Experimental Testingof MicrocontrollerBased Protection for ThreePhase Power Distribution Transformer
By ImranRiaz Dar
A thesissubmi ttedtothe Schoo lof GraduateStudiesinparti al fulfillmentof Masterof Engineering
Facultyof Engineer ingandAppliedScience MemorialUniversityof Newfoundla nd
November, 2011
StJohn ' s, Newfound land, Canada
Abstract
The differential protection techniqueisverypopularfor protecting powertransform ers of variou sratingsand configurationsand isbased on the differencesbetweenthe primary sideandsecondaryside currents.The differencecurrents contain inform ation when adequately processedand provide aclearpictureabout thetransformer operating conditions.Amo ng severalapproaches devel opedto processdifferential currents, harm onic analysis iswidelyemployed for severalutilities,industrial,comm erc ial and resid entialapplication s.
Theextraction of certain harmonic component spresentin the differentialcurrentscanbe criticalin distin guishin gbetweenthemagnetizinginrush currentandany internalfault current. Thediscrete Fourier transform can bethepreferred choice in the analysis that leads to an improvementin the protection ofpowertransformer s.The implementati on ofa digital filterin g approachisusually accomplishedusingmicroprocessorplatform s,which offeraccuracy,speed, reliabi lityandsimplicity for protection of distributiontransform ers.
This thesisimplements,for the firsttime,harmonic analysisofdifferenti al currents for protection ofpowertransform ersinamicrocontroll er. Theanalysis isbased onadiscrete Fourier transformand isrealizedusing ac-codefortestin gthe 3-phase laboratory transforme r. Performance s of the microprocessor digital relay show simple implementation,reliability,speed and accuracy for distribution typetransformers.
[ii]
Acknowledgement
Iam thankfultomy supervisor ProfessorDr. M.A.Rahm anfor hiscontinuous adv iceand assistance duringthe cour se ofgraduatestudies, researchworkand thewriting of this thesis.
I would liketopay special regard sto Dr. Saleh forhishelpwith my experimental research work.
Spec ial thank stomycolleagues and labtechnic ians for their support.
FinancialAssistance, in theformofa Teaching AssistantshipbytheFacult y of EngineeringandApplied Science,isworthmentionin ghere.
Finally, I mustmenti onthe patience ofmy wifewhostood by meforthelastthree years, asI wasdoing,my studies,and the samefeelingsgoformy children.
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Contents
Abstract
Acknow ledgeme nt ListofFigures I. Introdu ction
1.1 General
1.2 Summaryof Sho rtLiterature Review
1.3 Pertinent and BriefLiteratureReview on DigitalProtecti on
1.3.1 Digital Algor ithmsforPower Transformer Protecti on
1.3.2 Discrete FourierTransfor m(OFT)
1.3.3 ApplicationofDiscrete Fourier Transform (OFT)
in PowerDistributi on Transfor mer Protecti on
1.4 Purpose ofThisThes is 1.5 Outlineof The Thesis 2. Transfor mer Protecti on Concepts
2.1 Differenti alProtectionTechn ique 2.2TrippingCharac teristicsofDifferentialRelay
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iii viii
10 11 12 12 13
2.3MagnetizingInrush Current
