1、1-Power TransformersThe Vital component in Power SystemsContentnBasic PhysicsnComponentsnActive PartnCost DriversnTestingnOrder executionnManufacturing processnPacking and shippingnQualitynApplicationsWhy Power Transformers?nWith Transformers men can produce,transport and distribute electricity at t
2、he most convenient(economical!)Voltages.nA transformer is a component that receives power at one voltage and delivers virtually the same power at another voltage.ContentnBasic PhysicsnComponentsnActive PartnCost DriversnTestingnOrder executionnManufacturing processnPacking and shippingnQualitynAppli
3、cationsThe Transformer nThe main transformer input is one voltage and the power.100%POWERVOLTAGE 1A small part of the energy will leave the transformer as heat.VOLTAGE 299.7%POWERPower(MVA)=Sqrt3 x Voltage(kV)x Current(A)Thus the current is given indirectly!nThe main transformer output is another vo
4、ltage and virtually the same power.nA transformer may operate with the power flowing in any direction.Ex:100 MVA and 230 kV gives 251 AEx:100 MVA and 23 kV gives 2510 ABasic physicsThe Transformer Power enter through bushings at a certain voltageCoolers to dissipate the heat.Inside the tank are the
5、windings and coreTap Changer to regulateoutput voltageConservator to allow expansion of insulating oil.Three Phase SystemsnVarying voltages and currents means in time varying power.To obtain reduced power variation it is much better to generate and consume electric energy using 3 phases rather than
6、just one.nVirtually all existing power transformers were made for 3-phase systems.123Basic physicsThree Phase SystemsThree Phase Symetrical Voltage-1.5-1-0.500.511.5012345678 TimeVoltageUrUsUtThe yellow voltage is identical to the blue but it is displaced 120 degrees or rather 6.67 milliseconds(for
7、a 50 Hz system.)6.67 ms6.67 msThe purple voltage is identical to the yellow but it is displaced 120 degrees or rather 6.67 milliseconds(for a 50 Hz system.)The blue voltage is identical to the purple but it is displaced 120 degrees or rather 6.67 milliseconds(for a 50 Hz system.)6.67 msBasic physics
8、Three Phase Transformer n6 or more bushings.3 Low Voltage Bushings3 High Voltage BushingsPlus possible grounding bushings and/or test bushingsMain physical principlenWhat makes the transformer possible are two physical facts:n1)A current flowing through a conductor will create on its surroundings a
9、magnetic field.n2)A varying magnetic field will cause a voltage to appear on a neighboring conductor.Basic physicsSome useful formulasThe induced voltage in a transformerV=4,44*N*Btop*Afe*fWhere N is number of winding turnsBtop is flux density value(Tesla)Afe is core limb areaf is frequency of appli
10、ed voltageWeights and loss dependance of transformer power(same material utilization)Losses,Weights (Power)0,75Example:67 MVA unit,active part weight=46 ton,losses=200 kWA 100 MVA unit?Weight=(100/67)0,75*46 ton=62 tonLosses=(100/67)0,75*200 kW=270 kWBasic physicsContentnBasic PhysicsnComponentsnAct
11、ive PartnCost DriversnTestingnOrder executionnManufacturing processnPacking and shippingnQualitynApplicationsMain Active ComponentsnPower transformers will have a CORE to take care of the magnetic flux.nPower transformers will have WINDINGS to lead currents and to provide the correct voltage.nThe se
12、t formed by the windings and the core,plus the unavoidable electric insulators and mechanical frame,is called ACTIVE PART.nThere are two basic practical ways to put the CORE and the WINDINGS together in an ACTIVE PART;they are called the Core type design and the Shell type design respectively.Compon
13、entsCore Type and Shell Type TransformersCore TypeShell TypeABB global concept Trafostar is based on core type technologyComponentsThe CorenThe core will work as a cage for the magnetic flux.Most of the flux will be kept inside the core.nThe core is made with a very special electrical steel.The stee
14、l is made as thin(0.30 mm)insulated sheets which must be mounted one by one.nThe final goal is to minimize the size of the core and minimize the unavoidable losses.Magnetic FluxComponentsMost common core typesSingle phasecoresThree phasecores2 wounded limbs1 wounded limb and2 side limbs2 wounded lim
15、bs and2 side limbs3 wounded limbs3 wounded limbs and2 side limbsComponentsThe WindingsnThe currents will exclusively flow along copper conductors wound in coils or windings.nThe copper conductor is made of thin insulated strands.nBetween turns in a coil some insulation normally paper is needed.Somet
16、imes even spacers.This is due to the fact that there are big voltages between turns in a winding.CurrentsComponentsWinding typesDisc WindingMany turnsLow currentHigh voltageHelical WindingFew turnsHigh currentLow voltageLoop WindingSeparate layer winding with tappings to regulate voltageLow voltageH
17、igh voltageRegulation of High voltageExample:130+-9*1,25%/11 kVComponentsVoltage distribution along windingAt service voltageAt lightning impulse voltageWinding topWinding bottomVoltageDisc 1Disc NExample:Service voltage per disc=400kV/sqrt3*140 discs=1.65 kVFast transient voltages-like lightning su
