Advanced Practical Maintenance and Testing of Power Transformers Training Course

This course describes the fundamental principles, construction, operation, and protection of transformers.
It emphasizes the importance of safety when working with high-voltage equipment.

1. Transformer Fundamentals &
Protection Device Overview
Dr. Wael HOURANI
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2. 1.Pre-AssessmentandCourseOverview
•CourseOverview:
•Objective:Tounderstandthefundamentalprinciples,
construction,operation,andprotectionoftransformers.
•Safety:Emphasizetheimportanceofsafetywhenworkingwith
high-voltageequipment.

3. 2.TransformerFundamentals
Thissectionexplainsthecoreprinciplesofhowtransformerswork.
•PrincipleofOperation:itdependsonelectromagneticinductionandmutual
inductance.Atransformerworksontheprinciplethatachangingmagneticfieldinone
coil(theprimary)inducesavoltageinasecondcoil(thesecondary).
•TurnsRatio:Introducethefundamentalequation:Vp​/Vs​=Np​/Ns​=Is​/Ip​.Defineeach
variable:V(voltage),N(numberofturns),andI(current),withsubscriptsp(primary)
ands(secondary).
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4. 2.TransformerFundamentals
•Step-Upvs.Step-DownTransformers:
•Step-Up:Ns​>Np​.Thisincreasesvoltageanddecreasescurrent.Usedinpower
plants.
•Step-Down:Ns​<Np​.Thisdecreasesvoltageandincreasescurrent.Usedin
homesandfactories.
•Idealvs.RealTransformer:lossesinarealtransformerarethecorelosses(hysteresis
andeddycurrents)andthecopperlosses(RI
2
losses).
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5. 5
2.TransformerFundamentals

6. 3.TransformerConstruction,Applications,andComponentFunction
Thissectionshowsthephysicalaspectsoftransformers.
•Core:providealow-reluctancepathforthemagneticflux.Laminationreduceseddy
currents.
•Windings:typicallymadeofcopperoraluminum.Insulationisusedtopreventshort
circuits.
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7. 7
3.TransformerConstruction,Applications,andComponentFunction

8. •Bushings:High-voltageinsulatorsthatallowtheconductorstopassthroughthe
transformer’stankwithoutmakingelectricalcontactwithit.
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9. 3.TransformerConstruction,Applications,andComponentFunction
•CoolingSystem:Transformersgenerateheat.Thedifferentcoolingmethodsare:
•Oil-filled:Theoilinsulatesanddissipatesheat.
•Radiators/Fins:Increasethesurfaceareaforcooling.
•Fans:Usedforforced-aircoolinginlargerunits.
•Tank:Themainenclosurethatcontainsthecoreandwindings.
•ConservatorTank:Asmalltankontopof
largeoil-filledtransformersthatallowsfor
theexpansionandcontractionoftheoildue
totemperaturechanges.
Ithelpspreventmoisturefromentering
themaintank
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10. 4.TypesofTransformersinGridandIndustrialUse
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11. 4.TypesofTransformersinGridandIndustrialUse
Thissectioncategorizestransformersbasedontheirapplication.
•DistributionTransformers:Stepdownvoltageforlocaldistributiontohomesand
businesses(e.g.,from11kVto400V).
•PowerTransformers:Usedintransmissionnetworkstostepuporstepdownhigh
voltages(e.g.,from220kVto66kV).
•InstrumentTransformers:
•CurrentTransformers(CT):Usedtomeasurehighcurrentsbysteppingthem
downtoameasurablelevel.
•PotentialTransformers(PT):Usedtomeasurehighvoltagesbysteppingthem
down.
https://engineersguidebook.com/12-types-of-transformers-their-application/
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12. 5.OverviewofTransformerProtectionElements
Thissectioniscriticalforsafetyandreliability.
•Fuses:Thesimplestformofprotection.Fusesmeltandbreakthecircuitwhena
currentexceedsacertainvalue.Theyaretypicallyusedforsmallertransformers.
•Relays:Moreadvancedandprecise.Relaysdetectabnormalconditions(e.g.,
overcurrent,over-temperature)andsendasignaltoacircuitbreakertoopenthe
circuit.
•DifferentialRelays:Comparescurrententeringtheprimaryandleavingthe
secondary.Adifferenceindicatesaninternalfault.
•BuchholzRelay:Usedinoil-filledtransformers.Itdetectsgasbuildup,which
canindicateaninternalfault.
•SurgeArresters:Protectthetransformerfromhigh-voltagetransientscausedby
lightningstrikesorswitchingsurges.Theydivertthehigh-voltageenergytothe
ground.
•Grounding:Essentialforsafety.Groundingprovidesapathforfaultcurrentstosafely
dissipate,preventingdangerousvoltagesonthetransformer'scasing.
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13. Pre-Assessment Quiz
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14. 1.Atransformerhasaprimarywindingwith500turnsandasecondarywindingwith
2000turns.Ifthevoltageappliedtotheprimaryis120V,whatistheexpectedvoltage
atthesecondary?
A.120V
B.30V
C.480V
D.2400V
2.Whichofthefollowingcomponentsisprimarilyresponsibleforprovidingalow-
reluctancepathforthemagneticfluxinatransformer?
A.Thecoolingfins
B.Thewindings
C.Thebushings
D.Thelaminatedironcore
3.Whatisthemainpurposeofastep-downtransformerinaresidentialpower
distributionsystem?
A.Toincreasecurrentforindustrialmachinery
B.ToconvertACtoDC
C.Todecreasevoltageforsafehouseholduse
D.Toincreasevoltageforlong-distancetransmission 14