2.4Over-excitation 2.5 CurrentTransformerSaturation
2.6Three phase Transformer Protection 2.6.1CurrentTransfo rmers
2.6.2HarmonicRestraint s for DifferentialProtection
3. Hard wareDesign ofMicrocont roller-basedProtection of ThreePhase Power Transfo rmer
14 20
21 21
21 23
25
3.1 Introduction 25
3.2 Power Transformer 29
3.3CurrentTransformers 29
3.4 Relay ControlCircuit 30
3.5 SolidState Relay 32
3.6ScalingCircuit 33
3.7FilteringCircuit 38
3.8Microcon troller 45
4.SoftwareDesign of Microco ntroller based Three PhasePower TransformerProtection 47
[v]
4.1 Introduction 47
4.2SoftwareDesign 47
4.2.1OFT MethodforHarmonics Calculations 49
4.2.2Criteria for HarmonicCalculations 51
4.3Protection Scheme 52
5.ExperimentalTestingForDigitalProtection ofPower Transformer 53
5.1 Introduction 53
5.2MagnetizingInrush CurrentTests 55
5.3Internal FaultTests 59
5.3.1Phaseto GroundFault(Primaryside-secon dary unloaded) 59
5.3.2 Phaseto GroundFault(Primaryside-seco ndaryatequalresistive load) 62
5.3.3Phase to GroundFault(Primaryside-secon daryatdifferentresistiveload) 64
5.3.4 Phaseto PhaseFault(Primaryside-secondaryat noload) 66
5.3.5Phase to NeutralFault(Secondaryside-secon daryat equalresistive load) 69
5.3.6 Phaseto NeutralFault(Secondary side-seco ndaryat differentresistiveload)
71
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5.3.7Phase to Phase Fault(Secondary side-seco ndary-open)
5.3.8Phase to PhaseFault(Secondary side-secondary at equalresistive load) 73
76
5.3.9 Phase to PhaseFault(Secondary side-secondary at different resistiveload)
78
5.4 Summary of ExperimentalResults
6.Conclusionsand FutureWorks
6.1Conclusion
6.2FutureWorks
References
Appendices
A Design of ChebyshevFilter for Antialiasing
B MA TLAB Programme forInrush analysis
C Discrete FourierTransform (DFTs) Algorithm
o Real Time Testing
E Programme for Microcontro ller
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80
82
82
83
84
94
95
114
124
128
196
F Diagram sforMicrocont roller-16F877
List of Figures
201
Figure2.1:Biased Transformer DifferentialProtection 13
Figure2.2:DifferentialRelay Tripping Characteristics 14
Figure2.3: InputVoltageand Inrush CurrentwhenSwitchingAng le=0° 15
Figure2.4: Volta ge,flux and currentduringthemagneti zinginrush current. 16
Figure2.5:Deriv ati on of magnetizinginrush curre nt fromthe excitation characteristic 17
Figure2.6:Effectof residualfluxon inrush current,at saturationdensity 18
Figure2.7:Effect ofresidual flux oninrushcurren t,at90%saturationdensity 19
Figure2.8:Magnetizi ngCurveforSteelCore Transformers 20
Figure2.9: Differential protectio nSchemeforThreePhasePower Transfo rmer 22
Figure2.10:Basic circuit forharm on icrestraintrelay 23
Figure3.1:ExperimentalSetUp 26
Figure3.2:System BlockDiagram 27
Figure3.3:Experimenta lSet Up forInternal FaultsTests 28
[viii]
Figure3.4:Relay Contro lCircuit 31
Figure3.5:Equiva lent circu it ofSolidState Relay 33
Figure3.6:QuadOpera tio nalAmplifier 34
Figure3.7:Scalingcircuit 35
Figure3.8:ScalingcircuitwhenLow gain,RA=0 36
Figure3.9:Scalingcircuitwhen med ium gain,RA=2.5KO 37
Figure3.10:Scalingcircuit whenhigh gain,RA=5KO 38
