Testing of transformers and reactors.ppt

testing of transformers

1. TESTING OF
TRANSFORMERS
Tests on Bushing CTs
IR , Continuity and polarity, Ratio and Magnetization test
IR measurements
Ratio test
Vector group test
Floating Neutral point measurement
Magnetization current test
Magnetic balance test ( for 3 phase unit )
C & Tan delta measurement of bushings and windings

2. TESTING OF TRANSFORMERS
contd…
Winding resistance measurement
Tests on OLTC and Cooler controls
Alarm and trip tests
Core Isolation test
Testing of oil for DGA & other parameters
Oil filling , oil circulation and air venting

3. TESTS ON BUSHING CTs
Purpose
Thesetestsarecarriedouttoascertainthehealthinessofbushing
currenttransformeratthetimeoferectionandinservice.
Insulationresistancemeasurement
Insulation-resistancetestsisperformedbetweenCTwindingsand
togroundusingaMegger.Testvoltagewillbe500V.theneutral
groundmustberemovedandCTisolatedfromitsburden.Accepted
min.IRvalueis1M-Ohm
PolarityTest
DCKick/FlickTest:ThisisthetriedandtruemethodfortestingCT
polarity.Forthistest,adcbatterypack(6-10V),adcvoltmeteror
ammeter(preferablyanalog),andtestleadsarerequired.This
methodisaquickandeasytestforpolarityThestepsofthedc
kick/flicktestareasfollows:
ConnectthebatterypacktotheprimaryofbushingCTand
momentarydeflectionofpointerofvoltmeter(centreZero)
connectedtosecondaryisnoted.

4. Tests on Bushing CTs contd…
Closelywatchtheneedleoranalogscaleofthevoltmeter.It
shouldjumpinthepositivedirection.Thishappensina
fractionofasecond,sothemetermustbemonitoredvery
closely.Ifthevoltmeterkicksinthepositivedirection,the
polaritymarksarecorrect.Ifitkicksinthenegativedirection,
the polarity marks are incorrect.
Primaryandsecondaryleadsaresaidtohavesamepolarity
whenatagiveninstantthecurrententerstheprimaryleadin
questionandleavesthesecondaryleadinquestioninthe
samedirectionasthoughtwoleadsformedacontinuous
circuit.

5. Tests on Bushing CTs contd…
Ratiotest
TheCTratiotestdeterminestheCTaccuracy,andthe
resultsshouldbecomparedtotheaccuracyclass.The
stepsforaratio-verificationtestareasfollows:
Asuitablevoltage,belowsaturationisappliedtothe
secondary(fullwinding)andtheprimaryvoltageisread
withahighimpedancelowrangevoltmeter.
Connectavoltagesource(variabletransformer)and
voltmeteracrosstheCTsecondary.
ConnectavoltmeteracrosstheCTprimary.

6. Tests on Bushing CTs contd…
ApplyvoltagetotheCTsecondaryand
measurethesecondary(VM1)andprimary
voltages(VM2)simultaneously.Calculate
theratiobetweenthetwovoltages(VM1/
VM2).
ThisvoltageshouldmatchtheCTratio
(primary/secondary).Youcanrepeatthe
procedureaboveforeachtapofamultitap
CT,butIprefertotreattheCTtapsasI
wouldanautotransformer.Iapplyavoltage
acrossthemaximumratiotapandmeasure
allremainingtapstoacommonpoint.

7. Tests on Bushing CTs contd…
Magnetization test
Some of the following conditions can cause CT
saturation:
CT secondary burden greater than rated.
Extremely high current flowing through the
CT (fault current).
Current flowing through CT primaries with
open-circuit secondaries.
DC current flowing through either winding.