15. 1.Atransformerhasaprimarywindingwith500turnsandasecondarywindingwith
2000turns.Ifthevoltageappliedtotheprimaryis120V,whatistheexpectedvoltage
atthesecondary?
A.120V
B.30V
C.480V
D.2400V
2.Whichofthefollowingcomponentsisprimarilyresponsibleforprovidingalow-
reluctancepathforthemagneticfluxinatransformer?
A.Thecoolingfins
B.Thewindings
C.Thebushings
D.Thelaminatedironcore
3.Whatisthemainpurposeofastep-downtransformerinaresidentialpower
distributionsystem?
A.Toincreasecurrentforindustrialmachinery
B.ToconvertACtoDC
C.Todecreasevoltageforsafehouseholduse
D.Toincreasevoltageforlong-distancetransmission 15

16. 4.ABuchholzrelayisaprotectivedevicemostcommonlyassociatedwithwhichtype
oftransformer?
A.Dry-typetransformers
B.Oil-filledtransformers
C.Distributiontransformers
D.Instrumenttransformers
5.Whatistheprimaryfunctionofaconservatortankonalarge,oil-filledtransformer?
A.Tohousethetransformer'smainprotectionrelays
B.Toallowfortheexpansionandcontractionoftheoil
C.Toholdthetransformer'smainwindingsandcore
D.Tofilterandpurifythetransformer'sinsulatingoil
6.Whatisthekeyprinciplethatgovernstheoperationofatransformer?
A.Lenz'sLaw
B.Ohm'sLaw
C.TheLawofConservationofEnergy
D.Electromagneticinduction
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17. 4.ABuchholzrelayisaprotectivedevicemostcommonlyassociatedwithwhichtype
oftransformer?
A.Dry-typetransformers
B.Oil-filledtransformers
C.Distributiontransformers
D.Instrumenttransformers
5.Whatistheprimaryfunctionofaconservatortankonalarge,oil-filledtransformer?
A.Tohousethetransformer'smainprotectionrelays
B.Toallowfortheexpansionandcontractionoftheoil
C.Toholdthetransformer'smainwindingsandcore
D.Tofilterandpurifythetransformer'sinsulatingoil
6.Whatisthekeyprinciplethatgovernstheoperationofatransformer?
A.Lenz'sLaw
B.Ohm'sLaw
C.TheLawofConservationofEnergy
D.Electromagneticinduction
17