Figure3.11:Chebys hev lowpassfilter specifica tions 40
Figure3.12:Circuit diagram for 6thorde rChebys hev filter (Anti-alias ing filter) 44
Figure3.13:OutputofChebyshev filter 45
Figure 4.1 :FlowChart forDifferenti alRelayProtection 48
Figure 4.2:FlowChart forHarm on ics Calculations 50
Figure5.1:Photograph of Experi mentalset up 54
Figure 5.2:Exper imental inrush current response forphase A 56
Figure 5.3:Experi mentalcurrent harmonicsin phase A 57
Figure5.4: Exper imenta l magnetizing inrush current and response of electro nicswitch
58
fix]
Figure5.5:Experimental phaseto groundfaultand response ofthe electronicswitch 60
Figure5.6:Phase Ato groundfaultat noloadpowertransformer,harmonics and response
of electronicswitch. 61
Figure5.7:PhaseAto ground fault atequalresistive load (6000),powertransform er and
response ofelectronicswitch 62
Figure5.8:PhaseAto groundfaultatequal resistiveload (6000)-powertransformer,
harmonics and response ofelectronicswitch 63
Figure5.9:PhaseAto groundfaultatunequalresistiveload (600/1200/24000),power
transformer andresponseof electronicswitch 64
Figure5.10: PhaseAto groundfaultat unequal resistive load (600/1200/24000),power transformer, harmon ics and response of electronicswitch. 65
Figure5.11: PhaseAto PhaseBfault primary sideandsecondaryopenandresponseof
electronicswitch 67
Figure5.12:PhaseAtoPhaseBfaultprimarysideandsecondaryopen, harmonicsand
responseof electron icswitch 68
Figure5.13:Phasetoneutral faultcurrent(secondaryside) inPhaseAwhensecondary sideofpower transformeratequalresistive load (6000)andresponseof electronic switch.
[xl
69
Figure5.14: Phasetoneutralfault current (secondaryside)in PhaseA whensecondary sideof power transfo rmeratequal resistiveload (600n),harmonics andresponseof
electronicswitch 70
Figure5.15: Phasetoneutral faultcurrent(secondaryside) in Phase A whensecondary sideofpower transformerat unequal resistive load (600/1200/2400n)andresponseof electronicswitch.
71
Figure5.16:Phase toneutralfault current(secondaryside) in Phase C whensecondary side ofpowertransformer at unequal resistive load (600/1200/2400n)andresponseof
electronicswitch. 72
Figure5.17: PhasetoPhase faultcurrent(secondaryside) in Phase A whensecondaryside of powertransformer openand response of electronicswitch.
74
Figure 5.18:PhasetoPhase faultcurrent(secondaryside)in Phase A whensecondaryside ofpowertransformer openandresponseof electronicswitch.
75
Figure5.19: PhasetoPhase faultcurrent(secondaryside)in Phase Cwhen secondaryside ofpowertransformer atequal resistiveload (600n)andresponseof electronicswitch.
76
[xi]
Figure5.20: PhasetoPhasefault current(secondaryside)in Phase C whensecondaryside ofpowertransformer atequal resistiveload (600n)andresponseof electro nicswitch.
77
Figure5.21:PhasetoPhase faultcurrent(secondaryside)inPhase C whensecondaryside of powertransformer atdifferent resistive load (600/1200/2400n)and responseof
electronicswitch. 78
Figure 5.22:PhasetoPhase fault current (secondaryside)in Phase C whensecondaryside ofpowertransformer at differentresistiveload (600/12 00/2400n)and response of electronicswitch.
