelta.ThistestusesACandpursuestoknowlossangleofthetestedelement.

y Thismeasurementincludesinformationofthemoistureandcontaminationdegreeandemulatesthebehaviourandvoltageaggressionssimilartoserviceones.

y Itisimportanttotakenoteoftransformertemperatureandenvironmentalmoisture(surfaceleakage).

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VectorGroupTest:Dy11

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VectorGroupTest:Dy11

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Dy11

Vectorgrptest

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VectorGroupDy1

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VectorGroupDy1

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PartialDischargetest.

y Partialdischargearesmalldischargesthatappearinsideofthe dielectricasasignofitsdegeneration.Theyappearasaresultoftheincreaseoftheelectricfieldinsmallgaseousvoidsinsidetheoilandalsoinsidethepaper,epoxyor asaresultofthepresenceofmetalliccontamination,etc.Thisdischargeacceleratethethermaldegradationeffectsandleadsometimesexponentiallytothepowertransformerdestruction.

y Therearetwousualdetectionsystems,y Acousticsystemusessoundmechanicmanifestation(inultrasonicrangey Electric systemsdeterminatesdischargesandcorrelatesthemwithInsulation

healthiness.

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SomeotherTests,normallyperformedatManufacturersworks

TemperatureRiseTest LighteningImpulseTest VacuumAndPressureTest LightningImpulseWithstand SwitchingImpulseWithstand

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History/Trending

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Conclusion

Testingisanart&scienceofmanagingMachine,Manpower,Material,Quality,Time

y Peopleresponsibleforupkeepofelectricalapparatusneedtocultivateawatchfulattitudeto

y Look (eye) Listen (ear)y Smell (nose) Feel (finger/skin)

forsymptomsofimpendingtroubley Adaptnewtechniques.y Keeponlearningfromtheexperiencesworldwide,avoid

obsolescencey DevelopIntegratedPredictiveDiagnosticSystemy KeepitSystemOrientedtoensuresustainabilityy (Avoidsubjectivity&individualbias)

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Conclusion

y SafetyFirsty GoodTestequipmenty peopletohaveadeepknowledgey Takecareofenvironmentalconditionswhilecomparingresultsy Workpressureandshortdowntimey Keepyourmindcool

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SomeReferences

y NATIONALSTANDARDSIS2026(PARTIII)y IEC600765:2000.Powertransformers.Part5:Abilitytowithstandshortcircuit.y IEC60726:2003.Drytypepowertransformers.y IEC600763Testlevels&testmethodrequirementfordifferenttestsy ANSI/IEEEStd621995.IEEEGuideforDiagnosticFieldTestingofElectricPowerApparatus

Part1:OilFilledPowerTransformers,Regulators,andReactors.IEEEPowerEngineeringSociety1995.y ANSI/IEEEStdC57.12.901993.IEEEStandardTestCodeforLiquidImmersedDistribution.Power

andRegulatingTransformersandIEEEGuideforShortCircuitTestingofDistributionandPowerTransformers.

y ANSI/IEEEStd.C57.12.001993.IEEEStandardGeneralRequirementsforLiquidImmersedDistribution,PowerandRegulatingTransformers.

y EN601561995.Insulatingliquids.Determinationofthebreakdownvoltageatpowerfrequency.Testmethod..

y EN608141997.Insulatingliquids.Oilimpregnatedpaperandpressboard.DeterminationofwaterbyautomaticcoulometricKarlFischerTitration.

y EN605671992.Guideforthesamplingofgasesandofoilfromoilfilledelectricalequipmentandfortheanalysisoffreeanddissolvedgases .

y EN60599:1999.Mineraloilimpregnatedelectricalequipmentinservice Guidetotheinterpretationofdissolvedandfreegasesanalysis..

y EN61620:1999.Insulatingliquids.Determinationofthedielectricdissipationfactorbymeasurementoftheconductanceandcapacitance.Testmethod.

y EN61198:1994.Mineralinsulatingoils.Methodforthedeterminationof2furfuralandrelatedcompounds.

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y LIGHTNINGIMPULSETEST

TESTING OF TRANSFORMERS

TosimulatethesystemdisturbancesduetoLightningstroke Canberepresentedbythreebasicwaveshapes

FullwaveLI Choppedwave(Choppedontail) Choppedwave(Choppedonfront)

Lightningdisturbancesdonotalwayshavethesebasicwaveshapes.However,bydefiningtheamplitudeandshapeofthesewaves,itispossibletoestablishaminimumimpulsedielectricstrengththatatransformershouldmeet.

y Ifalightningdisturbancetravelssomedistancealongthelinebeforeitreachesatransformer,itswaveshapeapproachesthatofthefullwave.Itisgenerallyreferredtoas1.2/50wave.Awavetravelingalongthelinemightcauseflashoveracrossaninsulatorafterthepeakofthewavehasbeenreached.Thiswaveissimulatedbyachoppedwavewhichisofthesamemagnitudeasthefullwave.Ifalightningstrokehitsdirectlyatorveryneartoatransformerterminal,theimpulsevoltagemayrisesteeplyuntilitisrelievedbyaflashover,causingsudden,verysteepcollapseinvoltage.Thisconditionisrepresentedbythefrontofwave