18. 7.WhatistheprimarydifferencebetweenaPowerTransformerandaDistribution
Transformer?
A.Powertransformersareusedforresidentialareas,whiledistributiontransformers
areforindustrialuse.
B.Thereisnofunctionaldifferencebetweenthem.
C.Powertransformersaretypicallysmallerinsizeandcapacity.
D.PowertransformersoperateathighervoltagelevelsandhavehigherkVAratings.
8.Whatisthepurposeoflaminatingtheironcoreofatransformer?
A.Toreduceeddycurrentlosses
B.Toincreasethetotalvolumeofthecoreforbettercooling
C.Toprovideapathforthewindingstobesecured
D.Todecreasethemagneticfluxtoavoidsaturation
9.Inatransformer,whatistherelationshipbetweentheprimarycurrent(Ip​)and
secondarycurrent(Is​)forastep-uptransformer?
A.Therelationshipdependsonthetypeofwindingmaterial.
B.Theprimarycurrentisgreaterthanthesecondarycurrent(Ip​>Is​).
C.Theprimarycurrentisequaltothesecondarycurrent(Ip​=Is​).
D.Theprimarycurrentislessthanthesecondarycurrent(Ip​<Is​).
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19. 7.WhatistheprimarydifferencebetweenaPowerTransformerandaDistribution
Transformer?
A.Powertransformersareusedforresidentialareas,whiledistributiontransformers
areforindustrialuse.
B.Thereisnofunctionaldifferencebetweenthem.
C.Powertransformersaretypicallysmallerinsizeandcapacity.
D.PowertransformersoperateathighervoltagelevelsandhavehigherkVAratings.
8.Whatisthepurposeoflaminatingtheironcoreofatransformer?
A.Toreduceeddycurrentlosses
B.Toincreasethetotalvolumeofthecoreforbettercooling
C.Toprovideapathforthewindingstobesecured
D.Todecreasethemagneticfluxtoavoidsaturation
9.Inatransformer,whatistherelationshipbetweentheprimarycurrent(Ip​)and
secondarycurrent(Is​)forastep-uptransformer?
A.Therelationshipdependsonthetypeofwindingmaterial.
B.Theprimarycurrentisgreaterthanthesecondarycurrent(Ip​>Is​).
C.Theprimarycurrentisequaltothesecondarycurrent(Ip​=Is​).
D.Theprimarycurrentislessthanthesecondarycurrent(Ip​<Is​).
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20. 10.Whichofthefollowingisanessentialsafetyprotocolwhenperforming
maintenanceonatransformer?
A.Usingavoltmetertocheckforvoltagewhilethetransformerisenergized.
B.Lockout/Tagout(LOTO)procedures.
C.Usingawetclothtocleanthetransformer'sbushings.
D.Applyingashortcircuittothesecondarywindingstodrainanyresidualcharge.
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21. 6.Practical:ComponentIdentificationandSafetyClearances
Thisisahands-on,visualpartofthepresentation.
•ComponentIdentification:Useclearimagesoraphysicalmodelofatransformer.Ask
theaudiencetoidentifyandexplainthefunctionofthecore,windings,bushings,and
coolingfins.
•SafetyClearances:DiscusstherequireddistancesandPersonalProtectiveEquipment
(PPE).Emphasizethearcflashboundaryandtheimportanceofneverapproaching
energizedequipmentwithoutpropertrainingandgear.
•Lockout/Tagout(LOTO):Brieflyexplaintheprocedureforsafelyde-energizingand
lockingoutatransformerbeforemaintenancework.Thisisacriticalsafetyprotocol.
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22. Transformer Operating Principles &
Measurement Basics
Dr. Wael HOURANI

23. 1.Voltage/CurrentTransformationandPowerRelationships
•VoltageandCurrentTransformation:Atransformerworksbasedontheturnsratio
(Np​/Ns​),whichdictatesthechangeinvoltageandcurrentbetweentheprimary
andsecondarywindings.
•VoltageRatio:Thevoltageratioisdirectlyproportionaltotheturnsratio:
Vs​/Vp​=Ns​/Np
•CurrentRatio:Thecurrentratioisinverselyproportionaltotheturnsratio:
Is​/Ip​=Np​/Ns
•PowerRelationships:Inanidealtransformer,powerisconserved.Thepowerin
theprimarywindingequalsthepowerinthesecondarywinding,neglectinglosses.
•
Pp​=Ps
or
Vp×Ip​=Vs×Is
Thisrelationshipexplainswhyastep-uptransformer(whichincreasesvoltage)must
decreasecurrent,andastep-downtransformer(whichdecreasesvoltage)must
increasecurrent.
2