FigureA.I : Poleslocation forthe 6thorderChebyshev filter
79
101
FigureA.2:Magnitudeand phase responseofthree transfe rfunctio nsseparatelyfor the6th
order 102
FigureA3:Magnitudeand phase responseofthe6thorderChebyshev filter 103
FigureA.4: Filtercircuit 104
FigureAS:Antialiasingfilter-6thorderChebyshevfilter 109
FigureA6:Cascaded transfer functionsfor6thorderChebyshev filter 110
FigureA.7:Locationofpoles forcascadedtransferfunctionsfor 6thorderChebyshev filter
[xii]
III
FigureA.8:Magnitude response of 6thorderChebyshevfilter 112
FigureA.19:Phaseresponse of6thorderChebys hevfilter 113
Figure B.I:Input Voltage and Inrush CurrentwhenSwitchingAngle=00 liS
Figure B.2:InputVoltage and Inrush Curre nt when SwitchingAngle=4So 116
Figure B.3:InputVoltageand Inrush Current when SwitchingAngle=900 117
Figure B.4 :InputVoltageand Inrush Curre ntwhenSwitchingAngle=13So 118
Figure8.S:InputVoltageand Inrush Curre ntwhenSwitchingAngle=1800 119
Figure B.6:InputVoltageand Inrush Curre nt when SwitchingAngle=22So 120
Figure B.7:InputVoltageand Inrush Current when SwitchingAng le=2700 121
Figure B8:InputVoltageand Inrush Current whenSwitchingAngle=31So 122
Figure B9:InputVoltageand Inrush Curre nt when SwitchingAngle=3600 123
Figure 0.1:MATLABModel for Harm on ics Analysis 128
Figure 0.2:PhaseAto ground fault at Primary side,responseofthethreephases of differenti al currentat noloadpowertran sform er and Response ofelectronicswitch
129
Figure0.3 :PhaseB to groundfaultat noloadpowertransformer andresponseof electro nicswitch
[xiii)
130
Figure0.4: PhaseB to groundfaultatnoloadpowertransformer,harmonicsand
response of electron icswitch 131
Figure0.5:Phase Cto groundfaultatnoloadpower transformerandresponseof
electronicswitch 132
Figure0.6:Phase Cto groundfaultatnoloadpowertransformer,harmonics and
responseof electro nicswitch 133
Figure0.7: PhaseAto groundfault-Primaryside,responsesofthethree phasesof differential currentatequal resistiveload(6000),powertransform er andresponseof
electronicswitch. 134
Figure0.8:PhaseB to ground fault atequal resistiveload(6000) powertransformer and
responseof electronicswitch 135
Figure0.9:PhaseB to groundfaultatequalresistive load(6000) power transformer,
harmonics andresponseof electronicswitch. 136
Figure0.10:Phase Cto groundfaultat equal resistiveload(6000)powertransformer
and response of electro nicswitch 137
Figure0.11:Phase Cto groundfaultatequal resistiveload(6000) powertransformer, harmonics andresponseof electronicswitch
[xiv]
138
Figure 0.12:Response of thethreephases ofdifferential currentsat unequal resistive load (600/1200/2400n)powertransformer andresponseof electronicswitch
139
Figure 0.13:PhaseB to groundfaultat unequalresistiveload (600/1200/2400n),power
transformer andresponseof electronicswitch 140
Figure 0.14:PhaseB to groundfaultat unequal resistive load (600/1200/2400n),power transfor mer,harmon ics and response ofelectronicswitch. 141
Figure 0.15: Phase Cto groundfaultat unequalresistiveload (600/1200/2400n),power
transformer and response of electronicswitch 142
Figure 0.16:Phase Cto groundfaultat unequal resistive load (600/1200/2400n),power transformer,harmonics and response of electronicswitch. 143
Figure 0.17:Responses of the threephases of differential currentsforphasetophase fault,secondaryside open,power transformer andresponseof electronicswitch
144
Figure 0.18:Phase AtoPhaseBfaultat primary sideandsecondaryopenand response
of electronicswitch 145
Figure 0.19:Phase AtoPhaseBfaultat primary sideandsecondaryopen,harmonics and responseof electronicswitch.