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TESTSONTRANSFORMERS

y LIGHTNINGIMPULSETEST

1.2/50SLIWAVE CURRENTWAVEFORMDURINGLI

TYPICAL TEST LEVELS FOR LI TEST :H1 : 1300 kVp,

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TESTSONTRANSFORMERS

y LIGHTNINGIMPULSETEST

1.2/50SLIWAVECHOPPEDAT3.7S CURRENTWAVEFORMDURINGCHOPPEDLI

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SWITCHINGIMPULSETEST

y SwitchingimpulselevelisconductedtoverifytheintegrityoftransformerundervariousswitchingsurgesonAC/DCline.

y ThetypicalwaveshapeofSwitchingimpulseis250/2500SasperIEC60060&itsmagnitudeislowerthanthecorrespondingLIwavebutforlongerduration.

y Inviewofthedifficultiesingeneratingthesewaveshapesinlaboratoriesandmoretominimizethecoresaturationduringtestingoftransformers,IEC60076Part3hasspecifiedaswitchingimpulsewavehavingavirtualfronttimeofatleast100s,durationabove90%ofthespecifiedamplitudeofatleast200sandatotaldurationtothefirstzeropassageofatleast1000s.

y Unlikeinthecaseofacwinding,boththeterminalsofvalvewindingexperiencefulldcvoltageofthebridgetowhichtheyareconnected.Thereforevalvewindingistestedtoprovetheinsulationofthewindingwiththerespecttoground.Switchingimpulsesareappliedtothevalvewindingterminalsconnectedtogether.Theotherwindingterminalsnotundertestareshortcircuitedandgrounded.

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SWITCHINGIMPULSETEST

y WAVESHAPEOFSWITCHINGIMPULSEASPERIEC60060

STANDARD250/2500SSWITCHINGIMPULSEWAVE

TYPICAL SWITCHING IMPULSE LEVELS : H1 : 1080 kVp,X1-X2 : 1290 kVp, X3-X4: 980 kVp

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SWITCHINGIMPULSETEST

y SwitchingimpulselevelisconductedtoverifytheintegrityoftransformerundervariousswitchingsurgesonAC/DCline.

y ThetypicalwaveshapeofSwitchingimpulseis250/2500SasperIEC60060&itsmagnitudeislowerthanthecorrespondingLIwavebutforlongerduration.

y Inviewofthedifficultiesingeneratingthesewaveshapesinlaboratoriesandmoretominimizethecoresaturationduringtestingoftransformers,IEC60076Part3hasspecifiedaswitchingimpulsewavehavingavirtualfronttimeofatleast100s,durationabove90%ofthespecifiedamplitudeofatleast200sandatotaldurationtothefirstzeropassageofatleast1000s.

y Unlikeinthecaseofacwinding,boththeterminalsofvalvewindingexperiencefulldcvoltageofthebridgetowhichtheyareconnected.Thereforevalvewindingistestedtoprovetheinsulationofthewindingwiththerespecttoground.Switchingimpulsesareappliedtothevalvewindingterminalsconnectedtogether.Theotherwindingterminalsnotundertestareshortcircuitedandgrounded.

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SWITCHINGIMPULSETEST

y WAVESHAPEOFSWITCHINGIMPULSEASPERIEC60060

STANDARD250/2500SSWITCHINGIMPULSEWAVE

TYPICAL SWITCHING IMPULSE LEVELS : H1 : 1080 kVp,X1-X2 : 1290 kVp, X3-X4: 980 kVp

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Magnetic balance test Done on TRANSFORMER to see whether the transformer core is magnetically balanced or

not...AND ALSO WINDING CONDITION.... for example we r having delta HV and star winding LV side ... HV SIDE WE WIL BE HAVING

THREE BUSHINGS....THOSE R 1U,1V,1W.. HV SIDE WE WIL BE HAVING FOUR BUSHINGS.....2U,2V,2W,2N... WE CAN DO THIS MBT (MAGNETIC BALANCE TEST)WITH 1PHASE SUPPLY(1PHASE AND NEUTRAL(230V)) OR WITH 2-PHASE SUPPLY(2-PHASES(440V)) 1ST STEP OF MBT: GIVE POWER SUPPLY 1-PHASE OR 2 PHASE BETWEEN 1U AND 1V ,MEASURE THE VOLTAGES BETWEEN 1V-1W AND 1U-1W AND ALSO 2U-2N,2V-2N,2W-2N.. SO HERE IF THE CORE IS CORRECTLY BALANCED THEN WE WILL GET RESULTS AS FOLLOWS 1U-1V 1V-1W 1U-1W 2U-2N 2V-2N 2W-2N 440 300 140 90 50 40 THAT MEANS HV SIDE : 1U-1V = 1V-1W + 1U-1W I.E 440V = 300 + 140 AND ALSO LV SIDE : 2U-2N = 2V-2N + 2W-2N 90 = 50 + 40 STEP 2: AFTER DOING THIS ..INTERCHANGE THE CONNECTIONS BETWEEN 1V-1W AND MEASURE THE VALUES AS ABOVE... STEP 3: AFTER DOING THIS ..INTER CHANGE THE CONNECTIONS BETWEEN 1U-1W AND MEASURE THE VALUES AS ABOVE... IF ANY FAULT IS THERE IN TRANSFORMER WE CAN NOT FIND GOOD RESULTS.... IF THE WINDING IS DAMAGED .... WE WILL GET ZERO VALUES .... ex: HV SIDE 1U-1V = 1V-1W + 1U-1W 440V = 440 + 0 AND ALSO LV SIDE : 2U-2N = 2V-2N + 2W-2N 90 = 90 + 0 THIS IS THE ACTUAL APPLICATION OF MANGNETICA BALANCE TEST...

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