8. Tests on Bushing CTs contd…
TheexcitationtestisusedtoprovethattheCTisnot
saturatedandwilloperatewithinspecificationsatthe
ratedburden.Itisimportanttorememberwhen
comparingtestresultstotheburdenratingthatthe
burdenratingisaminimumvalue.TheCTcould
actuallyhaveahigherrating.Thishappensoftenin
transformerbushingapplications.
Asaturationtestisperformedbyapplyingacvoltage
totheCTsecondaryandincreasingthevoltagein
stepsuntiltheCTisinsaturation.Thetestvoltageis
slowlydecreasedtozerotodemagnetizetheCT.
Thetestresultsareplottedonalogarithmicgraph
andevaluatedbasedonthetransitionperiod
betweennormaloperationandsaturation.This
transitioniscalledthekneeofthecurveandis
directlyrelatedtothevoltageburdenratingofthe
CT.

9. IR MEASUREMENTS
Purpose
Thisisthesimplestandmostwidelyusedtestto
checkthesoundnessoftransformerinsulation.
Thistestrevealstheconditionofinsulationi.e
degreeofdrynessofpaperinsulation,presenceof
anyforeigncontaminantsinoilandalsoanygross
defectinsidethetransformer.
TheinstrumentusedisMegohmMeterpopularly
knownasMegger.IRvaluemeasurementsofEHV
transformerstobedonewith5KVMegger.

10. IR MEASUREMENTS contd….
Safety instructions
Thetestspecimenisdischargedbyshort
circuitingforaperiodatleastfourtimesas
longasthetestvoltagewasapplied.Before
barehandcontact,theabsenceofvoltage
shallbeconfirmedbymeasurement.
Precautions
Cleanthebushingporcelainsbywipingwitha
pieceofdrycloth.

11. IR MEASUREMENTS contd…
Leadwiresfromthebushinglineleadandtankearth
tomeggershallbeasshortaspossiblewithoutjoints
andshallnottouchtankoreachother.
Testingprocedure
IRmeasurementsshallbetakenbetweenthe
windingscollectivelyi.ewithallthewindingsbeing
connectedtogetherandtheearthedtankand
betweeneachwindingandthetank,therestofthe
windingsbeingearthed.Followingmeasurementsare
relevantforauto-transformersandReactors

12. IR MEASUREMENTS contd…
ForAutotransformer ForReactor
HV+IV/E HV/E
LV/E
HV+IV/LV
HV-HighVoltage,IV-IntermediateVoltage,LV–Low
Voltage,E-Earth
Recorddateandtimeofmeasurementsl.no&makeof
megger,oiltemperatureandIRvaluesatintervalsof15
seconds,1minuteand10minutes

13. IR MEASUREMENTS contd…
Evaluation of test results
Inair,IRvalueswillbenearly15-20timesmorethan
intransformeroilatthesametemperature.
Withevery10°Cdropintemperature,IRvalues
increasesbyapproximately1.5timesi.e.anIRvalue
of200M-ohmsatanoiltemp.of40°Cisequivalentto
300M-ohmsat30°C.
FollowingIRvaluesasathumbrulemaybe
consideredastheminimumsatisfactoryvaluesat
30°C(oneminutemeasurements)atthetimeof
commissioning

14. IR MEASUREMENTS contd…
Rated Voltage class Minumum desired
of Winding IR value at 1 min.
11 KV 300 M-ohms
33 KV 400 M-ohms
66 KV & above 500 M-ohms

15. IR MEASUREMENTS contd…
Polarisation Index
It is the ratio of the 10 min to 1 min readings.
Polarisation Index Insulation condition
Less than 1 Dangerous
1.0 –1.1 Poor
1.1 –1.25 Questionable
1.25 –2.0 Fair
Above 2.0 Good

16. RATIO TEST
Purpose
Todeterminetheturnsratiooftransformers.
Turnsratioistheratioofthenumberofturnsina
higherwindingtothatinalowerwinding.
PRECAUTIONS
ThevoltageshouldbeappliedonlyintheHigh
voltagewindinginordertoavoidunsafevoltage.