24. 2.CoreOperation,InsulationSystems,andMagneticBehavior
•CoreOperation:Thecoreismadeoflaminatedsteeltoprovidealow-reluctancepath
forthemagneticflux.Laminationminimizeseddycurrentlosses,whichareunwanted
circulatingcurrentsthatgenerateheat.
•InsulationSystems:Atransformer'sinsulationsystemiscrucialforsafetyand
longevity.
•SolidInsulation:Materialslikepaperandpressboardareusedtoinsulatethe
windingsfromeachotherandfromthecore.
•LiquidInsulation:Inoil-filledtransformers,mineraloilorasyntheticfluid
servestwofunctions:insulationandcooling.Ithelpsdissipatetheheat
generatedbythecoreandwindings.
•MagneticBehavior:Thechangingcurrentintheprimarywindingcreatesachanging
magneticfluxinthecore.Thisfluxlinkswiththesecondarywinding,inducinga
voltage.Thisistheprincipleofelectromagneticinduction.
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25. 3.SafetyMarginsandInsulationCoordination
•InsulationCoordination:Thisistheprocessofcoordinatingtheinsulationstrengthof
electricalequipmentwiththecharacteristicsoftheprotectivedevicestoensurethat
equipmentisnotdamagedbyovervoltages.
•OvervoltageProtection:TransformersaredesignedwithacertainBasicImpulseLevel
(BIL),whichisthemaximumvoltageimpulsetheycanwithstand.
•SurgeArresters:Thesedevicesarecriticalforovervoltageprotection.Theyare
connectedbetweenthehigh-voltagelineandtheground.Whenavoltagesurge
(e.g.,fromlightning)occurs,thearresterprovidesalow-resistancepathforthe
surgetotheground,protectingthetransformer.
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26. 3.SafetyMarginsandInsulationCoordination
•SurgeArresters:Thesedevicesarecriticalforovervoltageprotection.Theyare
connectedbetweenthehigh-voltagelineandtheground.Whenavoltagesurge
(e.g.,fromlightning)occurs,thearresterprovidesalow-resistancepathforthe
surgetotheground,protectingthetransformer.
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27. 4.Practical:Polarity,Ratio,andPhaseVerification
•PolarityTest:Thistestdeterminestherelativeinstantaneousdirectionofvoltagein
theprimaryandsecondarywindings.Thisiscriticalwhenconnectingmultiple
transformersinparallel.
•AdditivePolarity:Thevoltageofthesecondaryaddstothevoltageofthe
primary.
•SubtractivePolarity:Thevoltageofthesecondaryopposesthevoltageofthe
primary.
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28. 4.Practical:Polarity,Ratio,andPhaseVerification
•RatioTest:Thistestverifiestheturnsratioofthetransformertoensureitmatches
thenameplatespecifications.Itinvolvesapplyingaknownvoltagetotheprimaryand
measuringtheresultingsecondaryvoltage.
•PhaseVerification:Forthree-phasetransformers,phaseverificationensuresthe
correctphaserelationshipsforproperoperation.Thistestiscrucialtopreventshort
circuitswhenconnectingtransformerstoapowersystem.
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29. Transformer Design Impact & Loss
Reduction Strategies
Dr. Wael HOURANI

30. 1.FactorsInfluencingPerformance
Thissectionexploresthekeyvariablesthatimpactatransformer'soperational
efficiencyandlongevity.
Cooling:??????Theprimaryfunctionofatransformer'scoolingsystemistodissipatethe
heatgeneratedbyelectricallosses.Differentcoolingmethods(e.g.,oil-immersed,
forced-air,dry-type)havevaryingefficiencies.Thecoolingsystem'sdesignand
maintenancedirectlyinfluencethetransformer'sabilitytooperateunderhighloads
withoutoverheating,whichcandegradeinsulationandshortenitslifespan.
https://globecore.com/publications/cooling-of-transformer/
2