[xv]
146
Figure D.20:Phase AtoPhaseBfaultatprimary sideandsecondaryopenandresponse
ofelectronicswitch 147
Figure D.21:Phase AtoPhaseBfaultat primaryside andsecondaryopen,harmonics and
response of electronicswitch 148
Figure D.22:Responseof thethreephases of differentialcurrents for inrushmagnetizing inrus hcurrent when powertransform er secondaryside openand response of electron ic
switch 149
Figure D.23:MagnetizingInrush Current in PhaseBwhensecondarysideofpower
transformer openandresponseof electronicswitch ISO
FigureD.24:Magnetizing Inrush CurrentinPhaseBwhensecondarysideof power transform er open,harmonics and response of electronicswitch lSI
Figure D.25:Response ofthethreephases ofdifferential currents forinrushmagnetizing inrushcurrent when powertransformer secondaryside at equal resistiveload (6000)and
response of electronicswitch 152
Figure D.26:MagnetizingInrush Current in PhaseB when seconda rysideof power transformeratequalresistive load (6000)and response of electro nicswitch. 153
Figure D.27: Magnetizi ng Inrush Current in PhaseBwhensecondarysideofpower transformer atequal resistiveload(6000),harmonics and responseofelectronicswitch
154
[xvi]
FigureD.28:Magnetizing Inrush Current inPhase C whensecondarysideofpower transformer atequal resistiveload (600fi)andresponseof electronicswitch. 155
Figure D.29: Magnetizing Inrush Current in Phase C whensecondarysideofpower transformeratequalresistive load (600fi), harmonics and response of electronicswitch.
156
Figure D.30:Resp?n se of thethreephases ofdifferential currentsformagnetizing inrush current when powertransformer secondarysideatvariableresistive load (600/12 00/2400
fi) andresponseofelectro nicswitch 157
Figure D.31:MagnetizingInrush Current in Phase A whensecondarysideof power transformer at variableresistive load (600/1200/2400 fi) and response of electronic
switch. 158
Figure D.32: Magnetizing Inrush Current in Phase A whensecondarysideof power transformer atvariable resistiveload (600/1200/2400 fi),harmonics andresponseof
electronicswitch 159
Figure D.33:Magnetizing Inrush Current in PhaseB when secondarysideof power transformeratvariableresistive load (600/120 0/2400 fi) and response of electronic switch.
[xvii]
160
Figure0.34:MagnetizingInrush Current in PhaseBwhensecondarysideof power transformer atvariable resistiveload (600/I200/24000),harmonicsand response of
electronicswitch 161
Figure 0.35: Magnetizing InrushCurrentin Phase C whensecondarysideof power transform er atvariable resistiveload (600/I200/24000) andresponseof electronic
switch. 162
Figure 0.36:MagnetizingInrush Current in Phase Cwhen secondarysideof power transformerat variableresistiv eload(600/I2 00/24000),harmonic sand response of
electronicswitch 163
Figure 0.37:Response of thethreephases of differentialcurrentsforphaseto neutralfault when powertransformer secondarysideopenand response of electronicswitch
164
Figure 0.38:Phasetoneutral faultcurrent(secondaryside) in Phase A whensecondary sideofpowertransform er openand response of electro nicswitch 165
Figure 0.39:Phasetoneut ral faultcurrent(secondaryside) in Phase A whensecondary sideof powertransform er open, harmonics and response ofelectronicswitch. 166
Figure0.40:Phaseto neutralfault current(secondaryside) in PhaseB whensecondary side of powertransformer open and responseof electronic switch. 167
Figure0.41: Phaseto neutralfaultcurrent (secondaryside) in PhaseBwhensecondary sideof powertransform er open,harmonics and response ofelectronicswitch. 168
[xviii]
Figure 0.42:Phasetoneutralfaultcurrent (secondaryside) in Phase Cwhen secondary sideofpower transfo rmeropenandresponseof electronicswitch 169
Figure 0.43:Phasetoneutral fault current (secondaryside) in Phase C whensecondary sideofpower transfo rmeropen,harmonics and response of electronicswitch. 170
Figure 0.44:Response ofthethreephases ofdifferential currentsfor phasetoneut ral fault when powertransformer secondarysideatequal resistiveload (600n)and response of
electronicswitch. 171
Figure 0.45:Phasetoneutralfaultcurrent(secondaryside) in PhaseB whensecondary sideof powertransformer atequalresistiveload (600n)andresponse of electronic
switch. 172
Figure 0.46:Phasetoneutralfault current(secondaryside) in PhaseBwhensecondary side of power transfor meratequal resistiveload (600n),harmo nicsandresponseof
electronicswitch. 173
Figure 0.47:Phasetoneutral fault current (secondaryside) in PhaseC whensecondary side of power transform er at equalresistive load (600n)andresponseof electronic
switch. 174
Figure 0.48: Phase to neutralfaultcurrent(secondary side) in Phase C whensecondary sideofpowertransformer atequal resistiveload (600n),harmonics and response of electronicswitch.
