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Basics ofCurrent and Voltage TransformersPower Transmission and DistributionAuthorPage 2412007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Equivalent current transformer circuitI1jX1 R1I 2′ I1 N1N2jX2ImP1N1N2 UR2I2S1ZmZb2P2Ideal CTS2X1 Primary leakage reactanceR1 Primary winding resistanceX2 Secondary leakage reactanceZ0 Magnetizing impedanceR2 Secondary winding resistanceZb SecondaryAuthorloadNote:Page 2422007-08Power Transmission and DistributionNormally the leakage fluxes X1 and X2 can be neglectedAuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformer, simplified equivalent circuitjX21 : N2i2i1i'1 N21R2 i1LWL2 LWimZBR2i2Power Transmission and DistributionRBAuthorPage 2432007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformer:Phase displacement (δ) and current ratio error (ε)I1 .I1N1N2εN1I2N2I2δPower Transmission and DistributionZbAuthorImPage 2442007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT classes to IEC 60044-1: 5P or 10PPi I sec .2 R CTSpecification:300/1 A5P10, 30 VARatio In -Prim / In -Sek.5% accuracyat I n x InActual accuracy limit factorin operation is higher as the CTALF ' ALF is normally underburdened :Operating ALF: ALF‘RCT 5 OhmRated burden(nominal power) PNBAccuracylimit factor ALFPi PNBPi PBBDimension criterium:IALF ' SC max KTFPower TransmissionI n and DistributionKTF (over-dimensioning factor) considers the single sided CT over-magnetising due to the d.c.Authorcomponent in short circuit current ISC.KTF values required in practice depend on relay type and design.Recommendations are provided by manufacturers (see Application Guides)Page 2452007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Definition of the CT knee-point voltageU210 %IEC 60044-1 Class PXUKNformerly50 %British Standard BS3938:Class XPower Transmission and DistributionAuthorImPage 2462007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformer, Standard for steady-state performanceIEC 60044-1 specifies the following classes:Accuracy classCurrent errorat nominalcurrent (In)5P 1%10P 5%Angle error δat rated currentIn 60 minutesTotal error at n x In(rated accuracy limit)5%10 %Power Transmission and DistributionAuthorPage 2472007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformers, Standard for transient performanceIEC 60044-6 specifies four classes:ClassError at rated currentRatio errorTPX(closed iron core)TPYwith anti-remanenceair gapTPZlinear coreTPS2007-08RemanenceAngle error 0,5 % 30 minεˆ 10% 1,0 % 30 minεˆ 10% 1,0 % 180 18 minεˆ 10%Authorno limit 10 %Power Transmission and Distribution(a.c. current only)Special version forhigh impedance protectionAuthor(Knee point voltage, internal secondary resistance)closed iron corePage 248Maximum error atrated accuracy limitnegligibleNo limitCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformer saturationSteady-state saturation with AC currentPower Transmission and DistributionAuthorTransient saturation with offset currentPage 2492007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Transient CT saturation due to DC componentISCShort circuit currentDC flux non-saturatedΦSΦACfluxPowerTransmission and DistributionAuthorImPage 250Magnetising current2007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Course of CT-flux during off-set short-circuit currentΙP primary currenttt Bω TN TS TN 1 (e e TS )TN TSBˆd.c. component TSN TS TN T BMax 1 ω TS B TS tTNtotal fluxBt B Max BMaxtransientd.c. fluxB TS a.c. fluxAuthortB Page 2512007-08AuthorTN TSTS lnTS TN TNLW Power Transmission1and Distribution Ri RB ω tanδFor 50 Hz:TS 10900δ [min ][ms]Copyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT transient over-dimensioning factor KTFKTF60 TN KTF 1 ωTS TS TSTS TN (KTF 1 ωTN)500050Closed iron core100040TS [ms]5003025020100Linear core10Power Transmission and DistributionTN network time constant(short-circuit time constant)Author50Page 2522007-08100150200AuthorTN [ms]TS