Experimental testing of microcontroller based protection for three phase power distribution transformer

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Experimental Testing

of Microcontroller

Based Protection for

Three

Phase Power Distribution Transformer

By

ImranRiaz Dar

A thesissubmi ttedtothe Schoo lof GraduateStudiesinparti al fulfillmentof Masterof Engineering

Facultyof Engineer ingandAppliedScience MemorialUniversityof Newfoundla nd

November, 2011

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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.

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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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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

[vii] 80 82 82 83 84 94 95 114 124 128 196

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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

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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 6th_{orde r}_{Chebys 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

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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

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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

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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

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 6th

orderChebyshev filter

79

101

FigureA.2:Magnitudeand phase responseofthree transfe rfunctio nsseparatelyfor the6th

order 102

FigureA3:Magnitudeand phase responseofthe6th

orderChebyshev filter 103

FigureA.4: Filtercircuit 104

FigureAS:Antialiasingfilter-6th

orderChebyshevfilter 109

FigureA6:Cascaded transfer functionsfor6th

orderChebyshev filter 110

FigureA.7:Locationofpoles forcascadedtransferfunctionsfor 6th orderChebyshev filter

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FigureA.8:Magnitude response of 6th

orderChebyshevfilter 112

FigureA.19:Phaseresponse of6th_{orderChebys hev}_filter ₁₁₃

Figure B.I:Input Voltage and Inrush CurrentwhenSwitchingAngle=00 liS

Figure B.2:InputVoltage and Inrush Curre nt when SwitchingAngle

=4S

o 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

=31S

o 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

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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

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

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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.

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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

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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

Figure D.33:Magnetizing Inrush Current in PhaseB when secondarysideof power transformeratvariableresistive load (600/120 0/2400 fi) and response of electronic switch.

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Figure0.34:MagnetizingInrush Current in PhaseBwhensecondarysideof power transformer atvariable resistiveload (600/I200/24000),harmonicsand response of

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

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

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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

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

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.

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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

Figure0.52:Phasetoneutralfaultcurrent(second ary side) in Phase Cwhensecondary sideofpowertransform er at un-equ alresistiveload(600 /1200/2400fl) and respon se of

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

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.

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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.

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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

Figure 0.67:PhasetoPhasefaultcurrent (secondaryside) in Phase Cwhenseconda ry sideofpowertransform er at differentresistiveload (600/12 00/2400 .0.) and response of

Figure 0.68: Phaseto Phasefaultcurrent(secondaryside)inPhase C whensecondary sideofpowertransform er at different resistive load (600/1200/2400 0) and response of

FigureF.I.I:Pin DiagramforMicrocontroll er-16F877 201

Figure F.I.2:BlockDiagram ofProgramm able Contro ller(PIC 16F877 ) 202

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List of Tables

5.1: Inrushes andfaultsatvariousoperatingconditionsand respon ses ofTriac Switch.81

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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 and over-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

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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

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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

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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

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usin gtworecu rsivefilters[14].The ratio of 2nd

/ISIharm onicshasbeen obtained to distingui shbetweenthemagneti zinginrush andfault currents ofa pow ertransform er usingdigital filters[12,14]. Malik proposed thecross-correl ationalgorithm which involvescomputationof odd and evenfunctions of any harmonicsfrom the differential currentsamples [16]. The ratioof 2nd_/I_st

harmonicshasbeen usedasa thresholdvalueto declaretheinrushconditi on,if it ismorethan 17%.Schwe itze r usedthefiniteimpulse response (FIR) filtertoestim atetheharm oniccomp onents of thedifferenti alcurrent ofa powertransformerprotection to declarethe fault,if the ratioof 2nd

/Istharmonic

magnitudes is lessthan 17%[17]. Degens usedthe leastsquarecurve-fittingtechnique to find the ratioof 2nd

/Istharmoniccomponents for restraining an inrush trip in a power transformer[20].Rahmanextendedit toinclude theweightedleast square algorithm for betterrestraint oftheinrushtrip inanover-excited powerdistribut iontransform er[22]. Ramamoort yintrodu cedtheFourier transformtechniqu eto extract theharmonic comp onent s of currentfrom one cycle of inrush or faultcurrentsamples [II]. In these earlier digital protections of power transformers up to 16 currentsamples per cyclewere usedfor simulationsand field tests.

The rectangul artransformalgor ithminvolving computationof Fouriersineand cosine coefficients hasbeenusedbyRahman and Dash forfastbut reliableprotecti on ofpower transformer s[24].Theresponse of thealgorithmwasreasonabl y fast becau sethe calculation of the harmonic coefficientsup to the

s"

requires onlya fewadditions and subtractions.Itwasboth simulated and experim entall ytestedusingan Intel8085 microprocessor.Theseprotection schemes for powertransformer swereimplemented usin g12current samples per cycle[24] . Thorpand Phadkeutilized adiscr ete Fourier

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transform (OFT)algor ithm formicrop rocessorbasedprotect ion of powertransform ers using also12 current samples per cycle [23].

Many other digital algor ithms havebeenused for powertransform erprotection.These includetheKalm anfilteringtechniqu e[33, 38],spectralobserver technique[31J,Walsh functions [29],I-Iaar function[27],artificial neuralnetworks [51],waveletpacket transforms[55,57], waveletfilter banks[58], amulti-resolut ion signal decomp osit ion techn iqu e [53]and electro magnetic transientmodels[44],etc.