31. 1.FactorsInfluencingPerformance
LoadCycles:??????Atransformer'sefficiencyisnotconstant;itchangeswiththeload.
Thepointofmaximumefficiencyforatransformeroccurswhenitsno-loadlosses
(corelosses)areequaltoitsloadlosses(copperlosses).Thisisacriticaldesign
consideration,asatransformerisoftendesignedtooperatemostefficientlyatits
typicalload,whichmaybe50–70%ofitsfullcapacity.
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32. 1.FactorsInfluencingPerformance
Geometry:??????Thephysicalshapeandarrangementofatransformer'score,windings,
andtanksignificantlyinfluenceitsperformance.Thegeometryaffectsmagneticflux
distribution,whichinturnimpactsstraylosses,anditdictatestheeffectivenessofthe
coolingsystem.Properdesign,oftenoptimizedusingFiniteElementAnalysis(FEA),is
crucialforminimizinglossesandimprovingheatdissipation.
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33. 5
2.TransformerLossComponentsandReductionPractices
Thissectiondetailsthedifferenttypesoflossesthatoccurintransformersandthe
methodsusedtomitigatethem.
•No-LoadLosses(CoreLosses):Theselossesoccurwheneverthetransformeris
energized,regardlessoftheload.Theyarecausedbythealternatingmagneticfield
inthecore.
oHysteresisLoss:Energylostinrepeatedlymagnetizinganddemagnetizingthecore
material.
oEddyCurrentLoss:Heatgeneratedbycircularcurrentsinducedinthecore.
✓ReductionPractices:Toreducecorelosses,engineersusehigh-gradematerialslike
siliconsteeloramorphousmetal,whichhavelowhysteresisproperties.Theyalso
uselaminatedcores(thinsheetsseparatedbyaninsulatinglayer)toincrease
resistanceandminimizeeddycurrents.

34. 6
2.TransformerLossComponentsandReductionPractices
•LoadLosses(CopperLosses):Theselossesaredependentontheloadcurrent.
oI²RLoss:Theprimarycomponent,causedbytheresistanceofthewinding
conductors.Thislossincreaseswiththesquareofthecurrent.
oStrayLosses:Arisefromleakagemagneticfluxthatinduceseddycurrentsinthe
transformer'smetalliccomponents(e.g.,tank,clamps).
✓ReductionPractices:Tominimizeloadlosses,engineerscanincreasethecross-
sectionalareaoftheconductorstoreduceresistance,usehigh-puritycopper,or
optimizewindinggeometry.Propershieldingcanalsoreducestraylosses.

35. 7
3.PracticalAnalysis
Thissectionfocusesonthepracticalskillsofinterpretingdatatoevaluatetransformer
performance.
•AnalyzingLossData:Transformerlossesaretypicallymeasuredthroughtwo
standardtests:
oNo-LoadTest(Open-CircuitTest):Measurestheno-loadlosses.Avoltmeter,
ammeter,andwattmeterareusedtomeasurethepowerconsumedwhenthe
secondarywindingisopen.
oShort-CircuitTest:Measurestheloadlosses.Thesecondaryisshort-circuited,
andareducedvoltageisappliedtotheprimarytoachievetheratedcurrent.
Thepowermeasuredrepresentsthetotalloadlosses.

36. 8
3.PracticalAnalysis
•InterpretingPerformanceCurves:Performancecurves,suchasefficiencyvs.load
curves,areusedtovisualizeatransformer'sperformance.Byplottingthecalculated
efficiencyatdifferentloadlevels,youcanidentifytheloadatwhichthetransformer
operatesatitsmaximumefficiency.Analyzingthesecurveshelpsinmaking
informeddecisionsfortransformerselection,sizing,andloadmanagementto
maximizeenergysavingsandextendtheequipment'slifespan.