17. Ratio test contd…
Testingprocedures
KeepthetappositioninthelowestpositionandIVandLV
terminalsopen.
Apply3phase415VsupplyonHVterminals
Measurevoltagesappliedoneachphase(Phase-Phase)onHV
andIVterminalssimultaneously
Repeatabovestepsforeachofthetappositionseparately
RepeatabovestepsafterinterchangingthevoltmetersofHVand
IVwindingsandthenaveragethereadingsforfinalcalculationof
ratio.
TheabovetestcanalsobeperformedbyTransformerturns
rationmeter(TTR)
Twowindingsononephaseofatransformerareconnectedto
theinstrumentandintheinternalbridgeelementsarevariedto
produceanullindicationonthedetector

18. Ratio test contd…
Evaluation of results
Theturnsratiotoleranceshouldbewithin0.5%
Outoftoleranceratiomaybesymptomaticofshorted
turnsespeciallyifthereisanassociatedhigh
excitationcurrent.
Ratiomeasurementsmustbemadeonalltapsto
confirmtheproperalignmentandoperationofthetap
changers.
Openturnsintheexcitedwindingwillbeindicated
withverylowexcitingcurrentandnooutputvoltage

19. VECTOR GROUP TEST
Purpose
To determine the phase relationship and polarity of transformers.
Testing procedures
Connect Neutral point and LV phase with earth.
Join 1R1 of HV and 3R1 of tertiary.
Apply 415 V 3 phase supply to HV terminals
Measure voltage across the following terminals
1R1 –1Y1 3Y1 –1B1 3Y1 -N 2Y1 -N
1Y1 -1 B1 3Y1 –1Y1 1R1 –N 2B1 -N
1B1 –1 R1 3R1 –N 2R1 -N

20. Vector group test contd…
Evaluation of results
Ensure 2R1 –N = 2Y1 –N = 2B1 –N
Ensure 3R1 –N > 3Y1 –N > 3B1 –N
 3Y1 –1B1 >3Y1 –1Y1
The above confirms vector group Y naod 11

21. FLOATING NEUTRAL POINT
MEASUREMENT
Purpose
Thistestisconductedtoascertainpossibilityofshortcircuitina
winding.
Precaution
Nowindingterminalshouldbegrounded.
Testingprocedure
Keepthetapinnormalposition
Disconnectthetransformerneutralfromground.
Apply3phase415VintheHVwindingandmeasurevoltage
acrosseachoftheIVwindingandneutral,thenneutraland
ground.
Apply3phase415VtoIVwindingandmeasurevoltageacross
eachofthetertiarywindingterminalandneutral,thenneutral
andground.
Evaluationoftestresults
Forahealthytransformerthevoltagebetweenneutraland
groundshouldbezeroornegligible.

22. MAGNETISING CURRENT
TEST
Purpose
Thistestisperformedtolocatedefectinmagnetic
corestructure,shiftingofwindings,failuresinturnto
turninsulationorproblemsintapchangers.
Precautions
ThistestshouldbedonebeforeDCmeasurementsof
windingresistancetoreducetheeffectofresidual
magnetism.Thetransformermaybedemagnetised
beforecommencementofmagnetisingcurrenttest.

23. MAGNETISING CURRENT TEST
contd
Testing Procedure
KeepthetappositioninthelowestpositionandIVandLV
terminalsopen.
Apply3phase415VsupplyonHVterminals.
Measurethevoltagesappliedoneachphase(phase–phase)
onHVterminalsandcurrentineachphaseofHVterminal.
Aftercompletionoftheabovesteps,repeatthemeasurements
intappositionNormalaswellasinHighestposition.
KeepthetappositioninnormalandkeepHVandLVterminals
open.
Apply3phase415VsupplyonIVterminals
MeasurephasetophasevoltagebetweentheIVterminalsand
currentoneachoftheIVterminals.