In both simulation andexperimental testin g ofmodemdigitalprotecti on ofpower transform ersfollowin gthe IEEE standa rdofsix cyclesofthe 60Hzin NorthAmerica. The 16curren t samplespercycle whichcover 100 msfor6 cycles,areusedbecause of theavail abilit y ofhigh speedand largememorycapabil ities ofmodern computers.

A compa rativeanalysison variousalgorithmsfordifferential protectio nofthreephase powertransform ershasbeenmadeby HabibandMartin [32]. Performance indiceshave beencalcul ated on thebasis of computin gtime andsampling frequ enc yusing16 samplesper cycle.Rahm an and Jeyasur yahave systematica llycarriedoutanother compara tive studyof six algorithms,namelydiscret e Fourier transform ,rectan gular transform,Walshfunctions,Haarfunct ions,finiteimpulse responseand least square curvefittingtechniquesforthedigitalprotectio nof powertransform ersusingthe same mainfram ecomput er. The common sampling frequenc ywas 960 Hz and 16 current samples percyclewereused.Ithasbeen esta blishedin bothindepend ent studies that the Discrete Fourier transform isthemost efficientalgori thminterm s ofspeed,accuracy

(34)

and reliabilit y for theharm onicrestra int differential protection of powertransform ers. A brief analysisofthepopul arOFTtechn ique ispresentedinthenext sectio n.

1.3.1 Digital Algorithms for Power Transformer Protection

Forpowertransform erprotection,thereexist manydigital relayalgorithms.Some of the algorithmsarediscussed intheaboveparagraph.Mostoftheseare basedon the harmonic restraint.Genera lly these are similar to eachother. Butthe only difference is themethod of calculating theharmonic compone nts.Ithasbeen found that among these algori thms thediscrete Fourie r transform (OFT)isthe bestonein term s of speed, accuracy,memory requirementsandreliability [II,32 and33].Abrief descriptionof this techniqueisdiscussed inthesection below.

1.3.2 Discrete Fourier Transform (DFT)

Ramamoorth y wasthe first researcher who proposedthatthe fundamenta lcomponentof a curren twave can be extracted formthefaulttransient bycorrelatingthe storedsamples of sineandcosinewaves[II].Fourierseries were usedtodevelopthistheory.Fourier seriestheo rystates that any period ic funct ionf(t) having afinitenumber of discontin uitiesintheinterva lof (O,T)can bemodeled in the interva las:

J(t)=~+Lk=l (Ck cos(k wt )

+

Sk sin (k w t ) ) ........(1.1) where

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s,=

~J

;

f(t)sin( kw t) dt

Ck=~J

;

f(t)cos(kwt)dt... .... ... .(1.2) aoisthe de componentor average valueandSkandCk arethe sineandcosinefunctions

ofthe Fouriercoefficients,respec tive ly. Ifthe wave is sampledat equa ltimeintervals

andtimespace!J.T thenthereis a totalofN/!J.Tsamplesper cycle, the Sk and Ck can be

representedas:

Sk

=

;2:~:-J

x(n)sin

c:,n

).

.

C,=;2:~:-Jx(n)cos(2rr:n) wherex(n)isthe samp ledwaveformat the nth instant.

Amatrix presentatio nof theaboveequations is as follows:

S=CsX . C=CcX .. ... .(1.3) ...(1.4) ..(1.5) .. . ..(1.6)

whereS,C arethe vectors containingsineand cosine componentsof thesampled

waveforms,respectively.C,andC, are system matrices containingsinean~cosine

coefficients,respectively. X is thevectorcontai ning the sampledwaveformto be

analyzed.The dimension ofthe systemmatrices C,andCcareNxNwhereasvectorsC,S

andX are ofthelength N.Forthe protection ofthe powertransform er a data windowof

16 samp le per cycles(N=16)is selectedto get theall therequiredharmonics[37]. The

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[ COS cxn1:1X1) C e= COS cxn1:1X2)

(

2 X

~

X

1 X

16 )

COS -16' cosCxn1: 2X1) . cosCxn1: 2X2) . coscXn::X16)...

:::

i:

::~i:::l

]

(1.7) COSCxn~~6X16) [ s in cxn1:1X1) sincxn1:2X1)... ... sin

c

xn

:~6X1) ]

C

s= sin cxn1:1X2) sin

C

xn1

:2

X

2)

...

sincxn:~6X2)

...

(1.8) sin

C

X~

:~

X1

6)

sin

c

xn

::

x1

6) ..

.

..

.

sin

C

X;~~6X1

6)

From the systemmatricesC,and Ce,the amplitudeof thenthharmonic can be computed

Fk=~(S~

+

Cli)

whereFjisthe kthharmonicFouriercoefficientand k=1,2,3,...,N. (1.9)

1.3.3 Applicationof Discrete Fourier Transform (OFT) in Power Distributi onTransformerProtection

The OFTcoefficient s are of frequ ency selec tive filters and these coefficientsareused to extract thefrequencycom ponentsfromtime domain signa ls. In thepowerdistribution transformerprotection,threesuchfiltersare usedto extractthe fundamental,secondand fifth harmonic components.From the equation(1.9), these harmonicscan be calcul ated . Theseare FI, F2and

r

,

respectively.

