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3/2/2018Lesson 11 et332b.pptxET 332b Ac Motors, Generators and Power SystemsLESSON 11: TRANSFORMER NAME PLATEDATA AND CONNECTIONS1Lesson 11 et332b.pptxLEARNING OBJECTIVESAfter this presentation you will be able to: Identify transformer polarity using dot and conventional labeling.Explain and interpret information found on transformer nameplates.Compare and contrast the performance of three phase transformerconnections.Indentify the schematic symbols of potential and currenttransformer. List characteristics on these devices and explain howthey relate to performance.21
3/2/2018Lesson 11 et332b.pptxTRANSFORMER MARKINGS AND POLARITYDot Notation - Terminals marked with a dot are considered instantaneouslypositiveAI1 EAB0 degreeshift--BN1CI2 DN2Above Terminals A and C are positive at the same timeEABPhasor DiagramECD0 degrees3Lesson 11 et332b.pptxTRANSFORMER MARKINGS AND POLARITYA - EABB -CI2180 degreeshift DAbove Terminals A and D are positive at the same timePhasor DiagramEABECD180 degrees42
3/2/2018Lesson 11 et332b.pptxTRANSFORMER MARKINGS AND POLARITYConventional LabelingH high voltage sideX low voltage side H1 X10 phase shiftH2-X2-Terminal H1 more positive than H2. On secondary side X1 more positivethan X25Lesson 11 et332b.pptxTRANSFORMER MARKINGS AND POLARITYConventional LabelingH1H high voltage sideX low voltage side -X2180 phase shiftH2- X1 Note: Polarity of secondary is reversed creating 180 degree phase shiftacross transformer63
3/2/2018Lesson 11 et332b.pptxTRANSFORMER NAMEPLATE DATAVoltage ratingshigh side and low side values (no-load values)Additional Voltage MarkingsDash (-) Indicates voltages fromdifferent windingsSlant (/) voltages from same windingCross(x) voltages obtained by seriesor parallel connection of twopart windingsWye (Y) wye-connected windings7Lesson 11 et332b.pptxTRANSFORMER VOLTAGE RATINGSSingle Phase Examples240 V windingwith a center tap240/120240x120Two part winding that can beconnected in series for 240 V andparallel for 120 V240-120Three Phase Example12470-480Y-277 VA 240 V and separate120 V windingTwo winding transformer. Wyeconnected secondary with 277Vand 480 V available84
3/2/2018Lesson 11 et332b.pptxOTHER NAME PLATE DATAFrequencykVARated frequency of transformerRated Apparent power of transformerApparent power rating determined byconstruction and cooling- oil cooled. Oilalso provides insulationPower ratingsAO rating - natural convection air andoil.FA rating - natural convection of oil andforced airFOA rating - forced air and oil circulation9Lesson 11 et332b.pptxTRANSFORMER POWER RATINGSTypical ratings 30/40/50 MVA AO/FA/FOADry Type Insulated transformers (no oil) Typicalat voltages of 15 kV and belowClassesAA - dry type, self-cooled, natural convection of airAFA - Dry type, forced-air coolingAA/FA Dry type self-cooled/forced air cooledForced oil bycirculatingpumpsForced air byradiators andfansDual rating that requires fans105
3/2/2018Lesson 11 et332b.pptxOTHER NAME PLATE DATAPercentImpedanceTemperatureRiseImpedance measured at indicatedtemperature. Base S and V values are theratings of the transformerMaximum allowable temperature riseHeat effects the insulation Excessiveheating reduces life of transformerClassInsulating medium and cooling ( seeprevious slide)11Lesson 11 et332b.pptxBASIC IMPULSE LEVELBIL(Basic Impulse Level) Measures themaximum voltage stresses that thetransformer can handleImpulse tests used to simulate theeffects of lightning over-voltagesVoltage surges in excess of the BILcan cause insulation failures126
3/2/2018Lesson 11 et332b.pptxTHREE PHASE TRANSFORMER CONNECTIONSTransformers increase voltage, decreasing current in power systems.Lower I means less power loss due to I2R losses in lines, cables,transformers, etcIStep uptransformerVVIStep downtransformerGeneratorThree phase transformers3 single-phase units can form 3-phase bankorSingle three-phase transformer (3 separate cores in a single tank)Step up/down transformers are different connections of 1-f units or coils based onwye-delta configurations13Lesson 11 et332b.pptxTHREE PHASE TRANSFORMER CONNECTIONSExamples from the fieldThree-Phase Transformer SingleUnit, Substation TypeThree Phase BankThree individual Transformers147