18、rges creates non-linear voltage distributions within the windings ComponentsThe continouosly transposed cable(CTC)toMinimize load losses Improve short circuit withstand strengths(Glue)Used mostly in low voltage windings but is frequently also used in high voltage windingsTransposition=Change places
19、of each individual strand of the current carrying cable along the winding heightWinding cableComponentsHelical Low VoltageTypical winding for low voltageIt is often made with Netting-tape transposed cable in order to reduce the size and still have a good cooling.Product componentsFoil Low Voltage,us
20、ed in large distribution transformersLow voltage windingnCooling ducts between layersnHigh short circuit strengthnMinimum additional lossesnDump resistantComponentsDisc WindingTypical winding for HVNeeds transpositionsMay have center entryProduct componentsRegulating WindingMany taps because it will
21、 get connected to the tap-changerProduct componentsWhy Tap Changers?Voltages(U1 or U2)vary in a power network depending on the loading(I1 and I2).This variation must be kept within certain limits.The Turn ratio of a transformer located between two voltage systems can be used to stabilise the voltage
22、.The turn ratio of the transformer must be possible to change.Tap changer is therefore used to make the change of the turn ratio.N1=Number of turns to build up voltage U1N2=Number of turns to build up voltage U2N1/N2=Turn RatioU2=N1/N2*U1ComponentsOn load Tap ChangersIn-tank=Mounted inside of thetra
23、nsformer tankComponentsOn load Tap ChangersOn-tank=Mounted on the outside of thetransformer tankComponentsBushingsTo be able to bring high voltage and current from power line to the active part of the transformer bushings are neededComponentsDry BushingsComponentsBushingsCable box connectionsfor a v
24、ariuos types of power cables.ComponentsOther accessoriesnGas operated relay;to give warning and tripping due to gas production.nOil level indicator;located on oil expansion vessel or at ground level.nDehydrating breathers;Drying of air in contact with oil expansion system.nThermometers;Measurement o
25、f top oil and winding hot spot temperatures.nRelief device and sudden pressure relay;Detects overpressure in tank1.2.3.4.5.ComponentsOther accessoriesDehydrating breatherGas operated relayPressure relief deviceMan holeOil conservator with rubber sacSudden pressure relayBuilt in CT:s for temperature
26、indication,relay function,power measurementComponentsOther accessoriesOil level indicatorControl cabinetsThermometersComponentsOil expansion systemDue to temperature differences(-30 C to+70 C)Oil needs to expand.Typical value is 10%of oil volume in tankis needed for the oil conservator750 MVA transf
27、ormer has a conservatorvolume of 20 tons of oil.ComponentsContentnBasic PhysicsnComponentsnActive PartnCost DriversnTestingnOrder executionnManufacturing processnPacking and shippingnQualitynApplicationsThe Active PartnThe windings will be mounted on the limbs of the core.nAll support and distance e
28、lements are made of pressboard-a very fine and clean kind of cardboard.nPaper and pressboard must be thoroughly dried in an oven before putting the transformer in a tank.nThe tank will be filled with degasified mineral oil under vacuum.nClamp frames withstand the transformer weight.Active PartThe Ac
29、tive PartnCleats and leads connect windings to external bushings and tap changers.nTap changers affect the overall mechanical design and the relevant transformer dimensions.nTransformer clamp frame usually integrate the tap changer mechanical support.The Active PartCoreCore insulationCore ClampWindi
30、ngs Main insulationWindingsCleats and leadsTap ChangerBushingsActive PartThe active partCleats and Leads connecting winding outlets with tap changerActive PartContentnBasic PhysicsnComponentsnActive PartnCost DriversnTestingnOrder executionnManufacturing processnPacking and shippingnQualitynApplicat
31、ionsPower ratingEnvironmentalrestrictionsCoolingVoltage ratingMechanicalRequirementsImpedanceLoss evaluationRegulationInsulationlevelsTestsSpecificTemp.gradientsTransportlimitationaccelerationSeismicloadsFull ScaleSCC testCT:sTechnical parameters driving costsOverloadsNo of Voltage Systems Noiseleve
32、lDC currentsHarmonicsControl cubiclesCost DriversTransformer EconomyMaterial costs Winding copperCore laminationTankOilComponents like bushings,tap changer etc.Capitalized CostsGuaranteed no-load loss times the corresponding evaluationGuaranteed load loss times the corresponding evaluationDesign Cos
33、tsManufacturing CostsOver Head CostsTransport CostsSales Price levelComparison Price levelCost DriversComparison PriceTypical loss evaluation figuresNo load lossLoad lossGenerator Step Up Transformer3-4 kUSD/kW3-4 kUSD/kWSystem Intertie Transformer3-4 kUSD/kW1-2 kUSD/kWLow loss evaluationTransformer