In the case ofmagn etizin ginrush current theratio of F2/FIis greater than17.7%and it is lessthan17.7%for intern alfaults.For themagnetizing inrushcurre nt themagnitud eis quite high forthe first few cycles buttheratioF2/F)remains higher thanthe threshold levelof 17.7%and thustheprotection relay wouldrestrain from operation for magnet izinginrushconditions.For theinternalfaultsthe ratioF2/FIis less than the

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thresholdlevel of 17.7%and thus operates theprotectionrelay.In the overexcitatio n

andsaturat ioncasesthe ratioF2/F1mayfallbelowthethresholdlevel of17.7% and there exists no fault,thentheratio Fs/F1iscalcul ated anditsthresho ldvalue is 6.5%.If

the ratiois abovethanthislevel,thenthereisnofaultcondi tion anda restraintsignalis given totherelayfornottripping.

1.4 PurposeofThisThesis

There are several techniquesfor reali zingthe microp rocessorbaseddigitalprotection of

thepowertransform er. Each techniquehas offered someoperationaladvantages like speed,accuracy,reliabilityand size.The performances of somedifferentialprotection

techniqu eswereinvestigated off-line whileotherswereexperimentall y verified.Despit e

testingdifferenttransform ers,whichcausedsom eperform an ce variations, perform ance comparison between various protecti onstechniqu es are reported in the literatur e. The purpose ofthisthesis istorealizethemicrocontroll erbaseddigitalprotection system for alaborator y power transformerto achieve reliable,accurate,flexibl e andcost effective protect ion which iscomp act in size.The developeddigitalprotecti ve relayis basedon harm onic analysis,which isreali zed employing thediscrete Fourier transform (OFT).Thesecurrent signalsare passedthrough ananti-aliasing filterbeforebein g

digiti zedby the analog to digitalconverters (AOC)on theinput side of the

microprocessor.The dev elopedOFT-basedharmonic analysisisdesign edto extract the

1st,2nd ,and 5th

harmonics,presentin thedifferenti alcurrents.Thedistinctionbetween. faultand magnetizinginrushcurrentsis set based on theratios of2nd

to1stharm onics,as wellasthe 5th_t_o_I

stones .

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Thehard war epartconsists of acurrentto voltageisolatoras scalingcircuits to acquire thecurrent signalfrom currenttransform erson theprimar y andseco ndarysides of the testedpowertransform erin theMUNpowerresearchlaboratory.Anti-a liasingfiltersare usedto eliminate theunwanted signals.Certai n thresholdlevels are predefin ed according tothe transform erparamet ers.Theselevelsare definedin the softwa re.These param eters areverified accordin gtothe speedofthe processor.Allanalog todigital conve rsionstake placein the contro ller board.The Discrete FourierTransfo rm(OFT) is usedin thisprotect ion system.

1.5 Outline of thethesis

Chapter I:Itstatesthe problem .

Chapter2:It providesthenon-lin earbehaviourof apowertransform erthrough illustratingthe magneti zinginrushphenomen a, along with themagneti zing saturationof thecore and currenttransform er.Italso discusses overexcitation in powertransform ers.

Cha pter3:It prov idesthedetails of hard ware implementation.Itincludessca lingcircuit analog filter microcontroller, electroniccontrol circuitandelectronicswitch.

Chapter4:Itgivesthedetails of softwareexecutio nforfaultand no faultconditio ns (inrush) byusingharm on ics ratios.

Chapter 5:Harmonicsratiosare analyzedand testedfor theno fault(inrush)and fault conditi ons.Itprovides the realtimetesting of softwareand hardwar edesigns.These result s are evaluated in MATLABenvironment.

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Chapter 2 Power Transformer Protection Principles

Amongthe differentprotectiontechnique sthedifferentialprotectionmethodisthe mostpopular one.Inthe following sections thismethodisdiscussed.

2.1 Differential Protection Techniqu e

The differential protection techniqu eisbasedon extraction of informationfrom differentialcurrents ,whichare determinedas thedifferencesbetweentheprimaryand secondary currents.In norm alconditi onsthe differenti al currents musthave verysmall

values. Figure 2.1shows the schematic diagramfor abasicbiased differenti al circuit,

wherethenet currentflowing inthedifferentia lcircuitisused for protectionpurposes.

The netcurrentflow in the operating relay would operate the relay.Duetocurrent

transform er saturationor magneti zinginrushcurrent, theunbiasedprotective system mayoperate and thus causethe unnecessary trippingof the system .Thereforethe restraining coil is energised in the protection circuit to overcomethisproblem.

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S=Restraintcircuitcoil d=Operatecircuitcoil

Figure2.1:BiasedTransformerDifferential Protection [61]

2.2 Tripping Characteristics of Differential Relay

The tripping actionsof adifferential relayare determ inedbased onthe differential current

r

,

(2.1)

Thiscurrentrepresents thedifferencebetween the primary currentI,andsecondary current Iy •Thecurrent Idcan also have a value dueto:

1.Current transformersmismatch on primary andsecondary side.

2.Saturationin the core ofthe main transformer orthe currenttransfor mer.

3.Transien t disturbances suchasmagnetizinginrush orfaultcurrents,open supplyandover-exc itation.

In general,differentialprotection is usedforprotecting variouscompone nts in power systems .This differential protectionprovides,with high selectivityofthe protection

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differential relay,from internalfaultand shuntfaults.This protectionprincipleis used

for theprotect ionof generators,motors,power transformers and transmission lines.

Forpowertransform ers, wehaveto arrangespecial protectiontechniqu es,duetonon linear characteristicsof thetransformer. Thatisthereason,theratios ofthe 2nd

and5th harmonics to thefundamentalcomponentof thedifferentialcurrentare appliedfor restraining purposes[5].Figure 2.2 showsthe restrainingandtripping area of the differentialcurrentwithrespect to thebias current[61].