CT secondary time constantCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT with closed iron core,Over-dimensioning factor KTF‘ for specified time to saturation (tM)1514tMtM K‘td50 ms40 ms1030 ms20 ms6510 msPage 253202007-08406080TpAuthor Power Transmission and DistributiontM :KTF 1 ωTN 1 Author0K' TFtM 1 ωTN 1 - e T N XNRN100 [ ms]Copyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT over-dimensioning factor KTF (tM,TN)in the case of short time to saturation (tM)KTFtM4Θ (tM,TN)(el. degree) 16010 ms3.514031208 ms2.5tM100210 ms8 ms6 msPower Transmission and Distribution4 ms2 ms806 ms1.5160404 ms0.501802 ms0Page 254204060802007-08 TNAuthor0100in ms20Author020406080100Copyright Siemens AG 2007. All rights reserved.TPTDmsPTIN inSE

Current transformermagnetising and de-magnetisingΙPBtBMaxBB BR (BMax. BR ) eBR BMaxtTSBRPower Transmission and DistributionΙmAuthortPage 2552007-08AuthorΙmCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformerCourse of flux in the case of non-successful auto-reclosureBBmaxtF1 duration of 1st faultttF1tDTtF2tDT dead timePower Transmission and DistributiontF2 duration of 2nd faulttF1tDT tF2 tF2tF1tF2 Author N TSBmax ω TN TS TNTω 1 e TS e TS 1 (e(e TN e TS TN TSTN TSBˆ Page 2562007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformermagnetising curve and point of remanenceIBIIIIIup to 80% 10%negligibleH im wPower Transmission and DistributionI: closed iron core (TPX)II: core with anti-remanence air-gaps (TPY)AuthorPage 2572007-08AuthorIII: Linearised core (TPZ)Copyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformers TPX und TPYCourse of the flux with non-successful auto-reclosureBRBRclosed iron core (TPX)Power Transmission and Distributioncorewith antiremanence air-gaps (TPY)AuthortDTtF1Page 2582007-08tF2AuthortCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Standards of voltage transformersVT classes to IEC 60044-2ClassdesignationPermissible error at 0.05 ·UN and 1.0 · UNVoltage error FUAngle error δ3P 3.0 %120 minutes5P 6.0 %240 minutesAll 3P and 5P protection CTs mustadditionally comply with one of thebelow VT metering classes!VT classes for measurement IEC 60044-2ClassdesignationPage 259Permissible voltage error in %at 1.0 · UN0.10.20.10.20.50.513132007-08Permissible angle error in minutesat 1.0·UNPower Transmission and Distribution5AuthorAuthor102030Not determinedCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Capacitive voltage transformer,Equivalent circuit400 kVC1 5 nFC2 95 nF3RF20 kVLF3CE C 1 C 2C1U*1 UP C1 C2Page 2602007-08100 VCF3LORECTAuthorAuthorZBPower Transmission and DistributionRFRTLTLFCFU*2R'BL'BCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Transient performance of CVTs,Recommendations acc. to IEC 60044-5RatioMeasured voltage transients after fault atvoltage maximum and zero-crossing(Example)TimeTsin mstUP(t) Primary voltage2 Us 100%Classes3PT16PT13PT26PT23PT36PT310--- 25 420 10 10 240 10 2 260 10 0.6 2 10 0.2 290Legend:Us (t)Power Transmission and DistributionRecommendations to IEC 60044-5US(t) Secondary voltageAuthorPage 2612007-08(1)Fault inception(2)Aperiodic damping of US(t)(3)Periodic damping of US(t)AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT dimensioningK OD KTFPi PBNRCT RBNALF' ALF ALF Pi PBRCT RBActual connected burden : PB RB I2N2P PBRCT RB ALF' ALF ALF' iPi PBNRCT RBNwithIALF ' K OD KINTheory:K OD K TF K Re mK Re m 1% Remanence1 100Page 2622007-08rated CT burden:PBNinternal burden of the CT: Pi Ri I2N2RB Rl RR burden resistanceRl resistance of connecting cablesRR burden resistance of the relayNo saturationfor the totalshort-circuit duration:No saturation forthe specified time tM:Practice:AuthorK' TF BMaxXN 1 ωTN 1 RNBˆ PowertM Transmission andtMDistribution ωTN TS (e TN e TS K ' 'TF 1 TN TS Remanence only considered in extra high voltage systems (EHV)KTF-values acc. to relay manufacturers‘ guidesAuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Practical CT requirementsTransient