34、Lower costsHigher lossesHigh loss evaluationTransformerHigher costsLower lossesLowest comparison priceLowest comparison priceMore copper,Core steel,oil and bigger tankMore coolersCost DriversLoss evaluation exampleTransformer:100 MVA,220/69 kV with cooling ONAN(70%)/ONAF(100%)Loss evaluation:No load
35、=4000 USD/kW,Load and auxiliary=2500 USD/kWTwo designs have been given to a customerHigh loss alternativeWith lower Sales PriceLow loss alternativeWith higher Sales PriceSales Price:800 000 USD1 000 000 USDNo load loss evaluation:60 kW*4000 USD50 kW*4000 USDLoad and aux.loss evaluation:305 kW*2500 U
36、SD205 kW*2500 USDComparison Price:1,803 MUSD1,713 MUSDCost DriversLoss evaluationNo load losses exist when transformer is energized.Load losses are dependent on actual loading of transformer.Load loss evaluation therefore depends on customer forecast of future loading.Example:At 100%loading,load los
37、ses are 200 kWAt 70%loading,load losses are 98 kWLoad losses load current*2Cost DriversImpedancenReactance is a measure of the energy located in the leakage flux,i.e.,the flux outside the core.nCustomers normally specify it.The reasons they do are:nA suitable impedance will provide a good stability
38、range for the systemnImpedance can be used to limit the short-circuit current in a point of the network.Cost DriversImpedance and LossesLossesImpedanceLoad LossesNo-Load LossesHigh short circuit currentsMechanical forces:-inside the windings-between the windingsHigh leakage fluxHot-spots and cooling
39、 problemsImpedance and Active MaterialsLow ImpedanceHigh ImpedanceImpedanceCost DriversHigh short circuit currentsHigh leakage fluxMechanical forces:-inside the windings-between the windingsHot-spots and cooling problemsPriceImpedanceImpedance and Transformer CostsImpedanceCost DriversTransformer lo
40、ssesnNo-load losses take place mainly in the core sheets.It corresponds to about 25%of the transformer losses.nThe DC losses take place exclusively in windings.nThe Eddy/Stray losses take place in all pieces of metal inside the transformer(clamps,core,winding,tanks).A large part of it are generated
41、in the windings.nEspecially dangerous losses are the ones concentrated in a small volume since they may cause a hot-spot.No-loadLoadDC or ResistiveEddy or StrayCost DriversCoolingnIn principle the larger the losses in the Inner Circuit the larger the size of the Outer Circuit(coolers or radiators)nT
42、here is nevertheless a limit either due to the size of the coolers or to the impossibility of cooling a certain spot(hot-spot)in the Inner Circuit.nA pump to move the oil is often unnecessary.The generated heat will act as a siphon.FanoptionalPumpoptionalOuter CircuitInner CircuitOil immersedTankHea
43、tProduction(Core and Windings)HeatDissipationCost DriversCooling typesIECOld ANSIONANOAONAFFAOFAFFOAODAFFOAaOFWFFOWOil Natural Air NaturalRadiatorsOil Natural Air ForcedRadiators with FansOil Forced Air ForcedHeat Exchangers with pumps and fansOil Forced Water ForcedWater coolers with pumpsOil Direc
44、ted Air ForcedHeat Exchangers with pumps and fans+oil pipes intank to guide oil into the windingsCost DriversTemperaturesHeightTemperature gradient(K)Top of windingBottom of windingTop Oil TTAverageof oilBottom Oil TBTwoHot Spot THspAverageWinding temp TWGuaranteed Temperatures Average yearly temper
45、ature 20 CTw(mean winding temp.rise)65 K (85 C)TT(top oil temp.rise)60 K (80 C)THsp(winding hot spot temp.rise)78 K (98 C)THsp=TT+K1*Two according to IECCost DriversTRANSPORTABBContentnBasic PhysicsnComponentsnActive PartnCost DriversnTestingnOrder executionnManufacturing processnPacking and shippin
46、gnQualitynApplicationsWhy Transformer tests?nTo confirm that the transformer meets the customer specification,applicable standards and the guaranteed limits.nSuitable Tests will prove that the transformer is able to cope with the normal and abnormal conditions it may meet when in use.TestingnTempera
47、tures cannot exceed specification and standards limits during normal operation.nLosses cannot exceed losses figures specified by the customer.nTransformers have to withstand dielectric stresses of networks.nTransformers have to withstand short circuit currents due to faults over the networks or on t
48、erminals.Tests are cost drivers.It is essential to specify them clearly in the contract!TestingTransformer tests:nThe routine tests are carried out on all transformers.nThe type tests are carried out usually on one unit only,in case of a multiple order,to verify the design.nSpecial tests are carried
49、 out on customer request.TestingAll transformer are tested according to customer requirements and last IEC 60076,60146,60358 standards or ANSI standards.TestingTransformer routine tests:nWinding resistance.nVoltage ratio and phase displacement.nShort circuit impedance and load loss.nNo load losses a
50、nd current.nDielectric routine test(according to IEC 60076-3).nTest on OLTC where appropriate.Test definitions according IEC 60076-1Transformer type tests:nTemperature rise test.nDielectric type tests(according to 60076-3).TestingTransformer special tests:nDielectric special test(according to IEC 60