Tripping Area

O ' - - - ' - - - -- - - - ' - - - ' - - - ' - - L . - - - - ' - - - ' - - - ' o

Figure 2.2:Differential Relay Tripping Characteristics [61]

2.3 Magnetizing Inrush Current

An internalfault causessignificantcurrent to operatethe different ial relay.But inthe same time,magnetizinginrush, saturationof curre nt transform er andoverexcitatio nof current transform ermay alsoenergizethe trip coilof the differential relay.

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The resp on se ofthe inr ushcurrentdepends uponthe switchingang leof the inp ut voltage.Figure2.3 shows the samp leexampleofdecreasin g am plitudeoftheinrush currentwith timewhenswitchingang leof theinputvoltage is zero .Theother respon ses,with thedifferentswitchingang lesof theinputvoltageareshown in the appendixB.Thissing le phase simulation result of inrushcurrent is obtained usin g Matlab.Theresidua l fluxinthela mination coreofa3-phase tran sformerisnot conside red.

SwitchingAng le(Lamda)=O'andSatura tingAng le(T heta)=0radian 200,---r- - r- --,-- - , -- --,--- , - -- , - -- ,- - . .0.2

-200 -0.2

o

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 Angle(radian)

Figure2.3:Input Voltageand Inrush Current whenSwitch ing Ang le=0°

Whenthe tran sformer is switchedON.the stea dystate flux enters inthecoreof transfo rme r.Herethe secondaryof the transfo rmer is open.Thefluxis quadrat urewith theinputvoltage.neglecti ngthe resistance of thewindings.The flux in the core has

(43)

the sameshape but mayhavedifferent in amplitude and phase shift.Thestarting point of the steady fluxis also depend ent on residu alflux,Figur e 2.4.Total fluxatany instantwould bethe sumof steadystate flux and residu alflux. Transie ntfluxatany instant is equaltothemagnitude ofresidualflux.Itis analogo us tothe detransient comp on ent ofasymm etric al faultscurrents. Inrush would be zero at pointY, as the instantane ousfluxisthe sameasthe residual flux.Magnetizin g inrush current occurs whenthe polarity and magn itude ofthe residualflux donot agreewiththepolarityand magn itude of theinstantaneo us flux[5].Theamplitudeofthe inrushvariesdependin g upon theswitchingangle and the residu alflux.In thefigure 2.4,the switch isclosed at O.Theresidu alflux isin thepositivedirecti on. At pointX the steadystate flux is zero andat pointY the steadystateflux isequaltotheresidualflux.

Figure2.4:Voltage,fluxand currentduringthemagneti zing inrushcurrent.

The transformer is energise dat0 degree of switching.Ifthetransform er is energisedat point Y,there would notbe any inrush astheresidu alfluxis equal tothe steadystate

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FLUX

•

flux.Therewouldnotbe anytransientfluxat thispoint.If thetran sform er is energize d at the1800

position,thenthetransientfluxwould beproducedin theoppos ite direction[5].Theamount of theresidualflux isdeterminedby the hyster esis curveof the transformer.Theexcitation curve of figure2.5 can be used to establishthe inrush curve.Assume that sections OS andSParestraight lines.By extendi ng thepoints on instant aneou sfluxat900

degree, we get thepointl,and flux.Simi larlyextendingother points we get the entire curvefor inrushcurrent.

~ t5

~ ~

!i~~ t=K ~ ASSUMED P

~

k

-=-

~

-;;~.;~~~;~

-

t---7'-IF+-

.-+----+~-~--_:R~.!Qll.! ~~~

-!

~R

-

-Hf'----'---

M~~J~¥ ~'_L...I~_'ll__---I...,:_...r II) HYSTERESIS LOOP

Fig ure2.5: Derivation of magnetizing inrushcurrentfrom theexcitation characteristic [5]

If the residua lfluxisequalto thesaturation density,the inrushcurrentwouldbe asine wave whileoffsettingthe negativecycle.Herethefault current cannotbe distin gui shed from theinrushcurrent. But practicall ythe residu alfluxis always lessthanthe saturatio n densit y.Thisinrushwave form isdifferent from the fault current wavefor m.

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In the figure2.5 residu alfluxes are taken tobe40%ofthe rated flux.In themodern

stee lcore transform ertheresidu alflux is 90%ofthemaximum ratedflux.

3iO,---- ,---,- - - ,- ---,- - , -- ---,- ---,- ---,---,

300

TolliRux /lnrushcurrent

::: ~100 U ~.liO

loo

i 'lo

iO,r--i' - -- - - - - - -..- - - ' , - - - 1 4 Time (s) dO

Figure 2.6:Effectofresidu alflux oninrushcurrent,atsaturation density[5]

The damp ing effect duetothe resistanceofthesource and wind ing of thetransform er would redu cethe excitat ionvoltageand thusreduc etheinrushcurrent. This damping

effectistaken as a safetyfactoragainst the false trippin g ofdifferenti alrelays.Thisis

show nin figure 2.7.In modern stee lthesaturatio n density isfixedto140%ratedpeak flux. Thewidthoftheinrushhasreduc edto 3300

as compare d tothefigure 2.5 dueto

thedamp ing effect. Theharmon ic contentsoftheinrush show nin thefigu re 2.6areas follows[5]:

Fundamenta l=100%

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Third harm onic=7.4%

Theminimum possible seco nd harmonic component is16to17% ofthefundamental.