over-dimensioning factors KTF (AR not considered)Distance7SA6 and 7SA522Overcurrent7SJ56Close-in fault:KTF 5Fault atbalance point:KTF 2 ( 1 if TN 30ms)ALF‘ I setting/ IN, at least 20Internal faultExternal faultTransformer Differential7UT6KTF 0.75(Saturation freetime 4 ms)KTF 1.2(Saturation freetime 5 ms)Line differential7SD61KTF 0.5(Saturation freetime 3 ms)KTF 1.2Power Transmission and Distribution(Saturation freetime 5 ms)Bus differential7SS52Page 263AuthorKTF 0.5(Saturation freetime 3 ms)2007-08AuthorKTF 0.5(Saturation freetime 3 ms)Copyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT dimensioning forExample differential protection (1)1. Calculation of fault currents110 kV, 3 GVAF1110/20 kV40 MVAuT 12%F2OH-line:l 8 km,zL‘ 0,4 Ω/kmF3F4Net300/1A IT7UT61Transf. : IL7SD61 ILImpedances related to 20 kV:Impedances related to 110 kV:Net :200/1A200/1A1200/1AU N 2 kV 2 110 2 ZN 4.03 ΩSSC ' ' [MVA ] 3000U N 2 kV 2 Author uT [% ] 110 2 12 % ZT 36.3 ΩPN -T [MVA ] 10040 100Net :Transf. :U N 2 kV 2 Power Transmission20 2and DistributionZN 0.13 ΩSSC ' ' [MVA ] 3000U N 2 kV 2 uT [% ] 20 2 12 %ZT 1.2 ΩPN -T [MVA ] 10040 100Line : Z L l[km ] z L ' [Ω/km ] 8 0,4 3,2 ΩPage 2642007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT dimensioning forExample differential protection (2)1.1 U N / 3 1.1 110kV/ 3 17.3 kAZN4.03ΩF1I F1 F21.1 U N / 3 1,1 110kV/ 3I F2 1.73 kAZ N ZT4.03Ω 36.3ΩF3F4I F3 I F4 1.1 U N / 3 1.1 20kV/ 3 9.55 kAZ N ZT0.13Ω 1.2Ω1.1 U N / 31,1 20kV/ 3 2.8 kAZ N Z T Z L 0.13Ω 1.2Ω 3.2 ΩDimensioning of the 110 kV CTs for the transformer differential protection:Manufacturer recommends for relay 7UT61:The saturation free time of 3 mscorresponds to KTF 0,75See diagram, page 59Power Transmission and DistributionCriterion 1) therefore reads:I17300ALF' K TF F1 0,75 43IN3001) Saturation free time 4ms for internal faults2) Over-dimensioning factor KTF 1,2for through flowing currents (external faults)For criterion 2) we get:AuthorI1730ALF' K TF F2 1,2 7IN300The 110 kV CTs must be dimensioned according to criterion 1).Page 2652007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT dimensioning forExample differential protection (3)We try to use a CT type: 300/1, 10 VA, 5P?, internal burden 2 VA.Pi Poperation2 2.5ALF ALF ' 43 16.1Pi Prated2 10(Connected burden estimated to about 2.5 VA)Chosen, with a security margin : 300 /1 A, 5P20, 10 VA, R2 2 Ohm (Pi 2VA)Specification of the CTs at the 20 kV side of the transformer:It is good relaying practice to choose the same dimensioning as for the CTs on the 110 kV side:1200/1, 10 VA, 5P20, R2 2 Ohm (Pi 2VA)Dimensioning of the 20 kV CTs for line protection:1‘) Saturation free time 3ms for internal faults2‘) Over-dimensioning factor KTF 1.2for through flowing currents(externalfaults)PowerTransmissionand DistributionFor relay 7SD61, it is required:The saturation free time of 3 ms correspondsto KTF 0.5See diagram, page 59Criterion 1‘) therefore reads:I9550ALF' K TF F3 0.5 24IN200Page 2662007-08AuthorFor criterion 2‘) we get:I2800ALF' KTF F4 1.2 16.8IN200AuthorThe 20 kV line CTsmust be dimensionedaccording to criterion 1‘).Copyright Siemens AG 2007. All rights reserved.PTD SE PTI

CT dimensioning forExample differential protection (4)For the 20 kV line we have considered the CT type: 200/5 A, 5 VA, 5P?, internal burden ca. 1 VAPi Poperation1 1 ALF ' 24 8ALF 1 5Pi Prated(Connected burden about 1 VA)Specification of line CTs:We choose the next higher standard accuracy limit factor ALF 10 :Herewith, we can specify: CT Type TPX, 200/5 A, 5 VA, 5P10, R2 0.04 Ohm ( Pi 1 VA)Power Transmission and DistributionAuthorPage 2672007-08AuthorCopyright Siemens AG 2007. All rights reserved.PTD SE PTI

Current transformer Course of flux in the case of non-successful auto-reclosure t F1 DT t F2 t F1 duration of 1st fault t DT dead time t F2 duration of 2nd fault B t B max S F2 N F2 N S N S S DT F2 S F1 N F1 N S N S T t T t (ω 1 t t T t t (ω 1 .File Size: 410KBPage Count: 27