24. MAGNETISING CURRENT TEST
contd…
Evaluation of test results
Thesetofreadingsforcurrentmeasurementsineach
ofthetappositionshouldbeequal.Unequalcurrents
shallindicatepossibleshortcircuitsinwinding.
Resultsbetweensimilarsingle-phaseunitsshouldnot
varymorethan10%.Thetestvaluesontheoutside
legsshouldbewithin15%ofeachotherandvalues
forthecentrelegshouldnotbemorethaneither
outsidefora3phasetransformer.
Ifanoutoftolerancereadingisexperiencedwhile
turnsratio,windingresistanceandimpedancetests
arenormal,residualmagnetismshouldbe
suspected.

25. MAGNETIC BALANCE TEST
Purpose
Thistestisconductedtochecktheimbalance
inthemagneticcircuitonlyin3phaseX’mers.
Precautions
Nowindingterminalshouldbegrounded,
otherwiseresultswouldbeerraticand
confusing.

26. MAGNETIC BALANCE TEST
contd…
Testing Procedures
Keepthetapinnormaltapposition
DisconnecttheX’merneutralfromground
Applysinglephase230Vacrossonephase
ofIVwindingterminalandneutral(sayV1)
thenmeasurevoltageinothertwoIV
terminalsacrossneutral(sayV2&V3).
Repeatthetestforeachofthethreephases.
RepeattheabovetestforHVwindingalso.

27. MAGNETIC BALANCE TEST
contd…
Evaluation of results
Ensure that V1 = V2 + V3
Zero voltage or very negligile voltage induced
in other two windings should be investigated.

28. C AND TAN DELTA
MEASUREMENT( BUSHING)
Purpose
Insulationpowerfactorordissipationfactor(Tan
delta)andcapacitancemeasurementofbushing
provideanindicationofthequalityandsoundnessof
theinsulationinthebushing.
Precautions
Measurementmaybemadeatlowvoltagesusually
below10KV.
Connectiontooverheadbusatthebushingneedto
beremovedonlyifthebusaffectthereadings
appreciably.
Porcelainofthebushingsshallbecleananddry.

29. C AND TAN DELTA MEASUREMENT(
BUSHING)contd…
Testsshallnotbecarriedoutwhenthereiscondensationonthe
porcelainandwhentheRHisinexcessof80%.
Terminalsofthebushingsofeachwindingshallbeshortedtogether
usingbarebraidedcopperjumper.Thesejumpershallnotbeallowedto
sag.Transformerwindingsnottestedshallbegrounded.
Measureandrecordambienttemperatureandrelativehumidityfor
reference.
Ensurethatthetestspecimenisisolatedfromotherequipments.
Connectthegroundterminalofthetestsettoalowimpedanceearth
ground(toearthmatofthesub-station)

30. TypeTests
Routine Tests
Special Tests
Tan Delta & Capacitor Measurement

31. TypeTests
Routine Tests
Special Tests
Tan Delta & Capacitor Measurement
Mains & Commutating Mode

32. Routine Tests
Special Tests
Tan Delta Testing
For Null Condition
N
1
I
1
= N
2
I
2
For Capacitive Current
N
1
ICs = N2 ICx
N
1
VCs = N
2
VCx
Cx = (N1/N2)/Cs

33. TypeTests
Routine Tests
Special Tests
Tan Delta Measurement of Bushings

34. TypeTests
Routine Tests
Special Tests
Tan Delta Measurement of Bushings
For C
1
: Apply 10kV between HV and test tap on UST mode.
For C
2
: Apply 500 kV between Test tap and Ground with
HV Guarded on GSTg mode

35. TypeTests
Routine Tests
Special Tests
UST Mode
Earth will be connected to N Point
Hence only C
HL
will be measured

36. TypeTests
Routine Tests
Special Tests
GST Mode
Here measurement is made for the
Parallel combination of C
H
and C
HL

37. TypeTests
Routine Tests
Special Tests
GSTg mode
Here measurement is done for C
H
as L is guarded

38. C & Tan measurement contd…
Windings
ForICTs(2WindingTransformers),followingcombinationshall
betaken:
HV+IV&LVinUSTmode,bothMainsandreverseandaverage
tobetakentonullifyexternalinterference
HV+IV&Tank(LVtoGndtoGuard)inGSTgmode
LV&Tank(LVtoGndandHVtoGuard)inGSTgmode
-ForReactors,themeasurement shouldbedonein
HV/Tank+GndinGSTmode
-For3windingtransformer,measurementtobemadebetween
eachwindingandGndwithoneremainingwindingguardedand
thesecondremaoningwindinggroundedinGSTgtestmode