];0, - -- - ,- - , -- ---,- ---,-- ---.--- --.-- , - -- ,-, ]00 1;0 ~ionlXnsilY

J

/

.

f f -~ Time Is)

Figure2.7:Effect of residualflux on inrushcurrent,at 90%saturatio ndensity

Inrush current iscompl icatedin thethree-ph asetransform ers.Itcan be in morethan onephase.Thesephases canalso be affected bythe connec tionsandmagnetic couplingbetweenthephases. Themaximumamountof residual flux in eachphase would be 90% oftheratedflux.

The magnetizinginrush currentis significantlydepende nt upon the residua lfluxin the core. As shown in figure2.8thetransformerwiththemagnetic material, witha sharp knee point, has a flat magnetizingcurveabovetheknee point. Two differenttypes of thecorematerials are beingused. Thegrainorientedelectrica lstee l,(the newer type),

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has a greater portion of the residual magnetization as compared to the non-oriented electricalsteel (older type). Thegap of maximum residualflux is along the y-intercept

betweenthe1.4Wb/m2

and 1.8Wb/m2

.

NC\NC ,"type:",·ni..oJ·len _cdclcc tr-icnl~tcc l

Figure 2.8:MagnetizingCurvefor SteelCore Transformers [13]

2.4 Over -excitation

A common transformer with the standard iron laminationcoreis operated in the range

of 1.4Wb/m2

to 1.8Wb/m2

•When this flux goes beyond this limit then the transformer

is in an overexcited state and can cause transformer windingdamage in a few seconds.

This over excitatio n maybe duetohigh inputvoltage, low frequency,clearanceof the

fault.Only the fusecan protect this type offaul t. Underspec ificat ionof thefusemay

causethisfault.Thethird and fifth harmonics maydistinguishthe over-excitation and

faultor load current. The third harmonic can be filteredoutby the three phase current

transformer with delta connection on Y-side.The 5th

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preventthetripping from norm al overexcitation. 12to16 samples per cycleare required for good measurement of the5th

harmoni c.

2.5 Current Transformer Saturation

Current transform ers are usedtotakethesample currentforthedifferen tialrelays. Thesetransformer scan besaturatedwhen exces sive currentisconduct edthrough them.Thissaturationoccurs dueto slow decayin gde comp onents of interna land externa lfaults.The dc componentarises duetoinductivebehaviour of thecoilof the currenttransform er.Perc entagebiasing of thedifferentialrelayisappliedto overcome thisproblem[3).

2.6 Three Phase Transformer Protection

Three phasedifferenti alprotecti onismoredifficultthan the single phasetransformer protecti ontechnique. Here,threephases are tobeconsid eredon both sidesof the transform er.

2.6.1 Current Transformers (CTs)

Theseare the sensing partof the test equipment.Ittake sthecurrentfrom each phase fordata analysis.ThreeCTs are used at theprimary sideand three are used onthe secondaryside ofthe transform er.Inorde rtoblockthe zero sequencecurrenton externalground faultand tocomp ensat e for the 30 degreephase shift by Y/!'J. conn ecti on ofthemaintransform er,thethreetransform ers on thedelta (!'J.) sideofthe maintransformer are conn ectedin Y andYside if themain transform er is connected in delt a (!'J.).

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Figure2.9 showswhenanexternalground fault exists . If CTs areconnec ted in delta

there would notbe any zero sequencecurrentandifthese areinY-confi guration,it

wouldtriptherelay.The current in delta (6) connectedCTs would have current(1/'./3)

times the Y connecte dCTs.TheseCTshave tomatch the ratedcurrentof the

transformer (asthese currentofthemain transfo rmerinve rseratioof currentas

comp aringprimary andsecondary). Ifthetransform erhas tapchanging then the

differenti alprotection system mustfulfill therequ irement s.

Ci rc uit Cu rre n t Breaker Transformer

L Man ualsw ilch

Cu r re n t Circ u it Transformer Breaker

Figure2.9 Different ialProtection Scheme for Three PhasePower Transformer [4].

In thisthepowertransform erisdelta-starconn ected . On delta side the CTs are

connected instar andon the starside theCTsareconnected in deltaas shown in the

figure 2.9.Unde r norm al working condition sthe circulatingcurrents causedbythe

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conditions the balancewill nolon gerbethere and therelay willbe energize d totrip the circuit breakers on theprimary and seconda ryside.

2.6.2 Harmonic Restraint sfor DifferentialProtection

Magnetic Inrush currentappearsas aninterna lfaultcurre ntand itneedsto be addresse d.Norma l biasinginthedifferential protectionis ineffective.Harm onic restraintisrequ iredtopreventmisoperation ofrelays.In thestart, as show n in figure 2.10,harmonic restraintrectifie stheinrushcurrentandadds thesein percentage restraint,thro ugh inductor- capacitorfiltering.Thisisasingle-phaseprotection system but itcanbe extendedtoa three-phasesystem.This relayis adjusteduntil thesecond harmoniccrossesthecertainpredefinedlimit. Over-currentprotectionis alsoprovided asin the faultcurrent,direct currentoffsetand harm onicsandfault current itself for safety purposes[7]. PowerDistr ibuti on

Transform er

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Infigure2.11,a separate blocking relaywhosecontactsareinserieswithdifferential

relay,sothatblockingrelay wouldoperate ifthe second harmon icisbelow the

predefinedlimit[12].