3/2/2018Lesson 11 et332b.pptxDELTA-DELTA (D-D) CONNECTIONprimarysecondaryParallel coils are part of same transformer or corePhasediagramAdvantages: Absorbs unbalance of load. Unbalance I circulates in delta.Lose one transformer and can still operateDisadvantages: No natural neutral15Lesson 11 et332b.pptxWYE-WYE CONNECTIONCThree phase, four-wire , grounded wye primaryAAdvantages:BCATwo voltage levels availableGraded High voltage insulationEasy balancing between 1-fand 3-f 120120bDisadvantages:X1208208aX1bSingle phase short circuitcurrents passed. Zerosequence (3rd Harmonicspassed)Three 7200 - 240/120 V transformersPrimary VAn VBn VCn 7200 V H1 - H2 VLL 12470 V Total Bank power:Secondary Van Van Vcn 120 V X1 - X2 VLL 208 VS1 S2 S3 ST168
3/2/2018Lesson 11 et332b.pptxDELTA-DELTA (180O SHIFT)BThree phase, three-wire, delta c120120ban240240aAdvantagesIdeal for motor loads, 3-wire.Can tolerate single line shorts withno interruption.Traps 3rd harmonic currentsb120/208/2403 Transformers 7200 - 240/120 Vprimary VA VB VC 7200 V VLL 7200 VSecondary Vab Vbc Vca 240 V VLL 240 VDisadvantagesFull insulation required on H.V.Windings.T1 must be larger KVA whenServing 1-f load with 3-f.No natural ground pointUnbalanced connection whenserving 1-f and 3-f loads togetherBank powerS1 S2 S3 ST180o phase shift H1-H2 to X1-X317Lesson 11 et332b.pptxEXAMPLE FROM FIELD12.47 kV Line-toline primarySmaller sizeindicates lowerpower ratingSecondaryconnectionsThree-Phase BankUnequal Transformer Power Ratings189
3/2/2018Lesson 11 et332b.pptxDELTA-WYE CONNECTION- 30 PHASE SHIFTBThree phase, three-wire, delta primaryABCAH2H1H2H1Advantages:Single phase load easy tobalance. New neutral point isestablished. Two voltagelevels. Traps 3rd harmonicsCH2H1bX2X3X2X3X1X1X2X3Disadvantages:Full insulation required on H.V.winding of transformersX1an208120cn120120b208208aBank PowerS1 S2 S3 STc120/2083 Transformers 7200-240/120 VPrimary VLL Vp 7200 VSecondary Van Vbn Vcn 120 V Vab 208 V30o phase shift Van to VAn lag is standard19Lesson 11 et332b.pptxOPEN DELTA-OPEN DELTABThree phase, three-wire, delta primaryTwo transformers supplybalanced 3 phase voltagesand currents. Connectionitself is unbalancedABCAH2H1CH2H1Bank PowerX2X3X3X1X2ST X1b240120c120b240 240aIf S1 S2naS1 S23cn120/208/240ST 2S132 transformers 7200 - 240/120 VPrimary Vp VLL 7200 V Secondary VLL Vp 240 VTransformers exchange reactive power to provide balancedvoltages and currents to 3-phase loads2010
3/2/2018Lesson 11 et332b.pptxINSTRUMENT TRANSFORMERSHigh power measurement requires highly accurate transformers to reducethe levels or voltage and current to a safe range.Meter coil ratingsVoltage coils for voltmeters, power meters and watthour meters115 - 120 VacCurrent coils for ammeters, power meters and watthour meters2.5-5 A (100% overload allowed)(short time)In power systems, voltage levels 100’s of kV and thousands of ampsInstrument Transformers – converthigh voltages and currents tomeasurement levelsPotential transformers (PTs)Current transformers (CTs)21Lesson 11 et332b.pptxCURRENT TRANSFORMERS (CTS)Single turn primary - usually the conductor with the current to be measuredMulti-turn secondary - side connected to low power instrumentsSchematic SymbolsBushingtype CTCurrent transformer ratios200/5 200 amps input gives 5 ampsoutput (40/1)600/5 600 amps input gives 5 ampsoutput (120/1)Can overload rated values150-200% continuouslyStandardtype CT2211
3/2/2018Lesson 11 et332b.pptxPOTENTIAL TRANSFORMERS (PT’S)High accuracy transformers with minimum powerloss and very small voltage drop. Assume ideaoperation of both CTs and PTsPotential transformers reduces high voltages to110-120 Vac rangePotential Transformer Schematic SymbolSchematicsymbols shouldindicate polaritymarkTypical potential transformer ratios39,837-115 Vac6900- 115 VacCan exceedVin ratings byapproximately 10%23Lesson 11 et332b.pptxPT’S AND CT’S CHARACTERISTICSPower Ratings (burden)VA rating of all instruments connected can not exceedthis value or accuracy of transformers will sufferPhase ShiftTypically less than 1 degree of phase shift or less throughtransformerVoltage and Current MagnitudeMagnitude accuracy: Panel metering and protection 0.5%Revenue metering 0.1%2412
3/2/2018Lesson 11 et332b.pptxPT/CT SAFETY CONSIDERATIONSPTs - always ground secondary. Capacitive coupling cancause dangerously high voltage to develop on low voltage windingsCTs - NEVER OPEN LOAD ON CT CIRCUIT.ALWAYS SHORT CT SECONDARY TERMINALSBEFORE REMOVING LOAD. Can induce 2-6kV on open secondary leads2-6 kV25Lesson 11 et332b.pptxET 332b Ac Motors, Generators and Power SystemsEND LESSON 11: TRANSFORMER NAME PLATEDATA AND CONNECTIONS2613
Unequal Transformer Power Ratings Smaller size indicates lower power rating 12.47 kV Line-to-line primary Secondary connections . 3/2/2018 10 DELTA-WYE CONNECTION- 30 PHASE SHIFT Lesson 11_et332b.pptx 19 3 Transformers 7200-240/120 V Primary V LL V p