39. C & Tan measurement
contd…
Windings contd.
Thetestisperformedat10KV
Max.valueofTanofclassApaperinsulation,oil
impregnatedat20°Cshouldbe0.007.
RateofchangeofTanandcapacitanceshouldbe
marginal.Tanshouldnotvarymorethan0.001
frompreviousvalue.Capacitancevalueshouldbe
+10%,-5%
Tanmeasurementistemperaturedependentand
correctionfactorshouldbeapplied,if
measurementistakenattemperatureotherthan20
°C.

40. C & Tan measurement contd…
For ICTs :
Tan delta and Capacitance measurement of windings
should be done in combination of
HV+IV / LV+TANK+E
HV+IV+LV / TANK+E
LV/HV+IV+TANK+E IN GST TEST MODE
For Reactors
HV / TANK+E IN GST TEST MODE

41. WINDING RESISTANCE
MEASUREMENT
Purpose
Transformerwindingresistancearemeasuredinthefieldto
checkforanyabnormalitiesduetolooseconnections,broken
strandsandhighcontactresistanceintapchangers.
Precautions
Asthetransformerresistanceisoflowresistance,the
measurementhastobecarriedoutwiththehelpofKelvin
DoubleBridge/Transformerohmmeter
Toreducethehighinductiveeffectitisadvisabletousea
sufficientlyhighcurrenttosaturatethecore.Thiswillreducethe
timerequiredtogetastabilisedreading.
Itisessentialthatthetemperatureofthewindingsisaccurately
measured.

42. WINDING RESISTANCE
MEASUREMENT contd…
Testing Procedure
Forstarconnectedauto-transformerstheresistance
oftheHVsideismeasuredbetweenHVterminaland
IVterminal,thenbetweenIVterminalandneutral
Fordeltaconnectedwindings,suchastertiary
windingofautotransformers,measurementshallbe
donebetweenpairsoflineterminalsandresistanceof
windingshallbecalculatedasperthefollowing
formula
Resistanceperwinding=1.5xMeasuredvalue
Calculatetheresistanceat75°Casperthefollowing
formula
R75=Rt(235+75)/(235+t)WhereRtisthe
resistancemeasuredatwindingtemperature

43. WINDING RESISTANCE
MEASUREMENT contd…
Evaluation of test results
Expecteddeviationforthistestatsiteis5.0%
ofthefactorytestvalue.
Theresistancevalueobtainedshouldbe
comparedwiththefactorytestvalueincase
ofpre-commissioningandwithpre-
commissioningtestvalueincasethetestis
carriedoutduringroutinemaintenance.

44. OPERATIONAL TEST ON
OLTC
Purpose
OLTCaredesignedtobeoperatedwhilethetransformeris
energised.
OLTCbeingacurrentinterruptingdevicerequiresperiodic
inspectionandmaintenance.
Thefrequencyofinspectionisbasedontimeinservicemrange
ofuseandnumberofoperations.
Precautions
ThistestshallbecarriedoutduringshutdownperiodunderSFT
EnsureisolationofX’merforHVandLVsidewithphysical
openingofconcernedisolators.
Iftertiaryisconnected,ensureisolationofthesame

45. TAP CHANGER HAND
OPERATION
Checkhandoperationofthetapchanger
upanddownthefullrangebefore
electricaloperation.
Checkthecontinuityduringtapchanging
operationbyconnectingananalogue
multimeteracrossHV&IVbushingand
changethetappositionsfrommaximumto
minimum.