Harmonicpass Fundamentalpass

Figure2.11: Basic circuitfor harmonicblocking relay [12]

Inconclusion,thebasic elements of amodernpowertransformer relayare provided in thischapter.Herethe inductorandcapacitorare included toprovidetherestraintfor

therelay.Inthe nextchapters,theharmonicratios are usedfor the filteringof these

harmonics from the faults.The discrete Fourier transform isused as analgorithm for

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Chapter 3 Hardware Design of Microcontroller-based

Protection of Three Phase Power Transformer

3.1Introduction

Design of ahardware system is importa ntforthe power transfo rmerprotection.The design process invo lves boththe electro-mec han icalandelectron iccompone ntsfor the successfulprotectio nof apowertransform er.Inchapter2theprinciple of protection against the inrushcurrentsand the faultsis explained.Figure3.1shows the electrical schematics ofthelaboratory SkVA,3-phase powertransformer with tap changing

provisions.Thesystem blockdiagram is show n infigure3.2. Hold ing circuit,

multiplexing circuit, multiplexer and processor areincludedin theprogrammable controllercard.The pertin entdetails are givenin thefollowing sections.

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Three Phase 5kVA230/550·575·600V DeltalWye Power Distribution Transformer ThreePhase Powersupply Figure3.1:ExperimentalSetUp 26

(54)

Current Transformer

ThreePhase

I

Thr

ee

Ph

ase

Power Di

stribution

OutputPower~

Supply I

Tran

sformer

Figure3.2:System BlockDiagram

(55)

Three Phase 5kVA 230/550·575·600 VPower Distribution

Transformer.T

.estingSet.up

SP9 1

I

SP92

II

SP9 3

II

SPP11

h'''''!

111

Figure 3.3: ExperimentalSetUp for Internal FaultsTests.

It is noted that Spgl =Primary-side-Phaseto groundswi t ch1,Spg2=Primary-side-Phaseto ground switch2,Spg3=Primary-side-Phaseto groundswi t ch 3,SPP1=Primary-side -Pha se

to phaseswi t ch1,Spp2=Primary-side-Phaseto phase swi t ch2,SPP3=Primary-side-Phase

to phase swi t ch 3,Ssgl=Secondary -Phasetogroundswi t ch1,Ssg2=Secondary -Ph ase to ground switch2,Ssg3=Secondary -Phaseto groundswi t ch 3, SSP1=Secondary -Phase to

phaseswitch1,Ssp2=Secondary -Phaseto phaseswi t ch2and SSP3=Secondary -Phaseto phaseswitch3.

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3.2 Laboratory3-phase Power Transfor mer

The laboratorypowertransform erisprovidedby Siemens,Canada.Ithasthe follow ing

spec ifications:

Power=5kVA

Input Voltage=230V

OutputVoltages=550V, 575V,600V.3taps on the outputside.

Input windin gs=Deltaconn ected

Outputwindings=Y connected

Phase=3Phase

Typeof cooling=Air coolin g

3.3Current Transform ers

A totalof six curre nt transform ers (CTs) are usedto sense input andoutputcurre nts.One

CT isconnecte dineach phase for theprimaryandsecondarysides,respectively.To

overcomethezero sequencecurre nt, theprimary side three current transform ers are

connectedinaWye config uratio n,whereas the seco ndarysideofthelaboratorytest

transform er,thethreecurrenttransform ersare connected in a Deltaconfigur ation.The

shunt resistan ces at the outputof the current transform ers are included at both sidesof the

laboratorytransform er. Thevaluesof theshu ntresistancesare0.3

n

forallcurrent

transfo rmers,CTs.

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Therating of the currenttransformeris as follows:

Input=120 A

Output=5 A

Cooling=Air Cooled

Shuntresistance=0.3

n

3.4 Relay Control Circuit

Thesolidstate relayis contro lledthroughwiththe Darlingtonpair. Two NPN transistors are connectedtogetherto get thehigh curre ntgain. Thecircuitdiagramis shown in figure 3.4.Outputis givento eachsolidstaterelay.So, threecontrolcircuitsare utilizedfor the three phase powerlaboratory transfor merprotectionsystem.Input for thecontro lcircuit istaken from theprocessor.

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Vcc+ Vin+ Darlington Pair Voul

1

Figure 3.4:Relay ControlCircuit

Design Parameters:

DarlingtonPair oftwo NPN transistors

Model =2N2222A

TurnOnTime=35ns

TurnOff Time=285 ns

Vee=Collector Voltage=5V(DC)

R,=Base resistance=1 KO

(59)

Ie=Vee/Re=250mA

For

Vin=OV, VOU!=5V orHigh

Vin=2- IOV de,VOU!=OVorLow

3.5 Solid State Relay

Crydo msolidstate rela yisused forthe switching . Its specifica tio nsare:

Mod el No.D2425

InputVoltage=3-32 V(DC)

OutputVoltage-Max= 240 V(AC)

OutputCurre nt-Max= 25A

Theequivalent circuit of the solidstate relayis show ninfigure3.5.Two SCRs are coupled together. Gatesare triggered from the opto-coupler.Three relays are used, one foreach phase. Turnon timeforthis relay is .02ms.Itoperates in lessthanhalf cycle of the input current.

(60)

AC

Figure3.5:Equivalentcircuitof SolidState Relay

3.6 Scaling Circuit

Thecurrentfrom the curre nt transform erneedstobe sensedand tobe conve rtedin de voltage level for furthe revaluation.For thispurpose,LM324fourtee n pins chipis

selected.Ithasfour Op-amps,eachconsistswith thehigh gainand intern allyfrequ ency

compensated.Thecircuit diagramfor thisQuadOperatio nalAmplifie ris shown in figure

3.6.