[xix]
175
Figure0.49:Responseof the three phases of differential currents for phasetoneutral fault when powertransformer secondarysideat unequ alresistiveload (600/12 00/2400Q)and
response ofelectronicswitch S. 176
Figure0.50:Phasetoneutralfaultcurre nt(seco ndaryside) in PhaseB when secondary sideofpowertransform erat un-equ al resistive load (600/12 00/2400fl) and response of
electronicswitchS. 177
Figure0.51:Phasetoneutralfaultcurrent(seco ndaryside) in PhaseB when secondary side ofpowertransformerat un-equ alresistiveload (600/12 00/2400fl) and response of
electronicswitchS. 178
Figure0.52:Phasetoneutralfaultcurrent(second ary side) in Phase Cwhensecondary sideofpowertransform er at un-equ alresistiveload(600 /1200/2400fl) and respon se of
electronicswitchS. 179
Figure0.53:Phaseto neutr alfaultcurrent (seco ndaryside)in Phase C whenseconda ry side of powertransform er at un-equal resistive load(600/1200/2400fl) and response of
electronicswitch. 180
Figure0.54:Responses of thethreephases ofdifferent ialcurrentsforphasetophasefault whenpowertransform er secondary sideopen and respon seofelectronicswitch. 181
Figure0.55:Phaseto Phase fault current(secondaryside) in PhaseBwhensecondary sideofpowertransform eropen and responseofelectronicswitchS.
[xx]
Figure0.56:PhasetoPhase faultcurre nt(seconda ryside)in Phase Bwhensecondary side of powertransform er openandresponseof electro nicswitchS. 183
Figure0.57:PhasetoPhase faultcurre nt(seconda ryside) inPhaseC whensecondary side ofpowertransform er openandresponseof electro nicswitchS. 184
Figure0.58:PhasetoPhase faultcurren t(secondaryside)in Phase C whensecondary sideofpowertransformer openandresponseof electronicswitch. 185
Figure0.59:Responses ofthethree phasesofdifferentialcurrents forphaseto phasefault when powertransformer secondarysideat equal resistiveload (6000.) andresponseof
electro nicswitch. 186
Figure0.60:PhasetoPhase faultcurren t(seconda ryside) inPhase Bwhensecondary sideofpowertran sformer atequal resistiveload (6000.)and response of electronicswitch
S. 187
Figure0.61:Phase toPhase faultcurre nt(secondaryside)in PhaseBwhensecondary sideof powertransform er atequal resistive load(6000.) andresponseof electronicswitch
S. 188
Figure0.62:PhasetoPhase faultcurre nt(secondaryside) in Phase C whensecondary sideof power transforme ratequalresistive load (6000.)andresponseof electro nicswitch S.