46. TAP CHANGER HAND
OPERATION contd…
CheckthatthetapchangermovesonetapatatimebypushingRor
Lpushbutton.
Checkaccurateindicationofmechanicalpositionindicatorinall
positions.Alsochecktheremoteinstrumentoperatesinthesame
sequencetothesameaccuracy.
Checktheoperationoftapchangerincompletealarmincludingthe
flagrelaybywindingtheunitbyhandhalfwaythroughatap
change.
Checkthetapchangercounteroperatescorrectlyandrecordthe
readings.
Checktheremoteindicationandcontrolfacilityisprovedtothe
outgoingterminalsofthemarshallingkiosk.
Checktheoutofsteprelayoperationbymovingonetapchangerin
thethreephasebanktoonepositionoutofstepwithothertwo.The
tapchangerfaultyalarmshouldcome.Repeatforothertwophases.

47. Operational checks on Cooler
Cooler Control Switch
Auto control from WTI
Time delay for pump
Pump failure alarm
Auto start of standby pump
Stand pump indication
Cooler fail alarm
Fuse rating correct
Differential pressure alarm

48. VIBRATION MEASUREMENT ON
SHUNT REACTOR
Purpose
Tomeasurethevibrationsofcore/coilassemblyinthetankof
thereactor.Movementofcor-coilassemblyandshielding
structurecausedbythetimevaryingmagneticforcesresultsin
vibrationofthetankandancilliaryequipment
Testingmethod
Minimumno.ofreadingsrequiredshallbe48.
EachofthefoursidesoftheSRtankshallbedividedinto12
rectangularareasofapproximatelythesamesize.Theseareas
aresuitablymarkedandnumberedforreference.Vibration
readingsaremeasuredatthesepointsandrecorded.
Evaluationoftestresults
Theaverageamplitudeofalllocalmax.pointsshallnotexceed
60µm.Themax.amplitudewithinanyrectangularareashallnot
exceed200µm.

49. Protection & Alarm checks
Check for correct operation of protection by sorting contacts at MB / relay and
appearance of alarm at control panel in C.R.:
Buchholz relay/ OSR of Main tank / OLTC
WTI/ OTI
PRD
Low oil level in conservator
Tap change incomplete
Cooler fan/ pump fail

50. CORE ISOLATION TEST
Purpose
Thistestisrecommendedbeforetheunitisplacedinserviceor
followingmodificationstothetransformerthatcouldaffecttheintegrity
ofitscoreinsulationandatothertimesduringamajorinspection.
Testingprocedure
ThegroundconnectionterminalsfortheX’merorreactoractivepartare
locatedinabox.
Theterminalboxcontainsaterminalblockwiththreeterminals.
CL–connectedtocorelaminations
CC–connectedtocoreclamps
G–connectedtoground(thetank)
Forcoreinsulationgroundtest,disconnecttheclosinglinkthat
connectsthetwoterminalsCL-G.Apply2.5KVdirectvoltagebetween
CLandCC+G.Thetankshallbegroundedduringthetest.
Theinsulationvalueafter1minutetesttimeshallbeminimum100M-
ohmsforservicetransformersandmorethan1000M-ohmsfornew
transformers.

51. DGA & Oil parameters tests
OIL SAMPLE TO BE TAKEN FOR DGA AS PER
FOLLOWING FREQUENCY
BEFORE COMMISSIONING
24 HOURS AFTER 1ST CHARGING
15 DAYS AFTER 1ST CHARGING
ONE MONTH AFTER 1ST CHARGING
3 MONTHS AFTER 1ST CHARGING
THEREAFTER EVERY SIX MONTHS

52. Oil Parameters before energisation
Prior to energisation of transformer, the oil sample
shall be tested for following properties & acceptance
norms:
1.BDV (kV rms)-60 kV (Min.)
2.Moisture content -15 ppm (Max.)
3.Tan delta at 90 ºC-0.05 (Max.)
4.Resisitivity at 90 ºC-1*10-cm (Min.)
5.Interfacial Tension-0.03 N/m (Min.)