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Figure3.6:QuadOperationalAmplifier

Thegain of thisamplifieris calculatedas following:

Gain=~= 1+~

where RA=0to5kQ Vee=32V(de) RB=5kQ 34

(62)

Thecircuitdiagramforthe scalingcircuit is showninfigure3.7.The response of the scalingcircuit withdifferentvariableresistances(RA)is shownin figure 3.8,figure3.9 andfigure 3.10respectively.

LM324

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Time(s) - - 7

Figure3.8:ScalingcircuitwhenLow gain,RA

=

0

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Time(s) ---7

Figure3.9:Scalingcircuitwhen medium gain, RA=2.5KQ

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Time(s)

Figure3.10:Scalingcircuitwhen high gain, RA=5Kn

The scalingcircuitistested for each phaseof the 3-phaselaboratorypower transformer, both at primaryside and secondary side. Theoutput result s are found to be perfect. A total ofsix scalingcircuits are required togetsix inputstobe analyzed.

3.7 Filtering Circuit

Whenever the samplingfrequency isless than the twice thecut off frequencythe phenomenonof the downward frequency translationoccurs and it is calledaliasin g.Due to thiseffect the datain thesam pled frequencycannotbe linearlyrelated with an input

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continuo ussignal. To eliminate this errorin the sampledfrequency the.inputcontinuous

signal mustbefiltered out to get therequired signal.

For thepowertransform erdifferenti alprotection,thedesignmustdetectthemagnetizing inrushcurrentbydetectingthe second harm oniccomp onent,andforoverexcitationthe 5lh_h_{arm onic co}_mponent._{To avo}_{id th}_{e a}_{liasi ng}_effec_t_a_{nd p}_reserve_th_{e f}_{undamenta l} comp onent at60Hz,second harmonic at 120Hz, and5th

harm on ic at300Hz,alowpass

anti-alias ing filterisproposed.Acutofffrequency(fe)of 350Hzand bandstop frequency

(f5)of 450Hzare found tobepractic al.Toget all threefrequ enc ycomp onents and to

accommodate thedifferenti alprotection algorithm,a samplingfrequencyof960 Hz was

chosen.

TheChebys hev filter wasused for the anti-aliasing filter.Ithas a stee prolloff

charac teristic nearthecutoff freque ncy.Referr ing to the filter speci fica tio ns [46],

minimum attenuatio ninthe passbandis 0.3dBand maximum attenuationinstopbandis

20dB.TheChebys hev lowpassfilter speci ficationsare show nin figure 3.11.

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amin

Stop band

amax

L-_-=---.lo..L-- --l..-_~f(Hz) fc

Figure3.11:Chebys hev lowpass filterspecifica tions [46]

Umax=decibelattenuationin passband

Umin=decibelattenuation instop band

fc=Frequencyatwhichamaxoccurs fs=FrequencyatwhichGminoccurs

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Ananti-alias ing filterisrequir edto overcome the aliasi ngeffects,wheneverthe sam pling

frequency islessthanthetwicethe freq uencyofthecutofffrequ ency.To overcome these effec ts,the 6th

orderChe bys hevfilter isprop osed .Detail s ofthedesi gn aregive nin the appendixA.Thecircuit diagramfor the6th

orde rChe bys hev filter is show nin figu re 3.12.

Designparam etersare as follows:

{lmax =0.3 dll

CutOff Frequency=350 Hz

Funda mentalFreq uency=60 Hz

2nd

Harmon ic=120Hz

5th

Harm on ic=300Hz

Sto p Band Freq uency=450Hz

Orderof Che bys hev filter (n)is givenas:

{

~

Oa

1 ~

-

1

1 w

}

n=cosh? !!mM cosh -1_(-2.)

1010-1 we

n=6

The tran sfer functionofthe 6th

orde rChebys hev filteris:

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1.32 2 .1 0' 9

56+ 2 9 25 *55 +1.15 4 *107.54 +2.0 39 *1010. S3 + 3.257* 1013 .52+ 1.368*1019 (3.2)

Using this transfer function the frequency and phase respon seare found by simulation. The resultsof thesimulation are shown in figure 3.13.The filter responseshows that the cut off frequencyis 2199 rad/sec (350 Hz),which ensure sthat all three harmonicspass

throughthefilterand higher frequency componentswill be eliminated. The phase

responseis alsolinearfor passband.The program and detailedcalculationsare givenin

theappend ixA. Six LM324 chipsfor all the three phases at primary and secondaryside, respectivelyareused forthelaboratory testing.

Stage 1 R1

=

R2

=

10.55Kfl RA1=1kil RB 1=3.45kfl Sta ge 2 Wc2=1.7315*103 rad /s (3.3) (3.4)

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RA2

=

lkfl Ra2=4.38 kfl Stage 3 We3=2.254*l03ra d js Rs

=R6

=

4.3Kfl RA3= lkfl Ra3=4.8kfl 43 (3.5)

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Yin

I

RI R5 R6 You!