[xxi]
189
Figure0.63:PhasetoPhasefault current(seconda ryside)inPhase C whensecondary sideofpower transfor meratequal resistiveload (600.0.) and response of electronic
switch. 190
Figure 0.64:Responses ofthe threephases of differential currents forphasetophase fault when powertransform er seconda ryside different at resisti veload (600/1200/2400.0.)and
responseof electro nicswitch. 191
Figure 0.65:PhasetoPhase faultcurrent(seco nda ryside)in PhaseBwhe nsecondary side ofpowertransform erat differentresistiveload (600/12 00/2400 .0.) and response of
electro nicswitchS. 192
Figure 0.66:PhasetoPhasefault current(secondaryside) inPhaseBwhensecondary side ofpower transfo rme rat differentresistiveload (600/1200/2400.0.) andresponseof
electro nicswitchS. 193
Figure 0.67:PhasetoPhasefaultcurrent (secondaryside) in Phase Cwhenseconda ry sideofpowertransform er at differentresistiveload (600/12 00/2400 .0.) and response of
electro nicswitchS. 194
Figure 0.68: Phaseto Phasefaultcurrent(secondaryside)inPhase C whensecondary sideofpowertransform er at different resistive load (600/1200/2400 0) and response of
electronicswitchS. 195
FigureF.I.I:Pin DiagramforMicrocontroll er-16F877 201 Figure F.I.2:BlockDiagram ofProgramm able Contro ller(PIC 16F877 ) 202
[xxii]
List of Tables
5.1: Inrushes andfaultsatvariousoperatingconditionsand respon ses ofTriac Switch.81
[xxiii]
Chapter 1
Introduction
1.1Gene ral
Digitalprotectionhasbeen anareaof interestforthe lastthreedecades.In the early 1970' stheuse ofthecomput er was proposedfor the protectionofpower systems.
Effortsare beingmadetoincreasethereliabilit y,speed,economics and flexibil ity and to reduce thesize of the protection system.Advancementof the microproc essor,like microcontroll er,madeit possibleto achieve the objectivesofthe proposedprotection of a powerdistributiontransformer.
Theprotection of thepower transformerposesa challenge to protectionengineers,such as protectionfrommagnetizinginrushesin powertransform ersmadeupofiron laminati onsduring over-vo ltageexcitation.Simpleandordinaryelectro-mechanical relayshave failed toadequatelyfulfill these versatile requirements.These requirements includeprotectionthatidentifiesmagnetizin ginrushcurrent,saturation andover- excitation.Magnetizing inrushoccursat theinstance of switchingon thepower transformer, whereasoverexcitation isdueto voltagesurgesat inputor saturationof the ironlaminatedcore of thepowertransformer.Differentialcurrent protection isthe most populartechniquefor theprotection of the transformer. In the idealcase,these differentialcurrents are linear.However,the residu alfluxdensity andsaturationof iron laminationmaketheinrush current non-line ar and non-sinu soidal. The differential protectiontechniqueisusedfor digitalprotectionof thepower transformer.Itrequires
the samplingoftheinputcurrent data and evaluation according tothe algor ithm used.
These inputsample data aredigitallyfiltered by thealgori thmin theprocessor and decla resthefaultor nofaultlike inrush.
1.2 Summary of Short Literature Review
The powertransform eris oneofthemostessenti al componentsofapower system.
Power transform ers are broadl y classified as: (I) Generatin g transform er, (2) Transmi ssiontransform er, (3)Distributi ontransform er,in terms of power andvoltage ratings.The generatingtransformeris connected tothe generator directly and usually locatedjust outside thegenerating building. Thegenerator and thegenera ting transform er are calledtheunit transform er.The generatingtransformer is a step up powertransform erhavin gratin gs of13.8kVIl32kV (Line toLine), etc.Transmissio n transform ers are locatedin transm ission substations. These arestep up transform ers having standardvoltageratingsofI38kV/230kV /345kV/500kV1735kV.Attheload center(substatio n) thetransmi ssiontransform ers arestep downtypeshaving standard voltage ratin gsI32kV/66kV/33kV.The distribut iontransform ers are typicallylow voltage typeshaving standa rdvoltage ratin gs of33kVI12.4 7kV/4. 16kV/575V/208V(L- L) and IIOV (L-N).This thesisdeals withthe protection of distributi onpower transformer s.