C6

1

RA3 RB3

I

Stage3

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BodeDiagram II I I

""~

~

'"

<,

o

- - - - -i---,': ::

r-,

\

~

~-180 &.-360 -540 10'

Frequency(radJsec) Figure3.13:Outpu tof Chebyshevfilter

3.8 Microcont roller

Afterfiltering the analogsignal, it needstobe analyzedfor deci sionmakin g for faultand no fault condit ions. The filtered analogsignal isprocessedin themicrocont roller.Itis a 28/40-Pin, 8 bits CMOS flashMicroc ontrollerPIC16F8 77, which hasbeenusedfor the experimental prototypedigitalrela y.Thescaled, filtered, sampledsignal hastobe

presentedtothe analog-lo-digitalconverter for conversionto form anumber,whichcan be read bythemicroprocessor. An 8-ch annelIO-bitAIDconvertoriswithin the

microcont roller,makingit idealfor real-timeimplementationintheexperimentalrelay.

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Multiplexing ofthelar ge amountof inform ationinsing le line is donein themultiplexer in the contro ller.The microcontroller performs the power transfo rmer different ial protectionfunctio nsandother tasks definedin thesoftware. Port A is usedforthree input differe ntia l(filteredandscaled)curre ntsand port B isused for the output.Thesamethree inputsareusedto analyzethe data at theoscill oscope.Thepin diagram oftheused microcontroller is show nin figure.Fl.landtheblockdiagra m is show n infigureF.1.2 ofthe AppendixF.The mainfeatures of the microcontro llerare as follows:

8K*14Words Flash ProgramMemory

368*8 Bytes ofdatamemory (RAM)

256*8Bytes of EEPROMdata memory

A CompaqLaptop with 2GRAMand 80Gharddrive isusedfor writingthe programm e in the Clanguage and itisdownloadedtotheprocessor via an RS232C cablethrougha PIC down loader. At anyinstantafter scalingandfiltering thethree differentia lcurrent signalsare fedtothemicrocontroll erwhere analog to digitalconversio nand then multiplexing of thesignals are done.The signal is analyzedaccording tothediscrete Fouriertransform algori thmanda decision istaken for faultor no fault conditionas specifie din inthesoftware.Thissoftwareoperationwilldiscussin thenext chapter.

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Chapter 4 Software Design of Microcontroller-based Three

Phase Power Transformer Protection

4.1 Introduction

In modern relaystheuseofanalog and digitalelectronic circuit cardsaswell as computers is common. For laborator ytesting, aSkYA230/SS0-S7S-600Y,~-yconne cted threephase power transform er isanalyzed.

Irated.primary

=

(

~~~~o)

=

12.55A

4.2 Softwa re Design

(4.1)

The reare threeinput different ial currents totheprogramm able contro ller.TheseareIda. Idb.andIde.The processorisdoin gthecalcul ations forthe funda mental,2ndand5th harm oniccomp onents.Thediscret e Fourier transform techn iqueisusedto getthe harmonic s.Data format andscaling are requir edforlaborat or ySkYApowertransform er protection.Aflow chartfor thepowertransformerdifferenti alrelayisshown in figure 4.1.A stop for loop isapplicablewhenev erthere isinterruptsignal, appliedexternallyto themicrocontroller.

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Figure 4.1:Flow Chart for Differential Relay Protection

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4.2.1 DFT Method for Harmonics Calculation

Detail ed ca lculationsof the Fourier coeffic ientsare give nin the Appe ndixC.

S=Sinefunc tio n

C=Cosine functi on

N=16

11:=3.14.. .

j=1, 2 and5

k=a,band caredifferentialphases

Id=Differ enti al cur rent

For Funda me nta lCompo nent:

ForSeco nd Harm oni c:

For

s"

Harm onic:

49

(4.1)

(4.2)

(4.3)

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(4.5)

The flow chart of the subroutine for thisalgorithm isshown in the figure 4.2.

I

yo

Start

C ,---

)

1.Multiply Current Signal by DFT Sine Coefficients 2.Multiply Current Signal by DFT

Cosine Coefficients Retrieve Three Differential Current

Signals for DFT I from Program Memory

Calculate the Combined Harmonic Components

Return

Figure 4.2:FlowChartfor HarmonicCalculations-DiscreteFourierTransfor m Method

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4.2.2 Criteria forHarmonic Calculations

For Magneti zingInrush Current:

~ ~

0.177

Then no fault

For over excitation:

~~0.065

Hi

(4.6)

(4.7)

As the currentto voltage isolatorhas a rangefrom0to10Vde,the referencevoltagefor

instantaneo us tripp ing of thedigitalrelay5V deistaken asreferenceto savethetest

equipment.For Instantaneoustrippin g ofthe circuit breakers, voltage level settingfor

each phaseis asfollows:

(4.8)

(4.9)

(4.10)

where

Va=Reference voltage for phase A,

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Vb=Reference voltage forphaseB

andVc=Reference voltageforphase C

4.3 Protection Scheme

Different ial curre ntsamplesaretakenandfedtothememoryforreference. AOFT

scheme is applied forthecalculationof Fouriercoefficients. In norm aloperations the

digitalrelayis onand thetransform erisinoperation.The electronicrelaygives the

trippingsignalwheneverthereis short circuiting oraninternalfault.Equations(4.7)and

(4.8)areused for inrushcurrentconditions.Themicrocontroller ischeckingthese

conditions continu ously.Jf these conditionsarenotmet,then it conclud esthatitis afault

condition,and tripsthe relay.The input baud rateis 9600 samples per secondand the

windowsize for calculatin gtheharmon ics is 16per cycle. Thecode for the

microcon trolleris given inappendixE.Thesystem layout foranalyzing thedata in MATLABis show n infigure3.1. A fourchanne loscillosco pe isusedto observethe

three inpu tdifferenti al currentsandthe response ofthe electro nic relay.Somesample

resultsare discussedin chapter5.