There are two types of major over currentabnormalitiesin powertransformers. Thefirst one is thefault currentsand the otherone ismagneti zinginrush currents. The obje ctives ofall types of distributiontransformerprotecti onincludethe artandscienceof protectin g against the faultsin thetransformeritsel f andagainst highmagnetizinginrush
currents duringthe initialswitching.Theartofprotection also lies on the intelligent decisiontotrip acircuitbreaker againstany fault , while restrainingthe tripofthe circuit breaker during theinrush conditions.Thediscriminating features involve isolatingthe faultcurrentbut allowingthe inrush currentsfor thefirst six cycles or hundred milliseconds ofthe sixtyhertz (60Hz) power systemasperIEEE standard.
Many analogand digital circuitsare employedfor protection of power transformers . Thereexist numerous papersand methods in theliteraturefor power transformer protection.The earlierartofpowertransformer protectionis mechanicalor analogtypes overthe yearsuntilthe1980s[1-18].Systematicresearchesare being carriedoutat the MemorialUniversity'sPowerResearch Laboratoryfor developingandimprovingthe digital protection of power transformer, as a part of worldwide researchand developm ent of techniques forprotecting the powertransformers[19-64].Abrief review oftheliterature would highlightthe contributionofthe researches to achieve these goals.
In 1979,MUN undergraduate student,Eric Downtonhad successfu llyimplementedthe weightedleastsquare algorit hmsfor discriminatingbetween inrushand faultcurrents in powertransformers[22]. Gangopandhy,a MUN graduatestudent, did complete digital computer based simulationofthemagnetizing inrushandfaultcurrentsin 1980 [30].
Rahman,Dash,Phadke,Jeyasurya andYallacontributedsignificantlytowards digital protectionof power transforme rsby employingvarious techn iques andmethodsfor performin gharmonic analyses [16, 22, 24,30,33, 38, and40].
Ivi Harmanto,anothe rMUN graduatestudent,was thefirsttouse themicroprocessor based protectionof a power transformer in thelate 1980s[21,23 and 37].In recent years,intelligenthigh speed digital relayshave been introduced;employingartificial
neuralnetwork s,fuzz ylogics and wavelet techniqu esbasedprotection ofpower transform ers.Thesenewerintelligenttechniqu es wereinves tiga ted bymany otherMUN graduatestudentslikeZaman,Hoqu e,Darw ash,So and Salehon differentaspects for usingcomputer relayingof powersystems [47-50 , 62]. The uses of micro controllersare notwellresearch edfor distributi ontypepowertransform ers. The microcontroll erbased digitalrelayisnowconsidered ascheap andinexpensive for stand-alone protect ion of sing le and three phase power distribut ion transform ers. These cost effective microc ontroll errelays are conn ected in parallel with thestandard HRC (high interrupting capacit y) English Electricfuses.
1.3 Pertinentand Brief Literatu reRevie w on DigitalProt ecti on
Earlier protecti on schemesofpowertransform erprotecti on areboth electro-mec hanica l and analog electro nic types.Since the1960sresearchhasmovedtoward s computer relaying. Thecomputer relaying in powertransformerprotection hasbeenprogressin gin stepwith advancesin highspeed computing since the1970s.The digital algorithmsfor powertransformerprotectionhavebeenthe focu s of the resea rchincomputer relayin g.
Rock fell erintrodu ced adetail eddigital scheme forfaultprotect ionin the joint Pacific Gas&Electric(PG&E)and Westinghouse sub-station project[9].Digitalrelays for transformerprotecti on are based onaharmonicrestraintpercentagedifferent ial current principle.The harmonicanalysisiscarriedout fordigitalprocessing of the differential currentsamples.The primarydesi gn obje ctive isto find a fast and accuratealgorithmto quickl ycalculat ethefundamental,secondand higher,(pa rticularly thefifth)harm onic components fromthe currentsamples.Sykesand Morrison atte mpted to extractthe fundamental(151) andsecond harm onic (2nd)comp onents of different ial currentsamp les
